arXiv subjects

Jonathan J. Finley

Publications and source records attributed to Jonathan J. Finley.

At least 109 records · Page 6Linked to original sources

arXiv2017

Pulsed Rabi oscillations in quantum two-level systems: beyond the Area Theorem

The Area Theorem states that when a short optical pulse drives a quantum two-level system, it undergoes Rabi oscillations in the probability of scattering a single photon. In this work, we investigate the breakdown of the Area Theorem as both the pulse length becomes non-negligible and for certain pulse areas. Using simple quantum trajectories, we provide an analytic approximation to the photon emission dynamics of a two-level system. Our model provides an intuitive way to understand re-excitation, which elucidates the mechanism behind the two-photon emission events that can spoil single-photon emission. We experimentally measure the emission statistics from a semiconductor quantum dot, acting as a two-level system, and show good agreement with our simple model for short pulses. Additionally, the model clearly explains our recent results [K. Fischer and L. Hanschke, et al., Nature Physics (2017)] showing dominant two-photon emission from a two-level system for pulses with interaction areas equal to an even multiple of $π$.

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arXiv2017

Signatures of two-photon pulses from a quantum two-level system

The theoretical community has found interest in the ability of a two-level atom to generate a strong many-body interaction with light under pulsed excitation. Single-photon generation is the most well-known effect, where a short Gaussian laser pulse is converted into a Lorentzian single-photon wavepacket. However, recent proposals have surprisingly suggested that scattering with intense laser fields off a two-level atom may generate oscillations in two-photon emission that are out of phase with its Rabi oscillations, as the power of the pulse increases. Here, we provide an intuitive explanation for these oscillations using a quantum trajectory approach and show how they may preferentially result in emission of two-photon pulses. Experimentally, we observe signatures of these oscillations by measuring the bunching of photon pulses scattered off a two-level quantum system. Our theory and measurements provide crucial insight into the re-excitation process that plagues on-demand single-photon sources while suggesting the production of novel multi-photon states.

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arXiv2017

Direct exciton emission from atomically thin transition metal dichalcogenide heterostructures near the lifetime limit

We demonstrate the reduction of the inhomogeneous linewidth of the free excitons in atomically thin transition metal dichalcogenides (TMDCs) MoSe$_{2}$, WSe$_{2}$ and MoS$_{2}$ by encapsulation within few nanometer thick hBN. Encapsulation is shown to result in a significant reduction of the 10K excitonic linewidths down to $\sim3.5 \text{ meV}$ for n-MoSe$_{2}$, $\sim5.0 \text{ meV}$ for p-WSe$_{2}$ and $\sim4.8 \text{ meV}$ for n-MoS$_{2}$. Evidence is obtained that the hBN environment effectively lowers the Fermi level since the relative spectral weight shifts towards the neutral exciton emission in n-doped TMDCs and towards charged exciton emission in p-doped TMDCs. Moreover, we find that fully encapsulated MoS$_{2}$ shows resolvable exciton and trion emission even after high power density excitation in contrast to non-encapsulated materials. Our findings suggest that encapsulation of mechanically exfoliated few-monolayer TMDCs within nanometer thick hBN dramatically enhances optical quality, producing ultra-narrow linewidths that approach the homogeneous limit.

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arXiv2017

Electric-field switchable second-harmonic generation in bilayer MoS$_{2}$ by inversion symmetry breaking

We demonstrate pronounced electric-field-induced second-harmonic generation in naturally inversion symmetric 2H stacked bilayer MoS$_{2}$ embedded into microcapacitor devices. By applying strong external electric field perturbations ($|F| = \pm 2.6 MVcm^{-1}$) perpendicular to the basal plane of the crystal we control the inversion symmetry breaking and, hereby, tune the nonlinear conversion efficiency. Strong tunability of the nonlinear response is observed throughout the energy range ($E_ω \sim 1.25 eV - 1.47 eV$) probed by measuring the second-harmonic response at $E_{2ω}$, spectrally detuned from both the A- and B-exciton resonances. A 60-fold enhancement of the second-order nonlinear signal is obtained for emission at $E_{2ω} = 2.49 eV$, energetically detuned by $ΔE = E_{2ω} - E_C = -0.26 eV$ from the C-resonance ($E_{C} = 2.75 eV$). The pronounced spectral dependence of the electric-field-induced second-harmonic generation signal reflects the bandstructure and wave function admixture and exhibits particularly strong tunability below the C-resonance, in good agreement with Density Functional Theory calculations. Moreover, we show that the field-induced second-harmonic generation relies on the interlayer coupling in the bilayer. Our findings strongly suggest that the strong tunability of the electric-field-induced second-harmonic generation signal in bilayer transition metal dichalcogenides may find applications in miniaturized electrically switchable nonlinear devices.

