arXiv subjects

Hongyan Mei

Publications and source records attributed to Hongyan Mei.

18 recordsLinked to original sources

arXiv2026

Material selection for mid-infrared thin-film coatings and windows

We summarized the room-temperature optical properties for infrared-transparent materials, defining transparency windows for two different applications: thin-film coatings (absorption coefficient $\alpha < 10 cm^{-1}$) and windows ($\alpha < 1 cm^{-1}$). The transparency requirements for thin films are substantially less stringent, enabling the use of many more optical materials for a given wavelength range. To make an easy-to-use reference, we categorized materials by chemical group, crystallinity, and typical deposition technique, and discussed practical pros and cons such as chemical and thermal stability, mechanical hardness, and cost. The datasets and plotting scripts are provided so that users can regenerate transparency-window maps for different applications. If you do optical design at infrared wavelengths, we recommend that you print out the figures and stick them on your wall.

physics.optics↗

arXiv2026

Silicon-on-sapphire metasurfaces generate arrays of dark and bright traps for neutral atoms

We demonstrated crystalline silicon-on-sapphire (c-SOS) metasurfaces that convert a Gaussian beam into arrays of complex optical traps, including arrays of optical bottle beams that trap atoms in dark regions interleaved with bright tweezer arrays. The high refractive index and indirect band gap of crystalline silicon makes it possible to design high-resolution near-infrared ($\lambda>700$ nm) metasurfaces that can be manufactured at scale using CMOS-compatible processes. Compared with active components like spatial light modulators (SLMs) that have become widely used to generate trap arrays, metasurfaces provide an indefinitely scalable number of pixels, enabling large arrays of complex traps in a very small form factor, as well as reduced dynamic noise. To design metasurfaces that can generate three-dimensional bottle beams to serve as dark traps, we modified the Gerchberg-Saxton algorithm to enforce complex-amplitude profiles at the focal plane of the metasurface and to optimize the uniformity of the traps across the array. We fabricated and measured c-SOS metasurfaces that convert a Gaussian laser beam into arrays of bright traps, dark traps, and interleaved bright/dark traps.

physics.optics↗

arXiv2026

Anti-reflection coatings for highly anisotropic materials in the mid infrared

We develop and optimize thin anti-reflection coatings (ARCs) for highly anisotropic materials in the mid infrared. Unlike conventional ARCs that assume nearly isotropic refractive indices, this work fully integrates the anisotropic nature of materials into the design process. We describe two designs of thin ARCs for highly anisotropic materials: a single form-birefringent layer, and a planar bilayer. We realized the planar bilayer ARC experimentally, demonstrating excellent mid-infrared anti-reflectance across over a broad range of angles for all polarizations.

physics.optics↗

arXiv2025

Tuning the surface energy of fluorinated diamond-like carbon coatings via plasma immersion ion implantation plasma-enhanced chemical vapor deposition with 1,1,1,2-tetrafluoroethane

We demonstrate an environmentally friendly and scalable method to create fluorine-doped diamond-like carbon (F-DLC) coatings using plasma immersion ion implantation plasma-enhanced chemical vapor deposition (PIII-PECVD) with 1,1,1,2-tetrafluoroethane. F-DLC films tend to have low wettability and good mechanical flexibility, which make them suitable for applications in biomedical devices and antibiofouling surfaces. We report on the effects of fluorine incorporation on the surface chemistry, surface energy, and morphology of these coatings, showing that our method is effective in increasing the fluorine content in the F-DLC up to 40%. We show that the addition of fluorine leads to a decrease in surface energy, which is consistent with a reduction in surface wettability.

cond-mat.mtrl-sci↗

arXiv2024

Enhanced biochemical sensing with high-Q transmission resonances in free-standing membrane metasurfaces

