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Accepted papers to appear in an upcoming issue

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Control of phase of the magnetization precession excited by circularly polarized fs-laser pulses

Alexander Chernov, Mikhail Kozhaev, Anastasiya Khramova, Alexander Shaposhnikov, Anatoly Prokopov, Vladimir Berzhansky, Anatoliy Zvezdin, and Vladimir Belotelov

Doc ID: 341970 Received 09 Aug 2018; Accepted 24 Sep 2018; Posted 24 Sep 2018  View: PDF

Abstract: The inverse Faraday effect induced in magnetic films by ultrashort laser pulses allows excitation and control of spins at GHz and sub-THz frequencies. Frequency of the optically excited magnetization precession is easily tunable by the external magnetic field. On the other hand, initial phase of the precession marginally depends on magnetic field. Here we demonstrate an approach for the control of the precession phase by variation of the pump beam direction. In particular, we consider the case when the magnetization precession is excited by obliquely incident pump pulses in a magnetic dielectric film placed in the in-plane magnetic field. Theoretical consideration predicts that the initial phase should appear for non-zero in-plane component of the pump wavevector orthogonal to the external magnetic field. Experimental studies confirm this conclusion and reveal that the phase grows with increase of the in-plane wavevector component. Variation of phase by 15 deg. is demonstrated. Potentially, the phase could be changed even more pronouncedly by more than 90 deg. This work provides a simple way for additional manipulation with optically excited magnetization dynamics, which is of importance for different spintronic applications.

Analysis of optical coupling behaviors in two-dimensional implant-defined coherently coupled VCSEL arrays

Guanzhong Pan, Yiyang Xie, Chen Xu, Yibo Dong, Jun Deng, Hongda Chen, and Jie Sun

Doc ID: 340827 Received 26 Jul 2018; Accepted 20 Sep 2018; Posted 20 Sep 2018  View: PDF

Abstract: Optical coupling behaviors and associated effects in two-dimensional implant-defined coherently coupled VCSEL arrays (including small-area addressable three-element arrays and large-area 16-element in-phased arrays and 69-element out-of-phased arrays) are studied by experiments and theoretical calculations. Experiments reveal that the optical coupling between the array elements can enhance the output power of the array. In addition, by directly monitoring and analyzing the optical coupling behaviors in the active regions of the addressable arrays, it is found that the optical coupling can provide extra optical gain for the array elements via leaky optical field, which can reduce the threshold of the array elements. By receiving the strong leaky optical field from other array elements, the element without current injection can even be pumped to emit light. Moreover, optical coupling can cause unusual phenomena in large-area coherently coupled VCSEL arrays that the central elements lase prior to the outer elements when the arrays are biased, and the average injection current for each element required to lase is much smaller than the threshold of a single VCSEL. Theoretical calculations are taken to explain the experimental results. Exploring the optical coupling behaviors in these coherently coupled VCSEL arrays will provide a deeper understanding of the coupling mechanism of the array and is helpful for designing and fabricating coherently coupled VCSEL arrays with enhanced performance.

Bound states in the continuum and high-Q resonances supported by a dielectric ridge on a slab waveguide

Evgeni Bezus, Dmitry Bykov, and Leonid Doskolovich

Doc ID: 337954 Received 05 Jul 2018; Accepted 20 Sep 2018; Posted 20 Sep 2018  View: PDF

Abstract: We investigate the diffraction of guided modes of a dielectric slab waveguide on a simple integrated structure consisting of a single dielectric ridge on the surface of the waveguide. Numerical simulations based on aperiodic rigorous coupled-wave analysis demonstrate the existence of sharp resonant features and bound states in the continuum (BICs) in the reflectance and the transmittance spectra occurring at oblique incidence of a TE-polarized guided mode on the ridge. Using the effective index method, we explain the resonances by the excitation of the cross-polarized modes of the ridge. The formation of the BICs is confirmed using a theoretical model based on the coupled-wave theory. The model suggests that the BICs occur due to coupling of quasi-TE and quasi-TM modes of the structure. Simple analytical expressions for the angle of incidence and the ridge width predicting the location of the BICs are obtained. The existence of high-Q resonances and BICs makes the considered integrated structure promising for filtering, sensing, transformation of optical signals, and enhancing nonlinear light-matter interactions.

