arXiv subjects

Masaya Notomi

Publications and source records attributed to Masaya Notomi.

At least 37 records · Page 2Linked to original sources

arXiv2024

Optoelectronic recurrent neural network using optical-electrical-optical converters with RC delay

Optical neural network (ONN) has been attracting intense attention owing to their low latency and low-power consumption. Among the ONNs, optical recurrent neural network (RNN) enables low-power and high-speed time-series data processing using a compact loop structure. The loop losses need to be efficiently compensated so that the time-series information is maintained in the RNN operation. For this purpose, we focus on the optoelectronic RNN (OE-RNN) with optical-electrical-optical (OEO) converters to compensate for the loop losses. However, the effect of resistive-capacitive (RC) delay of OEO converters on the RNN performance is unclear. Here, we study in simulation an OE-RNN equipped with OEO converters with RC delay. We confirm that our modeled OE-RNN achieves the high training accuracy of time-series data classification even when RC delay is comparably large to the time interval of time-series data. Our analyses reveal that the accumulation of time-series data by RC delay does not degrade the RNN performance but rather can compensate for the degraded RNN performance due to loop losses. From the theoretical analysis referring to the gradient explosion and vanishing problems, we find the region related to loss and RC delay where the high training accuracy can be achieved. In simulation, we confirm this compensation effect in the large OE-RNN circuit up to 32$\times$32 scale. Our proposed scheme opens a new way of time-series data processing by utilizing RC delay for the optical computing and optical communication.

physics.optics↗

arXiv2024

EP restoration and fast-light edge states in photonic crystal waveguide with glide and time reversal symmetry

Exceptional points (EPs) in the propagation states give rise to the emergence of intriguing properties with the divergence of the group velocity. However, there have been no experimental reports due to the necessity of maintaining high levels of fabrication precision and the requisite high group velocity contrast. In our study, we propose a design of photonic crystal waveguide with glide and time reversal symmetry, and derive an effective Hamiltonian for edge states to realize fast-light edge states. We adopt a systematic method to generate EPs in edge states by introducing non-Hermitian perturbations to Dirac points guaranteed by glide symmetry, which ensures that EP modes are free from out-of-plane radiation losses. Then, our study reveals the conditions for the exact EP restoration and provides an analytical solution to offset the EP smoothing due to symmetry breaking, which drastically reduces the group velocity contrast. A good symmetry property of the photonic crystal waveguide allows us to derive the effective Hamiltonian as a simple form, and the EPs can be restored by adjusting the real part of the permittivity. Furthermore, we design a feasible photonic crystal slab waveguide incorporating graphene as the absorbing material, and numerically demonstrate a group velocity reaching $v_g = 3.3c$ near the EP, which is up to 25 times that of the original structure. Thanks to the short periodicity of photonic crystals, it's possible to reach the speed of light in vacuum with group velocity contrasts on the order of one digit. Our study paves an innovative way to manipulate the group velocity of light.

physics.optics↗

arXiv2024

Photonic topological phase transition induced by material phase transition

Photonic topological insulators (PTIs) have been proposed as an analogy to topological insulators in electronic systems. In particular, two-dimensional PTIs have gained attention for the integrated circuit applications. However, controlling the topological phase after fabrication is difficult because the photonic topology requires the built-in specific structures. This study experimentally demonstrates the band inversion in two-dimensional PTI induced by the phase transition of deliberately-designed nanopatterns of a phase-change material, Ge2Sb2Te5 (GST), which indicates the first observation of the photonic topological phase transition with changes in the Chern number. This approach allows us to directly alter the topological invariants, which is achieved by symmetry-breaking perturbation through GST nanopatterns with different symmetry from original PTI. The success of our scheme is attributed to the ultrafine lithographic alignment technologies of GST nanopatterns. These results demonstrate to control photonic topological properties in a reconfigurable manner, providing an insight into new possibilities for reconfigurable photonic processing circuits.

physics.optics↗

arXiv2023

Vector beam generation from standing hollow GaN nanowire lasers on sapphire substrate

We fabricated GaN based hollow nanowires standing upright on a sapphire substrate by the sublimation method and found that they exhibit laser oscillation at room temperature. These very long, hollow, nano-sized structures cannot be fabricated by other means. Furthermore, we determined the condition under which the fundamental mode is azimuthally polarized by investigating the dispersion of the hollow structure. Examination of the measured emission properties indicates that the hollow nanowire operates as a topological, vector-beam, light source.

