Photonic Cavity Research Articles

On the trade-off between mode volume and quality factor in dielectric nanocavities optimized for Purcell enhancement.

This study explores the effect of geometric limitations on the achievable Purcell factor for single emitters in dielectric structures by employing topology optimization as an inverse design tool to maximize the local density of states. Nanobeams of different lengths with varying fixed central bridge widths are considered to investigate the impact of footprint and geometric length-scale. In single-mode photonic cavities, the Purcell factor is known to be proportional to the ratio of the quality factor Q to the effective mode volume V. Analysis of the optimized nanocavities shows a trade-off between quality factor and mode volume as a function of geometric limitations. Crucially, the design exhibiting the largest Purcell enhancement does not have the highest Q nor the lowest V found in the design pool. On the contrary, it is found that Q consistently drops along with decreasing V as the minimum allowed geometric length-scale decreases while the Purcell factor increases. Finally, the study provides insight into the importance of Q and V for enhancing the Purcell factor under geometric limitations.

Strong superradiance of coherently coupled magnetic dipole emitters mediated by whispering gallery modes of a subwavelength all-dielectric cavity

The interaction of magnetic dipole (MD) emitters and common photonic cavities is usually weak, which is partially due to the low magnetic near field enhancements of the cavities. Here, we show that whispering gallery modes (WGMs) of a subwavelength dielectric cavity can not only greatly boost the emission rate of a MD emitter but also bring efficient couplings between coherent MD emitters. In a WGM cavity, the maximal emission rate ({\gamma}max) of a single emitter occurs at an antinode of the field pattern. The emission of the MD emitter can also be greatly affected by another coherent one depending on the magnetic field response of the WGM. The maximal contribution can also reach {\gamma}max. Notably, the cooperative emission rate of the coherent MD emitters does not decay with distance in the considered range due to the high-quality feature of a WGM. In contrast to the emission, the absorption of an emitter is hardly affected by the coherent couplings between emitters mediated by a WGM. The difference between the performances of emission and absorption is highly related to the excitation behaviors of WGMs. Our results are important for enhanced magnetic light-matter interactions.

Exploration of a PBG-based cavity structure for a multiple-beam extended interaction klystron

The feasibility of a photonic band-gap (PBG)-based interaction structure was analytically explored for a multiple-beam extended interaction klystron amplifier. This configuration accrues the advantages of the multiple-defect PBG-based extended interaction structure and multiple-beam operation. The mode configurations were analysed through 3D electromagnetic simulation and the applicability of the photonic band-gap cavity was studied for the multiple-beam operation of an extended interaction klystron amplifier. A typical six-defect cavity operating at around 83 GHz was designed for electron–wave interaction at 2π mode with 6 electron beams each carrying 300 mA current at the accelerating potential of 16.5 kV. A particle-in-cell simulation shows that an output power of ∼2 kW is possible with electronic efficiency of around 6.7%. A frequency-scaled-down interaction structure at Ku-band was fabricated and cold measurements were carried out to ascertain the feasibility. The measured values of the frequency for various modes, loaded quality factor and 3 dB bandwidth were found to be within 0.36%, 5.3% and 4.07%, respectively, against those from the simulation.

Scaling Theory of Wave Confinement in Classical and Quantum Periodic Systems.

Functional defects in periodic media confine waves-acoustic, electromagnetic, electronic, spin, etc.-in various dimensions, depending on the structure of the defect. While defects are usually modeled by a superlattice with a typical band-structure representation of energy levels, determining the confinement associated with a given band is highly nontrivial and no analytical method is known to date. Therefore, we propose a rigorous method to classify the dimensionality of wave confinement. Starting from the confinement energy and the mode volume, we use finite-size scaling to find that ratios of these quantities raised to certain powers yield the confinement dimensionality of each band. Our classification has negligible additional computational costs compared to a band structure calculation and is valid for any type of wave, both quantum and classical, and in any dimension. In the quantum regime, we illustrate our method on electronic confinement in 2D hexagonal boron nitride (BN) with a nitrogen vacancy, in agreement with previous results. In the classical case, we study a three-dimensional photonic band gap cavity superlattice, where we identify novel acceptorlike behavior. We briefly discuss the generalization to quasiperiodic lattices.

