Small Mode Volume Research Articles

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.

Ultrafast optical control using the Kerr nonlinearity in hydrogenated amorphous silicon microcylindrical resonators

Microresonators are ideal systems for probing nonlinear phenomena at low thresholds due to their small mode volumes and high quality (Q) factors. As such, they have found use both for fundamental studies of light-matter interactions as well as for applications in areas ranging from telecommunications to medicine. In particular, semiconductor-based resonators with large Kerr nonlinearities have great potential for high speed, low power all-optical processing. Here we present experiments to characterize the size of the Kerr induced resonance wavelength shifting in a hydrogenated amorphous silicon resonator and demonstrate its potential for ultrafast all-optical modulation and switching. Large wavelength shifts are observed for low pump powers due to the high nonlinearity of the amorphous silicon material and the strong mode confinement in the microcylindrical resonator. The threshold energy for switching is less than a picojoule, representing a significant step towards advantageous low power silicon-based photonic technologies.

Photonic crystal nanofishbone nanocavity

We propose a photonic crystal (PhC) nanofishbone (NFB) nanocavity, which confines an ultrahigh Q (~1.8 × 10(7)) transverse-magnetic (TM) mode. With thin slab thickness and only few PhC periods, the TM mode in NFB nanocavity shows higher Q, larger confinement factor and smaller mode volume than that in PhC nanobeam nanocavity, while the total etched-surface area is also significantly reduced. This PhC NFB nanocavity with very compact device size will be very beneficial for quantum cascade lasers, plasmonic nanolasers, and other applications needing high Q TM modes.

A Hollow‐Core Optical Cavity Built in a Three‐Layer Silicon Photonic Crystal

Hollow‐core (HC) optical microcavities differ from conventional ones in that they can strongly confine light in air. HC cavities simultaneously possessing high air–energy ratios (αair), high quality factors (Q), and small mode volumes (Veff) may enable on‐chip low‐concentration molecular sensing, high optical power delivery or storage, and low‐threshold optical oscillation or lasing when filled with materials of desirable nonlinearities. However, designing and fabricating such cavities has been challenging, especially when using only a small number of layers. Here, the design and fabrication of such an HC cavity in a three‐layer silicon photonic crystal (PhC) is reported. It supports a transverse magnetic (TM) cavity mode with αair ≈ 67% and Q ≈ 1.7 × 105 in the 1550 nm wavelength range. The in‐plane confinement is provided by the 2D photonic bandgap (PBG) of a dielectric rod array, while a pair of capping slabs with air–hole PhC patterns confine light in air in the vertical direction. The cavity allows detection of low‐concentration substances that induce refractive index changes as small as around 8.8 × 10−6. The proposed cavity design and its fabrication provide a new approach to three‐dimensional photonic confinement using a structure consisting of only a few layers.

Plasmon Hybridization and Field Confinement in Multilayer Metal–Dielectric Nanocups

Large-area arrays of dispersed multilayer gold–dielectric nanocups were fabricated by colloidal lithography and studied by extinction spectroscopy. Hybridization of the elemental plasmons of the individual nanocups gave rise to new resonance peaks in the visible and near-infrared regions of the extinction spectrum. Transmission electron microscopy was used to confirm the fabricated structure geometry, and the optical properties of the arrays were studied by UV–vis–NIR spectroscopy and finite-difference time-domain (FDTD) simulations. The nature of the resonances was elucidated from E-field plots and charge plots showing clear hybridized modes. We observe a dominant hybridized dipolar mode combining a bonding and antibonding mode at the two caps. A high-energy antibonding (antisymmetric) quadrupolar mode of an individual nanocup is revealed through hybridization with an elemental mode on the second nanocup. A low-energy tunable cavity mode with a very small mode volume is observed in the near-IR range.

