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Accepted papers to appear in an upcoming issue

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Design and Development of a Near-Infrared Plasmonic Gas Sensor based-on the Graphene Nano-Gratings

Morteza Maleki, Mahdiyeh Mehran, and Arash Mokhtari

DOI: 10.1364/JOSAB.401589 Received 02 Jul 2020; Accepted 23 Sep 2020; Posted 24 Sep 2020  View: PDF

Abstract: In this work, a gas sensor based on the plasmonic double-layer graphene nano-grating (GNG) structure with an enhanced figure of merit is presented in the near-infrared region. This structure includes double periodic graphene nanoribbon arrays, separated by a dielectric. The wavelength interrogation is employed to accurately investigate the behavior of the proposed structure for various physical and geometrical parameters including the array pitch, graphene nanoribbons width, refractive index of the intermediated dielectric between GNGs, the chemical potential of the graphene, and the angle of the incident light. For the optimized design, obtained sensitivity and figure of merit (FoM) are 430.91 nm/RIU and 174.68 RIU-1, respectively, when the finite element method is used for the simulations. In addition to the high performance and FoM, the structure is robust to the misalignment of two GNG layers offering a solution for practical gas sensing applications. To the best of our knowledge, the proposed GNG-based structure enjoys a boosted FoM compared to the previously-proposed integrated gas sensors as well as a practically feasible design for fabrication.

Optical solitons in saturable nonlinear medium in the presence of an asymmetric complex potential

Solange Cavalcanti and Frederico Moreira

DOI: 10.1364/JOSAB.403785 Received 30 Jul 2020; Accepted 23 Sep 2020; Posted 24 Sep 2020  View: PDF

Abstract: We report on the existence of families of stable spatial solitons in a saturable nonlinear medium characterized by a refractive index with asymmetric distribution of gain and loss. The properties of the nonlinear modes bifurcating from the eigenvalue of the underlying linear problem are thoroughly investigated. The eigenvalue ranges in the power-eigenvalue diagrams for different gain/loss profiles are inspected. We find that the saturable nonlinearity severely restricts these ranges as the eigen-values tend quite fast to an asymptotic profile, as power increases. Numerical simulations of the wave equations are carried out and examples of the dynamics of the asymmetric solitons obtained, exhibit a remarkable agreement with the analytic stability results

Realizing the ultra-bandwidth cross-polarization conversion by a double-layer metasurface

YuPeng Li, HaiFeng Zhang, Tong Yang, Tangyi Sun, and Li Zeng

DOI: 10.1364/JOSAB.402479 Received 10 Jul 2020; Accepted 22 Sep 2020; Posted 24 Sep 2020  View: PDF

Abstract: In this paper, a double-layer structure of ultra-bandwidth cross-polarization converter (PC) has been devised in line with the rationale of the metasurface. The project can bring about a superior 90° conversion effect of the incident wave when it is incident perpendicularly. This PC is an improvement and majorization of the single-layer PC with a bowknot surface structure, which can hit the mark of improving the polarization conversion ratio (PCR) in the low frequencies and enlarging the bandwidth. In this article, not only the basic principle of polarization conversion but also the discussion of parameters are mentioned. Besides, the point of view such as incidence angle and current are utilized for analysis. Ultimately, the cross-polarization conversion can be achieved in the range of 0.63-1.50 THz, whose relative bandwidth is 81.7% and the bandwidth is 0.87 THz. The proposed design may be worth applying in radar, communication, and imaging technologies.

Polarization-insensitive dielectric metalenses with different numerical apertures and off-axis focusing characteristics

Chen Deli and X Sun

DOI: 10.1364/JOSAB.400227 Received 15 Jun 2020; Accepted 22 Sep 2020; Posted 24 Sep 2020  View: PDF

Abstract: Optical metasurfaces with ultra-thin, planar structure can easily adjust the phase and polarization of light waves. Based on the principle of the resonant phase, a cubic nanopillar is designed to construct the polarization-insensitive metalens in the visible light range. By simulating the phase and transmission efficiency of cubic nanopillar with a width of 50~200 nm, the optimized heigth of nanopillar is obtained. Metalenses with different numerical apertures are designed to obtain the sub-wavelength focusing effect. The focusing performance of the same metalens is the same when the incident light is right-handed and left-handed circular polarization light respectively, which proves that the designed structure is insensitive to circular polarization light. At the same time, the different off-axis focusing metalenses which can achieve flexible focusing in the desired spatial location are also designed and investigated. This research provides a reference to design polarization-insensitive photonic integrated components and flexible imaging metalenses.

Dark soliton steering in $\mathcal{PT}$-symmetric couplers with third-order and intermodal dispersions

DIPTI MAHATO, Govindarajan Arjunan, and Amarendra Sarma

DOI: 10.1364/JOSAB.402606 Received 14 Jul 2020; Accepted 22 Sep 2020; Posted 22 Sep 2020  View: PDF

Abstract: We demonstrate steering dynamics of dark soliton in a parity-time ($\mathcal{PT}$) symmetric nonlinear directional coupler (NLDC) in the presence of third-order dispersion (TOD) and intermodal dispersion (IMD). A complete switch with an excellent efficiency at a very low critical power, even lower as compared to the bright soliton switching has been observed. The numerical results show that both TOD and IMD have no effect on soliton steering in $\mathcal{PT}$-symmetric couplers with coupling length $\pi/2$. But as we increase the coupling length to $2\pi$, IMD shows marginal effects for dark soliton steering in $\mathcal{PT}$-symmetric couplers while TOD shows no impact. Additionally, we have also studied the phase-controlled switching in $\mathcal{PT}$ couplers with two different coupling lengths and demonstrated its advantage over the power-controlled one.

Self focusing in nodal semimetals

Chao Zhang and Jack Zuber

DOI: 10.1364/JOSAB.403512 Received 22 Jul 2020; Accepted 21 Sep 2020; Posted 24 Sep 2020  View: PDF

Abstract: We analyize the Kerr index, associated critical power and focal length for nodal semimetals in a Weyl, Dirac and gapped semimetal phase. Our model produces a Kerr index of $10^{-15}-10^{-20}$m$^2$/W in the micro-meter wavelength range. Novel dependence of the Kerr index, critical power and focal length on temperature, chemical potential, band gap and node separation are readily obtained through our model and show that the tunability of the non-linear response of a nodal semimetal persists in its focusing phenomenon.

