As advancing technology pushes to further miniaturize systems while increasing processing power, optical structures which offer dynamic tunability are becoming ever more valuable. Diffractive gratings popularly offer high efficiencies and can be readily designed to provide polarization sensitivity, making them useful as dynamic structured optics. Recently, slanted wire gratings compatible with fabrication by two-photon polymerization were investigated for their ability to be mechanically tuned. Potential applications for this grating may be in mechanical sensing and beam splitting. In this study, we investigate an additional degree of tunability not previously considered by exploiting the polarization sensitivity as well as the mechanical. It is observed that the population of the −1st, 0th, and +1st transmitted orders are sensitive to changes between x- and y-axis polarization.
In this study, we report on the optical properties of a photochromic thiazolo[5,4-d]thiazole-embedded in polymer in the visible and near infrared spectral range determined using spectroscopic ellipsometry. The dielectric functions for the yellow (TTz2+) and blue (TTz0) states were extracted from a numerical wavelength-by-wavelength inversion of the experimental data. The extracted dielectric functions are in good agreement with a Kramers-Kronig consistent B-spline-based model analysis of the experimental data. The thiazolo[5,4-d]thiazole-embedded polymer exhibits several strong absorption bands from the visible to the near infrared spectral range depending upon their redox states.
Calibrating thermal detection systems for target recognition and accuracy can be challenging when live assets are not an option as a target. Infrared scene projection provides a cost effective and realistic alternative to assess missile capability. Infrared scene projection systems allow the generation of a thermally simulated scene for hardware-in-the-loop calibration of missile targeting. Previously, infrared scene projection technology has used resistor arrays, digital micromirror devices and laser diode arrays to name a few. Recent advancements in dynamic metamaterials provide a novel approach for the design of an infrared scene projection system. Reciprocal plasmonic metasurfaces are a metal-insulator-metal configuration of high aspect ratio dielectric pillars with sub-wavelength periodicity contained between a conductive top and bottom layer. Reciprocal plasmonic metasurfaces display an extreme sensitivity to ambient refractive index. This sensitivity in synergy with a conformal coating of a phase change material, such as vanadium dioxide, provide an excellent mechanism to implement a spatial light modulator as the scene generation component of an infrared scene projector. We report on the operating mechanism of the metasurface and characterize its sensitivity to changes in the ambient refractive index by applying a thin, conformal layer of aluminum oxide. We then expand on the experimental results by employing dielectric function data of optically characterized vanadium dioxide to inform calculations for predicting the effects of a thin conformal coating applied on the metasurface. Results indicate that pairing the sensitive metamaterial with the fast switching optical properties of vanadium dioxide provide a novel platform for infrared scene generation.
One-dimensional photonic crystals have been frequently used as optical filters and in sensing technology due to their ability to induce highly reflective photonic bandgaps. Conventionally, at least two materials are required to create the necessary dielectric contrast for photonic bandgaps to form. Recently, one-dimensional photonic crystals fabricated by two-photon polymerization have demonstrated the ability to induce photonic bandgaps with reflectances over 90%. Using this fabrication approach, dielectric contrast is achieved by altering the density of adjacent layers from a single photo-sensitive polymer. The success of this technique has led to the development of design modifications which allow additional spectral control of the photonic bandgap. In this study, we combine these concepts to develop a one-dimensional photonic crystal which includes a mechanical defect for the first time. Mechanical control of this defect allows for the presence of the transmissive defect mode to be actively shifted in and out of the photonic bandgap. The fabrication of this structure as well as its characterization is reported and discussed. The results of this study further support the use of one-dimensional photonic crystals in opto-mechanical applications where switchable narrow transmission bands are desired.
Recently, two-photon polymerization has been successfully employed to fabricate high-contrast one-dimensional photonic crystals. Using this approach, photonic bandgap reflectivities over 90% have been demonstrated in the infrared spectral range. As a result of this success, modifications to the design are being explored which allow additional tunability of the photonic bandgap. In this paper, a one-dimensional photonic crystal fabricated by two-photon polymerization which has been modified to include mechanical flexures is evaluated. Experimental findings suggest these structures allow mechanically induced spectral shifting of the entire photonic bandgap. These results support the use of one-dimensional photonic crystals fabricated by two-photon polymerization for opto-mechanical applications.
Plasmonic metasurfaces composed of arrays of rectangular metallic bars are well known for their strong optical response in the infrared spectral range. In this study, we explore the polarization sensitivity of plasmonic metasurfaces for encoding information. The polarization-sensitive optical response depends strongly on the orientation of the metallic bars allowing the encoding of information into the metasurface. Here we demonstrate that a 2-dimensional polarization encoded metasurface can be obtained by using mask-less two-photon polymerization techniques. This novel approach for the fabrication of plasmonic metasurfaces enables the rapid prototyping and adaptation of polarization sensitive metasurfaces for the encoding of multiplexed images.
