Two-dimensional Photonic Crystal Cavity Research Articles

Improved Q-factors of III-nitride-based photonic crystal nanocavities by optical loss engineering.

III-nitride-based two-dimensional photonic crystal (2D-PhC) cavities with high-quality factors (Q-factors) have a large potential application, however realized Q-factors in the visible wavelength regime have been relatively moderate. In this study, we demonstrate the design and fabrication of 2D-PhC cavities to achieve high Q-factors, especially in the visible range. From the comparison of numerical calculations and the experimental results, we discuss the dominant optical losses that limit the Q-factor of H3-type cavities formed in an Eu,O-codoped GaN film. Based on these results we designed 2D-PhC cavities which can effectively suppress these dominant losses. We fabricated 2D-heterostructures and show a high Q-factor of 10500 at a resonant wavelength of ∼660 nm, which is considerably larger than any existing GaN-based nano/micro-resonators in the visible region. This study provides design guidelines for the realization of high Q-factors in photonic crystal nanocavities based on III-nitride semiconductors.

Design considerations of III-nitride-based two-dimensional photonic crystal cavities with crystallographically induced disorder

We present a study on the improvement of quality factors (Q-factors) of III-nitride-based two-dimensional photonic crystal (2D-PhC) nanocavities. Although III-nitride-based 2D-PhC cavities have been studied for long time, they typically show relatively low experimental Q-factors due to disorders inevitably introduced during the fabrication processes. In order to realize high-Q-factors, it is necessary to introduce cavities that are tolerant to disorder. We numerically reproduce the experimental found disorder in simulations, and discuss the influence of disorder on Q-factors of several types of cavities. We demonstrate the suitability of these cavities for the realization of high-Q-factors, and show which structures have the largest tolerance for disorders.

Telecommunication-wavelength two-dimensional photonic crystal cavities in a thin single-crystal diamond membrane

We demonstrate two-dimensional photonic crystal cavities operating at telecommunication wavelengths in a single-crystal diamond membrane. We use a high-optical-quality and thin (∼300 nm) diamond membrane, supported by a polycrystalline diamond frame, to realize fully suspended two-dimensional photonic crystal cavities with a high theoretical quality factor of ∼8 × 106 and a relatively small mode volume of ∼2(λ/n)3. The cavities are fabricated in the membrane using electron-beam lithography and vertical dry etching. We observe cavity resonances over a wide wavelength range spanning the telecommunication O- and S-bands (1360–1470 nm) with Q factors of up to ∼1800. Our method paves the way for on-chip diamond nanophotonic applications in the telecommunication-wavelength range.

Vector beams in planar photonic crystal cavities with rotating air holes.

We report a method to generate angularly polarized vector beams with a topological charge of one by rotating air holes to form two-dimensional photonic crystal (PC) cavities. The mode volume and resonance wavelength of these cavities are tuned from ${0.33}{(\lambda /n)^3}$0.33(λ/n)3 to ${12}{(\lambda /n)^3}$12(λ/n)3 and in a wide range of 400 nm, respectively, by controlling the range of fixed air holes near the center of the structure. As a benefit, the half-maximum divergence angles of the vector beam can be widely changed from 90° to $\sim{60}^\circ $∼60∘. By adjusting the shift direction of the air holes in the PC cavities, optical vector beams with different far-field morphology are obtained. The scheme provides not only an alternative method to generate optical vector beams, but also an effective strategy to control far-field morphology and polarizations, which holds promising applications such as optical microscopy and micro-manipulation.

