arXiv subjects

J. J. Finley

Publications and source records attributed to J. J. Finley.

At least 19 recordsLinked to original sources

arXiv2026

Lasing from SOI-integrated GaAsSb nanowires via resonator-driven optical feedback

Silicon photonic integrated circuits critically depend on compact on-chip light sources, for which nanowire (NW) lasers are an attractive solution. However, their practical implementation is often limited by broad emission linewidths and poor frequency stability resulting from weak optical feedback. Here, we integrate individual GaAsSb NWs by transfer-printing onto silicon-on-insulator (SOI) racetrack resonators to realize optical feedback at silicon-transparent wavelengths. Finite-difference-time-domain simulations reveal efficient coupling between the hybrid NW-waveguide mode and the fundamental TE resonator mode, with calculated cavity Q-factors exceeding 10$^4$. Experimentally, we observe feedback-induced lasing emission at a low threshold (P$_{th}$) of 8.6 $\pm$ 1.8 $μ$J/cm$^2$. Compared to identical NW lasers without SOI resonator, the linewidth is reduced by more than a factor of four at 3P$_{th}$ and remains stable below 1.8 meV up to 5P$_{th}$. Our results demonstrate NW-based light sources on SOI and show that tailored resonator designs enable improved linewidth control and frequency stabilization.

physics.app-ph↗

arXiv2026

Ultra-high-vacuum cluster tool for epitaxial synthesis and optical spectroscopy of reactive 2D materials

The large-area synthesis of high-crystalline-quality two-dimensional (2D) materials is at the core of novel material integration for semiconductor technology. This effort relies on developing fabrication and characterization techniques that can uncover the material's intrinsic properties by preserving its pristine conditions. In this article, we present an all ultra-high-vacuum cluster for the growth using molecular beam epitaxy of 2D semiconductors that are unstable under ambient conditions and optical spectroscopy using low temperature (20 K) photoluminescence and Raman scattering. The optical chamber of the setup provides micrometer scale spatial resolution and the ability to scan the entire wafer. The performance of its setup regarding spatial resolution, temperature control over a temperature range of 20-300 K using a closed-cycle cryostat and long-term preservation are demonstrated using as-grown post-transition metal monochalcogenides. Furthermore, we introduce a deconvolution-based algorithm to recover spatial information under vibration using a system-specific point-spread function. This enables in situ analysis of the structural and optoelectronic properties of as-grown materials in their pristine form, providing rich and reproducible feedback for both fundamental studies and the optimization of scalable 2D material growth toward integration in advanced devices.

cond-mat.mtrl-sci↗

arXiv2026

Electrically tunable orbital coupling and quantum light emission from O-band quantum dot molecules

We present the observation of electrically tunable quantum coupling of orbital states in individual InAs/InGaAs quantum dot molecules emitting in the telecom O-band (~1300 nm). By tuning the static electric field along the growth axis of the QD-molecule, we observe pronounced anticrossings between excitonic transitions and determine the dependence of the interdot electron tunnel coupling on the interdot separation. As the electric field applied along the growth axis of the QD-molecules increases, positively charged exciton complexes sequentially emerge in the time-integrated emission spectra due to electron escape from the system while holes remain trapped. Moreover, for strong pumping, biexciton emission from the O-band molecules is identified. We demonstrate single-photon emission from the InAs/InGaAs QD-molecule emitting around 1300 nm with a g(2)(0) = 0.017(2) and explore the impact of tuning orbital coupling on the second-order correlation function.

cond-mat.mes-hall↗

arXiv2026

Electrically and optically active charge carrier traps in silicon-doped few-layer GaSe

