Large transmittance contrast via 90-degree sharp bends in square lattice glide-symmetric photonic crystal waveguides.

We demonstrate an intriguing transmittance contrast in a glide-symmetric square-lattice photonic crystal waveguide with a 90-degree sharp bend. The glide-symmetry gives rise to a degeneracy point in the band structure and separates a high-frequency and a low-frequency band. Previously, a similar large transmittance contrast between these two bands has been observed in glide-symmetric triangular- or honeycomb-lattice photonic crystals without inversion symmetry, and this phenomenon has been attributed to the valley-photonic effect. In this study, we demonstrate the first example of this phenomenon in square-lattice photonic crystals, which do not possess the valley effect. Our result sheds new light onto unexplored properties of glide-symmetric waveguides. We show that this phenomenon is related to the spatial distribution of circular polarization singularities in glide-symmetric waveguides. This work expands the possible designs of low-loss photonic circuits and provides a new understanding of light transmission via sharp bends in photonic crystal waveguides.

Nanocavity tuning and formation controlled by the phase change of sub-micron-square GST patterns on Si photonic crystals.

It has been well established that photonic crystal nanocavities with wavelength sized mode volume enable various integrable photonic devices with extremely small consumption energy and small footprint. In this study, we explore the possibility of non-volatile functionalities employing photonic crystal nanocavities and phase change material, Ge2Sb2Te5 (GST). Recently, non-volatile photonic devices based on GST have attracted significant interest and are expected to enable energy-efficient photonic processing, especially for optical computing. However, the device size and the area of GST in previous studies have been rather large. Here, we propose and fabricate Si photonic crystal nanocavities on which submicron-square GST patterns are selectively loaded. Because of the strong light confinement, extremely small area of GST is sufficient to manipulate the cavity mode. We have succeeded to fabricate 30-nm-thick and several-100nm-square GST blocks patterned at the center of photonic crystal cavity with a high alignment accuracy. We confirmed that the resonant wavelength and Q-factor of cavity modes are controlled by the phase change of GST. Moreover, cavity formation controlled by submicron-sized GST is also demonstrated by GST-loaded photonic-crystal line-defect waveguides. Our approach in which we place sub-micron-sized GST inside a photonic crystal nanocavity is promising for realizing extremely energy-efficient non-volatile integrable photonic devices, such as switches, modulators, memories, and reconfigurable novel devices.

Improved design and experimental demonstration of ultrahigh-Q C6-symmetric H1 hexapole photonic crystal nanocavities.

An H1 photonic crystal nanocavity (PCN) is based on a single point defect and has eigenmodes with a variety of symmetric features. Thus, it is a promising building block for photonic tight-binding lattice systems that can be used in studies on condensed matter, non-Hermitian and topological physics. However, improving its radiative quality (Q) factor has been considered challenging. Here, we report the design of a hexapole mode of an H1 PCN with a Q factor exceeding 108. We achieved such extremely high-Q conditions by varying only four structural modulation parameters thanks to the C6 symmetry of the mode, despite the need of more complicated optimizations for many other PCNs. Our fabricated silicon H1 PCNs exhibited a systematic change in their resonant wavelengths depending on the spatial shift of the air holes in units of 1 nm. Out of 26 such samples, we found eight PCNs with loaded Q factors over one million. The best sample was of a measured Q factor of 1.2 × 106, and its intrinsic Q factor was estimated to be 1.5 × 106. We examined the difference between the theoretical and experimental performances by conducting a simulation of systems with input and output waveguides and with randomly distributed radii of air holes. Automated optimization using the same design parameters further increased the theoretical Q factor by up to 4.5 × 108, which is two orders of magnitude higher than in the previous studies. We clarify that this striking improvement of the Q factor was enabled by the gradual variation in effective optical confinement potential, which was missing in our former design. Our work elevates the performance of the H1 PCN to the ultrahigh-Q level and paves the way for its large-scale arrays with unconventional functionalities.

Damage protection from focused ion beam process toward nanocavity-implemented compound semiconductor nanowire lasers.

A focused ion beam (FIB) can precisely mill samples and freely form any nanostructure even on surfaces with curvature, like a nanowire surface, which are difficult to implement by using conventional fabrication techniques, e.g. electron beam lithography. Thus, this tool is promising for nanofabrication; however, fabrication damage and contamination are critical issues, which deteriorate optical properties. In this work, we investigated the protective performance of Al2O3against the FIB process (especially by a gallium ion). Nanowires were coated with Al2O3as a hard mask to protect them from damage during FIB nanofabrication. To estimate the protective performance, their emission properties by photoluminescence measurement and time-resolved spectroscopy were compared with and without Al2O3coating conditions. From the results, we confirmed that the Al2O3coating protects the nanowires. In addition, the nanowires also showed lasing behavior even after FIB processing had been carried out to implement nanostructures. This indicates that their optical properties are well maintained. Thus, our study proves the usefulness of FIBs for future nanofabrication.

