Showing papers in "Optical Materials Express in 2020"

Dual-controlled switchable broadband terahertz absorber based on a graphene-vanadium dioxide metamaterial

Abstract: We propose a dual-controlled switchable broadband terahertz (THz) metamaterial absorber based on a hybrid of vanadium dioxide (VO2) and graphene that demonstrates strong polarization-independent characteristics and works well at a wide range of incidence angles. The peak absorptance of the proposed absorber can be tuned from 26 to 99.2% by changing the Fermi energy of the graphene; the absorptance can be dynamically tuned from 9 to 99.2% by adjusting the conductivity of the vanadium dioxide because of its unique insulator-to-metal transition characteristic. Using these two independent controls in tandem, we found that the state of the proposed absorber can be switched from absorption (>96%) to reflection (>73.5%), and the transmittance can be tuned from 0% to 65% while maintaining broad bandwidth (1.05-1.6 THz), resulting in a better-performing switchable broadband terahertz absorber. Furthermore, we have provided a discussion of the interference theory in which the physical mechanism of the absorption is explained from an optical point of view. The absorber achieves dual-controlled absorptance switching via two independently controllable pathways, offering a new method for switching and modulation of broadband THz radiation.

Design of a wideband terahertz metamaterial absorber based on Pythagorean-tree fractal geometry

Abstract: Broadband absorption in the terahertz regime is a challenge and onerous to realize with a single layer metasurface. Self-similarity in fractal structures are exploiting metamaterial characteristics that offer a promising platform to design wideband microwave and optical devices. This paper presents a metamaterial absorber that consists of fractal geometry of Pythagorean-tree. The proposed metamaterial absorber demonstrates the wideband absorptivity in a terahertz spectrum ranging from 7.5–10 THz. Both transverse electric (TE)–and transverse magnetic(TM)–mode are taken up under different obliquity incidence angles to deeply study the angular dependence on absorption features of the Pythagorean-tree fractal meta-absorber (PTFMA). A numerical approach of interference theory is employed to verify the simulation results of the designed PTFMA. Further, the performance of the PTFMA was analyzed in terms of the figure of merit (FOM) and operational bandwidth (OBW) for different geometric parameters. Furthermore, surface electric field patterns and current distributions were studied to understand the absorption mechanism of the suggested PTFMA. The designed absorber would be a promising contender for bolometers, THz detection, and communication.

Absorptive loss and band non-parabolicity as a physical origin of large nonlinearity in epsilon-near-zero materials

Abstract: For decades, nonlinear optics has been used to control the frequency and propagation of light in unique ways enabling a wide range of applications such as ultrafast lasing, sub-wavelength imaging, and novel sensing methods. Through this, a key thread of research in the field has always been the development of new and improved nonlinear materials to empower these applications. Recently, epsilon-near-zero (ENZ) materials have emerged as a potential platform to enhanced nonlinear interactions, bolstered in large part due to the extreme refractive index tuning (Δn∼ 0.1 - 1) of sub-micron thick films that has been demonstrated in literature. Despite this experimental success, the theory has lagged and is needed to guide future experimental efforts. Here, we construct a theoretical framework for the intensity-dependent refractive index of the most popular ENZ materials, heavily doped semiconductors. We demonstrate that the nonlinearity when excited below bandgap, is due to the modification of the effective mass of the electron sea which produces a shift in the plasma frequency. We discuss trends and trade-offs in the optimization of excitation conditions and material choice (such material loss, band structure, and index dispersion), and provide a figure of merit through which the performance of future materials may be evaluated. By illuminating the framework of the nonlinearity, we hope to propel future applications in this growing field.

High sensitivity and rapid response ultraviolet photodetector of a tetragonal CsPbCl 3 perovskite single crystal

Abstract: Inorganic perovskite has attracted great interest due to its excellent optoelectronic properties. There are much less low band gap halide perovskite semiconductors, and CsPbCl3 is one of a wide band gap semiconductor in the perovskite family. In this study, a 0.5-mm CsPbCl3 perovskite single crystal with tetragonal structure and a direct band gap of 2.86 ± 0.3 eV is synthesized by flash evaporation of CsCl-PbCl2 solution. An ultraviolet photodetector based on a CsPbCl3 single crystal is fabricated, showing a photoresponse in a wide wavelength range of 280–435 nm, with a maximum responsivity of 0.272 A/W at 410 nm. Rise and decay response times of the device are less than 28.4 and 2.7 ms, respectively. The good performance of this CsPbCl3 photodetector indicates promising applications in the field of UV optoelectronic devices.

