Showing papers in "Optical Materials Express in 2022"

Broadband Polarization-insensitive and Oblique-incidence Terahertz Metamaterial Absorber with Multi-layered Graphene

Abstract: Metamaterial absorbers have been widely studied in the past decade and their performances have been incessantly improved in the practical applications. In this paper, we present a broadband terahertz metamaterial absorber based on graphene-polyimide composite structure, and the structure consists of a metal substrate and graphene layers with different sizes separated by two polyimide dielectric layers. The simulation results show that the absorptance of the absorber is greater than 90% in 0.86–3.54 THz with the fractional bandwidth of 121.8%. The absorptance can be adjusted by changing the chemical potential of graphene. In addition, the absorber is insensitive to polarization and still has robust tolerance for the oblique incidence. The equivalent circuit model based on transmission line is introduced to analyze the physics of the designed absorber and the results are in good agreement with the simulations. We believe that the designed absorber is a potential competitive candidate in terahertz energy harvesting and thermal emission.

Emerging devices and packaging strategies for electronic-photonic AI accelerators: opinion

Abstract: The field of mimicking the structure of the brain on a chip is experiencing interest driven by the demand for machine intelligent applications. However, the power consumption and available performance of machine-learning (ML) accelerating hardware still leave much desire for improvement. In this letter, we share viewpoints, challenges, and prospects of electronic-photonic neural network (NN) accelerators. Combining electronics with photonics offers synergistic co-design strategies for high-performance AI Application-specific integrated circuits (ASICs) and systems. Taking advantages of photonic signal processing capabilities and combining them with electronic logic control and data storage is an emerging prospect. However, the optical component library leaves much to be desired and is challenged by the enormous size of photonic devices. Within this context, we will review the emerging electro-optic materials, functional devices, and systems packaging strategies that, when realized, provide significant performance gains and fuel the ongoing AI revolution, leading to a stand-alone photonics-inside AI ASIC ‘black-box’ for streamlined plug-and-play board integration in future AI processors.

Ultra-broadband nanostructured metamaterial absorber based on stacked square-layers of TiN/TiO₂

Abstract: Metamaterial-based nano-scale absorbers have been becoming very popular in the modern era due to efficiently absorbing solar radiation to revamp the performance characteristics of thermal emitters and solar thermophotovoltaics (STPV) systems. Here, we explore and implement an ultra-broadband nanostructured metamaterial absorber (NMMA), which comprises a stack of alternating nano-squares of TiN and TiO 2 mounted over the dielectric substrate backed by a metallic sheet. The numerical simulations and electromagnetic (EM) characteristics of the proposed NMMA have been investigated by employing the finite difference time domain (FDTD) EM tool. The numerical results indicate that the average absorption of the NMMA reaches 96% in the wavelength range from 200-3000 nm (from ultraviolet to mid-infrared), and the minimal absorption is also above 90% in a continuous large operating spectrum ranging from 200-2800 nm. Surprisingly, the absorption features of the designed nano-absorber remain stable under the influence of oblique incident-angles for both the polarization states (TE & TM). Furthermore, the proposed nano-absorber manifests polarization-insensitive behavior due to the presence of four-fold symmetry of the proposed structure. Large operational bandwidth, miniaturized structure, and the use of thermally stable refractory metal TiN make this NMMA an appealing candidate for the applications of thermal emission, solar thermophotovoltaics, and other opto-electronic devices. In addition, the design of this absorber is also scalable to other operating spectrums through carefully selecting the materials and optimizing the geometry of the proposed structure.

