Beam steering devices have gained extensive interests in the fields of optical interconnects, communications, displays and data storages. However, the challenge lies in obtaining an ultrafast beam steering structure in the optical regime. Here, we propose phase-array-like plasmonic resonators based on metal/phase-change materials (PCMs)/metal trilayers for all-optical ultrafast beam steering in the mid-infrared (MIR) region. We numerically demonstrate an angle beam steering of 11° for transmitted wave (front lobe) and 22° for reflected wave (back lobe) by switching between the amorphous and crystalline states of the PCM (Ge2Sb2Te5). A photothermal model is used to study the temporal variation of the temperature of the Ge2Sb2Te5 film to show potential for switching the phase of Ge2Sb2Te5 by optical heating. Generation of the beam steering in this structure exhibits a fast beam steering time of 3.6 ns under a low pump light intensity of 2.6 μW/μm2. Our design possesses a simple geometry which can be fabricated using standard photolithography patterning and is essential for exploiting the ultrafast beam steering in various applications in the MIR regime.
© 2015 Optical Society of America
Beam steering devices have attracted intensive attention, owing to the unique capability to control the direction of the incident beam . It is an important component in many interesting applications, such as optical communications , holographic image-generation technologies  and optical data storages . Particularly, the study of the mid-infrared (MIR) plasmonic beam steering structure is important for shaping the beam from quantum cascade lasers (QCLs), greatly reducing the divergence of emitted laser beam [5,6]. As such, these devices have potential applications in sensing and security [7, 8]. However, most of these beam steering devices show a slow operating speed, high energy consumption and limited steering angle . Here, phase-array-like plasmonic resonators based on metal/phase-change materials (PCMs)/metal trilayers have been proposed to demonstrate an ultrafast beam steering with a low energy consumption and wide steering angle in the MIR region.
Recently, the introduction of active elements, i.e. liquid crystals (LCs) , active silicon , thermally reconfigurable semiconductors  and PCMs , into optical phase array (OPA) has been demonstrated, resulting in the innovative beam-steering device for the infrared regime. The basic idea is to modify the phase of a reflected or transmitted wave by tuning the refractive index of the electro-optical material, and thus, to control direction and divergence of light. Yet, these schemes are not exploited to investigate the time response of the beam-steering. OPAs can be applied to a variety of new and exciting areas such as missile defense shield systems, if the time response of beam-steering is short. Therefore, we investigate ways to steer light fast. Implementation of fast beam-steering using OPA has been mostly limited to the phase arrays based on LCs . For example, Jau et al. experimentally presents an optically controllable beam-steering device, using cholesteric LCs doped with azobenzene. Such a device has a wide steering angle of ~19° but is limited by a slow switching speed of ~4 s . Engström et al. proposes ferroelectric LCs phased arrays to increase the speed of the beam steering. However, the response time of ~200 μs still limits its application . Very recently, Yoo et al. developed an optical phased array incorporating a high-index-contrast subwavelength grating (HCG). It exhibits microsecond-order beam steering, whereas has a narrow steering angle of 1.26° . All of these technologies have their own advantages and shortcomings and the choice of the tunable element is often a trade-off between the steering angle and response time. Particularly, these beam steering devices are not compatible with fast optical systems with pulses on the order of nanoseconds. It is still a formidable challenge to obtain an ultrafast, low-power beam-steering device with a wide steering angle and response time of nanoseconds in the optical regime.