Deterministically fabricated spectrally-tunable quantum dot based single-photon source

Abstract: Spectrally-tunable quantum light sources are key elements for the realization of long-distance quantum communication. A deterministically fabricated single-photon source with a photon extraction efficiency of η =(20 ± 2) %, a maximum tuning range of ΔE = 2.5 meV and a minimum g(2)(τ = 0) = 0.03 ± 0.02 is presented. The device consists of a single pre-selected quantum dot (QD) monolithically integrated into a microlens that is bonded onto a piezoelectric actuator via gold thermocompression bonding. Here, a thin gold layer simultaneously provides strain transfer and acts as a backside mirror for the QD-microlens to maximize the photon extraction efficiency. The QD-microlens structure is patterned via 3D in-situ electron-beam lithography (EBL), which allows us to pre-select and integrate suitable QDs based on their emission intensity and energy with a spectral accuracy of 1 meV for the final device. Together with strain fine-tuning, this enables the scalable realization of single-photon sources with identical emission energy. Moreover, we show that the emission energy of the source can be stabilized to µeV accuracy by closed-loop optical feedback. Thus, the combination of deterministic fabrication, spectral-tunability and high broadband photon-extraction efficiency makes the QD-microlens single-photon source an interesting building block for the realization of quantum communication networks.

Stitching-free 3D printing of millimeter-sized highly transparent spherical and aspherical optical components

Abstract: We demonstrate the fabrication of optical elements on the millimeter scale by stitching-free 3D printing via two-photon polymerization, using a commercial microfabrication system (Nanoscribe GmbH). Previous limitations are overcome by the use of a large writing field objective as well as a novel high transparency resist. The printed optical components are free of stitching defects due to a single step exposure and exhibit an unpreceded glass-like appearance due to the low absorption of the resist material throughout the entire visible wavelength range. We print aspherical focusing lenses, characterize and optimize their shape fidelity, and find their optical performance close to the simulated optimum. For comparison with commercially available glass lenses we also fabricate spherical half-ball lenses of different sizes. The imaging quality of the lenses is very similar, underpinning the powerfulness of our fabrication strategy.

Reduced optical losses in refractory plasmonic titanium nitride thin films deposited with molecular beam epitaxy

Abstract: Refractory plasmonic materials that have optical properties close to those of noble-metals and at the same time are environmentally friendly, commercially viable and CMOS-compatible could lead to novel devices for many thermo-photonic applications. Recently developed TiN thin films overcome some of the limitations of noble-metals, as their optical loss is larger than noble metals and conventional methods to deposit TiN films are not compatible for its integration with other semiconductors. In this work, high-quality epitaxial single-crystalline TiN thin films are deposited with plasma-assisted molecular beam epitaxy (MBE) that exhibit optical losses that are less than that of Au in most part of the visible (300 nm – 580 nm) and near-IR spectral ranges (1000 nm - 2500 nm). In addition, a large figure-of-merit for surface plasmon polariton (SPP) propagation length compared to the previously reported TiN films is achieved with the MBE-deposited films.

Broadband high-efficiency multiple vortex beams generated by an interleaved geometric-phase multifunctional metasurface

Abstract: Vortex beams have witnessed tremendous development in the past decade by exhibiting profound implications for both fundamental physics and a multitude of novel engineering applications. In this work, broadband high-efficiency multiple vortex beams with independent topological modes and inclination angles are generated leveraging an interleaved geometric-phase multifunctional metasurface operating in a very broadband frequency range. A set of meta-atoms are elaborately engineered to offer broadband high-efficiency complete phase control covering the entire 2π range. Multiple geometric-phase sub-arrays implemented by the designed meta-atoms are synthesized into one metasurface via a shared-aperture interleaved manner, in which each sub-array can be individually manipulated and serves as an independent channel for launching a vortex beam. According to the established design methodology, two vortex beams with topological modes of −1 and +2 and distinct inclination angles are generated by one metasurface. Experimental results are provided to corroborate the proposed mechanism for multiple vortex beams generation, which exhibit broadband and high-efficiency features. The presented multifunctional metasurface paves the way for the generation of broadband high-efficiency multiple vortex beams in the microwave, millimeter-wave and terahertz regions. This work is of significance for high-capacity wireless communication applications, high-efficiency manipulation of electromagnetic waves, and novel design of radar and imaging systems.