Coupling Strategies for Silicon Photonics Integrated Chips

Cosimo Lacava, Riccardo Marchetti, Lee Carroll, Kamil Gradkowski, and Paolo Minzioni

Doc ID: 338163 Received 06 Jul 2018; Accepted 18 Sep 2018; Posted 20 Sep 2018  View: PDF

Abstract: In the last 20 years, silicon photonics has revolutionized the field of integrated optics, providing a novel and powerful platform to build mass-producible optical circuits. One of the most attractive peculiarity of silicon photonics is its ability to supply extremely small optical components, whose typical dimensions are order of magnitude smaller than optical fiber devices. This dimension discrepancy makes the design of fiber-to chip interfaces extremely challenging and, over the years, has stimulated an incredibly large amount of research efforts in the field. Fiber-to-Silicon photonic chip interfaces can be broadly divided into two big categories: in-plane and out-of-plane couplers. Devices falling in the first category typically offer high coupling efficiency, large coupling bandwidth (in wavelength) and no polarization dependence, but they require relatively complex fabrication procedures and do not allow for wafer-scale testing.Conversely, out-of-plane device offer lower efficiency, limited bandwidth and are typically polarization dependent. However they are compatible with high-volume fabrication processes, and allow on-wafer accessing any part of the optical circuit. In this paper we review the current state of the art of optical couplers, aiming to give to the reader a comprehensive and broad view of the field, identifying advantages and disadvantages of each solution proposed.As fiber-to-chip couplers are inherently related to packaging technologies, and the co-design of optical packages has become essential, we also review in this document the main solutions currently used to package and assemble optical fibers with Silicon-photonic integrated circuits.

Broadband terahertz rotator with all-dielectric metasurface

Quanlong Yang, Xieyu Chen, Quan Xu, Chunxiu Tian, Yuehong Xu, Longqing Cong, Xueqian Zhang, Yanfeng Li, Caihong Zhang, Xixiang Zhang, Jiaguang Han, and Weili Zhang

Doc ID: 332335 Received 24 May 2018; Accepted 14 Sep 2018; Posted 20 Sep 2018  View: PDF

Abstract: Polarization manipulation is essential in developing cutting-edge photonic devices ranging from optical communication, displays to solar energy harvesting. Most previous works for efficient polarization control cannot avoid utilizing metallic components that inevitably suffer from large Ohmic loss and thus low operational efficiency. Replacing metallic components with Mie resonance based dielectric resonators will largely suppress the Ohmic loss toward high-efficiency metamaterial devices. Here, we propose an efficient approach for broadband, high-quality polarization rotation operating in transmission mode with all-dielectric metamaterials in the terahertz regime. By separating the orthogonal polarization components in space, we obtain rotated output waves with a conversion efficiency of 67.5%. The proposed polarization manipulation strategy shows impressive robustness and flexibility in designing metadevices of both linear and circular polarization incidences.

Low-Noise 1.3 µm InAs/GaAs Quantum Dot Laser Monolithically Grown on Silicon

Mengya liao, Siming Chen, Zhixin Liu, Yi Wang, Lalitha Ponnampalam, Jiang Wu, Mingchu Tang, Sam Shutts, Zizhuo Liu, Peter Smowton, Siyuan Yu, Alwyn Seeds, and Huiyun Liu

Doc ID: 341901 Received 08 Aug 2018; Accepted 13 Sep 2018; Posted 14 Sep 2018  View: PDF

