Session C03: Condensed Matter and Optical Physics I

How Light Makes Tiny Whirlpools in Materials

Brighter, steadier lasers have now allowed the reprogramming of materials on demand. In Floquet engineering, periodic light “dresses” electrons, creating new phenomena absent at equilibrium. Experiments have visualized light-made bands on topological insulators and induced a light-driven anomalous Hall effect in graphene [1], highlighting intensity, frequency, and polarization as key knobs. A newer lever is to shape the beam itself: vortex (twisted) light carries spin angular momentum (from circular polarization) and orbital angular momentum (from a corkscrew wavefront), adding spatial structure that can drive matter into novel, switchable states [2].

This talk presents a theory that describes the effects of irradiating the surface of a Dirac-like material with a monochromatic vortex beam. Using Floquet theory, we build a total angular-momentum operator that couples electrons and photons that is conserved only for circular polarization, enabling accurate computation of quasienergies and eigenstates. In the one-photon limit, the system maps onto an s-wave superfluid, predicting multiply quantized electronic vortices with characteristic Caroli–de Gennes–Matricon core levels; compact analytic expressions closely match full numerical simulations [3]. We then contrast these vortex-core states with topological edge states, track their evolution as polarization is tuned from circular to linear, and assess robustness to dilute impurities [4], illustrating how structuring light in time and space can write and control quantum textures for ultrafast, optically switchable electronic and topological devices.

[1] J. W. McIver, B. Schulte, F.-U. Stein, T. Matsuyama, G. Jotzu, G. Meier, and A. Cavalleri, Nat. Phys. 16, 38–41 (2020).

[2] Y. Shen, X. Wang, Z. Xie, C. Min, X. Fu, Q. Liu, M. Gong, and X. Yuan, Light Sci. Appl. 8, 90 (2019).

[3] L. I. Massaro, C. Meese, N. P. Sandler, and M. M. Asmar, Phys. Rev. B 111, 085402 (2025).

[4] T. Walsh, E. Caldwell, N. P. Sandler, and M. M. Asmar, in preparation.   

Thermo-optical Characterization of TPV selective emitters using ellipsometry.

TPV systems convert heat to electricity via photons emitted from thermal emitters with spectra matched to photovoltaic cells. Optimizing conversion efficiency requires precise determination of the complex refractive index (n, k) of emitters at elevated temperatures. This work characterized the thermo-optical properties of thermal emitters from room temperature to 600°C to support optimization of thermophotovoltaic (TPV) efficiency at ultra-high operating conditions (~1800 °C). We established accurate room-temperature optical models of aluminum nitride (AlN) ceramic and silicon carbide (4H-SiC) from UV to NIR, as well as aluminum oxide on an aluminum nitride (Al₂O₃/AlN) thin film/substrate pair in the transparent region. Spectroscopic ellipsometry and WVASE modeling yielded mean square errors (MSE) of 1.239, 4.08, and 29.89, respectively, across a 240–2500 nm spectrum. Measurements were performed while heating samples from 25 °C to 600 °C. The extracted room-temperature properties were validated against literature values. We further observed temperature-dependent trends in optical properties that will guide future TPV emitter optimization.   

Role of Functional Group in Electronic Structure and Optics of Sc2CTx MXene Electride

Two-dimensional graphene-like layered transition metal carbides, nitrides, or carbonitrides called MXenes obey the stoichiometric formula of M_n+1X_nT_x, where M is an early transition metal, X is C, N, or CN, and Tx is a functional group such as -O, -F, -OH, etc. that passivates the surface of the MXene. As the top-down wet method is the most commonly used method for synthesizing MXenes, one needs to passivate the crystal structure with functional groups such as -O, -F, and -H, bonded to the transition metal. Sc₂CT_x MXene has a unique electronic structure, characterized by anionic electrons that are unbound from any atomic nuclei within the crystal system but reside in the crystal’s periodic lattice vacancies, known as electrides. The electrons occupying those lattice vacancies wander in the interstitial spaces, which can be exploited in current technology that requires materials with low work functions and electron-emitting abilities. Among the few known MXene electrides, Sc₂CTx, a 3d transition metal MXene electride, is the one reported to exhibit semiconducting properties. We perform the first principles Density Functional Theory (DFT) calculations on pristine Sc₂C and its functionalized form. Calculation shows Sc₂C in its preferred crystal structure symmetry is metallic, while the functionalized ones are semiconducting. DFT calculations show that Sc₂CTx is a small band gap semiconductor for oxygen and fluorine terminations, but not for hydrogen termination.