Comparison and analysis of phase change materials-based reconfigurable silicon photonic directional couplers

Abstract: The unique optical properties of phase change materials (PCMs) can be exploited to develop efficient reconfigurable photonic devices. Here, we design, model, and compare the performance of programmable 1 × 2 optical couplers based on: Ge 2 Sb 2 Te 5 , Ge 2 Sb 2 Se 4 Te 1 , Sb 2 Se 3 , and Sb 2 S 3 PCMs. Once programmed, these devices are passive, which can reduce the overall energy consumed compared to thermo-optic or electro-optic reconfigurable devices. Of all the PCMs studied, our ellipsometry refractive index measurements show that Sb 2 S 3 has the lowest absorption in the telecommunications wavelength band. Moreover, Sb 2 S 3 -based couplers show the best overall performance, with the lowest insertion losses in both the amorphous and crystalline states. We show that by growth crystallization tuning at least four different coupling ratios can be reliably programmed into the Sb 2 S 3 directional couplers. We used this effect to design a 2-bit tuneable Sb 2 S 3 directional coupler with a dynamic range close to 32 dB. The bit-depth of the coupler appears to be limited by the crystallization stochasticity.

Polarization-controllable and angle-insensitive multiband Yagi-Uda-shaped metamaterial absorber in the microwave regime

Abstract: This paper reports a multiband Yagi-Uda shaped metamaterial absorber (YUMA) operating in X- and Ku-band of the microwave regime with the added functionalities of miniaturization, polarization controllability and wide incidence angle stability. The proposed YUMA shows three distinctive near-unity absorption peaks corresponding to 10.64 GHz, 12.08 GHz, and 14.09 GHz frequencies. The YUMA was analyzed under different oblique incidence angles for transverse electric (TE)-mode and different rotation angles of the top metasurface. The results showed that the proposed YUMA possesses multifunctional characteristics such as polarization controllability, and wide incidence angle stability. The comparison of simulated and measured results further demonstrates that the proposed absorber can be a potential candidate in polarization detection systems and transmissive polarizers. The proposed YUMA operating in the X- and Ku-band can have potential uses in several other applications, such as air traffic control, weather monitoring, military radar, and satellite communication.

Endurance of Chalcogenide Optical Phase Change Materials: a Review

Abstract: Chalcogenide phase change materials (PCMs) are truly remarkable compounds whose unique switchable optical and electronic properties have fueled an explosion of emerging applications in electronics and photonics. Key to any application is the ability of PCMs to reliably switch between crystalline and amorphous states over a large number of cycles. While this issue has been extensively studied in the case of electronic memories, current PCM-based photonic devices show limited endurance. This review discusses the various parameters that impact crystallization and re-amorphization of several PCMs, their failure mechanisms, and formulate design rules for enhancing cycling durability of these compounds.

Low-threshold all-optical nonlinear activation function based on Ge/Si hybrid structure in microring resonator

Abstract: Optical nonlinear activation function is an indispensable part of the optical neural network. While linear matrix computation has thrived in an integrated optical neural network, there are many challenges for nonlinear activation function on a chip such as large latency, high power consumption and high threshold. Here, we demonstrate that Ge/Si hybrid structure would be a qualified candidate owing to its property of CMOS-compatibility, low nonlinear threshold and compact footprint. Thanks to the strong thermal-optic effect of germanium in conjunction with micro-ring resonator, we experimentally demonstrate three different types of nonlinear function (Radial basis, Relu and ELU functions) with a lowest threshold of 0.74 mW among our measured nonlinear functions and they can work well with a repetition rate below 100 kHz. Simultaneous size shrinkage of germanium and resonance constraint inside germanium is proposed to speed up response time. Furthermore, we apply our measured nonlinear activation function to the task of classification of MNIST handwritten digit image dataset and improve the test accuracy from 91.8% to 94.8% with feedforward full-connected neural network containing three hidden layers. It proves that our scheme has potential in the future optical neural network.

Role of delay-times in delay-based photonic reservoir computing [Invited]

Abstract: Delay-based reservoir computing has gained a lot of attention due to the relative simplicity with which this concept can be implemented in hardware. However, unnecessary constraints are commonly placed on the relationship between the delay-time and the input clock-cycle, which can have a detrimental effect on the performance. We review the existing literature on this subject and introduce the concept of delay-based reservoir computing in a manner that demonstrates that no predefined relationship between the delay-time and the input clock-cycle is required for this computing concept to work. Choosing the delay-times independent of the input clock-cycle, one gains an important degree of freedom. Consequently, we discuss ways to improve the computing performance of a reservoir formed by delay-coupled oscillators and show the impact of delay-time tuning in such systems.

