Anisotropic materials that exhibit large birefringence in the mid-infrared (mid-IR) spectral range, particularly those that exhibit large in-plane birefringence, are of considerable interest for optical devices (e.g., waveplates and polarizers), yet the availability of such materials is limited in the infrared (IR) spectral range. Theoretical exploration is thus keenly sought to both identify layered 2D and bulk materials with large birefringence and to better understand the mechanisms underlying the response. In this work, we investigate anisotropic CrSBr by first-principles calculations, where following validation of our computational methods by analysis of the optical absorption and of Raman and infrared spectra, we predict giant birefringence values, with Delta n exceeding 5 in the near-IR and over 2 in the mid-IR in layered semiconductor bulk CrSBr, the highest, to our knowledge, reported to date. We employed the structure of our CrSBr grown crystalline material as the starting structure for optimization. Thorough analyses of the electronic structure reveal the origin of the birefringence. The in-plane giant birefringence persists in the ultrathin monolayer (<1 nm thickness), still covering a broad portion of the IR spectral range. Our results, predicting a very large birefringence in CrSBr, may offer a promising material solution for optical devices in the mid-IR.

We probe the excited-state dynamics of a platinum-acetylide chromophore dissolved in ormosil glasses in the concentration range of 0.1-400 mM to gain a better understanding of how the environment of the dye reflects upon the overall kinetics observed. At 0.1 mM, ground-state absorption, fluorescence, excited-state absorption (ESA), and triplet ESA reproduce solution behavior. Above ≥10 mM, a weak 485 nm ground-state band appears, consistent with a nominally forbidden S0 → T1 transition, and steady-state emission shows quenched fluorescence with enhanced phosphorescence. Following 355 nm flash photolysis, high-concentration samples initially exhibit triplet ESA identical to the 0.1 mM case, but a blue-shifted triplet ESA develops at longer delays; direct excitation of the 485 nm band yields the same blue-shifted spectrum, confirming aggregation effects. Kinetically, the 0.1 mM sample displays a single triplet lifetime, whereas ≥10 mM samples require two. The shorter lifetime at all loadings follows a Freundlich adsorption dependence, consistent with monomer binding to ormosil sites, while the longer lifetime is attributed to aggregation. Ultrafast transient absorption (TA) resolves two ESA bands whose energy separation and relative areas suggest intramolecular exciton coupling between ligand-localized transitions. Fitting the data with exciton theory gives the interligand transition-dipole angle and the excitonic splitting; both evolve with concentration and pump-probe delay, reflecting symmetry breaking, intersystem crossing, and charge-transfer reorganization. At ∼1 mM, the time-dependent band separation is consistent with excimer formation, whereas no excimer signatures are observed at ≥10 mM. These results establish a quantitative structure-dynamics-concentration relationship: aggregation and ormosil-induced microphase separation create coexisting free and aggregated populations that modulate exciton coupling (dipole geometry and splitting) and govern the triplet photophysics.

CdSiP2 crystals are used in optical parametric oscillators to produce tunable output in the mid-infrared. As expected, the performance of the OPOs is adversely affected by residual optical absorption from native defects that are unintentionally present in the crystals. Electron paramagnetic resonance (EPR) identifies these native defects. Singly ionized silicon vacancies (VSi−) are responsible for broad optical absorption bands peaking near 800, 1033, and 1907 nm. A fourth absorption band, peaking near 630 nm, does not involve silicon vacancies. Exposure to 1064 nm light when the temperature of the CdSiP2 crystal is near 80 K converts VSi− acceptors to their neutral and doubly ionized charge states (VSi0 and VSi2−, respectively) and greatly reduces the intensities of the three absorption bands. Subsequent warming to room temperature restores the singly ionized charge state of the silicon vacancies and brings back the absorption bands. Transitions responsible for the absorption bands are identified, and a mechanism that allows 1064 nm light to remove the singly ionized charge state of the silicon vacancies is proposed.

In previous work, we have introduced an analytical approach that utilizes the dispersion relation for an infinite periodic multilayer structure to predict the performance of finite multilayer structures. We have validated the accuracy of our predictions by demonstrating numerical agreement with other established simulation methods, such as the transfer matrix method, and through experimental confirmation. In this work, we employ dispersion relations to first illustrate that metallo-dielectric structures, as opposed to multilayer dielectric-dielectric structures, can efficiently yield a sharp-edge transmittance spectrum profile, with control over both sides of the bandpass cutoff edges. Our approach also enables the calculation of effective permittivity without relying on traditional homogenization techniques. Next, utilizing the concept of effective permittivity, we illustrate that increasing the thickness of specific dielectric layers within MD structures leads to narrower passbands without significant loss in transmission, demonstrating the potential of this approach for engineering the transmittance spectrum of bandpass filters in the visible and near-IR regions. The capability to achieve a sharp-edge filter with a limited number of layers further underscores the cost-effectiveness of such bandpass filters.

