The effect of various thicknesses (6nm-74nm) on the optical, electrical, and surface topology properties of Tin-doped Indium Oxide (ITO) thin conductive films (TCF) on Si and glass substrates has been studied. The ITO thin films were prepared by direct-current (DC) sputtering at room temperature using 1-meter diameter high vacuum coating chamber at GSFC. The Ar partial pressure was 2 × 10-3 Torr and DC power was kept at 200W. The film resistivity was measured by a four-point probe method at room temperature. Transmittance of ITO films on glass were characterized in the 200-2500nm range through optical spectrophotometry. Optical properties (n, k) were derived through ellipsometry. The surface topology and morphology were examined by scanning white light interferometry (SWILI) and atomic force microscopy (AFM). As an exemplary result, the transmittance of glass substrate coated with a 22.9 nm thick ITO thin film, normalized to the transmittance of the bare glass substrate, is 0.985 at λ=550nm and 0.988 at λ=1064nm. This sample presents a resistivity of 7.29 × 10-4 Ω-cm, and a surface roughness of 0.4 nm.
We investigate the optical properties of two-dimensional monolayer films of MoSe2 and WSe2 (each 0.7nm thick) assembled on SiO2/Si substrates (285nm/0.5mm thick). These films are interesting because they are direct band gap semiconductors that have large excitonic responses. However, due to numerous challenges, including the lack of a quick, contactless, and reliable method, obtaining the optical constants and exciton binding energies in-situ remains a difficult endeavor. Here, we report the optical properties based on contactless ellipsometry to retrieve the optical constants (n,k) and excitonic properties of both monolayers (MoSe2 and WSe2). The optical properties of these materials away from the exciton (~700 nm) are generally not well understood. In this work, we will explore the optical response of these films over a broad range that includes the UV/visible and near infrared (200-2000nm) in order to understand if there are other spectral regions with a strong or tunable refractive index. The current samples are intrinsic without doping. The SiO2 on the Si substrate would be used as a gate capacitor which would allow to vary the density by ~1011-1012 cm-2. These transition metal dichalcogenides (TMD) offer new possibilities for designing modern photonic and optoelectronic components.
The Wide-Field instrument (WFI) for the Roman Space Telescope (RST) features an imaging camera that comprises the Wide-Field Channel (WFC) with several bandpass filters, a spectroscopic dispersion unit called the Grism, and a Prism Assembly (PA), which took the place of the descoped Integral-Field Channel (IFC) assembly. The PA system consists of two prism elements made from S-TIH1 glass (P1) and CaF2 substrate (P2) that together will provide slitless low resolution spectroscopy with a spectral resolution R < 70 at all wavelengths, and R < 170 for wavelengths λ < 0.8 μm, across the full field. One key feature of the P1 element is the application of a bandpass coating that operates in the 0.75-1.8 μm spectral region. The extension of the bandpass towards short wavelengths greatly enhances the capabilities of RST for studies of stellar populations that provides additional means of testing in supernova studies. We have used spectroscopic techniques such as a double-beam monochromator and Fourier Transform InfraRed (FTIR) spectroscopy to characterize the spectral performance of the bandpass coatings of the P1 element. The coating technology used to produce these bandpass optical coatings has been demonstrated in the successful mission of the Mars Perseverance Rover in February of 2021.
The far-infrared (IR) region is rich with information needed to characterize interstellar dust and to investigate the cold outer planets of the solar system and their icy moons. The proposed sub-orbital observatory the balloon experiment for galactic infrared science (BEGINS) will utilize cryogenic instruments to map spectral energy distributions (SEDs) of interstellar dust in the Cygnus molecular cloud complex. A future high priority flagship mission Uranus Orbiter and Probe carrying a net flux radiometer (NFR) will study the in situ heat flux of the icy giants atmosphere to 10 bar pressure. These instruments require far-IR filters to define the instrument spectral bandwidths. Our ultimate goal is to define the instrument bands of BEGINS and the NFR with linear-variable filters (LVFs) and discrete-variable filters (DVFs). The LVFs and DVFs will be made of metal mesh band-pass filters (MMBF) comprised of a 100 nm thick gold film with cross-shaped slots of varying sizes along a silicon (Si) substrate with cyclic olefin copolymer (COC) anti-reflection (AR) coatings. We present our progress towards LVFs and DVFs with simulated and measured transmission of a room temperature, non-AR coated, single-band 44 µm MMBF filter. We have successfully fabricated, measured, and modeled a non-AR coated, room temperature 44 µm MMBF. The transmission at room temperature and non-AR coated was measured to be 27% with a resolving power of 11. When COC-AR coated on both sides the transmission is expected to increase to 69% with a resolving power of ten.
