Jin Liu

High-Q optomechanical GaAs nanomembranes

We demonstrate that suspended single-crystal GaAs nanomembranes exhibit mechanical Q-factors exceeding 2u2009×u2009106 at room temperature, which makes them a very promising platform for optomechanics. Because of the completely removed substrate and their millimeter-scale lateral size, the membranes can be incorporated in macroscopic optical cavities for quantum optomechanics experiments. We have measured the mechanical mode spectrum and spatial profiles and find good agreements with theory. Our work paves the way for optomechanical experiments with direct band gap semiconductors in which not only radiation pressure but also other mechanisms involving embedded light emitters could be exploited for quantum optical control of massive mechanical systems.

Deterministic implementation of a bright, on-demand single-photon source with near-unity indistinguishability via quantum dot imaging

Deterministic techniques enabling the implementation and engineering of bright and coherent solid-state quantum light sources are key for the reliable realization of a next generation of quantum devices. Such a technology, at best, should allow one to significantly scale up the number of implemented devices within a given processing time. In this work, we discuss a possible technology platform for such a scaling procedure, relying on the application of nanoscale quantum dot imaging to the pillar microcavity architecture, which promises to combine very high photon extraction efficiency and indistinguishability. We discuss the alignment technology in detail, and present the optical characterization of a selected device which features a strongly Purcell-enhanced emission output. This device, which yields an extraction efficiency of η = (49 ± 4) %, facilitates the emission of photons with (94 ± 2.7) % indistinguishability.

Efficient fiber-coupled single-photon source based on quantum dots in a photonic-crystal waveguide

Many photonic quantum information processing applications would benefit from a high brightness, fiber-coupled source of triggered single photons. Here, we present a fiber-coupled photonic-crystal waveguide single-photon source relying on evanescent coupling of the light field from a tapered out-coupler to an optical fiber. A two-step approach is taken where the performance of the tapered out-coupler is recorded first on an independent device containing an on-chip reflector. Reflection measurements establish that the chip-to-fiber coupling efficiency exceeds 80 %. The detailed characterization of a high-efficiency photonic-crystal waveguide extended with a tapered out-coupling section is then performed. The corresponding overall single-photon source efficiency is 10.9 % ± 2.3 %, which quantifies the success probability to prepare an exciton in the quantum dot, couple it out as a photon in the waveguide, and subsequently transfer it to the fiber. The applied out-coupling method is robust, stable over time, and broadband over several tens of nanometers, which makes it a highly promising pathway to increase the efficiency and reliability of planar chip-based single-photon sources.

A comparison between experiment and theory on few-quantum-dot nanolasing in a photonic-crystal cavity

We present an experimental and theoretical study on the gain mechanism in a photonic-crystal-cavity nanolaser with embedded quantum dots. From time-resolved measurements at low excitation power we find that four excitons are coupled to the cavity. At high excitation power we observe a smooth low-threshold transition from spontaneous emission to lasing. Before lasing emission sets in, however, the excitons are observed to saturate, and the gain required for lasing originates rather from multi-excitonic transitions, which give rise to a broad emission background. We compare the experiment to a model of quantum-dot microcavity lasers and find that the number of excitons that must be included to fit the data largely exceeds the measured number, which shows that transitions involving the wetting layer can provide a surprisingly large contribution to the gain.