Here, we numerically demonstrate that the propagation direction of light can be optically controlled over a wide angle within a few nanoseconds using PCMs in the MIR region. Our structure is composed of gradient phase-array-like plasmonic resonators based on metal/dielectric/metal (MDM) multilayers, and a prototypical PCM: Ge2Sb2Te5, is selected as the dielectric layer. It is known that when two metal layers are placed closer than the surface plasmon polaritons (SPPs) attenuation length, the SPPs propagating along each of the two metal-dielectric interfaces of MDM structures couple to each other, and thus interfering constructively, which can strongly concentrate light inside the internal dielectric layer [15,16]. As soon as the refractive index of the dielectric layer is changed, the resonance of the MDM structure would be altered. Therefore, such a structure is capable of controlling the phase shifter for beam-steering devices. In this work, a broad steering angle of the transmitted light (from 259° to 270°) and reflected light (from 74° to 96°) in the MIR region can be obtained by switching between the amorphous and crystalline states of the Ge2Sb2Te5 . A heat model is used to examine the temporal variation of the temperature of the Ge2Sb2Te5 layer in gradient phase-array-like MDM structure. It shows that the temperature of the amorphous Ge2Sb2Te5 layer can be raised from room temperature to > 433 K (phase transition point of Ge2Sb2Te5) [18,19] in just 3.6 ns, with a low pumping light intensity of 2.6 μW/μm2. Therefore, our proposed structure can rapidly steer the light with a low power consumption in the optical region. This work paves the way for ultrafast beam steering system with a wide beam-steering angle. The structure possesses a simple geometry that can be fabricated using standard photolithography patterning. Finally, it should be noted that PCMs do not require any energy to maintain the structural state of the material. Thus, once the device has been switched, it will retain the beam direction until it is switched again. This obviously makes the proposed beam steering design interesting from a 'green technology' perspective.
2. Structure and design
Figure 1(a) shows the design of the gradient MDM strips array. The resonance strip consists of two Au layers separated by a Ge2Sb2Te5 interlayer. The MDM strip is assumed infinite along the z direction and β is a cross-section plane of the structure (x-y plane). The structure is considered to be suspended in a vacuum in order to simplify the model. The structure is excited by a plane wave source, propagating along the y direction with the E field polarized along the x-axis and H field along z-axis. Au is selected as the metal due to its stability against oxidation and low ohmic loss. The geometry of the cross-section of the strip is displayed in Fig. 1(b), where the thicknesses of Au and Ge2Sb2Te5 layers are tm = 40 nm and td = 250 nm, respectively. The lattice period is fixed at d = 500 nm. The resonant elements are chosen with decreasing widths in order to introduce different phase shifts, where w1 = 230, w2 = 200, w3 = 180 and w4 = 150 nm. A simple Drude model is used for the dielectric constant of Au, whereis the plasma frequency and is the scattering frequency for Au .
Ge2Sb2Te5 is a semiconductor chalcogenide alloy with a crystallization temperature of ≈433 K and a melting temperature of ≈873K . The amorphous Ge2Sb2Te5 dielectric interlayer will be crystallized if it is heated above its crystallization temperature, but without reaching the melting temperature. An amorphization process of Ge2Sb2Te5 involves the melting and rapid quenching of the Ge2Sb2Te5 back to its amorphous phase. This amorphization process can be achieved by a femtosecond (fs) laser with a relatively strong fluence, where the fs laser generates a very high density of electron hole pairs in the Ge2Sb2Te5, which subsequently thermalize and melt the Ge2Sb2Te5 . Fast cooling quenches the melt into the amorphous state. Therefore, Ge2Sb2Te5 is a promising candidate to realize modulation functionality, since it allows for an ultrafast reversible switching between its amorphous and crystalline states in the infrared region [22, 23]. The real, and imaginary, parts of the dielectric function for the different structural states of Ge2Sb2Te5 were obtained from the published Fourier transform infrared spectroscopy data in  and for the MIR spectral range the dielectric function is shown in Fig. 2. At a wavelength of 2000 nm, Ge2Sb2Te5 shows a pronounced change in the dielectric function during the reversible structural transformation from amorphous to crystalline. Thus transforming the state between amorphous and crystalline has potential for manipulating the transmission (reflection) phase of the gradient MDM strip array, hence steering the transmitted (reflected) wave. It should be mentioned that the reversible electronic structural state transition in Ge2Sb2Te5 is highly repeatable and more than a billion cycles have been experimentally demonstrated in data storage devices . With these unique properties, the Ge-Sb-Te system is of great interest for tunable plasmonics and nanophotonics [25–32]. Moreover, the operating wavelength of the proposed beam steering structure can be blue shifted to the near-infrared (NIR) region. This is because that Ge2Sb2Te5 possesses a pronounced change in the dielectric constant during its reversible phase transition in the optical communication band of 1550 nm, as shown in Fig. 2 .
