Resolving Photon Numbers Using Ultra-High-Resolution Timing of a Single Low-Jitter Superconducting Nanowire Detector

Photon-number-resolving (PNR) detectors are a key enabling technology in photonic quantum information processing. Here, we demonstrate the PNR capacity of conventional superconducting nanowire single-photon detectors by performing ultra-high-resolution time-tagging of the detector-generated electrical pulses. This method provides a viable approach for PNR with high detection efficiency and a high operational repetition rate. We present the implementation of such a PNR detector in the telecom C-band and its characterization by measuring the photon-number statistics of coherent light with tunable intensity. Additionally, we demonstrate the capabilities of the detection method by measuring photon-number correlations of non-classical states.Comment: 6 pages, 8 figure

Strong amplitude-phase coupling in submonolayer quantum dots

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 109, 201102 (2016) and may be found at https://doi.org/10.1063/1.4967833.Submonolayer quantum dots promise to combine the beneficial features of zero- and two-dimensional carrier confinement. To explore their potential with respect to all-optical signal processing, we investigate the amplitude-phase coupling (α-parameter) in semiconductor optical amplifiers based on InAs/GaAs submonolayer quantum dots in ultrafast pump-probe experiments. Lateral coupling provides an efficient carrier reservoir and gives rise to a large α-parameter. Combined with a high modal gain and an ultrafast gain recovery, this makes the submonolayer quantum dots an attractive gain medium for nonlinear optical signal processing

Sideband pump-probe technique resolves nonlinear modulation response of PbS/CdS quantum dots on a silicon nitride waveguide

For possible applications of colloidal nanocrystals in optoelectronics and nanophotonics, it is of high interest to study their response at low excitation intensity with high repetition rates, as switching energies in the pJ/bit to sub-pJ/bit range are targeted. We develop a sensitive pump-probe method to study the carrier dynamics in colloidal PbS/CdS quantum dots deposited on a silicon nitride waveguide after excitation by laser pulses with an average energy of few pJ/pulse. We combine an amplitude modulation of the pump pulse with phase-sensitive heterodyne detection. This approach permits to use co-linearly propagating co-polarized pulses. The method allows resolving transmission changes of the order of 10(-5) and phase changes of arcseconds. We find a modulation on a sub-nanosecond time scale caused by Auger processes and biexciton decay in the quantum dots. With ground state lifetimes exceeding 1 mu s, these processes become important for possible realizations of opto-electronic switching and modulation based on colloidal quantum dots emitting in the telecommunication wavelength regime

Gain dynamics of quantum dot devices for dual-state operation

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 104, 261108 (2014) and may be found at https://doi.org/10.1063/1.4885383.Ground state gain dynamics of In(Ga)As-quantum dot excited state lasers are investigated via single-color ultrafast pump-probe spectroscopy below and above lasing threshold. Two-color pump-probe experiments are used to localize lasing and non-lasing quantum dots within the inhomogeneously broadened ground state. Single-color results yield similar gain recovery rates of the ground state for lasing and non-lasing quantum dots decreasing from 6 ps to 2 ps with increasing injection current. We find that ground state gain dynamics are influenced solely by the injection current and unaffected by laser operation of the excited state. This independence is promising for dual-state operation schemes in quantum dot based optoelectronic devices.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Biexciton fine structure in monolayer transition metal dichalcogenides

The optical properties of atomically thin transition metal dichalcogenide (TMDC) semiconductors are shaped by the emergence of correlated many-body complexes due to strong Coulomb interaction. Exceptionalelectron-holeexchangepredestinesTMDCstostudyfundamentalandappliedproperties of Coulomb complexes such as valley depolarization of excitons and fine-structure splitting of trions. Biexcitons in these materials are less understood and it has been established only recently that they are spectrally located between exciton and trion. Here we show that biexcitons in monolayer TMDCs exhibit a distinct fine structure on the order of meV due to electron-hole exchange. Ultrafast pump-probe experiments on monolayer WSe2 reveal decisive biexciton signatures and a fine structure in excellent agreement with a microscopic theory. We provide a pathway to access biexciton spectra with unprecedented accuracy, which is valuable beyond the class of TMDCs, and to understand even higher Coulomb complexes under the influence of electron-hole exchange.This work was supported by Deutsche Forschungsge- meinschaft (DFG) within CRC 1558 and RTG 2247. A. W. A. acknowledges funding from DFG via grant No. AC290-2/1. The spectroscopic experiments were jointly supported by NSF DMR1306878 (A.S.) and NSF MR- SEC program DMR-1720595 (K.T.). X.L. gratefully ac- knowledges support from the Welch foundation (F-1662).Center for Dynamics and Control of Material

