Advanced technologies for quantum photonic devices based on epitaxial quantum dots

Abstract

Quantum photonic devices are candidates for realizing practical quantum computers and networks. The development of integrated quantum photonic devices can greatly benefit from the ability to incorporate different types of materials with complementary, superior optical or electrical properties on a single chip. Semiconductor quantum dots (QDs) serve as a core element in the emerging modern photonic quantum technologies by allowing on-demand generation of single-photons and entangled photon pairs. During each excitation cycle, there is one and only one emitted photon or photon pair. QD photonic devices are on the verge of unfolding for advanced quantum technology applications. In this review, we focus on the latest significant progress of QD photonic devices. We first discuss advanced technologies in QD growth, with special attention to droplet epitaxy and site-controlled QDs. Then we overview the wavelength engineering of QDs via strain tuning and quantum frequency conversion techniques. We extend our discussion to advanced optical excitation techniques recently developed for achieving the desired emission properties of QDs. Finally, the advances in heterogeneous integration of active quantum light-emitting devices and passive integrated photonic circuits are reviewed, in the context of realizing scalable quantum information processing chips.

Keywords: epitaxial growth technology; heterogeneous photonic integration; quantum dot; quantum photonic device; wavelength tuning.

Figure 1.

(a-c) A 1 μm × 1 μm AFM scan of a quantum dot (QD) sample, along with a close-up atomic force microscopy (AFM) map (b) and height profiles (c) of a typical single QD. Reproduced with permission.^[89] Copyright 2017, American Chemical Society. (d) Cross-sectional 3D view of an AFM image of a nanohole in an AlGaAs layer, along with the sketch of the sample structure and the top view of the AFM measurement result; (e-f) Microphotoluminescence spectrum of a representative QD under non-resonant excitation (e) and resonant two-photon excitation (f). Reproduced with permission.^[103] Copyright 2017, Springer Nature.

Figure 2.

(a) Schematic of a pattern consisting of a 3×3 array of holes initially etched onto the GaAs substrate; (b) Integrated PL intensity map (integration range of 900 nm to 1000nm) of InAs dots grown in the initial pattern; (c-d) AFM height images of one site-controlled dot (c) and a pair of site-controlled dots (d); Reproduced with permission.^[120] Copyright 2008, American Institute of Physics. (e) Schematic view of a fully processed and overgrown structure with site-controlled QD; (f) AFM image of two site-controlled QDs positioned over a buried stressor with an aperture diameter of ≈700 nm. Reproduced with permission.^[116] Copyright 2017, American Institute of Physics. (g) AFM image of a cleaved quantum dot sample grown on GaAs (111)B substrate, along with high-resolution scanning electron microscopy image of the sample after the post-growth substrate removal procedure. Reproduced with permission.^[72] Copyright 2013, Springer Nature.

Figure 3.

(a) Schematic of a clad nanowire (NW) quantum dot tapered at the top. (b) Top-view SEM image of a clad NW showing the in-plane hexagonal symmetry of its core and the embedded quantum dot (blue circle); (c) SEM picture of an array of nanowires, showing their homogeneous positioning. Reproduced with permission.^[133] Copyright 2014, American Chemical Society.

Figure 4.

Fabrication flow of piezo substrate strain tuning device. (a) The fabrication flow starts with an as-grown QDs sample. (b) Structured gold metal is deposited on top. (c) (d) Mesa of QDs nano-membrane is prepared by vertical etching of diluted sulfuric acid and then the sacrificial layer is removed with HF etching. (e) In the last step, the sample chip is flipped onto a gold-coated piezo chip and QDs membranes are transferred to the piezo chip by thermal compression bonding. (f) The final device can be electrically wired and is ready to use.

Figure 5.

(a) Colored-coded PL intensity of QD1 as a function of emission energy and voltage applied to the piezo substrate. Reproduced with permission.^[148] Copyright 2010, American Physical Society. (b) The FSS tuning behavior for a QD whose anisotropic axis is aligned with strain direction. Reproduced with permission.^[164] Copyright 2014, American Chemical Society.

Figure 6.

(a) Three independent voltages (V1, V2, V3) applied across pairs of legs and the top (grounded) contact allow the in-plane stress in the QD membrane to be controlled. Reproduced with permission.^[167] Copyright 2015, American Physical Society. (b) A three-dimensional stressor that can be used to independently tune the FSS and exciton energy in QDs. Reproduced with permission.^[168] Copyright 2015, American Physical Society. (c) An artistic sketch of orthogonal strain engineering chip based on structured piezo film. (d) Four top electrodes are marked as A, B, C and D. A cross-section of the designed device. Piezo film is free standing at the center. (e) Exciton energy shifts are plotted against the voltage applied on legs B and D. Reproduced with permission.^[170] Copyright 2016, Springer Nature.

Figure 7.

Sketch of FSS is plotted as a function of the exciton wavelength (energy), at different values of biased voltage V_BD. The solid lines are theoretical fits. Exciton energy at which FSS ~0 is tuned by 3.7 meV. Reproduced with permission.^[170] Copyright 2016, Springer Nature.

Figure 8.

(a) Illustration of the sum frequency generation in periodically poled lithium niobate (PPLN) waveguides. Quasi phase matching (QPM) is achieved by choosing the proper poling period to compensate the phase mismatch of the nonlinear process. Reproduced with permission.^[175] Copyright 2012, American Institute of Physics. (b) Detection scheme based on quantum frequency conversion: single photons in the telecom band are upconverted to the visible range through efficient sum frequency generation and subsequently detected using silicon single-photon avalanche diode (SPAD). (c) Comparison of the two detection schemes for the telecom-band single photons: Direct detection using InGaAs single-photon detectors (left), and QFC-based detection scheme using SPAD (right). The detection dynamic range has been improved by a factor of 25. Reproduced with permission.^[179] Copyright 2010, Springer Nature.

