Search

This paper demonstrates a five-layer InAs/InP quantum-dash semiconductor optical amplifier (QDash-SOA), which will be integrated into microwave-photonic on-chip devices for millimeter-wave (mmWave) over fibre wireless networking systems. A thorough investigation of the QDash-SOA is conducted regarding its communication performance at different temperatures, bias currents, and input powers. The investigation shows a fibre-to-fibre (FtF) small-signal gain of 18.79 dB and a noise figure of 6.3 dB. In a common application with a 300 mA bias current and 25 °C temperature, the peak FtF gain is located at 1507.8 nm, which is 17.68 dB, with 3 dB gain bandwidth of 56.6 nm. Furthermore, the QDash-SOA is verified in a mmWave radio-over-fibre link with QAM (32 Gb/s 64-QAM 4-GBaud) and OFDM (250 MHz 64-QAM) signals. The average error vector magnitude of the QAM and OFDM signals after a 2 m wireless link could be as low as 8.29% and 6.78%, respectively. These findings highlight the QDash-SOA's potential as a key amplifying component in future integrated microwave-photonic on-chip devices. [ABSTRACT FROM AUTHOR]

An on-demand source of bright entangled photon pairs is desirable for quantum key distribution (QKD) and quantum repeaters. The leading candidate to generate such pairs is based on spontaneous parametric down-conversion (SPDC) in non-linear crystals. However, its pair extraction efficiency is limited to 0.1% when operating at near-unity fidelity due to multiphoton emission at high brightness. Quantum dots in photonic nanostructures can in principle overcome this limit, but the devices with high entanglement fidelity (99%) have low pair extraction efficiency (0.01%). Here, we show a measured peak entanglement fidelity of 97.5% ± 0.8% and pair extraction efficiency of 0.65% from an InAsP quantum dot in an InP photonic nanowire waveguide. We show that the generated oscillating two-photon Bell state can establish a secure key for peer-to-peer QKD. Using our time-resolved QKD scheme alleviates the need to remove the quantum dot energy splitting of the intermediate exciton states in the biexciton-exciton cascade. Spontaneous parametric down-conversion, the standard technique for generating entangled photons, is limited by low pair extraction efficiencies at near-unity fidelity. The authors show quantum dots in nanowires efficiently emit an oscillating state with near-unity entanglement fidelity and propose a time-resolved quantum key distribution protocol. [ABSTRACT FROM AUTHOR]

Single photon sources operating on-demand at telecom wavelengths are required in fiber-based quantum secure communication technologies. In this work, we demonstrate single photon emission from position-controlled nanowire quantum dots emitting at λ > 1530 nm. Emission in the C-band is achieved by composition engineering of an InAsxP1−x dot-in-a-rod structure. Using above-band pulsed excitation, we obtain single photon purities of g (2) (0) = 0.062. These results represent an important step toward the scalable manufacture of high efficiency, high rate single photon emitters in the telecom C-band. [ABSTRACT FROM AUTHOR]

We present here a performance comparison of quantum-dash (Qdash) semiconductor amplifiers (SOAs) with three, five, eight, and twelve InAs dash layers grown on InP substrates. Other than the number of Qdash layers, the structures were identical. The eight-layer Qdash SOA gave the highest amplified spontaneous emission power (4.3 dBm) and chip gain (26.4 dB) at 1550 nm, with a 300 mA CW bias current and at 25 °C temperature, while SOAs with fewer Qdash layers (for example, three-layer Qdash SOA), had a wider ASE bandwidth (90 nm) and larger 3 dB gain saturated output power (18.2 dBm) in a shorter wavelength range. The noise figure (NF) of the SOAs increased nearly linearly with the number of Qdash layers. The longest gain peak wavelength of 1570 nm was observed for the 12-layer Qdash SOA. The most balanced performance was obtained with a five-layer Qdash SOA, with a 25.4 dB small-signal chip gain, 15.2 dBm 3 dB output saturated power, and 5.7 dB NF at 1532 nm, 300 mA and 25 °C. These results are better than those of quantum well SOAs reported in a recent review paper. The high performance of InAs/InP Qdash SOAs with different Qdash layers shown in this paper could be important for many applications with distinct requirements under uncooled scenarios. [ABSTRACT FROM AUTHOR]