Optical links on silicon photonic chips using ultralow-power consumption photonic-crystal lasers.

Ultrashort-distance optical interconnects are becoming increasingly important due to continuous improvements in servers and high-performance computers. As light sources in such interconnects, directly modulated semiconductor lasers with an ultrasmall active region are promising. In addition, using Si waveguides is important to provide low loss optical links with functions such as wavelength filtering and switching. In this paper, we demonstrate a wafer-scale heterogeneous integration of lambda-scale embedded active-region photonic-crystal (LEAP) lasers and Si waveguides, achieved through precise alignment. We numerically and experimentally demonstrated the coupling design between the LEAP lasers and Si waveguides; it is important to match propagation constants of Si waveguides and wavenumber of the optical cavity modes. The LEAP lasers exhibit an ultralow threshold current of 13.2-µA and 10-Gbit/s direct modulation. We also achieved the first data transmission using an optical link consisting of a LEAP laser, Si waveguide, and photodetector and obtained an averaged eye diagram at a bit rate of 10 Gbit/s with a bias current of 150 µA.

Designs toward synchronization of optical limit cycles with coupled silicon photonic crystal microcavities.

A driven high-Q Si microcavity is known to exhibit limit cycle oscillation originating from carrier-induced and thermo-optic nonlinearities. We propose a novel nanophotonic device to realize synchronized optical limit cycle oscillations with coupled silicon (Si) photonic crystal (PhC) microcavities. Here, coupled limit cycle oscillators are realized by using coherently coupled Si PhC microcavities. By simulating coupled-mode equations, we theoretically demonstrate mutual synchronization (entrainment) of two limit cycles induced by coherent coupling. Furthermore, we interpret the numerically simulated synchronization in the framework of phase description. Since our proposed design is perfectly compatible with current silicon photonics fabrication processes, the synchronization of optical limit cycle oscillations will be implemented in future silicon photonic circuits.

Designs toward synchronization of optical limit cycles with coupled silicon photonic crystal microcavities: erratum.

Correction for the citation of a reference.

Generation and Annihilation of Topologically Protected Bound States in the Continuum and Circularly Polarized States by Symmetry Breaking.

We demonstrate by breaking the C_{6} symmetry for higher-order at-Γ bound states in the continuum (BICs) with topological charge -2 in photonic crystals (i) deterministic generation of off-Γ BICs from the at-Γ BIC and (ii) a variety of pair-creation and annihilation processes of circularly polarized states with opposite topological charges and the same handedness. To explain these phenomena, we introduce the handedness-wise topological charge quantized to a half-integer. The handedness-wise charge gives a unified picture of various phenomena involving BICs and circularly polarized states.

Mid-Infrared Lasing of Single Wurtzite InAs Nanowire.

Mid-infrared (MIR) photonics is a developing technology for sensing materials by their characteristic MIR absorptions. Since silicon (Si) is a low-loss material in most of the MIR region, Si photonic structures have been fabricated to guide and confine MIR light, and they allow us to achieve sensitive and integrated sensing devices. However, since the implementation of MIR light sources on Si is still challenging, we propose a thick indium arsenide (InAs) nanowire as an MIR laser that can couple to Si photonic structures with material manipulation. In this study, thick InAs nanowires are grown on an indium phosphide substrate with a self-catalyst vapor-liquid-solid method and transferred to gold-deposited SiO2/Si substrates. Low-temperature microphotoluminescence (PL) spectroscopy shows that InAs nanowires exhibit broad PL peaking at a wavelength of around 2.6 µm (3850 cm-1 in frequency), which corresponds to the bandgap energy of wurtzite InAs. At high optical pump fluences, single InAs nanowire exhibits sharp emission peaks, while their integrated intensity and polarization degree increase abruptly at the threshold pump fluence. These nonlinear behaviors indicate that the MIR lasing action takes place in the InAs nanowire in its cavity mode. Our demonstration of the MIR nanowire laser expands the wavelength coverage and potential application of semiconductor nanowires.

Reconfigurable nanocavity formation in graphene-loaded Si photonic crystal structures.

We propose and numerically demonstrate that a reconfigurable nanocavity can be created in a graphene-loaded Si photonic crystal waveguide. The cavity formation is caused by the local mode-gap modulation induced by electrostatic gate-tuning of graphene. Although most recent graphene photonic devices are based on a change in the imaginary part of the refractive index, here we make use of a change in the real part of the refractive index for gated graphene. We clarify that nanocavities can be formed in two different cases, red-shifted and blue-shifted tunings. These novel formation mechanisms enable us to create and annihilate a nanocavity in a reconfigurable way by varying the gate voltage, which is promising for novel control in photonic processing.

