ZnSe Substrates Research Articles

High-reliability infrared broadband thin-film polarizing beam splitter with ZnSe compensation layers.

Thin-film polarizing beam splitters (PBSs) fulfill a pivotal role in laser beam splitting, modulation, shaping and isolation. In this study, a high-reliability infrared broadband thin-film PBS was developed. To correct for tensile stress in Ge/YbF3 multilayer coatings, ZnSe compensation layers were incorporated in the multilayer design. The effects of different symmetrical periods on the spectral properties of the infrared PBS were systematically discussed. The infrared PBS operated at 45° and in the long-wave infrared (LWIR) band. Using the percent of optical extrema monitoring (POEM) strategy combined with the high-temperature optical constants (HTOC) of Ge film, the infrared PBS was precisely fabricated on ZnSe substrates. Subsequently, the spectral performance and film reliability of the infrared PBS were carefully characterized. Specifically, the transmittance of p-polarization surpassed 96%, while the extinction ratio exceeded 100:1 within the 10.6 ± 0.15 µm band. The infrared PBS demonstrated commendable environmental reliability, in addition to exhibiting excellent spectral characteristics.

Low-loss hybrid germanium-on-zinc selenide waveguides in the longwave infrared

Abstract The longwave infrared (LWIR) range, which spans from 6 µm to 14 µm, is appealing for sensing due to strong molecular fingerprints in this range. However, the limited availability of low-loss materials that can provide higher-index waveguiding and lower-index cladding in the LWIR range presents challenges for integrated photonics. In this work, we introduce a low-loss germanium-on-zinc selenide (GOZ) platform that could serve as a versatile platform for nanophotonics in the LWIR. By bonding high-quality thin-film germanium (Ge) to a zinc selenide (ZnSe) substrate, we demonstrate transparency from 2 µm to 14 µm and optical losses of just 1 cm−1 at 7.8 µm. Our results demonstrate that hybrid photonic platforms could be invaluable for overcoming the losses of epitaxially grown materials and could enable a wide range of future quantum and nonlinear photonics.

Transform-Limited Sub-100-fs Cr:ZnS Laser with a Graphene-ZnSe Saturable Absorber

In this work, we present ultrashort pulse generation from passively mode-locked Cr:ZnS laser with a monolayer graphene-coated ZnSe substrate exhibiting high nonlinearity. The femtosecond Cr:ZnS laser produces output power up to 330 mW at a 233 MHz repetition rate. Even in the presence of an uneven negative dispersion profile, the enhanced self-phase modulation by the ZnSe substrate of the graphene saturable absorber enables the polycrystalline Cr:ZnS laser to produce slightly chirped 99 fs pulses at 2373 nm. With extracavity dispersion compensation using a mixture of 3 mm and 2 mm thick ZnSe plates, the pulse width was compressed from 99 fs to 73 fs, resulting in an improved time–bandwidth product from 0.431 to 0.318. Assuming a sech2 pulse shape (0.315), the pulses were almost transform-limited. These results indicate that utilizing a graphene saturable absorber on a substrate with high nonlinearity presents an effective method for developing sub-100 fs solid-state lasers within the mid-IR spectral range.

Improving transmittance of long-wave infrared guided-mode resonant filters

The long-wave infrared (LWIR) spectral region spanning from 8 to 12 μm is useful for many scientific and industrial applications. Many of these applications require use of either a bandpass or a bandstop filter that can be realized by the guided-mode resonance (GMR) effect with subwavelength periodic features in layered dielectric materials transparent in the LWIR. The GMR filters operating in the LWIR region are fabricated by depositing an amorphous germanium (Ge) film to form a zero-contrast (ZC) waveguide-grating (WGG) on a polished zinc selenide (ZnSe) substrate. In general, the backside of a ZnSe substrate with refractive index 2.41 is uncoated causing a 17% Fresnel-reflection loss in the light transmitted through the filter due to a large impedance mismatch at the ZnSe/air interface. Because we use such filters in the LWIR laser experiments for notch filtering, to improve the filter transmittance we used ZnSe substrates coated on one-side with broadband antireflection coating (ARC) covering the 7 to 12 μm spectral range to fabricate GMRFs with one-dimensional (1D) Ge ZC WGG. We employed high-spatial resolution e-beam lithography and reactive-ion etching nanofabrication techniques to achieve high-performance large-area (12 × 12 mm2) 1D notch filters with subwavelength periods. We characterized polarization dependent spectral performance of the prototype filters with both coherent and incoherent incident light using a tunable quantum cascade laser system that spans the 7 to 12 μm region, and a Fourier transform infrared spectrometer with collimated incident beam to achieve close to 15% improvement in the peak transmittance as well as significant reduction in coherent noise compared to our earlier results with GMRFs without ARC. Here, we present the filter design simulation and measurement results.