Rare-earth doped mixed sesquioxides for ultrafast lasers [Invited]

Abstract: Sesquioxides are outstanding host materials for rare-earth doped laser gain media. Unfortunately, their very high melting points make it challenging for them to be fabricated in high quality. Recently, we demonstrated that some mixed sesquioxides exhibit significantly reduced melting temperatures compared to their constituents. This enables their growth by the established Czochralski method yielding rare-earth doped mixed sesquioxides of high optical quality. Due to their inhomogeneously broadened gain spectra caused by the intrinsic disorder, mixed sesquioxides are very promising for the generation and amplification of ultrashort pulses. To envisage the potential of this emerging class of gain materials, this paper reviews the spectroscopic as well as continuous wave and pulsed laser properties of crystalline and ceramic rare-earth doped mixed cubic sesquioxides of the form (Sc x ,Lu y ,Y z ) 2 O 3 with x + y + z = 1.

Progress in water-based metamaterial absorbers: a review

Abstract: Increasing attention on microwave ultra-broadband metamaterial absorbers has been paid due to their promising applications. While most microwave ultra-broadband metamaterial absorbers developed so far are based on metallic resonant structures, dispersive dielectric water-based metamaterial opens a simpler and more versatile route for the construction of polarization- and angle- insensitive ultra-broadband absorption. Here, we review the recent progress of water-based metamaterial absorbers by providing an illustration of the mechanisms to realize ultra-broadband, tunable and multi-functional absorption. We also address the further development direction and some potential novel applications.

Tunable terahertz hybrid metamaterials supported by 3D Dirac semimetals

Abstract: By utilizing the three-dimensional Dirac semimetal (DSM)-strontium titanate (SrTiO 3 , STO) elliptical hybrid metamaterials, the tunable Fano resonances were systematically analyzed in the THz regime, for example, the effects of asymmetric degrees, DSM Fermi levels, and operation frequencies. Interestingly, an obvious Fano peak is observed by introducing a displacement (asymmetric degree) between STO and DSM resonators. In particular, the amplitude modulation depth (MD) of the Fano transmission peak (reflection dip) is 49.5% (86.65%) when the asymmetric degree ranges from 0 to 20 µm. Furthermore, on the condition that the asymmetric degree is larger than 10 µm, the LC resonance is also excited with an extraordinary Q –factor of more than 25. Additionally, by modifying the Fermi level of DSM layer, the amplitude MD of Fano transmission peak (reflection dip) is 32.86% (67.26%). The results facilitate our understanding of the tunable mechanisms of DSM metamaterials and potentially promote the development of novel plasmonic devices, including filters, modulators and sensors.

Designing Highly Sensitive Exposed CoreSurface Plasmon Resonance Biosensor

Abstract: With technological advancement, photonic crystal fibers (PCFs) are effectively used to design miniaturized, flexible, and efficient biosensors. This paper proposes an exposed core PCF biosensor based on widely known surface plasmon resonance (SPR) phenomena. An external sensing mechanism is followed to characterize the sensing performance within the refractive index (RI) range between 1.28 and 1.40. Metal strip (gold (Au) and titanium dioxide (TiO 2 )) is deposited on the outer surface only along the four channels instead of the entire surface, which could decrease the difficulties associated with the metal deposition on the entire circular surface. Simulating the sensor using finite element method based COMSOL Multiphysics software, we find tremendous amplitude sensitivity of 7420.69 RIU −1 and wavelength sensitivity of 87,000 nm/RIU. In addition, the sensor offers the highest resolution of 7.7×10 −6 RIU, the figure of merit of 1011.63 RIU −1 , signal to noise ratio of 10.05 dB, the detection accuracy of 0.016598 nm −1 , and detection limit of 102.23 nm. However, the promising sensing performance indicates that the proposed sensor could be implemented effectively to detect different biological and chemical substances.