Stimulated and spontaneous four-wave mixing in silicon-on-insulator coupled photonic wire nano-cavities

We report on four-wave mixing in coupled photonic crystal nano-cavities on a silicon-on-insulator platform. Three photonic wire cavities are side-coupled to obtain three modes equally separated in energy. The structure is designed to be self-filtering, and we show that the pump is rejected by almost two orders of magnitude. We study both the stimulated and the spontaneous four-wave mixing processes: owing to the small modal volume, we find that signal and idler photons are generated with a hundred-fold increase in efficiency as compared to silicon micro-ring resonators.

Mass-producible and efficient optical antennas with CMOS-fabricated nanometer-scale gap.

Optical antennas have been widely used for sensitive photodetection, efficient light emission, high resolution imaging, and biochemical sensing because of their ability to capture and focus light energy beyond the diffraction limit. However, widespread application of optical antennas has been limited due to lack of appropriate methods for uniform and large area fabrication of antennas as well as difficulty in achieving an efficient design with small mode volume (gap spacing < 10nm). Here, we present a novel optical antenna design, arch-dipole antenna, with optimal radiation efficiency and small mode volume, 5 nm gap spacing, fabricated by CMOS-compatible deep-UV spacer lithography. We demonstrate strong surface-enhanced Raman spectroscopy (SERS) signal with an enhancement factor exceeding 108 from the arch-dipole antenna array, which is two orders of magnitude stronger than that from the standard dipole antenna array fabricated by e-beam lithography. Since the antenna gap spacing, the critical dimension of the antenna, can be defined by deep-UV lithography, efficient optical antenna arrays with nanometer-scale gap can be mass-produced using current CMOS technology.

Resonant Point Tracking and Locking Technique Based on Microsphere Resonator

Abstract. Microsphere resonator is widely studied for the high quality factor Q and small mode volume. Aimed at the sensor applications, the resonant point tracking and locking method is proposed for measuring the resonant frequency shift. To avoid external noises, such as air flow and vibration, spot-packaged coupling structure is adopted. The robustness of microsphere-taper coupling unit is greatly improved. And this lays a foundation for resonant point tracking and locking. By employing modulation method, the demodulation signal is obtained. To lock the laser frequency to resonant point, PID circuits will adjust the center frequency of laser until the intensity of demodulation signal becomes zero. In the experiment, resonant tracking and locking is realized on the rotating platform with rotation rate 2 r/s.

Designs of photonic crystal nanocavities for stimulated Raman scattering in diamond

Diamond is a good candidate for producing Raman laser due to its high first-order Raman gain coefficient. Since its Raman shift (~1,332.5 cm−1) is large compared to other solid-state materials, it is possible to produce a Raman frequency converter using diamond crystals. Photonic crystals can be employed for confining photons within periodic structures, the scale of which is on the order of the incident wavelength, making it convenient for integrating all-optical circuits. Combining the merits of both diamond and photonic crystals, we present two designs of photonic crystal nanocavities (in hexagonal and square lattice structures) which can produce stimulated Raman lasing with low-threshold power. After optimizing the photonic bandgaps, triple resonant modes with high Q and small modal volume are realized in each design by tuning the radii of dot defects in the nanocavities. Numerical simulations show that for such designs, the threshold power for generating Raman lasers in the range of a few hundred nano-Watts can be achieved.

Numerical analysis on the feasibility of a multi-layered dielectric sphere as a three-dimensional photonic crystal

The radiative decay rate of a dipole emitter inside the core of a multi-layered dielectric sphere is theoretically investigated. It is shown that, when the thickness of each layer coincides with a quarter wavelength, the multi-layered sphere has a great potential to work as a three-dimensional photonic crystal with a high quality factor and a small mode volume. From the analysis of the dipole position dependence of a radiative decay rate, we show that a smaller core radius, a quarter wavelength at the smallest, is more suitable for real applications. The investigation on the tolerance for thickness nonuniformity reveals that the thickness variation of 10% is tolerable.

