Ultrasmall Mode Volume Research Articles

Feasibility of efficient room-temperature solid-state sources of indistinguishable single photons using ultrasmall mode volume cavities

Highly efficient sources of indistinguishable single photons that can operate at room temperature would be very beneficial for many applications in quantum technology. We show that the implementation of such sources is a realistic goal using solid-state emitters and ultrasmall mode volume cavities. We derive and analyze an expression for photon indistinguishability that accounts for relevant detrimental effects, such as plasmon-induced quenching and pure-dephasing. We then provide the general cavity and emitter conditions required to achieve efficient indistinguishable photon emission, and also discuss constraints due to phonon sideband emission. Using these conditions, we propose that a nanodiamond negatively charged silicon-vacancy center combined with a plasmonic-Fabry-Perot hybrid cavity is an excellent candidate system.

A Slotted Photonic Crystal Nanobeam Cavity for Simultaneous Attainment of Ultra-High Q-Factor and Sensitivity

An optical refractive index (RI) sensor is proposed based on a slotted photonic crystal nanobeam cavity. Three rows of parabolic-tapered air-holes are used to design the cavity and an embedded narrow slot is used to obtain a highly localized optical field in a low refractive index region in order to produce a strong light matter interaction. The number and the radii of the air holes, the width of the slot, and the number of rows are optimized to address the most challenging issue of RI sensor design, i.e. to obtain a high Q-factor and sensitivity, simultaneously. The plane wave expansion method has been used to facilitate the design procedure and a 3-D finite difference time domain method has been used to obtain the simulation outcome. The outcomes for the optimized cavity show an ultra-high Q-factor $\sim 4.3\times 10^{7}$ along with a sensitivity ~975 nm/RIU at a wavelength 1534 nm, for which the figure-of-merit is calculated as $2.73\times 10^{7}$ . However, the slot-width can be varied to obtain a Q-factor of $8\times 10^{7}$ and sensitivity of 1100 nm/RIU individually. The device also has an ultra-small mode volume of the order of 0.0253 $(\lambda /n)^{3}$ . Hence, all together, the sensor is expected to be a promising candidate for the lab-on-chip sensor application.

Hybrid photonic–plasmonic crystal nanocavity sensors

We have investigated a hybrid photonic–plasmonic crystal nanocavity consisting of a silicon grating nanowire adjacent to a metal surface with a gain gap between them. The hybrid plasmonic cavity modes are highly confined in the gap due to the strong coupling of the photonic crystal cavity modes and the surface plasmonic gap modes. Using finite-element method (FEM), guided modes of the hybrid plasmonic waveguide (WG) were numerically determined at a wavelength of 1550 nm. The modal characteristics such as WG confinement factors and modal losses of the fundamental hybrid plasmonic modes were obtained as a function of groove depth at various gap heights. Furthermore, the band structure of the hybrid crystal modes corresponding to a wide band gap of 17.8 THz is revealed. To enclose the optical energy effectively, a single defect was introduced into the hybrid crystal. At a deep subwavelength defect length as small as 270 nm, the resonant mode exhibits a high quality factor of 567 and an ultrasmall mode volume of 1.9 × 10− 3 (λ/neff)3 at the resonance wavelength of 1550 nm. Compared to conventional photonic crystal nanowire cavities in the absence of a metal surface, the factor Q/Vm is significantly enhanced by about 15 times. The designed hybrid photonic–plasmonic cavity sensors exhibit distinguished characteristics such as sensitivity of 443 nm/RIU and figure of merit of 129. The proposed nanocavities open new possibilities for various applications with strong light–matter interaction, such as biosensors and nanolasers.