Excitation of high Q toroidal dipole resonance in an all-dielectric metasurface

Abstract: In metamaterial systems, toroidal dipole (TD) plays an important role in determining their optical properties. Here, we proposed an all-dielectric metasurface consisting of two silicon split-ring resonators (SRRs) that can support strong TD resonance. The TD resonance is excited by TD moments both inside the unit cell and between the neighboring unit cells, and can be easily manipulated by altering the gap size or distance of the SRRs, leading to powerful electric and magnetic near-field enhancement. In addition, symmetric unprotected TD bound state in the continuum (TD-BIC) was achieved in closed-ring-resonator (CRR) metasurface, and transformed into leaky resonances with ultrahigh Q factors by adjusting the distance of CRRs. The proposed structure provides a good platform for us to better understand the coupling of SRRs, which is useful for the design and application of TD metasurfaces in biological sensors, nonlinear interactions and other photonic devices.

Engineering the absorption spectra of thin film multilayer absorbers for enhanced color purity in CMY color filters

Abstract: A thin film of dielectric material on metal provides a simple and cost-effective platform for absorbing light of a specific wavelength that can be desirably tuned by tailoring the thin film thickness. This property of controlled absorption can lead to realizing various exciting applications such as absorbers and color filters. The primary concern, however, in using such multilayer configurations for color filtering is color purity, which is generally low as compared to patterned resonant structures that employ costly nanofabrication techniques. We report a practical design technique to achieve filters of cyan, magenta, and yellow (CMY) with enhanced color purity, polarization-insensitive, and angle-insensitive functionalities. The design involves dielectric thin film layer sandwiched between an ultra-thin metal-layer and ground plane. We demonstrate several multilayer material configurations that provide advantages over the current state-of-the-art color filters in terms of color purity. The proposed devices can find applications in high-resolution color printing, digital imaging, holographic displays, and sensing.

Phase engineering with all-dielectric metasurfaces for focused-optical-vortex (FOV) beams with high cross-polarization efficiency

Abstract: Metasurfaces, the two-dimensional (2D) metamaterials, facilitate the implementation of abrupt phase discontinuities using an array of ultrathin and subwavelength features. These metasurfaces are considered as one of the propitious candidates for realization and development of miniaturized, surface-confined, and flat optical devices. This is because of their unprecedented capabilities to engineer the wavefronts of electromagnetic waves in reflection or transmission mode. The transmission-type metasurfaces are indispensable as the majority of optical devices operate in transmission mode. Along with other innovative applications, previous research has shown that Optical-Vortex (OV) generators based on transmission-type plasmonic metasurfaces overcome the limitations imposed by conventional OV generators. However, significant ohmic losses and the strong dispersion hampered the performance and their integration with state-of-the-art technologies. Therefore, a high contrast all-dielectric metasurface provides a compact and versatile platform to realize the OV generation. The design of this type of metasurfaces relies on the concept of Pancharatnam-Berry (PB) phase aiming to achieve a complete 2π phase control of a spin-inverted transmitted wave. Here, in this paper, we present an ultrathin, highly efficient, all-dielectric metasurface comprising nano-structured silicon on a quartz substrate. With the help of a parameter-sweep optimization, a nanoscale spatial resolution is achieved with a cross-polarized transmission efficiency as high as 95.6% at an operational wavelength of 1.55 µm. Significantly high cross-polarized transmission efficiency has been achieved due to the excitation of electric quadrupole resonances with a very high magnitude. The highly efficient control over the phase has enabled a riveting optical phenomenon. Specifically, the phase profiles of two distinct optical devices, a lens and Spiral-Phase-Plate (SPP), can be merged together, thus producing a highly Focused-Optical-Vortex (FOV) with a maximum focusing efficiency of 75.3%.