Fast gain and phase recovery of semiconductor optical amplifiers based on submonolayer quantum dots

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 107, 201102 (2015) and may be found at https://doi.org/10.1063/1.4935792.Submonolayer quantum dots as active medium in opto-electronic devices promise to combine the high density of states of quantum wells with the fast recovery dynamics of self-assembled quantum dots. We investigate the gain and phase recovery dynamics of a semiconductor optical amplifier based on InAs submonolayer quantum dots in the regime of linear operation by one- and two-color heterodyne pump-probe spectroscopy. We find an as fast recovery dynamics as for quantum dot-in-a-well structures, reaching 2 ps at moderate injection currents. The effective quantum well embedding the submonolayer quantum dots acts as a fast and efficient carrier reservoir.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, BauelementeDFG, 87159868, GRK 1558: Kollektive Dynamik im Nichtgleichgewicht: in kondensierter Materie und biologischen Systeme

Breitbandige ultraschnelle Spektroskopie an gemischdimensionalen InAs/GaAs-Systemen

This thesis deals with light matter interaction and electron dynamics in coupled systems of nanostructured III-V semiconductors, particularly with quantum dots embedded in quantum wells. Its focus is on the role of dimensionality and the characterization of quantum dot states with respect to their two- and many-level nature. The sample under investigation is a semiconductor optical amplifier under standard operating conditions, in particular at room temperature. An essential part of the work and a prerequisite of the experimental results is the re–development of a setup for heterodyne detection of laser pulses. Compared to the established pump–probe experiment, the duration of a single measurement was reduced by a factor of 100. This allowed the introduction of simultaneous multi–power measurements that are used to investigate the non–linear response of the system. More than that, several novel techniques have been developed: The Frequency Resolved Optical Short–pulse Characterization by Heterodyning (FROSCH) for pulse shape analysis, the sideband pump–probe technique for low signal measurements, and finally a method for two–dimensional coherent spectroscopy. All these techniques can be used on the same setup without significant realignment. A detailed description is given in part II. In a first set of experiments we were able to observe Rabi oscillations of the quantum dot exciton ground state at room temperature as a typical property of two–level systems. These observations on a femtosecond timescale were enabled by the FROSCH technique when the ground state oscillations caused a characteristic pulse shape modification. On the other hand, the many–level characteristics dominated a large set of pump–probe measurements presented in chapter 10. Under the assumption of a linear superposition of zero–dimensional quantum dot states on the one hand and two- or three-dimensional continuum states on the other hand, it was not possible to describe the electron dynamics. The introduction of “crossed excitons” into the model allowed for a consistent explanation of the gain dynamics. These crossed excitons are formed by Coulomb attraction of a zero–dimensional single carrier state and another one of higher dimensionality. The crossed excitons affect the optical transitions as well as the carrier diffusion in the quantum well. The apparent contradiction in these findings with respect to the two- and many-level characteristics is resolved in the final experimental part by two–dimensional coherent spectroscopy. This technique uses white laser pulses and merges all technical approaches developed in the previous parts. This allows the observation of the coherent coupling of states with large energy separation. The reliability of the technique is proven by the extraction of dephasing times that are in excellent agreement with literature values. In the two–dimensional spectra we find distinct signatures of crossed excitons that can be investigated in detail by this novel technique.Diese Dissertation setzt sich mit der Licht–Materie–Wechselwirkung und der Elektronendynamik in gekoppelten Systemen von nanostrukturierten