(Invited) Achieving High-Power Single-Mode Operation in Vertical-Cavity Surface-Emitting Lasers Via Scalable, Higher-Order Mode Suppression Techniques

Achieving high-power single-mode operation in Vertical-Cavity Surface-Emitting Lasers (VCSELs) has received renewed interest because of applications that span facial recognition and 3D imaging in mobile telephones, Light Detection and Ranging (LiDAR) for autonomous vehicles, and Pulse Amplitude Modulation (PAM-4) for high-speed (>100 Gbps) data transmission within data centers. A key challenge with oxide-confined VCSELs is that the strong refractive index contrast between the oxidized ~AlAs aperture and surrounding AlGaAs layers combined with the oxide aperture size create a photonic cavity that fundamentally supports multiple transverse modes. When the size of the aperture is reduced to create a single-mode cavity, the output power is limited by the electrically pumped gain region within the aperture. There is thus a fundamental trade-off between size and power which manifests itself in a limited capability to simultaneously achieve high power and single mode operation. Various methods have been employed by groups working on VCSELs to overcome this limitation. Neglecting external cavity methods which are fundamentally not scalable, the most promising results have been achieved by groups using etched surface relief where a pattern is etched in the top layer or layers to change the standing wave pattern in the cavity and reduce modal gain by lowering the magnitude of the electric field in regions overlapping with the quantum wells, which provide laser gain. A downside of this approach is that material is removed, impacting the uniformity of current injection across the area of the device. A key question of interest is therefore how to achieve mode suppression in large area devices in a way that does not impact current injection. In this presentation, novel methods for mode control that rely on spatial variation of modal gain across the area of the VCSEL aperture will be discussed, and results on high-power single-mode operations of VCSELs will be presented. Two distinct mode control methods will be reviewed. The first method of mode control uses deposited optical coatings on the top surface of the VCSEL to spatially modify the electric field standing wave in the quantum well heterostructure region. Through patterning the coating in either a pillar or anulus, specific modes can be suppressed by selectively changing the gain available for those modes. These filters have been made using both SiO2/TiO2 stacks and alternatively a single layer of silicon. The capability to pattern these layers in a wafer-scale process makes this method attractive for volume manufacturing. The second method of mode control utilizes impurity-induced layer disordering via patterned diffusion of Zn to selectively intermix Distributed Bragg Reflector (DBR) layers, thereby creating a top laser mirror whose reflectivity varies spatially across the aperture. Designing the diffused region to maximize overlap with high-order modes while minimizing overlap with the fundamental mode provides mode suppression and enhanced single-mode power. An added benefit of this approach is a reduction in resistance through the device top DBR layers brought about by the higher p-doping level in the Zn-diffused regions, minimizing Ohmic losses and heat generation. Through the use of strain control in the diffusion mask, diffusion front shape is also shown to be controllable in a way that can be used to minimize the impact of the diffused region on fundamental mode reflectivity. Tensile and compressive strain of diffusion masks are shown to have differing effects on vertical (into the device) versus lateral (across the area of the device under the mask) diffusion, allowing more precise spatial control of reflectivity. Using these methods, single-mode output powers in excess of 10 mW have been demonstrated, with side-mode suppression ratios in excess of 30 dB. These results will be discussed, as well as further work to enhance VCSEL performance. Figure 1