Hyperbolic metamaterial-based metal-dielectric resonator-antenna designs for GHz photon collection rates from wide-range solid-state single photon sources

Nadeem Ahmed, Saba Akhtar, and Faraz Inam

DOI: 10.1364/JOSAB.402890 Received 15 Jul 2020; Accepted 20 Sep 2020; Posted 22 Sep 2020  View: PDF

Abstract: Solid-state single photon sources (SPS) based on quantum dots as well as color centers in diamonds, silicon-carbide have promises for application in emerging quantum technologies. Many of these technologies however demand photon rates in the GHz range, thereby rendering the use of these single photon sources where the maximum observed count rates are limited to few tens of MHz. Here we first study the performance of hyperbolic metamaterial based 5-layered metal-dielectric resonator antenna structures with metallic as well as hybrid metal-dielectric antennas in the wavelength range of 600 to 1000 nm. The performance of these resonator-antenna structures was analyzed for the Purcell enhancement, quantum efficiency (QE), collection efficiency (CE) and normalized collected photon counts (NCPC). The hybrid metal-dielectric antenna helps in providing the directivity to the dipole emission, thereby significantly improving the collection efficiency. We then present the novel design of a 5-layered metal-dielectric pillar resonator. This resonator structure with a metallic cylindrical antenna over the top showed significantly large fluorescence enhancement values. The Purcell factor was observed to reach close to 1600 at 680 nm corresponding to the central peak of the NV centre spectrum. The NCPC value reached close to 550 at 680 nm. The maximum CE from the structure was observed to be around 60% with the maximum QE reaching close to 80%. With the above performance, the detected photon count rates for a solid-state SPS is expected to be well into the GHz range. Our designs show a state-of-the art improvement in the antenna performance for SPS with properties very close to a practical SPS.

Transition cancellations of ⁸⁷Rb and ⁸⁵Rb atoms in a magnetic field setting new standards

Artur Aleksanyan, Rodolphe Momier, Emil Gazazyan, Aram Papoyan, and Claude LEROY

DOI: 10.1364/JOSAB.403862 Received 30 Jul 2020; Accepted 17 Sep 2020; Posted 17 Sep 2020  View: PDF

Abstract: We have analyzed the magnetic field dependencies of intensities of all the optical transitions between magnetic sublevels of hyperfine levels, excited with σ+, π and σ- polarized light, for the D₁ and D₂ lines of ⁸⁷Rb and ⁸⁵Rb atoms. Depending on the type of transition and the quantum numbers of involved levels, the Hamiltonian matrices are of 1×1, 2×2, 3×3 or 4×4 dimension. As an example, analytical expressions are presented for the case of 2×2 dimension matrices for D₁ line of both isotopes. Eigenvalues and eigenkets are given, and the expression for the transition intensity as a function of B has been determined. It is found that some π transitions of ⁸⁷Rb and ⁸⁵Rb get completely canceled for certain, extremely precise, values of B. No cancellation occurs for σ+ or σ- transitions of D₁ line. For matrices with size over 2×2, analytical formulas are heavy, and we have performed numerical calculations. All the B values cancelling σ+, π and σ- transitions of D₁ and D₂ lines of ⁸⁷Rb and ⁸⁵Rb are calculated, with an accuracy limited by the precision of the involved physical quantities. We believe our modeling can serve as a tool for determination of standardized values of magnetic field. The experimental implementation feasibility and its possible outcome are addressed. We believe the experimental realization will allow to increase precision of the physical quantities involved, in particular the upper state atomic levels energy.

Evolution of Bessel–Gaussian beam modeled by fractional Schrödinger equation

yagang zhang, zhenkun wu, Jingmin Ru, Feng Wen, and yuzong gu

DOI: 10.1364/JOSAB.399840 Received 08 Jun 2020; Accepted 17 Sep 2020; Posted 18 Sep 2020  View: PDF

Abstract: In this study, we theoretically and numerically investigate the evolution of one-dimensional (1D) and two-dimensional (2D) Bessel–Gaussian (BG) optical beams in a linear and nonlocal nonlinear fractional Schrödinger equation (FSE). As the propagation distance increases, the linear propagation dynamics of the BG beams undergo an initial compression phase before they split into two sub-beams. The sub-beams with a saddle shape separate from each other and their interval increases linearly. However, when the FSE carries nonlocal nonlinearity, BG beams follow a zigzag trajectory in real space, which corresponds to a modulated anharmonic oscillation in momentum space. In the 2D case, the input chirped BG beam first evolves into a filament in real space and then into a ring structure; if the input is a superposed BG beam carrying orbital angular momentum, the rule fulfilled in evolution is similar to that for a single one, and it forms a funnel-like structure, with periodic inversion and variable rotation. Our studies increased understanding of the FSE and nonlinear optics, which can potentially contribute to further developments in the fabrication of light modulators.

Incidence angle-dependent broadband chiral metamaterial for near-infrared light absorption

Junxing Fan, Dong Xiao, Ting Lei, and Xiaocong Yuan

DOI: 10.1364/JOSAB.403623 Received 23 Jul 2020; Accepted 17 Sep 2020; Posted 18 Sep 2020  View: PDF

Abstract: The ability to spin-selectively absorb circularly polarized light (CPL) plays a critical role in various photonic devices. Here we propose and investigate a broadband chiral metamaterial composed of asymmetric split-ring resonators (SRRs), showing a wide spin-selective absorption band from 950 nm to 1200 nm with pronounced circular dichroism (CD) up to 20°. We demonstrate that the broadband absorption spectra originate from induced dual chiral resonance modes. Meanwhile, the two different resonances can be adjusted independently, suggesting great flexibility to the designed chiral absorption band for different purposes. Also, the chiral-selective absorption performance is highly dependent on the oblique incident angle due to the extrinsic chirality. The chiral resonance modes can either be enhanced or destructed under oblique incidence. Such angle-dependent broadband chiral metamaterials may find potential applications for spin-orbit communications, chiral detection, polarimetric imaging, and biosensors.

Asymmetric transmission in bilayer chiral metasurfaces for both linearly and circularly polarized waves

Yi Ren, Chun Jiang, and Bin Tang

DOI: 10.1364/JOSAB.401783 Received 01 Jul 2020; Accepted 16 Sep 2020; Posted 16 Sep 2020  View: PDF

Abstract: In this paper, we numerically and theoretically propose an architecture with bilayer chiral metasurfaces which can achieve asymmetric transmission (AT) for both linearly and circularly polarized waves in the near-infrared region. The research results show that the structure can obtain a maximum AT ~ 0.65 and polarization conversion ratio (PCR) over 80% for linearly polarized waves. In addition, dual-band AT can even be achieved for circularly polarized waves. The electric field distribution and current vector are presented to explain the physical mechanisms of the AT effects. Our research work may have some potential applications in designing optical nonreciprocal devices, such as isolators, polarizers, rotators, etc.