ℏω versus ℏk: dispersion and energy constraints on time-varying photonic materials and time crystals [Invited]

Abstract: Photonic time-varying systems have attracted significant attention owing to their rich physics and potential opportunities for new and enhanced functionalities. In this context, the duality of space and time in wave physics has been particularly fruitful to uncover interesting physical effects in the temporal domain, such as reflection/refraction at temporal interfaces and momentum-bandgaps in time crystals. However, the characteristics of the temporal/frequency dimension, particularly its relation to causality and energy conservation ($\hbar \omega$ is energy, whereas $\hbar \boldsymbol{k}$ is momentum), create challenges and constraints that are unique to time-varying systems and are not present in their spatially varying counterparts. Here, we overview two key physical aspects of time-varying photonics that have only received marginal attention so far, namely temporal dispersion and external power requirements, and explore their implications. We discuss how temporal dispersion, an inherent property of any causal material, makes the fields evolve continuously at sharp temporal interfaces and may limit the strength of fast temporal modulations and of various resulting effects. Furthermore, we show that changing the refractive index in time always involves large amounts of energy. We derive power requirements to observe a time-crystal response in one of the most popular material platforms in time-varying photonics, i.e., transparent conducting oxides, and we argue that these effects are almost always obscured by less exotic nonlinear phenomena. These observations and findings shed light on the physics and constraints of time-varying photonics, and may guide the design and implementation of future time-modulated photonic systems.

Nanoparticles in optical fiber, issue andopportunity of light scattering

Abstract: Since its first creation, glass has always fascinated with its optical properties, its ability to let light through without being invisible. One of the most spectacular achievements of optical glass is the optical fiber for which considerable work has been done to make it as transparent as possible. However, for twenty years, contrary to this quest for transparency, nanoparticles have been inserted into optical fibres. First designed to develop new lasers and amplifiers, the lowest possible particle-induced light scattering then sought has for the last four years, on the contrary, been exacerbated in order to develop new sensors.

ℏω versus ℏk: Dispersion and Energy Constraints on Time-Varying Photonic Materials and Time Crystals

Abstract: Photonic time-varying systems have attracted significant attention owing to their rich physics and potential opportunities for new and enhanced functionalities. In this context, the duality of space and time in wave physics has been particularly fruitful to uncover interesting physical effects in the temporal domain, such as reflection/refraction at temporal interfaces and momentum-bandgaps in time crystals. However, the characteristics of the temporal/frequency dimension, particularly its relation to causality and energy conservation ( (cid:126) 𝜔 is energy, whereas (cid:126) 𝒌 is momentum), create challenges and constraints that are unique to time-varying systems and are not present in their spatially varying counterparts. Here, we overview two key physical aspects of time-varying photonics that have only received marginal attention so far, namely temporal dispersion and external power requirements, and explore their implications. We discuss how temporal dispersion, an inherent property of any causal material, makes the fields evolve continuously at sharp temporal interfaces and may limit the strength of fast temporal modulations and of various resulting effects. Furthermore, we show that changing the refractive index in time always involves large amounts of energy. We derive power requirements to observe a time-crystal response in one of the most popular material platforms in time-varying photonics, i.e., transparent conducting oxides, and we argue that these effects are almost always obscured by less exotic nonlinear phenomena. These observations and findings shed light on the physics and constraints of time-varying photonics, and may guide the design and implementation of future time-modulated photonic systems.