3D printed multimode-splitters for photonic interconnects

Abstract: Photonic waveguides are promising candidates for implementing parallel, ultra-fast and ultra-low latency interconnects. Such interconnects are an important technological asset for example for next generation optical routing, on and intra-chip optical communication, and for parallel photonic neural networks. We have recently demonstrated dense optical integration of multi-mode optical interconnects based on 3D additive manufacturing using two-photon-polymerization. The basis of such interconnects are 3D optical splitters, and here we characterize their performance against their splitting ratio, geometry, and conditions of the direct laser writing. Optical losses and splitting uniformity of 1 to 4, 1 to 9 and 1 to 16 splitters are evaluated at 632 nm. We find that, both, the uniformity of splitting ratios as well as the overall losses depend on the separation between the output waveguides as well as on the hatching distance (surface quality) of the 3D printing process.

Tailored nanocomposites for 3D printed micro-optics

Abstract: Optical polymers cover only a rather narrow range of optical properties. This is a limiting factor for the design of polymer-based optical systems such as smartphone cameras. Moreover, it also poses a problem for femtosecond two-photon lithography, which is a state-of-the-art technology to 3D print high-quality optics from photopolymers. To overcome the limitations of conventional polymers, we introduce nano-inks based on the commonly used photopolymers IP-DIP and IP-S as polymer matrix and zirconium dioxide (ZrO2) nanoparticles. We show that the refractive index and dispersion of these nano-inks can be purposefully tailored by varying the constituent materials and the volume fraction of the nanoparticles. Furthermore, we demonstrate the suitability of our nano-inks for optical applications by 3D printing single micro-lenses and a multi-material achromatic Fraunhofer doublet. Our findings confirm that nanocomposites expand the range of optical properties that are accessible for polymer-based systems and allow for the design of tailored optical materials.

Tunable optical materials for multi-resonant plasmonics: from TiN to TiON [Invited]

Abstract: Alternative plasmonic materials are gaining more and more interest since they deliver a plethora of advantages in designing of optical metadevices. Among other alternatives, titanium nitride (TiN) has shown an exceptional combination of encouraging properties, such as CMOS- and bio-compatibility, high carrier concentration, tunability and outstanding robustness (high mechanical, chemical and temperature durability). Optical constants of TiN can be tuned at the synthesis stage. This allows for the adjustment of the spectral position of a plasmon resonance within the visible and near-infrared (NIR) range in order to match the desired working wavelength of a particular device. Together, these factors made TiN a popular material of choice in a diversity of recent plasmonic applications. Titanium oxynitride (TiON), which can be produced through the oxidation of TiN, have a great potential to build upon the success of TiN. Recently, it has been demonstrated that TiON thin films can exhibit a negative double-epsilon-near-zero (2ENZ) dielectric function. This unusual behavior of the permittivity opens up novel opportunities for the excitation of the plasmon resonance at several distinct frequencies within the visible and NIR region. Multi-resonant plasmonic components are beneficial for applications, where the enhanced light-matter interaction at multiple frequencies is demanded, such as nonlinear optics, up- and down-conversion, wavelength multiplexing and broadband absorption. This work begins with a brief survey of the recent progress in plasmonics made with TiN-based structures. Then we focus on TiON thin films with the 2ENZ behavior by discussing their potential in plasmonics. The experimental approaches useful for characterization of TiON thin films and the corresponding results are analyzed. These results are valuable for the development of 2ENZ plasmonic materials with large figure-of-merits in a diversity of applications. We believe that 2ENZ media is a powerful concept for multi-resonant plasmonics that will augment the functionalities and extend the operation bandwidth of plasmonic devices.

Tunable graphene-based terahertz absorber via an external magnetic field

Abstract: A terahertz absorber that can be dynamically tuned via an external magnetic field is proposed. The absorber is composed of periodic gold-disks and an underlying graphene sheet on a dielectric/gold reflector substrate. Simulated results reveal that a new absorption peak appears under the perpendicularly applied external magnetic field. The new absorption peak under a 10 T magnetic field red-shifts from 14.22 THz to 4.47 THz as the Fermi level of graphene increases from 0.1 eV to 0.3 eV. At a fixed Fermi level, the new absorption peak blue shifts as the magnetic field increases. The new absorption peak can be enhanced by using multilayer graphene. The absorber is polarization independent. These results may promote the development and applications of flexibly tunable terahertz absorbers.

