Record Purcell factors in ultracompact hybrid plasmonic ring resonators

Emission enhancement of erbium in a reverse nanofocusing waveguide

Since Purcell's seminal report 75 years ago, electromagnetic resonators have been used to control light-matter interactions to make brighter radiation sources and unleash unprecedented control over quantum states of light and matter. Indeed, optical resonators such as microcavities and plasmonic antennas offer excellent control but only over a limited spectral range. Strategies to mutually tune and match emission and resonator frequency are often required, which is intricate and precludes the possibility of enhancing multiple transitions simultaneously. In this letter, we report a strong radiative emission rate enhancement of Er³⁺-ions across the telecommunications C-band in a single plasmonic waveguide based on the Purcell effect. Our gap waveguide uses a reverse nanofocusing approach to efficiently enhance, extract and guide emission from the nanoscale to a photonic waveguide while keeping plasmonic losses at a minimum. Remarkably, the large and broadband Purcell enhancement allows us to resolve Stark-split electric dipole transitions, which are typically only observed under cryogenic conditions. Simultaneous radiative emission enhancement of multiple quantum states is of great interest for photonic quantum networks and on-chip data communications.

Hyperbolic whispering-gallery phonon polaritons in boron nitride nanotubes

Light confinement in nanostructures produces an enhanced light-matter interaction that enables a vast range of applications including single-photon sources, nanolasers and nanosensors. In particular, nanocavity-confined polaritons display a strongly enhanced light-matter interaction in the infrared regime. This interaction could be further boosted if polaritonic modes were moulded to form whispering-gallery modes; but scattering losses within nanocavities have so far prevented their observation. Here, we show that hexagonal BN nanotubes act as an atomically smooth nanocavity that can sustain phonon-polariton whispering-gallery modes, owing to their intrinsic hyperbolic dispersion and low scattering losses. Hyperbolic whispering-gallery phonon polaritons on BN nanotubes of ～4 nm radius (sidewall of six atomic layers) are characterized by an ultrasmall nanocavity mode volume (V_m ≈ 10^-10λ₀³ at an optical wavelength λ₀ ≈ 6.4 μm) and a Purcell factor (Q/V_m) as high as 10¹². We posit that BN nanotubes could become an important material platform for the realization of one-dimensional, ultrastrong light-matter interactions, with exciting implications for compact photonic devices.

Thermal tunable silicon valley photonic crystal ring resonators at the telecommunication wavelength

Tunable ring resonators are essential devices in integrated circuits. Compared to conventional ring resonators, valley photonic crystal (VPC) ring resonators have a compact design and high quality factor (Q-factor), attracting broad attention. However, tunable VPC ring resonators haven't been demonstrated. Here we theoretically demonstrate the first tunable VPC ring resonator in the telecommunication wavelength region, the resonance peaks of which are tuned by controlling the temperature based on the thermal-optic effect of silicon. The design is ultracompact (12.05 µm by 10.44 µm), with a high Q-factor of 1281.00. By tuning the temperature from 100 K to 750 K, the phase modulation can reach 7.70 π, and the adjustment efficiency is 0.062 nm/K. Since thermal tuning has been broadly applied in silicon photonics, our design can be readily applied in integrated photonic circuits and will find broad applications. Furthermore, our work opens new possibilities and deepens the understanding of designing novel tunable VPC photonic devices.

Design and simulation of a plasmonic density nanosensor for polarizable gases

In this paper, an optical method of measuring the mass density of polarizable gases is proposed using a plasmonic refractive index nano-sensor. Plasmonic sensors can detect very small changes in the refracting index of arbitrary dielectric materials. However, attributing them to a specific application needs more elaboration of the material's refractive index unit's (RIU) relation with the introduced application. In a gaseous medium, the optical properties of molecules are related to their dipole moment polarizability. Hence, the theoretical index-density relation of Lorentz-Lorenz is applied in the proposed sensing mechanism to interpret changes in the gas' refractive index and to changes in its density. The proposed plasmonic mass density sensor shows a sensitivity of 348.8nm/(gr/cm³) for methane gas in the visible light region. This sensor can be integrated with photonic circuits for gas sensing purposes.