Plasmon-assisted low-threshold nanolasers

Low power consumption is of great importance for on-chip optical applications of nanolasers. To reduce the lasing threshold, a high quality factor and strong Purcell effect are vital, while the latter is often hindered by the above-diffraction-limit mode volume of photonic cavity, and its improvement generally requires complicated cavity design. We show that by coupling to an off-resonant plasmonic antenna, the lasing threshold of a photonic cavity can be reduced over a wide range of parameters where the plasmon-assisted coherent light-matter interaction dominates over the plasmon-induced dissipation. We develop a semiclassical model of a two-level nanolaser for such a hybrid plasmonic-photonic cavity, which is consistent with the quantum description of the spectral density of a hybrid cavity. The analytical expression of the lasing threshold is derived, which can be utilized to optimize the system parameters associated with cavities designed to obtain the minimum lasing threshold. Through comprehensive study, we find that compared to all-dielectric photonic cavity, a hybrid cavity can have prominent advantages in lasing with lower threshold, narrower emission linewidth, and sustaining the higher-power output of laser field. Our paper demonstrates the possibility of utilizing a hybrid plasmonic-photonic cavity to fabricate room-temperature low-threshold nanolasers.

Optical Transparency Induced by a Largely Purcell Enhanced Quantum Dot in a Polarization-Degenerate Cavity.

Optically active spin systems coupled to photonic cavities with high cooperativity can generate strong light-matter interactions, a key ingredient in quantum networks. However, obtaining high cooperativities for quantum information processing often involves the use of photonic crystal cavities that feature a poor optical access from the free space, especially to circularly polarized light required for the coherent control of the spin. Here, we demonstrate coupling with a cooperativity as high as 8 of an InAs/GaAs quantum dot to a fabricated bullseye cavity that provides nearly degenerate and Gaussian polarization modes for efficient optical accessing. We observe spontaneous emission lifetimes of the quantum dot as short as 80 ps (an ∼15 Purcell enhancement) and a ∼80% transparency of light reflected from the cavity. Leveraging the induced transparency for photon switching while coherently controlling the quantum dot spin could contribute to ongoing efforts of establishing quantum networks.

Tuning of silicon nitride micro-cavities by controlled nanolayer deposition

Integration of single-photon emitters (SPEs) with resonant photonic structures is a promising approach for realizing compact and efficient single-photon sources for quantum communications, computing, and sensing. Efficient interaction between the SPE and the photonic cavity requires that the cavity's resonance matches the SPE’s emission line. Here we demonstrate a new method for tuning silicon nitride (Si3N4) microring cavities via controlled deposition of the cladding layers. Guided by numerical simulations, we deposit silicon dioxide (SiO2) nanolayers onto Si3N4 ridge structures in steps of 50 nm. We show tuning of the cavity resonance exceeding a free spectral range (FSR) of 3.5 nm without degradation of the quality-factor (Q-factor) of the cavity. We then complement this method with localized laser heating for fine-tuning of the cavity. Finally, we verify that the cladding deposition does not alter the position and spectral properties of nanoparticles placed on the cavity, which suggests that our method can be useful for integrating SPEs with photonic structures.

Vibrational Polaritons in Disordered Molecular Ensembles.

Disorder is an intrinsic attribute of any realistic molecular system. It is known to lead to localization, which hampers efficient transport. It was recently proposed that in molecular ensembles strongly coupled to photonic cavities, moderate disorder leads to delocalization and increases of the transport and chemical reaction rates. Vibrational polaritons involve molecular vibrations hybridized with an infrared cavity. When the coupling strength largely exceeds the molecular inhomogeneity, polaritons are unaffected by disorder. However, in many experiments, such a homogeneous limit does not apply. We investigated vibrational polaritons involving molecular ensembles with systematically modified disorder. Counterintuitively, moderate disorder leads to an increase in Rabi splitting and the modification of the polariton bandwidths. Experimental spectroscopic data agree with a Tavis–Cummings-like model that suggests enhanced delocalization of the reservoir states occurs via the admixture of the cavity mode. Our results provide new insights into the paradigm of disorder-induced cavity-assisted delocalization in molecular polaritons.

