Photonic Crystal Slab Cavities Research Articles

Double-coupled slotted photonic crystal slab cavities through a high-frequency mechanical oscillator

Double-coupled slotted photonic crystal slab cavities through a high-frequency mechanical oscillator

Near-Field Imaging of Magnetic Complex Mode Volume

The non-Hermitian nature of confined photonic modes is described by the electric complex modal volume, VE, which represents a key parameter that leads to counterintuitive effects, such as negative modal contribution to the local density of states and non-Lorentzian lineshapes. Here, we address the magnetic counterpart of VE by means of near-field perturbation experiments in a photonic crystal slab cavity. We study the relevant role played by the imaginary part of the magnetic modal volume, VH, which can increase the quality factor of the confined modes by means of a local external magnetic perturbation. We show how a mapping of the spatial distribution of both the real and imaginary parts of VH can be inferred by near-field experiments employing Al-covered near-field tips. Our findings deepen the role of the magnetic component of light and could open a new route in employing metamaterials, magnetic quantum emitters, and topological photonics.

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.

Doubly resonant χ(2) nonlinear photonic crystal cavity based on a bound state in the continuum

Photonic nanostructures simultaneously maximizing spectral and spatial overlap between fundamental and second-harmonic confined modes are highly desirable for enhancing second-order nonlinear effects in nonlinear media. These conditions have thus far remained challenging to satisfy in photonic crystal cavities because of the difficulty in designing a band gap at the second-harmonic frequency. Here, we solve this issue by using instead a bound state in the continuum at that frequency, and we design a doubly-resonant photonic crystal slab cavity with strongly improved figures of merit for nonlinear frequency conversion when compared to previous photonic crystal designs. Furthermore, we show that the far-field emission at both frequencies is highly collimated around normal incidence, which allows for simultaneously efficient pump excitation and collection of the generated nonlinear signal. Our results could lead to unprecedented conversion efficiencies in both parametric down conversion and second harmonic generation in an extremely compact architecture.

Extreme terahertz electric-field enhancement in high-Q photonic crystal slab cavity with nanoholes.

A one-dimensional photonic-crystal (PC) cavity with nanoholes is proposed for extreme enhancement of terahertz (THz) electric fields using the electromagnetic (EM) boundary conditions. Both slot (for the perpendicular component of the electric displacement field) and anti-slot (for the parallel component of the electric field) effects contribute to the considerable field enhancement. The EM energy density can be enhanced by a factor of (εh/εl)2 in the high-refractive-index material, where εh and εl are the permittivities of the high- and low-refractive-index materials, respectively. Correspondingly, the mode volume can be reduced by a factor of 288, compared with a conventional THz PC cavity, and is three orders of magnitude smaller than the diffraction limitation. Further, the proposed THz cavity design also supports modes with high quality factors (Q) > 104, which induces strong Purcell enhancement by a factor exceeding 106. Our THz cavity design is feasible and attractive for experimental demonstrations, because the semiconductor layer in which the EM is maximized can naturally be filled with quantum-engineered active materials. Thus, the proposed design can possibly be used to develop room-temperature coherent THz radiation sources.

Disorder effects on the coupling strength of coupled photonic crystal slab cavities

We study the effects of disorder on the coupling strength of coupled photonic crystal slab cavities by considering fully-3D electromagnetic calculations. Specifically, we investigate two coupled L3 cavities at 30° and 60° configurations, where the coupling strength J (or photon hopping) is extracted from the simulations in the presence of disorder. We found that the relative fluctuations of the photon hopping are more sensitive to disorder effects than the corresponding fluctuations in the eigenfrequencies of the coupled cavities. Furthermore, for the typical range of disorder in state-of-the-art devices, the J fluctuations are found to increase linearly as a function of the disorder amplitude. This allows one to set upper bounds to the amplitude of fabrication imperfections, for which the coupling predicted by design can still be expected, on average, in a fabricated device.

Anderson Localization in Disordered LN Photonic Crystal Slab Cavities

We present a detailed theoretical study of the effects of structural disorder on LN photonic crystal slab cavities, ranging from short to long length scales, using a fully-3D Bloch mode expansion technique. We compute the optical density of states, quality factors and effective mode volumes of the cavity modes, with and without disorder, and compare with the localized modes of the corresponding disordered photonic crystal waveguide. We demonstrate how the quality factors and effective mode volumes saturate at a specific cavity length and become bounded by the corresponding values of the Anderson modes appearing in the disordered waveguide. By means of the intensity fluctuation criterion, we observe Anderson localization for cavity lengths larger than around L31, and show that the field confinement in the disordered LN cavities is mainly determined by the local characteristics of the structural disorder as long as the confinement region is far enough from the cavity mirrors and the effective mode localization length is much smaller than the cavity length; under this regime, the disordered cavity system becomes insensitive to changes in the cavity boundaries and a good agreement with the intensity fluctuation criterion is found for localization. Surprisingly, we find that the Anderson localized modes do not appear as new disorder-induced resonances in the main spectral region of the LN cavity modes, and, moreover, the disordered DOS enhancement is largest for the disordered waveguide system with the same length. These results are fundamentally interesting for applications such as lasing and cavity-QED, and provide new insights into the role of the boundary condition on finite-size slow-light waveguides. They also point out the clear failure of using models based on the cavity boundaries/mirrors and a single slow-light Bloch mode to describe cavity systems with large N.

