High-efficiency Photon Research Articles

Inserting colloidal quantum dots into GaN mesa-array subsurface porous structures through electrochemical etching for display color conversion application

High-efficiency photon color conversion is an approach of great potential for implementing color display. Inspired by the observation of emission enhancement in a nanoscale cavity, a novel technique to fabricate an array of color converter by mixing colloidal quantum dots (QDs) with the electrolyte of an electrochemical etching (ECE) process is demonstrated. In this process, QDs flow with the electrolyte into the etched subsurface nanoscale porous structure (PS) and settle inside. Since the PS formation and hence QD insertion are controlled by the flow path of the applied electric current in the ECE process, this technique can be used for fabricating any graphic pattern. The nanostructure of such a QD-inserted mesa is examined to confirm QD insertion. Although only single-color mesa arrays are demonstrated in this paper, this technique can be used for fabricating a multiple-color mesa array if a QD or a light-emitting nanoparticle of higher thermal stability is available.

Micro-columnar CsBr:Eu storage phosphor for high-efficiency high energy photon radiography panels

Micro-columnar CsBr:Eu storage phosphor for high-efficiency high energy photon radiography panels

Highly efficient visible-light-driven CdS-loaded ZnO-GaN nanowire photoanode fabricated on Si for H2 evolution

GaN nanowires (NWs) are considered promising photoelectrode materials owing to their direct bandgap, which a high-efficiency photon absorption. Additionally, their conduction and valence bands straddle the water redox level, making them ideal candidates for solar water splitting without external biasing. Moreover, they have a large reactive surface area, which promotes charge separation. However, their practical applicability is limited because of their poor stability and low solar-to-hydrogen conversion efficiency, which can be mitigated by loading a passivation layer and cocatalyst onto the GaN NW surfaces. In this study, CdS/ZnO/GaN NW photoanodes were fabricated on Si substrates for photoelectrochemical water splitting. Vapor–liquid–solid GaN NWs were used as the core material on a Si substrate passivated with a ZnO conformal layer, and CdS was adopted as a cocatalyst. Compared to a ZnO/GaN NW photoanode, the CdS/ZnO/GaN NW photoanode showed a 10-fold increase in photocurrent density and a 47-fold increase in photoconversion efficiency under 1-Sun illumination. We propose that ZnO and CdS inhibit the photocorrosion of bare GaN NWs and thus reduce the surface defects, thereby lowering the photogenerated charge recombination rate. As a result, the stability of GaN NWs in a high pH electrolyte increases, leading to an efficient H2 evolution.

Non-collinear generation of ultra-broadband parametric fluorescence photon pairs using chirped quasi-phase matching slab waveguides.

Many optical quantum applications rely on broadband frequency correlated photon pair sources. We previously reported a scheme for collinear emission of high-efficiency and ultra-broadband photon pairs using chirped quasi-phase matching (QPM) periodically poled stoichiometric lithium tantalate (PPSLT) ridge waveguides. However, collinearly emitted photon pairs cannot be directly adopted for applications that are based on two-photon interference, such as quantum optical coherence tomography (QOCT). In this work, we developed a chirped QPM device with a slab waveguide structure. This device was designed to produce spatially separable (photon pair non-collinear emission) parametric fluorescence photon pairs with an ultra-broadband bandwidth in an extremely efficient manner. Using a non-chirped QPM slab waveguide, we observed a photon pair spectrum with a full-width-at-half-maximum (FWHM) bandwidth of 26 nm. When using a 3% chirped QPM slab waveguide, the FWHM bandwidth of the spectrum increased to 190 nm, and the base-to-base width is 308 nm. We also confirmed a generation efficiency of 2.4×106 pairs/(μW·s) using the non-chirped device, and a efficiency of 8×105 pairs/(μW·s) using the 3% chirped device under non-collinear emission conditions after single-mode fiber coupling. This is, to the best of our knowledge, the first report of frequency correlated photon pairs generation using slab waveguide device as a source. In addition, using slab waveguides as photon pair sources, we performed two-photon interference experiments with the non-chirped device and obtained a Hong-Ou-Mandel (HOM) dip with a FWHM of 7.7 μm and visibility of 98%. When using the 3% chirped device as photon pair source, the HOM measurement gave a 2 μm FWHM dip and 74% visibility.