Heterogeneous HI-V/Si 3 N 4 integration for quantum photonic circuits

Photonic integration is as an enabling technology for photonic quantum science, providing great experimental scalability, stability, and functionality. Although the increasing complexity of quantum photonic circuits has allowed proof-of-principle demonstrations of quantum computation, simulation, and metrology[1], further development is severely limited by the on-chip photon flux that can be made available from external quantum light sources[2]. Overcoming such limitations would allow a significant scaling of quantum photonic experiments, and enable quantum-level investigation of many physical processes observable on-chip through nanophotonic and nanoplasmonic structures (e.g., Kerr, optomechanical, single-photon nonlinearities). Towards such goals, we have developed a scalable, heterogeneous III-V/Si3N4 integration platform for quantum photonic circuits based on passive Si3N4 waveguides which directly incorporate nanophotonic single-photon sources based on self-assembled InAs quantum dots (QDs)[3]. InAs quantum dots constitute the most promising solid-state triggered single-photon sources to date[4], while SÍ3N4 waveguides offer low-loss propagation, tailorable dispersion and high Kerr nonlinearities which can be used for linear and nonlinear optical signal processing down to the quantum level. In our platform, the building blocks of which are shown in Fig. 1(a), active GaAs waveguide-based geometries containing InAs QDs are designed to efficiently capture QD-emitted single-photons. Captured photons, confined within the GaAs core, are then transferred with high efficiency into a passive Si3N4 waveguide network via adiabatic mode transformers. Figure 1(b) shows an example device fabricated with our platform: a GaAs microring resonator containing InAs quantum dots, evanescently coupled to a GaAs bus waveguide, which is in turn coupled to an underlying Si3N4 waveguide through adiabatic mode-transformers. The photoluminescence spectrum for this device, in Fig. 1(b), shows that a single QD exciton near 1125 nm, coupled to a microring whispering-gallery mode, acts as a source of single-photons that are launched directly into the Si3N4 waveguide. This geometry also allows us to effectively control the QD spontaneous emission decay lifetime by spectrally detuning the WGM with respect to the QD, as shown in Fig. 1(d).

Heterogeneous III-V / Si 3 N 4 integration for scalable quantum photonic circuits

We develop a scalable heterogeneous integration platform for quantum photonic circuits based on Si3N4 waveguides and on-chip, self-assembled InAs quantum dot-based single-photon sources. Hybrid waveguides, photonic crystals, and microring resonators are demonstrated.

A heterogeneous III-V/Si3N4 quantum photonic integration platform (Conference Presentation)

Photonic integration is an enabling technology for photonic quantum science, offering greaternscalability, stability, and functionality than traditional bulk optics. Here, we describe a scalable,nheterogeneous III-V/silicon integration platform to produce Si3N4 photonic circuits incorporatingnGaAs-based nanophotonic devices containing self-assembled InAs/GaAs quantum dots. Wendemonstrate pure single-photon emission from individual quantum dots in GaAs waveguidesnand cavities - where strong control of spontaneous emission rate is observed - directly launchedninto Si3N4 waveguides with > 90 % efficiency through evanescent coupling. To date, InAs/GaAsnquantum dots constitute the most promising solid state triggered single-photon sources, offeringnbright, pure and indistinguishable emission that can be electrically and optically controlled.nSi3N4 waveguides offer low-loss propagation, tailorable dispersion and high Kerr nonlinearities,ndesirable for linear and nonlinear optical signal processing down to the quantum level. Wencombine these two in an integration platform that will enable a new class of scalable, efficientnand versatile integrated quantum photonic devices.

A heterogeneous III-V / Si_3N_4 quantum photonic integration platform

We develop a heterogeneous III-V/Si3N4 integration platform for photonic integrated circuits incorporating on-chip, InAs quantum dot-based single-photon sources.

Optoelectronic cooling of mechanical modes in a semiconductor nanomembrane

Optical cavity cooling of mechanical resonators has recently become a research frontier where cooling of the vibrational motion of the resonators has been realized via photo-thermal force [1] and subsequently via radiation pressure [2–4]. One of the ultimate goals is reaching the vibrational ground state allowing quantum mechanical states of vibration - and the field offers a potential for hybrid quantum devices, and optical engineering of the phonon degrees of freedom. We present a new cavity cooling mechanism in semiconductors due to an optical excitation of carriers above the bandgap and the stress that these carriers introduce in the crystal lattice. The new method proves very power efficient and could potentially help in enabling ground state cooling of vibrational modes of semiconductor resonators. A laser cooled narrow-band phonon bath may open up a new avenue for photonics.

Few-quantum-dot lasing in photonic crystal nanocavities

A very smooth lasing transition in photonic crystal nanocavities with embedded quantum dots is observed and compared to the theory. Decay rate measurements reveal that only a few quantum dots are feeding the cavity.