Nanostructured hybrid plasmonic waveguide in a slot structure for high-performance light transmission

Squeezing light to nanoscale is the most vital capacity of nanophotonic circuits processing on-chip optical signals that allows to significantly enhance light-matter interaction by stimulating various nonlinear optical effects. It is well known that plasmon can offer an unrivaled concentration of optical energy beyond the optical diffraction limit. However, the progress of plasmonic technology is mainly hindered by its ohmic losses, thus leading to the difficulty in building large-area photonic integrated circuits. To significantly increase the propagation distance of light, we develop a new waveguide structure operating at the telecommunication wavelength of 1,550 nm. It consists of a nanostructured hybrid plasmonic waveguide embedded in a high-index-contrast slot waveguide. We capitalize on the strong mode confinement of the slot waveguide and reduce mode areas with the nanostructured hybrid plasmonic configuration while maintaining extremely low ohmic losses using a nanoscale metal strip. The proposed design achieves a record propagation distance of 1,115 µm while comparing with that of other designs at a mode area of the order of 10^-5A₀ (A₀ is the diffraction-limited area). The mode characterization considering fabrication imperfections and spectral responses show the robustness and broadband operation range of the proposed waveguide. Moreover, we also investigated the crosstalk to assess the density of integration. The proposed design paves the way for building nanophotonic circuits and optoelectronic devices that require strong light-matter interaction.

Strong Purcell effect in deep subwavelength coaxial cavity with GeSn active medium

We propose a deep subwavelength plasmonic cavity based on a metal-coated coaxial structure with Ge_0.9Sn_0.1 as the active medium. A fundamental surface plasmon polariton mode is strongly confined on the sidewall of the metal core, with the quality factor up to 5×10³ at 10 K. By reducing the cavity dimension to a few nanometers, this cavity mode shows a strong plasmon binding with the mode volume down to 8×10^-10 (λ/n)³, and significant size-dependent damping caused by the non-local optical response. The Purcell factor is achieved as high as 2×10⁹ at 10 K and 7×10⁸ at 300 K. This cavity design provides a systematic guideline of scaling down the cavity size and enhancing the Purcell factor. Our theoretical demonstration and understanding of the subwavelength plasmonic cavity represent a significant step toward the large-scale integration of on-chip lasers with a low threshold.

LiNbO3 waveguide with embedded Ag nanowire and 3L MoS2 for strong light confinement and ultra-long propagation length in the visible spectral range

A vertical slot LiNbO₃ waveguide with an Ag nanowire and 3L MoS₂ embedded in the low-refractive index slot region is proposed for the purpose of improving light confinement. We find that the proposed waveguide has a novel dielectric based plasmonic mode, where local light field is enhanced by the Ag nanowire. The mode exhibits an extremely large figure of merit (FoM) of 6.5×10⁶, one order of magnitude larger than that the largest FoM of any plasmonic waveguide reported in the literature to date. The waveguide also has an extremely long propagation length of 84 cm in the visible wavelength at 680 nm. Furthermore, the waveguide has a low sub-micro bending loss and can be directly connected to all-dielectric waveguides with an extremely low coupling loss. The proposed vertical slot LiNbO₃ waveguide is a promising candidate for the realization of ultrahigh integration density tunable circuits in the visible spectral range.