Optical metasurfaces provide novel solutions to label-free biochemical sensing by localizing light resonantly beyond the diffraction limit, thereby selectively enhancing light-matter interactions for improved analytical performance. However, high-Q resonances in metasurfaces are usually achieved in the reflection mode, which impedes metasurface integration into compact imaging systems. Here, we demonstrate a novel metasurface platform for advanced biochemical sensing based on the physics of the bound states in the continuum (BIC) and electromagnetically induced transparency (EIT) modes, which arise when two interfering resonances from a periodic pattern of tilted elliptic holes overlap both spectrally and spatially, creating a narrow transparency window in the mid-infrared spectrum. We experimentally measure these resonant peaks observed in transmission mode (Q~734 at ~8.8 um) in free-standing silicon membranes and confirm their tunability through geometric scaling. We also demonstrate the strong coupling of the BIC-EIT modes with a thinly coated PMMA film on the metasurface, characterized by a large Rabi splitting (32 cm-1) and biosensing of protein monolayers in transmission mode. Our new photonic platform can facilitate the integration of metasurface biochemical sensors into compact and monolithic optical systems while being compatible with scalable manufacturing, thereby clearing the way for on-site biochemical sensing in everyday applications.

physics.optics↗

arXiv2024

Preventing overfitting in infrared ellipsometry using temperature dependence: fused silica as a case study

Fitting oscillator models to variable-angle spectroscopic ellipsometry (VASE) data can lead to non-unique, unphysical results. We demonstrate using temperature-dependent trends to prevent overfitting and ensure model physicality. As a case study, we performed mid-infrared VASE measurements on fused silica (SiO2) of various grades, from room temperature to 600 {\deg}C. We fitted oscillator models independently at each temperature, and confirmed the model's physical validity by observing the expected monotonic trends in vibrational oscillator parameters. Using this technique, we generated a highly accurate dataset for the temperature-dependent complex refractive index of fused silica for modeling mid-infrared optical components such as thermal emitters.

physics.optics↗

arXiv2024

Self-referencing photothermal common-path interferometry to measure absorption of Si3N4 membranes for laser-light sails

Laser-light sails are a spacecraft concept wherein lightweight "sails" are propelled by high-intensity lasers. We investigated the near-infrared absorption of free-standing membranes of stoichiometric silicon nitride (Si$_3$N$_4$), a candidate sail material. To resolve the small but non-zero optical loss, we used photothermal common-path interferometry (PCI), for which we developed a self-referencing modality where a PCI measurement is performed twice: once on a bare membrane, and a second time with monolayer graphene deposited on the membrane. The graphene increases the absorption of the sample by orders of magnitude, such that it can be measured by ellipsometry, without significantly affecting the thermal properties. We measured the absorption coefficient of Si$_3$N$_4$ to be (1.5-3) $\times$ 10$^{-2}$ cm$^{-1}$ at 1064 nm, making it a suitable sail material for laser intensities as high as ~10 GW/m$^2$. By comparison, silicon-rich "low stress" SiN$_x$ (x~1), with a measured absorption coefficient of approximately 8 cm$^{-1}$, is unlikely to survive such high laser intensities. Our self-referencing technique enables testing of low-loss membranes of various materials for laser sails and other applications.

physics.optics↗

arXiv2024

Atomic magnetometry using a metasurface polarizing beamsplitter in silicon on sapphire

We demonstrate atomic magnetometry using a metasurface polarizing beamsplitter fabricated on a silicon-on-sapphire (SOS) platform. The metasurface splits a beam that is near-resonant with the rubidium atoms (795 nm) into orthogonal linear polarizations, enabling measurement of magnetically sensitive circular birefringence in a rubidium vapor through balanced polarimetry. We incorporated the metasurface into an atomic magnetometer based on nonlinear magneto-optical rotation and measured sub-nanotesla sensitivity, which is limited by low-frequency technical noise and transmission loss through the metasurface. To our knowledge, this work represents the first demonstration of SOS nanophotonics for atom-based sensing and paves the way for highly integrated, miniaturized atomic sensors with enhanced sensitivity and portability.