Simulation technique of quantum optical emission process from multiple two-level atoms based on classical numerical method.

In this paper, we report a numerical method for analyzing optical radiation from a two-level atom. The proposed method can consistently consider the optical emission and absorption process of an atom and also the interaction between atoms through their interaction with a radiation field. The numerical model is based on a damping oscillator description of a dipole current, which is a classical model of atomic transition and is implemented with a finite-difference time-domain method. Using the method, we successfully simulate the spontaneous emission phenomena in a vacuum, where the interaction between an atom and a radiated field plays an important role. We also simulate the radiation from an atom embedded in a photonic crystal (PhC) cavity. As a result, an atom-cavity field interaction is sucessfuly incorporated in the simulation, and the enhancement of the optical emission rate of an excited atom is explained. The method considers the effect of the interaction between atoms through the radiated field. We simulate the optical emission process of the multiple atoms and show that an enhancement of the emission rate can occur owing to an atom-atom interaction (superradiance) (R. H. Dicke, Phys. Rev. 93, 99 [1954]). We also show that the emission rate is suppressed by the effect of the destructive dipole-dipole interaction under an out-of-phase excitation condition (subradiance).

Wideband slow short-pulse propagation in one-thousand slantingly coupled L3 photonic crystal nanocavities.

Coupled cavities have been used previously to realize on-chip low-dispersion slow-light waveguides, but the bandwidth was usually narrower than 10 nm and the total length was much shorter than 1 mm. Here we report long (0.05-2.5 mm) slow-light coupled cavity waveguides formed by using 50, 200, and 1,000 L3 photonic crystal nanocavities with an optical volume smaller than (λ/n)3, slanted from Γ-K orientation. We demonstrate experimentally the formation of a single-mode wideband coupled cavity mode with a bandwidth of up to 32nm (4THz) in telecom C-band, generated from the ultra-narrow-band (~300 MHz) fundamental mode of each L3 nanocavity, by controlling the cavity array orientation. Thanks to the ultrahigh-Q nanocavity design, coupled cavity waveguides longer than 1 mm exhibited low loss and allowed time-of-flight dispersion measurement over a bandwidth up to 22 nm by propagating a short pulse over 1,000 coupled L3 nanocavities. The highly-dense slanted array of L3 nanocavity demonstrated unprecedentedly high cavity coupling among the nanocavities. The scheme we describe provides controllable planar dispersion-managed waveguides as an alternative to W1-based waveguides on a photonic crystal chip.

Room temperature continuous-wave nanolaser diode utilized by ultrahigh-Q few-cell photonic crystal nanocavities.

Few-cell point-defect photonic crystal (PhC) nanocavities (such as LX and H1 type cavities), have several unique characteristics including an ultra-small mode volume (Vm), a small device footprint advantageous for dense integration, and a large mode spacing advantageous for high spontaneous-emission coupling coefficient (ß), which are promising for energy-efficient densely-integratable on-chip laser light sources enhanced by the cavity QED effect. To achieve this goal, a high quality factor (Q) is essential, but conventional few-cell point-defect cavities do not have a sufficiently high Q. Here we adopt a series of modified designs of LX cavities with a buried heterostructure (BH) multi-quantum-well (MQW) active region that can achieve a high Q while maintaining their original advantages and fabricate current-injection laser devices. We have successfully observed continuous-wave (CW) lasing in InP-based L1, L2, L3 and L5 PhC nanocavities at 23°C with a DC current injection lower than 10 µA and a bias voltage lower than 0.9 V. The active volume is ultra-small while maintaining a sufficiently high confinement factor, which is as low as ~10-15 cm3 for a single-cell (L1) nanocavity. This is the first room-temperature current-injection CW lasing from any types of few-cell point-defect PhC nanocavities (LX or H1 types). Our report marks an important step towards realizing a nanolaser diode with a high cavity-QED effect, which is promising for use with on-chip densely integrated laser sources in photonic networks-on-chip combined with CMOS processors.

Photonic Topological Insulating Phase Induced Solely by Gain and Loss.

We reveal a one-dimensional topological insulating phase induced solely by gain and loss control in non-Hermitian optical lattices. The system comprises units of four uniformly coupled cavities, where the successive two have loss; the others experience gain, and they are balanced under two magnitudes. The gain and loss parts are effectively dimerized, and a bulk band gap, topological transition, midgap topological edge, and interface states in finite systems can all be achieved by controlled pumping. We also clarify non-Hermitian topological invariants and edge states in gapless conditions.

Design of nanowire-induced nanocavities in photonic crystal disks.