Diode optical frequency comb lasers are promising compact solutions to generate high-speed optical pulses for applications in high spectral efficiency wavelength division multiplexing transmission with advanced modulation formats. In this paper, an InAs/InP quantum dot (QDot) C-band single-section passively mode-locked laser (MLL) based broadband optical frequency comb source with a free spectral range of 28.4 GHz is presented. The device exhibits less than 1.5 MHz optical linewidth (phase noise) over 56 channels and 2.1 fs pulse-to-pulse timing jitter with a central wavelength of 1550 nm. Using this comb, we demonstrate an aggregate data transmission capacity of 12.5 Terabit/s over 100 km of standard single mode fiber by employing dual-polarization with 16 QAM modulation format. This investigation shows the viability for semiconductor QDot MLLs to be used as low-cost optical source in Terabit/s or higher optical networks. [ABSTRACT FROM AUTHOR]

Resonant excitation of the biexciton state in an InAsP quantum dot by a phase-coherent pair of picosecond pulses allows preparing time-bin entangled pairs of photons via the biexciton–exciton cascade. We show that this scheme can be implemented for a dot embedded in an InP nanowire. The underlying physical mechanisms can be represented and quantitatively analyzed by an effective three-level open system master equation. Simulation parameters including decay and intensity dependent dephasing rates are extracted from experimental data, which in turn let us predict the resulting entanglement and optimal operating conditions. [ABSTRACT FROM AUTHOR]

We study experimentally and theoretically the in-plane magnetic field dependence of the coupling between dots forming a vertically stacked double dot molecule. The InAsP molecule is grown epitaxially in an InP nanowire and interrogated optically at millikelvin temperatures. The strength of interdot tunneling, leading to the formation of the bonding-antibonding pair of molecular orbitals, is investigated by adjusting the sample geometry. For specific geometries, we show that the interdot coupling can be controlled in-situ using a magnetic field-mediated redistribution of interdot coupling strengths. This is an important milestone in the development of qubits required in future quantum information technologies. [ABSTRACT FROM AUTHOR]

We present a compact, fibre-coupled single photon source using gradient-index (GRIN) lenses and an InAsP semiconductor quantum dot embedded within an InP photonic nanowire waveguide. A GRIN lens assembly is used to collect photons close to the tip of the nanowire, coupling the light immediately into a single mode optical fibre. The system provides a stable, high brightness source of fibre-coupled single photons. Using pulsed excitation, we demonstrate on-demand operation with a single photon purity of 98.5% when exciting at saturation in a device with a source-fibre collection efficiency of 35% and an overall single photon collection efficiency of 10%. We also demonstrate "plug and play" operation using room temperature photoluminescence from the InP nanowire for room temperature alignment. [ABSTRACT FROM AUTHOR]

A systematic study of the emission spectra of single InAsP nanowire quantum dots in position-controlled InP photonic nanowire waveguides is presented. Using excitation power-dependent photoluminescence and correlation measurements, we distinguish between the different excitonic complexes responsible for s-shell emission. From measurements of over 40 nominally identical devices, we obtain a standard deviation of ∼ 5 meV in the emission energy of excitons, biexcitons, and charged exciton photons. The mean biexciton binding energy was 1.9 meV with a standard deviation of ∼ 0.8 meV. The experimental spectra are understood using atomistic multi-million atom theory of neutral and charged multi-exciton complexes implemented in the design tool QNANO. [ABSTRACT FROM AUTHOR]

Hexagonal photonic crystal microcavities with missing-hole defects were fabricated in suspended InP membranes. Embedded InAs quantum dots were utilized as broadband emitters to characterize the modes of the cavities. Photoluminescence emission consists of two orthogonally polarized peaks corresponding to the two dipole modes of the hexagonal defect cavity of reduced symmetry. The emission wavelength ranges from 745 to 840 meV, depending on the crystal structure, and quality factors are up to 850. Finite-difference time-domain simulations reproduce the cavity mode energies and the quality factor dependence on the crystal structure, but predict quality factors systematically lower. The experimental quality factors and mode splittings are associated with a slight ellipticity of the lattice holes. [ABSTRACT FROM AUTHOR]

We present an experimental route to engineer the exciton energies of single quantum dots in nanowires. By integrating the nanowires onto a piezoelectric crystal, we controllably apply strain fields to the nanowire quantum dots. Consequently, the exciton energy of a single quantum dot in the nanowire is shifted by several meVs without degrading its optical intensity and single-photon purity. Second-order autocorrelation measurements are performed at different strain fields on the same nanowire quantum dot. The suppressed multi-photon events at zero time delay clearly verify that the quantum nature of single-photon emission is well preserved under external strain fields. The work presented here could facilitate on-chip optical quantum information processing with the nanowire based single photon emitters. [ABSTRACT FROM AUTHOR]