Mechanical tuning of defect modes in a polymer-based one-dimensional photonic crystal was demonstrated for the terahertz (THz) spectral range. A sharp defect mode in the photonic bandgap was achieved by symmetrically enclosing a defect layer with two identical pairs of alternating compact and low-density layers. By adjusting the thickness of the defect layer, the spectral position of the defect mode within the photonic bandgap was easily controlled. Normal incidence transmission spectroscopy in a spectral range from 82 to 125 GHz was used to determine the THz spectral response for different defect layer thicknesses. The transmission data were analyzed using stratified optical layer model calculations. Spectral shift of the center frequency of the narrow transmission peak within a distinct photonic bandgap was observed in the experimental transmission spectra. The shift was achieved by mechanical tuning of the defect layer thickness. A good agreement between the relevant model parameters and the corresponding design parameters was found.
Structured surfaces composed of subwavelength-sized features offer multifunctional properties including antireflective characteristics that are increasingly important for the development of micro-optical components. Here, three-dimensional (3-D) direct laser writing, via two-photon polymerization, is used to fabricate planoconvex spherical microlenses with antireflective structured surfaces. The surfaces are composed of subwavelength-sized conicoid structures, which are arranged fully conformal to the convex surface of the microlenses. The dimensions of the conicoid structures are optimized to effectively reduce Fresnel reflection loss over a wide band in the near-infrared spectral range from 1.4 to 2.2 μm, with a maximum reduction at 1.55 μm. Infrared reflection and transmission measurements are used, in combination with 3-D finite element calculations, to investigate the performance of the microlenses. The experimental results reveal that in the spectral range from 1.4 to 2.2 μm an effective suppression of the Fresnel reflection loss at the convex surface of spherical microlenses can be achieved. The transmittance enhancement is ranging from 1% to 3% for spherical microlenses with antireflective structured surfaces, in comparison to an uncoated reference.
We report on the first successful generalized Mueller-matrix ellipsometry measurements in the THz-frequency domain using the high-brilliance THz synchrotron radiation source IRIS at the electron storage ring BESSY, Germany. Generalized Ellipsometry, which is known as a powerful tool for measurement of optical constants including anisotropy and which was previously used in the FIR to VUV spectral range, is now employed for the first time to investigate condensed matter samples in the frequency range from 0.9 to 8 THz (30 to 650 cm-1). Exemplarily, results obtained from bound and unbound charge-carrier investigations in low-dimensional semi- and superconducting systems are presented. Future applications of this technique for investigation of charge-carrier dynamics in magnetic fields are envisioned.
This work presents a spectroscopic ellipsometry study of phonon and polariton modes in zincblende group-III-group-V semiconductor layer structures. Contributions to the dielectric function due to infrared-active polar phonon modes and coupled longitudinal-phonon-plasmon modes are differentiated and quantified upon model lineshape analysis. Interface Fano-, Brewster- and surface-guided modes are assigned upon solution of the surface polariton dispersion relation for layered structures, and addressed by experiment. We explain the physical origin of the Berreman-effect.
In accord with the Drude model, the free-carrier contribution to the dielectric function at infrared wavelengths is proportional to the ratio of the free-carrier concentration N and the effective mass m, and the product of the optical mobility μ and m. Typical infrared optical experiments are therefore sensitive to the free-carrier mass, but determination of m from the measured dielectric function requires an independent experiment, such as an electrical Hall-effect measurement, which provides either N or μ. However, doped zincblende III-V-semiconductors exposed to a strong external magnetic field exhibit non-symmetric magneto-optical (MO) birefringence, which is inversely proportional to m. Therefore, if the spectral dependence of the MO dielectric function tensor is known, the parameters N, μ and m can be determined independently from optical measurements alone. Generalized Ellipsometry (GE) measures three complex-valued ratios of normalized Jones matrix elements, from which the individual tensor elements of the dielectric function of arbitrarily anisotropic materials in layered samples can be reconstructed. We present the application of GE at far-infrared (FIR) wavelengths for measurement of the FIR-MO-GE parameters, and determine the MO dielectric function of GaAs for wavelengths from 100 μm to 15 μm. We obtain the free electron mass and mobility results in excellent agreement with results obtained from Hall-effect and Shubnikov-de-Haas experiments. (F)IR-MO-GE may open up new avenues for non-destructive characterization of free-carrier properties in complex semiconductor heterostructures.
A new global approach, called 'Generalized Ellipsometry', is now capable to characterize the optical and structural properties of general anisotropic layered systems, including absorption, and can be applied, in general, to determine the linear response tensor elements for wavelengths from the far IR to the deep UV. This technique enables new insights into physical phenomena of layered anisotropic mediums, and can provide precise structural and optical data of novel compound materials. Experimental results are presented for stibnite single crystals as example for an arbitrary biaxial absorbing material, a wurtzite GaN thin film with uniaxial anisotropy grown on sapphire, a spontaneously atomically ordered III-V semiconductor alloy thin film, and a sculptured titanium dioxide film with symmetrically dielectric tensor properties.
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