Controlling the mode profile of photonic crystal nanobeam cavities with mix-and-match unit cells

Sami Halimi, Zhongyuan Fu, Francis Afzal, Joshua Allen, Shuren Hu, and Sharon Weiss

DOI: 10.1364/JOSAB.398574 Received 05 Jun 2020; Accepted 16 Sep 2020; Posted 17 Sep 2020  View: PDF

Abstract: We report simulations and experimental measurement of a photonic crystal (PhC) designed with different unit cell geometries in a single device. This “mix-and-match” approach enables enhanced mode manipulation by incorporating non-traditional unit cell shapes into a one-dimensional PhC nanobeam cavity. Inclusion of a bowtie-shaped unit cell in the center of a mix-and-match PhC nanobeam cavity comprised elsewhere of either circular or antislot unit cells leads to a two order of magnitude reduction in the mode volume of the cavity while maintaining a similar quality factor.

A Silicon Carbide photonic platform based on suspended subwavelength waveguides

Francesco Garrisi, Ioannis Chatzopoulos, Robert Cernansky, and Alberto Politi

DOI: 10.1364/JOSAB.403170 Received 20 Jul 2020; Accepted 16 Sep 2020; Posted 17 Sep 2020  View: PDF

Abstract: Silicon Carbide (SiC) displays a unique combination of optical and spin-related properties that make it interesting for photonics and quantum technologies.However, guiding light by total internal reflection can be difficult to achieve, especially when SiC is grown as thin films on higher index substrates, like Silicon.Fabricating suspended, subwavelength waveguides requires a single lithography step and offers a solution to the confinement problem, while preserving the design flexibility required for a scalable and complete photonic platform.Here we present a design for such platform, that can be used for both classical and quantum optics operation. We simulate the key optical components and analyze how to exploit the high nonlinearities of SiC and its defects.

Frequency comb solutions for driven $\chi(2)$ optical microresonators

Evgeny Podivilov, Boris Sturman, and Ingo Breunig

DOI: 10.1364/JOSAB.402007 Received 03 Jul 2020; Accepted 15 Sep 2020; Posted 16 Sep 2020  View: PDF

Abstract: Frequency combs in $\chi(2)$ optical microresonators caused by cascaded second-order nonlinear processes attract nowadays a great research interest. In contrast to $\chi(3)$ resonators, two light amplitudes relevant to the first and second harmonics (FH and SH), two dispersion coefficients, and a considerable difference of the FH and SH group velocities (the walk-off) haveto be taken into account to investigate localized coherent structures (solitons) propagating with a common velocity. Finding such comb solutions taking into account an external pumping is a crucial step towards the $\chi(2)$ combs. We report on two new families of driven soliton-combsolutions for $\chi(2)$ microresonators. They are strongly localized corresponding to spectrally broad combs and possess well defined FH and SH frequency detunings, the propagation velocities, and, generally, nonzero spatial backgrounds.

Multi-channel mode-selective silicon photonic add/drop multiplexer with phase change material

Yihui Wei, Ming Zhang, and Daoxin Dai

DOI: 10.1364/JOSAB.400897 Received 25 Jun 2020; Accepted 15 Sep 2020; Posted 16 Sep 2020  View: PDF

Abstract: A multi-channel mode-selective silicon photonic add/drop multiplexer is proposed by introducing asymmetric directional couplers (ADCs) assisted with phase change material (PCM) strips. Each ADC consists of a multimode bus waveguide and a singlemode access waveguide with a thin PCM strip on the top. For the present mode-selective add/drop multiplexer, the PCM of Ge2Sb2Se4Te1 (GSST) is used and the PCM-loaded access waveguide is designed optimally by engineering the effective index and the modal field profile. When the switching state of the add/drop multiplexer is OFF with crystal GSST, all the mode-channels go through with very low excess losses. Otherwise, when it is ON by switching the GSST-PCM strip to be amorphous, the desired TE mode-channel can selectively be added to or dropped from the multimode bus waveguide, while the other mode-channels still stay and propagate in the multimode bus waveguide. The simulated results show that the designed mode-selective silicon photonic add/drop multiplexer utilizing single ADC have low excess losses of < 0.5 dB and low intermode crosstalks of <−15 dB over the C-band for all the mode-channels. Furthermore, using the design with cascaded ADCs for each mode-selective add/drop switch, the crosstalks can be suppressed to <−20 dB and the excess losses are less than 1 dB over an 80 nm-broad wavelength band from 1505 nm to 1585 nm.

Wideband two-process frequency conversion under stimulated Raman adiabatic passage via a continuum of dark intermediate states

Pragati Aashna and Krishna Thyagarajan

DOI: 10.1364/JOSAB.401416 Received 29 Jun 2020; Accepted 14 Sep 2020; Posted 16 Sep 2020  View: PDF

Abstract: In this paper we propose a novel hybrid planar-channel waveguide configuration with the channel waveguide being submerged within the planar region to study two simultaneous three wave mixing processes under stimulated Raman adiabatic passage. In our study, an input frequency is converted to an output frequency lying very close to or very far from the input frequency via an intermediate frequency which is in the form of a continuum of modes in the planar region while all the other frequencies propagate as guided modes of the channel waveguide. The continuum of modes at the intermediate frequency allows the simultaneous phase matching to be satisfied over a wide range of wavelengths, hence leading to a wideband and efficient conversion from input to the output frequency without any significant power accumulation at the intermediate stage which is ensured through the counterintuitive as well as adiabatic variation in the nonlinear coupling strengths.

THz Power Optimization and Analysis of Plasmonic Unbiased Photoconductive THz Emitters coupled to Novel Spiral-like Dipole Antenna

M. J. Mohammad-Zamani

DOI: 10.1364/JOSAB.398675 Received 26 May 2020; Accepted 14 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: We propose a new spiral-like dipole antenna-coupled and unbiased THz photomixer with the plasmonic unsymmetrical interdigitated electrodes (UIEs). Each electrode pair is made of subwavelength metal/semiconductor/metal (MSM) structures constituting heterogeneous Schottky contacts. From the optical view and analysis of the electrical transport in the semiconductor, by applying a systematic procedure in order to achieve optimum design, it is possible to benefit from advantages of having high THz photocurrent derived from the plasmonic enhanced optical absorption and hence photogeneration as well as severe built-in shottcky field in the photoconductor. Furthermore, from the electromagnetic view, by introducing an output resonant THz antenna having highly match impedance with photomixer device, the efficiency of free-space coupling has significantly been enhanced. By exploiting of all aspects of design, it has been provided to achieve the THz power of 409 µW mW from the spiral-like dipole antenna coupled to the proposed plasmonic UIEs with 10×10 µm2 active area and pitch of Λ=395 nm. This is more than one order of magnitude larger THz power than the highest THz power radiated from per same area of a conventional dipole antenna-coupled unbiased array of the near-field emitters. Also, this improvement is more than three orders of magnitudes higher THz power as compared with the power radiated from a non-plasmonic far-field operating antenna-less THz chip of the same size.

