Optical properties of chalcogenide topological insulators (TIs), namely, Bi2Se3 (BS) and Bi2Te3 (BT) were studied across the NIR to MIR spectral ranges. In this spectral range, the experimentally measured optical constants revealed an extremely high permittivity values amounting to refractive indices as high as n≈11 and n≈6.4, for BT and BS respectively. These ultra-high index values were then utilized for demonstrating ultracompact, deep-subwavelength nanostructures (NSs), with unit cell sizes down to ~λ/10. Finally, using scattering-type Scanning Near-field Optical Microscopy (s-SNOM), local variations in the optical constants of these nanostructured TIs were studied. Nanoscale phase mapping on a BS NS revealed the role of the imaginary component of the refractive index in the observed phase shifts, varying from as low as ~0.37π to a maximum of ~2π radians across a resonance. This work thus highlights the potential of TIs as a low-loss, high index material for ultracompact nanophotonics.
In nanophotonic, small mode volumes, narrow resonance linewidths and field enhancements, fundamentally
scales with refractive index values and are key for many implementations involving light-matter interactions.
Topological insulators (TI) are a class of insulating materials that host topologically protected surface states, some of which exhibit very high permittivity values. In this talk, I will present my group’s latest results on chalcogenide metaphotonics. I start by discussing Chalcogenide Bi2Te3 and Bi2Se3 TIs nanostructures. Using polarized far-field and near field Nanospectroscopy we reveal that Bi2Se3 nanobeams exhibit mid-infrared resonant modes with 2π phase shifts across the resonance. We further demonstrate that Bi2Te3 metasurfaces exhibit deep-subwavelength resonant modes utilizing their record high index value peaking at n~11. Finally we discuss how the anomalous thermo-optic effect in lead chalcogenide can be harnessed for implementing temperature invariant metasurfaces
In nanophotonics, temperature variations shift the optical response through the thermo-optic (TO) effect (dn/dT) and may be detrimental in certain applications. We present a comprehensive solution to overcome the problem of optical constants thermal dispersion in nanophotonic and meta-optic devices. By using hybrid meta-atoms composed from two materials with positive and negative TO coefficients, we engineer metasurfaces with zero effective thermal dispersion (dneff/dT≈0). We demonstrate temperature independent resonant frequency, amplitude and phase response in metasurfaces, operating across a broad temperature range (ΔT=500K). Controlling the sign and magnitude of TO dispersion extends the capabilities of light manipulation in nanophotonic systems.
KEYWORDS: Nanostructures, Refractive index, Near field optics, Spectroscopy, Resonance enhancement, Phase shifts, Oscillators, Optical properties, Niobium, Near field
In nanophotonic, small mode volumes, narrow resonance linewidths and field enhancements, fundamentally scales with refractive index values and are key for many implementations involving light-matter interactions. Topological insulators (TI) are a class of insulating materials that host topologically protected surface states, some of which exhibit very high permittivity values. In this talk, I will discuss our latest results on Bi2Te3 and Bi2Se3 TI nanostructures. Using polarized far-field and near field nanospectroscopy we reveal that Bi2Se3 nanobeams exhibit mid-infrared resonant modes with 2π phase shifts across the resonance. We further demonstrate that Bi2Te3 metasurfaces exhibit deep subwavelength resonant modes utilizing their record high index value peaking at n~11.
In nanophotonics, small mode volume and high-quality factor (Q-factor) resonances fundamentally scales with high refractive index values. Topological insulators (TI) are a new class of materials possessing narrow bulk bandgap and gapless Dirac surface states, and exhibit ultra high permittivity values. In this work, I will discuss our latest results on Bi2Te3 and Bi2Se3 TI meta-optics. Using polarized far-field and near field Nanospectroscopy we reveal that Bi2Se3 nanobeams exhibit mid-infrared resonant modes with 2pi phase shifts across the resonance. We further demonstrate that Bi2Te3 metasurfaces exhibit deep subwavelength resonant modes utilizing their record high index value peaking at n~11.
Topological insulators (TIs) are a new class of condensed matter system that host topologically protected surface states, leading to dissipationless electron transport. This intrinsic characteristic makes them potential candidate for quantum computing owing to their ability to preserve quantum coherence. Recently, these systems and the concept of topology have been embraced by the photonics community as well. In this work, we study the mid-infrared optical properties of high index (n~5.2) TI bismuth selenide (Bi2Se3) nanobeams (NBs), grown by chemical vapor deposition. Using Finite-difference time-domain (FDTD) simulations and FTIR nanospectroscopy, we find that these NBs support size-tunable Mie-resonant modes across the infrared (~1-16 µm). Furthermore, polarized measurements reveal that the total optical response of these deep subwavelength NBs is composed of TE and TM resonant mode. Finally, near-filed studies are also carried out to understand the effect of topological phase.