physics.optics↗

arXiv2024

Molten flux growth of single crystals of quasi-1D hexagonal chalcogenide BaTiS3

BaTiS3, a quasi-1D complex chalcogenide, has gathered considerable scientific and technological interest due to its giant optical anisotropy and electronic phase transitions. However, the synthesis of high-quality BaTiS3 crystals, particularly those featuring crystal sizes of millimeters or larger, remains a challenge. Here, we investigate the growth of BaTiS3 crystals utilizing a molten salt flux of either potassium iodide, or a mixture of barium chloride and barium iodide. The crystals obtained through this method exhibit a substantial increase in volume compared to those synthesized via the chemical vapor transport method, while preserving their intrinsic optical and electronic properties. Our flux growth method provides a promising route towards the production of high-quality, large-scale single crystals of BaTiS3, which will greatly facilitate advanced characterizations of BaTiS3 and its practical applications that require large crystal dimensions. Additionally, our approach offers an alternative synthetic route for other emerging complex chalcogenides.

cond-mat.mtrl-sci↗

arXiv2024

Trapping light in air with membrane metasurfaces for vibrational strong coupling

Optical metasurfaces can manipulate electromagnetic waves in unprecedented ways at ultra-thin engineered interfaces. Specifically, in the mid-infrared (mid-IR) region, metasurfaces have enabled numerous biochemical sensing, spectroscopy, and vibrational strong coupling (VSC) applications via enhanced light-matter interactions in resonant cavities. However, mid-IR metasurfaces are usually fabricated on solid supporting substrates, which degrade resonance quality factors (Q) and hinder efficient sample access to the near-field electromagnetic hotspots. Besides, typical IR-transparent substrate materials with low refractive indices, such as CaF2, NaCl, KBr, and ZnSe, are usually either water-soluble, expensive, or not compatible with low-cost mass manufacturing processes. Here, we present novel free-standing Si-membrane mid-IR metasurfaces with strong light-trapping capabilities in accessible air voids. We employ the Brillouin zone folding technique to excite tunable, high-Q quasi-bound states in the continuum (q-BIC) resonances with our highest measured Q-factor of 722. Leveraging the strong field localizations in accessible air cavities, we demonstrate VSC with multiple quantities of PMMA molecules and the q-BIC modes at various detuning frequencies. Our new approach of fabricating mid-IR metasurfaces into semiconductor membranes enables scalable manufacturing of mid-IR photonic devices and provides exciting opportunities for quantum-coherent light-matter interactions, biochemical sensing, and polaritonic chemistry.

physics.optics↗

arXiv2024

Infrared Optical Anisotropy in Quasi-1D Hexagonal Chalcogenide BaTiSe3

Polarimetric infrared detection bolsters IR thermography by leveraging the polarization of light. Optical anisotropy, i.e., birefringence and dichroism, can be leveraged to achieve polarimetric detection. Recently, giant optical anisotropy was discovered in quasi-1D narrow-bandgap hexagonal perovskite sulfides, A1+xTiS3, specifically BaTiS3[1,2] and Sr9/8TiS3[3,4]. In these materials, the critical role of atomic-scale structure modulations[4,5] in the unconventional electrical[5,6], optical[7,8], and thermal[7,9] properties raises the broader question of other materials that belong to this family. To address this issue, for the first time, we synthesized high-quality single crystals of a largely unexplored member of the A1+xTiX3 (X = S, Se) family, BaTiSe3. Single-crystal X-ray diffraction determined the room-temperature structure with the P31c space group, which is a superstructure of the earlier reported[10] P63/mmc structure. The crystal structure of BaTiSe3 features antiparallel c-axis displacements similar to BaTiS3,[2] but is of lower symmetry. Polarization-resolved Raman and Fourier transform infrared (FTIR) spectroscopy were used to characterize the optical anisotropy of BaTiSe3, whose refractive index along the ordinary (perpendicular to c) and extraordinary (parallel to c) optical axes was quantitatively determined by combining ellipsometry studies with FTIR. With a giant birefringence {\Delta}n~0.9, BaTiSe3 emerges as a new candidate for miniaturized birefringent optics for mid-wave infrared to long-wave infrared imaging.

