Small Mode Volume Research Articles

Whispering‐gallery microcavities with unidirectional laser emission

AbstractOptical whispering‐gallery mode (WGM) microcavities featuring ultrahigh Q factors and small mode volumes enhance significantly the interaction between light and matter, becoming an excellent platform for achieving ultralow‐threshold microlasers. However, the emission of traditional WGMs is isotropic due to the rotational symmetry of cavity geometries, which hinders the potential photonics applications. In this review, the progress in WGM microcavities towards unidirectional laser emission is summarized. When a subwavelength scatterer is placed on the boundary of the microcavity, the unidirectional emission occurs due to the collimation effect of the microcavity‐enhanced scattering field. Furthermore, microcavities deformed from the circular shapes can not only produce the chaos‐assisted unidirectional emission, but also maintain high Q factors by special design and fabrication processes. Finally, gratings along the circumference of the WGM microdisk or microring can scatter the WGMs in the vertical direction. The review also lists several important applications of these types of microcavities, such as wide‐band laser illumination source, free‐space coupling, evanescent‐field enhancement, optical energy storage, and sensing. image

Compact metamaterial-enclosed wireless sensors with subtle perception of internal physical events

We propose and theoretically investigate a metamaterial/metasurface-enclosed electrically small antenna (ESA), perfectly matched to the input stage of a radio-frequency circuitry. By tailoring the effective-medium properties of a low-profile metamaterial/metasurface cloak, the electromagnetic power can be maximally delivered to a 50 Ω load in a narrow frequency window, efficiently utilizing the available design volume. This ESA system, although having a small modal volume, is highly resonant, and its resonance frequency is sensitively shifted by material and volume perturbations within the cloaked region. We envision that this cloaked ESA system may make efficient and miniaturized wireless sensor nodes, probing internal information (e.g., liquid-level sensor) for various medical, environmental, and industrial applications.

Enhanced nonlinear interaction in a microcavity under coherent excitation.

The large field enhancement that can be achieved in high quality factor and small mode volume photonic crystal microcavities leads to strengthened nonlinear interactions. However, the frequency shift dynamics of the cavity resonance under a pulsed excitation, which is driven by nonlinear refractive index change, tends to limit the coupling efficiency between the pulse and the cavity. As a consequence, the cavity enhancement effect cannot last for the entire pulse duration, limiting the interaction between the pulse and the intra-cavity material. In order to preserve the benefit of light localization throughout the pulsed excitation, we report the first experimental demonstration of coherent excitation of a nonlinear microcavity, leading to an enhanced intra-cavity nonlinear interaction. We investigate the nonlinear behavior of a Silicon-based microcavity subject to tailored positively chirped pulses, enabling to increase the free carrier density generated by two-photon absorption by up to a factor of 2.5 compared with a Fourier-transform limited pulse excitation of equal energy. It is accompanied by an extended frequency blue-shift of the cavity resonance reaching 19 times the linear cavity bandwidth. This experimental result highlights the interest in using coherent excitation to control intra-cavity light-matter interactions and nonlinear dynamics of microcavity-based optical devices.

Theoretical Study on Surface Mode in Photonic Crystal Fishbone Nanocavity

We propose and theoretically investigate a novel 1-D photonic crystal fishbone (FB) that can sustain surface waves. By designing a nanocavity in an FB, the confined surface mode with a high quality factor (∼ 105) and extremely concentrated field near the FB surface (small mode volume, ∼ 2.3 × 10−2 (λ/2)3) cause strong interactions between light and the surrounding medium for optical sensing and manipulation. In simulation, as an optical sensor, the proposed design achieved a high index sensitivity of 650 nm/ RIU and minimum detectable index variation of 2 × 10−5. As optical tweezers, a simulated injected optical threshold power of only 80 μW is needed for stably trapping a polystyrene sphere (PS) 100 nm in diameter. In addition, a method of selectively trapping a PS of specific size is theoretically proposed via our design. We believe that our proposed FB nanocavity with a surface mode would provide enhanced features for on-chip optical sensors and tweezers.

Nonlinear Fano-Resonant Dielectric Metasurfaces.

Strong nonlinear light-matter interaction is highly sought-after for a variety of applications including lasing and all-optical light modulation. Recently, resonant plasmonic structures have been considered promising candidates for enhancing nonlinear optical processes due to their ability to greatly enhance the optical near-field; however, their small mode volumes prevent the inherently large nonlinear susceptibility of the metal from being efficiently exploited. Here, we present an alternative approach that utilizes a Fano-resonant silicon metasurface. The metasurface results in strong near-field enhancement within the volume of the silicon resonator while minimizing two photon absorption. We measure a third harmonic generation enhancement factor of 1.5 × 10(5) with respect to an unpatterned silicon film and an absolute conversion efficiency of 1.2 × 10(-6) with a peak pump intensity of 3.2 GW cm(-2). The enhanced nonlinearity, combined with a sharp linear transmittance spectrum, results in transmission modulation with a modulation depth of 36%. The modulation mechanism is studied by pump-probe experiments.