We present a study of various compositions of the chalcogenide family used for static and active metasurfaces. We start with large area CVD grown amorphous spherical Selenium nanoparticles on various substrates and show that their Mie-resonant response spans the entire mid-infrared (MIR) range. By coupling Se Mie-resonators to ENZ substrates we demonstrate an order of magnitude increase in quality factor. Next, we investigate topological insulators Bi2Se3 and Bi2Te3 metasurfaces. We study the optical constants of single crystal Bi2Te3 in the NIR to the MIR range, followed by fabrication and characterization of metasurface disk arrays. We show that these high permittivity metasurfaces can yield very large absorption resonances using deep subwavelength structures. Finally, we demonstrate ultra-wide dynamic tuning of PbTe meta-atoms and metasurfaces, utilizing the anomalously large thermo-optic coefficient and high refractive index of this material.
Efficient light manipulation at subwavelength scales in the mid-infrared (MIR) region is essential for various applications and can be harnessed from intrinsic low-loss dielectric resonators. Here, we demonstrate the fabrication of truncated spherical selenium (Se) resonators with tunable high-quality (Q) factor Mie resonances. Large area amorphous Se subwavelength resonators of varying sizes were grown on different substrates, using a novel CVD process. We demonstrate size-tunable Mie resonances spanning the 2-16 µm range, for single isolated resonators and large area ensembles, respectively. We show strong tunable absorption resonances (90%) in ensembles of resonators in a significantly broad MIR range. Moreover, by coupling resonators to epsilon-near-zero (ENZ) substrates, we engineer high-Q resonances as high as Q=40. We also show the resonance pinning effect near the substrate ENZ value, which is manifested in size-independent resonances.
Chalcogenide based materials are excellent candidates for implementing static and dynamic meta-optics as they possess very high permittivities and support large modulation of optical constants through various mechanisms such as, phase-change, photon-darkening, laser writing and anomalous thermo-optic effects. We present a study of various chalcogenide compositions used for static and active metasurfaces. We start with large area CVD grown amorphous Selenium nanoparticles on various substrates and show that their Mie-resonant response spans the entire mid-infrared range. By coupling Se Mie-resonators to ENZ substrates we demonstrate an order of magnitude increase in quality factor. Next, we investigate topological insulators Bi2Te3 metasurfaces and demonstrate that these high permittivity metasurfaces can yield very large absorption resonances that are tunable in the infrared range. Finally, we demonstrate ultra-wide dynamic tuning of PbTe metasurface resonators.
We present our latest progress in the study of different tunable and active mechanisms in various materials that exhibit large modulation of optical constants and are used to implement active resonators and metasurfaces. We first discuss tuning of infrared Mie-resonant Si and Ge metasurfaces by modulating their free carrier density. We then move to discuss thermo-optic (TO) effects in Si, Ge and InSb and demonstrate tuning of Mie resonances by more than the resonance linewidth. Exploiting the peak TO coefficient of Si near its bandgap, we realize reconfigurable metasurfaces and tunable metafilters. We also show that phase transition materials such as VO2 can be used to implement active devices. We demonstrate electrically tunable Ge on VO2 resonators acting both as amplitude and phase modulators. Finally, we demonstrate ultra-wide dynamic tuning of PbTe meta-atoms. Taking advantage of the anomalously large TO coefficient and high refractive index of PbTe, we demonstrate high-quality factor resonances that are tuned by several linewidths with temperature modulation as small as ΔT~10K. We reveal that the origin for this exceptional tunability is due to an increased TO coefficient of PbTe at low temperatures. When combined into metasurface arrays these effects can be exploited in ultra-narrow active notch filers and metasurface phase shifters that require only few-kelvin modulation. We also study photoluminescence properties of lead chalcogenides from single antenna resonators and metasurface arrays towards the implementation of infrared emitting metasurfaces.
Metasurfaces manipulate light through engineering the amplitude, phase and polarization across arrays of meta-atom antenna resonators. Adding tunability and active functionality to metasurface components would boost their potential and unlock a vast array of new application possibilities such as dynamic beam steering, LIDAR, tunable metalenses, reconfigurable meta-holograms and many more. We present here high-index reconfigurable meta-atoms, resonators and metasurfaces that can dynamically and continuously tune their frequency, amplitude and phase, across the infrared spectral ranges. We utilize narrow linewidth resonances along with peak performance of tunable mechanisms for efficient and practical reconfigurable devices.