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arXiv2016

Surface acoustic wave regulated single photon emission from a coupled quantum dot-nanocavity system

A coupled quantum dot--nanocavity system in the weak coupling regime of cavity quantumelectrodynamics is dynamically tuned in and out of resonance by the coherent elastic field of a $f_{\rm SAW}\simeq800\,\mathrm{MHz}$ surface acoustic wave. When the system is brought to resonance by the sound wave, light-matter interaction is strongly increased by the Purcell effect. This leads to a precisely timed single photon emission as confirmed by the second order photon correlation function $g^{(2)}$. All relevant frequencies of our experiment are faithfully identified in the Fourier transform of $g^{(2)}$, demonstrating high fidelity regulation of the stream of single photons emitted by the system.

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arXiv2016

Stark effect spectroscopy of mono- and few-layer MoS$_{2}$

We demonstrate electrical control of the A-exciton interband transition in mono- and few-layer $MoS_{2}$ crystals embedded into photocapacitor devices via the DC Stark effect. Electric field dependent low-temperature photoluminescence spectroscopy reveals a significant tuneability of the A-exciton transition energy, up to $\sim 16 meV$, from which we extract the mean DC exciton polarisability $<\barβ>_{N} = (0.58 \pm 0.25) \cdot 10^{-8} DmV^{-1}$. The exciton polarisability is shown to be layer-independent, indicating a strong localisation of both electron and hole wave functions in each individual layer.

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arXiv2016

Integrated superconducting detectors on semiconductors for quantum optics applications

Semiconductor quantum photonic circuits can be used to efficiently generate, manipulate, route and exploit non-classical states of light for distributed photon based quantum information technologies. In this article, we review our recent achievements on the growth, nanofabrication and integration of high-quality, superconducting niobium nitride thin films on optically active, semiconducting GaAs substrates and their patterning to realise highly efficient and ultrafast superconducting detectors on semiconductor nanomaterials containing quantum dots. Our state-of-the-art detectors reach external detection quantum efficiencies up to 20% for ~4nm thin films and single photon timing resolutions <72ps. We discuss the integration of such detectors into quantum dot loaded, semiconductor ridge waveguides, resulting in the on-chip, time-resolved detection of quantum dot luminescence. Furthermore, a prototype quantum optical circuit is demonstrated that enabled the on-chip generation of resonance fluorescence from an individual InGaAs quantum dot, with a line width <15 $μ$eV displaced by 1mm from the superconducting detector on the very same semiconductor chip. Thus, all key components required for prototype quantum photonic circuits with sources, optical components and detectors on the same chip are reported.

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arXiv2015

Ultrafast photodetection in the quantum wells of single AlGaAs/GaAs-based nanowires

We investigate the ultrafast optoelectronic properties of single Al0.3Ga0.7As/GaAs-core-shell-nanowires. The nanowires contain GaAs-based quantum wells. For a resonant excitation of the quantum wells, we find a picosecond photocurrent which is consistent with an ultrafast lateral expansion of the photogenerated charge carriers. This Dember-effect does not occur for an excitation of the GaAs-based core of the nanowires. Instead, the core exhibits an ultrafast displacement current and a photo-thermoelectric current at the metal Schottky contacts. Our results uncover the optoelectronic dynamics in semiconductor core-shell nanowires comprising quantum wells, and they demonstrate the possibility to use the low-dimensional quantum well states therein for ultrafast photoswitches and photodetectors.

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arXiv2015

Quantum Fingerprints in Higher Order Correlators of a Spin Qubit

The spin of an electron in a semiconductor quantum dot represents a natural nanoscale solid state qubit. Coupling to nuclear spins leads to decoherence that limits the number of allowed quantum logic operations for this qubit. Traditional approach to characterize decoherence is to explore spin Relaxation and the spin echo, which are equivalent to the studies of the spins 2nd order time-correlator at various external conditions. Here we develop an alternative technique by showing that considerable information about dynamics can be obtained from direct measurements of higher than the 2nd order correlators, which to date have been hindered in semiconductor quantum dots. We show that such correlators are sensitive to pure quantum effects that cannot be explained within the classical framework, and which allow direct determination of ensemble and quantum dephasing times with only repeated projective measurements without the need for coherent spin control. Our method enables studies of pure quantum effects, including tests of the Leggett-Garg type inequalities that rule out local hidden variable interpretation of the spin qubit dynamics.

