Searcharxiv⌕ Search

arXiv subjects

Thomas E. Murphy

Publications and source records attributed to Thomas E. Murphy.

At least 19 recordsLinked to original sources
arXiv2025

Ultrasensitive Polarization-Resolved Probing of Transient Dynamics in MoS$_2$ on Silicon Nitride Microresonators

We present an ultrasensitive technique for probing transient optical changes in atomically thin molybdenum disulfide (MoS$_2$) layers integrated onto silicon nitride (Si$_3$N$_4$) ring resonators. The MoS$_2$ is illuminated by a femtosecond laser, while a tunable near-infrared (NIR) continuous-wave laser probes the microresonator resonance. The NIR light polarization can be adjusted to either transverse electric (TE, parallel to the 2D material) or transverse magnetic (TM, perpendicular), a configuration that is impossible to achieve with conventional normal-incidence pump-probe techniques. By capturing the transmitted signal on a fast oscilloscope, we detect transient optical shifts with unprecedented sensitivity, observing phenomena over time scales ranging from picoseconds to microseconds. Our results reveal both a rapid, carrier-induced nonlinear optical shift in the resonance, and a slower thermo-optic transient. The ability to simultaneously measure these fast and slow dynamics offers new insight into the complex optoelectronic behavior of 2D materials when integrated with microresonators. This method provides a significant advance over traditional pump-probe approaches, enabling the detection of exceedingly small transient signals and opening new avenues for exploring the optical properties of atomically thin materials. Our findings highlight the potential of this approach for investigating polarization-dependent nonlinear effects, with applications in photonics, sensing, and optoelectronics.

physics.optics↗
arXiv2024

Terahertz Antenna Impedance Matched to a Graphene Photodetector

Developing low-power, high-sensitivity photodetectors for the terahertz (THz) band that operate at room temperature is an important challenge in optoelectronics. In this study, we introduce a photo-thermal-electric (PTE) effect detector based on quasi-free standing bilayer graphene (BLG) on a silicon carbide (SiC) substrate, designed for the THz frequency range. Our detector's performance hinges on a quasi-optical coupling scheme, which integrates an aspherical silicon lens, to optimize impedance matching between the THz antenna and the graphene p-n junction. At room temperature, we achieved a noise equivalent power (NEP) of less than 300 $pW/\sqrt{Hz}$. Through an impedance matching analysis, we coupled a planar antenna with a graphene p-n junction, inserted in parallel to the nano-gap of the antenna, via two coupling capacitors. By adjusting the capacitors and the antenna arm length, we tailored the antenna's maximum infrared power absorption to specific frequencies. The sensitivity, spectral properties, and scalability of our material make it an ideal candidate for future development of far-infrared detectors operating at room temperature.

physics.ins-det↗
arXiv2023

Strong transient magnetic fields induced by THz-driven plasmons in graphene disks

Strong circularly polarized excitation opens up the possibility to generate and control effective magnetic fields in solid state systems, e.g., via the optical inverse Faraday effect or the phonon inverse Faraday effect. While these effects rely on material properties that can be tailored only to a limited degree, plasmonic resonances can be fully controlled by choosing proper dimensions and carrier concentrations. Plasmon resonances provide new degrees of freedom that can be used to tune or enhance the light-induced magnetic field in engineered metamaterials. Here we employ graphene disks to demonstrate light-induced transient magnetic fields from a plasmonic circular current with extremely high efficiency. The effective magnetic field at the plasmon resonance frequency of the graphene disks (3.5 THz) is evidenced by a strong (~1°) ultrafast Faraday rotation (~ 20 ps). In accordance with reference measurements and simulations, we estimated the strength of the induced magnetic field to be on the order of 0.7 T under a moderate pump fluence of about 440 nJ cm-2.

cond-mat.mes-hall↗
arXiv2022

Non-local polarization alignment and control in fiber using feedback from correlated measurements of entangled photons

Quantum measurements that use the entangled photons' polarization to encode quantum information require calibration and alignment of the measurement bases between spatially separate observers. Because of the changing birefringence in optical fibers arising from temperature fluctuations or external mechanical vibrations, the polarization state at the end of a fiber channel is unpredictable and time-varying. Polarization tracking and stabilization methods originally developed for classical optical communications cannot be applied to polarization-entangled photons, where the separately detected photons are statistically unpolarized, yet quantum mechanically correlated. We report here a fast method for automatic alignment and dynamic tracking of the polarization measurement bases between spatially separated detectors. The system uses the Nelder-Mead simplex method to minimize the observed coincidence rate between non-locally measured entangled photon pairs, without relying on classical wavelength-multiplexed pilot tones or temporally interleaved polarized photons. Alignment and control is demonstrated in a 7.1 km deployed fiber loop as well as in a controlled drifting scenario.