Understanding defects in atomically thin van der Waals (vdW) semiconductors is essential for advancing their use in next-generation optoelectronic and photovoltaic devices. Here, we apply a combination of various impedance spectroscopy techniques to two-dimensional (2D) vdW GaSe doped with silicon (Si) to reconstruct deep trap states across the full bandgap. Deep-level transient spectroscopy reveals three distinct deep states 0.31, 0.88, and 1.40 eV below the conduction band edge. Complementary deep-level optical spectroscopy and photocapacitance measurements identify three deep states at 1.4 and 1.8 eV below the conduction band edge, and 2.0 eV above the valence band edge, with thermal admittance spectroscopy providing additional verification and further resolving two trap states, at 0.16 eV above the valence band edge and at 0.26 eV below the conduction band edge. By comparing the experimentally extracted ionization energies with the predictions of density functional theory, our results attribute these trap states primarily to Si-related defects and metal vacancies. This work presents a comprehensive defect map of Si-doped GaSe, providing critical insights into carrier trapping mechanisms that are essential for optimizing the design of 2D material-based devices for industrial applications.

cond-mat.mtrl-sci↗

arXiv2025

Unveiling the growth mode diagram of GaSe on sapphire

The growth of two-dimensional epitaxial materials on industrially relevant substrates is critical for enabling their scalable synthesis and integration into next-generation technologies. Here we present a comprehensive study of the molecular beam epitaxial growth of gallium selenide on 2-inch c-plane sapphire substrates. Using in-situ reflection high-energy electron diffraction (RHEED), in-situ Raman spectroscopy, optical and scanning electron microscopies, we construct a diagram of the gallium selenide growth modes as a function of substrate temperature (530-650 °C) and Se/Ga flux ratio (5-110). The growth mode diagram reveals distinct regimes, including the growth of layered post-transition metal monochalcogenide GaSe with an unstrained in-plane lattice constant of 0.371$\pm$0.001 nm and a partial epitaxial alignment on sapphire. This work demonstrates a RHEED-based pathway for synthesizing gallium selenide of specific phase and morphology, and the construction of a phase diagram for high vapor pressure III-VI compounds that can be applied to a wide range of other metal chalcogenide materials.

cond-mat.mtrl-sci↗

arXiv2025

Magnetic brightening of light-like excitons in a monolayer semiconductor

Monolayer transition-metal dichalcogenides, such as WSe$_2$, are direct gap, multi-valley semiconductors. Long-range electron-hole exchange interactions mix the valleys, yielding dispersion relations for massive ($\propto Q^2$) as well as light-like ($\propto Q$) excitons. We report magneto-photoluminescence spectroscopy of excitons in the monolayer semiconductor WSe$_2$ to $B = \pm25$T. The magnetic field-dependent line shape of the neutral exciton reveals the emergence of a new blue-detuned emission peak in both field orientations. Analyzing the distinct magnetic field-dependent shifts of both peaks facilitates the identification of the emergent feature as a spin-singlet with a significantly smaller reduced exciton mass as compared to the neutral exciton. The intensity of the emergent feature increases with magnetic field according to $\propto B^2$, as expected for a linear dispersion relation. The density-dependent diamagnetic shift ratios of both features follow the expected density dependence of the electron-hole exchange interactions. We interpret our observations within a picture of magnetic-field-induced coupling between the bright massive and quasi dark light-like exciton, leading to its brightening.

cond-mat.mes-hall↗

arXiv2023

Optimizing the growth conditions of Al mirrors for superconducting nanowire single-photon detectors

We investigate the growth conditions for thin (less than 200 nm) sputtered aluminum (Al) films. These coatings are needed for various applications, e.g. for advanced manufacturing processes in the aerospace industry or for nanostructures for quantum devices. Obtaining high-quality films, with low roughness, requires precise optimization of the deposition process. To this end, we tune various sputtering parameters such as the deposition rate, temperature, and power, which enables 50 nm thin films with a root mean square (RMS) roughness of less than 1 nm and high reflectivity. Finally, we confirm the high quality of the deposited films by realizing superconducting single-photon detectors integrated into multi-layer heterostructures consisting of an aluminum mirror and a silicon dioxide dielectric spacer. We achieve an improvement in detection efficiency at 780 nm from 40 % to 70 % by this integration approach.

physics.optics↗

arXiv2022

The dependence of timing jitter of superconducting nanowire single-photon detectors on the multi-layer sample design and slew rate

We investigated the timing jitter of superconducting nanowire single-photon detectors (SNSPDs) and found a strong dependence on the detector response. By varying the multi-layer structure, we observed changes in pulse shape which are attributed to capacitive behaviour affecting the pulse heights, rise times and consequently timing jitter. Moreover, we developed a technique to predict the timing jitter of a single device within certain limits by capturing only a single detector pulse, eliminating the need for detailed jitter measurement using a pulsed laser when a rough estimate of the timing jitter is sufficient.