Photonic neuromorphic computing using vertical cavity semiconductor lasers

Abstract: Photonic realizations of neural network computing hardware are a promising approach to enable future scalability of neuromorphic computing. The number of special purpose neuromorphic hardware and neuromorphic photonics has accelerated on such a scale that one can now speak of a Cambrian explosion. Work along these lines includes (i) high performance hardware for artificial neurons, (ii) the efficient and scalable implementation of a neural network’s connections, and (iii) strategies to adjust network connections during the learning phase. In this review we provide an overview on vertical-cavity surface-emitting lasers (VCSELs) and how these high-performance electro-optical components either implement or are combined with additional photonic hardware to demonstrate points (i-iii). In the neurmorphic photonics context, VCSELs are of exceptional interest as they are compatible with CMOS fabrication, readily achieve 30% wall-plug efficiency, >30 GHz modulation bandwidth and multiply and accumulate operations at sub-fJ energy. They hence are highly energy efficient and ultra-fast. Crucially, they react nonlinearly to optical injection as well as to electrical modulation, making them highly suitable as all-optical as well as electro-optical photonic neurons. Their optical cavities are wavelength-limited, and standard semiconductor growth and lithography enables non-classical cavity configurations and geometries. This enables excitable VCSELs (i.e. spiking VCSELs) to finely control their temporal and spatial coherence, to unlock terahertz bandwidths through spin-flip effects, and even to leverage cavity quantum electrodynamics to further boost their efficiency. Finally, as VCSEL arrays they are compatible with standard 2D photonic integration, but their emission vertical to the substrate makes them ideally suited for scalable integrated networks leveraging 3D photonic waveguides. Here, we discuss the implementation of spatially as well as temporally multiplexed VCSEL neural networks and reservoirs, computation on the basis of excitable VCSELs as photonic spiking neurons, as well as concepts and advances in the fabrication of VCSELs and microlasers. Finally, we provide an outlook and a roadmap identifying future possibilities and some crucial milestones for the field.

Deep Learning for design of 3D ChiralPlasmonic Metasurfaces

Abstract: Chiral plasmonic metasurfaces are promising for enlarging the chiral signals of biomolecules and improving the sensitivity of bio-sensing. However, the design process of the chiral plasmonic nanostructures is time consuming. Deep learning has been playing a key role in the design of photonic devices with high time efficiency and good design performance. This paper proposes a deep neural network (DNN) to achieve forward prediction and inverse design for 3D chiral plasmonic metasurfaces, and further improve the training speed and performance by the transfer learning method. Once the DNNs are trained using a part of the sampled data from the parameter space, the circular dichroism (CD) spectra can be predicted within the time on milliseconds (about 3.9 ms for forward network and 5.6 ms for inverse network) with high prediction accuracy. The inverse design was optimized by taking more spectral information into account and extracting the critical features using the one-dimensional convolutional kernel. The aforementioned trained network for one handedness can accelerate the training speed and improve performance with small datasets for the opposite handedness via the transfer learning method. The proposed approach is instructive in the design process of chiral plasmonic metasurfaces and could find applications in exploring versatile complex nanophotonic devices efficiently.

Electrically tunable Goos-Hänchen shift intwo-dimensional quantum materials

Abstract: We theoretically investigate the tunable Goos-Hänchen (GH) shifts in silicene subjected to an external electric field and circularly polarized light. The prominent feature of these 2D quantum materials is the tunable bandgap that can be tuned by an external electric field or by irradiating circular polarized light beam. Using angular spectrum analysis, we obtain the analytical expressions for the spin and valley polarized spatial and angular GH shifts. We find that tuneable giant spatial and angular GH shifts exhibit extreme values near Brewster’s angles and away from the optical transition frequencies in the silicene. We demonstrate that both positive and negative giant GH shifts can be achieved in the graphene family by tuning the electric field and circularly polarized light in distinct topological regimes. Due to the topological properties of these materials, the GH shift is sensitive to the coupled spin and valley indices of the Dirac fermions as well as to the number of closed gaps. We further demonstrated that topology and spin-orbit interactions play a crucial role in beam shifts and topological quantum phase transitions of the silicene can be comprehensively and efficiently probed through GH shift at the nanoscale.

