L3 Photonic Crystal Cavities Research Articles

Global optimization of an encapsulated Si/SiO_2 L3 cavity with a 43 million quality factor

We optimize a silica-encapsulated silicon L3 photonic crystal cavity for ultra-high quality factor by means of a global optimization strategy, where the closest holes surrounding the cavity are varied to minimize out-of-plane losses. We find an optimal value of Q_c=4.33times 10^7, which is predicted to be in the 2 million regime in presence of structural imperfections compatible with state-of-the-art silicon fabrication tolerances.

Tunable all-optical microwave filter with high tuning efficiency.

We propose and experimentally demonstrate a continuously tunable all-optical microwave filter based on a photonic crystal (PC) L3 cavity. Due to the small cavity mode volume and prominent optical properties, the required power to arouse the cavity nonlinear effects is low as microwatt level. Moreover, the cavity resonance could be continuously shifted by finely adjusting the input powers. Therefore, under optical single sideband modulation, the frequency interval between the optical carrier and cavity resonance could be controllable. In this case, the central frequency of the microwave photonic filter (MPF) could be continuously tuned with low power consumption. To the best of our knowledge, the experimental tuning efficiency of 101.45 GHz/mW is a record for on-chip tunable all-optical microwave filters. With dominant features of all-optical control, ultra-high tuning efficiency (101.45 GHz/mW), large rejection ratios (48 dB) and compact footprint (100 µm2), the proposed silicon nanocavity is competent to process microwave signals, which has many useful applications in on-chip energy-efficient microwave photonic systems.

Radiative coupling and decay dynamics of two two-level quantum emitters in arbitrary dielectric nanostructures

Photon-mediated coherent interaction determines the decay dynamics of quantum emitters and facilitates on-chip quantum manipulation for quantum information processing. We derive a general formalism to efficiently and flexibly simulate the radiative coupling between two two-level quantum emitters in arbitrary dielectric nanostructures, based on which, the decay dynamics of the two quantum emitters can be obtained. We demonstrate this formalism to investigate the two quantum dots embedded in the photonic crystal L3 cavity. The anticrossing behavior in the evolution spectrum and the vacuum Rabi oscillation in the populations of the two quantum dots can be simulated in both zero and small dot-dot detuning case. Our formalism can serve as a flexible and efficient theoretical tool and may be further developed to handle the cases for different initial states, multiple quantum emitters, and arbitrary metallic nanostructures.

Numerical analysis of subwavelength field effects in photonic crystal slab cavities

Surface coupling of single quantum emitters to optical cavities consisting of a photonic crystal slab is a delicate yet crucial task for photonic quantum applications. By coupling through the evanescent surface field only small Purcell factors can be achieved. Here, we propose to introduce a pit in the slab to position the emitter closer to the mode field maximum. Photonic crystal slab L3 cavities are investigated with respect to quality factor and Purcell effect, using finite element calculations in the frequency domain. That way the spatial distribution of the Purcell factor can be calculated. Introducing a small sized pit to the surface of the photonic crystal cavity can evoke subwavelength field effects, confining the field maximum inside the pit. By engineering a pit in the center of the cavity the Purcell factor can be increased from 176 to 1331, albeit reducing the Q factor from 20769 to 16696.

Silicon photonic crystal cavities at near band-edge wavelengths

We demonstrate photonic crystal L3 cavities with a resonant wavelength of around 1.078 μm on an undoped silicon-on-insulator, designed to enhance spontaneous emission from phosphorus donor-bound excitons. We have optimised a fabrication recipe using readily available process materials such as polymethyl methacrylate as a soft electron-beam mask and a Chemical Vapour Deposition grown oxide layer as a hard mask. Our bilayer resist technique efficiently produces photonic crystal cavities with a quality factor (Q) of ∼5000 at a wavelength of 1.078 μm, measured using cavity reflection measurements at room temperature. We observe a decrease in Q as the cavity resonance shifts to shorter wavelengths (Q≲3000 at wavelengths <1.070 μm), which is mostly due to the intrinsic absorption of silicon.