quant-ph↗
arXiv2022

Dark Current and Single Photon Detection by 1550nm Avalanche Photodiodes: Dead Time Corrected Probability Distributions and Entropy Rates

Single photon detectors have dark count rates that depend strongly on the bias level for detector operation. In the case of weak light sources such as novel lasers or single-photon emitters, the rate of counts due to the light source can be comparable to that of the detector dark counts. In such cases, a characterization of the statistical properties of the dark counts is necessary. The dark counts are often assumed to follow a Poisson process that is statistically independent of the incident photon counts. This assumption must be validated for specific types of photodetectors. In this work, we focus on single-photon avalanche photodiodes (SPADs) made for 1550nm. For the InGaAs detectors used, we find the measured distributions often differ significantly from Poisson due to the presence of dead time and afterpulsing with the difference increasing with the bias level. When the dead time is increased to remove the effects of afterpulsing, it is necessary to correct the measured distributions for the effects of the dead time. To this end, we apply an iterative algorithm to remove dead time effects from the probability distribution for dark counts as well as for the case where light from an external weak laser source (known to be Poisson) is detected together with the dark counts. We believe this to be the first instance of the comprehensive application of this algorithm to real data and find that the corrected probability distributions are Poisson distributions in both cases. We additionally use the Grassberger-Procaccia algorithm to estimate the entropy production rates of the dark count processes, which provides a single metric that characterizes the temporal correlations between dark counts as well as the shape of the distribution. We have thus developed a systematic procedure for taking data with 1550nm SPADs and obtaining accurate photocount statistics to examine novel light sources.

physics.optics↗
arXiv2020

Spatiotemporal Characterization of Nonlinear Interactions between Selectively Excited Radially Symmetric Modes of a Few-Mode Fiber

Nonlinear propagation of signals in single-mode fiber is well understood, and is typically observed by measuring the temporal profile or optical spectrum of an emerging signal. In multimode fibers, the nonlinearity has both a spatial and a temporal element, and a complete investigation of the interactions between propagating modes requires resolving the output in both space and time. We report here spatiotemporal measurements of two nonlinear interacting LP$_{0m}$ modes of a step-index few-mode fiber (FMF). We describe a method to selectively excite two propagating modes through the use of a phase-mask directly patterned on the entrance face of the fiber. The output is resolved by raster-scanning a near-field tapered single-mode optical fiber probe that is connected to a high-speed detector. The results show that in the presence of nonlinearity, the output exhibits a spatiotemporal character that cannot be adequately characterized by a camera image or pulse shape alone.

physics.optics↗
arXiv2020

Control of hot-carrier relaxation time in Au-Ag thin films through alloying

The plasmon resonance of a structure is primarily dictated by its optical properties and geometry, which can be modified to enable hot-carrier photodetectors with superior performance. Recently, metal-alloys have played a prominent role in tuning the resonance of plasmonic structures through chemical composition engineering. However, it has been unclear how alloying modifies the time dynamics of generated hot-carriers. In this work, we elucidate the role of chemical composition on the relaxation time of hot-carriers for the archetypal Aux Ag1-x thin-film system. Through time-resolved optical spectroscopy measurements in the visible wavelength range, we measure composition-dependent relaxation times that vary up to 8x for constant pump fluency. Surprisingly, we find that the addition of 2% of Ag into Au films can increase the hot carrier lifetime by approximately 35% under fixed fluence, as a result of a decrease in optical loss. Further, the relaxation time is found to be inversely proportional to the imaginary part of the permittivity. Our results indicate that alloying is a promising approach to effectively control hot-carrier relaxation time in metals.

physics.optics↗
arXiv2019

Guiding and confining of light in a two-dimensional synthetic space using electric fields

Synthetic dimensions provide a promising platform for photonic quantum simulations. Manipulating the flow of photons in these dimensions requires an electric field. However, photons do not have charge and do not directly interact with electric fields. Therefore, alternative approaches are needed to realize electric fields in photonics. One approach is to use engineered gauge fields that can mimic the effect of electric fields and produce the same dynamical behavior. Here, we demonstrate such an electric field for photons propagating in a two-dimensional synthetic space. We achieve this using a linearly time-varying gauge field generated by direction-dependent phase modulations. We show that the generated electric field leads to Bloch oscillations and the revival of the state after a certain number of steps dependent on the field strength. We measure the probability of the revival and demonstrate good agreement between the observed values and the theoretically predicted results. Furthermore, by applying a nonuniform electric field, we show the possibility of waveguiding photons. Ultimately, our results open up new opportunities for manipulating the propagation of photons with potential applications in photonic quantum simulations.