Multifunctional Space-Time Phase Modulated Graphene Metasurface

Mohammad Mahdi Kashef and Zahra Ghattan Kashani

DOI: 10.1364/JOSAB.401333 Received 29 Jun 2020; Accepted 14 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: Metasurfaces provide special features for manipulating electromagnetic wavefronts that are not possible with conventional optical devices. A common approach in designing metasurfaces has been the use of spatially varying metallic and/or dielectric nanoantennas separated with a subwavelength distance to obtain the required local phase change yielding the desired optical performance. In this paper, we propose a space-time phase modulation technique based on a graphene metasurface with the possibility of actively manipulating the electromagnetic wavefront. In this technique, we utilize graphene microribbon arrays that exhibit resonant behavior at terahertz (THz) frequencies. By applying an alternating voltage with a particular modulation frequency and phase, the time-dependent changes in the complex refractive indices of the graphene ribbons can be induced. This phenomenon results in the active control of the reflection amplitude and phase, and the generation of the harmonic frequencies in the output reflection spectra. Theoretically, by using the Floquet analysis it is shown that the reflected wave has harmonic frequencies and the phase of the reflection wave at each harmonic component changes through changing the modulation phase of each graphene ribbon. The performance of the wavefront manipulation technique is evaluated using the Finite Difference Time Domain (FDTD) method and the circuit model. The results of the proposed circuit model are in good agreement with those of the full-wave simulation. Additionally, the applications of the proposed space-time phase modulated graphene metasurface for realizing an anomalous reflector and a lens with a tunable focal length are explained in detail.

Coherent control of microwave pulse propagation based on gain assisted electromagnetically induced transparency in superconducting circuits.

MIR AYAZ, Sajid Qamar, and Shahid Qamar

DOI: 10.1364/JOSAB.399961 Received 11 Jun 2020; Accepted 14 Sep 2020; Posted 16 Sep 2020  View: PDF

Abstract: Based on gain assisted electromagnetically induced transparency (EIT), we propose a scheme for coherent manipulation and storage of a microwave pulse propagating through a linear array of superconducting artificial atoms (SAAs). In an EIT based system, signal operates in an absorption mode and its attenuation cannot be avoided during propagation. This results in low transmissivity of the medium. We show that based on gain assisted EIT, the attenuation can be ameliorated and a probe pulse can be amplified while it propagates through the medium with significant reduction in its group velocity. The linear array of SAAs can thus act as a gain assisted EIT based coherent microwave memory scheme with high fidelity.

Gaussian pulse Behavior in the Nonlocal Kerr-Like Media: Application to the generation of compact bright pulses

Kamgaing Mabou William, Chancelor POKAM, and YEMELE David

DOI: 10.1364/JOSAB.400730 Received 22 Jun 2020; Accepted 13 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: The dynamics of the gaussian pulse, usually generated by most of the Mode-locked Lasers is studied in the nonlinear waveguide supporting the propagation of compact bright (CB) pulse-like signals. By means of the collective variables (CVs) approach, the state equations describing the evolution of the gaussian pulse parameters are derived. It appears that the system exhibits an equilibrium state in which the pulse width is independent of the amplitude as it is the case for the CB pulse. In this state, the gaussian pulse propagates without modification of its parameters. However, in the presence of the small kick, namely induced by a weak linear dispersion, this pulse evolves towards the CB pulse highlighting the unstable character of this equilibrium state and more interesting the possibility of generation of the CB pulse by means of this waveguide operating in the anomalous dispersion regime. In addition, the newly formed CB pulse can exhibit the breathing mode with the frequency depending on the strength of the linear dispersion. Similarly, the gaussian pulse is subjected to the compression phenomena when its width is much high compared to the value at the equilibrium state, followed by the emission of CB of very small amplitude.

Tunable terahertz radiation generation using the beating of twosuper- Gaussian laser beams in the collisional nanocluster plasma

Robabeh Nemati Siahmazgi and Saed Jafari

DOI: 10.1364/JOSAB.404146 Received 04 Aug 2020; Accepted 11 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: In this paper, generation of terahertz (THz) radiation by the beating of two super- Gaussian laser beams in a nanocluster plasma is investigated, theoretically. The electric field of laser beams interact with the nanocluster, leads to the ionization of the cluster atoms, and producing nanoplasma. Interaction of laser beams with the electronic clouds of nanoplasma generates ponderomotive force that leads to the creation of a macroscopic electron current at the beat frequency which can generate THz radiation. The THz wave equations and THz efficiency are analytically derived in nanocluster plasma medium. The effects of the density and radius of nanoclusters, the laser beam width and intensity of super- Gaussian lasers on the THz radiation efficiency has been investigated in a nanoplasma- based THz emitter. The results indicated the maximum value of the field amplitude for THz radiation is the near the frequency of , that is, when the beat- wave frequency approaches the effective plasmon frequencies of the nanoplasma. In addition, it was found that the best situation that can be obtained the THz radiation occurs at frequency .

Dynamics of Coupled Two Qubits Interacting with Two-Photon Transitions via Nondegenerate Parametric Amplifier: Non-local Correlations under decoherence

Abdel-Baset Mohamed, Ahmed Farouk, M. F. Yassen, and Hichem Eleuch

DOI: 10.1364/JOSAB.405098 Received 10 Aug 2020; Accepted 11 Sep 2020; Posted 14 Sep 2020  View: PDF

Abstract: In this study, an analytical solution for a novel intrinsicnoise model represented by two coupled qubits, the Su(1, 1) andSu(2) Lie group, is investigated. Each qubit interacts with atwo-mode parametric amplifier through a non-degenerate 2-photonprocess when the two-mode system is initially in asuperposition of generalized Barut-Girardello coherent state. Thenonlinearity of the unitary interaction and the initial two-modefields lead to the generation of different quantum correlations (QCs),which are measured by log-negativity and skew informationquantifiers, uncertainty-induced non-locality, and local quantumuncertainty (LQU). The generated QC of the unitary interaction depends notonly on the two-qubit coupling, but also on the intrinsic noiseand the initial coherent intensity. Our results show that theability of the two-qubit coupling to protect and enhance therobustness and generation of the QCs depends on thesuperposition and the coherent intensity of the initial Su(1, 1)state. Furthermore, the sudden birth and death of the entanglementlog-negativity and the sudden variations of the LQU depend on theintrinsic noise and the two-qubit coupling.