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.

Effective bichromatic potential for ultra-high Q-factor photonic crystal slab cavities

We introduce a confinement mechanism in photonic crystal slab cavities, which relies on the superposition of two incommensurate one-dimensional lattices in a line-defect waveguide. It is shown that the resulting photonic profile realizes an effective quasi-periodic bichromatic potential for the electromagnetic field confinement yielding extremely high quality (Q) factor nanocavities, while simultaneously keeping the mode volume close to the diffraction limit. We apply these concepts to pillar- and hole-based photonic crystal slab cavities, respectively, and a Q-factor improvement by over an order of magnitude is shown over existing designs, especially in pillar-based structures. Thanks to the generality and easy adaptation of such confinement mechanism to a broad class of cavity designs and photonic lattices, this work opens interesting routes for applications where enhanced light–matter interaction in photonic crystal structures is required.

Sensitive All-Optical Channel-Drop Sensor in Photonic Crystals

We report the results of a study of an optical sensor based on a channel-drop technique with two cascaded cavities in photonic-crystal slabs. Quality factors and intensities of the resonant modes of the sensor were analyzed with three-dimensional simulations. With the introduction of a reflector in the bus-channel and by control of the coupling between the two cavities and the drop-channel, the drop efficiency can be remarkably increased. In addition to the simulation, the two cavity sensor is fabricated and tested for optical response to water and oil infiltration. Both direct visual imaging and quantitative analysis were applied in experiment. A difference of refractive index $\Delta n = 0.12$ between water and oil samples results in a wavelength shift of 18.3 nm, which greatly matches the simulation result of 20 nm and indicates a sensitivity of 153 nm RIU−1. Both resonant peaks for water and oil infiltration have good selectivity in their transmission spectrum. The contrast between the broadband output of the bus-channel and the highly wavelength-selective outputs of the drop-channel opens opportunities for the two cascaded-cavity system as a fundamental building block for a multiplex drop-channel array for all-optical sensing, which can be widely used for bio/chemical detection and environmental monitoring.

Automated optimization of photonic crystal slab cavities.

Thanks to their high quality factor, combined to the smallest modal volume, defect-cavities in photonic crystal slabs represent a promising, versatile tool for fundamental studies and applications in photonics. In paricular, the L3, H0, and H1 defects are the most popular and widespread cavity designs, due to their compactness, simplicity, and small mode volume. For these cavities, the current best optimal designs still result in Q-values of a few times 105 only, namely one order of magnitude below the bound set by fabrication imperfections and material absorption in silicon. Here, we use a genetic algorithm to find a global maximum of the quality factor of these designs, by varying the positions of few neighbouring holes. We consistently find Q-values above one million – one order of magnitude higher than previous designs. Furthermore, we study the effect of disorder on the optimal designs and conclude that a similar improvement is also expected experimentally in state-of-the-art systems.

Modeling of Fano resonances in the reflectivity of photonic crystal cavities with finite spot size excitation

We study the reflectivity spectra of photonic crystal slab cavities using an extension of the scattering matrix method that allows treating finite sizes of the spot of the excitation beam. The details of the implementation of the method are presented and then we show that Fano resonances arise as a consequence of the electromagnetic interference between the discrete contribution of the fundamental cavity mode and the continuum contribution of the light scattered by the photonic crystal pattern. We control the asymmetry lineshape of the Fano resonance through the polarization of the incident field, which determines the relative phase between the two electromagnetic contributions to the interference. We analyse the electric field profile inside and outside of the crystal to help in the understanding of the dependence on polarization of the reflectivity lineshape. We also study with our implementation the dependence of the Fano resonances on the size of the incident radiation spot.

Ultrasmall modal volume and high Q factor optimization of a photonic crystal slab cavity

We investigate the photonic crystal slab cavities with different symmetries (C2,σv, C3,σv, C6,σv), and try to find out the rules for increasing the Q factor and reducing the effective modal volume value. The results indicate that the cavity with C2,σv symmetry may be the most suitable candidate, which has high Q factor and small effective modal volume. Based on the cavity with C2,σv symmetry, we use the improved geometry projection method to optimize the shapes of neighbor holes of the cavity and obtain a new slab cavity structure with a high Q value of 5.15 × 105 and small effective modal volume value of 0.09(λ0/n)3. The performance of such a structure is better than in previous reports, which is significant for developing photonic crystal cavity based devices.