Metasurfaces allow unprecedented control of light through engineering the amplitude, phase and polarization across arrays of meta-atom resonators. Adding dynamic tunability to metasurface components would boost their potential and unlock a vast array of new application possibilities such as dynamic beam steering, LIDAR, tunable metalenses and reconfigurable meta-holograms, to name a few. We present here high-index reconfigurable metaatoms, resonators and metasurfaces that can dynamically and continuously tune their frequency, amplitude and phase, across the near to mid-infrared spectral ranges. We highlight the importance of narrow linewidth resonances along with peak performance of tunable mechanisms for efficient and practical reconfigurable devices.
Optical antenna metasurfaces have attracted substantial attention in recent years, as they may enable new classes of planar optical elements. However, actively tuning nanoantenna resonances, whether dielectric or plasmonic, remains an unresolved challenge. In this work, we investigate tuning mid-infrared (MIR) Mie resonances in semiconductor subwavelength particles by directly modulating the permittivity with free charge carriers. Using femtosecond laser ablation, we fabricate spherical silicon and germanium particles of varying sizes and doping concentrations. Single-particle infrared spectra reveal electric and magnetic dipole, quadrupole, and hexapole resonances. We first demonstrate size-dependent Si and Ge Mie resonances spanning the entire mid-infrared (2-16 μm) spectral range. We subsequently show doping-dependent resonance frequency shifts that follow simple Drude models. Taking advantage of the large doping dependence of Si and Ge MIR permittivities, we demonstrate a huge tunability of Mie resonance wavelengths (up to ~ 9 μm) over a broad 2-16 μm MIR range. This tuning range corresponds to changes of permittivity as large as 30 within a single material system, culminating in the emergence of plasmonic modes at high carrier densities and long wavelengths. We also demonstrate dynamic tuning of intrinsic semiconductor antennas using thermo-optic effects. These findings demonstrate the potential for actively tuning infrared Mie resonances, thus providing an excellent platform for tunable metamaterials.
Dielectric optical antenna resonators have recently emerged as a viable alternative to plasmonic resonators for metamaterials and nanophotonic devices, due to their ability to support multipolar Mie resonances with low losses. In this work, we experimentally investigate the mid-infrared Mie resonances in Si and Ge subwavelength spherical particles. In particular, we leverage the electronic and optical properties of these semiconductors in the mid-infrared range to design and tune Mie resonators through free-carrier refraction.
Si and Ge semiconductor spheres of varying sizes of 0.5-4 μm were fabricated using femtosecond laser ablation. Using single particle infrared spectroscopy, we first demonstrate size-dependent Si and Ge Mie resonances spanning the entire mid-infrared (2-16 μm) spectral range. Subsequently we show that the Mie resonances can be tuned by varying material properties rather than size or geometry. We experimentally demonstrate doping-dependent resonance frequency shifts that follow simple Drude models of free-carrier refraction. We show that Ge particles exhibit a stronger doping dependence than Si due to the smaller effective mass of the free carriers. Using the unique size and doping dispersion of the electric and magnetic dipole modes, we identify and demonstrate a size regime where these modes are spectrally overlapping. We also demonstrate the emergence of plasmonic resonances for high doping levels and long wavelengths. These findings demonstrate the potential for tuning infrared semiconductor Mie resonances by optically or electrically modulating charge carrier densities, thus providing an excellent platform for tunable electromagnetic metamaterials.
We report the demonstration of single mode AgClxBr1-x channel waveguides for mid-infrared range. The waveguides were made by the deposition of AgClxBr1-x layers on top of a Ag/Ti/SiO2/Si substrate, followed by photolithographic and lift-off processing. We showed that these waveguides operate in a single mode for the 6-14 μm band. The propagation losses of 20 dB/cm were measured at λ=10.6 μm using the cut-back method. We discuss the possible propagation losses mechanisms and show that the waveguide sidewall roughness is likely the major contributor for these losses. Using this fabrication process we have also realized Y-couplers and splitters. The development of these waveguides is a crucial step towards realizing on-chip AgClxBr1-x mid-infrared integrated optical circuits which will be used for applications such as chemical sensing and spectro-interferometry for planet detection.
In the astrophysical context of the search for Earth-like extrasolar planets, an important research effort has been done for
the realization of single-mode integrated optics devices for mid-infrared space-based interferometry. Preparatory projects
like FKSI [3], where rejection of high order modes is required to a level better than 40dB, will need photonic devices
that achieve modal filtering and beam combination in the mid-IR band. In this context, we present results on midinfrared
planar integrated optic beam combiners characterized at LAOG using chalcogenide and silver halide materials.
We show results on FTS measurements, allowing to determine the single mode spectral domain, as well as interference
fringes obtained from Y-junctions realized on these materials.
Modal filters are necessary to the proposed high-performance mid-infrared nulling interferometers, because they can
help achieve deeper interferometric nulls. Silver halide fibers of composition AgClxBr1-x(0
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