Polarization properties of stimulated Brillouin scattering generators in linear birefringent fibers

Mark Bowers

DOI: 10.1364/JOSAB.402899 Received 17 Jul 2020; Accepted 10 Sep 2020; Posted 10 Sep 2020  View: PDF

Abstract: The transient theory of stimulated Brillouin scattering (SBS) is developed for optical fibers with constant modal birefringence along its length and arbitrary polarization of the pump and Stokes fields. SBS is initiated by thermally excited acoustic waves distributed along the length of the optical fiber, and the generated Stokes field fluctuates in time. It is shown that when a single-frequency pump field is launched such that the fiber is nearly equally excited along the slow and fast axes, the generated Stokes field is partially polarized for fiber lengths longer than its polarization beat length, due to temporal fluctuations in power and polarization. The Stokes field degree of polarization is reduced as the birefringence of the fiber is increased. For launched pump polarizations resulting in non-equal excitation along the slow and fast axes of the fiber, the generated Stokes field is preferentially linearly polarized along the principal axis that exhibits the highest Brillouin gain for fibers with birefringence such that its length is greater than twice its polarization beat length. As the fiber birefringence is reduced, the generated Stokes field state of polarization moves towards the launched pump polarization state, with the opposite sense of rotation. Numerical results are presented to quantify these effects as a function of single-pass Brillouin gain, fiber birefringence, and launched pump polarization state.

Design of resonant-cavity thin film structures with complex active layers

Andrew Sarangan

DOI: 10.1364/JOSAB.404894 Received 10 Aug 2020; Accepted 10 Sep 2020; Posted 22 Sep 2020  View: PDF

Abstract: A systematic approach to designing resonant-cavity distributed bragg reflector (DBR) structures with complex-index active regions is developed in this paper. The technique is based on the effective reflectance index of multilayer thin films. While the design of DBR structures with real refractive index films is quite straight-forward, the use of complex refractive index in the cavity requires a different approach. In this paper we show that a complex active region requires an asymmetric reflector geometry along with a separate phase compensator. This technique is illustrated using phase change materials (PCM) and metals in the resonant cavity.

PT-symmetric non-Hermitian AB-stacked photonic bilayer graphene

Zhao Jin, Di Zhang, xue-si li, Lian-Lian Zhang, Dong-Ze Fan, and Wei-Jiang Gong

DOI: 10.1364/JOSAB.398686 Received 26 May 2020; Accepted 09 Sep 2020; Posted 09 Sep 2020  View: PDF

Abstract: We theoretically investigate the band structure of the $\cal PT$-symmetric non-Hermitian photonic bilayer graphene, by considering the co-existence of the onsite energy detuning and $\cal PT$-symmetric imaginary potentials. It is found that $\cal PT$ symmetry causes the system's energy spectra to display salient features, which include the various energy-band degeneracy and the deformation of the band structure in the vicinity of the Dirac point. When this system is strained, the effect of $\cal PT$ symmetry tends to be weakened. The results in this work can help to describe the band structures of the $\cal PT$-symmetric two-dimensional photonic-lattice systems.

Generation and control of phase-locked Bessel beams with a persistent non-interfering region

Zhanna Rodnova, Tobias Saule, Richard Sadlon, Edward McManus, Nicholas May, Xiaoming Yu, Sina Shahbazmohamadi, and Carlos Trallero

DOI: 10.1364/JOSAB.400801 Received 07 Jul 2020; Accepted 08 Sep 2020; Posted 09 Sep 2020  View: PDF

Abstract: We present a novel method for the generation of multiple Bessel-like (Bessel-Gauss) beams with tilted wavefronts whose optical phases can be controlled independently. In the far field multiple field-free regions develop that are persistent to interference effects and the optical phase. The extent of these field-free regions can be controlled through the tilt angle of the wavefront with a 0.1 mrad precision. Experimentally we employ a spatial light modulator, a lens and an axicon, each suitable for narrow-band (continuous-wave) and broadband operation. Because of the simplicity in the optical elements the number of non-interfering Bessel-like beams can be scaled on demand for applications that require multiple sources for coherent imaging or multiple intense beams over a controllable region with the added benefit of a field-free region for detection. Here, for reasons of clarity, we characterized two beams.

Wavefront aberration correction utilizing liquid crystal alignment in geometric-phase Fresnel lens

Ryusei Momosaki, Kazunari Ashikawa, Kentaro Ohkoshi, Moritsugu Sakamoto, Kohei Noda, Tomoyuki Sasaki, Nobuhiro Kawatsuki, Yoshichika Tanaka, Takeya Sakai, Yukitoshi Hattori, and Hiroshi Ono

DOI: 10.1364/JOSAB.404724 Received 05 Aug 2020; Accepted 08 Sep 2020; Posted 09 Sep 2020  View: PDF

Abstract: We propose two types of wavefront aberration correction in geometric-phase Fresnel lenses (GPFLs). First, in the situations that the incident light on a GPFL has a wavefront aberration represented by the Zernike polynomial, the wavefront aberration is eliminated by setting the phase distribution that cancels the phase distribution of the incident light to GPFL. Second, the aberrations that occurs when a light wave is obliquely incident on the GPFL is compensated by a correction method that incorporates an optimization design using a genetic algorithm. These methods have been demonstrated not only by the theory but also by experiments using imaging systems in which the designed and fabricated GPFLs are utilized. These wavefront aberration correction method can be expected to overcome the wavefront aberration caused by the arrangement of the optical elements in an optical system and contribute to expanding the application range of GPFLs.

Optical single-sideband polarization modulator based on Sagnac interferometer and its applications in radio-over-fiber systems

Mohsen Ganjali, NAJMEH SAFAVI, Mohammadreza Qashqaei, and S.Esmail Hosseini

DOI: 10.1364/JOSAB.398804 Received 28 May 2020; Accepted 06 Sep 2020; Posted 10 Sep 2020  View: PDF

Abstract: A novel scheme to implement an optical single-sideband (OSSB) polarization modulator (PolM) is proposed and theoretically demonstrated in this paper. The proposed OSSB-PolM is based on the phase modulation technique in two consecutive Sagnac interferometers (SIs). It is shown that by applying four equal-power radio frequency (RF) signals with appropriate phases to electro-optic modulators and align the polarization state of the modulated signal in the proper direction, an OSSB-PolM signal will be generated. The proposed scheme can find many microwave photonic (MWP) applications. To show this, an OSSB modulator with tunable optical carrier-to-sideband ratio (OCSR), an OSSB-suppressed carrier (OSSB-SC) modulator/optical frequency shifter, and a full-ranged MWP phase shifter are proposed and demonstrated. The proposed structure includes several important advantages. For instance, the system is free from the limitations caused by optical/electrical filters. Since the propagating lights in an SI traverse the identical optical path in clockwise (CW) and counterclockwise (CCW) directions, the system is immune to environmental perturbations. Furthermore, there is no need for a bias control circuit, so the system has a stable operation. Besides the proposed scheme can be extended to multi-channel applications in array antennas. Theoretically calculations have been verified by simulation results.