Ultrawideband axion search using a Faraday haloscope

Dark matter is a major constituent of our Universe and the axion is a prime candidate. In this article, it is shown that by exploiting the axion induced magnetization in a magnetic rod and the Faraday effect, axions in the mass range $500--5000\text{ }\text{ }\mathrm{\ensuremath{\mu}}\mathrm{eV}$, a part of which ($>3500\text{ }\text{ }\mathrm{\ensuremath{\mu}}\mathrm{eV}$) is currently inaccessible to experiments, can be searched for using the same experimental setup in a year using the existing technologies. The magnetic rod is placed inside a high-finesse optical cavity, which by confining the probe light inside it increases the interaction time and thus enhances the Faraday effect. This rotates the plane of polarization of the probe light sufficiently and produces a robust signal. Axions of different mass are selected using a dc magnetic field. Detection is carried out by counting photons in the optical domain using a readily available and high quantum efficiency ($\ensuremath{\approx}1$) photon counter in a noise-free environment. An optical interferometric scheme that could provide spectroscopic information about the axion to be searched is also proposed.

Manipulating Exciton Dynamics toward Simultaneous High-Efficiency Narrowband Electroluminescence and Photon Upconversion by a Selenium-Incorporated Multiresonance Delayed Fluorescence Emitter.

Multiresonance thermal activated delayed fluorescence (MR-TADF) materials with an efficient spin-flip transition between singlet and triplet excited states remain demanding. Herein, we report an MR-TADF compound (BN-Se) simultaneously possessing efficient (reverse) intersystem crossing (ISC/RISC), fast radiative decay, close-to-unity quantum yield, and narrowband emission by embedding a single selenium atom into a common 4,4'-diazaborin framework. Benefitting from the high RISC efficiency accelerated by the heavy-atom effect, organic light-emitting diodes (OLEDs) based on BN-Se manifest excellent performance with an external quantum efficiency of up to 32.6% and an ultralow efficiency roll-off of 1.3% at 1000 cd m-2. Furthermore, the high ISC efficiency and small inherent energy loss also render BN-Se a superior photosensitizer to realize the first example of visible (λex > 450 nm)-to-UV (λem < 350 nm) triplet-triplet annihilation upconversion, with a high efficiency (21.4%) and an extremely low threshold intensity (1.3 mW cm-2). This work not only aids in designing advanced pure organic molecules with fast exciton dynamics but also highlights the value of MR-TADF compounds beyond OLED applications.

Synthesis and characterization of red-emitting Yb/Ho-CaSiO3 upconversion phosphors

Host material plays an important role in obtaining efficient photon upconversion and downshifting luminescence. Generally, fluoride and oxyfluoride glasses-based materials are used for high-efficiency photon upconversion. However, the poor thermal stability of fluoride glasses and the toxicity of fluorine ions limit their applications. In this report, Yb/Ho-doped CaSiO3 wollastonite phosphors have been demonstrated as efficient red-emitting upconversion phosphors. The phosphors have been synthesized by microwave hydrothermal process followed by heat treatment at 1050 ​°C in the air environment. 2M phase of the β-wollastonite has been confirmed by X-ray diffraction and Raman spectroscopy while the existence of the Yb and Ho dopants in the CaSiO3 lattice has been confirmed by X-ray photoelectron spectroscopy. The synthesized samples showed strong red upconversion emission centered at 662 ​nm and near-infrared downshifting emissions at 2016 ​nm upon 980 ​nm excitation. The emissions were found to depend significantly on the Ho concentration. Temporal evolution of the main emission bands was investigated to show that the energy transfer upconversion from Yb to Ho ions was responsible for the efficient upconversion and downshifting phenomenon.

Realization of efficient 3D tapered waveguide-to-fiber couplers on a nanophotonic circuit.