Experimentally, repetitively, reversibly switching the state of Ge2Sb2Te5 embedded in the proposed MDM structure may sound challenging, since many noble metals are known to diffuse into Ge2Sb2Te5 at the high temperatures and this may limit the cycleability of the devices , such that the back and forth phase transition may collapse the structure, hence mixing the MDM trilayer. To solve this problem, we suggest that a future design, with improved cycleability, should include a diffusion barrier between the Ge2Sb2Te5 and Au nano strips. Or alternatively, new phase change materials, that may switch between two crystalline states without melting, such as interfacial phase change materials (IPCMs), should be used .
A finite-width MDM strip represents a plasmonic resonator, in which the counter propagating SPPs at the top and bottom metal-dielectric interfaces are efficiently reflected by the ends of the strip. It can form a resonant standing-wave pattern hence localizing the light within the dielectric interlayer when the counter-propagating SPPs interfere constructively. The resonance condition is given by
where is the wave vector of the incident light, the effective refractive index of the amorphous MDM strip, w the width of the strip, m an integer, and φ a phase acquired upon reflection of the internal SPPs at the end of the strip [7,16,34]. Considering the properties of the SPPs modes supported by an amorphous Ge2Sb2Te5 dielectric layer () sandwiched by Au layers at the excitation wavelength of 2000 nm, is 4.6 derived from the explicit dispersion relation . The w is then calculated to be 230 0nm using Eq. (1), where m = 1 and φ = 0.06π for a first-order resonance. The structure is simulated by a commercial software (Lumerical FDTD Solutions), which is based on Finite Difference Time Domain (FDTD) method. The uniform FDTD mesh size is adopted, the mesh size is the same along all Cartesian axes: Δx = Δy = 2 nm, which is sufficient to minimize the numerical errors arising from the FDTD method. The computational domain has perfectly matched layers (PMLs) and absorbing boundaries in all directions.
3. Results and discussions
To deflect the wavefront of the transmitted beam, a variation of the transmission phase is desired. In Fig. 3, it is clear that the transmission phase dispersion can be modulated by varying the width of the MDM strip. Moreover, the spectra of the transmission phase can be massively red-shifted when the structural state of Ge2Sb2Te5 switches from amorphous to crystalline. Figure 3(a) shows the scheme of the single MDM strip. As can be seen in Fig. 3(b), a shift of the transmission phase appears near 2000 nm for the amorphous MDM strip with various widths. For the widest amorphous MDM strip (w = 230 nm), the transmission phase exhibits the sharpest shift compared to those for the narrower strips (w = 200, 180 and 150 nm). In Fig. 3(c), the transmission phase of the crystalline MDM strip for w = 230, 200, 180 and 150 nm respectively is presented. A large tuning range of approximately 1000 nm for the phase shift is shown to be possible. Importantly, changing the structural state of the Ge2Sb2Te5 offers a significant transmission phase tunability (~50%) which can be useful in switching on/off beam-steering. For example, transiting the Ge2Sb2Te5 from the amorphous to crystalline states, the spectra of the transmission phase (shown in Fig. 3(c)) does not possess a rapid shift at λ = 2000 nm and thus resulting in a small value of steering angle (namely, switch off beam-steering), as shown in Fig. 6(a).