Impact of dipole orientation on strongly-coupled system composed of a single quantum dot and a photonic crystal L3 cavity

The strong coupling interaction realizes light manipulation at the single-photon level, while the dipole orientation plays a crucial role in the light–matter interaction in anisotropic environments. We investigate the impact of dipole orientation on the strongly-coupled system (SCS) composed of a single quantum dot and a photonic crystal L3 cavity. For both the ideal and the disordered SCS, as the quantum dot deflects from the ideal direction aligning with the cavity mode, the coupling strength and the vacuum Rabi splitting decrease even vanish, the anti-crossing behavior weakens even degrades to crossing behavior, and the vacuum Rabi oscillation period elongates even degrades to the irreversible decay. The dipole orientation has no impact on the coherence time of the SCS. These findings provide significant guidance for the manipulation of the SCS and the characterization of the dipole orientation of the quantum dot.

Dual effects of disorder on the strongly-coupled system composed of a single quantum dot and a photonic crystal L3 cavity

Light-matter interaction in the strong coupling regime enables light control at the single-photon level. We develop numerical method and analytical expressions to calculate the decay kinetics of an initially excited two-level quantum emitter in dielectric nanostructure and single-mode cavity, respectively. We use these methods to discover the dual effects of disorder on the strongly-coupled system composed of a single quantum dot and a photonic crystal L3 cavity. The quality factor is sensitive to disorder, while the g factor and vacuum Rabi splitting are robust against disorder. A small amount of disorder may either decrease or increase the light localization and the light-matter interaction. Our methods offer flexible and efficient theoretical tools for the investigation of light-matter interaction, especially cavity quantum electrodynamics. Our findings significantly lower the requirements for optimization effort and fabrication precision and open up many promising practical possibilities.

Ultra-wide-band structural slow light

The ability of using integrated photonics to scale multiple optical components on a single monolithic chip offers key advantages to create miniature light-controlling chips. Numerous scaled optical components have been already demonstrated. However, present integrated photonic circuits are still rudimentary compared to the complexity of today’s electronic circuits. Slow light propagation in nanostructured materials is a key component for realizing chip-integrated photonic devices controlling the relative phase of light and enhancing optical nonlinearities. We present an experimental record high group-index-bandwidth product (GBP) of 0.47 over a 17.7 nm bandwidth in genetically optimized coupled-cavity-waveguides (CCWs) formed by L3 photonic crystal cavities. Our structures were realized in silicon-on-insulator slabs integrating up to 800 coupled cavities, and characterized by transmission, Fourier-space imaging of mode dispersion, and Mach-Zehnder interferometry.

Diamond in a Nanopocket: A New Route to a Strong Purcell Effect.

Light emission from the color centers in diamonds can be significantly enhanced by their interaction with optical microcavities. In the conventional chip-based hybrid approach, nanodiamonds are placed directly on the surface of microcavity chips created using fabrication-matured material platforms. However, the achievable enhancement due to the Purcell effect is limited because of the evanescent interaction between the electrical field of the cavity and the nanodiamond. Here, we propose and statistically analyze a diamond in a nanopocket structure as a new route to achieve a high enhancement of light emission from the color center in the nanodiamond, placed in an optical microcavity. We demonstrate that by creating a nanopocket within the photonic crystal L3 cavity and placing the nanodiamond in, a significant and a robust control over the local density of states can be obtained. The antinodes of the electric field relocate to the nanosized air gaps within the nanopocket, between the nanodiamond and the microcavity. This creates an elevated and uniform electric field across the nanodiamond that is less sensitive to perturbations in the shape and orientation of the nanodiamond. Using a silicon nitride photonic crystal L3 cavity and aiming at silicon-vacancy and nitrogen-vacancy color centers in diamond, we performed a statistical analysis of light emission, assuming random positions of color centers and dipole moment orientations. We showed that in cavities with experimentally feasible quality factors, the diamond in the nanopocket structure produces Purcell factor distributions with mean and median that are tenfold larger compared to what can be achieved when the diamond is on the surface of the microcavity.