Figure 9.

(a) Scanning electron micrograph of a 40-μm-radius silicon nitride microresonator (left) and the schematic of the four-wave mixing Bragg scattering process implemented (right): the two pumps are in the 1550 nm band, while the signal and the frequency converted idlers are all in the 930 nm band. Reproduced with permission.^[187] Copyright 2016, Springer Nature. The frequency translation is an integer number of the free spectral range of the microresonator. (b) The upper figure shows the signal transmission based on a swept tunable laser for several different pump powers, going from over coupling in the linear case (green line) to critical coupling (blue line) and finally under coupling (red dotted line) as the pump powers increase. The lower figure plots the output spectrum of the frequency converter with a 1 FSR separation between the two pump lasers (frequency translation ≈ 572 GHz) for a total pump power of 20 mW on-chip (10 mW each). The power in the 930 nm band is normalized by the input signal power, corresponding to the on-chip conversion efficiency for a narrowband input signal. Reproduced with permission.^[188] Copyright 2019, American Physical Society.

Figure 10.

(a) (b) (c) Above-band resonant, quasi-resonant and resonant optical excitation schemes. (d) The spectrum of photons from above-band excitation. (e) The spectrum of photons from quasi-resonant excitation. (f) The Mollow-triplet spectrum of photons from resonant excitation. Reproduced with permission.^[190] Copyright 2013, Science China Press.

Figure 11.

(a) Setup of the confocal microscopy without polarization suppression.^[204] Copyright 2019, American Chemical Society. (b) The confocal microscopy setup with polarization suppression for resonant excitation.^[204] Copyright 2019, American Chemical Society. (c) Schematic for orthogonal excitation and detection.^[202] Copyright 2007, American Physical Society.

Figure 12.

(a) Power-dependent resonance fluorescence count rate for three different excitation methods. (b) Time-dependent resonance fluorescence count rates under external modulation of laser power as indicated by the dark line in the inset. Reproduced with permission.^[206] Copyright 2014, American Chemical Society.

Figure 13.

(a) Spectrum of a GaAs QD under phonon-assisted two-photon excitation for optical detuning of the laser energy and pulse length. Reproduced with permission.^[211] Copyright 2017, American Chemical Society. (b) Power dependent studies of the resonant TPE. The results of the phonon-assisted excitation scheme are shown as green circles. Reproduced with permission.^[211] Copyright 2017, American Chemical Society. (c) The ratio of the experiment data: 2X/X. Reproduced with permission.^[210] Copyright 2017, American Physical Society.

Figure 14.

(a) Conceptual quantum photonic circuit composed of a waveguide interferometric network with a directly integrated GaAs nanophotonic device containing a single InAs quantum dot. The zoomed-in image of the GaAs device region (inside the dashed boundary box) shows details of the geometry and operation principle. The light-matter interaction section of the device promotes efficient coupling between the InAs quantum dot and a confined optical mode (here, a wave confined in a GaAs waveguide). Adiabatic mode transformers allow light from the QD in the light-matter interaction region to be efficiently transferred to a Si₃N₄ waveguide. (b) Fabrication process in the wafer-bonding approach. The bonded GaAs / Si₃N₄ wafer is shown inside the dotted line, schematically at the top, and imaged in a cross-sectional scanning electron micrograph. After wafer bonding, two subsequent electron-beam lithography and etch steps (first the GaAs layer, then the Si₃N₄) are used to define the geometry in (a). (c) GaAs microring resonator coupled to a GaAs bus waveguide terminated into mode transformers fabricated through the process in (b). (d) Photoluminescence spectrum for the microring in (c), showing single quantum dot transition coupled to a whispering-gallery mode. Inset: second-order correlation showing antibunching characteristic of single-photon emission. Reproduced with permission.^[226] Copyright 2017, Springer Nature.

Figure 15.

Schematics of hybrid photonic circuit platforms produced through pick-and-place techniques, including passive waveguides and quantum dot-based nanophotonic single-photon sources. (a) InP NW containing InAsP QD, encapsulated in a SiN waveguide and capped with a layer of polymethyl acrylate (PMMA). The nanowires were produced through a selective-area and vapor-liquid-solid epitaxy process.^[129] Reproduced with permission.^[239] Copyright 2016, American Chemical Society. (b) InP nanobeam with embedded InAs QDs, placed above a Si waveguide on a SiO₂ waveguide. Reproduced with permission.^[240] Copyright 2017, American Chemical Society. (c) GaAs photonic crystal cavity containing InAs QDs, placed over a GaAs waveguide on a SiO₂ substrate, spaced from it by distance d, by way of a planarized spin-on-glass (SOG) layer. Reproduced with permission.^[242] Copyright 2018, Optical Society of America. (d) Schematic of quantum memories based on diamond nanobeams with NV centers, coupled to SiN waveguides. Reproduced with permission.^[238] Copyright 2014, American Physical Society. (e) Right panel: illustration of NbN superconducting nanowire single-photon detector on a SiN membrane being transferred onto a silicon-on-insulator photonic waveguide. Right panel: Schematic of a photonic chip with four waveguide-integrated detectors (A1, A2, B1 and B2). Reproduced with permission.^[256] Copyright 2015, Springer Nature.