Vertical microcavities with highQand strong lateral mode confinement

We show that strong lateral photon confinement in a vertical Fabry-P\'erot microcavity can be achieved without sacrificing the quality factor $Q$ or the need for pillar structures. A submicrometer Gaussian-shaped defect inserted between two distributed Bragg reflectors confines the photons on resonance in a small modal volume $V$ on the order of ${(\ensuremath{\lambda}/n)}^{3}$ while maintaining a $Q$ up to ${10}^{5}$. This design minimizes light scattering induced by lateral dielectric discontinuities, and therefore our simulations demonstrate that the $Q/V$ is increased by nearly two orders of magnitude relative to cavities with an embedded mesa defect. We further discuss possible fabrication strategies for such devices, which are well suited for the study of cavity quantum electrodynamics with either organic or inorganic quantum emitters, and hold promise for the implementation of scalable quantum photonic networks.

Cavity-enhanced optical detection of carbon nanotube Brownian motion

Optical cavities with small mode volume are well-suited to detect the vibration of sub-wavelength sized objects. Here we employ a fiber-based, high-finesse optical microcavity to detect the Brownian motion of a freely suspended carbon nanotube at room temperature under vacuum. The optical detection resolves deflections of the oscillating tube down to 70 pm/Hz1/2. A full vibrational spectrum of the carbon nanotube is obtained and confirmed by characterization of the same device in a scanning electron microscope. Our work extends the principles of high-sensitivity optomechanical detection to molecular scale nanomechanical systems.

Ultra small mode volume defect cavities in spatially ordered and disordered metamaterials

In this letter, we study metamaterials made out of resonant electric wires arranged on a spatial scale much smaller than the free space wavelength, and we show that they present a hybridization band that is insensible to positional disorder. We experimentally demonstrate defect cavities in disordered and ordered samples and prove that, analogous to those designed in photonic crystals, those cavities can present very high quality factors. In addition, we show that they display mode volumes much smaller than a wavelength cube, owing to the deep subwavelength nature of the unit cell. We underline that this type of structure can be shrunk down to a period close of a few skin depth. Our approach paves the way towards the confinement and manipulation of waves at deep subwavelength scales in both ordered and disordered metamaterials.

Novel high-Q modes in thick 2D photonic crystal slabs

The periodic nature of photonic crystals (PCs) (Yablonovitch 1987 Phys. Rev. Lett. 58 2059–62; John 1987 Phys. Rev. Lett. 58 2486–9) has been extensively exploited for the past quarter of a century using photonic bandgap (PBG) effects to manipulate photons in engineered electromagnetic structures. Structures such as photonic crystal nanocavities are widely considered to be key in realizing future nanoscale optoelectronic devices. These cavities are capable of creating resonant modes with high-quality factor (Q) and small mode volume, in other words a large Purcell factor (Purcell 1946 Phys. Rev. 69 681), and have been widely researched in the two-dimensional photonic crystal slab (PCS) defect cavity configuration (Painter et al 1999 J. Opt. Soc. Am. B 16 275–85). Here, we demonstrate for the first time how three confinement mechanisms are thought to coincide to give rise to a high-Q resonance for a slab containing a modified L3 defect where the slab thickness is such that the guiding in the slab is no longer single moded (Tandaechanurat et al 2008 Opt. Express 16 448–55). This is in contrast to the conventional design approach, where the PCS thickness is chosen to be of the order of half a lattice constant to ensure that a PBG exists to confine cavity modes strongly within the slab (Painter et al 1999 J. Opt. Soc. Am. B 16 275–85; Johnson et al 1999 Phys. Rev. B 60 5751–8; Khankhoje et al 2010 Nanotechnology 21 065202). These newly identified high-Q modes can be important in terms of the fabrication of slabs and other devices such as vertical pillars since they allow high-Q factors in thicker and more fabrication tolerant geometries.