Cavity-mode calculation of L3 photonic crystal slab using the effective index perturbation method

This paper mainly describes that the effective index perturbation (EIP) technique in combination with two-dimensional (2D)/three-dimensional (3D) PWE methods predicts resonant mode frequencies and mode patterns for the L3 photonic crystal slab cavity, which substantially reduces the computation time and computer memory without losing accuracy. The simulation results show that when the perturbed effective index by matching dielectric band edge is used, the fundamental resonant mode has small calculation frequency error; however, when the effective index obtained by the standard effective index method (EIM) is used, it is appropriate for predicting higher-order resonant modes, and the micro-photoluminescence experiments verify the predictions of the theoretical model. In addition, mode profiles of the L3 cavity with both the displacement of the end-holes and the base cavity structure can also be accurately predicted.

Reconfigurable, Defect-Free, Ultrahigh-Q Photonic Crystal Microcavities for Sensing

We propose a new approach for creating reconfigurable high-Q cavities in defect-free photonic crystal slabs (PCSs). The approach relies on selective air-hole infiltration in otherwise defect-free PCSs. We show that using this method we can design ultrahigh-Q microcavities, with Q˜106. Numerical calculations indicate a large number of high-Q modes with high sensitivity, which are ideal for simultaneous, multi-parameter refractive index-based sensing.

Properties Investigation for Single-Defect Square-Lattice Photonic Crystal Slab Cavity in Crystal Material Application

Compact and high-quality cavities are essential building blocks in constructing nano-photonic systems and quantum information systems. So, it is important in understanding the properties of localized modes produced by disorder in two-dimensional photonic crystals cavity. Single defect square lattice photonic crystal slab cavity is taken as the investigation object, the relationship between structure parameters (lattice constant a, radius of hole r and slab thickness) and properties parameters (quality factor Qt, effective mode volume Vmode) is discussed for a optimization design of high performance cavity. The results of this investigation are useful reference for single defect square lattice photonic crystal slab cavity in photonic crystal material application.

Optimized light–matter interaction and defect hole placement in photonic crystal cavity sensors

Photonic crystal slab cavities were investigated for increased light-matter interaction based on selective placement of sublattice hole sized defect holes inside L3 cavities. A multiple-hole defect (MHD) consisting of three defect holes placed in the regions of highest cavity mode field intensity were demonstrated through finite-difference time-domain simulations and experiments to exhibit the strongest light-matter interaction without introducing significant scattering losses. Compared to an L3 cavity without defect holes, these strategically designed three-hole MHD cavities presented higher quality factor and more than double the resonance wavelength shift upon exposure to a thin oxide and two small chemical molecules.

Study on properties of the H1 photonic crystal slab cavity using the effective index perturbation method

In this paper the effective index perturbation technique in combination with two-dimensional (2D)/three-dimensional (3D) plane wave expansion methods is used to predict resonant mode frequencies of donor-like and acceptor-like H1 photonic crystal slab cavity, and their results are very close to the ones obtained by three-dimensional finite difference time domain method. For donor-like H1 photonic cavity, when the perturbed effective index by matching dielectric band edge is used, there is a relatively small frequency difference; however, for acceptor-like H1 photonic cavity, the matching criterion should be set at middle band position. The effective index perturbation method can not only save computation time and computer memory with reducing dimensionality (from 3D to 2D), but also ensure the accuracy of the simulation results, which is substantially important for the extensive application of photonic crystal slab cavity.

Finite-Difference Time-Domain Analysis of Photonic Crystal Slab Cavities with Two-Level Systems

In this paper, we report the numerical simulation of an atom-cavity interaction within photonic crystal nano-cavities. The numerical model is based on a damping oscillator description of a dipole current and it is implemented with a finite-difference time-domain method. Using the method, we successfully simulate the atom-cavity mode field interactions of a two-level system embedded in a photonic crystal cavity under several coupling strength conditions. We show that enhancement and suppression of optical emission rate from a two-level system are also shown by this model.

Phonon-Mediated Coupling ofInGaAs/GaAsQuantum-Dot Excitons to Photonic Crystal Cavities

We demonstrate that the emission characteristics of site-controlled InGaAs/GaAs single quantum dots embedded in photonic crystal slab cavities correspond to single confined excitons coupled to cavity modes, unlike previous reports of similar systems based on self-assembled quantum dots. By using polarization-resolved photoluminescence spectroscopy at different temperatures and a theoretical model, we show that the exciton-cavity interaction range is limited to the phonon sidebands. Photon-correlation and pump-power dependence experiments under nonresonant excitation conditions further establish that the cavity is fed only by a single exciton.