Liquid crystalline (LC) materials, particularly, polymers allow generating molecular orientation patterns with submicrometer resolution. By that, materials and fabrication technology have been developed to reach half-wave (HW) retardation for visible and even infrared wavelengths thus ensuring feasibility of geometrical phase optical components that are practically 100% efficient. Such LC polymer films allow rich architectures for designing spectrally selective optical components and polarization-independent systems. Thin LC films with customized optical functions make promising basis for high throughput and high definition display systems.

Powder-based second harmonic generation measurements are a fast way to characterize semi-quantitatively non-linear properties of crystalline materials. Here, we include temperature control to study the behavior of chalcophosphates across phase transitions.

Unlike any other technology, planar optical components based on geometrical phase modulation can combine fundamentally 100% efficiency in broad spectral bandwidths and for wide range of angles. Their truly planar and smooth structure ensures no haze and provides an opportunity for anti-reflection coatings further enhancing transmission. As, essentially, diffractive waveplates, they allow integration of multiple layers for complex spectral and polarization engineering. Achieving perfection is not easy though since it requires meeting tight tolerances on a multitude of fabrication processes and materials. We will present the results of tolerance analysis for different architectures of geo-phase optics, and will discuss the performance of some of most challenging components we have been fabricating.

Zinc germanium diphosphide (ZnGeP2) is a ternary semiconductor best known for its nonlinear optical properties. A primary application is optical parametric oscillators operating in the mid-infrared region. Controlled donor doping provides a method to minimize the acceptor-related absorption bands that limit the output power of these devices. In the present study, a ZnGeP2 crystal is doped with selenium during growth. Selenium substitutes for phosphorus and serves as a deep donor. Significant concentrations of native defects (zinc vacancies, germanium-on-zinc antisites, and phosphorous vacancies) are also present in the crystal. Electron paramagnetic resonance (EPR) is used to establish the atomic-level model for the neutral charge state of the selenium donor. The S = 1/2 signal from the neutral donors is produced at 6 K by illuminating with 633 nm light (electrons excited from doubly ionized Zn vacancies convert SeP+ donors to SeP0 donors). A g matrix, with principal values of 2.088, 2.203, and 1.904, is extracted from the angular dependence of the EPR spectrum. The principal-axis direction associated with the 1.904 principal value is close to a Se–Ge bond. This indicates an asymmetric distribution of unpaired spin density around the selenium ion and thus predicts the deep donor behavior.

The dispersion relation for electromagnetic/optical wave propagation based on the Helmholtz equation for an infinite one-dimensional metallo-dielectric structure is derived using the Bloch theorem and heuristically modified to include material dispersion. We investigate the connection between the dispersion relation of an infinite metallo-dielectric structure with the transmittance characteristics of finite metallo-dielectric structures. The dispersion relation is used to determine the center wavelength and bandwidth as a function of the material properties and the thicknesses of the metal and dielectric layers. These estimates are found to be in excellent agreement with the values obtained from numerically calculated transmittance spectra using the transfer matrix method for finite metallo-dielectric structures with the same building units. The dispersion relation calculations and simulations for the transmittance are done for an ideal case where the real part for the refractive index of the metal and imaginary part for the refractive index of the dielectric are zero, and also with actual values of the real and imaginary parts of the refractive index for the metal and dielectric obtained from literature, instead of using the canonical Drude model for the metal. It is shown that the real part of the dispersion relation for the actual case is almost identical to the ideal case in the visible and NIR range, implying that essential information on the center wavelength and bandwidth can be obtained from the ideal dispersion relation. It is also found that an ideal metal and dielectric give near-unity transmittance in the passband. It is predominantly the presence of a finite real part of the refractive index of the metal that introduces attenuation. The effective refractive index of the structure can also be determined. Oscillations present in the transmittance spectrum can be explained as a Fabry–Perot effect. Approximate simple estimates of the center wavelength and bandwidth can be useful in initiating intelligent designs of finite metallo-dielectric filter structures for fabrication and characterization.

Thin-film LC based geo-phase optics have previously been demonstrated to drastically reduce size, weight, and power requirements for large-aperture optical systems while providing non-mechanical functionality for discrete values - for example, digital switching of the beam steering angle. In this work, we present a series of geo-phase Alvarez-Lohmann lens systems capable of analog tuning of focal length through lateral translation or azimuthal rotation. For circularly polarized inputs, diffraction efficiencies greater than 90% were observed over the visible spectrum with some wavelengths exceeding 99% for lateral embodiments. Both cylindrical and spherical Alvarez-Lohman systems were fabricated with focal lengths varying from infinity to 110 mm with increasing lateral displacement. These LC-based geo-phase optical systems could enable a new generation of low-cost, high-performance, and ruggedized dynamic optical components.

The directed self-assembly of liquid crystal polymers enables significantly enhanced optical properties compared to conventional isotropic optical materials. The transmissive properties of these thin polymeric films have been extensively studied, but the exploration of reflective liquid crystalline polymeric materials films has received little attention due to challenges in producing large, high-quality self-assembled films. We explore here for the first time stacked polymer cholesteric liquid crystal thin films as a pathway to generate reflective diffractive optics that act on both hands of circularly polarized light independently. We present 3 unique exemplar cases with increasing design complexity to communicate the widespread potential of these unique thin polymer films.