Photonic Crystal Nanobeam Cavity With Stagger Holes for Ultrafast Directly Modulated Nano-Light-Emitting Diodes

A photonic crystal nanobeam cavity with stagger holes in InP/InGaAsP/InP heterostructure is proposed for ultrafast directly modulated nano-light-emitting diodes (nanoLEDs). With stagger holes, the quality factor Q can be engineered in the range of 102 ~ 104 while keeping a small mode volume (Veff). As a result, the modulation speed of nanoLEDs can be dramatically improved by a small Veff to enhanced spontaneous emission (SpE) rate and a moderate Q to counterbalance SpE lifetime and photon lifetime of the cavity. In our simulation, the direct modulation bandwidth could be higher than 60 GHz with optimal Q value of 2150 and Veff of 2.3( λ0/2n)3.

Single ion coupled to an optical fiber cavity.

We present the realization of a combined trapped-ion and optical cavity system, in which a single Yb(+) ion is confined by a micron-scale ion trap inside a 230 μm-long optical fiber cavity. We characterize the spatial ion-cavity coupling and measure the ion-cavity coupling strength using a cavity-stimulated Λ transition. Owing to the small mode volume of the fiber resonator, the coherent coupling strength between the ion and a single photon exceeds the natural decay rate of the dipole moment. This system can be integrated into ion-photon quantum networks and is a step towards cavity quantum electrodynamics based quantum information processing with trapped ions.

Experimental demonstration of a four-port photonic crystal cross-waveguide structure

We report the design and fabrication of a four-port InP photonic crystal cavity-waveguide structure in which two crossing waveguides intersect in a cavity. Transmission measurements show that by exploiting mode-gap effects, high cross-talk suppression between the two waveguides can be obtained. In addition, the waveguides couple to two distinct cavity resonances with different quality-factors as well as small mode volumes. This structure is promising for realizing ultra-fast, low-energy optical switches or memories.

High Q H1 photonic crystal nanocavities with efficient vertical emission

We report on newly-designed H1-type photonic crystal (PhC) nanocavities that simultaneously exhibit high Q factors, small mode volumes, and high external coupling efficiencies (η([perpendicular])) of light radiated above the PhC membrane. Dipole modes of the H1 PhC nanocavities, which are doubly-degenerate and orthogonally-polarized in theory, are investigated both by numerical calculations and experiments. Through modifying the sizes and positions of the air-holes near to the defect cavity, a Q factor of 62,000 is achieved, accompanied with an improved η([perpendicular]) of 0.38 (assuming an objective lens with a numerical aperture of 0.65). A further increase of η([perpendicular]) to more than 0.60 is observed at the expense of slight degradation of Q factor (down to 50,000). We further experimentally confirm the increase of both Q and η([perpendicular]), using micro-photoluminescence measurements, and demonstrate high Q factors up to 25,000: the highest value ever reported for dipole modes in H1 PhC nanocavities.

Low-threshold ultraviolet solid-state laser based on a Ce^3+:LiCaAlF_6 crystal resonator

A low-threshold solid-state UV laser using a whispering gallery mode (WGM) resonator constructed from UV transparent crystalline material is demonstrated. Using a Ce3+:LiCaAlF6 resonator, we observe broad bandwidth lasing (280-330 nm) with a low threshold intensity of 7.5×10(9) W/m(2) and an effective slope efficiency of ~25%. The lasing time delay dynamics in the pulsed operation mode are also observed and analyzed. Additionally, a LiCaAlF(6) WGM resonator with Q=2×10(7) at 370 nm is realized. The combination of this high Q and the small WGM mode volume significantly lowers the pump power threshold compared to traditional cavity designs, opening the door for both tunable continuous-wave and mode-locked operation.

A hybrid plasmonic microresonator with high quality factor and small mode volume

We propose a novel hybrid plasmonic microcavity which is composed of a silver nanoring and a silica toroidal microcavity. The hybrid mode of the proposed hybrid plasmonic microcavity due to the coupling between the surface plasmon polaritons (SPPs) and the dielectric mode is demonstrated with a high quality factor (>1000) and an ultrasmall mode volume (∼0.8 μm3). This microcavity shows great potential in fundamental studies of nonlinear optics and cavity quantum electrodynamics (cQED) and applications in low-threshold plasmonic microlasers.