Spoof Surface Plasmon Analysis Based on Marcatili’s Method

Mohammad Ali Khosrovani Moghaddam and Amir Ahmad Shishegar

DOI: 10.1364/JOSAB.402160 Received 07 Jul 2020; Accepted 06 Sep 2020; Posted 08 Sep 2020  View: PDF

Abstract: Motivated by surface plasmon polariton waveguides in optical regime, spoof surface plasmons (SSP) waveguides have received a lot of attention in terahertz and millimeter wave frequencies. Most of researches on these kinds of waveguides are numerical. However, some limited analytical work can be seen in literature. In this paper, one type of SSP waveguide which is composed of rectangular corrugation with finite lateral width on the ground is considered and an analytical method, which is inspired by Marcatili’s method, is proposed in order to calculate the dispersion curve of the first mode. The results of this analytical method and a numerical commercial eigenmode solver is compared. The accuracy of this method by varying the different geometrical parameters is shown. Finally, it is shown that this method works well on predicting the electromagnetic fields of the first mode.

Modified optical response of biased semiconductor nanowires within nonlocal hydrodynamic framework

Tianyu Dong, Xiaoke Gao, Ke Yin, Chun Xu, and Xikui Ma

DOI: 10.1364/JOSAB.402316 Received 08 Jul 2020; Accepted 06 Sep 2020; Posted 08 Sep 2020  View: PDF

Abstract: Semiconductors and their oxides, when properly doped, are potential promising plasmonic material alternatives due to their special properties such as low-loss and tunability. The hydrodynamic theory has been applied to describe the nonlocal response of pint-sized nano-structures even when several different kinds of charge carriers are considered, but when an external static magnetic field is presented the interplay between the gyrotropy and nonlocality needs to be considered, which is important and critical for semiconductors. We derive an analytical approach to calculate the optical properties of a plasmonic semiconductor nanowire in an external dc magnetic field within the multi-fluid hydrodynamic framework. The extended nonlocal Mie theory to magnetized multi-fluid plasmas predicts the existence of multiple acoustic and optical longitudinal modes within the multi-fluid hydrodynamic theory and the resonance splitting due to the applied bias magnetic field. We further focus on the nonlocal magneto-plasmonic response of nanowires that consist of thermally excited InSb, and predict the modified Zeeman splitting of the plasmonic extinction resonances due to the interplay between nonlocality and gyrotropy.

Numerical modelling of intracavity coherent anti-Stokes Raman laser

Cong Wang and Dongxiang Lv

DOI: 10.1364/JOSAB.402253 Received 09 Jul 2020; Accepted 05 Sep 2020; Posted 08 Sep 2020  View: PDF

Abstract: The space-dependent rate equations of intracavity coherent anti-Stokes Raman lasers are deduced by adding the term describing the coherent anti-Stokes scattering effect to the rate equations of the intracavity Raman laser. The intracavity photon densities of the fundamental and first Stokes lights and the initial population-inversion density are assumed to be Gaussian spatial distributions. The rate equations are normalized and solved numerically, and a group of general curves is generated. The influences of normalized parameters on the pulse parameters of the output anti-Stokes laser are investigated. The normalized rate-equation theory and numerical results will be of help in the design of high-performance intracavity coherent anti-Stokes Raman lasers.

Non-stationary testing of avalanche-like behavior of up-conversion luminescence

Mikhael Korolkov

DOI: 10.1364/JOSAB.400997 Received 24 Jun 2020; Accepted 05 Sep 2020; Posted 08 Sep 2020  View: PDF

Abstract: Nonlinear coupling of pumping rates in the pump channels can produce effects very similar to the photon avalanche in up-conversion luminescence in absence of true avalanche. Stationary luminescence intensity can depend on the pump intensity practically in the same way. Here we show that time-dynamics for true avalanche and for avalanche-like process are drastically different. Based on measuring the development times of stationary luminescence for several pump intensities, a simple test is proposed for revealing the nonlinear process underlying the observed avalanche-like effect.

Analysis of quantum noise of gain-saturated optical parametric amplifier based on equivalent input noise model

Kyo Inoue

DOI: 10.1364/JOSAB.401612 Received 02 Jul 2020; Accepted 03 Sep 2020; Posted 08 Sep 2020  View: PDF

Abstract: A theoretical approach to evaluate quantum noise in gain-saturated optical parametric amplifiers (OPAs) is presented. An equivalent input noise model that considers quantum noise is introduced, and a phenomenologically approximated expression of the signal gain is presented, to obtain the analytical formulas of the probability density function and the variance of signal fluctuations. Using our approach, the noise figure of gain-saturated OPAs can be analytically evaluated.

Theory of double-resonance alignment magnetometers based on atomic high-order multipole moments using effective master equations

Pan-li Qi, xu-xing geng, Guo-qing Yang, guang-ming huang, and Gaoxiang Li

DOI: 10.1364/JOSAB.404651 Received 05 Aug 2020; Accepted 03 Sep 2020; Posted 08 Sep 2020  View: PDF

Abstract: We derive the approximation and limitations of the well-known three-step method, which is a theoretical study of the resonance signals obtained in double resonance spectroscopy using linearly polarized light. Starting from a full master equation that contains the relaxation processes caused by the optical pumping ($\Gamma_{L}$) and the collision ($\Gamma_{g}$) between alkali-metal atoms and between alkali-metal atoms and the buffer gas, we have obtained the precise expressions of the relaxation rates $ \Gamma_{kq} $ of multipole moment $ m_{kq} $ , which provide guidance and reference for experiments of laser-detected magnetic resonance. The analytic expressions of the absorption signals are discussed when the alignment multipole moments $m_{2,q}$ are coupled to $m_{4,q}$ by relaxation processes in different RF magnetic field. Our approachs are in excellent agreement with the numerical results of the master equation. Meanwhile, we give the general application range of the three-steps method is $\Gamma_{L}<\frac{1}{2}\Gamma_{g} $ . The physical mechanism of the splitting in DC-signal is discussed with strong RF magnetic field. These results are valid for arbitrary light power levels and for arbitrary orientations of the offset magnetic field $B_{0}$ .

