arXiv subjects

Matthew W. Day

Publications and source records attributed to Matthew W. Day.

13 recordsLinked to original sources

arXiv2026

Finite-momentum Cooper plasmons in superconducting terahertz microcavities

The phase mode of a superconductor's order parameter encodes fundamental information about pairing and dissipation, but is typically inaccessible at low frequencies due to the Anderson-Higgs mechanism. Superconducting samples thinner than the London penetration depth, however, support a gapless phase mode whose dispersion can be reshaped by a proximal screening layer. Here, we theoretically and experimentally show that this screened phase mode in a superconducting thin film integrated into on-chip terahertz circuitry naturally forms a superconducting microcavity that hosts resonant finite-momentum standing waves of supercurrent density, which we term Cooper plasmons. We measure two Cooper plasmons in a superconducting NbN microcavity and demonstrate that their resonance frequencies and linewidths independently report the density of participating carriers and plasmon's dissipation at finite momenta. Our results reveal an emergent collective mode of an integrated superconductor-circuit system and establish design principles for engineering or suppressing Cooper plasmons in superconducting terahertz devices and circuits.

cond-mat.supr-con↗

arXiv2026

Response regimes in on-chip THz spectroscopy

On-chip THz spectroscopy enables quantitative measurements of the optical conductivity of sub-wavelength 2D materials by tightly confining THz fields in metallic transmission line structures interfaced to the material. However, because the probed structures are smaller than the THz wavelength, finite-size and environmental effects can strongly influence the measured response. Here, we identify the conditions under which a metallic sample exhibits a genuine Drude response and when finite-size and environmental effects must be considered. We further introduce and characterize an additional regime, the Phantom-Drude response, which mimics Drude behavior but instead originates from the superposition of multiple finite-momentum plasmonic resonances. If unrecognized, this regime can lead to misinterpretation of intrinsic material properties. We systematically show how the Phantom-Drude response can emerge and demonstrate its sensitivity to sample dimensions, transmission line geometry, material shape, and gate properties, providing practical guidelines to avoid this regime in future on-chip THz measurements.

cond-mat.mes-hall↗

arXiv2025

Monolithic optoelectronic circuit design for on-chip terahertz applications

We demonstrate a monolithic coplanar stripline platform for on-chip terahertz (THz) generation, transmission, and detection, addressing key challenges of mode purity, bandwidth, and referencing. Capacitive coupling of the photoconductive generator switch enforces pure odd-mode propagation, increases THz field strength, and extends the operational frequency range, achieving 0.05-1.4 THz. Our architecture enables fully monolithic fabrication with amorphous silicon switches, in situ field referencing, and galvanic isolation between generation and detection. Finite-element simulations and experiments confirm that suppressing parasitic modes improves signal integrity, providing a robust platform for high-fidelity THz spectroscopy, ultrafast electronics, and nanoscale quantum materials research.

physics.optics↗

arXiv2025

Indirect Excitons and Many-body Interactions in InGaAs Double Quantum Wells

Spatially indirect excitons in semiconductor quantum wells are relevant to basic research and device applications because they exhibit enhanced tunability, delocalized wave functions, and potentially longer lifetimes relative to direct excitons. Here we investigate the properties of indirect excitons and their coupling interactions with direct excitons in asymmetric InGaAs double quantum wells using optical multidimensional coherent spectroscopy and photoluminescence excitation spectroscopy. Analyses of the spectra confirm a strong influence of many-body effects, and reveal that excited-state zero-quantum coherences between direct and indirect excitons in the quantum wells dephase faster than the much higher-energy single-quantum coherences between excitonic excited states and ground states. The results also suggest an important energy-dependent role of continuum states in mediating system dynamics, and they indicate that dephasing mechanisms are associated with uncorrelated or anticorrelated energy-level fluctuations.

cond-mat.mes-hall↗

arXiv2025

Purcell enhancement of photogalvanic currents in a van der Waals plasmonic self-cavity

Cavities provide a means to manipulate the optical and electronic responses of quantum materials by selectively enhancing light-matter interaction at specific frequencies and momenta. While cavities typically involve external structures, exfoliated flakes of van der Waals (vdW) materials can form intrinsic self-cavities due to their small finite dimensions, confining electromagnetic fields into plasmonic cavity modes, characterized by standing-wave current distributions. While cavity-enhanced phenomena are well-studied at optical frequencies, the impact of self-cavities on nonlinear electronic responses--such as photogalvanic currents--remains largely unexplored, particularly in the terahertz regime, critical for emerging ultrafast optoelectronic technologies. Here, we report a self-cavity-induced Purcell enhancement of photogalvanic currents in the vdW semimetal WTe$_2$. Using ultrafast optoelectronic circuitry, we measured coherent near-field THz emission resulting from nonlinear photocurrents excited at the sample edges. We observed enhanced emission at finite frequencies, tunable via excitation fluence and sample geometry, which we attribute to plasmonic interference effects controlled by the cavity boundaries. We developed an analytical theory that captures the cavity resonance conditions and spectral response across multiple devices. Our findings establish WTe$_2$ as a bias-free, geometry-tunable THz emitter and demonstrate the potential of self-cavity engineering for controlling nonlinear, nonequilibrium dynamics in quantum materials.