Abstract: We report low-noise, high-performance single transverse mode 1.3 µm InAs/GaAs quantum dot (QD) lasers monolithically grown on silicon (Si) using molecular beam epitaxy. The fabricated narrow ridge-waveguide Fabry-Perot (FP) lasers have achieved a room-temperature continuous-wave (CW) threshold current of 12.5 mA and high CW temperature tolerance up to 90 ºC. An ultra-low relative intensity noise (RIN) of less than -150 dB/Hz is measured in the 4-16 GHz range. Using this low-noise Si-based laser, we then demonstrate 25.6 Gb/s data transmission over 13.5 km SMF-28. These low-cost FP laser devices are promising candidates to provide cost-effective solutions for use in uncooled Si photonics transmitters in inter/hyper data centers and metropolitan data links.

PbS quantum dots as a saturable absorber for ultrafast laser

Ling Yun, Yang Qiu, Conghao Yang, Jie Xing, Ke-Han Yu, Xiang-xing Xu, and Wei Wei

Doc ID: 340424 Received 23 Jul 2018; Accepted 11 Sep 2018; Posted 13 Sep 2018  View: PDF

Abstract: Low-dimensional nanomaterials, owing to their unique and versatile properties, are very attractive for enormous electronic and optoelectronic applications. PbS quantum dots (QDs), characterized by large Bohr radius and size tunable bandgap, are especially interesting for photonic applications in the near-infrared region. Here, oleic acid capped colloidal PbS QDs as a saturable absorber are investigated for ultrashort-pulse generation. The PbS QDs exhibit outstanding nonlinear saturable absorption properties at 1550 nm: a modulation depth up to 44.5% and a thermal damage threshold larger than 31 mJ/cm2. By incorporating PbS QDs into fiber laser, transform-limited soliton pulse as short as 559 fs with a bandwidth of 4.78 nm is realized at 1563 nm. Numerous applications may benefit from the nonlinear saturable absorption properties of PbS QDs, such as near-infrared pulsed lasers, modulators, and so on.

All-fiber passively mode-locked laser using nonlinear multimode interference of step-index multimode fiber

Tao Chen, Qiaoli Zhang, Yaping Zhang, Xi Li, Haikun Zhang, and Wei Xia

Doc ID: 340879 Received 31 Jul 2018; Accepted 11 Sep 2018; Posted 13 Sep 2018  View: PDF

Abstract: We experimentally demonstrate for the first time an all-fiber passively mode-locked laser operation based on the nonlinear multimode interference of step-index multimode fiber. Such a structure couples the light in and out of the multimode fiber via single-mode fibers, and its physical mechanisms for saturable absorption have been analyzed theoretically based on the third-order nonlinear Kerr effect of multimode fiber. Using the nonlinear multimode interference structure with 48.8-mm length step-index multimode fiber, the modulation depth has been measured to be ~5 %. The passively mode-locked laser output pulses have the central wavelength of 1596.66 nm, the bandwidth of 2.18 nm, the pulsewidth of ~625 fs, and the fundamental repetition rate of 8.726 MHz. Futhermore, the influence of total cavity dispersion on the optical spectrum, pulse width and output power are investigated systematically by adding different lengths of single-mode fiber and dispersion compensation fiber in the laser cavity.

Broadband Nonlinear Optical Resonance and All-Optical Switching of Liquid Phase Exfoliated WSe2

Yue Jia, Youxian Shan, Leiming Wu, Xiaoyu Dai, Dianyuan Fan, and Yuanjiang Xiang

Doc ID: 341643 Received 06 Aug 2018; Accepted 07 Sep 2018; Posted 07 Sep 2018  View: PDF