Spin measurements of NV centers coupled to a photonic crystal cavity

Nitrogen-vacancy (NV) centers feature outstanding properties such as a spin coherence time of up to 1 s as well as a level structure offering the possibility to initialize, coherently manipulate, and optically read-out the spin degree of freedom of the ground state. However, only about 3% of their photon emission is channeled into the zero phonon line (ZPL), limiting both the rate of indistinguishable single photons and the signal-to-noise ratio (SNR) of coherent spin-photon interfaces. We here report on the enhancement of the SNR of the optical spin read-out achieved by tuning the mode of a two-dimensional photonic crystal (PhC) cavity into resonance with the NV-ZPL. PhC cavities are fabricated by focused ion beam milling in thin reactive ion etched ultrapure single crystal diamond membranes featuring modes with Q-factors of up to 8250 at mode volumes below one cubic wavelength. NV centers are produced in the cavities in a controlled fashion by a high resolution atomic force microscope implantation technique. On cavity resonance, we observe a lifetime shortening from 9.0 ns to 8.0 ns as well as an enhancement of the ZPL emission by almost one order of magnitude. Although on resonance the collection efficiency of ZPL photons and the spin-dependent fluorescence contrast are reduced, the SNR of the optical spin read-out is almost tripled for the cavity-coupled NV centers.

Selective tuning of optical modes in a silicon comb-like photonic crystal cavity

Abstract Realizing multiply resonant photonic crystal cavities with large free spectral range is key to achieve integrated devices with highly efficient nonlinear response, such as frequency conversion, four-wave mixing, and parametric oscillation. This task is typically difficult owing to the cavity modes’ sensitivity to fabrication disorder, which makes it hard to reliably achieve a comb-like spectrum of equally spaced modes even when a perfect matching is theoretically predicted. Here we show that a comb-like spectrum of up to eight modes with very high quality factor and diffraction limited volumes can be engineered in the bichromatic-type potential of a two-dimensional photonic crystal cavity fabricated in a thin silicon membrane. To cope with the tight tolerance in terms of frequency spacings and resonance linewidths, we develop a permanent post-processing technique that allows the selective tuning of individual confined modes, thus achieving an almost perfect frequency matching of high Q resonances with record finesse in silicon microresonators. Our experimental results are extremely promising in view of ultra-low power nonlinear photonics in silicon.

High-sensitivity broad free-spectral-range two-dimensional three-slot photonic crystal sensor integrated with a 1D photonic crystal bandgap filter.

We present a novel high-sensitivity broad free-spectral-range (FSR) two-dimensional three-slot photonic crystal sensor integrated with a 1D photonic crystal tapered nanobeam bandgap filter (1DPC-TNBF) based on thin-film silicon. Designed to lie in the wavelength at around 1550nm, the resonance of the two-dimensional photonic crystal three-slot cavity (2DPC-TSC) shows strong light-matter interaction in the slot region, which enhances the bulk refractive index sensitivity of the sensor significantly. The simulated sensitivity is over 900nm/refractive index unit (RIU). By connecting an additional 1DPC-TNBF to a 2DPC-TSC in series, the high-order modes are suppressed, which means only a fundamental mode exists with a broad FSR over 200nm. Thus, the proposed structure is promising in designing lab-on-a-chip applications, especially in compact parallel-integrated sensor arrays.

Cavity-enhanced harmonic generation in silicon rich nitride photonic crystal microresonators

We report second and third harmonic generation in photonic crystal cavities fabricated in a suspended silicon-rich nitride membrane under resonant continuous-wave excitation at telecom wavelength. Two-dimensional photonic crystal cavities with a far-field optimized line-width modulated design were employed. A quality factor at fundamental wavelength as high as Q = 1.3 × 104 and a coupling efficiency ηc ≈ 30% enabled us to exploit the cavity field enhancement to achieve the generation efficiencies ρSH = (4.7 ± 0.2) × 10−7 W−1 and ρTH = (5.9 ± 0.3) × 10−5 W−2. The absence of saturation effects at high power and the transparency of the device at the second harmonic wavelength suggest the absence of two-photon absorption and related detrimental effects.

On-chip dynamic time reversal of light in a coupled-cavity system

We theoretically and experimentally demonstrate dynamic, all-linear time-reversal of infrared light in planar optical circuits for the first time. We propose that the oscillatory motion of the light stored in cavities can be time-reversed by fast nonadiabatic tuning of the frequency of eigenmodes of a coupled cavity system and experimentally demonstrate it using a system consisting of distant high-Q-factor two-dimensional photonic crystal cavities between which effective direct couplings are formed via line-defect waveguides. We also analyze the loss and methods to reduce the loss, as well as a theory that expands our system to realize general time-reversal operation for any input light.