cond-mat.mtrl-sci↗

arXiv2023

Giant Modulation of Refractive Index from Picoscale Atomic Displacements

Structural disorder has been shown to enhance and modulate magnetic, electrical, dipolar, electrochemical, and mechanical properties of materials. However, the possibility of obtaining novel optical and optoelectronic properties from structural disorder remains an open question. Here, we show unambiguous evidence of disorder in the form of anisotropic, picoscale atomic displacements modulating the refractive index tensor and resulting in the giant optical anisotropy observed in BaTiS$_3$, a quasi-one-dimensional hexagonal chalcogenide. Single crystal X-ray diffraction studies reveal the presence of antipolar displacements of Ti atoms within adjacent TiS$_6$ chains along the c-axis, and three-fold degenerate Ti displacements in the a-b plane. $^{47/49}$Ti solid-state NMR provides additional evidence for those Ti displacements in the form of a three-horned NMR lineshape resulting from a low symmetry local environment around Ti atoms. We used scanning transmission electron microscopy to directly observe the globally disordered Ti a-b plane displacements and find them to be ordered locally over a few unit cells. First-principles calculations show that the Ti a-b plane displacements selectively reduce the refractive index along the ab-plane, while having minimal impact on the refractive index along the chain direction, thus resulting in a giant enhancement in the optical anisotropy. By showing a strong connection between structural disorder with picoscale displacements and the optical response in BaTiS$_3$, this study opens a pathway for designing optical materials with high refractive index and functionalities such as large optical anisotropy and nonlinearity.

cond-mat.mtrl-sci↗

arXiv2023

Colossal optical anisotropy from atomic-scale modulations

In modern optics, materials with large birefringence ({\Delta}n, where n is the refractive index) are sought after for polarization control (e.g. in wave plates, polarizing beam splitters, etc.), nonlinear optics and quantum optics (e.g. for phase matching and production of entangled photons), micromanipulation, and as a platform for unconventional light-matter coupling, such as Dyakonov-like surface polaritons and hyperbolic phonon polaritons. Layered "van der Waals" materials, with strong intra-layer bonding and weak inter-layer bonding, can feature some of the largest optical anisotropy; however, their use in most optical systems is limited because their optic axis is out of the plane of the layers and the layers are weakly attached, making the anisotropy hard to access. Here, we demonstrate that a bulk crystal with subtle periodic modulations in its structure -- Sr9/8TiS3 -- is transparent and positive-uniaxial, with extraordinary index n_e = 4.5 and ordinary index n_o = 2.4 in the mid- to far-infrared. The excess Sr, compared to stoichiometric SrTiS3, results in the formation of TiS6 trigonal-prismatic units that break the infinite chains of face-shared TiS6 octahedra in SrTiS3 into periodic blocks of five TiS6 octahedral units. The additional electrons introduced by the excess Sr subsequently occupy the TiS6 octahedral blocks to form highly oriented and polarizable electron clouds, which selectively boost the extraordinary index n_e and result in record birefringence ({\Delta}n > 2.1 with low loss). The connection between subtle structural modulations and large changes in refractive index suggests new categories of anisotropic materials and also tunable optical materials with large refractive-index modulation and low optical losses.

physics.optics↗

arXiv2022

Heavily doped zinc oxide with plasma frequencies in the telecommunication wavelength range

We demonstrate heavy and hyper doping of ZnO by a combination of gallium (Ga) ion implantation using a focused ion beam (FIB) system and post-implantation laser annealing. Ion implantation allows for the incorporation of impurities with nearly arbitrary concentrations, and the laser-annealing process enables dopant activation close to or beyond the solid-solubility limit of Ga in ZnO. We achieved heavily doped ZnO:Ga with free-carrier concentrations of ~10^21 cm^(-3), resulting in a plasma wavelength of 1.02 um, which is substantially shorter than the telecommunication wavelength of 1.55 um. Thus, our approach enables the control of the plasma frequency of ZnO from the far infrared down to 1.02 um, providing a promising plasmonic material for applications in this regime.