We report the realization of efficiently coupled 3D tapered waveguide-to-fiber couplers (TWCs) based on standard lithography techniques. The 3D TWC design is capable of achieving highly efficient flat-cleaved fiber to silicon nitride photonic waveguide coupling, with T ≈ 95% polarization-insensitive coupling efficiency, wide bandwidth, and good misalignment tolerance. Our fabricated 3D TWCs on a functional nanophotonic circuit achieve T ≈ 85% coupling efficiency. Beyond applications in high-efficiency photon coupling, the demonstrated 3D lithography technique provides a complementary approach for mode field shaping and effective refractive index engineering, potentially useful for general applications in integrated photonic circuits.

Three-Dimensional Electrical Control of the Excitonic Fine Structure for a Quantum Dot in a Cavity.

The excitonic fine structure plays a key role for the quantum light generated by semiconductor quantum dots, both for entangled photon pairs and single photons. Controlling the excitonic fine structure has been demonstrated using electric, magnetic, or strain fields, but not for quantum dots in optical cavities, a key requirement to obtain high source efficiency and near-unity photon indistinguishability. Here, we demonstrate the control of the fine structure splitting for quantum dots embedded in micropillar cavities. We propose and implement a scheme based on remote electrical contacts connected to the pillar cavity through narrow ridges. Numerical simulations show that such a geometry allows for a three-dimensional control of the electrical field. We experimentally demonstrate tuning and reproducible canceling of the fine structure, a crucial step for the reproducibility of quantum light source technology.

Enhanced single photon emission in silicon carbide with Bull’s eye cavities

The authors demonstrate a Bull’s eye cavity design that is composed of circular Bragg gratings and micropillar optical cavity in 4H silicon carbide (4H-SiC) for single photon emission. Numerical calculations are used to investigate and optimize the emission rate and directionality of emission. Thanks to the optical mode resonances and Bragg reflections, the radiative decay rates of a dipole embedded in the cavity center is enhanced by 12.8 times as compared to that from a bulk 4H-SiC. In particular, a convergent angular distribution of the emission in far field is simultaneously achieved, which remarkably boost the collection efficiency. The findings of this work provide an alternative architecture to manipulate light–matter interactions for achieving high-efficient SiC single photon sources towards applications in quantum information technologies.

Enhanced emission directivity from asymmetrically strained colloidal quantum dots.

Current state-of-the-art quantum dot light-emitting diodes have reached close to unity internal quantum efficiency. Further improvement in external quantum efficiency requires more efficient photon out-coupling. Improving the directivity of the photon emission is considered to be the most feasible approach. Here, we report improved emission directivity from colloidal quantum dot films. By growing an asymmetric compressive shell, we are able to lift their band-edge state degeneracy, which leads to an overwhelming population of exciton with in-plane dipole moment, as desired for high-efficiency photon out-coupling. The in-plane dipole proportion determined by back-focal plane imaging method is 88%, remarkably higher than 70% obtained from conventional hydrostatically strained colloidal quantum dots. Enhanced emission directivity obtained here opens a path to increasing the external quantum efficiencies notably.

Optimal detection of ultra-broadband bi-photons with quantum nonlinear SU(1,1) interference

The visibility of nonlinear SU(1,1) interference directly reflects the nonclassical properties of entangled bi-photons and squeezed light with practically unlimited bandwidth, high efficiency and ultra-high photon flux, orders of magnitude beyond the abilities of standard photo-detectors. We study experimentally the dependence of the SU(1,1) visibility on the phase matching conditions and beam parameters in a free-space configuration, and show that maximal SU(1,1) visibility requires extreme collinear conditions, which deviate from the conditions for maximal nonlinear conversion. We demonstrate near-ideal visibility of ∼95% (limited only by internal loss) in an ultra-broadband SU(1,1), interferometer with over 120 THz of squeezed light bandwidth. Utilizing this analysis we demonstrate efficient detection of the spectral phase of single-cycle bi-photons and precise compensation of the dispersion over a full octave of bandwidth.

