Circular Bragg Reflector Research Articles

On‐Demand Indistinguishable and Entangled Photons Using Tailored Cavity Designs

AbstractThe biexciton‐exciton emission cascade commonly used in quantum‐dot systems to generate polarization entanglement yields photons with intrinsically limited indistinguishability. In the present work, it focuses on the generation of pairs of photons with high degrees of polarization entanglement and simultaneously high indistinguishability. It achieves this goal by selectively reducing the biexciton lifetime with an optical resonator. It demonstrates that a suitably tailored circular Bragg reflector fulfills the requirements of sufficient selective Purcell enhancement of biexciton emission paired with spectrally broad photon extraction and twofold degenerate optical modes. The in‐depth theoretical study combines (i) the optimization of realistic photonic structures solving Maxwell's equations from which model parameters are extracted as input for (ii) microscopic simulations of quantum‐dot cavity excitation dynamics with full access to photon properties. It reports non‐trivial dependencies on system parameters and use the predictive power of the combined theoretical approach to determine the optimal range of Purcell enhancement that maximizes indistinguishability and entanglement to near unity values, here specifically for the telecom C‐band at 1550 nm.

Single quantum dot selection and tailor-made photonic device integration using nanoscale focus pinspot.

Among the diverse platforms of quantum light sources, epitaxially grown semiconductor quantum dots (QDs) are one of the most attractive workhorses for realizing quantum photonic technologies owing to their outstanding brightness and scalability. However, the spatial and spectral randomness of most QDs severely hinders the construction of large-scale photonic platforms. In this work, we present a methodology to deterministically integrate single QDs with tailor-made photonic structures. We apply a nondestructive luminescence picking method termed as nanoscale focus pinspot (NFP) using helium ion microscopy to reduce the luminous QD density while retaining the surrounding medium. We extract only a single QD emission out of the high-density ensemble QDs. Then the tailor-made photonic structure of a circular Bragg reflectors (CBR) are designed and deterministically integrated with the selected QD. Given that the microscopy can image with nanoscale resolution and apply NFP in situ, photonic devices can be deterministically fabricated on target QDs. The extraction efficiency of the NFP-selected QD emission is improved by 25 times after the CBR integration. Since the NFP method only controls the luminescence without destroying the medium, it is applicable to various photonic structures such as photonic waveguides or photonic crystal cavities regardless of materials. This article is protected by copyright. All rights reserved.

Graded circular Bragg reflectors: a semi-analytical retrieval of approximate pitch profiles from Mueller-matrix data

Graded pitch profiles are found in structurally chiral materials like cholesteric liquid crystals (CLC) and in the cuticle of some scarab beetles. In most cases, the pitch profile is determined from electron microscopy techniques. Recently, it was shown that approximate pitch profiles in the cuticle of scarab beetles can be retrieved through an analysis of the spectral dependence of maxima and minima in normalized Mueller-matrix data. The analysis relies on basic concepts of interference in thin films, properties of optical modes in chiral systems, and the condition for circular Bragg reflection. In this work, the consistency of the procedure is demonstrated by analysis of normalized Mueller matrices of circular Bragg reflectors calculated for three predefined pitch profiles with (1) a stepwise decrease, (2) a stepwise increase and, (3) an exponential increase. The procedure does not require knowledge of the full Mueller matrix and can be used for non-destructive analysis of pitch in CLC, beetle cuticle and similar structures.

Mueller-matrix modeling of the architecture in the cuticle of the beetle Chrysina resplendens

Circular Bragg reflectors have the ability of reflecting the cohanded circular-polarization mode of the inherent helicoidal structures. Cuticles of some plants and beetles are examples of natural circular Bragg reflectors. In many cases, the period or pitch of the helicoidal structure shows spatial variation across the cuticle (pitch profile). Among scarab beetles, the special architecture in the cuticle of the Chrysina resplendens comprising a birefringent layer sandwiched between two helicoidal layers reflects both right- and left-handed circular-polarization states. In this work, the modeling of Mueller-matrix data is applied to investigate polarization properties of this exceptional structure by using pitch profile and optical functions reported in the literature. Reflectance spectra for circular-polarization states are explained in terms of the phase shift introduced by the birefringent layer in a phasor plot. The azimuth-dependent polarization properties are investigated at oblique incidence for unpolarized light.