physics.optics↗

arXiv2023

Optical non-Hermitian skin effect in two-dimensional uniform media

The non-Hermitian skin effect (NHSE) is a novel localization phenomenon in certain non-Hermitian systems with gain and/or loss. Most of previous works study the non-Hermitian skin effect in periodic systems. However, electromagnetic waves often propagate within uniform materials without periodic modulation, and it has not been clear whether the optical NHSE occurs in uniform media such as bulk materials and electromagnetic metamaterials. Here we establish the theory of the optical NHSE in non-Hermitian anisotropic media. We show that the NHSE occurs even in uniform media with appropriate anisotropy and material loss. The localization of non-Hermitian skin modes are completely determined by an effective gauge potential caused by the anisotropy of a dielectric tensor. On the basis of the theory, we propose subwavelength multilayer metamaterials as a novel platform for the optical NHSE. We also propose a new concept of stationarily-excited skin modes whose frequencies are forced to be real in non-Hermitian systems. We find that the NHSE occurs even under the condition that the frequency is forced to be real, which implies that the NHSE we propose is observable under stationary excitation. Our work presents a general theory of the NHSE in homogeneous systems, and pave the way to realize the optical NHSE in bulk materials and metamaterials.

physics.optics↗

arXiv2023

Improved design and experimental demonstration of ultrahigh-Q C${}_\text{6}$-symmetric H1 hexapole photonic crystal nanocavities

An H1 photonic crystal nanocavity 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 nanocavity with a $Q$ factor exceeding $10^8$. We achieved such extremely high-$Q$ conditions by designing only four structural modulation parameters thanks to the ${\rm C_{6}}$ symmetry of the mode, despite the need of more complicated optimizations for many other nanocavities. The fabricated silicon photonic crystal nanocavities 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 cavities with loaded $Q$ factors over one million ($1.2 \times 10^6$ maximum). 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 $4.5 \times 10^8$, which is two orders of magnitude higher than in the previous studies. Our work elevates the performance of the H1 nanocavity to the ultrahigh-$Q$ level and paves the way for its large-scale arrays with unconventional functionalities.

physics.optics↗

arXiv2022

Probing the Ginzburg-Landau Potential for Lasers Using Higher-order Photon Correlations

Lasing transition is known to be analogous to the second-order phase transition. Furthermore, for some cases, it is possible to define the Ginzburg-Landau (GL) potential, and the GL theory predicts the photon statistical properties of lasers. However, the GL potential for lasers is surprising, because lasers are operating far from equilibrium. In this paper, we theoretically examine the validity of the GL theory for lasers in terms of various parameters, particularly, the ratio between photon and carrier lifetimes. For this purpose, we use stochastic rate equations and higher-order photon correlation functions. With higher-order photon correlation measurements, we can check whether or not laser dynamics are described by the GL theory. We demonstrate that, for low-$β$ lasers, the GL theory is applicable even when the photon lifetime is comparable to the carrier lifetime and that photon-carrier relaxation oscillation is the fundamental origin of the breakdown of the GL theory, which can be understood in the framework of center manifold reduction.

cond-mat.mes-hall↗

arXiv2022

Imaginary couplings in non-Hermitian coupled-mode theory: Effects on exceptional points of optical resonators

Exceptional point (EP) degeneracies in coupled cavities with gain and loss provide on-chip photonic devices with unconventional features and performance. However, such systems with realistic structures often miss the exact EPs even in simulation, and the mechanism of this EP disruption has yet to be thoroughly identified. Here, we extend the coupled-mode theory of one-dimensional non-Hermitian resonator arrays to study the effects of the imaginary part of the inter-cavity coupling, which is a second-order term and attributed to material amplification, absorption, and radiation. By taking an appropriate gauge for the model, we clarify that the imaginary coupling components have a symmetric form in the effective Hamiltonian and hence represent non-Hermiticity. These additional factors can lift the gain- and loss-based EP degeneracies. However, they are proportional to the sum of the imaginary permittivities for involved cavity pairs. Thus, when the amplification and absorption of adjacent cavities are balanced, their contribution to the imaginary coupling is canceled, and the EP singularity can be restored. Radiation-induced imaginary couplings measure the change in net radiation loss by the interference between cavity modes. Their impact on the EP can also be counteracted by small cavity resonance detuning even in loss-biased cases. We show and analyze eligible simulation examples based on photonic crystal nanocavities, and highlight design of an ideal EP degeneracy that is protected by generalized PT symmetry and induced by radiation.