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arXiv2015

Photocurrents in a Single InAs Nanowire/ Silicon Heterojunction

We investigate the optoelectronic properties of single indium arsenide nanowires, which are grown vertically on p-doped silicon substrates. We apply a scanning photocurrent microscopy to study the optoelectronic properties of the single heterojunctions. The measured photocurrent characteristics are consistent with an excess charge carrier transport through mid-gap trap states, which form at the Si/InAs heterojunctions. Namely, the trap states add an additional transport path across a heterojunction, and the charge of the defects changes the band bending at the junction. The bending gives rise to a photovoltaic effect at a small bias voltage. In addition, we observe a photoconductance effect within the InAs nanowires at large biases.

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arXiv2015

Dynamic acousto-mechanical control of a strongly coupled photonic molecule

Two-dimensional photonic crystal membranes provide a versatile planar architecture for integrated photonics to control the propagation of light on a chip employing high quality optical cavities, waveguides, beamsplitters or dispersive elements. When combined with highly non-linear quantum emitters, quantum photonic networks operating at the single photon level come within reach. Towards large-scale quantum photonic networks, selective dynamic control of individual components and deterministic interactions between different constituents are of paramount importance. This indeed calls for switching speeds ultimately on the system's native timescales. For example, manipulation via electric fields or all-optical means have been employed for switching in nanophotonic circuits and cavity quantum electrodynamics studies. Here, we demonstrate dynamic control of the coherent interaction between two coupled photonic crystal nanocavities forming a photonic molecule. By using an electrically generated radio frequency surface acoustic wave we achieve optomechanical tuning, demonstrate operating speeds more than three orders of magnitude faster than resonant mechanical approaches. Moreover, the tuning range is large enough to compensate for the inherent fabrication-related cavity mode detuning. Our findings open a route towards nanomechanically gated protocols, which hitherto have inhibited the realization in all-optical schemes.

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arXiv2014

Directional and dynamic modulation of the optical emission of an individual GaAs nanowire using surface acoustic waves

We report on optical experiments performed on individual GaAs nanowires and the ma- nipulation of their temporal emission characteristics using a surface acoustic wave. We find a pronounced, characteristic suppression of the emission intensity for the surface acoustic wave propagation aligned with the axis of the nanowire. Furthermore, we demonstrate that this quenching is dynamical as it shows a pronounced modulation as the local phase of the surface acoustic wave is tuned. These effects are strongly reduced for a SAW applied in the direction perpendicular to the axis of the nanowire due to their inherent one-dimensional geometry. We resolve a fully dynamic modulation of the nanowire emission up to 678 MHz not limited by the physical properties of the nanowires.

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arXiv2014

Dynamic Acoustic Control of Individual Optically Active Quantum Dot-like Emission Centers in Heterostructure Nanowires

We probe and control the optical properties of emission centers forming in radial het- erostructure GaAs-Al0.3Ga0.7As nanowires and show that these emitters, located in Al0.3Ga0.7As layers, can exhibit quantum-dot like characteristics. We employ a radio frequency surface acoustic wave to dynamically control their emission energy and occupancy state on a nanosec- ond timescale. In the spectral oscillations we identify unambiguous signatures arising from both the mechanical and electrical component of the surface acoustic wave. In addition, differ- ent emission lines of a single quantum dot exhibit pronounced anti-correlated intensity oscilla- tions during the acoustic cycle. These arise from a dynamically triggered carrier extraction out of the quantum dot to a continuum in the radial heterostructure. Using finite element modeling and Wentzel-Kramers-Brillouin theory we identify quantum tunneling as the underlying mech- anism. These simulation results quantitatively reproduce the observed switching and show that in our systems these quantum dots are spatially separated from the continuum by > 10.5 nm.