(Invited) Direct Band Gap Germanium

Germanium is an indirect band gap material. This property is a significant disadvantage to realize an efficient laser emitter with this material. This feature can nonetheless be circumvented by strain engineering of the band structure. It is well known that tensile strain can reduce the energy difference between the conduction band valleys and ultimately lead to a band inversion. With a direct band gap material, one should expect to obtain optical gain with a significantly reduced threshold as compared to unstrained or weakly-strained Ge. Consequently, the integration of a group IV emitter on a silicon chip could become more realistic. In this presentation, we will show that direct band gap germanium can be obtained by transferring tensile strain with nitride stressor layers. We have experimentally determined when the cross-over from indirect to direct band gap occurs using temperature-dependent photoluminescence measurements. This cross-over was found to occur for a biaxial tensile strain of 1.67%, the theoretical values varying between 1.5 and 2% in the literature. We will show that this level of strain transfer is compatible with the fabrication of optical micro-resonators with germanium microdisks. Tensile-strained germanium microdisks with circular Bragg reflectors have been successfully fabricated. They exhibit high quality factors for the quasi-radial modes. These types of structures are ideal candidates to realize microlasers with pure germanium. We will discuss the latest progress in this direction.

(Invited) Direct Band Gap Germanium

We show that direct band gap germanium can be obtained with external silicon nitride stressor layers. The cross-over from indirect to direct band gap is found to occur for an equivalent 1.67% biaxial tensile strain. Both whispering gallery modes and quasi-radial modes are observed with tensile-strained Ge microdisks. Quality factors up to 7100 have been measured around 2 µm wavelength. We demonstrate that circular Bragg reflectors can significantly enhance the quality factors of quasi-radial modes in strained germanium microdisks.

Tensile-strained germanium microdisks with circular Bragg reflectors

We demonstrate the combination of germanium microdisks tensily strained by silicon nitride layers and circular Bragg reflectors. The microdisks with suspended lateral Bragg reflectors form a cavity with quality factors up to 2000 around 2 μm. This represents a key feature to achieve a microlaser with a quasi-direct band gap germanium under a 1.6% biaxial tensile strain. We show that lowering the temperature significantly improves the quality factor of the quasi-radial modes. Linewidth narrowing is observed in a range of weak continuous wave excitation powers. We finally discuss the requirements to achieve lasing with these kind of structures.

Sum decomposition of Mueller-matrix images and spectra of beetle cuticles.

Spectral Mueller matrices measured at multiple angles of incidence as well as Mueller matrix images are recorded on the exoskeletons (cuticles) of the scarab beetles Cetonia aurata and Chrysina argenteola. Cetonia aurata is green whereas Chrysina argenteola is gold-colored. When illuminated with natural (unpolarized) light, both species reflect left-handed and near-circularly polarized light originating from helicoidal structures in their cuticles. These structures are referred to as circular Bragg reflectors. For both species the Mueller matrices are found to be nondiagonal depolarizers. The matrices are Cloude decomposed to a sum of non-depolarizing matrices and it is found that the cuticle optical response, in a first approximation can be described as a sum of Mueller matrices from an ideal mirror and an ideal circular polarizer with relative weights determined by the eigenvalues of the covariance matrices of the measured Mueller matrices. The spectral and image decompositions are consistent with each other. A regression-based decomposition of the spectral and image Mueller matrices is also presented whereby the basic optical components are assumed to be a mirror and a circular polarizer as suggested by the Cloude decomposition. The advantage with a regression decomposition compared to a Cloude decomposition is its better stability as the matrices in the decomposition are determined a priori. The origin of the depolarizing features are discussed but from present data it is not possible to conclude whether the two major components, the mirror and the circular polarizer are laterally separated in domains in the cuticle or if the depolarization originates from the intrinsic properties of the helicoidal structure.