Polarization independent electron-beam written 2-D longwave infrared guided-mode resonant filters

We fabricated guided mode resonance filters (GMRFs) with two-dimensional (2-D) gratings operating in the 8 to 12 µm long-wave infrared (LWIR) region by depositing amorphous germanium (Ge) film to form a zero-contrast (ZC) waveguide-grating (WGG) on polished zinc selenide (ZnSe) substrates with and without antireflection coating (ARC). We employed high-spatial resolution e-beam lithography and reactive-ion etching (RIE) nanofabrication techniques. We characterized the fabricated filters for their polarization independent spectral performance using a tunable quantum cascade laser (QCL) system and a modified Fourier transform infrared (FTIR) spectrometer. Here, we will present both theoretical and experimental results and their comparison.

Surface oxidation of hydrophobic ZnSe for enhanced growth of atomic layer deposited aluminum oxide

The growth of atomic layer deposited (ALD) Al2O3 on planar ZnSe substrates is studied using in situ spectroscopic ellipsometry. An untreated ZnSe surface requires an incubation period of 27 cycles of ALD Al2O3 before film growth is observed. Pretreating the surface with an ultraviolet generated ozone lowers the incubation to 17 cycles, whereas a plasma-enhanced ALD Al2O3 process can further lower the incubation period to 13 cycles. The use of ozone or plasma-activated oxygen species on ZnSe is found to create ZnO and SeO2, which are responsible for converting ZnSe from a hydrophobic to a hydrophilic surface. The interfacial layer between Al2O3 and ZnSe is mapped using high-resolution transmission electron microscopy and scanning transmission electron microscopy/energy dispersive spectroscopy. SeO2 is volatile and leaves a zinc-rich interface, which is 4.3 nm thick for the ultraviolet generated ozone pretreated sample and 2.5 nm for the plasma-enhanced ALD process.

A Symmetrical Terahertz Triple-Band Metamaterial Absorber Using a Four-Capacitance Loaded Complementary Circular Split Ring Resonator and an Ultra-Thin ZnSe Substrate

In this research work, a symmetrical four-capacitance loaded complementary circular split ring resonator is proposed, which uses an ultra-thin Zinc Selenide (ZnSe) substrate to realize a low-profile triple-band metamaterial (MTM) perfect absorber for application in the terahertz (THz) frequency range. The electromagnetic properties of the proposed structure were calculated and investigated using the Finite Integration Technique (FIT). The proposed structure exhibited three highly absorptive (nearly perfect) peaks at the resonance frequencies of 15.68 THz, 37.48 THz, and 39.55 THz. Furthermore, the absorber was found to be insensitive to the polarization and incident wave angles, due to its symmetrical design. The effects of the conductor type, substrate thickness, unit cell dimension, resonator gap, and substrate type on the reflection and absorption spectra were investigated. To validate the numerical results, the proposed design was analyzed using High-Frequency Simulation Software (HFSS) and Advanced Design System (ADS). The surface current, electric field, and magnetic field distributions at the three-resonance frequency were analyzed. It was concluded that the overall performance of the proposed MTM structure was superior compared to those reported in the literature. The proposed design could be a good candidate for application in stealth technology, imaging, and thermal energy harvesting.

Continuously tunable middle-IR bandpass filters based on gradient metal-hole arrays for multispectral sensing and thermography