physics.optics↗

arXiv2021

Exceptional point induced unidirectional radiation from non-Hermitian plasmonic structures

Non-Hermitian (NH) photonics has attracted considerable attention from researchers owing to exotic properties that originate from the parity-time ($\mathcal{PT}$) phase transition and exceptional points (EPs). To date, the $\mathcal{PT}$ phase transition, EPs, and circling around EPs have been investigated in many photonic systems. However, few studies focused on the singular nature of the EP eigenstates of the Hamiltonian matrices. Moreover, the switching of an EP eigenstate based on the sign of the coupling constant and its manifestation in physical phenomena have not yet been investigated. In this paper, we propose and numerically demonstrate a unidirectional radiation phenomena manifested by the formation of Huygens dipoles using the singular EP eigenstates in coupled plasmonic systems. Two types of EPs corresponding to positive and negative signs of the coupling constants are realized using dipole-dipole coupling. We show that the Huygens dipole is formed at the EP condition and its radiation direction can be controlled by choosing the sign of the coupling. The unique photonic functionality, which originates directly from the singular eigenstate at the EP uncovers a new aspect of NH photonic physics and devices.

physics.optics↗

arXiv2021

Emulating the local Kuramoto model with an injection-locked photonic crystal laser array

The Kuramoto model is a mathematical model for describing the collective synchronization phenomena of coupled oscillators. We theoretically demonstrate that an array of coupled photonic crystal lasers emulates the Kuramoto model with non-delayed nearest-neighbor coupling (the local Kuramoto model). Our novel strategy employs indirect coupling between lasers via additional cold cavities. By installing cold cavities between laser cavities, we avoid the strong coupling of lasers and realize ideal mutual injection-locking with effective non-delayed dissipative coupling. First, after discussing the limit cycle interpretation of laser oscillation, we demonstrate the synchronization of two indirectly coupled lasers by numerically simulating coupled-mode equations. Second, by performing a phase reduction analysis, we show that laser dynamics in the proposed device can be mapped to the local Kuramoto model. Finally, we briefly demonstrate that a chain of indirectly coupled photonic crystal lasers actually emulates the one-dimensional local Kuramoto chain. We also argue that our proposed structure, which consists of periodically aligned cold cavities and laser cavities, will best be realized by using state-of-the-art buried multiple quantum well photonic crystals.

physics.optics↗

arXiv2021

Low- and high-$β$ lasers in Class-A limit: photon statistics, linewidth, and the laser-phase transition analogy

Nanocavity lasers are commonly characterized by the spontaneous coupling coefficient $β$ that represents the fraction of photons emitted into the lasing mode. While $β$ is conventionally discussed in semiconductor lasers where the photon lifetime is much shorter than the carrier lifetime (class-B lasers), little is known about $β$ in atomic lasers where the photon lifetime is much longer than the other lifetimes and only the photon degree of freedom exists (class-A lasers). We investigate the impact of the spontaneous coupling coefficient $β$ on lasing properties in the class-A limit by extending the well-known Scully-Lamb master equation. We demonstrate that, in the class-A limit, all the photon statistics are uniquely characterized by $β$ and that the laser phase transition-like analogy becomes transparent. In fact, $β$ perfectly represents the "system size" in phase transition. Finally, we investigate the laser-phase transition analogy from the standpoint of a quantum dissipative system. Calculating a Liouvillian gap, we clarify the relation between $β$ and the continuous phase symmetry breaking.

cond-mat.mes-hall↗

arXiv2020

Observing exceptional point degeneracy of radiation with electrically pumped photonic crystal coupled-nanocavity lasers

Controlling gain and loss of coupled optical cavities can induce non-Hermitian degeneracies of eigenstates, called exceptional points (EPs). Various unconventional phenomena around EPs have been reported, and expected to incorporate extra functionalities into photonic devices. The eigenmode exactly under the EP degeneracy is also predicted to exhibit enhanced radiation. However, such responses have yet to be observed in on-chip lasers, because of both the limited controllability of their gain and loss and the lifting of degeneracy by pump-induced cavity detuning. Here, we report the first non-Hermitian nanophotonic platform based on two electrically pumped photonic crystal lasers and its spontaneous emission at an EP degeneracy. Systematically tuned and independent current injection to our wavelength-scale active heterostructure cavities enables us to demonstrate the clear EP phase transition of their spontaneous emission, accompanied with the spectral coalescence of coupled modes and reversed pump dependence of the intensity. Furthermore, we find experimentally and confirm theoretically the peculiar squared Lorentzian emission spectrum very near the exact EP, which indicates the four-fold enhancement of the photonic local density of states induced purely by the degeneracy. Our results open a new pathway to engineer the light-matter interaction by non-Hermiticity and explore larger reconfigurable laser arrays for further non-Hermitian features and physics.