Mechanism study of terahertz radiation regulation in multi-color laser field

Shengfeng Wang, Haicheng Xiao, and Yan Peng

DOI: 10.1364/JOSAB.399626 Received 08 Jun 2020; Accepted 03 Sep 2020; Posted 08 Sep 2020  View: PDF

Abstract: By establishing a theoretical model for terahertz (THz) generation from a laser-induced air plasma filament and the subsequent propagation process, we study the physical mechanisms of THz wave radiation regulation in the multi-color femtosecond laser field. It is proved that the total THz radiation in the far field are affected by three factors: the ionized electron density, transient current distribution along the laser filament and coherent superposition between THz waves generated by each point alone the filament. The results show that the wavelength, relative phase, energy ratio, pulse width, laser frequency ratio and frequency combination of multi-color laser are all correlated to their radiated THz energy with different changing law. The proper regulation of a three-color filament can realize a fortyfold increase in the generated THz pulse energy as compared with the conventional terahertz generation by a two-color filament. These results provide the theoretical basis for the enhancement of THz radiation energy, and are of great significance for clarifying the physical process of laser filament.

Narrowband and ultra-wideband modulation instability in nonlinear metamaterial waveguides

Nicolas Linale, Pablo Fierens, Santiago Hernandez, Juan Bonetti, and Diego Grosz

DOI: 10.1364/JOSAB.393464 Received 23 Mar 2020; Accepted 02 Sep 2020; Posted 02 Sep 2020  View: PDF

Abstract: Waveguides based on metamaterials may exhibit strongly frequency-dependent nonlinearities. In this work, we focus on the phenomenon of modulation instability in this type of waveguides, departing from a new modeling equation that ensures strict conservation of both the energy and the photon number of the parametric process. In particular, we analyse the case of a waveguide with a linearly frequency-dependent nonlinear coefficient, revealing unique features such as narrowband and ultra-wideband gain spectra, and the suppression of the power cutoff giving rise to an ever-growing MI gain. These markedly distinct regimes are enabled by self-steepening (SS) and manifest themselves depending upon the magnitude and sign of the SS parameter. We believe these findings to be most relevant in the context of mid-IR supercontinuum sources.

Geometric phase of dielectric multilayers

Julio Gutierrez-Vega

DOI: 10.1364/JOSAB.404078 Received 30 Jul 2020; Accepted 02 Sep 2020; Posted 02 Sep 2020  View: PDF

Abstract: We characterize the geometric and dynamic phase introduced by a lossless dielectric multilayer into an elliptically polarized input wave with oblique incidence. The phases are conveniently mapped on the Poincar\'{e} sphere as a function of the Stokes vector of the input field. The geometric phase interval of the transmitted wave is determined by the difference of the phases of the overall transmission coefficients for the TE and TM waves. The same conclusion is obtained for the reflected wave. To exemplify the theory, we show the surface plots of the geometric phase intervals as a function of the incident angle and the phase thickness of a single-layer and a periodic multilayer.

Analysis of frequency-shifting loops in integer and fractional Talbot conditions: electro-optic vs acousto-optic modulation

Hongzhi Yang, Marc Brunel, Marc Vallet, Haiyang Zhang, and changming zhao

DOI: 10.1364/JOSAB.389801 Received 06 Feb 2020; Accepted 01 Sep 2020; Posted 02 Sep 2020  View: PDF

Abstract: Frequency-shifting loops (FSL) are analyzed theoretically in cases where the intracavity modulator induces two side-bands at each round-trip, a situation that can be commonly obtained with electro-optic intensity or phase modulators. Using a simple model, we discuss the ability of such loops to perform real-time Fourier transformation, in the integer Talbot condition, or pulse repetition rate enhancement, in the fractional Talbot condition. The results are compared to the established acousto-optic FSL with pure frequency shift. We show that, in spite of a more complicated situation resulting from the dual sideband modulation, pulse repetition rate amplification can be obtained with an amplitude modulator, and frequency-to-time mapping can be obtained with a phase modulator. This opens new routes to high-frequency manipulation of microwave-optical signals with high-bandwidth (multi-GHz) modulators.

Strong Second-harmonic generation in dielectric optical nanoantennas resulting from the hybridization of magnetic dipoles and lattice resonances

Zhanghua Han, Hui Jiang, and Yang Jiancai

DOI: 10.1364/JOSAB.402624 Received 14 Jul 2020; Accepted 31 Aug 2020; Posted 02 Sep 2020  View: PDF

Abstract: Magnetic dipole resonances supported by high refractive index dielectric nanoparticles have appeared recently as a new alternative to plasmonic nanostructures to manipulate light-matter interactions. The volume field enhancement within the particles significantly boosts some nonlinear processes, mitigating the need of conventional phase matching conditions. In this paper, we show that by using an AlGaAs nano-cylinder array whose lattice resonance spectrally matches the magnetic resonance of individual particle, the two resonances will couple with each other, leading to a strong hybridization of them and helping achieve a stronger circulation of the electric fields within the particles. As a result of this mode hybridization, the efficiency of second harmonic generation can be further enhanced by more than three orders of magnitude at a low pump laser power density of 0.053GW/cm² compared to that from an individual nano-cylinder.

Emission of R6G dye in Fabry-Perot cavities in weak and strong coupling regimes

Md Omar Faruk, Nelly Jerop, and Mikhail Noginov

DOI: 10.1364/JOSAB.403612 Received 23 Jul 2020; Accepted 31 Aug 2020; Posted 02 Sep 2020  View: PDF

Abstract: We have studied spectra and angular distribution of emission in Fabry-Perot cavities formed by two silver mirrors separated by a layer of PMMA polymer doped with R6G dye in low (20 g/l) and high (200 g/l) concentrations. The frequency of emission radiated to a cavity mode was larger at large outcoupling angles – the “rainbow” effect. At the same time, the angle of the strongest emission was also determined by the cavity size: the larger the cavity – the larger the angle. The angular distribution of emission is commonly dominated by two symmetrical lobes (located at the intersection of the 3D emission cone with a horizontal plane) pointing to the leftand to the right of the normal to the sample. Despite the strong Stokes shift in R6G dye, the branch of the cavity dispersion curve obtained in the emission experiment is positioned above the one obtained in the reflection(extinction) experiment. Some dye molecules are poorly coupled to cavity modes. Their emission has very broad angular distribution with the maximum at θ=0o. The signatures of strong cavity-exciton coupling were observed at high dye concentration (200 g/l) but not at low concentration (20 g/l). The evidence of the effect of strong coupling on emission is exemplified by a strong difference in the angular distribution of emission in two almost identical cavities, one with and another without strong coupling. Most importantly, we have demonstrated the possibility to control the ground state concentration, the coupling strength, and the dye emission spectra with Q-switched laser pulses.

