Investigation of cavity modes and direct observation of Purcell enhancement in 2D photonic crystal defect microcavities

We demonstrate our ability to control and manipulate the optical modes in 2D Photonic Crystal Defect cavities and investigate their coupling to InGaAs self-assembled quantum dots. Our results enable us to probe the nature of individual cavity modes and directly investigate cavity QED phenomena. For the lowest mode volume cavities investigated, consisting of a single missing air hole within a hexagonal lattice, we have measured a clear Purcell enhancement of the light-matter interaction in the weak coupling regime. For QDs on-resonance with localized cavity modes this translates to a shortening of the quantum dot spontaneous emission lifetime by a factor 2 when compared to off-resonance dots.     

Positioning photonic crystal cavities to single InAs quantum dots

We have investigated the optical properties of planar photonic crystal cavities formed by removing a single hole from a two-dimensional square lattice of air holes etched through a thin GaAs slab. We have demonstrated cavity resonances with quality factors (Q’s) as high as 8500, using an internal light source provided by an ensemble of InAs quantum dots (QDs) grown by molecular beam epitaxy (MBE). The high-Q modes are confined to a very small mode volume, V = 0.7(λ/n)³, making them attractive to study in the context of cavity quantum electrodynamics with single QDs, where a high is needed to observe the strong coupling between an electronic state of the dot and the optical cavity mode. To this end, we have developed an accurate and robust alignment technique that positions a photonic crystal cavity to a single QD with 25 nm resolution. We present the details of this new technology and demonstrate its effectiveness by strategically positioning a number of QDs within photonic crystal cavities at points where the electric field intensity is high.     

Collective coupling of randomly dispersed oscillators with cavities filled with zero-index metamaterials

In cavity quantum electrodynamics, it is hard to enhance the coupling strength between quantum dot (QD) and cavity, owing to the limited choice of QDs and the positional uncertainty brought by the inhomogeneous cavity fields. In this paper, we randomly distribute N oscillators with oscillating strength G = G ₀ into a cavity filled with a zero-index metamaterial (ZIM). Because of the enhanced uniform fields, each oscillator couples to the field maximum and the N oscillators are equivalent to one oscillator with effective N G ₀. This provides a way to enhance the coupling strength just by adding the number of QDs. Both simulation and experiment demonstrate the adjustable coupling strength in ZIM-filled cavities.     

High quality factor nitride-based optical cavities: microdisks with embedded GaN/Al(Ga)N quantum dots

We compare the quality factor values of the whispering gallery modes of microdisks (μ-disks) incorporating GaN quantum dots (QDs) grown on AlN and AlGaN barriers by performing room temperature photoluminescence (PL) spectroscopy. The PL measurements show a large number of high Q factor resonant modes on the whole spectrum, which allows us to identify the different radial mode families and to compare them with simulations. We report a considerable improvement of the Q factor, which reflects the etching quality and the relatively low cavity loss by inserting QDs into the cavity. GaN/AlN QDs-based μ-disks show very high Q values (Q>7000) whereas the Q factor is only up to 2000 in μ-disks embedding QDs grown on the AlGaN barrier layer. We attribute this difference to the lower absorption below bandgap for AlN barrier layers at the energies of our experimental investigation.     

Polar optical phonon states and their electron-phonon coupling properties in a wurtzite nitride quantum dot

Within the framework of the macroscopic dielectric continuum model, the interface-optical-propagating (IO-PR) mixing phonon modes of a quasi-zero-dimensional (Q0D) wurtzite cylindrical quantum dot (QD) structure are derived and studied. The approximative analytical-phonon-states of IO-PR mixing modes are given. It is found that there are two types of IO-PR mixing phonon modes, i.e. ρ-IO/z-PR mixing modes and the z-IO/ρ-PR mixing modesexisting in Q0D wurtzite QDs. And each IO-PR mixing modes also have symmetrical and antisymmetrical forms. Via a standard procedure of field quantization, the Fröhlich Hamiltonians of electron-(IO-PR) mixing phonons interaction are obtained. And the orthogonal relations of polarization eigenvectors for these IO-PR mixing modes are also deduced. Numerical calculations of dispersive relation and electron-phonon coupling properties on a wurtzite GaN cylindrical QD are carried out. The behaviors that the IO-PR mixing phonon modes in wurtzite QDs reduce to the IO modes and PR modes in wurtzite QW and QWR systems are analyzed deeply from both of the viewpoints of physics and mathematics. The result shows that the present theories of polar mixing phonon modes in wurtzite cylindrical QDs are consistent with the phonon modes theories in wurtzite QWs and QWR systems. The coupling properties of electron-(IO-PR) mixing modes interactions are studied and analyzed in detail. An abnormal increase of electron-phonon coupling strength are observed as the azimuthal quantum numbers and order of phonon modes increase, which is ascribed to the modulation effect of different dielectric functions of wurtzite crystals in radius- and axial-directions. The analytical electron-phonon interaction Hamiltonians obtained here are useful for further investigating phonon influence on optoelectronics properties of wurtzite Q0D QD structures.     