cond-mat.mes-hall↗

arXiv2022

Moiré straintronics: a universal platform for reconfigurable quantum materials

Large scale two-dimensional (2D) moiré superlattices are driving a revolution in designer quantum materials. The electronic interactions in these superlattices, strongly dependent on the periodicity and symmetry of the moiré pattern, critically determine the emergent properties and phase diagrams. To date, the relative twist angle between two layers has been the primary tuning parameter for a given choice of constituent crystals. Here, we establish strain as a powerful mechanism to in-situ modify the moiré periodicity and symmetry. We develop an analytically exact mathematical description for the moiré lattice under arbitrary in-plane heterostrain acting on any bilayer structure. We demonstrate the ability to fine-tune the moiré lattice near critical points, such as the magic angle in bilayer graphene, or fully reconfigure the moiré lattice symmetry beyond that imposed by the unstrained constituent crystals. Due to this unprecedented simultaneous control over the strength of electronic interactions and lattice symmetry, 2D heterostrain provides a powerful platform to engineer, tune, and probe strongly correlated moiré materials.

cond-mat.mes-hall↗

arXiv2021

Wafer-Scale Epitaxial Modulation of Quantum Dot Density

Precise control of the properties of semiconductor quantum dots (QDs) is vital for creating novel devices for quantum photonics and advanced opto-electronics. Suitable low QD-density for single QD devices and experiments are challenging to control during epitaxy and are typically found only in limited regions of the wafer. Here, we demonstrate how conventional molecular beam epitaxy (MBE) can be used to modulate the density of optically active QDs in one- and two- dimensional patterns, while still retaining excellent quality. We find that material thickness gradients during layer-by-layer growth result in surface roughness modulations across the whole wafer. Growth on such templates strongly influences the QD nucleation probability. We obtain density modulations between 1 and 10 QDs/$μm^{2}$ and periods ranging from several millimeters down to at least a few hundred microns. This novel method is universal and expected to be applicable to a wide variety of different semiconductor material systems. We apply the method to enable growth of ultra-low noise QDs across an entire 3-inch semiconductor wafer.

cond-mat.mtrl-sci↗

arXiv2019

Direct Bandgap Emission from Hexagonal Ge and SiGe Alloys

Silicon crystallized in the usual cubic (diamond) lattice structure has dominated the electronics industry for more than half a century. However, cubic silicon (Si), germanium (Ge) and SiGe-alloys are all indirect bandgap semiconductors that cannot emit light efficiently. Accordingly, achieving efficient light emission from group-IV materials has been a holy grail in silicon technology for decades and, despite tremendous efforts, it has remained elusive. Here, we demonstrate efficient light emission from direct bandgap hexagonal Ge and SiGe alloys. We measure a subnanosecond, temperature-insensitive radiative recombination lifetime and observe a similar emission yield to direct bandgap III-V semiconductors. Moreover, we demonstrate how by controlling the composition of the hexagonal SiGe alloy, the emission wavelength can be continuously tuned in a broad range, while preserving a direct bandgap. Our experimental findings are shown to be in excellent quantitative agreement with the ab initio theory. Hexagonal SiGe embodies an ideal material system to fully unite electronic and optoelectronic functionalities on a single chip, opening the way towards novel device concepts and information processing technologies.