cond-mat.mes-hall↗

arXiv2025

Resolving self-cavity effects in two-dimensional quantum materials

Two-dimensional materials and van der Waals (vdW) heterostructures host many strongly correlated and topological quantum phases on the $\sim$ meV energy scale. Direct electrodynamical signatures of such states are thus expected to appear in the terahertz (THz) frequency range (1 THz $\sim$ 4 meV). Because the typical size of vdW heterostructures ($\sim$10 $μm$) is much smaller than the diffraction limit of THz light, probing THz optical conductivities necessitates the use of near-field optical probes. However, interpreting the response of such near-field probes is complicated by finite-size effects, the presence of electrostatic gates, and the influence of the probe itself on material dynamics -- all of which conspire to form polaritonic self-cavities, in which interactions between THz electromagnetic fields and material excitations form discretized standing waves. In this paper, we demonstrate the relevance of self-cavity effects in 2D materials and derive an analytical framework to resolve these effects using the emerging experimental technique of time-domain on-chip THz spectroscopy. We show that by pairing experiments with the analytical theory, it is possible to extract the THz conductivity and resolve collective mode dynamics far outside the light cone, with $\sim μm$ in-plane and $\sim nm$ out-of-plane resolution. This study lays the groundwork for studying quantum phases and cavity effects in vdW heterostructures and 2D quantum materials.

cond-mat.str-el↗

arXiv2025

Probing Complex Decoherence Processes in Materials for Quantum Applications

The primary consideration in developing new material platforms for quantum applications is to optimize coherence. Despite its importance, decoherence processes remains challenging to experimentally interrogate and quantify. In this Perspective, we first introduce the concept of decoherence in quantum systems and conventional techniques to assess decoherence at optical frequencies. We then introduce multidimensional coherent spectroscopy as a unique probe capable of revealing the full complexity of decoherence dynamics in realistic circumstances. To contextualize the techniques discussed here, demonstrative examples in two prototypical quantum systems, namely colloidal nanocrystals and vacancy centers in diamond, are provided.

quant-ph↗

arXiv2024

Nonperturbative Nonlinear Transport in a Floquet-Weyl Semimetal

Periodic laser driving, known as Floquet engineering, is a powerful tool to manipulate the properties of quantum materials. Using circularly polarized light, artificial magnetic fields, called Berry curvature, can be created in the photon-dressed Floquet-Bloch states that form. This mechanism, when applied to 3D Dirac and Weyl systems, is predicted to lead to photon-dressed movement of Weyl nodes which should be detectable in the transport sector. The transport response of such a topological light-matter hybrid, however, remains experimentally unknown. Here, we report on the transport properties of the type-II Weyl semimetal T$\mathrm{_d}$-MoTe$_\mathrm{2}$ illuminated by a femtosecond pulse of circularly polarized light. Using an ultrafast optoelectronic device architecture, we observed injection currents and a helicity-dependent anomalous Hall effect whose scaling with laser field strongly deviate from the perturbative laws of nonlinear optics. We show using Floquet theory that this discovery corresponds to the formation of a magnetic Floquet-Weyl semimetal state. Numerical ab initio simulations support this interpretation, indicating that the light-induced motion of the Weyl nodes contributes substantially to the measured transport signals. This work demonstrates the ability to generate large effective magnetic fields ($>$ 30T) with light, which can be used to manipulate the magnetic and topological properties of a range of quantum materials.

cond-mat.mes-hall↗

arXiv2021

Hidden Silicon-Vacancy Centers in Diamond

We characterize a high-density sample of negatively charged silicon-vacancy (SiV$^-$) centers in diamond using collinear optical multidimensional coherent spectroscopy. By comparing the results of complementary signal detection schemes, we identify a hidden population of \ce{SiV^-} centers that is not typically observed in photoluminescence, and which exhibits significant spectral inhomogeneity and extended electronic $T_2$ times. The phenomenon is likely caused by strain, indicating a potential mechanism for controlling electric coherence in color-center-based quantum devices.

quant-ph↗

arXiv2021

Coherent Interactions Between Silicon-Vacancy Centers in Diamond

We report coherent interactions within an ensemble of silicon-vacancy color centers in diamond. The interactions are ascribed to resonant dipole-dipole coupling. Further, we demonstrate control over resonant center pairs by using a driving optical pulse to induce collective, interaction-enabled Rabi-oscillations in the ensemble. Non-resonant center pairs do not undergo collective oscillations.

quant-ph↗

arXiv2021

Using silicon-vacancy centers in diamond to probe the full strain tensor

An ensemble of silicon vacancy centers in diamond (\ce{SiV-}) is probed using two coherent spectroscopy techniques. Two main distinct families of \ce{SiV-} centers are identified using multidimensional coherent spectroscopy, and these families are paired with two orientation groups by comparing spectra from different linear polarizations of the incident laser. By tracking the peak centers in the measured spectra, the full diamond strain tensor is calculated local to the laser spot. Such measurements are made at multiple points on the sample surface and variations in the strain tensor are observed.

cond-mat.mtrl-sci↗

arXiv2020

Radio frequency polarization modulation based on an optical frequency comb

We propose a method to generate stabilized radio-frequency polarization modulation based on optical frequency combs. Two pulse trains with the same repetition rate and different offset frequencies generate arbitrary polarization states that are modulated at the offset frequency difference. Long-term stability of the polarization modulation is demonstrated with the modulation frequency at frep/2. Modulation at frep/4 is also demonstrated to show the flexibility of the technique. We employ an electrical delay line to fine-tune the polarization states that constitute the time-dependent modulation.

physics.optics↗

arXiv2020

Quantum-Well Laser Diodes for Frequency Comb Spectroscopy

We demonstrate simple optical frequency combs based on semiconductor quantum well laser diodes. The frequency comb spectrum can be tailored by choice of material properties and quantum-well widths, providing spectral flexibility. Finally, we demonstrate the mutual coherence of these devices by using two frequency combs on the same device to generate a radio-frequency dual comb spectrum.

physics.optics↗