III-V–Halbleitern am Beispiel in Quantenfilme eingebetteter Quantenpunkte auseinander. Dabei wird insbesondere der Einfluss der Dimensionalität untersucht und die Frage nach der Charakterisierung von Quantenpunktzuständen hinsichtlich ihrer Eigenschaften als Zwei- bzw. Viel-Niveau-Systemen behandelt. Als Probe dient ein optischer Halbleiter-Verstärker, der unter typischen Anwendungsbedingungen untersucht wird, also insbesondere bei Raumtemperatur. Wesentlicher Bestandteil der Arbeit und Grundlage der experimentellen Ergebnisse war die Weiterentwicklung eines Aufbaus zur heterodynen Detektion von Laserpulsen. Dabei konnte für die etablierte Pump-Probe-Technik die nötige Messzeit um einen Faktor 100 verkürzt werden. Dies ermöglichte auch die Einführung simultaner Messungen mit abgestuften Anregungsintensitäten zur Untersuchung nichtlinearer Antworten des Systems. Darüber hinaus wurden mehrere neue Techniken entwickelt: Die “Frequency Resolved Optical Short-pulse Characterization by Heterodyning” (FROSCH) zur Analyse von Pulsformen, die Seitenband-Pump-Probe zur Messung sehr kleiner Signale und zuletzt ein Aufbau zur zweidimensionalen kohärenten Spektroskopie. All diese Techniken sind ohne nennenswerte Umbauarbeiten am selben Versuchsstand nutzbar und im Teil II der Dissertation detailiert beschrieben. Zunächst konnten mit der FROSCH–Technik Rabi-Oszillationen des Exzitonen-Grundzustandes als typische Eigenschaft eines Zwei-Niveau-Systems in Quantenpunkten bei Raumtemperatur nachgewiesen werden. Die Beobachtung dieses Effekts auf der Zeitskala von Femtosekunden wurde durch die FROSCH-Technik ermöglicht. Das Oszillieren des Grundzustandes führt dabei zu einer charakteristischen Pulsverformung. Im Gegensatz dazu zeigte sich die Viel-Niveau-Charakteristik in einer Vielzahl von Pump-Probe- Messungen, die in Abschnitt 10 präsentiert werden. Die erhobenen Daten konnten dabei nicht im Rahmen einer linearen Überlagerung von nulldimensionalen Zuständen einerseits und zwei- bzw. dreidimensionalen Zuständen andererseits erklärt werden. Durch die Einführung gekreuzter Exzitonen (“crossed excitons”), die einen durch Coulomb-Weschselwirkung gebundenen Zustand je eines null- und eines höherdimensionalen Einzelladungsträgerzustands darstellen, konnte eine konsistente Erklärung für die beobachtete Verstärkungsdynamik geliefert werden. Dies gilt sowohl in Hinblick auf die Anregung optischer Übergänge als auch bezüglich der Ladungsträgerdiffusion im Quantenfilm. Die Vereinigung dieser widersprüchlich anmutenden Ergebnisse hinsichtlich Zwei- oder Viel-Niveau-Charakteristik erfolgt im letzten experimentellen Teil mit Hilfe der zweidimensionalen kohärenten Spektroskopie. Diese Technik vereint alle zuvor entwickelten technischen Ansätze und ermöglicht durch den Einsatz von weißen Laserpulsen die Identifikation kohärenter Kopplungen von Übergängen mit großem energetischen Abstand. Die Zuverlässigkeit der Technik wird durch die Extraktion von Kohärenzzeiten der Quantenpunkte und den Abgleich mit Literaturdaten belegt. Darüber hinaus weisen die zweidimensionalen Spektren deutliche Signaturen gekreuzter Exzitonen auf, womit erstmalig eine direkte Beobachtung dieser optischen Übergänge möglich wird

Fast time-domain diffuse correlation spectroscopy with superconducting nanowire single-photon detector: system validation and in vivo results

Abstract Time-domain diffuse correlation spectroscopy (TD-DCS) has been introduced as an advancement of the “classical” continuous wave DCS (CW-DCS) allowing one to not only to measure depth-resolved blood flow index (BFI) but also to extract optical properties of the measured medium without using any additional diffuse optics technique. However, this method is a photon-starved technique, specially when considering only the late photons that are of primary interest which has limited its in vivo application. In this work, we present a TD-DCS system based on a superconducting nanowire single-photon detector (SNSPD) with a high quantum efficiency, a narrow timing response, and a negligibly low dark count noise. We compared it to the typically used single-photon avalanche diode (SPAD) detector. In addition, this system allowed us to conduct fast in vivo measurements and obtain gated pulsatile BFI on the adult human forehead