This work is supported by the Department of Energy BES-RENEW award number DE-SC0024611   

Light-driven Superlattices in Dirac-like Systems

Optical driving offers fine control of matter, yet conventional approaches, such as tuning intensity, frequency, and polarization, use only part of light’s capability. Spatially structured beams add a powerful knob, imprinting spatiotemporal order. We study graphene irradiated by a monochromatic beam with a one-dimensional (1D), periodically modulated intensity. Moreover, we drive our material with circularly polarized light, which in contrast to standard linear polarization will result in an effective mass. Utilizing Floquet theory, we construct the space–time–dependent Floquet Hamiltonian and, in the high-frequency limit, derive a stroboscopic effective description with 1D spatial periodicity. We solve analytic transcendental equations to obtain the photo-dressed quasi-energy spectrum and provide a complete characterization of the emergent light-induced states. Our description reveals how spatially structured periodic drives shape the spectrum and electronic states.   

Investigation into Designing, Fabricating, and Characterizing Electrochromic Thin Films in the Visible Range.

Electrochromic thin films are optical coatings capable of reversibly modulating their transmittance, reflectance, and absorbance under an applied voltage, making them suitable for applications such as smart windows, electronic displays, and tunable optical filters. Transition metal oxides like WO₃ and NiO serve as primary electrochromic layers and are fabricated via RF magnetron sputtering, which produces uniform, dense, and chemically stable films with precisely controlled thicknesses. Multilayer thin f ilm stacks can be engineered to enhance optical performance by tailoring optical constants and exploiting interference effects. Characterization in the visible spectrum is performed using a Cary Spectrophotometer to determine optical constants and properties for low-absorbing dielectric films. For highly absorbing films, variable-angle reflectometry with a 661 nm laser is employed, and the optical constants are extracted using the matrix transfer method. By carefully controlling film thickness, complex refractive index, and multilayer design, high optical contrast, rapid switching, and long-term stability can be achieved, enabling optimized performance for diverse optical engineering applications.Electrochromic thin films are optical coatings capable of reversibly modulating their transmittance, reflectance, and absorbance under an applied voltage, making them suitable for applications such as smart windows, electronic displays, and tunable optical filters. Transition metal oxides like WO₃ and NiO serve as primary electrochromic layers and are fabricated via RF magnetron sputtering, which produces uniform, dense, and chemically stable films with precisely controlled thicknesses. Multilayer thin film stacks can be engineered to enhance optical performance by tailoring optical constants and exploiting interference effects. By carefully controlling film thickness, complex refractive index, and multilayer design, high optical contrast, rapid switching, and long-term stability can be achieved, enabling optimized performance for diverse optical engineering applications.   

X-ray spectroscopy of highly charged Ni-like and neighboring ions of Bi, Nd and Pr

The electron beam ion trap (EBIT) at the National Institute of Standards and Technology (NIST) was used to confine and probe Bi, Nd and Pr ions at different electron beam energies and densities near their Ni-like ionization potentials. Time resolved spectra were recorded by a transition edge sensor (TES) x-ray microcalorimeter array of detectors [1]. The non-Maxwellian plasma was modeled by collisional-radiative calculations to reliably predict the spectral emission of the ion cloud [2].

Aided by these collisional-radiative calculations, the analysis of the spectra has led to the identification of several new transitions in the Ni-like isoelectronic sequence, along with new lines in the Cu-like and Zn-like sequence. These identifications include forbidden magnetic octupole (M3) and electric quadrupole (E2) transitions, which can greatly affect plasma conditions [3]. By using the quantum defect formula, we identified a series of high n Rydberg transitions converging towards the ionization limit for the Ni-like ions.

References

[1] P. Szypryt, G. C. O’Neil, E. Takacs, et al. Rev. Sci. Instrum. 90 (2019) 23107

[2] Y. Ralchenko and Y. Maron, J. Quant. Spec. and Rad. Trans. 71 (2001) 609

[3] T. Burke , E. Takacs, Dipti, A. Hosier, et. al. Eur. Phys. J. D (2024) 78