New Approach of Plasmonically Induced Reflectance in a Planar Metamaterial for Plasmonic Sensing Applications

We theoretically demonstrate and investigate plasmonically induced reflectance (PIR) in a new planar metamaterial with two completely different approaches. Here, we not only show that broken symmetry is a general strategy to create electromagnetically induced reflectance (EIR)-like effect but also demonstrate that the nanoplasmonic EIR can be realized even without broken symmetry via the excitation of the higher-order plasmonic modes in the same designed planar metamaterial. In nanophotonics, plasmonic structures enable large field strengths within small mode volumes. Therefore, combining EIR with nanoplasmonics would open up the way toward ultracompact sensors with extremely high sensitivity. In the second approach of creating the PIR of our proposed nanostructure, the restrictions on size are partially relaxed, making fabrication much easier. Their interactions and coupling between plasmonic modes are investigated in detail by analyzing field distributions and spectral responses. Also, we show that the PIR frequency position depended very sensitively on the dielectric surrounding. Furthermore, the narrow and fully modulated PIR features due to the extraordinary reduction of damping may serve for designing novel devices in the field of chemical and biomedical sensing.

Purcell enhancement and focusing effects in plasmonic nanoantenna arrays

This paper presents measured fluorescence results for PMMA dye-coated 5×5 gold plasmonic nanoantenna arrays. We use numerical electromagnetic modeling to show how array size and element spacing can be used to control emitted beam shape and compare this with experimental data. The Friis formula from RF antenna theory is used to calculate the intensity enhancement produced by the array. A figure of merit is then developed, which accounts for the very small mode volume from which the array emission is occurring.

Acousto-optic coupling in phoxonic crystal nanobeam cavities with plasmonic behavior.

Acousto-optic (AO) coupling in a two-layer GaAs/Ag heterogeneous phoxonic crystal nanobeam cavity with plasmonic behavior is studied numerically. Because of the Ag metal layer, the cavity structure hybridizes photons and surface plasmons, squeezing the optical energy into small regions near the GaAs/Ag interface; the phononic cavity modes can be simultaneously tailored to highly match the photonic cavity modes at reduced regions in the cavity. Consequently, AO coupling is enhanced at near-infrared wavelengths. Boosting of the interface effect by the acoustic displacement field mainly contributes to the AO coupling enhancement. The simultaneous small photonic mode volume and high spatial matching of photonic and phononic cavity modes enhance the photonic resonance wavelength shift by one order of magnitude. This study enables applications of strong AO or photon-phonon interaction in subwavelength nano-structures.

Single quantum dot controls a plasmonic cavity’s scattering and anisotropy

Plasmonic cavities represent a promising platform for controlling light-matter interaction due to their exceptionally small mode volume and high density of photonic states. Using plasmonic cavities for enhancing light's coupling to individual two-level systems, such as single semiconductor quantum dots (QD), is particularly desirable for exploring cavity quantum electrodynamic (QED) effects and using them in quantum information applications. The lack of experimental progress in this area is in part due to the difficulty of precisely placing a QD within nanometers of the plasmonic cavity. Here, we study the simplest plasmonic cavity in the form of a spherical metallic nanoparticle (MNP). By controllably positioning a semiconductor QD in the close proximity of the MNP cavity via atomic force microscope (AFM) manipulation, the scattering spectrum of the MNP is dramatically modified due to Fano interference between the classical plasmonic resonance of the MNP and the quantized exciton resonance in the QD. Moreover, our experiment demonstrates that a single two-level system can render a spherical MNP strongly anisotropic. These findings represent an important step toward realizing quantum plasmonic devices.

Graphene nanophotonic sensors

Graphene is known to possess a host of remarkable properties such as a zero bandgap at its Dirac point, broadband saturable optical absorption, ballistic carrier transport at room temperature, as well as extremely high stiffness and thermal conductivity. This has in turn made it a material of interest for many applications, ranging from fundamental physics studies to electronic devices. From a photonics perspective, graphene’s ability to support surface plasmon-polaritons with extremely small mode volumes in the infrared spectral regime and beyond renders it an ideal platform for strongly enhanced light–matter interactions at deeply subwavelength size scales. Together with its large bandwidth of operation, as well as intrinsic chemical stability and affinity to organic molecules, graphene serves as a natural candidate for numerous optics-based sensing applications. This article reviews recent works that highlight the various advantages of graphene in an optical sensing context. Specifically, it focuses on how the passive functionalization of graphene can improve the performance of existing optical sensors, and how its use as an active signal transduction element could lead to various novel or hybrid devices that extend the functionalities of traditional sensors.