Continuously tunable middle-infrared bandpass filters have been developed based on gradient metal-hole arrays with two distinct geometries. The rotation filter relies on an array of metal holes with gradually changing periods and hole sizes in the azimuthal direction, while the translation filter exploits a metal-hole array with a linear gradient. The filters are fabricated in a Ti film on a ZnSe substrate using electron-beam nanolithography. They are characterized experimentally using Fourier-transform infrared spectroscopy, and the observed results are compared with numerical predictions of the finite element method. The developed filters offer wide spectral tunability when operating with a focused beam. Particularly, the central wavelength of the transmission band is tunable in the [Formula: see text] range, for the rotation filter, and in the [Formula: see text] range for the translation one, as a linear function of the filter angular or linear displacement. The filters feature relatively broad bandwidths of [Formula: see text], while their spectral contrast and energy efficiency depend on the gradient type. The filter spectral response function shape and the extent of its spectra tunability can be further optimized by judicious design of the hole geometry and the metal-hole array gradient, respectively. The developed filters hold strong potential in the infrared multispectral sensing and imaging, thanks to their conceptual simplicity. Considering the linearity of Maxwell’s equations and availability of appropriate technologies for the fabrication of gradient arrays of sub-wavelength metal holes, the developed concept can be translated to other spectral ranges.

10.6 μm saturable absorption and optical isolation of graphene

In this paper, the nonlinear absorption and optical isolation properties of graphene on a ZnSe substrate were studied at 10.6 μm. The relationships between graphene saturable absorption parameters, the damage threshold and number of layers were studied by experiment. The results show that single-layer graphene on a ZnSe substrate has a higher damage threshold and lower nonsaturable loss coefficient than multilayer graphene and is more suitable for high-power MOPA-CO2 laser systems. The simulation results show that 90% of the small-signal noise light can be suppressed by 50 passes through single-layer graphene, and the ratio of the noise to the main pulse can be reduced from 1% to 0.15%.

Highly sensitive and recyclable surface-enhanced Raman scattering (SERS) substrates based on photocatalytic activity of ZnSe nanowires

The semiconductor-based SERS substrates have attracted considerable attention due to their unique optical and reusable biosensor applications. However, most substrates generally exhibit poor enhancement effects and are non-reusable, needing a complicated fabrication process. In the present study, novel recyclable SERS substrates were synthesized from ZnSe nanowires using chemical vapor deposition. SERS performance was enhanced via introducing the defects by adding Se dopant in the ZnSe and achieved the value of EF 6.92 × 107 for the Zn/Se ratio 1:1.2. The SERS efficiency of the fabricated ZnSe nanowire was probed by rhodamine 6 G (R6G) and methylene blue (MB), presenting excellent stability and reaching the sensitivity level of 10−11 M. The photocatalytic degradation of R6G molecules were experimentally investigated with Raman mapping tests using a visible light source to explore the recyclability of the SERS substrates. Our study demonstrates that the proposed ZnSe nanostructure possesses the capability to reuse up to 6 times with preserving strong SERS signal reproducibility. The bifunctional recyclable ZnSe substrate not only presents a novel method to truly enhance the SERS performance, photocatalytic behavior but is also considerable for food safety application.

A “Janus” double sided mid-IR photodetector based on a MIM architecture

We present a mid-IR (λ ∼ 8.3 μm) quantum well infrared photodetector (QWIP) fabricated on a mid-IR transparent substrate, allowing photodetection with illumination from either the front surface or through the substrate. The device is based on a 400 nm-thick GaAs/AlGaAs semiconductor QWIP heterostructure enclosed in a nanostructured metal–insulator–metal cavity and hosted on a mid-IR transparent ZnSe substrate. Metallic stripes are symmetrically patterned by e-beam lithography on both sides of the active region. The detector spectral coverage spans from λ ∼ 7.15 to λ ∼ 8.7 μm by changing the stripe width L—from L = 1.0 to L = 1.3 μm—thus frequency-tuning the optical cavity mode. Both micro-FTIR passive optical characterizations and photocurrent measurements of the two-port system are carried out. They reveal a similar spectral response for the two detector ports with an experimentally measured TBLIP of ∼200 K.

An ultrathin and dual band metamaterial perfect absorber based on ZnSe for the polarization-independent in terahertz range

In this work, a new metamaterial design is proposed to yield an ultra-thin and dual band metamaterials perfect absorber (MPA) to be operated in the frequency range from 15 to 35 THz. The proposed structure is consisted of a copper resonator deposited on a very thin Zinc Selenide ZnSe (0.6 μm) substrate, where the backside of the structure is covered with a metal plate to block the transmission of electromagnetic waves. Computer Simulation Technology (CST) was used to design and investigate the proposed structure. The absorption response of the proposed structure was found to be high enough with absorptivity of 98.44 and 99.28 at 22.46 THz and 28.95 THz, respectively. Results showed that the absorber is insensitive to the incident angle of 0°–60° in both transverse electric (TE) and transverse magnetic (TM) modes, respectively. The MPA was seen to be highly independent on the angles of polarization of the incident waves. The working mechanism of the proposed design was revealed by multiple reflection interference theory and a good agreement was confirmed between the calculated and simulated results. The proposed design can be used for possible applications of stealth technology and imaging.