We propose a novel type of nanowire (NW)-induced nanocavity based on photonic crystal disks, and we investigate its design by three-dimensional finite-difference time-domain calculations. We detail the confinement principle used in such a cavity and discuss the influence of geometric and material parameters on the cavity performance. Finally, we report on an optimized design presenting a quality factor Q=7.2×104, a mode volume as small as Vm=2.2(λ/nrNW)3, and a large confinement factor of the electric field energy in the NW Γ=65%, which shows good prospects for the realization of efficient NW-based nanolasers operating in the ultraviolet and visible ranges.

Enhanced electron-hole droplet emission from surface-oxidized silicon photonic crystal nanocavities.

We have observed electron-hole droplet (EHD) emission enhanced by silicon photonic crystal (Si PhC) nanocavities with a surface oxide. The EHD is employed as a massive emitter that remains inside the nanocavity to achieve efficient cavity-emitter coupling. Time-resolved emission measurements demonstrate that the surface oxide greatly reduces the nonradiative annihilation of the EHDs and maintains them in the PhC nanocavities. It is found that the surface-oxidized Si PhC nanocavity enhances EHD emission in addition to the Purcell enhancement of the resonant cavity, which will contribute to works on Si light emission and the cavity quantum electrodynamics of electron-hole condensates.

Design of nanowire-induced nanocavities in grooved 1D and 2D SiN photonic crystals for the ultra-violet and visible ranges.

Nanowire-induced SiN photonic crystal (PhC) nanocavities specifically designed for the ultra-violet and visible range are investigated by three-dimensional finite-difference time-domain calculations. As opposed to their silicon PhC counterpart, we find that the formation of nanowire-induced two-dimensional (2D) SiN PhC nanocavities is more challenging because of the low refractive index of SiN. We thus discuss optimization strategies to circumvent such difficulties and we investigate the influence of critical design parameters such as PhC geometry, as well as nanowire geometry and position. We also propose a novel nanowire-induced cavity design based on one-dimensional (1D) nanobeam PhCs. We finally report on nanowire-induced nanocavity designs in 1D (resp. 2D) PhCs presenting quality factors as high as Qc = 5.1 x 104 (resp. Qc = 2.5 x 104 with a mode volume Vm=1.8(λ/nrNW)3 (resp. Vm=5.1(λ/nrNW)3), which show good prospects for light-matter interaction in the near-ultraviolet and visible ranges.

Nanowire-nanoantenna coupled system fabricated by nanomanipulation.

Here we demonstrate the combination of a semiconductor nanowire and a plasmonic bowtie nanoantenna. A subwavelength InP nanowire was placed precisely in the middle of the nanogap of a gold bowtie nanoantenna with a nanomanipulator installed in a focused ion beam system. We observed a significantly large enhancement (by a factor of 110) of the photoluminescence intensity from this coupled system when the excitation wavelength was at the plasmonic resonance with its polarization parallel to the nanoantenna. Moreover, simulation results revealed that this large enhancement was caused by an interesting interplay between the plasmonic resonance of the nanoantenna and the breakdown of the field suppression effect in the subwavelength nanowire. Our results show that the combination of a nanowire and a nanoantenna gives us a new degree of freedom to design light-matter interactions on a nanoscale.

Optomechanical oscillator pumped and probed by optically two isolated photonic crystal cavity systems.

Optomechanical control of on-chip emitters is an important topic related to integrated all-optical circuits. However, there is neither a realization nor a suitable optomechanical structure for this control. The biggest obstacle is that the emission signal can hardly be distinguished from the pump light because of the several orders' power difference. In this study, we designed and experimentally verified an optomechanical oscillation system, in which a lumped mechanical oscillator connected two optically isolated pairs of coupled one-dimensional photonic crystal cavities. As a functional device, the two pairs of coupled cavities were respectively used as an optomechanical pump for the lumped oscillator (cavity pair II, wavelengths were designed to be within a 1.5 µm band) and a modulation target of the lumped oscillator (cavity pair I, wavelengths were designed to be within a 1.2 µm band). By conducting finite element method simulations, we found that the lumped-oscillator-supported configurations of both cavity pairs enhance the optomechanical interactions, especially for higher order optical modes, compared with their respective conventional side-clamped configurations. Besides the desired first-order in-plane antiphase mechanical mode, other mechanical modes of the lumped oscillator were investigated and found to possibly have optomechanical applications with a versatile degree of freedom. In experiments, the oscillator's RF spectra were probed using both cavity pairs I and II, and the results matched those of the simulations. Dynamic detuning of the optical spectrum of cavity pair I was then implemented with a pumped lumped oscillator. This was the first demonstration of an optomechanical lumped oscillator connecting two optically isolated pairs of coupled cavities, whose biggest advantage is that one cavity pair can be modulated with an lumped oscillator without interference from the pump light in the other cavity pair. Thus, the oscillator is a suitable platform for optomechanical control of integrated lasers, cavity quantum electrodynamics, and spontaneous emission. Furthermore, this device may open the door on the study of interactions between photons, phonons, and excitons in the quantum regime.