Data-Enhanced Deep Greedy Optimization Algorithm for the On-Demand Inverse Design of TMDC-Cavity Heterojunctions.

A data-enhanced deep greedy optimization (DEDGO) algorithm is proposed to achieve the efficient and on-demand inverse design of multiple transition metal dichalcogenides (TMDC)-photonic cavity-integrated heterojunctions operating in the strong coupling regime. Precisely, five types of photonic cavities with different geometrical parameters are employed to alter the optical properties of monolayer TMDC, aiming at discovering new and intriguing physics associated with the strong coupling effect. Notably, the traditional rigorous coupled wave analysis (RCWA) approach is utilized to generate a relatively small training dataset for the DEDGO algorithm. Importantly, one remarkable feature of DEDGO is the integration the decision theory of reinforcement learning, which remedies the deficiencies of previous research that focused more on modeling over decision making, increasing the success rate of inverse prediction. Specifically, an iterative optimization strategy, namely, deep greedy optimization, is implemented to improve the performance. In addition, a data enhancement method is also employed in DEDGO to address the dependence on a large amount of training data. The accuracy and effectiveness of the DEDGO algorithm are confirmed to be much higher than those of the random forest algorithm and deep neural network, making possible the replacement of the time-consuming conventional scanning optimization method with the DEDGO algorithm. This research thoroughly describes the universality, interpretability, and excellent performance of the DEDGO algorithm in exploring the underlying physics of TMDC-cavity heterojunctions, laying the foundations for the on-demand inverse design of low-dimensional material-based nano-devices.

Cooper pair splitter in a photonic cavity: Detection of Andreev scatterings

We simulated the radiative response of the cavity quantum electrodynamics (QED) coupled to the double quantum dot Cooper pair splitter and analyzed its spectral dependence to get insight into dynamics of the Cooper pair transfers. The model is confined to the energy subspace where two entangled electrons are transferred to two normal electrodes through the interdot singlet state on two proximitized quantum dots. Our research is focused on the Andreev scatterings in the subgap regime, for which the local charge susceptibility $\mathrm{\ensuremath{\Pi}}({\ensuremath{\omega}}_{p})$ is derived, by means of Keldysh Green functions, in a whole bias voltage range. In particular, in the large voltage limit, the spectrum of $\mathrm{\ensuremath{\Pi}}({\ensuremath{\omega}}_{p})$ is expressed by a simple analytical formula, which shows various dissipation processes related with photon-induced transitions between the Andreev bound states.

Nanostructured GaAs /( Al,Ga ) As Waveguide for Low-Density Polariton Condensation from a Bound State in the Continuum

Exciton-polaritons are hybrid light-matter states that arise from strong coupling between an exciton resonance and a photonic cavity mode. As bosonic excitations, they can undergo a phase transition to a condensed state that can emit coherent light without a population inversion. This aspect makes them good candidates for thresholdless lasers, yet short exciton-polariton lifetime has made it difficult to achieve condensation at very low power densities. In this sense, long-lived symmetry-protected states are excellent candidates to overcome the limitations that arise from the finite mirror reflectivity of monolithic microcavities. In this work we use a photonic symmetry-protected bound state in the continuum coupled to an excitonic resonance to achieve state-of-the-art polariton condensation threshold in a $\mathrm{Ga}\mathrm{As}$/($\mathrm{Al},\mathrm{Ga}$)$\mathrm{As}$ waveguide. Most important, we show the influence of fabrication control and how surface passivation via atomic layer deposition provides a way to reduce exciton quenching at the grating sidewalls.