Abstract: As a kind of two-dimensional (2D) transition metal dichalcogenides (TMDCs) material, tungsten diselenide (WSe2) has attracted increasing attentions owing to its gapped electronic structure, relatively high carrier mobility, and valley pseudospin, all of which show its valuable nonlinear optical properties. There are few studies on the nonlinear optical properties of WSe2 and the correlation with its electronic structure. In this paper, the effects of spatial self-phase modulation (SSPM) and distortion influence of WSe2 ethanol suspensions are systematically studied, namely the nonlinear refractive index and the third order nonlinear optical effect. We got the WSe2 dispersions SSPM distortion formation mechanism, and through it, we calculated the nonlinear refractive index n2, the non-linear susceptibility χ(3) and their wavelength dependence under the excitation of 457nm, 532 nm, and 671nm lasers. Moreover, by using of its strong and broadband non-linear optical response, all-optical switching of two different laser beams due to the spatial cross-phase modulation have been realized experimentally. Our results are useful for the future optical devices, such as all-optical switching, all-optical information conversion, and so on.

Accurate Extraction of Fabricated Geometry Using Optical Measurement

Yufei Xing, Jiaxing Dong, Sarvagya Dwivedi, Umar Khan, and Wim Bogaerts

Doc ID: 338244 Received 10 Jul 2018; Accepted 05 Sep 2018; Posted 06 Sep 2018  View: PDF

Abstract: We experimentally demonstrate extraction of silicon waveguide geometry with sub-nanometer accuracy using optical measurements. Effective and group indices of silicon on insulator (SOI) waveguides are extracted from the optical measurements. An accurate model linking the geometry of an SOI waveguide to its effective and group indices is used to extract the linewidths and thicknesses within respective errors of 0.37 nm and 0.26 nm on a die fabricated by the IMEC multi-project wafer (MPW) services. A detailed analysis of the setting of the bounds for the effective and group indices is presented to get the right extraction with improved accuracy.

Superior Multiphoton Absorption Properties in Colloidal Mn-Doped CsPbCl3 Two-dimensional Nanoplatelets

Tingchao He, Junzi Li, Xin Qiu, Shuyu Xiao, and Xiaodong Lin

Doc ID: 338572 Received 12 Jul 2018; Accepted 05 Sep 2018; Posted 07 Sep 2018  View: PDF

Abstract: We have studied the two- and three-photon absorption (2PA and 3PA) properties of Mn-doped CsPbCl3 two-dimensional nanoplatelets (2D NPs) and cubic NCs. Compared with their cubic counterparts, the Mn-doped 2D NPs exhibit stronger quantum confinement effects that can more efficiently enhance their dopant-carrier exchange interactions and their MPA. More specifically, compared with their cubic counterparts, the maximum volume-normalized 2PA and 3PA cross-sections of the 2D NPs were 2.88 and 2.4 times greater, up to 412 GM/nm3 in the visible light band and 75×10−80 cm6∙s2∙photon−2/nm3 in the second biological window, respectively.

2.8 μm all-fiber Q-switched and mode-locked lasers with black phosphorus

Guoqiang Xie, Zhipeng Qin, Jingui Ma, Peng Yuan, and Liejia Qian

Doc ID: 341246 Received 03 Aug 2018; Accepted 02 Sep 2018; Posted 05 Sep 2018  View: PDF

Abstract: In the past years, rare-earth-doped fluoride fiber lasers (FFLs) have developed rapidly in the mid-infrared (mid-IR) region. However, due to the lack of fiber optic devices and challenge of fluoride fiber splicing, most of mid-IR FFLs have been demonstrated with free-space optic elements, limiting the advantages of all-fiber laser for flexible delivery, stability, and compactness. Here, we report, to the best of our knowledge, the first pulsed all-fiber FFL in the mid-IR region. By taking advantage of the integration of black phosphorus flake, stable Q-switched and mode-locked pulses were obtained at 2.8 μm wavelength. We believe that this all-fiber design will promote the application of pulsed FFL in the mid-IR region.