Q factor limitation at short wavelength (around 300 nm) in III-nitride-on-silicon photonic crystal cavities

III-nitride-on-silicon L3 photonic crystal cavities with resonances down to 315 nm and quality factors (Q) up to 1085 at 337 nm have been demonstrated. The reduction of the quality factor with decreasing wavelength is investigated. Besides the quantum well absorption below 340 nm, a noteworthy contribution is attributed to the residual absorption present in thin AlN layers grown on silicon, as measured by spectroscopic ellipsometry. This residual absorption ultimately limits the Q factor to around 2000 at 300 nm when no active layer is present.

Spontaneous emission from dipole-forbidden transitions in semiconductor quantum dots

We theoretically investigate the multipolar effects on the dipole-forbidden transitions of a semiconductor quantum dot. An approximated expression for the decay rate of these transitions is derived. Unlike the general theory of the spontaneous emission beyond the dipole approximation, the distinct roles of the emitter and the vacuum electric field in the transition rate are here clearly recognizable and can be separately optimized. We illustrate the potential of this formalism by calculating the spontaneous emission decay rate of an InAs/GaAs quantum dot embedded into two realistic nanostructures---an L3 photonic crystal cavity and a plasmonic dimer antenna. The obtained results show that, although the two structures provide an enhancement of the same order of magnitude, the plasmonic antenna constitutes a more promising candidate for the experimental observation of the dipole-forbidden transitions of a quantum dot.

Integrated quantum photonic sensor based on Hong-Ou-Mandel interference.

Photonic-crystal-based integrated optical systems have been used for a broad range of sensing applications with great success. This has been motivated by several advantages such as high sensitivity, miniaturization, remote sensing, selectivity and stability. Many photonic crystal sensors have been proposed with various fabrication designs that result in improved optical properties. In parallel, integrated optical systems are being pursued as a platform for photonic quantum information processing using linear optics and Fock states. Here we propose a novel integrated Fock state optical sensor architecture that can be used for force, refractive index and possibly local temperature detection. In this scheme, two coupled cavities behave as an "effective beam splitter". The sensor works based on fourth order interference (the Hong-Ou-Mandel effect) and requires a sequence of single photon pulses and consequently has low pulse power. Changes in the parameter to be measured induce variations in the effective beam splitter reflectivity and result in changes to the visibility of interference. We demonstrate this generic scheme in coupled L3 photonic crystal cavities as an example and find that this system, which only relies on photon coincidence detection and does not need any spectral resolution, can estimate forces as small as 10(-7) Newtons and can measure one part per million change in refractive index using a very low input power of 10(-10)W. Thus linear optical quantum photonic architectures can achieve comparable sensor performance to semiclassical devices.

Gallium nitride L3 photonic crystal cavities with an average quality factor of 16 900 in the near infrared

Photonic crystal point-defect cavities were fabricated in a GaN free-standing photonic crystal slab. The cavities are based on the popular L3 design, which was optimized using an automated process based on a genetic algorithm, in order to maximize the quality factor. Optical characterization of several individual cavity replicas resulted in an average unloaded quality factor Q = 16 900 at the resonant wavelength λ∼1.3 μm, with a maximal measured Q value of 22 500. The statistics of both the quality factor and the resonant wavelength are well explained by first-principles simulations including fabrication disorder and background optical absorption.

Doubly resonant photonic crystal cavities in gallium nitride for fluorescence sensing

This paper presents the modeling and design of a photonic crystal (PhC) cavity in gallium nitride on sapphire. Using the 3D finite difference time domain method, we explore the suitability of asymmetric dielectric layer structures as a platform for L3 PhC cavities. The paper explores the origin of losses and modification to the cavity modes introduced by asymmetric layers. Based upon this analysis, a cavity design is proposed, which is optimized to provide enhancement for fluorescent molecule detection. A central air hole in the cavity allows the positioning of molecules in a high field region. Also, the hole modifies the cavity modes to have a smaller mode volume. Both of these properties are used to simultaneously enhance the absorption and spontaneous emission of fluorescent molecules.