Micro-electromechanical-system-tuned resonant filters spanning the 8-12 µm band.

The spectral band covering ∼8-12µm is atmospherically transparent and therefore important for terrestrial imaging, day/night situational awareness systems, and spectroscopic applications. There is a dearth of tunable filters spanning the band. Here, we propose and demonstrate a new, to the best of our knowledge, tunable-filter method engaging the fundamental physics of the guided-mode resonance (GMR) effect realized with a non-periodic lattice. The polarization-dependent filter is fashioned with a one-dimensional Ge grating on a ZnSe substrate and interrogated with a ∼1.5mm Gaussian beam to show clear transmittance nulls. To expand the tuning range, the device parameters are optimized for sequential operation in TM and TE polarization states. The theoretical model exhibits a tunable range exceeding 4 µm, thus covering the band fully. In the experiment, a prototype device exhibits a spectral range of 8.6-10.0 µm in TM and 9.9-11.7 µm in TE polarization or >3µm total. With additional efforts in fabrication, we expect to achieve the full range.

Investigation of the laser induced damage thresholds of all-dielectric and metal-dielectric mirrors for a continuous wave at 10.6 μm

Thin films have many applications in the field of optics and photonics. One of these applications is the fabrication of high-reflection mirrors for high-power lasers. High-power lasers also require mirrors with high laser induced damage threshold (LIDT). In this research, firstly, considering the principles of simulation, we have tried to design two types of mirrors, all-dielectric and metal-dielectric. Therefore, they are suitable in terms of both the reflectance spectrum and LIDT at 10.6 μm. To simulate and fabricate these two types of mirrors, ZnSe and Ge have been used as substrates and for thin film layers, Ge and ZnS have been used. In metal-dielectric mirrors, gold layers have also been used on the substrate as the metal layers. After design and simulation, the process of mirrors' fabrication was done by electron beam evaporation technique and the reflectance spectrum of the samples was taken, which shows the reflectance of all-dielectric mirrors is more than 99% and that of Metal-dielectric mirrors is more than 99.9%. Finally, measuring the LIDT of these two types of mirrors was performed based on ISO21254-1 2011 standard which shows that the metal-dielectric mirror with ZnSe substrate is the most suitable mirror for continuous lasers at 10.6 μm.

Transfer mechanisms in semiconductor hybrids with colloidal core/shell quantum dots on ZnSe substrates

Hybrid systems consisting of colloidal CdS/ZnS core/shell quantum dots on ZnSe semiconductor substrates have been studied by continuous-wave and nanosecond time-resolved photoluminescence. On the basis of kinetic calculations, we studied the interplay between the possible transfer processes in these hybrids. The considered transfer mechanisms were resonance energy transfer, photon reabsorption, electron and hole tunneling. Depending on the size of the CdS cores the dominating transfer mechanism is changing. Carrier tunneling was found only for quantum dots in direct contact to the substrate. For large quantum dots a hole tunneling was found, whereas in case of small dots the fast electron tunneling is decisive. Eventually, we were able to determine the conduction band offset between CdS and ZnSe to 0.56 eV at 10 K.

In Situ Monitoring of Alkanethiol Self‐Assembly onto Zinc Selenide: The Role of Substrate Pretreatment and Its Implication in Bacterial Attachment

AbstractThe interface between pioneer sessile bacteria and a supporting substrate can be probed in situ and at the molecular scale by infrared spectroscopy in the attenuated total reflection mode (ATR‐FTIR). Here, a self‐assembled monolayer (SAM) of amino‐terminated alkanethiol is formed on the internal reflection element (IRE) composed of zinc selenide, and the attachment of model bacterium Lactobacillus rhamnosus GG (LGG) is subsequently studied. The impact of the beforehand surface preparation of the IRE on the SAM is studied on ZnSe substrates (i) cleaned by exposure to ozone/UV, (ii) acid cleaned, or (iii) coated with a thin gold film. The surface properties of the obtained substrates are analyzed by atomic force and electron microscopies, and elastic ion backscattering spectrometry. The kinetics of the formation and the organization of the formed SAMs are strongly surface dependent, as evidenced with ATR‐FTIR. Acid‐cleaned and gold‐coated IREs are the least and most favorable substrates for alkanethiol SAM formation, respectively. Regardless of differences in SAM properties, the average degree of LGG attachment is similar on all functionalized substrates. The molecular organization of LGG cells, however, is substrate‐dependent suggesting a possible effect of SAM organization on the bacteria–substrate interface.