physics.optics↗
arXiv2019

Surface Plasmon Assisted Control of Hot-Electron Relaxation Time

Surface plasmon mediated hot carrier generation is widely utilized for the manipulation of the electron-photon interactions in many types of optoelectronic devices including solar cells, photodiodes, and optical modulators. A diversity of plasmonic systems such as nanoparticles, resonators, and waveguides have been introduced to enhance hot carrier generation; however, the impact of the propagating surface plasmons on hot carrier lifetime has not been clearly demonstrated. Here, we systematically study the hot carrier relaxation in thin film gold (Au) samples under surface plasmon coupling with the Kretschmann configuration. We observe that the locally confined electric field at the surface of the metal significantly affects the hot carrier distribution and electron temperature, which results in a slowing of the hot electrons relaxation time, regardless of the average value of the absorbed power in the Au thin film. This result could be extended to other plasmonic nanostructures, enabling the control of hot carrier lifetimes throughout the optical frequency range.

physics.optics↗
arXiv2019

Effect of defect-induced cooling on graphene hot-electron bolometers

At high phonon temperature, defect-mediated electron-phonon collisions (supercollisions) in graphene allow for larger energy transfer and faster cooling of hot electrons than the normal, momentum-conserving electron-phonon collisions. Disorder also affects the heat flow between electrons and phonons at very low phonon temperature, where the phonon wavelength exceeds the mean free path. In both cases, the cooling rate is predicted to exhibit a characteristic cubic power law dependence on the electron temperature, markedly different from the T^4 dependence predicted for pristine graphene. The impact of defect-induced cooling on the performance of optoelectronic devices is still largely unexplored. Here we study the cooling mechanism of hot-electron bolometers based on epitaxial graphene quantum dots where the defect density can be controlled with the fabrication process. The devices with high defect density exhibit the cubic power law. Defect-induced cooling yields a slower increase of the thermal conductance with increasing temperature, thereby greatly enhancing the device responsivity compared to devices with lower defect density and operating with normal-collision cooling.

cond-mat.mes-hall↗
arXiv2019

Optical Control of Chiral Charge Pumping in a Topological Weyl Semimetal

Solids with topologically robust electronic states exhibit unusual electronic and optical transport properties that do not exist in other materials. A particularly interesting example is chiral charge pumping, the so-called chiral anomaly, in recently discovered topological Weyl semimetals, where simultaneous application of parallel DC electric and magnetic fields creates an imbalance in the number of carriers of opposite topological charge (chirality). Here, using time-resolved terahertz measurements on the Weyl semimetal TaAs in a magnetic field, we optically interrogate the chiral anomaly by dynamically pumping the chiral charges and monitoring their subsequent relaxation. Theory based on Boltzmann transport shows that the observed effects originate from an optical nonlinearity in the chiral charge pumping process. Our measurements reveal that the chiral population relaxation time is much greater than 1 ns. The observation of terahertz-controlled chiral carriers with long coherence times and topological protection suggests the application of Weyl semimetals for quantum optoelectronic technology.

cond-mat.mes-hall↗
arXiv2019

A synthetic gauge field for two-dimensional time-multiplexed quantum random walks

Temporal multiplexing provides an efficient and scalable approach to realize a quantum random walk with photons that can exhibit topological properties. But two dimensional time-multiplexed topological quantum walks studied so far have relied on generalizations of the Su-Shreiffer-Heeger (SSH) model with no synthetic gauge field. In this work, we demonstrate a 2D topological quantum random walk where the non-trivial topology is due to the presence of a synthetic gauge field. We show that the synthetic gauge field leads to the appearance of multiple bandgaps and consequently, a spatial confinement of the random walk distribution. Moreover, we demonstrate topological edge states at an interface between domains with opposite synthetic fields. Our results expand the range of Hamiltonians that can be simulated using photonic random walks.

physics.optics↗
arXiv2018

Delayed Dynamical Systems: Networks, Chimeras and Reservoir Computing

We present a systematic approach to reveal the correspondence between time delay dynamics and networks of coupled oscillators. After early demonstrations of the usefulness of spatio-temporal representations of time-delay system dynamics, extensive research on optoelectronic feedback loops has revealed their immense potential for realizing complex system dynamics such as chimeras in rings of coupled oscillators and applications to reservoir computing. Delayed dynamical systems have been enriched in recent years through the application of digital signal processing techniques. Very recently, we have showed that one can significantly extend the capabilities and implement networks with arbitrary topologies through the use of field programmable gate arrays (FPGAs). This architecture allows the design of appropriate filters and multiple time delays which greatly extend the possibilities for exploring synchronization patterns in arbitrary topological networks. This has enabled us to explore complex dynamics on networks with nodes that can be perfectly identical, introduce parameter heterogeneities and multiple time delays, as well as change network topologies to control the formation and evolution of patterns of synchrony.