Revisiting the absorption and transmission properties of coupled open waveguides

Keng Chou and Lei Chen

Doc ID: 330305 Received 26 Apr 2018; Accepted 31 Aug 2018; Posted 05 Sep 2018  View: PDF

Abstract: Coupled open waveguides are widely used in modern photonic devices, such as microstructured fiber filters and sensors. Their absorption and transmission spectra are the most important factors determining the overall properties of the photonic devices. The imaginary parts of their eigenvalues have been commonly used to calculate the absorption and consequently the transmission spectra. Here we show that this formulism is generally incorrect and not consistent with the simulation results using the beam propagation method. We revisit the fundamental theory for the absorption of open waveguides and present a general formulism. We found that the parity-time-symmetry transition, which has been conventionally ignored, plays a critical role in the properties of the coupled waveguide. The absorption and transmission properties are highly dependent on the physical length of the system. On the basis of our findings, optimal approaches for designing photonic sensors and filters are presented.

Broadband athermal waveguides and resonators for datacom and telecom applications

Liuqing He, Yuhao Guo, Zhaohong Han, Kazumi Wada, Jurgen Michel, Anuradha Agarwal, Lionel Kimerling, Guifang Li, and Lin Zhang

Doc ID: 332312 Received 21 May 2018; Accepted 31 Aug 2018; Posted 05 Sep 2018  View: PDF

Abstract: The high temperature sensitivity of silicon material index limits the applications of silicon-based micro-ring resonators in integrated photonics. To realize a low but broadband temperature-dependence-wavelength-shift (TDWS) microring resonator, designing a broadband athermal waveguide becomes a significant task. In this work, we propose a broadband athermal waveguide which shows a low effective thermo-optical coefficient (TOC) of ±1×10-6 /K from 1400 to 1700 nm. The proposed waveguide shows low-loss performance and stable broadband athermal property, when it is applied to ring resonators, and the bending loss of ring resonators with a radius of >30 μm is 0.02 dB/cm.

Facile actively control of pulsed erbium-doped fiber laser using modulation depth tunable carbon nanotubes

xintong xu, Shuangchen Ruan, jianpang zhai, Ling Li, Jihong Pei, and Zikang Tang

Doc ID: 335051 Received 12 Jun 2018; Accepted 31 Aug 2018; Posted 05 Sep 2018  View: PDF

Abstract: We experimentally demonstrate modulation depth tunable carbon nanotubes saturable absorber (SA) to the actively control of erbium-doped pulsed fiber laser. The well-aligned carbon nanotubes that fabricated in the nanochannels of zeolite AlPO4-5 (AFI) are used as SA to generate switchable erbium-doped pulsed fiber laser. Both Q-switched and mode-locked pulsed laser are obtained by changing a polarization controller under 330 mW pump power. In addition, the repetition rate of the Q-switched and the pulse duration of the mode-locked pulsed laser can be easily adjusted according the modulation depth varies. Moreover, the soliton molecules can also be obtained when the modulation depth of SA is 4.5 %.

Raman Spectroscopy Regulation in van der Waals Crystals

Wei Zheng, Yanming Zhu, Fadi Li, and Feng Huang

Doc ID: 336335 Received 29 Jun 2018; Accepted 31 Aug 2018; Posted 05 Sep 2018  View: PDF

Abstract: Raman spectroscopy is a versatile tool widely used for comprehensive probe of crystals information. However, generally when applied in narrow band-gap van der Waals crystals, it is liable to form a “bug” especially in transition-metal-dichalcogenides (TMDs). That is, several resonant-Raman-scattering (RS) modes will inevitably appear in the Raman spectra with strong intensity, interfering the desired signal of optical-phonon (OP) modes. Here, we proposed the cross-section polarized Raman scattering capable of regulating the intensity of RS modes in accordance with quasi-sinusoidal rules. Typically, for MoS2 and WS2, when the polarization vector of excited light is along the c-axis of crystal, all RS modes are nearly "expunged" completely from the Raman spectra. The mechanism is that the absorption of most TMDs with space group of R3m for the light polarized along the c-axis is infinitesimal, thus forming a small coupling intensity of electronic states excited optically and acoustic-phonon modes at M point, which in turn restraining the appearance of RS modes. The regulating strategy proposed can be applied to other van der Waals crystals, so as to obtain high signal-to-noise ratio Raman spectrum.