Finite-size and quantum effects in plasmonics:manifestations and theoretical modelling

Abstract: The tremendous growth of the field of plasmonics in the past twenty years owes much to the pre-existence of solid theoretical foundations. Rather than calling for the introduction of radically new theory and computational techniques, plasmonics required, to a large extent, application of some of the most fundamental laws in physics, namely Maxwell’s equations, albeit adjusted to the nanoscale. The success of this description, which was triggered by the rapid advances in nanofabrication, makes a striking example of new effects and novel applications emerging by applying known physics to a different context. Nevertheless, the prosperous recipe of treating nanostructures within the framework of classical electrodynamics and with use of macroscopic, bulk material response functions (known as the local-response approximation, LRA) has its own limitations, and inevitably fails once the relevant length scales approach the few- to sub-nm regime, dominated by characteristic length scales such as the electron mean free path and the Fermi wavelength. Here we provide a review of the main non-classical effects that emerge when crossing the border between the macroscopic and atomistic worlds. We study the physical mechanisms involved, highlight experimental manifestations thereof and focus on the theoretical efforts developed in the quest for models that implement atomistic descriptions into otherwise classical-electrodynamic calculations for mesoscopic plasmonic nanostructures.

Magnetically-sensitive nanodiamond thin-films on glass fibers

Abstract: By assembling 140 nm-sized fluorescent nanodiamonds (FNDs) in a thin-film on (3-aminopropyl) triethoxysilane functionalized glass surface, we prepare magnetically-sensitive FND-fiber probes for endoscopy. The obtained FND layers show good uniformity over large surfaces and are characterized using confocal, fluorescence, and atomic force microscopes. Further, FNDs are assembled on single large-core multimode optical fibers and imaging fiber bundles end face to detect optically detectable magnetic resonance (ODMR) signals. The ODMR signals are recorded through the fiber’s far end in magnetic fields between 0 to 2.5 mT. A multi-channel sensor is demonstrated with the capability of parallel-in-time mapping and instantaneous readout from individual pixel and enabling magnetic mapping at high spatial resolution. Results of this study are promising for early stage detection in bio-diagnostic applications.

Time-varying electromagnetic media: opinion

Abstract: In this opinion article, we briefly summarize some of the background materials and recent developments in the field of temporal and spatiotemporal media and provide our opinion on some of potential challenges, opportunities, and open research questions for manipulation of fields and waves in four dimensions.

Realization of tunable dual-type quasi-bound states in the continuum based on Dirac semimetal metasurface

Abstract: In this study, two types of tunable quasi-bound states in the continuum (BIC) based on Dirac semimetal metamaterial (DSM) in the terahertz (THz) band are proposed in the same metasurface. The symmetry-protected BICs are achieved by altering the structural symmetry. The accidental BICs are realized by adjusting the structural parameters, and the quality factor (Q factor) of the corresponding quasi-BICs can be as high as 175. To better understand the excitation mechanism of the quasi-BIC, we investigated the magnetic field distribution and current distribution of the BIC and quasi-BIC, respectively. The results showed that the accidental BIC and the symmetry-protected BIC have a common resonance mode, and the two LC resonance modes are coupled with each other, which causes a strong resonance. The dynamic modulation of the transmission amplitude is achieved by changing the Fermi energy of the DSM at a nearly constant resonant frequency, and the difference in amplitude modulation is about 46%. Based on the idea of amplitude modulation, the design of the Dirac semimetal film (DSF) metasurface array for stereoscopic graphic display is realized.

Review of integrated magneto-optical isolators with rare-earth iron garnets for polarization diverse and magnet-free isolation in silicon photonics

Abstract: Passive optical isolators are needed in silicon photonics but unavailable due to challenges in rare-earth iron garnet processing and integration. Material challenges include incompatibility with silicon and high annealing temperatures, and design challenges include a need for polarization diversity and a preference for no external magnetic bias. These challenges have restricted optical isolation to discrete modules that require physical pick and place of bulk garnet pieces. This review presents developments in the processing of magneto-optical garnets on Si and the enhancement of their Faraday rotation that enables small footprint isolators on silicon waveguide structures. For example, seedlayers and/or new garnet compositions have enabled monolithic Si integration, and in some cases, hybrid integration of garnet-on-garnet or transfer-printed garnet nanosheets enable reduced on-chip thermal processing. Integrated isolators that utilize non-reciprocal phase shift (NRPS) or non-reciprocal mode conversion (NRMC) have been demonstrated to have isolation ratios up to 30 dB, insertion loss as low as 9 dB, polarization diversity and magnet-free operation in the desired telecommunication wavelengths. The advances in materials, processing techniques, and isolator designs shown here will pave the way for on-chip isolators and novel multi-lane photonic architectures.