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arXiv2014

Imaging surface plasmon polaritons using proximal self-assembled InGaAs quantum dots

We present optical investigations of hybrid plasmonic nanosystems consisting of lithographically defined plasmonic Au-waveguides or beamsplitters on GaAs substrates coupled to proximal self-assembled InGaAs quantum dots. We designed a sample structure that enabled us to precisely tune the distance between quantum dots and the sample surface during nano-fabrication and demonstrated that non-radiative processes do not play a major role for separations down to $\sim 10 nm$. A polarized laser beam focused on one end of the plasmonic nanostructure generates propagating surface plasmon polaritons that, in turn, create electron-hole pairs in the GaAs substrate during propagation. These free carriers are subsequently captured by the quantum dots $\sim 25 nm$ below the surface, giving rise to luminescence. The intensity of the spectrally integrated quantum dot luminescence is used to image the propagating plasmon modes. As the waveguide width reduces from $5 μm$ to $1 μm$, we clearly observe different plasmonic modes at the remote waveguide end, enabling their direct imaging in real space. This imaging technique is applied to a plasmonic beamsplitter facilitating the determination of the splitting ratio between the two beamsplitter output ports as the interaction length $L_i$ is varied. A splitting ratio of $50:50$ is observed for $L_i\sim 9\pm1 μm$ and $1 μm$ wide waveguides for excitation energies close to the GaAs band edge. Our experimental findings are in good agreement with mode profile and finite difference time domain simulations for both waveguides and beamsplitters.

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arXiv2014

Radio frequency occupancy state control of a single nanowire quantum dot

The excitonic occupancy state of a single, nanowire-based, heterostructure quantum dot is dynamically programmed by a surface acoustic wave. The quantum dot is formed by an interface or thickness fluctuation of a GaAs QW embedded in a AlGaAs shell of a GaAs-AlGaAs core-shell nanowire. As we tune the time at which carriers are photogenerated during the acoustic cycle, we find pronounced intensity oscillations of neutral and negatively charged excitons. At high acoustic power levels these oscillations become anticorrelated which enables direct acoustic programming of the dot's charge configuration, emission intensity and emission wavelength. Numerical simulations confirm that the observed modulations arise from acoustically controlled modulations of the electron and electron-hole-pair concentrations at the position of the quantum dot.

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arXiv2014

Optical study of lithographically defined, subwavelength plasmonic wires and their coupling to embedded quantum emitters

We present an optical investigation of surface plasmon polaritons propagating along nanoscale Au-wires, lithographically defined on GaAs substrates. A two-axis confocal microscope was used to perform spatially and polarization resolved measurements in order to confirm the guiding of surface plasmon polaritons over lengths ranging from $5-20 μm$ along nanowires with a lateral dimension of only $\approx 100 nm$. Finite difference time domain simulations are used to corroborate our experimental observations and highlight the potential to couple proximal quantum dot emitters to propagating plasmon modes in such extreme sub-wavelength devices. Our findings are of strong relevance for the development of semiconductor based integrated plasmonic and active quantum plasmonic nanosystems that merge quantum emitters with nanoscale plasmonic elements.

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arXiv2013

Acoustically regulated carrier injection into a single optically active quantum dot

We study the carrier injection into a single InGaAs/GaAs quantum dot regulated by a radio frequency surface acoustic wave. We find that the time of laser excitation during the acoustic cycle programs both the emission intensities and time of formation of neutral $(X^0)$ and negatively charged $(X^-)$ excitons. We identify underlying, characteristic formation pathways of both few-particle states in the time-domain experiments and show that both exciton species can be formed either with the optical pump or at later times by injection of single electrons and holes "surfing" the acoustic wave. All experimental observations are in excellent agreement with calculated electron and hole trajectories in the plane of the two-dimensional wetting layer which is dynamically modulated by the acoustically induced piezoelectric potentials. Taken together, our findings provide insight on both the onset of acousto-electric transport of electrons and holes and their conversion into the optical domain after regulated injection into a single quantum dot emitter.

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arXiv2013

On-chip time resolved detection of quantum dot emission using integrated superconducting single photon detectors

We report the routing of quantum light emitted by self-assembled InGaAs quantum dots (QDs) into the optical modes of a GaAs ridge waveguide and its efficient detection on-chip via evanescent coupling to NbN superconducting nanowire single photon detectors (SNSPDs). Individual QD light sources embedded within such integrated nano-photonic circuits are highly attractive for the realization of quantum photonic circuits for many applications in photonic information science. Here, we demonstrate that the waveguide coupled SNSPDs primarily detect QD luminescence with scattered photons from the excitation laser being negligible by comparison. The SNSPD detection efficiency from the evanescently coupled waveguide modes is shown to be two orders of magnitude higher when compared with operation under normal incidence illumination. Furthermore, in-situ time resolved measurements show an average exciton lifetime of 0.93 \pm 0.03 ns when recorded with the integrated detector with an ultrafast timing jitter of only 72 \pm 2 ps showing the great potential of this highly integrated quantum optics system.

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