Spontaneous emission of two quantum dots in a single-mode cavity

The spontaneous emission spectrum from two quantum dots （QDs） that are strongly coupled with a single-mode nanocavity is investigated using rigorous numerical calculations and simple analytical solutions of quantum dynamics. The emission spectra both from the side and along the axis of the cavity are considered. Modification of two parameters, the coupling strength and the detuning between the transition frequencies of the two quantum dots, allows us to efficiently control the shape of the spontaneous emission spectrum. Different profiles and their physical origins can be well understood in the dressed-state picture for the light-QD interaction in the on-resonance and off-resonance situations. In the on-resonance situation, the emission spectra exhibit symmetric features, and they are not altered by the asymmetry in the coupling pa- rameters. The axis spectra show two emission peaks while the side spectra have three emission peaks. In the off-resonance situation, the emission spectra always show an asymmetrical three-peak feature. When the two QDs have different decay parameters, singular features （a peak or a dip） can take place at the frequency of the cavity mode, and this is attributed to the unbalanced process of the emission and absorption of a single photon.     

二维GaAs 基光子晶体微腔的制作与光谱特性分析

研究了以InAs量子点为有源区的二维GaAs基光子晶体微腔的设计与制作,测试并分析了室温下微腔的光谱特性.观察到了波长约为1137 nm,谱线半高宽度约为1 nm的尖锐低阶谐振模式发光峰.我们比较了不同刻蚀条件下光子晶体微腔的发光谱线,结果表明空气孔洞截面的垂直度是影响光子晶体微腔发光特性的重要因素之一.通过调节干法刻蚀工艺,改变空气孔半径与晶格常数的比率,可以在较大范围内调节谐振模式发光峰位置,达到谐振模式与量子点发光峰调谐的目的.     

Enhanced emission of single quantum dot formed by interface fluctuations in photonic-crystal microcavities

We fabricated photonic-crystal (PhC) microcavities tuned to GaAs quantum dots (QDs) formed by interface fluctuation for the first time and observed the spontaneous emission enhancement in a weak coupling regime. A QD is a very thin GaAs quantum well (QW), and its interface steps exhibit quantum dot-like behavior. The emission intensity from the PhC cavity was stronger than that from the area where no PhC pattern was fabricated and the overall shape of the photoluminescence (PL) agreed with the cavity mode calculated with the three-dimensional (3D) finite-difference time domain (FDTD) method. The spontaneous emission enhancement factor was 10.     

等效零折射率材料微腔中均匀化腔场作用下的简正模劈裂现象

研究了零折射率材料微腔中人造原子与腔模的相干耦合现象.首先通过数值模拟的方法研究了在二维光子晶体微腔中填充阻抗匹配的零折射率材料后腔模的场分布.结果表明零折射率材料的引入使得原本以驻波场形式存在的腔模分布在整个微腔中变得近似均匀且值最大.其次,将人造原子放入腔中的不同位置并与腔模耦合,结果从频谱上观察到腔模的劈裂与人造原子在腔中的位置无关.最后,利用微波实验,通过开口谐振环等效的人造原子与一维复合左右手传输线等效的零折射率材料微腔之间的耦合验证了仿真结果的准确性.该结果为腔量子电动力学中量子点对位难的问题提供了新的方案,同时零折射率材料微腔也为今后研究原子与光子之间的相互作用提供了一个新的平台.     

The Properties of the Polarons’ Ground State in Coupling Spherical Quantum Dots

International Journal of Theoretical Physics - Investigations on the properties of polarons in coupling quantum dots (QDs) are useful for the designs of quantum devices and applications of QDs....     