Progress and prospects of III-nitride optoelectronic devices adopting lift-off processes

Lift-off processes have been developed as the enabling technology to free the epitaxial III-nitride thin film from a conventional growth substrate such as sapphire and silicon in order to realize a variety of novel device designs and structures not otherwise possible. An epitaxial lift-off (ELO) process can be adopted to transfer the entire film to an arbitrary foreign substrate to achieve various functions, including enhancement of device performance, improvement of thermal management, and to enable flexibility among others. On the other hand, partial ELO techniques, whereby only a portion of the thin-film is detached from the substrate, can be employed to realize unconventional device structures or geometries, such as apertured, pivoted, and flexible devices, which may be exploited for various photonic structures or optical cavities. This paper reviews the development of different lift-off strategies and processes for III-nitride materials and devices, followed by a perspective on the future directions of this technology.

Temperature-induced stochastic resonance in Kerr photonic cavities for frequency shift

Driven nonlinear photonic cavities are widely studied because they exhibit many interesting effects, such as nonreciprocity, thermal effects, and frequency conversion. Specifically, adding noise to a modulated nonlinear system can lead to stochastic resonance (SR), which corresponds to periodic transitions between stable states. In this work, we study the outgoing power and spectra from a nonlinear-driven photonic cavity coupled to an external port. Using a Langevin framework, we show that the system temperature induces SR in the bistable regime, which we study in detail to exploit for enhanced frequency shift. In this way, the thermal fluctuations of the system itself can function as a driver for effective sideband generation, enabling shift efficiencies of up to 40%. We extensively explore various regimes in order to understand and maximize the process.

Unveiling atom-photon quasi-bound states in hybrid plasmonic-photonic cavity

Abstract Dissipation, often associated with plasmons, leads to decoherence and is generally considered fatal for quantum nonlinearities and entanglement. Counterintuitively, by introducing a dissipative plasmonic nanoantenna into a typical cavity quantum electrodynamics (QED) system, we unveil the wide existence of the atom-photon quasi-bound state (qBS), a kind of exotic eigenstate with anomalously small decay, in the hybrid plasmonic-photonic cavity. To derive the analytical condition of atom-photon qBS, we formulate a quantized two-mode model of the local density of states by connecting the interacting uncoupled cavity modes to the macroscopic QED. With resonant plasmon-photon coupling, we showcase the single-atom qBS that improves the efficiency of single-photon generation over one order of magnitude; and the two-atom qBS that significantly enhances spontaneous entanglement generation compared with a bare photonic cavity. Notably, such single-atom and multi-atom qBS can be simultaneously accessed in realistic plasmonic-photonic cavities, providing a versatile platform for advanced quantum technologies, such as quantum light sources, quantum computation, and quantum information.

Fingerprints of Qubit Noise in Transient Cavity Transmission.

Noise affects the coherence of qubits and thereby places a bound on the performance of quantum computers. We theoretically study a generic two-level system with fluctuating control parameters in a photonic cavity and find that basic features of the noise spectral density are imprinted in the transient transmission through the cavity. We obtain analytical expressions for generic noise and proceed to study the cases of quasistatic, white and 1/f^{α} noise in more detail. Additionally, we propose a way of extracting the noise power spectral density in a frequency band only bounded by the range of the qubit-cavity detuning and with an exponentially decaying error due to finite measurement times. Our results suggest that measurements of the time-dependent transmission probability represent a novel way of extracting noise characteristics.

Multistable excitonic Stark effect

The optical Stark effect is a tell-tale signature of coherent light--matter interaction in excitonic systems, wherein an irradiating light beam tunes exciton transition frequencies. Here we show that, when excitons are placed in a nanophotonic cavity, the excitonic Stark effect can become highly nonlinear, exhibiting multivalued and hysteretic Stark shifts that depend on the history of the irradiating light. This multistable Stark effect (MSE) arises from feedback between the cavity mode occupation and excitonic population mediated by the Stark-induced mutual tuning of the cavity and excitonic resonances. Strikingly, the MSE manifests even for very dilute exciton concentrations and can yield discontinuous Stark shift jumps of order meV. We expect that the MSE can be realized in readily available transition metal dichalcogenide excitonic systems placed in planar photonic cavities at modest pump intensities. This phenomenon can provide new means to engineer coupled states of light and matter that can persist even in the single exciton limit.