physics.optics↗

arXiv2020

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.

physics.optics↗

arXiv2020

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 charge and same handedness. These processes are well explained by the conservation of handedness-wise topological charges, showing topologically robustness of these phenomena. The results indicate a new control of topological aspect of far-field polarization vectors.

physics.optics↗

arXiv2019

Active topological photonics

Topological photonics has emerged as a novel route to engineer the flow of light. Topologically-protected photonic edge modes, which are supported at the perimeters of topologically-nontrivial insulating bulk structures, have been of particular interest as they may enable low-loss optical waveguides immune to structural disorder. Very recently, there is a sharp rise of interest in introducing gain materials into such topological photonic structures, primarily aiming at revolu-tionizing semiconductor lasers with the aid of physical mechanisms existing in topological physics. Examples of re-markable realizations are topological lasers with unidirectional light output under time-reversal symmetry breaking and topologically-protected polariton and micro/nano-cavity lasers. Moreover, the introduction of gain and loss provides a fascinating playground to explore novel topological phases, which are in close relevance to non-Hermitian and parity-time symmetric quantum physics and are in general difficult to access using fermionic condensed matter systems. Here, we review the cutting-edge research on active topological photonics, in which optical gain plays a pivotal role. We discuss recent realizations of topological lasers of various kinds, together with the underlying physics explaining the emergence of topological edge modes. In such demonstrations, the optical modes of the topological lasers are deter-mined by the dielectric structures and support lasing oscillation with the help of optical gain. We also address recent researches on topological photonic systems in which gain and loss themselves essentially influence on topological prop-erties of the bulk systems. We believe that active topological photonics provides powerful means to advance mi-cro/nanophotonics systems for diverse applications and topological physics itself as well.

physics.optics↗

arXiv2019

Ultrafast and energy-efficient all-optical switching with graphene-loaded deep-subwavelength plasmonic waveguides

All-optical switches have attracted attention because they can potentially overcome the speed limitation of electric switches. However, ultrafast, energy-efficient all-optical switches have been challenging to realize due to the intrinsically small optical nonlinearity in existing materials. As a solution, we propose graphene-loaded deep-subwavelength plasmonic waveguides (30 nm x 20 nm). Thanks to extreme light confinement, we have significantly enhanced optical nonlinear absorption in graphene, and achieved ultrafast all-optical switching with a switching energy of 35 fJ and a switching time of 260 fs. The switching energy is four orders of magnitudes smaller than that in previous graphene-based devices and is the smallest value ever reported for any all-optical switch operating at a few picoseconds or less. This device can be efficiently connected to conventional Si waveguides and employed in Si photonic integrated circuits. We believe that this graphene-based device will pave the way towards on-chip ultrafast and energy-efficient photonic processing.

physics.optics↗

arXiv2019

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 the an atom-atom interaction (superradiance)(R. H. Dicke, Phys. Rev. {\bf 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).

physics.optics↗

arXiv2019

All-optical dynamic modulation of spontaneous emission rate in hybrid optomechanical cavity quantum electrodynamics systems

Recent nanofabrication technologies have miniaturized optical and mechanical resonators, and have led to a variety of novel optomechanical systems in which optical and mechanical modes are strongly coupled. Here we hybridize an optomechanical resonator with two-level emitters and successfully demonstrate all-optical dynamic control of optical transition in the two-level system by the mechanical oscillation via the cavity quantum-electrodynamics (CQED) effect. Employing copper-doped silicon nanobeam optomechanical resonators, we have observed that the spontaneous emission rate of excitons bound to copper atoms is dynamically modulated by the optically-driven mechanical oscillation within the time scale much shorter than the emission lifetime. The result is explained very well with an analytical model including the dynamic modulation of the Purcell effect and the exciton population. To the best of our knowledge, this is the first demonstration of a dynamic modulation of the spontaneous emission rate by mechanical oscillations. Our achievement will open up a novel field of hybrid optomechanical CQED systems in which three body--optical transitions, optical resonance modes, and mechanical resonance modes--are strongly coupled and will pave the way for novel hybrid quantum systems.

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