Topological whispering gallery modes in two-dimensional photonic crystal cavities

Utilizing the robust transport properties of the topological photonic crystal interface, we experimentally realize two-dimensional topological photonic crystal cavities, where discrete whispering gallery modes can propagate unidirectionally along the cavity circumference. Different from traditional cavities, these topological whispering galley modes are insensitive to cavity shapes. Our microwave demonstration has a good agreement with numerical simulations. Using pure dielectrics, by scaling down to the optical wavelength, an optical directional coupler based on the same topological photonic crystal scheme is also proposed. We here show that topological photonics can provide more novel designs for optical devices.

Superconducting nanowire single-photon detector implemented in a 2D photonic crystal cavity

The combination of efficient single-photon generation, manipulation, and detection on a single chip poses a major challenge for quantum photonics and all-optical quantum computing. Among a multitude of detection technologies, waveguide-integrated superconducting nanowire single-photon detectors stand out as they promise near-unity detection efficiencies at outstanding timing accuracy and speed. Here, by exploiting the concept of critical coupling, we present the integration of a short nanowire into a two-dimensional double heterostructure photonic crystal cavity to realize an integrated single-photon detector with excellent performance metric. The complete detector characterization reveals on-chip detection efficiencies of almost 70% at telecom wavelengths, recovery times of 480 ps, and vanishingly low dark count rates. Our design paves the way for the implementation of compact on-chip detector arrays and time-multiplexed single-detector schemes.

Photonic crystal slab cavity simultaneously optimized for ultra-high Q/V and vertical radiation coupling

We present a design for a two-dimensional photonic crystal slab cavity in which the electric field localization is due to an extra hole in the lattice, as opposed to the more standard procedure of removal of holes. This leads to a tighter field confinement and a mode volume that is several times smaller than that of conventionally used designs. Through small modifications of the holes around the cavity, we optimize the theoretical quality factor (Q) to an ultra-high value of 20.9 × 106 and furthermore illustrate the possibility for high coupling efficiency to free-space modes in the vertical direction, while keeping a high Q of 3.7 × 106.

Reproducible fabrication and characterization of diamond membranes for photonic crystal cavities

Single crystal diamond has turned out to be a promising material for applications in quantum information processing for its ability to host color centers with optimal properties such as narrow emission lines or long-lived spin states. However, the mechanical properties of diamond like its hardness and chemical resistance make the fabrication of photonic devices challenging up to date. In this article, a method is presented to produce thin, free-standing single crystal diamond membranes. These membranes are characterized with four different techniques complementing one another and detecting defects or thickness variations in the diamond films with high accuracy. The precise characterization allows for reproducible fabrication of two-dimensional photonic crystal cavities by focused ion beam milling in the as-produced membranes. Furthermore, optimizations in focused ion beam milling are presented, reducing imperfections of the fabricated photonic crystal cavities. Together with post-processing and implantation of color centers, these diamond nanostructures are a building block for cavity coupling experiments toward reliable spin–photon interfaces or optomechanical devices.

Polymer-based Photonic Crystal Cavity Sensor for Optical Detection in the Visible Wavelength Region

In this study, a polymer-based two-dimensional photonic crystal (PhC) cavity for visible-light-based optical-sensing applications was designed and fabricated for the first time. The PhC cavity configuration was designed to operate at 650 nm, and fabricated with a polymer (resist) on a silicon substrate using electron-beam lithography. For investigating sensing applications based on shifting of condition exhibiting a photonic bandgap (PBG), the polymer monolayer deposition (layer-by-layer method) was monitored as the light-intensity change at the cavity position. Consequently, the monolayer-level detection of polyions was achieved.

Integrated III-V Photonic Crystal--Si waveguide platform with tailored optomechanical coupling.