Multifaceted anapole: from physics to applications- Invited

Abstract: The optical anapole state resulting from interference of the electric and toroidal moments is of much interest due to its nonradiating nature. Interference of optical modes supported by a diverse range of Mie-resonant structures has found many applications, such as in biosensors and optical communication. This review provides an overview of the recent progress of anapole states in photonics. After a brief historical background, a complete mathematical description is presented. It allows one to clearly demonstrate and identify the existence of anapole states and highlight their fundamental properties. Then, we focus on the excitation of anapoles in photonics and discuss the relation to other states, such as bound states in the continuum. Finally, we discuss a series of advances that uncover the anapole potential in various applications, from nonlinear photonics and lasing to optical communication and sensing.

Tunable and switchable bifunctional meta-surface for plasmon-induced transparency and perfect absorption

Abstract: Tunable multi-function metasurfaces have become the latest research frontiers in planar optics. In this study, a dynamically tunable plasmon-induced transparency (PIT) structure based on a graphene split-ring resonator and graphene ribbon is proposed. The influences of the structural parameters and graphene Fermi energy on the PIT response were investigated both analytically and numerically simulations. The inclusion of an additional vanadium dioxide (VO 2 ) substrate layer enables the metasurface to achieve dynamic switching between PIT and perfect absorption using the phase change property of VO 2 . The new metasurface device exhibits the PIT effect when the VO 2 layer is in an insulating state and acts as a perfect absorber when it is in a metallic state. Moreover, the response of the two functions can be easily adjusted dynamically by changing the Fermi energy of graphene. In addition, both functions were highly sensitive to changes in the ambient refractive index. The results of this work have potential applications in slow-light devices, optical switches, modulators, perfect absorbers, highly sensitive sensors, and multifunctional devices.

Bandgap engineering, monolithic growth, and operation parameters of GaSb-based SESAMs in the 2 – 2.4 µm range

Abstract: We present the detailed growth and characterization of novel GaSb-based semiconductor saturable absorber mirrors (SESAMs) operating in the 2–2.4 µm spectral range. These SESAMs at different wavelengths are bandgap engineered using ternary material compositions and without strain compensation. We observe that even when the thickness of quantum wells (QWs) exceeds the critical thickness we obtain strain relaxed SESAMs that do not substantially increase nonsaturable losses. SESAMs have been fabricated using molecular beam epitaxy with a AlAs 0.08 Sb 0.92 /GaSb distributed Bragg reflector (DBR) and strained type-I In x Ga 1-x Sb or type-II W-like AlSb/InAs/GaSb QWs in the absorber region. All the type-I SESAMs show excellent performance, which is suitable for modelocking of diode-pumped semiconductor, ion-doped solid-state, and thin-disk lasers. The recovery time of the type-II SESAM is too long which can be interesting for laser applications. The dependence of the SESAM design, based on its QW number, barrier material, and operation wavelength are investigated. A detailed characterization is conducted to draw conclusions from macroscopic nonlinear and transient absorption properties at different wavelengths in the 2–2.4 µm range for the corresponding devices.