Submegahertz spectral width photon pair source based on fused silica microspheres

High-efficiency submegahertz bandwidth photon pair generators will enable the field of quantum technology to transition from laboratory demonstrations to transformational applications involving information transfer from photons to atoms. While spontaneous parametric processes are able to achieve high-efficiency photon pair generation, the spectral bandwidth tends to be relatively large, as defined by phase-matching constraints. To solve this fundamental limitation, we use an ultrahigh quality factor ( Q ) fused silica microsphere resonant cavity to form a photon pair generator. We present the full theory for the spontaneous four-wave mixing (SFWM) process in these devices, fully taking into account all relevant source characteristics in our experiments. The exceptionally narrow (down to kilohertz-scale) linewidths of these devices result in a reduction in the bandwidth of the photon pair generation, allowing submegahertz spectral bandwidth to be achieved. Specifically, using a pump source centered around 1550 nm, photon pairs with the signal and idler modes at wavelengths close to 1540 and 1560 nm, respectively, are demonstrated. We herald a single idler-mode photon by detecting the corresponding signal photon, filtered via transmission through a wavelength division multiplexing channel of choice. We demonstrate the extraction of the spectral profile of a single peak in the single-photon frequency comb from a measurement of the signal–idler time of emission distribution. These improvements in device design and experimental methods enabled the narrowest spectral width ( Δ ν = 366 kHz ) to date in a heralded single-photon source based on SFWM.

Monte Carlo simulation fused with target distribution modeling via deep reinforcement learning for automatic high-efficiency photon distribution estimation

Particle distribution estimation is an important issue in medical diagnosis. In particular, photon scattering in some medical devices extremely degrades image quality and causes measurement inaccuracy. The Monte Carlo (MC) algorithm is regarded as the most accurate particle estimation approach but is still time-consuming, even with graphic processing unit (GPU) acceleration. The goal of this work is to develop an automatic scatter estimation framework for high-efficiency photon distribution estimation. Specifically, a GPU-based MC simulation initially yields a raw scatter signal with a low photon number to hasten scatter generation. In the proposed method, assume that the scatter signal follows Poisson distribution, where an optimization objective function fused with sparse feature penalty is modeled. Then, an over-relaxation algorithm is deduced mathematically to solve this objective function. For optimizing the parameters in the over-relaxation algorithm, the deep Q -network in the deep reinforcement learning scheme is built to intelligently interact with the over-relaxation algorithm to accurately and rapidly estimate a scatter signal with the large range of photon numbers. Experimental results demonstrated that our proposed framework can achieve superior performance with structural similarity &gt; 0.94 , peak signal-to-noise ratio &gt; 26.55 dB , and relative absolute error &lt; 5.62 % , and the lowest computation time for one scatter image generation can be within 2 s.

Correlation between geometric parametric instability sidebands in graded-index multimode fibers.

The spectral analysis of the light propagating in normally dispersive graded-index multimode fibers is performed under initial noisy conditions. Based on the obtained spectra with multiple simulations in the presence of noise, we investigate the correlation in energy between the well-separated spectral sidebands through both the scattergrams and the frequency-dependent energy correlation map and find that conjugate couples are highly correlated while cross-combinations exhibit a very poor degree of correlation. These results reveal that the geometric parametric instability processes associated with each sideband pair occur independently from each other, which can provide significant insights into the fundamental dynamical effect of the geometric parametric instability and facilitate the future implementation of high-efficiency photon pair sources with reduced Raman decorrelations.

Engineering Sensitized Photon Upconversion Efficiency via Nanocrystal Wavefunction and Molecular Geometry.

Triplet energy transfer from inorganic nanocrystals to molecular acceptors has attracted strong attention for high-efficiency photon upconversion. Here we study this problem using CsPbBr3 and CdSe nanocrystals as triplet donors and carboxylated anthracene isomers as acceptors. We find that the position of the carboxyl anchoring group on the molecule dictates the donor-acceptor coupling to be either through-bond or through-space, while the relative strength of the two coupling pathways is controlled by the wavefunction leakage of nanocrystals that can be quantitatively tuned by nanocrystal sizes or shell thicknesses. By simultaneously engineering molecular geometry and nanocrystal wavefunction, energy transfer and photon upconversion efficiencies of a nanocrystal/molecule system can be improved by orders of magnitude.