The resonances at which incident wave couples to SPPs can be determined from the dispersion diagram for the MDM layers [15, 35]. In the MDM layers, the internal SPP mode resonates on the inner surfaces of the metal layers and the external SPP mode resonates on the outer surfaces of the metal layers. In Fig. 4(a), we have calculated the SPP modes dispersion relation of Au- amorphous Ge2Sb2Te5-Au trilayer with the Au film thickness tm = 40 nm and middle Ge2Sb2Te5 film thickness td = 250 nm, where the tm = 40 nm and td = 250 nm are optimized to achieve the target wavelength of 2000 nm for the outer SPP mode. In Fig. 4(b), the transmittance spectra of the gradient amorphous MDM strips array is depicted together with the dispersion relation of the Au- amorphous Ge2Sb2Te5- Au trilayer, where the scheme of the gradient MDM strips array is presented in Fig. 1. Figure 4(a) shows that the (1,0) external mode and (1,0) internal mode in the simple trilayer appear at 1900 and 2450 nm respectively. Both modes are in a good agreement with the two transmittance peaks at the resonance wavelength of 2000 and 2270 nm in the gradient MDM strip array, shown in Fig. 4(b). The small remaining discrepancies are because the dispersion relation of the SPP modes used as matching condition in the MDM films does not take into account the presence of the resonant elements, which cause resonance shifts . As can be seen in Fig. 4(b), a high transmittance exists around the resonance wavelengths of 2000 and 2270 nm. Moreover, the transmittance around 2000nm for the (1,0) outer SPP mode is more significant than that around 2270 nm for the (1,0) inner SPP mode, hence steering the transmitted wave with a lower loss. Therefore, here we consider the phase control at the outer order (1,0) mode of 2000 nm. In particular, Fig. 4(d) shows the two transmittance peaks can red shift to 2893 and 3266 nm when the amorphous state changes to the crystalline state.
We start from four MDM strips with the equal width of w1 = 230 nm, where the dielectric layer is amorphous Ge2Sb2Te5 as illustrated in Fig. 5(a). Figure 5(b) presents the normalized emission pattern of the structure at λ = 2000 nm under normal incidence. It shows that both the front and back lobes are symmetric with respect to their centers since the MDM strips with the same width cannot provide varied phase shift. Here, PMLs are adopted for all dimensions to absorb scattering waves at boundaries. A near-to-far field transformation within Lumerical FDTD has then been used to calculate the far field radiation pattern of the structure .
To investigate the oblique wavefront of the transmitted beam achieved by the gradient MDM strips array, Fig. 6(a) shows the normalized radiation patterns of the gradient MDM strips array at λ = 2000 nm for both the amorphous and crystalline states. In the case of the amorphous structure (shown in blue solid line), the inclined angles of 259° and 74° for both the transmitted (front lobe) and reflected waves (back lobe) are obtained, respectively. By transiting the phase of Ge2Sb2Te5 from the amorphous to crystalline shown in red solid line, an angle steering of 11° is achieved from 259° to 270° for the front lobe. However, the radiation intensity of the front lobe for the crystalline state has decreased due to the off-resonance at λ = 2000 nm. Moreover, the steering performance for the back lobe is much stronger (angular steering of 22° from 74° to 96°) compared to that of the front lobe. Nevertheless, the radiation intensities of the back lobes are weaker than those of the front lobes for both amorphous and crystalline states. Figure 6(b) shows the trapped plasmon mode absorbance spectra of the structure for different states of Ge2Sb2Te5, where absorbance A = 1 − R − T, R is reflectance and T is transmittance. The variable dielectric constant of Ge2Sb2Te5 gives rise to a concomitant tunability in the absorbance. It can be seen that the main absorbance dip of the structure is broader in the crystalline state due to increased damping of the plasmon resonance .