Entanglement transfer from two-mode squeezed vacuum light to spatially separated mechanical oscillators via dissipative optomechanical coupling

In this paper, we propose a scheme to generate an entangled state between two spatially separated movable mirrors by injecting the two-mode squeezed optical reservoir to the dissipative optomechanics, in which the movable mirrors can modulate the linewidth of the cavity modes. When the coupling between the mirrors and the corresponding cavity modes is weak, the two driven cavity fields can respectively behave as the squeezed-vacuum reservoir for the two movable mirrors by utilizing the effect of completely destructive interference of quantum noise. Thus the mechanical modes are prepared in a two-mode squeezed vacuum state. Moreover,when the coupling between the two mirrors and the cavities modes is strong, the entanglement between the two movable mirrors decreases because photonic excitation can preclude the completely destructive interference of quantum noise, but the movable mirrors are still entangled.     

Quantum coherence transfer between an optical cavity and mechanical resonators

This study highlights the theoretical investigation of quantum coherence in mechanical oscillators and its transfer between the cavity and mechanical modes of an optomechanical system comprising an optical cavity and two mechanical oscillators that,in this study, were simultaneously coupled to the optical cavity at different optomechanical coupling strengths. The quantum coherence transfer between the optical and mechanical modes is found to depend strongly on the relative magnitude of the two optomechanical couplings. The laser power, decay rates of the cavity and mechanical oscillators, environmental temperature, and frequency of the mechanical oscillator are observed to significantly influence the investigated quantum coherences. Moreover,quantum coherence generation in the optomechanical system is restricted by the system's stability condition, which helps sustain high and stable quantum coherence in the optomechanical system.     

Single photon sources with single semiconductor quantum dots

In t.his contribution, we briefly recall the basic concepts of quantum optics and properties of semicon- ductor quantum clot. （QD） which a.re necessary to the nnderstanding of the physics of single-photon generation with single QDs. Firstly, we address the theory of quantmn emitter-cavity system, the fluorescence and optical properties of semiconductor QDs, and the photon statistics as well as opti- cal properties of the QDs. We then review the localizatioll of single semiconductor QDs in quantum confined optical microcavity systems to achieve their overall optical properties and perfornances in terms of strong coupling regime, elfieiency, directionality, and polarization control. Furthermore, we will discuss the recenl, progress on the fabrication of single photon sources, and various a.pproaehes for embedding single QDs into mieroca,vities or photonic crystal nanoeavities and show how to ex- tend the wavelength range. We focus in part;icular on new generations of electrically driven QD single photon source leading to high repetition rates, efficiencies at elevated temperature operation. Besides strong eoupling regime, and high collection new development;s of room temperature sin- gle photon emission in the strong coupling regime are reviewed. The generation of indistinguishable photons and remaining challenges for pract ical single-photon sources are also discussed.     

Long wavelength strain-engineered InAs multi-layer stacks quantum dots laser diode on GaAs substrate

The growth of GaAs based 1.5 ??m multi-layer stacked InAs quantum dots (QDs) has been investigated by solid-source molecular beam epitaxy (SSMBE), which was very important devices for transmission window. Owing to a strong electronic coupling between the QDs layers and the quantum wells (QWs), and antimony (Sb) introduced by for long-wavelength semiconductor lasers were obtained. The device structure for QDs laser diodes (LDs) with a cavity length of 1000 ??m and stripe width of 100 ??m as well as the device fabrication results will also be presented. The output performance was achieved with continuous wave (CW) operation, the measurement were from 20 to 60°C with a temperature step of 10°C. The threshold current density was 168 A/cm², and the CW operating up to 20 mW at room temperature (RT) was achieved.     

Statistical theory of a quantum emitter strongly coupled to Anderson-localized modes

A statistical theory of the coupling between a quantum emitter and Anderson-localized cavity modes is presented based on a dyadic Green's function formalism. The probability of achieving the strong light-matter coupling regime is extracted for an experimentally realistic system composed of InAs quantum dots embedded in a disordered photonic crystal waveguide. We demonstrate that by engineering the relevant parameters that define the quality of light confinement, i.e., the light localization length and the loss length, strong coupling between a single quantum dot and an Anderson-localized cavity is within experimental reach. As a consequence, confining light by disorder provides a novel platform for quantum electrodynamics experiments.     