cond-mat.mes-hall↗

arXiv2019

Ultra-compact photodetection in atomically thin MoSe$_2$

Excitons in atomically-thin semiconductors interact very strongly with electromagnetic radiation and are necessarily close to a surface. Here, we exploit the deep-subwavelength confinement of surface plasmon polaritons (SPPs) at the edge of a metal-insulator-metal plasmonic waveguide and their proximity of 2D excitons in an adjacent atomically thin semiconductor to build an ultra-compact photodetector. When subject to far-field excitation we show that excitons are created throughout the dielectric gap region of our waveguide and converted to free carriers primarily at the anode of our device. In the near-field regime, strongly confined SPPs are launched, routed and detected in a 20nm narrow region at the interface between the waveguide and the monolayer semiconductor. This leads to an ultra-compact active detector region of only ~0.03$μm ^2$ that absorbs 86% of the propagating energy in the SPP. Due to the electromagnetic character of the SPPs, the spectral response is essentially identical to the far-field regime, exhibiting strong resonances close to the exciton energies. While most of our experiments are performed on monolayer thick MoSe$_2$, the photocurrent-per-layer increases super linearly in multilayer devices due to the suppression of radiative exciton recombination. These results demonstrate an integrated device for nanoscale routing and detection of light with the potential for on-chip integration at technologically relevant, few-nanometer length scales.

physics.app-ph↗

arXiv2019

Impact of intrinsic and extrinsic imperfections on the electronic and optical properties of MoS2

Intrinsic and extrinsic disorder from lattice imperfections, substrate and environment has a strong effect on the local electronic structure and hence the optical properties of atomically thin transition metal dichalcogenides that are determined by strong Coulomb interaction. Here, we examine the role of the substrate material and intrinsic defects in monolayer MoS2 crystals on SiO2 and hBN substrates using a combination of scanning tunneling spectroscopy, scanning tunneling microscopy, optical absorbance, and low-temperature photoluminescence measurements. We find that the different substrates significantly impact the optical properties and the local density of states near the conduction band edge observed in tunneling spectra. While the SiO2 substrates induce a large background doping with electrons and a substantial amount of band tail states near the conduction band edge of MoS2, such states as well as the high doping density are absent using high quality hBN substrates. By accounting for the substrate effects we obtain a quasiparticle gap that is in excellent agreement with optical absorbance spectra and we deduce an exciton binding energy of about 480 meV. We identify several intrinsic lattice defects that are ubiquitious in MoS2, but we find that on hBN substrates the impact of these defects appears to be passivated. We conclude that the choice of substrate controls both the effects of intrinsic defects and extrinsic disorder, and thus the electronic and optical properties of MoS2. The correlation of substrate induced disorder and defects on the electronic and optical properties of MoS2 contributes to an in-depth understanding of the role of the substrates on the performance of 2D materials and will help to further improve the properties of 2D materials based quantum nanosystems.

cond-mat.mtrl-sci↗

arXiv2018

Coexistence of weak and strong coupling with a quantum dot in a photonic molecule

We study the emission from a molecular photonic cavity formed by two proximal photonic crystal defect cavities containing a small number (<3) of In(Ga)As quantum dots. Under strong excitation we observe photoluminescence from the bonding and antibonding modes in excellent agreement with expectations from numerical simulations. Power dependent measurements reveal an unexpected peak, emerging at an energy between the bonding and antibonding modes of the molecule. Temperature dependent measurements show that this unexpected feature is photonic in origin. Time-resolved measurements show the emergent peak exhibits a lifetime $τ_M=0.75 \, \pm 0.1 \, ns $, similar to both bonding and antibonding coupled modes. Comparison of experimental results with theoretical expectations reveal that this new feature arises from a coexistence of weak- and strong-coupling, due to the molecule emitting in an environment whose configuration permits or, on the contrary, impedes its strong-coupling. This scenario is reproduced theoretically for our particular geometry with a master equation reduced to the key ingredients of its dynamics. Excellent qualitative agreement is obtained between experiment and theory, showing how solid-state cavity QED can reveal new regimes of light-matter interaction.

cond-mat.mes-hall↗

arXiv2017

The dielectric impact of layer distances on exciton and trion binding energies in van der Waals heterostructures

The electronic and optical properties of monolayer transition-metal dichalcogenides (TMDs) and van der Waals heterostructures are strongly subject to their dielectric environment. In each layer the field lines of the Coulomb interaction are screened by the adjacent material, which reduces the single-particle band gap as well as exciton and trion binding energies. By combining an electrostatic model for a dielectric hetero-multi-layered environment with semiconductor many-particle methods, we demonstrate that the electronic and optical properties are sensitive to the interlayer distances on the atomic scale. Spectroscopical measurements in combination with a direct solution of a three-particle Schrödinger equation reveal trion binding energies that correctly predict recently measured interlayer distances.