Improved hybrid plasmonic microcavity laser

In this paper, an improved hybrid surface plasmon nanolaser with a gain medium ridge and a layer of air gap is proposed. In order to achieve low propagation loss and sub-wavelength field confinement, a triangular air gap and a 50 nm microcavity end face silver mirror are adopted in this structure, and the combination of this particular triangular structure and silver mirror effectively improves the performance of nano-laser. In this paper, we numerically simulate the waveguide by using the finite-element method. The COMSOL multiphysics software is a superior numerical simulation software to simulate the real physical phenomena based on the finite element method. On the basic of the COMSOL multiphysics software, a two-dimensional cross-section model and a three-dimensional model are built, the transmission performance and microcavity performance of the improved structure are analyzed in detail at a working wavelength of 1550 nm. Some quantities including the electric field distribution, transmission length, normalized mode field area, average energy density, foundation modal volume, quality factor of the structure, threshold gain, quality factor, effective modal volume, and Purcell factor are considered here which are dependent on the dielectric constant and geometrical parameters. The results indicate that on a two-dimensional scale, the contradiction between transmission loss and transmission distance can be effectively solved by the guidance of Fom value, and the IHPM laser structure with optimal transmission characteristics is obtained under the guidance of quality factor and foundation modal volume. A deep sub-wavelength constraint on light is achieved:the propagation length of the electromagnetic mode reaches a millimeter level and the longest distance can reach 1.29 mm. When testing the microcavity performance of the laser separately on a two-dimensional scale and three-dimensional scale, the high quality factor, low gain threshold, ultra-small effective mode volume of 0.001092 μm3 and ultra-high Purcell factor of 8.29×105 are obtained by adjusting the structural parameters and plating a 50 nm-thick silver layer on the end face of the laser microcavity. Compared with the previous structure without air gaps, the designed structure has a low laser lasing threshold and strong micro-cavity local capability when these two structural parameters are unified. The designed hybrid surface plasmon nanolaser may serve as a fundamental building block for various functional photonic components and can have applications such as in sensing, nanofocusing, and nanolasing.

Graphene-supported plasmonic whispering-gallery mode in a metal-coated microcavity for sensing application with ultrahigh sensitivity

We propose and numerically investigate the plasmonic whispering-gallery mode (WGM) with high-quality (Q) factor (as high as 285) and ultra-small mode volume (as low as 0.04μm3) in the hybrid plasmonic microcavity with a sandwiched and electrically controlled graphene. The theoretical results present that the resonant wavelength of hybrid plasmonic microcavity dramatically changes and the corresponding intrinsic loss exhibits a distinct peak by electrically adjusting the permittivity of graphene around the epsilon-near-zero (ENZ) point. The influence of graphene with different layers on the characteristic of hybrid plasmonic microcavity is also analyzed. As a potential application, the plasmonic WGM microcavity with the sandwiched graphene could be applied for a refractometer with the sensitivity of higher than 1000 nm per refraction index unit (nm/RIU), and large figure of merit. The sensitivity can also be tuned by the electrically controlled graphene.

Ultrasmall Mode Volumes of Multilayered Hyperbolic Metamaterial Nanocavities in the Visible Range

Designing optical nanocavities with high quality factors (Q) or small mode volume (Vm) has important applications in the field of quantum information due to the strong light-matter coupling phenomena. The multilayered hyperbolic metamaterial (HMM) nanocavities have been designed and numerically investigated to achieve the small mode volume. Unique dispersion relations of our HMM nanocavity enable propagation of large wave vectors result in strong wave confinement. The excitation of coupled localized surface plasmons, which have achieved ultrasmall mode volume below 10 –6 λ3 or high Q/Vm around 107 λ–3 causing better electric fields confinement. These properties can be demonstrated by rationally the geometrical parameters and may have potential applications in light-matter interaction.

Ultrahigh Purcell Factor, Improved Sensitivity, and Enhanced Optical Force in Dielectric Bowtie Whispering-Gallery-Mode Resonators

We propose and theoretically investigate an all-dielectric bowtie whispering-gallery-mode (WGM) resonator which consists of two tip-to-tip coupled semiconductor nanorings with triangular cross section separated by low refractive index material gap. Mode splitting of symmetric WGM and antisymmetric WGM is observed and analyzed. Due to extremely field enhancements by the “slot” and “tip” effects, strong localization of light in the gap of symmetric WGMs leads to ultrasmall modal volume of $0.3\,\mu {\rm{m}}^{3}$ . This value is two orders of magnitude smaller than that in toroidal microresonator which is the typical WGM microcavity with tight mode confinement. Importantly, large amount of light confined in the gap in bowtie WGM resonator not only suppresses the radiation loss (radiation-loss-related quality factor is above $10^{8}$ ), but also makes it as an ideal platform for Purcell effect enhancement, ultrasensitive sensing, and optical trapping of nanoparticles. Calculation results show that Purcell factor can reach to $10^{8}$ at room temperature when assuming quantum dots or atoms placed in the gap. Refractive index sensitivity is improved to 700 nm/RIU as compared with conventional slot waveguide with rectangular cross section. The optical gradient force is greatly enhanced and allows efficient trapping of single nanoscale particle with diameter of 5 nm even at a relatively large gap of 100 nm.

Ultra-compact tunable silicon nanobeam cavity with an energy-efficient graphene micro-heater.

