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1. Unique prospects of phase change material Sb ₂ Se ₃ for ultra-compact reconfigurable nanophotonic devices

   https://doi.org/10.1364/OME.435979  

   Jia, Wei ; Menon, Rajesh ; Sensale-Rodriguez, Berardi ( August 2021 , Optical Materials Express)

   In this work, we explore inverse designed reconfigurable digital metamaterial structures based on phase change material Sb₂Se₃for efficient and compact integrated nanophotonics. An exemplary design of a 1 × 2 optical switch consisting of a 3 µm x 3 µm pixelated domain is demonstrated. We show that: (i) direct optimization of a domain containing only Si and Sb₂Se₃pixels does not lead to a high extinction ratio between output ports in the amorphous state, which is owed to the small index contrast between Si and Sb₂Se₃in such a state. As a result, (ii) topology optimization, e.g., the addition of air pixels, is required to provide an initial asymmetry that aids the amorphous state's response. Furthermore, (iii) the combination of low loss and high refractive index change in Sb₂Se₃, which is unique among all phase change materials in the telecommunications 1550 nm band, translates into an excellent projected performance; the optimized device structure exhibits a low insertion loss (∼1.5 dB) and high extinction ratio (>18 dB) for both phase states.
2. Suppressed electronic contribution in thermal conductivity of Ge2Sb2Se4Te

   https://doi.org/10.1038/s41467-021-27121-x  

   Aryana, Kiumars ; Zhang, Yifei ; Tomko, John A. ; Hoque, Md Shafkat Bin ; Hoglund, Eric R. ; Olson, David H. ; Nag, Joyeeta ; Read, John C. ; Ríos, Carlos ; Hu, Juejun ; et al ( December 2021 , Nature Communications)

   Integrated nanophotonics is an emerging research direction that has attracted great interests for technologies ranging from classical to quantum computing. One of the key-components in the development of nanophotonic circuits is the phase-change unit that undergoes a solid-state phase transformation upon thermal excitation. The quaternary alloy, Ge₂Sb₂Se₄Te, is one of the most promising material candidates for application in photonic circuits due to its broadband transparency and large optical contrast in the infrared spectrum. Here, we investigate the thermal properties of Ge₂Sb₂Se₄Te and show that upon substituting tellurium with selenium, the thermal transport transitions from an electron dominated to a phonon dominated regime. By implementing an ultrafast mid-infrared pump-probe spectroscopy technique that allows for direct monitoring of electronic and vibrational energy carrier lifetimes in these materials, we find that this reduction in thermal conductivity is a result of a drastic change in electronic lifetimes of Ge₂Sb₂Se₄Te, leading to a transition from an electron-dominated to a phonon-dominated thermal transport mechanism upon selenium substitution. In addition to thermal conductivity measurements, we provide an extensive study on the thermophysical properties of Ge₂Sb₂Se₄Te thin films such as thermal boundary conductance, specific heat, and sound speed from room temperature to 400 °C across varying thicknesses.
3. Orientation dependence of electronic properties of antimony selenide nanowires

   https://doi.org/10.1088/2632-959X/ac929d  

   Vadapoo, Rajasekarakumar ; Krishnan, Sridevi ; Yilmaz, Hulusi ; Marin, Carlos ( September 2022 , Nano Express)

   We present a comprehensive DFT study of size-dependent atomic and electronic properties of antimony selenide (Sb₂Se₃) nanowires in three main crystallographic directions. Our calculations show a significant enhancement in the band gap of wires oriented in [100] and [010] directions due to confinement effects, however the band gap of [001] oriented wires is reduced with respect to bulk. We attribute this anomaly in band gap reduction to the surface reconstructions in these nanostructures. These surface reconstructions are similar to the polyhedral distortions observed in bulkSb₂Se₃under high pressure leading to the insulator-metal transition related to the topological insulating states and then at lower temperature (8K) to superconductivity.
4. Visible and near-infrared programmable multi-level diffractive lenses with phase change material Sb ₂ S ₃

   https://doi.org/10.1364/OE.452472  

   Jia, Wei ; Menon, Rajesh ; Sensale-Rodriguez, Berardi ( January 2022 , Optics Express)

   In this paper, we discuss flat programmable multi-level diffractive lenses (PMDL) enabled by phase change materials working in the near-infrared and visible ranges. The high real part refractive index contrast (Δn ∼ 0.6) of Sb 2 S 3 between amorphous and crystalline states, and extremely low losses in the near-infrared, enable the PMDL to effectively shift the lens focus when the phase of the material is altered between its crystalline and amorphous states. In the visible band, although losses can become significant as the wavelength is reduced, the lenses can still provide good performance as a result of their relatively small thickness (∼ 1.5λ to 3λ). The PMDL consists of Sb 2 S 3 concentric rings with equal width and varying heights embedded in a glass substrate. The height of each concentric ring was optimized by a modified direct binary search algorithm. The proposed designs show the possibility of realizing programmable lenses at design wavelengths from the near-infrared (850 nm) up to the blue (450 nm) through engineering PMDLs with Sb 2 S 3 . Operation at these short wavelengths, to the best of our knowledge, has not been studied so far in reconfigurable lenses with phase-change materials. Therefore, ourmore »results open a wider range of applications for phase-change materials, and show the prospect of Sb 2 S 3 for such applications. The proposed lenses are polarization insensitive and can have the potential to be applied in dual-functionality devices, optical imaging, and biomedical science.« less
5. Millimetre-long transport of photogenerated carriers in topological insulators

   https://doi.org/10.1038/s41467-019-13711-3  

   Hou, Yasen ; Wang, Rui ; Xiao, Rui ; McClintock, Luke ; Clark Travaglini, Henry ; Paulus Francia, John ; Fetsch, Harry ; Erten, Onur ; Savrasov, Sergey Y. ; Wang, Baigeng ; et al ( December 2019 , Nature Communications)

   Excitons are spin integer particles that are predicted to condense into a coherent quantum state at sufficiently low temperature. Here by using photocurrent imaging we report experimental evidence of formation and efficient transport of non-equilibrium excitons in Bi_2-xSb_xSe₃nanoribbons. The photocurrent distributions are independent of electric field, indicating that photoexcited electrons and holes form excitons. Remarkably, these excitons can transport over hundreds of micrometers along the topological insulator (TI) nanoribbons before recombination at up to 40 K. The macroscopic transport distance, combined with short carrier lifetime obtained from transient photocurrent measurements, indicates an exciton diffusion coefficient at least 36 m² s⁻¹, which corresponds to a mobility of 6 × 10⁴ m² V⁻¹ s⁻¹at 7 K and is four order of magnitude higher than the value reported for free carriers in TIs. The observation of highly dissipationless exciton transport implies the formation of superfluid-like exciton condensate at the surface of TIs.