The beam-steering of the gradient MDM strips array is due to the reversible amorphous - crystalline phase transition of the Ge2Sb2Te5 dielectric interlayer, which can be obtained by optical heating. To study the heat induced beam steering performance of the structure, a heat transfer model developed from our previous work , is used to obtain the temporal variation of temperature of Ge2Sb2Te5 layer using the Finite Element Method (FEM) solver within COMSOL. Here, the gradient structure is set up in COMSOL identical to that shown in Fig. 1. A Gaussian pulse is used as the excitation source to evaluate the required time to switch from the amorphous to crystalline state of the Ge2Sb2Te5. In particular, the Gaussian source has a repetition rate, fr = 25 kHz and pulse duration of 2.6 ns. The light fluence shining on the sample from a single pulse is written as Fig. 6(b). The temperature dependent thermal conductivity of Ge2Sb2Te5 are obtained from experiment data in . The heat source power is given by:Figure 7(a) shows Qs(r, t) and the temperatures of the amorphous Ge2Sb2Te5 dielectric interlayer, for the different width resonators located at the center of Gaussian beam. It shows that the temperature within the amorphous Ge2Sb2Te5 dielectric interlayer is a function of the incident radiation flux and can exceed the amorphous to crystalline phase transition temperature of 433 K after 3.6 ns under a threshold incident flux of 2.6 μW/μm2. Namely, this structure enables an ultrafast beam steering time of 3.6 ns under a low pump light intensity of 2.6 μW/μm2. The temperature starts dropping after 5.8 ns before the next pulse arrives, due to heat dissipation into the surroundings. Figure 7(b) shows the temperature distribution of the MDM strip array at 3.6 ns along the β plane, where the dominant temperature gradient (depicted by green arrows) is toward the top and bottom Au films.
Ge2Sb2Te5 can form intermediate states, containing regions of both amorphous and crystalline phases, so-called partial crystallization [39,40], which is attractive for obtaining continuously tunable photonic devices. It has been shown that an ultrafast laser i.e. fs lasers enables cumulative switching of Ge2Sb2Te5 layer, where rapid heat diffusion as well as the repeatable energy dose can control the partial crystallization process [41,42]. By repeatedly illuminating the same area of the Ge2Sb2Te5 film by the fs laser pulse, a continuous change of the refractive index of the Ge2Sb2Te5 is achieved [43,44], where the change of the refractive index (or the degree of crystallization) is verified by infrared reflectivity measurements of the switched area. Here, to show the function of the continuous beam steering in the proposed structure, a few more simulations are performed to continuously change the directions of the reflected and transmitted light from the propagation direction of the incident wave, by means of partial crystallization in the Ge2Sb2Te5 dielectric interlayer. For instance, we split the Ge2Sb2Te5 into two layers (one crystalline and one amorphous) and gradually change the relative thickness of each whilst keeping the total thickness of both layers constant (td = 250 nm). This may actually match the experimental fs laser switching where the surface is preferentially switched. By increasing the time of the laser pulse, thicker layers are crystallized. Figure 8(a) shows that the directional angles of 74°, 79°, 86°, 94°, and 96° for the reflected beams as well as 259°, 264°, 266°, 268°, and 270° for the transmitted beams at 𝛌 = 2000 nm can be achieved by crystallizing thicker Ge2Sb2Te5 layer, where the thicknesses of the crystalline Ge2Sb2Te5 films are of tC = 0, 100, 170, 220 and 250 nm accordingly. Figure 8(b)-8(c) show the relationships of the directional angles of the reflected waves and transmitted waves with respect to tC, respectively. It is clear that varying tC allows direct control over the directions of both reflected and transmitted beams. Particularly, a large and continuous beam steering of 22° for the reflected beam has potential for controlling the wavefront of the reflected beam. This could enable applications in free-space optical inter/intra chip interconnects.
We numerically demonstrate a low-power and rapid MIR beam steering using an array of Au/Ge2Sb2Te5/Au multilayer strips. The different widths in each strip element with amorphous Ge2Sb2Te5 exhibit the different phase shifts, hence resulting in a deflection angle of 259° for the transmitted light and 74° for the reflected light. Importantly, a wide beam-steering range of 11° for the transmitted light and 22° for the reflected light are observed by switching the structural phase from the amorphous to crystalline. A thermal model is constructed to resolve the transient temperature variation in the structure during a photothermal process. Our model predicts that a fast beam-steering response time of 3.6 ns can be achieved under a low power intensity of 2.6 μW/μm2. These findings could lead to a new avenue for designing ultralow-power and ultrafast photonics beam-steering devices.
We acknowledge the financial support from National Natural Science Foundation of China (Grant No. 61172059 and 51302026).
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