We propose and experimentally demonstrate an ultra-compact silicon photonic crystal nanobeam (PCN) cavity with an energy-efficient graphene micro-heater. Owing to the PCN cavity with an ultra-small optical mode volume of 0.145 µm3, the light-matter interaction is greatly enhanced and the thermo-optic (TO) tuning efficiency is increased. The TO tuning efficiency is measured to be as high as 1.5 nm/mW, which can be further increased to 3.75 nm/mW based on numerical simulations with an optimized structure. The time constants with a rise time constant of τrise = 1.11 μs and a fall time constant of τfall = 1.47 μs are obtained in the experiment.

Simultaneously high-Q and high-sensitivity slotted photonic crystal nanofiber cavity for complex refractive index sensing

We proposed and theoretically investigated a one-dimensional slotted photonic crystal nanofiber cavity (SPCNC) for complex refractive index sensing in a gaseous environment. Three-dimensional finite-difference time-domain simulations show that a high quality factor (Q) of 1.45×106 and a high sensitivity of 756 nm/refractive index unit (RIU) can be simultaneously achieved for the detection of the real part of the refractive index (RI) of the surrounding gas. In addition, we demonstrated that the SPCNC possesses the capability of detecting the change of the imaginary part of the RI. A high sensitivity of 1525 nm/RIU is obtained, and Q is found to decrease with increasing absorption strength but still remains as high as 1.1×104 for strong absorption. We believe our proposed SPCNC with excellent sensing performances, ultra-small mode volume, and compact footprint may offer the potential to develop multiplexed sensing techniques for applications in multielement mixture detection.

Surface plasmon polariton nanocavity with ultrasmall mode volume

We present a plasmonic nanocavity structure, consisting of a gallium phosphide (GaP) cylinder penetrating into a rectangular silver plate, and study its properties using a finite element method (FEM). An ultrasmall mode volume of is achieved, which is more than 200 times smaller than the previous ultrasmall mode volume plasmonic nanodisk resonators. Meanwhile, the quality factor of the plasmonic nanocavity is about 38.2 and is over two times greater than the ultrasmall mode volume plasmonic nanodisk resonators. Compared to the aforementioned plasmonic nanodisk resonators, a more than one-order of magnitude larger Purcell factor of is achieved. We determined the resonant modes of our plasmonic nanocavity are dipolar plasmon modes by analyzing the electric field properties. In addition, we investigate the dependence of the optical properties on the refractive index of the cavity material and discuss the effect of including the silica (SiO2) substrate. Our work provides an alternative approach to achieve ultrasmall plasmonic nanocavity of interest in applications to many areas of research, including device physics, nonlinear optics and quantum optics.

Strong Light-Matter Interactions in Single Open Plasmonic Nanocavities at the Quantum Optics Limit.

Reaching the quantum optics limit of strong light-matter interactions between a single exciton and a plasmon mode is highly desirable, because it opens up possibilities to explore room-temperature quantum devices operating at the single-photon level. However, two challenges severely hinder the realization of this limit: the integration of single-exciton emitters with plasmonic nanostructures and making the coupling strength at the single-exciton level overcome the large damping of the plasmon mode. Here, we demonstrate that these two hindrances can be overcome by attaching individual J aggregates to single cuboid Au@Ag nanorods. In such hybrid nanosystems, both the ultrasmall mode volume of ∼71 nm^{3} and the ultrashort interaction distance of less than 0.9nm make the coupling coefficient between a single J-aggregate exciton and the cuboid nanorod as high as ∼41.6 meV, enabling strong light-matter interactions to be achieved at the quantum optics limit in single open plasmonic nanocavities.

Self-Similar Nanocavity Design with Ultrasmall Mode Volume for Single-Photon Nonlinearities.

We propose a photonic crystal nanocavity design with self-similar electromagnetic boundary conditions, achieving ultrasmall mode volume (V_{eff}). The electric energy density of a cavity mode can be maximized in the air or dielectric region, depending on the choice of boundary conditions. We illustrate the design concept with a silicon-air one-dimensional photon crystal cavity that reaches an ultrasmall mode volume of V_{eff}∼7.01×10^{-5}λ^{3} at λ∼1550 nm. We show that the extreme light concentration in our design can enable ultrastrong Kerr nonlinearities, even at the single-photon level. These features open new directions in cavity quantum electrodynamics, spectroscopy, and quantum nonlinear optics.