Photonic cavity mode tuning in porous silicon-based microcavities by He+ and H+ ion irradiation

The present investigation reports the optical characteristics of the porous Si (PSi) based microcavities before and after energetic He+ and H+ ion irradiations. These PSi microcavities were fabricated by the galvanostatic electrochemical etching process and irradiated with 35 keV He+ and H+ ions with three different ion fluences: 1 × 1015, 5 × 1015, and 1 × 1016 ions/cm2. Significant color contrast is evident in the reflection images after ion irradiation. These reflection spectra of the optical microcavities are systematically investigated before and after ion irradiations. The dominant resonant cavity peak of the microcavity shows a notable shift of ∼28–48 and ∼17–26 nm toward the higher wavelength region with He+ and H+ ion irradiations at various ion fluences, respectively. The relative changes in the cavity wavelengths are about ∼5%–10% and ∼3%–5% for He+ and H+ ion irradiations, respectively. The redshift in the reflectance spectra is attributed to modification in the refractive index of microcavities induced by He+ and H+ ions. These experimental results compare well with the ion propagation and transfer matrix method simulations. The observed changes in the optical properties arise due to surface modification of the Si–Si and Si–O bonds and thereby refractive index modification of individual PSi layers of the microcavity. This study establishes that low-energetic ions produce broadly optically tunable and photonic structures suitable for optoelectronic applications.

Dynamically actuated soft heliconical architecture via frequency of electric fields

Dynamic electric field frequency actuated helical and spiral structures enable a plethora of attributes for advanced photonics and engineering in the contemporary era. Nevertheless, leveraging the frequency responsiveness of adaptive devices and systems within a broad dynamic range and maintaining restrained high-frequency induced heating remain challenging. Herein, we establish a frequency-actuated heliconical soft architecture that is quite distinct from that of common frequency-responsive soft materials. We achieve reversible modulation of the photonic bandgap in a wide spectral range by delicately coupling the frequency-dependent thermal effect, field-induced dielectric torque and elastic equilibrium. Furthermore, an information encoder prototype without the aid of complicated algorithm design is established to analogize an information encoding and decoding process with a more convenient and less costly way. A technique for taming and tailoring the distribution of the pitch length is exploited and embodied in a prototype of a spatially controlled soft photonic cavity and laser emission. This work demonstrates a distinct frequency responsiveness in a heliconical soft system, which may not merely inspire the interest in field-assisted bottom-up molecular engineering of soft matter but also facilitate the practicality of adaptive photonics.

Plasmonic-photonic hybrid configuration on optical fiber tip: Toward low-cost and miniaturized biosensing probe

Owing to the label-free and real-time biosensing advantages, the nanostructured plasmonic biosensors on optical fiber tips have attracted increasing attention as potential plug-and-play remote sensing probes available for limited environments. Various nanostructures have been developed by either top-down or bottom-up approaches on optical fiber tips. However, a trade-off between the sensing performance and manufacturing cost of these plasmonic fiber-optic sensors usually limited their further application. In this context, we propose and systematically investigate a novel plasmonic-photonic hybrid configuration, which dramatically boosts the sensing performance and simplifies the fabrication process. The hybrid configuration integrated onto the fiber tip consists of a dielectric coating acting as the photonic cavity and dispersed Au nanoparticles serving as the plasmonic nanostructures. As a result of the coupling between localized surface plasmon resonance and the cavity interference mode, a hybrid plasmonic-photonic resonance is successfully excited and proved to have serval unique features including enhanced sensitivity, high stability, and precise tunability of the resonance frequencies. Finally, as a proof-of-concept for biosensing, we experimentally demonstrate that the proposed fiber-optic sensors can achieve real-time monitoring of the biomolecular binding interactions at nanomolar concentration, confirming the potential of the plasmonic-photonic configuration in developing miniaturized high-performance sensing probes.