Optomechanical systems, in which the vibrations of a mechanical resonator are coupled to an electromagnetic radiation, have permitted the investigation of a wealth of novel physical effects. To fully exploit these phenomena in realistic circuits and to achieve different functionalities on a single chip, the integration of optomechanical resonators is mandatory. Here, we propose a novel approach to heterogeneously integrate arrays of two-dimensional photonic crystal defect cavities on top of silicon-on-insulator waveguides. The optomechanical response of these devices is investigated and evidences an optomechanical coupling involving both dispersive and dissipative mechanisms. By controlling the optical coupling between the waveguide and the photonic crystal, we were able to vary and understand the relative strength of these couplings. This scalable platform allows for an unprecedented control on the optomechanical coupling mechanisms, with a potential benefit in cooling experiments, and for the development of multi-element optomechanical circuits in the framework of optomechanically-driven signal-processing applications.

All-optical on-chip sensor for high refractive index sensing

A highly sensitive sensor design based on two-dimensional photonic crystal cavity is demonstrated. The geometric structure of the cavity is modified to gain a high quality factor, which enables a sensitive refractive index sensing. A group of slots with optimized parameters is created in the cavity. The existence of the slots enhances the light–matter interactions between confined photons and analytes. The interactions result in large wavelength shifts in the transmission spectra and are denoted by high sensitivities. Experiments show that a change in refractive index of Δn ∼ 0.12 between water and oil sample 1 causes a spectral shift of 23.5 nm, and the spectral shift between two oil samples is 5.1 nm for Δn ∼ 0.039. These results are in good agreement with simulations, which are 21.3 and 7.39 nm for the same index changes.

Ultrahigh-Q modes in anisotropic 2D photonic crystal

In this work, we design a two-dimensional photonic crystal cavity made with a substrate of an anisotropic material. We consider triangular lattice photonic crystal made from air holes in tellurium. The cavity itself is then created by three missing holes in the centre. Using the three-dimensional finite-difference time-domain simulation and optimization of the geometrical parameters and the symmetric displacement of the edge air holes on the quality factor, the cavity’s structural parameters yield an ultrahigh-Q mode cavity with quality factor Q = 2.95 × 1011 for a filling factor r/a = 0.45 and lateral displacement of 10 nm. This shows great enhancement compared with previous studies in which silicon material has been used. The designed structure can be helpful in a number of applications associated with photonic crystal cavities, including quantum information processing, filters, and nanoscale sensors.

Optical properties of metamaterial-based devices modulated by a liquid crystal

Due to the fact that it is possible to manipulate light with photonic crystals (PCs), PCs hold a great potential for designing new optical devices. There has been an increase in research on tuning the optical properties of PCs to design devices. We presented a numerical study of optical properties of metamaterial-based devices by liquid crystal infiltration. The plane wave expansion method and finite-difference time-domain method for both TE and TM modes revealed optical properties in photonic crystal structures in an air background for a square lattice. E7 type has been used as a nematic liquid crystal and SrTiO3 as a ferroelectric material. We showed the possibility of the metamaterials for a two-dimensional photonic crystal cavity on a ferroelectric base infiltrated with a nematic liquid crystal.

All-optical on-chip sensor for high refractive index sensing in photonic crystals

In this paper we demonstrate an optical sensor designed to detect material infiltrations with relatively high indices, based on a two-dimensional photonic crystal cavity structure. The locations and sizes of the holes surrounding a L3 cavity were modified to increase the Q factor to a value of 1500 with a high refractive index infilling of . With precise design and simulation, we overcome the difficulty of low index contrast, and observe a very clear wavelength shift of 10.4 nm in simulation and 12.4 nm in experiment between water and oil samples at resonance.

All-optical tunable filters based on optomechanical effects in two-dimensional photonic crystal cavities

All-optical tunable filters are basic elements for various micro-optical circuits. Obtaining all-optical tunability remains a challenge for micro-optical circuits. Optical forces with significant effects in nanophotonic systems provide new ways for wavelength tuning. In this Letter, the optomechanical effects in two-dimensional photonic crystal cavities are investigated. Simulations based on the finite element method demonstrate that forces arise in single and coupled cavities with movable rods inside. The optical force controls the positions of the movable rods and, thus, the resonance wavelength of the cavity, based on which tunable filter is designed. The operating wavelength of the cavity or the filter for the signal can be tuned by a control light with a different frequency. The results have potential applications for various integrated circuits.