nlin.AO↗
arXiv2017

Experiments with arbitrary networks in time-multiplexed delay systems

We report a new experimental approach using an optoelectronic feedback loop to investigate the dynamics of oscillators coupled on large complex networks with arbitrary topology. Our implementation is based on a single optoelectronic feedback loop with time delays. We use the space-time interpretation of systems with time delay to create large networks of coupled maps. Others have performed similar experiments using high-pass filters to implement the coupling; this restricts the network topology to the coupling of only a few nearest neighbors. In our experiment, the time delays and coupling are implemented on a field-programmable gate array, allowing the creation of networks with arbitrary coupling topology. This system has many advantages: the network nodes are truly identical, the network is easily reconfigurable, and the network dynamics occur at high speeds. We use this system to study cluster synchronization and chimera states in both small and large networks of different topologies.

nlin.CD↗
arXiv2017

Ultra-broadband photodetectors based on epitaxial graphene quantum dots

Graphene is an ideal material for hot-electron bolometers, due to its low heat capacity and weak electron-phonon coupling. Nanostructuring graphene with quantum dot constrictions yields detectors with extraordinarily high intrinsic responsivity, higher than 1x10^9 V/W at 3K. The sensing mechanism is bolometric in nature: the quantum confinement gap causes a strong dependence of the electrical resistance on the electron temperature. Here we show that this quantum confinement gap does not impose a limitation on the photon energy for light detection and these quantum dot bolometers work in a very broad spectral range, from terahertz, through telecom to ultraviolet radiation, with responsivity independent of wavelength. We also measure the power dependence of the response. Although the responsivity decreases with increasing power, it stays higher than 1x10^8 V/W in a wide range of absorbed power, from 1 pW to 0.4 nW.

cond-mat.mes-hall↗
arXiv2017

Recommendations and illustrations for the evaluation of photonic random number generators

The never-ending quest to improve the security of digital information combined with recent improvements in hardware technology has caused the field of random number generation to undergo a fundamental shift from relying solely on pseudo-random algorithms to employing optical entropy sources. Despite these significant advances on the hardware side, commonly used statistical measures and evaluation practices remain ill-suited to understand or quantify the optical entropy that underlies physical random number generation. We review the state of the art in the evaluation of optical random number generation and recommend a new paradigm: quantifying entropy generation and understanding the physical limits of the optical sources of randomness. In order to do this, we advocate for the separation of the physical entropy source from deterministic post-processing in the evaluation of random number generators and for the explicit consideration of the impact of the measurement and digitization process on the rate of entropy production. We present the Cohen-Procaccia estimate of the entropy rate $h(ε,τ)$ as one way to do this. In order to provide an illustration of our recommendations, we apply the Cohen-Procaccia estimate as well as the entropy estimates from the new NIST draft standards for physical random number generators to evaluate and compare three common optical entropy sources: single photon time-of-arrival detection, chaotic lasers, and amplified spontaneous emission.

physics.optics↗
arXiv2016

Tunable Ultrafast Thermal Relaxation in Graphene Measured by Continuous-Wave Photomixing

Hot electron effects in graphene are significant because of graphene's small electronic heat capacity and weak electron-phonon coupling, yet the dynamics and cooling mechanisms of hot electrons in graphene are not completely understood. We describe a novel photocurrent spectroscopy method that uses the mixing of continuous-wave lasers in a graphene photothermal detector to measure the frequency dependence and nonlinearity of hot-electron cooling in graphene as a function of the carrier concentration and temperature. The method offers unparalleled sensitivity to the nonlinearity, and probes the ultrafast cooling of hot carriers with an optical fluence that is orders of magnitude smaller than in conventional time-domain methods, allowing for accurate characterization of electron-phonon cooling near charge neutrality. Our measurements reveal that near the charge neutral-point the nonlinear power dependence of the electron cooling is dominated by disorder-assisted collisions, while at higher carrier concentrations conventional momentum-conserving cooling prevails in the nonlinear dependence. The relative contribution of these competing mechanisms can be electrostatically tuned through the application of a gate voltage -- an effect that is unique to graphene.

cond-mat.mes-hall↗