This paper describes a cryogenic optical testbed developed to characterize µ-Spec spectrometers in a dedicated dilution refrigerator (DR) system. μ-Spec is a far-infrared integrated spectrometer that is an analog to a Rowland-type grating spectrometer. It employs a single-crystal silicon substrate with niobium microstrip lines and aluminum kinetic inductance detectors (KIDs). Current designs with a resolution of R = λ/Δλ = 512 are in fabrication for the EXCLAIM (Experiment for Cryogenic Large Aperture Intensity Mapping) balloon mission. The primary spectrometer performance and design parameters are efficiency, NEP, inter-channel isolation, spectral resolution, and frequency response for each channel. Here we present the development and design of an optical characterization facility and preliminary validation of that facility with earlier prototype R=64 devices. We have conducted and describe initial optical measurements of R = 64 devices using a swept photomixer line source. We also discuss the test plan for optical characterization of the EXCLAIM R = 512 μ-Spec devices in this new testbed.
The cryogenic etching of the black silicon (BSI) has been demonstrated as a superior absorber in par with other ultraabsorbers such as carbon nanotubes in the visible and near-infrared spectrum. In this work, we discuss the fabrication, modeling, and characterization of the BSI targeting the 2.5-5 microns range. We investigated a series of cryogenic parameters such as temperature, pressure, oxygen flow rate, power, and etching duration and fabricated a series of uniformly etched wafers. Additionally, we established a three-dimensional mathematical model of a unit cell and manipulated the silicon needle geometry and shape. Our preliminary results show five orders of magnitude specular reflectance in the infrared region. The technique employed here could be used to scale the etching process and enhance the absorption in the far-infrared and submillimeter range.
Pyroelectric materials show a change in their spontaneous polarization due to the temperature variations. This property makes these materials unique for sensing radiation in the infrared (IR) broad range. Here, we report the deposition and characterization of pyroelectric Calcium Lead Titanate (PCT) thin films for using them to fabricate pyroelectric detectors. PCT films were deposited on both silicon and Si/SiN/Ti/Au substrates at 13 mTorr pressure by 200W Radio Frequency (RF) sputtering in Ar+O2 environment for four hours. Substrates were kept at variable temperatures starting from 550 ºC up to 800 ºC during the deposition. The PCT films were annealed at 550, 600, 650 and 700 ºC in O2 environment for 15 minutes. X-ray diffraction (XRD) results confirm the polycrystalline nature of these films. Energy dispersive spectroscopy (EDS) function of scanning electron microscope (SEM) was done to determine the elemental composition of PCT films. Our EDS result reveals the presence of the elements such as Calcium, Lead, Titanium , and Oxygen in the thin films. Moreover, it shows that the films are stoichiometric (Ca0.43Pb0.57)TiO3 (Ca/Ti=0.5, Pb/Ti=0.66). The film thicknesses were measured using a Dektak model XT profilometer which ranges from ~ 250 to 400 nm. The surface morphology obtained from SEM and atomic force microscopy confirms the crack-free nature of our films as well as their smoothness and low surface roughness. Temperature dependence of capacitance, pyroelectric current, and pyroelectric coeeficient were investigated for different PCT films. Our results show that films deposited at 550ºC and 600 ºC demonstrate better quality and larger values of pyroelectric coefficient. On the other hand, the capacitance fabricated on the PCT films at 550 ºC showed the highest value of pyroelectric current and pyroelectric coefficient which are 14 pA and 50 μC/m2K respectively at higher temperature.
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