Lasing in hybrid metal-Bragg nanocavities

We report room-temperature lasing from an optically pumped subwavelength-scale cylindrical InGaAsP pillar surrounded by circular Bragg reflectors on a metal substrate with a dielectric spacer layer. By taking advantage of wide in-plane photonic bandgaps and proper vertical antiresonances, three dielectric Bragg pairs produce a sufficient optical feedback capable of low threshold lasing from the fundamental TE011 mode. A large spontaneous emission coupling into the lasing mode is obtained from the cavity-enhanced Purcell effects and effective suppression of nonlasing modes.

Optical thin-film linear polarization selector fabricated by oblique angle deposition

A linear polarization selector was made from a three-section sculptured thin film deposited using an oblique angle deposition technique. In this device, the circular Bragg reflector with a left-handed helical structure was sandwiched in between two quarter-wave plates with opposite in-plane birefringence. Within the Bragg regime, the normal incident S-polarized light on this device was reflected, whereas the incident P-polarized light was transmitted through it. The microstructure of the linear polarization selector was also examined by scanning electron microscopy.

Increasing the coupling efficiency of a microdisk laser to waveguides by using well designed spiral structures

In this article, we optimize the coupling efficiency from a GaAs microdisk resonator into a single mode spiral waveguide. A classical microdisk resonator coupling light into a nonevanescent straight waveguide reaches a typical coupling efficiency of 67%. We show that the introduction of a spiral waveguide that works both as a waveguide and circular Bragg reflector can improve such efficiency to almost 90%. The same structure with the addition of a taper can couple up to 80% of the generated power into a slot waveguide.

Linear polarization-discriminatory state inverter fabricated by oblique angle deposition

In this paper, we report a linear polarization-discriminatory state inverter made of three-layer sculpture thin film fabricated by oblique angle deposition technique. The first and third layers are quarter-wave plates of zigzag structure and the middle of them is a circular Bragg reflector of left-handed helical structure. It is found that the normal incidence of P-polarized light on this polarization-discriminatory state inverter becomes the S-polarized light at output, while the incident S-polarized light of wavelength lying in the Bragg regime is reflected. The microstructure of the linear polarization-discriminatory state inverter is also investigated by using a scanning electron microscope.

Lasing properties of disk microcavity based on a circular Bragg reflector

The lasing properties of quantum well structures, where the cavity is defined in the plane of the wells by circular Bragg reflectors are investigated. Diffraction of the in-plane lasing modes into the vertical direction by the circular distributed Bragg reflector (DBR) allows the simultaneous measurement of near-field emission patterns and emission spectra, allowing unambiguous assignment of azimuthal quantum numbers to the lasing modes. The radial quantum number is determined by fitting the lasing spectrum to theory. Lasing is shown to occur in modes whose wave vector is mainly radial, confined by the circular DBR structure, rather than in whispering gallery type modes which are mainly azimuthal.

High-finesse disk microcavity based on a circular Bragg reflector

We made disk-shaped microcavities of approximately 10 μm2 in area in a GaAs/AlGaAs waveguide structure by etching deep vertical concentric trenches. The trenches form a circular Bragg-like reflector that confines light in the remaining two lateral dimensions. We demonstrate from photoluminescence excited in the waveguide the confinement of discrete disk modes whose wave vector is mainly radial, in contrast with whispering gallery modes. Their quality factors up to Q=650 indicate in-plane reflectivities approaching 90%. In the near infrared, this represents a demonstration of wavelength-scale light confinement based on photonic crystal effects in two dimensions.