Tuning and switching effects of quasi-BIC statescombining phase change materials withall-dielectric metasurfaces

Abstract: Emission enhancement of quantum emitters is particularly relevant in the development of single-photon sources, which are key elements in quantum information and quantum communications. All-dielectric metasurfaces offer a route towards strong enhancement of local density of optical states via engineering of high quality factor optical modes. In particular, the recently proposed concept of quasi-bound states in the continuum (quasi-BIC) allows for precise control of such resonances in lattices with an asymmetric unit cell. Still, the spectral band of emission enhancement is usually fixed by the geometric parameters of the metasurface. Here, we propose to utilize phase change materials to tune the properties of light-emitting metasurfaces designed to support quasi-BIC states in the telecom wavelength range. In our design, a thin layer of a phase change material, which provides strong contrast of refractive index when switched from the amorphous to the crystalline state, is located on top of the resonators made of amorphous silicon (a-Si). Depending on the selected phase change material, we numerically demonstrate different functionalities of the metasurface, In particular, for low-loss Sb 2 Se 3 we evidence spectral tuning effects, whereas for Ge 2 Sb 2 Te 5 , we report an “on”/“off” switching effect of the quasi-BIC resonance. Furthermore, we investigate the influence of the crystallization fraction and the asymmetry parameter of the metasurface on the results. This work provides concrete design blueprints for switchable metasurfaces, offering new opportunities for nanophotonics devices or integrated photonic circuits.

Polarimetry Analysis and Optical Contrast of Sb₂S₃ Phase Change Material

Abstract: Phase-change materials (PCMs) are the cornerstone for the development of reconfigurable and programmable photonic devices. Sb 2 S 3 has been recently proposed as an interesting PCM due to its low-losses in the visible and near-IR. Here, we report the use of imaging polarimetry and spectroscopic ellipsometry to reveal and directly measure the optical properties of Sb 2 S 3 both in crystalline and amorphous states obtained upon crystallization by annealing in the air of chemical bath deposited amorphous Sb 2 S 3 . The Mueller Matrix polarimetric analysis reveals the strong anisotropy of the Sb 2 S 3 crystallites which crystallize in radial spherulitic domains in contrast to the optical isotropy of the amorphous films. A refractive index contrast of Δ n = 0.5 is demonstrated while maintaining low-losses at telecommunications C-band, i.e., λ = 1550 nm.

First-principles investigation of amorphous Ge-Sb-Se-Te optical phase-change materials

Abstract: Chalcogenide phase-change materials (PCMs) are promising candidates for nonvolatile memory and neuromorphic computing devices. The recently developed Ge 2 Sb 2 Se 4 Te 1 alloy shows superior properties in terms of low optical loss and higher thermal stability with respect to the flagship Ge 2 Sb 2 Te 5 alloy, making this new quaternary alloy a suitable candidate for high-performance optical switches and modulators. In this work, we carry out ab initio calculations to understand how selenium substitution modifies the local structure and the optical response of the amorphous quaternary alloys. We consider four amorphous Ge 2 Sb 2 Se x Te 5- x (GSST) alloys with x = 1 to 4 and show that the substitution of selenium content induces a gradual reduction in the calculated refractive indices, which is in agreement with experimental observation. This improvement on optical loss stems from the increased band gap size, which is attributed to the larger Peierls-like distortion and the stronger charge transfer in the Se-richer amorphous GSST alloys.

Mode-locked erbium-doped fiber laser based on a mechanically exfoliated ReS₂ saturable absorber onto D-shaped optical fiber

Abstract: In this work, we report a femtosecond mode-locking Erbium-doped fiber laser using mechanically exfoliated rhenium disulfide (ReS 2 ) deposited onto the polished surface of a D-shaped optical fiber. By performing the polarization and saturable absorption measurements, the sample exhibited a polarization extinction ratio of 10 dB (90%) and nonlinear transmittance variation of 3.40%. When incorporated into the cavity as a saturable absorber (SA), the passive mode-locking performance of 220 fs was achieved. This is the best mode-locking performance ever reported in literature achieved with all-fiber based ReS 2 SA. By using density functional theory (DFT) calculations, we obtained the electronic states and the optical absorption spectrum at 1550 nm attributed by defects in the ReS 2 structures, which is consistent with its linear and nonlinear optical absorption in the laser mode-locking mechanism.