Analysis of an electro-optic modulator based on a graphene-silicon hybrid 1D photonic crystal nanobeam cavity.

We propose and numerically study an on-chip graphene-silicon hybrid electro-optic (EO) modulator operating at the telecommunication band, which is implemented by a compact 1D photonic crystal nanobeam (PCN) cavity coupled to a bus waveguide with a graphene sheet on top. Through electrically tuning the Fermi level of the graphene, both the quality factor and the resonance wavelength can be significantly changed, thus the in-plane lightwave can be efficiently modulated. Based on finite-difference time-domain (FDTD) simulation results, the proposed modulator can provide a large free spectral range (FSR) of 125.6 nm, a high modulation speed of 133 GHz, and a large modulation depth of ~12.5 dB in a small modal volume, promising a high performance EO modulator for wavelength-division multiplexed (WDM) optical communication systems.

Advances and Prospects for Whispering Gallery Mode Microcavities

Microlasers have experienced tremendous development in the past decade and become an essential part in laser evolution, as miniature lasers provide strong optical confinement and feature greatly enhanced light–matter interactions. Among all the configurations, whispering gallery mode (WGM) microcavities and microlasers exhibit outstanding optical performances with high quality factors and small mode volumes, thus ensuring low lasing thresholds. In addition, some unique properties inherent to WGM cavities, like bi‐directional propagation and an evanescent field that spans several hundred nanometers across the boundary, can be exploited for novel applications. Therefore, designing and engineering innovative WGM microcavities and microlasers has attracted increasing research interest. The fundamentals and characteristics of WGM are introduced here, and then the developments and current status of WGM microcavities and microlasers are reviewed in terms of the evolution of fabrication techniques and built‐up materials. In particular, the melting of glassy materials in early studies, top‐down and bottom‐up approaches with semiconductors, coating structures, as well as flexible, soft microresonators in recent years are presented. Finally, the application prospects of microlasers including the wavelength manipulation, sensing and microresonator coupling, are discussed.

Topographic control of open-access microcavities at the nanometer scale.

Open-access optical microcavities are emerging as an original tool for light-matter studies thanks to their intrinsic tunability and the direct access to the maximum of the electric field along with their small mode volume. In this article, we present recent developments in the fabrication of such devices demonstrating topographic control of the micromirrors at the nanometer scale as well as a high degree of reproducibility. Our method takes into account the template shape as well as the effect of the dielectric mirror growth. In addition, we present the optical characterization of these microcavities with effective radii of curvature down to 4.3 µm and mode volume of 16×(λ/2)(3). This work opens the possibility to fully engineer the photonic potential depending on the required properties.

Ultra-low threshold gallium nitride photonic crystal nanobeam laser

We report exceptionally low thresholds (9.1 μJ/cm2) for room temperature lasing at ∼450 nm in optically pumped Gallium Nitride (GaN) nanobeam cavity structures. The nanobeam cavity geometry provides high theoretical Q (>100 000) with small modal volume, leading to a high spontaneous emission factor, β = 0.94. The active layer materials are Indium Gallium Nitride (InGaN) fragmented quantum wells (fQWs), a critical factor in achieving the low thresholds, which are an order-of-magnitude lower than obtainable with continuous QW active layers. We suggest that the extra confinement of photo-generated carriers for fQWs (compared to QWs) is responsible for the excellent performance.

Optimization of Q-factor in nonlinear planar photonic crystal nanocavity incorporating hybrid silicon/polymer material

We present a novel design for a high quality (Q)-factor, nonlinear planar photonic crystal (PhC) nanocavity incorporating a silicon/polymer material that is well suited to ultrafast all-optical switching. The hybrid nanocavity is created in the centre of a triangular lattice planar PhC made from silicon using the three-missing-holes point defect (L3). It is formed by infiltrating the air hole array of the PhC with polymer and by depositing a polymer layer on top of a PhC membrane. To determine the hybrid nonlinear cavity performance, we analyze the dependence of the refractive index (RI) of the top cladding on the Q-factor and resonant wavelength. The results show that, when the top cladding RI is increased from 1.5 to 1.6, corresponding to that of polymer materials, the Q-factor decreases markedly (Q < 103). Optimization of the hybrid nonlinear silicon/polymer cavity design by modulating the structure parameters yields a high Q-factor of 54 000 with a small modal volume across the telecommunications band. In addition, the field distribution of the resonant mode indicates that the radiation loss is sufficiently small. Due to the overwhelmingly large Kerr nonlinearity of polymer over silicon, this structure configuration design shows considerable promise as regards the realization of ultrafast response speed in small-sized all-optical switching integrated devices.