A structure consisting of the 2d black phosphorus (BP) film and a rugate filter (RF) divided by a liquid crystal (LC) layer is theoretically studied as a system in which Tamm plasmon-polaritons (TPP) can be excited. The spectral distribution of the reflection coefficient of the system in the RF band gap region is calculated for the cases when the band gap is in the regions of 1 and 10 THz. It is shown that, in both cases, the reflection coefficient dip associated with TPP excitation near the 2d BP film appears in the RF band gap region. The spectral position and magnitude of the reflection coefficient dip depend on the harmonic profile parameters of the RF refractive index, concentration and effective mass of the 2d BP charge carriers, and the LC layer thickness and refractive index. It was found that the TPP wavelength is a periodic function of the LC layer thickness and its refractive index, and with their increase, two or more TPP can appear at wavelengths within the RF band gap. The possibility of controlling the TPP spectral position using an external electric field is noted.

Technology of geometrical phase optics has been matured to enable customization and fabrication even in large aperture sizes within minutes enhancing development efficiency of novel optical systems. It is also the most environmentally friendly optics technology requiring only milligrams of material per square inches.

We show applications of our analytical approach to predict the performance of multilayer metallo-dielectric bandpass filters, which also enables estimation of their effective permittivity without relying on homogenization techniques. The approach is based on the one-dimensional dispersion relation for an infinite metallo-dielectric structure that accounts for the complex nature of the permittivities for the metal and dielectric constituents. The dispersion relation clearly reveals the band structure (often comprising multiple passbands), directly provides transmittance characteristics such as center wavelengths and bandwidths and enables the calculation of effective propagation constant and effective attenuation. In this work, we evaluate the dispersion relations for metallo-dielectric structures with complex refractive index data for the metal, viz., Ag, acquired from different sources to show the differences in the center wavelength and the cutoff wavelengths. We verify the accuracy of our method numerically by comparing the transmittance spectrum of finite metallo-dielectric structures using the transfer matrix method. We also plot the dispersion relation using Al as the metal and show the differences in the dispersion relations of the infinite structure and the transmittances of the finite structures relative to Ag. Extension to determination of dispersion relations for other polarizations, viz., transverse magnetic, is discussed, along with corresponding transmittance spectra for oblique incidence.

Cadmium silicon phosphide, CdSiP2 (CSP), exhibits the highest d-coefficient (d36 = 85 pm/V) among all practical nonlinear optical crystals. Its large band gap of 2.45 eV allows for 1-micron pumping with widely-available Nd- and Yb-based laser sources, and its dispersion properties are such that a 1-um pump yields non-critically phase-matched temperature-tunable output between 6.2-6.5 um (an attractive range for minimally-invasive laser surgery). However, residual 1-um absorption losses in CSP are not insignificant (0.16-0.2 cm-1). In this work we focused on identifying, and ultimately minimizing, the point defects responsible for these losses by correlating EPR spectra with polarized absorption near 1-um.

The outspread of bacterial pathogens causing severe infections and spreading rapidly, especially among hospitalized patients, is worrying and represents a global public health issue. Current disinfection techniques are becoming insufficient to counteract the spread of these pathogens because they carry multiple antibiotic-resistance genes. For this reason, a constant need exists for new technological solutions that rely on physical methods rather than chemicals. Nanotechnology support provides novel and unexplored opportunities to boost groundbreaking, next-gen solutions. With the help of plasmonic-assisted nanomaterials, we present and discuss our findings in innovative bacterial disinfection techniques. Gold nanorods (AuNRs) immobilized on rigid substrates are utilized as efficient white light-to-heat transducers (thermoplasmonic effect) for photo-thermal (PT) disinfection. The resulting AuNRs array shows a high sensitivity change in refractive index and an extraordinary capability in converting white light to heat, producing a temperature change greater than 50 °C in a few minute interval illumination time. Results were validated using a theoretical approach based on a diffusive heat transfer model. Experiments performed with a strain of Escherichia coli as a model microorganism confirm the excellent capability of the AuNRs array to reduce the bacteria viability upon white light illumination. Conversely, the E. coli cells remain viable without white light illumination, which also confirms the lack of intrinsic toxicity of the AuNRs array. The PT transduction capability of the AuNRs array is utilized to produce white light heating of medical tools used during surgical treatments, generating a temperature increase that can be controlled and is suitable for disinfection. Our findings are pioneering a new opportunity for healthcare facilities since the reported methodology allows non-hazardous disinfection of medical devices by simply employing a conventional white light lamp.

CdSiP2 (CSP) is a nonlinear optical material used for mid-infrared generation. For nonlinear optical materials, absorption bands associated with point defects often limit output power. We use electron paramagnetic resonance (EPR) to monitor paramagnetic charge states of defects. In CSP crystals, EPR shows singly ionized silicon vacancies (VSi-) initially present are eliminated by exposure to 1064 nm light. Our results suggest that 1064 nm light converts VSi- acceptors to nonparamagnetic doubly ionized (VSi2-) and neutral (VSi0) charge states. A thermal activation energy of 0.23 eV describes the recovery of the VSi- signal including at room temperature.