Optomechanical dynamics in detuned whispering-gallery modes cavity

The dynamics of a microresonator in detuned whispering-gallery modes (WGM) cavity opto-mechanical system are investigated by the quantum Langevin equation. A WGM cavity coupling to two parallel waveguides is devised to study the transmission and reflection of this system. In single mode WGM cavity, without optomechanical coupling, both the transmission and reflection of the cavity present a Lorentzian dip and peak. When the coupling between the cavity mode and mechanical mode is considered, the transmission and reflection of the optomechanical cavity show “W” and “M” shape mode splitting. Moreover, under the action of a controlling and a probe laser, the output field at the probe frequency presents electromagnetically induced transparency (EIT)-like spectrum in the system. We give the physical origin of EIT-like and the pump-probe response for the WGM shares all the features of the Λ system in atoms. Further, due to backscattering, the two traveling waves in WGM are coupled with a rate γ. The transmission and reflection of the optomechanical cavity display three modes splitting in the spectra with optomechanical coupling between the two cavity modes and the mechanical mode.     

量子点操控的光子探测和圆偏振光子发射

半导体量子点是研究光子与电子态相互作用的优选固态体系,并在光子探测和发射两个方向上展现出独特的技术机遇.其中基于量子点的共振隧穿结构被认为在单光子探测方面综合性能最佳,但受到光子数识别、工作温度两个关键性能的制约.利用腔模激子态外场耦合效应,有望获得圆偏振态可控的高频单光子发射.本文介绍作者提出的量子点耦合共振隧穿（QD-cRTD）的光子探测机理,利用量子点量子阱复合电子态的隧穿放大,将QD-cRTD光子探测的工作温度由液氦提高至液氮条件,光电响应的增益达到10⁷以上,并具备双光子识别能力;同时,由量子点能级的直接吸收,原型器件获得了近红外的光子响应.在量子点光子发射机理的研究方面,作者实现了量子点激子跃迁和微腔腔模共振耦合的磁场调控,在Purcell效应的作用下增强激子自旋态的自发辐射速率,从而增强量子点中左旋或右旋圆偏振光的发射强度,圆偏度达到90%以上,形成一种光子自旋可控发射的新途径.     

InAs/GaAs量子点1.3 μm单光子发射特性

采用双层耦合量子点的分子束外延生长技术生长了InAs/GaAs量子点样品,把量子点的发光波长成功地拓展到1.3 μm.采用光刻的工艺制备了直径为3 μm的柱状微腔,提高了量子点荧光的提取效率.在低温5 K下,测量得到量子点激子的荧光寿命约为1 ns;单量子点荧光二阶关联函数为0.015,显示单量子点荧光具有非常好的单光子特性;利用迈克耳孙干涉装置测量得到单光子的相干时间为22 ps,对应的谱线半高全宽度为30 μeV,且荧光谱线的线型为非均匀展宽的高斯线型.     

Implementing of Quantum Cloning with Spatially Separated Quantum Dot Spins

We propose some schemes for implementing optimal symmetric (asymmetric) 1 → 2 universal quantum cloning, optimal symmetric (asymmetric) 1 → 2 phase-covariant cloning, optimal symmetric 1 → 3 economical phase-covariant cloning and optimal symmetric 1 → 3 economical real state cloning with spatially separated quantum dot spins by choosing the single-qubit rotation angles appropriately. The decoherences of the spontaneous emission of QDs, cavity decay and fiber loss are suppressed since the effective long-distance off-resonant interaction between two distant QDs is mediated by the vacuum fields of the fiber and cavity, and during the whole process no system is excited.     

利用高阶拉盖尔-高斯横模精确测量法布里-珀罗腔内原子的运动轨迹

研究了冷原子与法布里-珀罗腔内拉盖尔-高斯横模强耦合相互作用体系的透射光谱, 分析了透射光谱与原子在腔中运动轨迹的关系. 结果表明, 与厄米特-高斯横模相比, 拉盖尔-高斯横模的腔场与原子的最大耦合系数几乎不随阶数的增加而变化, 使得探测光谱的对比度受模式阶数的影响较小. 在拉盖尔-高斯横模场分布的圆环边缘附近, 原子运动轨迹的微小偏移会引起透射光谱的很大变化, 因此在这些位置可以实现原子运动轨迹的高精度探测.