On-Chip Photonic Transistor Device and Biomolecule Mass Sensor Based on a Whispering Gallery Mode Cavity Optomechanical System

Whispering gallery mode (WGM) cavities, due to their high-quality factors, small mode volumes, and simple fabrications, have potential applications in photonic devices and sensing. In this paper, we present an all-optical transistor based on a coupled realistic WGM cavity optomechanical system. In the system, the transmission of the signal field is manipulated by the optical control power, and the signal field can be efficiently attenuated or amplified depending on the power of a second gating (control) field, which indicates a promising candidate for single-photon transistors. In addition, we theoretically propose a mass sensor based on this WGM cavity optomechanical system. The mass of external biomolecules (such as smallpox virus) deposited onto WGM cavity can be measured conveniently by tracking the mechanical resonance frequency shifts due to mass changes in the probe transmission spectrum. Through detecting the mass of viruses, the kinds of different viruses can be identified. This WGM cavity optomechanical system may have potential chip scale applications in quantum information processing, quantum measurement, and on-chip sensing devices.

Design of Bottom-Emitting Metallic Nanolasers Integrated With Silicon-On-Insulator Waveguides

We propose and analyze a bottom-emitting metallic nanolaser with a cavity size smaller than one wavelength in three dimensions. We optimize the nanolaser performance by adjusting the thickness of SiN x insulator layer, pillar radius, p-cladding, and n-cladding layers. With a proper design, we can achieve the TE01 mode lasing with an extremely small effective mode volume of 0.135 (λ0/n)3 and an ultra low threshold gain of 247 cm−1. The nanolaser is integrated with a silicon-on-insulator waveguide through a bottom-connected grating coupler. By properly modifying the grating coupler period, fill factor, shift position, and grating height, a high coupling efficiency of 88.5% can be acquired.

Ultrastrong Mode Confinement in ZnO Surface Plasmon Nanolasers.

Nanolasers with an ultracompact footprint can provide high-intensity coherent light, which can be potentially applied to high-capacity signal processing, biosensing, and subwavelength imaging. Among various nanolasers, those with cavities surrounded by metals have been shown to have superior light emission properties because of the surface plasmon effect that provides enhanced field confinement capability and enables exotic light-matter interaction. In this study, we demonstrated a robust ultraviolet ZnO nanolaser that can operate at room temperature by using silver to dramatically shrink the mode volume. The nanolaser shows several distinct features including an extremely small mode volume, a large Purcell factor, and a slow group velocity, which ensures strong interaction with the exciton in the nanowire.

Investigation of defect cavities formed in three-dimensional woodpile photonic crystals

We report the optimisation of optical properties of single defects in three-dimensional (3D) face-centred-cubic (FCC) woodpile photonic crystal (PC) cavities by using plane-wave expansion (PWE) and finite-difference time-domain (FDTD) methods. By optimising the dimensions of a 3D woodpile PC, wide photonic band gaps (PBG) are created. Optical cavities with resonances in the bandgap arise when point defects are introduced in the crystal. Three types of single defects are investigated in high refractive index contrast (Gallium Phosphide-Air) woodpile structures and Q-factors and mode volumes ($V_{eff}$) of the resonant cavity modes are calculated. We show that, by introducing an air buffer around a single defect, smaller mode volumes can be obtained. We demonstrate high Q-factors up to 700000 and cavity volumes down to $V_{eff}<0.2(\lambda/n)^3$. The estimates of $Q$ and $V_{eff}$ are then used to quantify the enhancement of spontaneous emission and the possibility of achieving strong coupling with nitrogen-vacancy (NV) colour centres in diamond.

Enhanced magnetic Purcell effect in room-temperature masers.

Recently, the world’s first room-temperature maser was demonstrated. The maser consisted of a sapphire ring housing a crystal of pentacene-doped p-terphenyl, pumped by a pulsed rhodamine-dye laser. Stimulated emission of microwaves was aided by the high quality factor and small magnetic mode volume of the maser cavity yet the peak optical pumping power was 1.4 kW. Here we report dramatic miniaturization and 2 orders of magnitude reduction in optical pumping power for a room-temperature maser by coupling a strontium titanate resonator with the spin-polarized population inversion provided by triplet states in an optically excited pentacene-doped p-terphenyl crystal. We observe maser emission in a thimble-sized resonator using a xenon flash lamp as an optical pump source with peak optical power of 70 W. This is a significant step towards the goal of continuous maser operation.