High-efficiency photon–electron coupling resonant emission in GaN-based microdisks on Si**Project supported by the National Key R&D Program of China (Grant Nos. 2016YFB0400102 and 2016YFB0400602), the National Natural Science Foundation of China (Grant Nos. 61674076, 61422401 and 51461135002), the Collaborative Innovation Center of Solid State Lighting and Energy-Saving Electronics, Open Fund of the State Key Laboratory on Integrated Optoelectronics (Grant No.

Resonance effects caused by the photon–electron interaction are a focus of attention in semiconductor optoelectronics, as they are able to increase the efficiency of emission. GaN-on-silicon microdisks can provide a perfect cavity structure for such resonance to occur. Here we report GaN-based microdisks with different diameters, based on a standard blue LED wafer on a Si substrate. A confocal photoluminescence spectroscopy is performed to analyze the properties of all microdisks. Then, we systematically study the effects of radial modes and axial modes of these microdisks on photon–electron coupling efficiency by using three-dimensional finite-difference time-domain simulations. For thick microdisks, photon–electron coupling efficiency is found to greatly depend on the distributions of both the radial modes and the axial modes, and the inclined sidewalls make significant influences on the axial mode distributions. These results are important for realization of high-efficiency resonant emission in GaN-based microcavity devices.

High-Rate, High-Fidelity Entanglement of Qubits Across an Elementary Quantum Network.

We demonstrate remote entanglement of trapped-ion qubits via a quantum-optical fiber link with fidelity and rate approaching those of local operations. Two ^{88}Sr^{+} qubits are entangled via the polarization degree of freedom of two spontaneously emitted 422nm photons which are coupled by high-numerical-aperture lenses into single-mode optical fibers and interfere on a beam splitter. A novel geometry allows high-efficiency photon collection while maintaining unit fidelity for ion-photon entanglement. We generate heralded Bell pairs with fidelity 94% at an average rate 182 s^{-1} (success probability 2.18×10^{-4}).

The influence of PT-symmetric degree on extraordinary optical properties of one-dimensional periodic optical waveguide networks

In this paper, we design two kinds of one-dimensional (1D) equal-length two-segment-connected periodic triangular optical waveguide networks with different PT-symmetric degrees, one-dimensional PT-symmetric triangular optical waveguide network (1D PT-TOWN) and one-dimensional non-PT-symmetric triangular optical waveguide network (1D nPT-TOWN). It is found that no matter whether the network is PT-symmetric or non-PT-symmetric, as long as a PT-symmetric unit exists, both gain and loss photonic nonpropagation modes will be created and their frequencies are all the same. Consequently, ultrastrong extraordinary transmissions can be generated by two networks, which can arrive at 2.2×106 and 8.7×104 respectively. However, because the PT-symmetric degree of the former is higher than the latter, the transmission of the former is two orders of magnitude larger than the latter. Therefore, The PT-symmetric degree influences seriously on the photonic modes distribution and transmission of the network. Our research may deepen people’s understanding of PT symmetry, which has potential application to the design of high-efficiency photon energy storage device, optical switches, optical amplifiers, and so on.

Design of spontaneous parametric down-conversion in integrated hybrid SixNy-PPLN waveguides.

High-efficient and high-purity photon sources are highly desired for quantum information processing. We report the design of a chip-scale hybrid SixNy and thin film periodically-poled lithium niobate waveguide for generating high-purity type-II spontaneous parametric down-conversion (SPDC) photons in the telecommunication band. The modeled second harmonic generation efficiency of 225% W-1 • cm-2 is obtained at 1560nm. Joint spectral analysis is performed to estimate the frequency correlation of SPDC photons, yielding intrinsic purity with up to 95.17%. The generation rate of these high-purity photon pairs is estimated to be 2.87 × 107 pairs/s/mW within the bandwidth of SPDC. Our chip-scale hybrid waveguide design has the potential for large-scale on-chip quantum information processing and integrated photon-efficient quantum key distribution through high-dimensional time-energy encoding.