(Invited) Optical Signatures of Carbon Nanotube Excitons Coupled to Plasmonic Nanocavities and Metallo-Dielectric Antennas

Single-walled carbon nanotubes (SWCNTs) have recently gained tremendous interest as a nanomaterial for next generation optoelectronics and quantum photonic devices, in particular through the demonstration of quantum light emission at room temperature in doped SWCNTs [1]. For further advances and to enable practical applications in quantum information processing light enhancement strategies are required. To this end plasmonic nanocavities such as bowtie antennas feature nanometer gaps that can support ultra-small mode volumes which are particularly attractive to enhance the exciton emission of single-walled carbon nanotubes (SWCNTs). In this talk we will discuss several technique to achieve the required spectral, spatial, and orientational resonance between SWCNT excitons and nanoplasmonic modes. Both time integrated and time-resolved measurements reveal characteristic signatures demonstrating pronounced coupling in the Purcell regime. As a test system we employ co-polymer wrapped carbon nanotubes for which we have recently shown that the polymer backbone can lead to efficient acoustic-phonon localization implying long exciton coherence times and ultra-narrow linewidth [2]. In addition, we will discuss SWCNT excitons coupled to metallo-dielectric antennas and evaluate the light collection efficiency enhancements achievable with this approach. Finally, we will discuss the prospects of achieving transform limited single photons in these systems. [1] X. Ma, N.F. Hartmann, J.K.S. Baldwin, S.K. Doorn, and H. Htoon, “Room-temperature single-photon generation from solitary dopants of carbon nanotubes”, Nature Nanotech 10, 671–675 (2015). [2] I. Sarpkaya, Ehsaneh D. Ahmadi, Gabriella D. Shepard, Kevin S. Mistry, Jeffrey L. Blackburn, and S. Strauf, “Strong Acoustic Phonon Localization in Copolymer-Wrapped Carbon Nanotubes”, ACS Nano 9, 6383 (2015).

Sensing of microparticles based on a broadband ultrasmall microcavity in a freely suspended microfiber

We theoretically design and experimentally realize a broadband ultrasmall microcavity for sensing a varying number of microparticles whose diameter is 2 μm in a freely suspended microfiber. The performance of the microcavity is predicted by the theory of one-dimensional photonic crystals and verified by the numerical simulation of finite-difference time domain and the experimental characterization of reflection and transmission spectra. A penetrating length into the reflectors as small as about four periods is demonstrated in the numerical simulation, giving rise to an ultrasmall effective mode volume that can increase the sensitivity and spatial resolution of sensing. Moreover, a reflection band as large as 150 nm from the reflectors of the microcavity has been realized in silica optical microfiber in the experiment, which highly expands the wavelength range of sensing. Our proposed microcavity integrated into a freely suspended optical fiber offers a convenient and stable method for long-distance sensing of microparticles without the need for complicated coupling systems and is free from the influence of substrates.

Metal-Substrate-Mediated Plasmon Hybridization in a Nanoparticle Dimer for Photoluminescence Line-Width Shrinking and Intensity Enhancement.

Metal-film-coupled nanoparticles with subnanometer particle-film gaps possess an ultrasmall mode volume, responsible for a variety of intriguing phenomena in plasmonic nanophotonics. Due to the large radiative loss associated with dipolar coupling, however, the plasmonic-film-coupled nanocavities usually feature a low-quality factor, setting an ultimate limit of the increased light-matter interaction strength. Here, we demonstrate a plasmonic nanocavity composed of a metal-film-coupled nanoparticle dimer, exhibiting a significantly improved quality factor. Compared to a silica-supported dimer, the spectral line width of the nanocavity plasmon resonance is reduced by a factor of ∼4.6 and is even smaller than its monomer counterpart (∼30% reduction). Comprehensive theoretical analyses reveal that this pronounced resonance narrowing effect can be attributed to intense film-mediated plasmon hybridization between the bonding dipolar and quadrupolar gap modes in the dimer. More importantly, the invoking of the dark quadrupole resonance leads to a giant photoluminescence intensity enhancement (∼200 times) and dramatic emission line-width narrowing (∼4.6 times), compared to the silica-supported dimer. The similar spectral characteristics of the measured plasmonic scattering and photoluminescence emission indicate that the radiative decay of the coupled plasmons in the nanocavity is the origin of the observed photoluminescence, consistent with a proposed phenomenological model. Numerical calculations show that the intensity enhancement is mainly contributed by the dimer-film gap rather than the interparticle gap. These findings not only shed more light on the hybridized interaction between plasmon modes but also deepen the understanding of photoluminescence emission in coupled plasmonic nanostructures.