Ultraviolet-B persistent luminescence and thermoluminescence of bismuth ion doped garnet phosphors

Abstract: Ultraviolet persistent luminescence technology holds potential for some new applications where ultraviolet emission is needed but constant external excitation is unavailable. Despite the promising applications, not much is known about such luminescence. Here we report ultraviolet-B (290−320 nm) persistent luminescence phenomenon in isostructural Y3Ga5O12:Bi3+ and Y3Al5O12:Bi3+ phosphors. We further investigate the luminescence by measuring thermoluminescence of the two phosphors. Our spectral results indicate that conventional thermoluminescence measurement cannot directly evaluate the electron population in the traps of Y3Ga5O12:Bi3+, in which the ultraviolet emission is suppressed at high temperature due to a thermal ionization quenching. We believe that the insight of the present trap performance is transferable to other ultraviolet persistent phosphors.

Phonon effects in quantum dot single-photon sources

Abstract: Semiconductor quantum dots are inevitably coupled to the vibrational modes of their host lattice. This interaction reduces the efficiency and the indistinguishability of single-photons emitted from semiconductor quantum dots. While the adverse effects of phonons can be significantly reduced by embedding the quantum dot in a photonic cavity, phonon-induced signatures in the emitted photons cannot be completely suppressed and constitute a fundamental limit to the ultimate performance of single-photon sources based on quantum dots. In this paper, we present a self-consistent theoretical description of phonon effects in such sources and describe their influence on the figures of merit.

Layer-by-layer spray coating of a stacked perovskite absorber for perovskite solar cells with better performance and stability under a humid environment

Abstract: Perovskite is an emerging material for high performance solar cell application with low-cost solution-processable fabrication. As an ink, perovskite composition can be easily modified to create semi-transparent solar cells for window replacement. To enable scalable large-scale production, the spray process is one of the major candidates. In this work, we developed sequential spray deposition (SSD) to create double layer absorbers from different dimensional perovskites. SSD, for the first time, achieves layer-by-layer deposition of different perovskite materials for stacked architecture. To demonstrate the benefits, we spray-coated lower dimension, more stable perovskite onto high performance yet sensitive 3D semi-transparent perovskite. SSD performed under a humid environment (40 - 50% RH) brings about better film stability and retains good performance of 3D perovskite. Sequential spray deposition opens new routes for various stacking designs and large-scale production under economical ambient conditions.

Temperature dependent spectroscopic study of Yb3+-doped KG(WO4)2, KY(WO4)2, YAlO3 and YLiF4 for laser applications

Abstract: We present a study on temperature dependent spectroscopic data for Yb:KGW, Yb:KYW and Yb:YLF between 80 K and 280 K and Yb:YAP between 100 K and 300 K. Absorption and emission cross sections are determined. The latter ones are obtained by using a combination of the McCumber relation and the Fuchtbauer-Ladenburg equation. Fluorescence lifetimes are measured within a setup optimized for the suppression of re-absorption and compared to the radiative lifetimes calculated from the previously determined cross sections to cross check the validity of the measurements. The cross sections are evaluated with regard to the materials’ potential for supporting the generation of ultra-short laser pulses, low quantum defect lasing and requirements for suitable diode laser pump sources.

Submicrometer periodic poling of lithium niobate thin films with bipolar preconditioning pulses

Abstract: Periodically poled second-order nonlinear materials with submicrometer periods are important for the development of quasi-phase matched backward-wave nonlinear optical processes. Interactions involving counter-propagating waves exhibit many unique properties and enable devices such as backward second harmonic generators, mirrorless optical parametric oscillators, and narrow-band quantum entangled photon sources. Fabrication of dense ferroelectric domain gratings in lithium niobate remains challenging, however, due to lateral domain spreading and merging. Here, we report submicrometer periodic poling of ion-sliced x-cut magnesium oxide doped lithium niobate thin films. Electric-field poling is performed using multiple bipolar preconditioning pulses that improve the poling yield and domain uniformity. The internal field is found to decrease with each preconditioning poling cycle. The poled domains are characterized by piezoresponse force microscopy. A fundamental period of 747 nm is achieved.