Discrete optical propagation in one-dimensional synthetic mesh lattice

Zengrun Wen, Baole Lu, Kaile Wang, Xinyuan Qi, and Jintao Bai

DOI: 10.1364/JOSAB.402915 Received 15 Jul 2020; Accepted 28 Aug 2020; Posted 31 Aug 2020  View: PDF

Abstract: Synthetic mesh lattice (SML) with temporally controlled potential is a versatile platform for realizing wave dynamics associated with physical areas of optics and quantum physics. Here, discrete optics in one-dimensional synthetic photonic lattice is investigated systematically, in which the light behavior is highly analogous to those in evanescently coupled one-dimensional discrete waveguides. Such a synthetic dimension is constructed with position-dependent periodic effective gauge fields based on Aharonov-Bohm effect arsing from the phase accumulation of the fiber loops. By tuning the phase accumulation and coupling coefficient of the coupler, the band translation and gap property can be modulated which further results in the impulse and tailored Gaussian wave packet responses as well as Talbot recurrences. In addition, Bloch oscillations and Anderson localization can also be obtained when the phase accumulations are linearly changed and weakly modulated in random, respectively. The periodic effective gauge fields configuration in our protocol enables SML to be a research platform for one-dimensional dynamically modulated elements or even non-Hermitian waveguides.

Giant resonant enhancement of optical binding of dielectric disks

Evgeny Bulgakov, Konstantin Pichugin, and Almas Sadreev

DOI: 10.1364/JOSAB.402659 Received 14 Jul 2020; Accepted 28 Aug 2020; Posted 15 Sep 2020  View: PDF

Abstract: Two-parametric variation over the aspect ratio of each disk anddistance between disks gives rise to numerous events of avoidedcrossing of resonances of individual disks. For these events thehybridized anti-bonding resonant modes can acquire a morphologyclose to the Mie resonant mode with high orbital momentum ofequivalent sphere. The $Q$ factor of such resonance can exceed the$Q$ factor of isolated disk by two orders in magnitude. We showthat dual incoherent counter propagating coaxial Bessel beams withpower $1mW/\mu m^2$ with frequency resonant to such a anti-bondingmodes result in unprecedented optical binding forces up to decadesof nano Newtons for silicon micron size disks. We show also that amagnitude and sign of optical forces strongly depend on thelongitudinal wave vector of the Bessel beams.

Probing ultrafast reaction mechanisms of photo-excited dithizone through transient absorption spectroscopy and computational CASSCF studies

Adebayo Adeniyi, Xavier von Stein, Gurthwin Bosman, Christine Steenkamp, Trevor Chiweshe, Karel Von Eschwege, and Jeanet Conradie

DOI: 10.1364/JOSAB.399082 Received 02 Jun 2020; Accepted 22 Aug 2020; Posted 24 Aug 2020  View: PDF

Abstract: Dithizone with its derivatives and complexes is characterized by seven classes of chromic reactions, which make it a remarkable candidate for diverse sensor applications. We now present dithizone's chromic response to photo-excitation as studied by ultrafast laser spectroscopy, conventional density functional theory and complete active space self-consistent field (CASSCF) calculations. After vertical excitation to the first excited state the reaction proceeds within 240 fs towards two conical intersections; the first leads to isomerization within 1.6 ps, while the second leads to excited state proton transfer within 12.4 ps. Additionally, dynamics through the conical intersections allow for efficient repopulation of the reactant ground state. Theoretical calculations using M062X and CASSCF shed light on the geometries of the different ground and excited states involved in the femtosecond laser pulse excitation of dithizone.

Optical spin polarization in ruby enhancesslow light by high-contrast transient spectralhole-burning

Hans Riesen, Alexander Rebane, Wayne Hutchison, and Steffen Ganschow

DOI: 10.1364/JOSAB.402214 Received 09 Jul 2020; Accepted 21 Aug 2020; Posted 21 Aug 2020  View: PDF

Abstract: The spin-lattice relaxation time in the 4A2 ground state of Cr3+ in ruby can be on the order of magnitude of seconds at liquid helium temperatures in low magnetic fields B||c. This allows for highly efficient optical pumping of the spin levels in this system. Such pumping can then in turn be employed to drastically increase the optical density of the inhomogeneously broadened R1(±3/2) or R1(±1/2) lines and also to narrow the spectral holewidthbecause Cr-Cr electron-spin flips (cross relaxation) are suppressed in the ground state leading to longer dephasing times T2* . Both effects can be employed to further slow down thegroup velocity of light pulses propagating through a transient spectral hole.

Numerical study of sum frequency ultrashort pulse compression in borate crystals

Martin Duda, Ondrej Novak, Martin Smrz, Antonio Lucianetti, Vaclav Kubecek, and Tomas Mocek

DOI: 10.1364/JOSAB.401657 Received 30 Jun 2020; Accepted 31 Jul 2020; Posted 10 Sep 2020  View: PDF

Abstract: Second harmonic generation (SHG) of picosecond pulses in Type II phase-matched nonlinear optical crystals is not widely employed because of the lower efficiencies compared to the Type I phase matching scheme. The limited efficiencies come from the difference between group velocities (GV difference) of the ordinary and extraordinary polarized input pulses. However, if the input pulses are delayed before the nonlinear crystal, short second harmonic pulse can be generated with a slowly widening temporal overlap. Furthermore, this GV difference can be controlled by tilting of the pulse fronts and optimal GV difference can be obtained to achieve powerful output pulses with durations order of magnitude lower than those of the input pulses. In this work, we present numerical results of SHG pulse compression in a BBO nonlinear crystal, which is ideal for SHG of high power 1030 nm thin-disk lasers. The pulse compression is controlled by predelay and tilting of the pulse fronts. We find optimal parameters to achieve 5-fold increase in output power and 20-fold pulse compression of 1.7 ps input pulses. Finally, we consider the experimental aspects of the group velocity control.

From African "Tam-Tam" to Nonlinear Optics

Timoleon Kofane, Conrad Bertrand Tabi, Moubissi Alain-Brice, and Tchawoua Clement

DOI: 10.1364/JOSAB.399177 Received 01 Jun 2020; Accepted 18 Jul 2020; Posted 30 Jul 2020  View: PDF

Abstract: Before the introduction of modern technological communication, signaling over a distance was often a very good way to pass messages, especially in difficult terrain such as the mountains or sparsely populated plains or forests, the "tam-tam" type of drums (known as the talking drum), in Africa were sometimes used to elaborate code systems to pass even complex information. With the explosion of information traffic due to the Internet, electronic commerce, computer networks, multimedia, voice, data, and video, the need for a transmission medium with the bandwidth capabilities for handling such vast amounts of information is paramount. Fiber optics, with its comparatively infinite bandwidth, has proven to be the solution. An overview of such palliative and revolutionary communication method is given here, with emphasis on the main contributions from Africa.

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