cond-mat.mtrl-sci↗

arXiv2016

Long-term mutual phase locking of picosecond pulse pairs generated by a semiconductor nanowire laser

The ability to generate phase-stabilised trains of ultrafast laser pulses by mode-locking underpins photonics research in fields such as precision metrology and spectroscopy. However, the complexity of conventional mode-locked laser systems, combined with the need for a mechanism to induce active or passive phase locking between resonator modes, has hindered their realisation at the nanoscale. Here, we demonstrate that GaAs-AlGaAs nanowire lasers are capable of emitting pairs of phase-locked picosecond laser pulses when subject to non-resonant pulsed optical excitation with a repetition frequency up to ~200GHz. By probing the two-pulse interference that emerges within the homogeneously broadened laser emission, we show that the optical phase is preserved over timescales extending beyond ~30ps, much longer than the emitted laser pulse duration (~2ps). Simulations performed by solving the optical Bloch equations produce good quantitative agreement with experiments, revealing how the phase information is stored in the gain medium close to transparency. Our results open the way to applications such as on-chip, ultra-sensitive Ramsey comb spectroscopy.

cond-mat.mes-hall↗

arXiv2016

A few-emitter solid-state multi-exciton laser

We report a combined experimental and theoretical study of non-conventional lasing from higher multi-exciton states of a few quantum dot-photonic crystal nanocavity. We show that the photon output is fed from saturable quantum emitters rather than a non-saturable background despite being rather insensitive to the spectral position of the mode. Although the exciton transitions of each quantum dot are detuned by up to $160$ cavity linewidths, we observe that strong excitation populates a multitude of closely spaced multi-exciton states, which partly overlap spectrally with the mode. The limited number of emitters is confirmed by a complete saturation of the mode intensity at strong pumping, providing sufficient gain to reach stimulated emission, whilst being accompanied by a distinct lasing threshold. Detailed second-order photon-correlation measurements unambiguously identify the transition to lasing for strong pumping and, most remarkably, reveal super-thermal photon bunching with $g^{(2)}(0)>2$ below lasing threshold. Based on our microscopic theory, a pump-rate dependent $β$-factor $β(P)$ is needed to describe the nanolaser and account for the interplay of multi-exciton transitions in the few-emitter gain medium. Moreover, we theoretically predict that the super-thermal bunching is related to dipole-anticorrelated multi-exciton recombination channels via sub- and super-radiant coupling below and above lasing threshold, respectively. Our results provide new insights into the microscopic light-matter-coupling of spatially separated emitters coupled to a common cavity mode and, thus, provides a complete understanding of stimulated emission in nanolasers with discrete emitters.

cond-mat.mes-hall↗

arXiv2016

Surface plasmon resonance spectroscopy of single bowtie nano-antennas using a differential reflectivity method

We report on the structural and optical properties of individual bowtie nanoantennas both on glass and semiconducting GaAs substrates. The antennas on glass (GaAs) are shown to be of excellent quality and high uniformity reflected by narrow size distributions with standard deviations for the triangle and gap size of $σ_s^{glass}=4.5nm$ ($σ_s^{GaAs}=2.6nm$) and $σ_g^{glass}=5.4nm$ ($σ_g^{GaAs}=3.8nm$), respectively. The corresponding optical properties of individual nanoantennas studied by differential reflection spectroscopy show a strong reduction of the localised surface plasmon polariton resonance linewidth from $0.21eV$ to $0.07eV$ upon reducing the antenna size from $150nm$ to $100nm$. This is attributed to the absence of inhomogeneous broadening as compared to optical measurements on nanoantenna ensembles. The inter-particle coupling of an individual bowtie nanoantenna, which gives rise to strongly localised and enhanced electromagnetic hotspots, is demonstrated using polarization-resolved spectroscopy, yielding a large degree of linear polarization of $ρ_{max}\sim80\%$. The combination of highly reproducible nanofabrication and fast, non-destructive and non-contaminating optical spectroscopy paves the route towards future semiconductor-based nano-plasmonic circuits, consisting of multiple photonic and plasmonic entities.

cond-mat.mes-hall↗