Valine Radiolysis by H+, He+, N+, and S15+ MeV Ions

Radiolysis of biomolecules by fast ions has interest in medical applications and astrobiology. The radiolysis of solid D-valine (0.2–2 μm thick) was performed at room temperature by 1.5 MeV H+, He+, N+, and 230 MeV S15+ ion beams. The samples were prepared by spraying/dropping valine-water-ethanol solution on ZnSe substrate. Radiolysis was monitored by infrared spectroscopy (FTIR) through the evolution of the intensity of the valine infrared 2900, 1329, 1271, 948, and 716 cm−1 bands as a function of projectile fluence. At the end of sample irradiation, residues (tholins) presenting a brownish color are observed. The dependence of the apparent (sputtering + radiolysis) destruction cross section, σd, on the beam stopping power in valine is found to follow the power law σd = aSen, with n close to 1. Thus, σd is approximately proportional to the absorbed dose. Destruction rates due to the main galactic cosmic ray species are calculated, yielding a million year half-life for solid valine in space. Data obtained in this work aim a better understanding on the radioresistance of complex organic molecules and formation of radioproducts.

Characterization of e-beam evaporated Ge, YbF3, ZnS, and LaF3 thin films for laser-oriented coatings.

Thin films of Ge, ZnS, YbF3, and LaF3 produced using e-beam evaporation on ZnSe and Ge substrates were characterized in the range of 0.4-12 µm. It was found that the Sellmeier model provides the best fit for refractive indices of ZnSe substrate, ZnS, and LaF3 films; the Cauchy model provides the best fit for YbF3 film. Optical constants of Ge substrate and Ge film as well as extinction coefficients of ZnS, YbF3, LaF3, and ZnSe substrate are presented in the frame of a non-parametric model. For the extinction coefficient of ZnS, the exponential model is applicable. Stresses in Ge, ZnS, YbF3, and LaF3 were estimated equal to (-50)MPa, (-400)MPa, 140 MPa, and 380 MPa, respectively. The surface roughness does not exceed 5 nm for all films and substrates.

Polarization sensitivity of ZnSe single crystals based structures

The work is devoted to study of polarization photosensitivity of structure on the basis of ZnSe single crystals. To produce such photosensitive structure we used heat treatment of ZnSe substrates on dry air. The polarization photosensitivity was detected and studied in relation to their manufacturing conditions and photoregistration configuration. We conclude that such photosensitive structure can be used as polarimetric short wavelength photodetector.

Thick hydride vapor phase epitaxial growth of ZnSe on GaAs (1 0 0) vicinal and orientation patterned substrates

Zinc selenide is attractive for numerous optical applications in the mid and longwave infrared from solid state lasers to non-linear optical frequency conversion. Its use has been limited by the availability of single crystal and epitaxial materials. To meet these challenges we have developed a low-pressure hydride vapor phase epitaxy (HVPE) process for growth of high quality thick ZnSe layers on GaAs (1 0 0) vicinal and orientation-patterned GaAs (OP-GaAs) substrates. We show growth of >300 µm thick layers at growth rates >50 µm/hr. With only 0.26% lattice mismatch between ZnSe and GaAs, epitaxial layers with crystal quality comparable to bulk ZnSe substrates are achieved. X-ray reciprocal space maps show the ZnSe to be fully relaxed. Cross-sectional TEM imaging analysis shows stacking faults at the interface that propagate in to the growing film consistent with a full relaxed film. Surface morphology shows the presence of pits that have their long axis perpendicular to [1–10] direction. Low temperature photoluminescence reveals band to band emission that consisted of the 2.797 eV direct band gap along with the strong exciton emission at 2.802 eV. Growth on OP templates revealed low fidelity domains of thickness >50 µm. This work presents a process to produce high quality thick layers potentially suitable for laser and frequency conversion for defense and civilian applications.