Compact, submilliwatt, 2 × 2 silicon thermo-optic switch based on photonic crystal nanobeam cavities

We propose and experimentally demonstrate a 2×2 thermo-optic (TO) crossbar switch implemented by dual photonic crystal nanobeam (PCN) cavities within a silicon-on-insulator (SOI) platform. By thermally tuning the refractive index of silicon, the resonance wavelength of the PCN cavities can be red-shifted. With the help of the ultrasmall mode volumes of the PCN cavities, only ∼0.16 mW power is needed to change the switching state. With a spectral passband of 0.09 nm at the 1583.75 nm operation wavelength, the insertion loss (IL) and crosstalk (CT) performances were measured as IL(bar)=−0.2 dB, CT(bar)=−15 dB, IL(cross)=−1.5 dB, and CT(cross)=−15 dB. Furthermore, the thermal tuning efficiency of the fabricated device is as high as 1.23 nm/mW.

Raman Spaser in a Plasmonic Nanoantenna Embedded with Raman-Active Nanoparticle

We report on a novel type of spaser which is based on the nonlinear optical Raman gain rather than stimulated emission. It consists of a plasmonic nanoantenna as the cavity and a highly Raman-active diamond nanoparticle as the gain medium. Ultrasmall mode volume accompanied by strong field enhancement and high Raman gain enable Raman spaser-based nanolasing. By using the model describing the oscillation of Raman mode in micro/nanocavity and the calculations for plasmonic nanocavity characteristics, we theoretically study the dependency of spasing threshold on the structural parameters. We believe that our work will trigger the extensive studies of nonlinear optical spasers.

Purcell-Enhanced Single-Photon Emission from Nitrogen-Vacancy Centers Coupled to a Tunable Microcavity

Optical microcavities are a powerful tool to enhance spontaneous emission of individual quantum emitters. However, the broad emission spectra encountered in the solid state at room temperature limit the influence of a cavity, and call for ultra-small mode volume. We demonstrate Purcell-enhanced single photon emission from nitrogen-vacancy (NV) centers in nanodiamonds coupled to a tunable fiber-based microcavity with a mode volume down to $1.0\,\lambda^{3}$. We record cavity-enhanced fluorescence images and study several single emitters with one cavity. The Purcell effect is evidenced by enhanced fluorescence collection, as well as tunable fluorescence lifetime modification, and we infer an effective Purcell factor of up to 2.0. With numerical simulations, we furthermore show that a novel regime for light confinement can be achieved, where a Fabry-Perot mode is combined with additional mode confinement by the nanocrystal itself. In this regime, effective Purcell factors of up to 11 for NV centers and 63 for silicon vacancy centers are feasible, holding promise for bright single photon sources and efficient spin readout under ambient conditions.

A broadband tapered nanocavity for efficient nonclassical light emission.

We present the design of a tapered nanocavity, obtained by sandwiching a photonic wire section between a planar gold reflector and a few-period Bragg mirror integrated into the tapered wire. Thanks to its ultrasmall mode volume (0.71 λ3/n3), this hybrid nanocavity largely enhances the spontaneous emission rate of an embedded quantum dot (Purcell factor: 6), while offering a wide operation bandwidth (full-width half-maximum: 20 nm). In addition, the top tapered section shapes the cavity far-field emission into a very directive output beam, with a Gaussian spatial profile. For realistic taper dimensions, a total outcoupling efficiency to a Gaussian beam of 0.8 is predicted. Envisioned applications include bright sources of non-classical states of light, such as widely tunable sources of indistinguishable single photons and polarization-entangled photon pairs.

Ultrasmall Mode Volumes in Plasmonic Cavities of Nanoparticle-On-Mirror Structures.

The mode volume and Purcell factor are two important parameters to assess the performance of optical nanocavities. Achieving small mode volumes and high Purcell factors for nanocavity structures while simplifying their fabrication has been a major task to realize high-performance and large-scale photonic devices and systems. Different optical resonators based on nanoparticle-on-mirror (NPoM) structures are systematically analyzed, which are easy to fabricate and flexible to use. Direct comparison of these optical resonators is made through finite-difference time-domain (FDTD) simulations. The achievement of ultrasmall mode volumes below 10-7 (λ/n)3 based on the NPoM structure through FDTD simulations is demonstrated by rationally selecting the structural parameters. Such NPoM structures provide a decent Purcell factor on the order of 107 , which can effectively enhance spontaneous emission and facilitate a number of photonic applications. The simulation results are confirmed by dark field scattering and second-harmonic generation measurements. This work is scientifically important and offers practical guidelines for the design of optical resonators for state-of-the-art optical and photonic devices.