We assess the influence of the degree of quantum confinement on the carrier recovery times in semiconductor optical amplifiers (SOAs) through an experimental comparative study of three amplifiers, one InAs-InGaAsP-InP quantum dot (O-D), one InAs-InAlGaAs-InP quantum dash (1-D), and one InGaAsP-In- GaAsP-InP quantum well (2-D), all of which operate near 1.55-μm wavelengths. The short-lived (around 1 ps) and long-lived (up to 2 ns) amplitude and phase dynamics of the three devices are characterized via heterodyne pump-probe measurements. The quantum-dot device is found to have the shortest long-lived gain recovery (~80 ps) as well as gain and phase changes indicative of a smaller linewidth enhancement factor, making it the most promising for high-bit-rate applications. The quantum-dot amplifier is also found to have reduced ultrafast transients, due to a lower carrier density in the dots. The quantum-dot gain saturation characteristics and temporal dynamics also provide insight into the nature of the dot energy-level occupancy and the interactions of the dot states with the wetting layer. [ABSTRACT FROM AUTHOR]

Hexagonal lattice photonic crystal microcavities with modified single missing air-hole defects were fabricated in suspended InAs/InP quantum dot membranes. The cavity modes predicted from finite-difference time-domain simulations are observed in photoluminescence measurements. The resonant energies of the defect modes are tuned across the band gap of the photonic crystal through modifications of the size and position of the inner ring holes surrounding the defect. Up to a 20-fold enhancement of the quality factor of the modes are observed as they are tuned across the band gap, with measured Q values of up to 6000. [ABSTRACT FROM AUTHOR]

For nanowire-based sources of non-classical light, the rate at which photons are generated and the ability to efficiently collect them are determined by the nanowire geometry. Using selective-area vapour-liquid-solid epitaxy, we show how it is possible to control the nanowire geometry and tailor it to optimise device performance. High efficiency single photon generation with negligible multi-photon emission is demonstrated using a quantum dot embedded in a nanowire having a geometry tailored to optimise both collection efficiency and emission rate. [ABSTRACT FROM AUTHOR]

A process is described where the position, size, and cladding of an InP nanowire with an embedded InAsP quantum dot are determined by design through lithography, processing, and growth. The vapor-liquid-solid growth mode on a patterned substrate is used to grow the InP core and defines the quantum dot size to better than ±2 nm while selective-area growth is used to define the cladding thickness. The clad nanowires emit efficiently in the range λ=0.95-1.15 μm. Photoluminescence measurements are used to quantify the dependence of the excitonic energy level structure on quantum dot size for diameters 10-40 nm. [ABSTRACT FROM AUTHOR]

The threshold current density, Jth, and its radiative component, Jrad, in 1.55 μm InAs/InP (100) quantum dot lasers are measured as a function of temperature and hydrostatic pressure. We find that Jrad is relatively temperature insensitive. However, Jth increases significantly with temperature leading to a characteristic temperature T0=72 K over the range 220-290 K. Nonradiative recombination accounts for up to 94% of Jth at T=293 K. Jth decreases with increasing pressure by 35% over 8 kbar causing an increase in T0 from 72 to 88 K. The results indicate that nonradiative Auger recombination determines temperature behavior of these devices and T0 value. [ABSTRACT FROM AUTHOR]

The authors study the quality factor Q of the x-dipole mode in single missing hole defect photonic crystal microcavities in InAs/InP quantum dot membranes as a function of the structural design parameters. Photoluminescence experiments show an optimized Q in excess of 28 000 for a wavelength close to λ=1550 nm. This is to be compared with a Q of 57 000 determined by finite difference time domain calculations. The fabrication tolerances necessary to achieve experimental Q values close to those predicted by theory are identified. [ABSTRACT FROM AUTHOR]

A method to fine-tune photonic crystal defect cavities is developed based on successive oxidation and wet etching cycles. Photonic crystal microcavities based on InP membranes are oxidized using an ultraviolet (UV)/ozone treatment, and the oxide is subsequently removed using a hydrofluoric acid solution. Each oxidation/etch cycle consumes a thin layer of InP directly exposed to the UV/ozone, enlarging the radius of holes in the photonic crystal and decreasing the membrane thickness. The method is applied to single missing air-hole defect cavities with embedded InAs quantum dots, permitting measurement of the resonant frequency tuning in emission. Defect mode energies were found to blueshift 1.74 meV per cycle, consistent with finite-difference time-domain simulations. A tuning range of 33 meV was obtained after 20 cycles. [ABSTRACT FROM AUTHOR]