Photonic molecules: tailoring the coupling strength and sign.

We demonstrate a large tuning of the coupling strength in Photonic Crystal molecules without changing the inter-cavity distance. The key element for the design is the "photonic barrier engineering", where the "potential barrier" is formed by the air-holes in between the two cavities. This consists in changing the hole radius of the central row in the barrier. As a result we show, both numerically and experimentally, that the wavelength splitting in two evanescently-coupled Photonic Crystal L3 cavities (three holes missing in the ΓK direction of the underlying triangular lattice) can be continuously controlled up to 5× the initial value upon ∼ 30% of hole-size modification in the barrier. Moreover, the sign of the splitting can be reversed in such a way that the fundamental mode can be either the symmetric or the anti-symmetric one without altering neither the cavity geometry nor the inter-cavity distance. Coupling sign inversion is explained in the framework of a Fabry-Perot model with underlying propagating Bloch modes in coupled W1 waveguides.

Silicon optical diode based on cascaded photonic crystal cavities

An all-silicon passive optical diode based on optical nonlinearity in cascaded photonic crystal (PhC) L3 cavities is proposed and demonstrated. A nonreciprocal transmission ratio (NTR) of 30.8dB and insertion loss of 8.3dB are realized in the device. The device has a relatively broad 17dB operation bandwidth of 0.08nm, and at least 16dB of NTR is achieved when input power varies between -6.25 and -2.95 dBm. A nonlinear couple mode model for cascaded PhC cavities is established to analyze the behavior of the device, and numerical simulation results are in good agreement with the experiment results.

Mode analysis of photonic crystal L3 cavities in self-suspended lithium niobate membranes

We report on a multimodal analysis of photonic crystal L3 cavities milled in lithium niobate free-standing membranes. The classical L3 cavity geometry is compared to an L3 cavity containing a second lattice superimposed on the primary one. Those two different geometries are investigated in terms of vertical radiation and quality (Q) factor for each mode of the cavities. Depending on the cavity geometry, some modes undergo an enhancement of their vertical radiation into small angles while other modes experience a higher Q factor. Experimental characterizations are corroborated by three-dimensional finite difference time domain simulations.

1D and 2D arrays of coupled photonic crystal cavities with a site-controlled quantum wire light source

We investigated experimentally 1D and 2D arrays of coupled L3 photonic crystal cavities. The optical modes of the coupled cavity arrays are fed by a site-controlled quantum wire light source. By performing photoluminescence measurements and relying on near-field calculation of the cavitiy modes, we evidence optical coupling between the cavities as well as supermode delocalization. In particular, for small cavity separations, fabrication induced disorder effects are shown to be negligible compared to optical coupling between cavities.

Lumped Circuit Heat Model of Photonic Crystal Membrane Nanocavities

An InP L3 photonic crystal cavity on a suspended membrane has a characteristic time of ~ 1 μs, which is 5.4× slower than the thermal cavity lifetime. To explain this mismatch, we use a lumped circuit model containing not only the cavity temperature, but also an average temperature of the membrane. Comparison of this model to a 1D COMSOL simulation, results in a thermal time constant of the membrane that is 7.9× slower than the thermal cavity lifetime, and slows down the transient response. The membrane behaves similar to other membrane cavity types.

Vacuum Rabi splitting in a coupled system of single quantum dot and photonic crystal cavity: effect of local and propagation Green’s functions

We investigate the light emission characteristics for single two level quantum dot (QD) in a realistic photonic crystal (PC) L3 cavity based upon the local coupling strength between the QD and cavity together with the Green's function in which the propagation function related to the position of the detector is taken into account. We find for a PC cavity that the line shape of the propagation function in frequency domain is identical to that of the cavity and independent on the detector's position. We confirm that this identity is not influenced by the horizontal decay of the cavity. Furthermore, it is revealed that the vacuum fluorescence spectrum of the coupled system never give the triplet in strong coupling regime. Our work demonstrates that the experimental spectral-triplet in coupled system of single QD and PC cavity cannot be individually understood by vacuum Rabi splitting without including other physics mechanism.