Enabling switchable and multifunctional terahertz metasurfaces with phase-change material

Abstract: Achieving switchable and diversified functionalities in a single metasurface has garnered great research interest for potential terahertz applications. Here, we propose and demonstrate a phase-change metasurface that simultaneously supports broadband electromagnetically induced transparency (EIT) and broadband nearly perfect absorption, depending on the phase state of a phase change material-vanadium dioxide (VO2). The phase-change metasurface is composed of a VO2 nanofilm, a quartz spacer and gold split-square-ring resonators with VO2 nanopads embedded into the splits. When VO2 is in its insulating phase at room temperature, a broadband EIT window (maximum transmittance reaching 83%) with a bandwidth of 0.27 THz (relative bandwidth 30%) can be observed. Alternatively, when VO2 transforms into its fully metallic phase, the EIT functionality will be switched off and instead, the metasurface operates as a broadband absorber with the total absorption exceeding 93% and a bandwidth of 0.5 THz (relative bandwidth 74%). The electric and magnetic field distributions indicate that the broadband EIT stems from the bright-bright mode coupling and the broadband absorption arises from the excitation and superposition of two resonances within a metal-insulator-metal cavity. The design scheme is scalable from terahertz to infrared and optical frequencies, enabling new avenues towards switchable and multifunctional meta-devices.

Mid-infrared 10-µJ-level sub-picosecond pulse generation via stimulated Raman scattering in a gas-filled revolver fiber

Abstract: A sub-picosecond mid-infrared laser based on hollow-core silica fiber is demonstrated for the first time. By using deuterium-filled revolver fiber as an active medium, we realized efficient two-cascade Raman conversion 1.03 → 1.49 → 2.68 µm pumped by chirped pulses of a femtosecond ytterbium laser. The gas fiber Raman laser generates ∼920 fs pulses at 2.68 µm with output pulse energy as high as 10 µJ. It is shown that SRS can dominate other nonlinearities even in highly transient regime implemented in the mid-IR. The approach used may be applied to develop mid-IR laser sources of various types, such as frequency combs, supercontinuum and few-cycle pulse sources.

Polymer dispersed liquid crystals with electrically controlled light scattering in the visible and near-infrared ranges

Abstract: We developed polymer-dispersed liquid crystals (PDLCs) that effectively scatter light both in the visible and near-infrared ranges simultaneously. Such PDLCs are characterized by an optimal size distribution of nematic liquid crystal droplets within 0.4 − 3 µm, which is achieved due to the specially selected copolymer, the elaborated liquid crystal material as well as the proper cooling mode from the isotropic phase. These PDLC films provide electrically controlled light scattering modulation in the spectral range 300 − 2300 nm with the response time around 10 ms.

Biosensing applications of all-dielectric SiO2-PDMS meta-stadium grating nanocombs

Abstract: Thin film grating meta-stadium nanocombs were fabricated and experimentally investigated for the purpose of glucose monitoring. The method of ellipsometry was used to study the sensitivity of the structure to the alterations in glucose concentration in aqueous solution. The existence of Tamm surface waves was demonstrated at the interface of two dielectric mediums (PDMS and SiO2) with acceptable resolution. The results revealed the best sensitivity achieved at a 48° angle of incidence over 350 − 450 nm visible wavelength span when the glucose concentration was varied in the range of 50 mg/l to 100 mg/l. Though the present work emphasizes on the monitoring of glucose, the structure can be used for sensing applications of other biological fluids as well.

Spatially controlled fabrication of single NV centers in IIa HPHT diamond

Abstract: Single NV centers in HPHT IIa diamond are fabricated by helium implantation through lithographic masks. The concentrations of created NV centers in different growth sectors of HPHT are compared quantitatively. It is shown that the purest {001} growth sector (GS) of HPHT diamond allows to create groups of single NV centers in predetermined locations. The {001} GS HPHT diamond is thus considered a good material for applications that involve single NV centers.