We present optical characterization measurements from three-dimensional pillar microcavities comprising an InP spacer layer sandwiched between two SiO2/Ta2O5 Bragg reflectors. InAs quantum dots placed at the center of the InP spacer are used as broadband emitters. Emission into the lowest-order mode is observed at an energy of 820 meV in the wide-pillar limit, with shifts of approximately 30 meV in the ground state and more than 80 meV in higher-order modes for pillars of 2 μm diameter. Excellent agreement is found between the experimentally observed cavity mode energies as a function of pillar diameter and the predictions of guided-mode theory. Cavity quality factors in excess of 2200 are obtained for large diameter pillars, with a decrease to approximately 500 for pillars of 2 μm diameter. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report the achievement of a semiconductor tube laser that can operate in the optical communication wavelength range for applications in the emerging Si-photonics. Such nanoscale devices are fabricated from self-organized InAs/InGaAsP quantum dot nanomembranes through a strain-driven self-rolling mechanism using standard photolithography process. Under continuous wave optical pumping, the devices exhibit an ultralow lasing threshold of ∼1.26 μW at 82 K, with multiple emission wavelengths in the S band of optical communications. The spontaneous emission coupling factor and Purcell factor are estimated to be ∼0.30 and ∼4.8, respectively. [ABSTRACT FROM AUTHOR]

Controlling the emission wavelength of quantum dots (QDs) over large surface area wafers is challenging to achieve directly through epitaxial growth methods. We have investigated an innovative post growth laser-based tuning procedure of the emission of self-assembled InAs QDs grown epitaxially on InP (001). A targeted blue shift of the emission is achieved with a series of iterative steps, with photoluminescence diagnostics employed between the steps to monitor the result of intermixing. We demonstrate tuning of the emission wavelength of ensembles of QDs to within approximately ±1 nm, while potentially better precision should be achievable for tuning the emission of individual QDs. [ABSTRACT FROM AUTHOR]

We report on the site-selected growth of bright single InAsP quantum dots embedded within InP photonic nanowire waveguides emitting at telecom wavelengths. We demonstrate a dramatic dependence of the emission rate on both the emission wavelength and the nanowire diameter. With an appropriately designed waveguide, tailored to the emission wavelength of the dot, an increase in the count rate by nearly 2 orders of magnitude (0.4 to 35 kcps) is obtained for quantum dots emitting in the telecom O-band, showing high single-photon purity with multiphoton emission probabilities down to 2%. Using emission-wavelength-optimized waveguides, we demonstrate bright, narrow-line-width emission from single InAsP quantum dots with an unprecedented tuning range of 880 to 1550 nm. These results pave the way toward efficient single-photon sources at telecom wavelengths using deterministically grown InAsP/InP nanowire quantum dots. [ABSTRACT FROM AUTHOR]

A major step toward fully integrated quantum optics is the deterministic incorporation of high quality single photon sources in on-chip optical circuits. We show a novel hybrid approach in which preselected III-V single quantum dots in nanowires are transferred and integrated in silicon based photonic circuits. The quantum emitters maintain their high optical quality after integration as verified by measuring a low multiphoton probability of 0.07 ± 0.07 and emission line width as narrow as 3.45 ± 0.48 GHz. Our approach allows for optimum alignment of the quantum dot light emission to the fundamental waveguide mode resulting in very high coupling efficiencies. We estimate a coupling efficiency of 24.3 ± 1.7% from the studied single-photon source to the photonic channel and further show by finite-difference time-domain simulations that for an optimized choice of material and design the efficiency can exceed 90%. [ABSTRACT FROM AUTHOR]

In this Letter, we present entanglementgenerated from a novel structure: a single InAsP quantum dot embeddedin an InP nanowire. These structures can grow in a site-controlledway and exhibit high collection efficiency; we detect 0.5 millionbiexciton counts per second coupled into a single mode fiber witha standard commercial avalanche photo diode. If we correct for theknown setup losses and detector efficiency, we get an extraction efficiencyof 15(3) %. For the measured polarization entanglement, we observea fidelity of 0.76(2) to a reference maximally entangled state aswell as a concurrence of 0.57(6). [ABSTRACT FROM AUTHOR]

We report on the ultraclean emission from single quantumdots embeddedin pure wurtzite nanowires. Using a two-step growth process combiningselective-area and vapor–liquid–solid epitaxy, we growdefect-free wurtzite InP nanowires with embedded InAsP quantum dots,which are clad to diameters sufficient for waveguiding at λ∼ 950 nm. The absence of nearby traps, at both the nanowiresurface and along its length in the vicinity of the quantum dot, manifestsin excitonic transitions of high spectral purity. Narrow emissionline widths (30 μeV) and very-pure single photon emission witha probability of multiphoton emission below 1% are achieved, bothof which were not possible in previous work where stacking fault densitieswere significantly higher. [ABSTRACT FROM AUTHOR]