Supermode Hybridization: A Material-Independent Route toward Record Schottky Detection Sensitivity Using <0.05 μm3 Amorphous Absorber Volume

Schottky photodetectors are attractive for CMOS-compatible photonic integrated circuits, but the inability to simultaneously optimize the metal emitter thickness for photon absorption and hot carrier emission limits the detection efficiency and sensitivity. Here, we propose and experimentally demonstrate a supermode hybridization waveguiding effect that can overcome the trade-off. By introducing structural asymmetry into coupled plasmonic nanostructures, hybridization-induced field enhancement can help ultrathin metal emitters to achieve greater optical absorption than bulk counterparts. Despite the use of amorphous materials with higher transport losses, our hybridized Schottky detectors demonstrate higher responsivity per device volume compared to crystalline-based and unhybridized Schottky designs with broadband (1.5-1.6 μm) and athermal (15-100 °C) behavior as well as record sensitivity of -55 dBm that approaches Ge counterparts that are 36 times larger. The hybridization effect can be utilized across diverse nanomaterial platforms to facilitate light-matter interaction, paving the way toward backend-compatible, chip-integrated photonics with greater manufacturing flexibility.

Role of the Exciton-Polariton in a Continuous-Wave Optically Pumped CsPbBr3 Perovskite Laser

Lead halide perovskites have emerged as excellent optical gain materials for solution-processable and flexible lasers. Recently, continuous-wave (CW) optically driven lasing was established in perovskite crystals; however, the mechanism of low-threshold operation is still disputed. In this study, CW-pumped lasing from one-dimensional CsPbBr₃ nanoribbons (NBs) with a threshold of ∼130 W cm^-2 is demonstrated, which can be ascribed to the large refractive index induced by the exciton-polariton (EP) effect. Increasing the temperature reduces the exciton fraction of EPs, which decreases the group and phase refractive indices and inhibits lasing above 100 K. Thermal management, including reducing the NB height to ∼120 ± 60 nm and adopting a high-thermal-conductivity sink, e.g., sapphire, is critical for CW-driven lasing, even at cryogenic temperatures. These results reveal the nature of ultralow-threshold lasing with CsPbBr₃ and provide insights into the construction of room-temperature CW and electrically driven perovskite macro/microlasers.

Monolithic Plasmonic Waveguide Architecture for Passive and Active Optical Circuits

Guided-wave plasmonic circuits are promising platforms for sensing, interconnection, and quantum applications in the subdiffraction regime. Nonetheless, the loss-confinement trade-off remains a collective bottleneck for plasmonic-enhanced optical processes. Here, we report a unique plasmonic waveguide architecture that can alleviate such trade-off and improve the efficiencies of plasmonic-based emission, light-matter-interaction, and detection simultaneously. Specifically, record experimental attributes such as normalized Purcell factor approaching 10⁴, 10 dB amplitude modulation with <1 dB insertion loss and fJ-level switching energy, and photodetection sensitivity and internal quantum efficiency of -54 dBm and 6.4% respectively have been realized within our amorphous-based, coupled-mode plasmonic structure. The ability to support multiple optoelectronic phenomena while providing performance gains over existing plasmonic and dielectric counterparts offers a clear path toward reconfigurable, monolithic plasmonic circuits.

Vertically Coupled Plasmonic Racetrack Ring Resonator for Biosensor Applications

Plasmonic chemical and biological sensors offer significant advantages such as really compact sizes and extremely high sensitivity. Biosensors based on plasmonic waveguides and resonators are some of the most attractive candidates for mobile and wearable devices. However, high losses in the metal and complicated schemes for practical implementation make it challenging to find the optimal configuration of a compact plasmon biosensor. Here, we propose a novel plasmonic refractive index sensor based on a metal strip waveguide placed under a waveguide-based racetrack ring resonator made of the same metal. This scheme guarantees effective coupling between the waveguide and resonator and low loss light transmittance through the long-range waveguide. The proposed device can be easily fabricated (e.g., using optical lithography) and integrated with materials like graphene oxide for providing adsorption of the biomolecules on the sensitive part of the optical elements. To analyze the properties of the designed sensing system, we performed numerical simulations along with some analytical estimations. There is one other interesting general feature of this sensing scheme that is worth pointing out before looking at its details. The sensitivity of the considered device can be significantly increased by surrounding the resonator with media of slightly different refractive indices, which allows sensitivity to reach a value of more than 1 μm per refractive index unit.