Lightweight metasurface mirror of silicon nanospheres [Invited]

Abstract: Many experiments in modern quantum optics require the implementation of lightweight and near-perfect reflectors for noise reduction and high sensitivity. Another important application of low mass and high reflectivity mirrors is related to the development of solar or laser-driven light sails for acceleration of ultra-light spacecrafts to relativistic velocities. Here, we present numerical results and theoretical analysis of a metasurface mirror consisting of periodically arranged silicon nanospheres embedded in a polymer. In the absence of material losses or disorder, this mirror demonstrates absolute 100% reflection at a single wavelength, which can be tuned by changing nanosphere dimensions or periodicity (for example, by mechanical stretching). We show that high reflectivity can be reached due to electric or magnetic dipole resonant responses of Si nanoparticles in the metasurface. Dependence of mirror reflectivity on surrounding conditions, nanoparticle sizes, and the disorder in the array is studied and discussed. The optimization and simulation procedures presented in this work can be used for the development of other optical devices with functional characteristics determined by the resonant interaction of light with metasurfaces made of nanospheres.

Third-order nonlinear optical properties of Ge-As-Te chalcogenide glasses in mid-infrared

Abstract: Third-order nonlinear optical properties of Ge10AsxTe90-x chalcogenide glasses were investigated utilizing the Z-scan method at the mid-infrared wavelengths of 2.5 and 3.0 µm. The compositional dependence of the third-order nonlinearity was analyzed, and their correlation with the refractive index and the optical bandgap was discussed. The results show that nonlinear refractive index n2 can be significantly enhanced by the addition of tellurium, and larger n2 values are observed at 3.0 µm rather than 2.5 µm due to the two-photon resonance effect, and the maximum of n2 is 4.96 × 10−13 cm2/W at the composition of Ge10As20Te70. In addition, the experimental results are in good agreement with the semi-empirical Miller’s rule whilst the variation of dispersive n2 values are in relatively good coincidence with the theoretical model by Sheik-Bahae et.al. for direct bandgap semiconductors.

Flexible random laser from dye doped stretchable polymer film containing nematic liquid crystal

Abstract: Random lasers (RLs) offer new functionalities inaccessible with conventional lasers, such as an alterable shape and an easy integration with flexible optoelectronic devices. Here, we demonstrate a stretchable and threshold tunable RL by modulating the order degree of the nematic liquid crystal (NLC) that is caused by the alignment of polymer chain under tensile force. The lasing thresholds show a “U” shape curve variation trend, which is attributed to the competition between the partial orientation of the NLC molecules and the reduction of the dye and NLC densities. The results are further confirmed by the power Fourier transform (PFT) spectrum analysis. This work evokes deeper understanding of the effect of order degree on RLs and extends the applications of polymer polyvinylidene fluoride (PVDF) on tunable RLs.

Tunable photonic devices by 3D laser printing of liquid crystal elastomers

Abstract: Liquid crystal elastomers (LCEs) are highly suitable materials for the fabrication of flexible photonic elements due to their ability for directional actuation induced by external stimuli. 3D laser printing (3DLP) is a well-established method to realize complex photonic architectures. In this paper, we present the technological adaptations necessary to combine the actuation-controlled flexibility of LCE with the design options inherent to 3DLP to realize a platform for tunable photonics. The role of birefringence of the LCE in the 3DLP fabrication is addressed and theoretically modelled. We demonstrate how LCEs can be used both as a flexible substrate for arrays of rigid photonic elements and as a material for tunable photonic structures itself. Flexible coupling of two optical whispering gallery mode cavities and full spectral tunability of a single cavity are presented as exemplary applications.

Design of bi-tunable triple-band metamaterial absorber based on Dirac semimetal and vanadium dioxide

Abstract: A bi-tunable triple-band metamaterial absorber based on Dirac semimetal films (DSFs) and vanadium dioxide (VO2) is presented. When VO2 is in the fully metallic state, the proposed absorber presents three distinctive absorption peaks in the terahertz range with absorptance 97%. Because the conductivity of VO2 changes from 100000 to 10 S/m, the reflectance and absorptance intensities achieve dynamic tunability at the three absorption peaks, and the proposed triple-band absorber exhibits a switchable function by the insulation-to-metal transition of VO2. Moreover, the frequencies of the three absorption peaks can also be tuned by varying the Fermi energies of the DSFs.