physics.app-ph↗

arXiv2022

Tuning carrier density and phase transitions in oxide semiconductors using focused ion beams

We demonstrate spatial modification of the optical properties of thin-film metal oxides, zinc oxide and vanadium dioxide as representatives, using a commercial focused ion beam (FIB) system. Using a Ga+ FIB and thermal annealing, we demonstrated variable doping of a band semiconductor, zinc oxide (ZnO), achieving carrier concentrations from 10^18 cm-3 to 10^20 cm-3. Using the same FIB without subsequent thermal annealing, we defect-engineered a correlated semiconductor, vanadium dioxide (VO2), locally modifying its insulator-to-metal transition (IMT) temperature by range of ~25 degrees C. Such area-selective modification of metal oxides by direct writing using a FIB provides a simple, mask-less route to the fabrication of optical structures, especially when multiple or continuous levels of doping or defect density are required.

cond-mat.mtrl-sci↗

arXiv2022

Orientation Controlled Anisotropy in Single Crystals of Quasi-1D BaTiS3

Low-dimensional materials with chain-like (one-dimensional) or layered (twodimensional) structures are of significant interest due to their anisotropic electrical, optical, thermal properties. One material with chain-like structure, BaTiS3 (BTS), was recently shown to possess giant in-plane optical anisotropy and glass-like thermal conductivity. To understand the origin of these effects, it is necessary to fully characterize the optical, thermal, and electronic anisotropy of BTS. To this end, BTS crystals with different orientations (aand c-axis orientations) were grown by chemical vapor transport. X-ray absorption spectroscopy (XAS) was used to characterize the local structure and electronic anisotropy of BTS. Fourier transform infrared (FTIR) reflection/transmission spectra show a large inplane optical anisotropy in the a-oriented crystals, while the c-axis oriented crystals were nearly isotropic in-plane. BTS platelet crystals are promising uniaxial materials for IR optics with their optic axis parallel to the c-axis. The thermal conductivity measurements revealed a thermal anisotropy of ~4.5 between the c- and a-axis. Time-domain Brillouin scattering showed that the longitudinal sound speed along the two axes is nearly the same suggesting that the thermal anisotropy is a result of different phonon scattering rates.

cond-mat.mtrl-sci↗

arXiv2021

Efficient generation of optical bottle beams

Optical bottle beams can be used to trap atoms and small low-index particles. We introduce a figure of merit for optical bottle beams, specifically in the context of optical traps, and use it to compare optical bottle-beam traps obtained by three different methods. Using this figure of merit and an optimization algorithm, we identified optical bottle-beam traps based on a Gaussian beam illuminating a metasurface that are superior in terms of power efficiency than existing approaches. We numerically demonstrate a silicon metasurface for creating an optical bottle-beam trap.

physics.optics↗

arXiv2019

Guidelines for measurement of thermal emission

Thermal emission is the radiation of electromagnetic waves from hot objects. The promise of thermal-emission engineering for applications in energy harvesting, radiative cooling, and thermal camouflage has recently led to renewed research interest in this topic. There is a substantial need for accurate and precise measurement of thermal emission in a laboratory setting, which can be challenging in part due to the presence of background emission from the surrounding environment and the measurement instrument itself. This is especially true for measurements of emitters at temperatures close to that of the environment, where the impact of background emission is relatively large. In this paper, we describe, recommend, and demonstrate general procedures for thermal-emission measurements that are applicable to most experimental conditions, including less-common and more-challenging cases that include thermal emitters with temperature-dependent emissivity and emitters that are not in thermal equilibrium.

physics.optics↗