TiS2-based saturable absorber for ultrafast fiber lasers

X. Zhu, S Chen, Meng Zhang, L Chen, Qing Wu, J Zhao, Q Jiang, Zheng Zheng, and Han Zhang

Doc ID: 336027 Received 28 Jun 2018; Accepted 20 Aug 2018; Posted 23 Aug 2018  View: PDF

Abstract: We fabricate titanium disulfide (TiS2) using a liquid exfoliation method and subsequently TiS2-based device by optically depositing the TiS2 material onto the microfiber. This device exhibits strong nonlinear saturable absorption property with an optical modulation depth of 8.3% at 1560 nm. With the implementation of this all-fiber TiS2-based saturable absorber, we demonstrate that both mode-locking and Q-switching operation can be obtained in a turn-key all-fiber erbium-doped laser cavity. Our findings constitute the first example of the TiS2-based saturable absorber for ultrashort pulse generation, and highlight the great potential of such devices into two-dimensional nanomaterials related photonics.

Sequential trapping of single nanoparticles using a gold plasmonic nanohole array

Xue Han, Viet Giang Truong, Prince Thomas, and Sile Nic Chormaic

Doc ID: 330950 Received 04 May 2018; Accepted 02 Aug 2018; Posted 28 Aug 2018  View: PDF

Abstract: We have used a gold nanohole array to trap single polystyrene nanoparticles, with a mean diameter of 30 nm, into separated hot spots located at connecting nanoslot regions. A high trap stiffness of approximately 0.85 fN/(nm∙mW) at a low incident laser intensity of ~0.51 mW/µm2 at 980 nm was obtained. The experimental results were compared to the simulated trapping force and a reasonable match was achieved. This plasmonic array is useful for lab-on-a-chip applications and has particular appeal for trapping multiple nanoparticles with predefined separations or arranged in patterns in order to study interactions between them.

Passively Q-switched fs-laser-written thulium waveguide laser based on evanescent field interaction with carbon nanotubes

Esrom Kifle, Pavel Loiko, Javier Vazquez de Aldana, Carolina Romero, Airan Rodenas, Sun Young Choi, Ji Eun Bae, Fabian Rotermund, Viktor Zakharov, Andrei Veniaminov, Magdalena Aguilo, Francesc Diaz, Uwe Griebner, Valentin Petrov, and Xavier Mateos

Doc ID: 336536 Received 29 Jun 2018; Accepted 17 Jul 2018; Posted 17 Jul 2018  View: PDF

Abstract: Surface channel waveguides (WGs) were fabricated in a monoclinic Tm3+:KLu(WO4)2 crystal by femtosecond direct laser writing (fs-DLW). The WGs consisted of a half-ring cladding with a diameter of 50 and 60 µm located just beneath the crystal surface. They were characterized by confocal laser microscopy and µ-Raman spectroscopy, indicating a reduced crystallinity and stress-induced birefringence of the WG cladding. In the continuous-wave mode, under Ti:Sapphire laser pumping at 802 nm, the maximum output power reached 171.1 mW at 1847.4 nm corresponding to a slope efficiency η of 37.8% for the 60 µm diameter WG. The WG propagation loss was 0.7±0.3 dB/cm. The top surface of the WGs was spin-coated by a PMMA film containing randomly-oriented (spaghetti-like) arc-discharge single-walled carbon nanotubes serving as a saturable absorber based on evanescent field coupling. Stable passively Q-switched (PQS) operation was achieved. The PQS 60 µm diameter WG laser generated a record output power of 150 mW at 1846.8 nm with η = 34.6%. The conversion efficiency with respect to the CW mode was 87.6%. The best pulse characteristics (energy / duration) were 105.6 nJ / 98 ns at a repetition rate of 1.42 MHz.

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