The 100 most-cited articles in The Japan Society of Applied Physics, 2019

Focused surface plasmon (FSP) sensing [1] has been widely used to characterize various samples pertaining to the high spatial resolution and single shot measurement of local refractive index. The theoretical analysis of surface plasmon with the consideration of the anisotropy in the sample is sparse in literature and is the motivation for the present work. In this study, we calculate the spatial frequency distribution of the reflected light as a response of FSP against the anisotropic sample by assuming that the fast axis of the anisotropic sample lies on the substrate surface which replicates cellular conditions. The influence of a thin layer of silica coated on the metal film is evaluated as the morphological property of the biological sample cultured on the substrate can be affected by the chemical effect (hydrophobic property) of the metal surface.

We present on the development of ultrafast fiber lasers and their applications in nonlinear optical imaging and precision measurements. These compact femtosecond lasers have enabled multiphoton imaging of biological samples [1], precision rotation measurement [2], and gas sensing applications [3, 4]. Our multiphoton microscopes have enabled label free determination of the stages of dysplasia in Barrett’s esophagus [5], pancreatic cancer margin detection [6], and examined the nonlinear optical properties of materials [7]. We have also achieved real-time dual-comb spectroscopy measurements of gases without the need for complex servo locking systems with a high signal to noise ratio [8]. With these systems, we can gather sample information not available with traditional microscopy, create precision rotation sensing systems unlike any existing market solution, and resolve narrow absorption lines without the need for complex equipment.

Dual-comb spectroscopy techniques are superior to a conventional Fourier transform spectroscopy in terms of the acquisition time, sensitivity, and resolution. Owing to its excellent capabilities, it has been applied to several useful applications. The conventional dual-comb system employs two independent mode-locked lasers for the generation of two optical frequency combs (OFCs) with slightly different repetition rates (Δfrep), and a complex servo system for high mutual coherence were needed. Despite the capabilities of dual-comb spectroscopy, the use of the dual-comb technique is still mainly restricted to experts for OFC. In recent years, a dual-comb laser, which emits two OFCs from a single laser cavity with a small Δfrep, has attracted attention owing to its desirable properties such as passive mutual coherence. Recently, dual-wavelength [1], bidirectional [2, 3], and polarization-multiplexed [4] mode-locked Er-fiber lasers have demonstrated as dual-comb fiber lasers. In previous works [1, 2], the demonstrated spectral bandwidth of the dual-comb fiber laser was of the order of a…

An optical vortex [1] carries an annular intensity profile and an orbital angular momentum, characterized by a topological charge l, owing to its helical wavefront, and it provides us a variety of potential research opportunities, including chiral fabrications [2-4], high-speed communications and scanning microscopes with a spatial resolution beyond the diffraction limit. The above-mentioned applications require strongly frequency versatility of the vortex light sources, thus, a THz vortex source will offer entirely new fundamental sciences and advanced technologies for THz photonics.

The explosive increase in internet traffic requires cost-effective optical networks between and within datacenters. However, to build these cost-effective networks, we need to fundamentally change how we manufacture InP-based photonic devices. InP/Si photonic integrated circuits (PICs) using silicon-photonics technologies are promising for such short-distance communications. Silicon-photonics technologies enable the integration of several functional devices except for lasers on a single compact chip, which can dramatically reduce assembly and packaging costs. Therefore, a cost-effective way to integrate InP-based active devices on a silicon-based platform is strongly desired. In this context, we have developed a new III-V/Si integration scheme in which an InP membrane is directly bonded to SiO2/Si substrate (InP-on-insulator) as an epitaxial template. Compared with a direct-growth scheme, this scheme solves the problems of lattice mismatch and the formation of anti-phase boundaries. In addition, we overcame the crystal degradation due to the difference in coefficient of thermal…

The electromagnetic excitations propagating in a wave-like manner along the interface between two different media and evanescently confined in the direction perpendicular to the interface are known as surface plasmons [1], [2], [3]. In this manuscript, propagation of Electric and Magnetic fields are analysed for hybrid Plasmonic ring waveguide. A plasmonic ring waveguide based on Silicon-on-insulator technology is taken in to consideration consisting of a 220 nm vertical height fabrication technology. The ring structure comprises of a bulk Gold (Au) material at the central ring cavity with a slot adjacent to silicon material is analysed for its field and intensity propagation.

Recently, as the display rapidly develops, a lot of research and development related to display technologies are being carried out. In particular, organic light emitting diode (OLED) display devices have complicated optical structures on the nanoscale and different luminescent properties depending on the polarization of light. To analyze this complicated OLED displays, the methods for Electromagnetic (EM) field analysis such as the finite difference domain (FDTD), the finite element method (FEM) and the rigorous coupled-wave analysis (RCWA) are generally used. In this study, we used Fourier modal method (FMM) [1] based on RCWA to calculate the EM field. However, to calculate the EM field of a large-area display using the FMM, it is necessary to use a high-resource computer or a special methodology for parallel computation. Therefore, we propose a method for parallel process of FMM that can perform calculations on general level calculation systems such as a personal computer (PC).

A cancer genome therapy is one kind of the precision medicine, which is an idea to analyze a patient’s genetic information and optimize medical treatment for each patient. To evaluate the analyzed tissue includes the information of the cancer gnome, quantitative evaluation of the ratio of cancer cells to normal cells in the tissue is required. However, evaluation conventionally takes more than two days because formalin fixed paraffin embedded (FFPE) blocks are firstly prepared, which was followed by dyeing.

Dual-comb technique is very attractive in terms of high speed spectroscopy. Recently, dual comb lasers that generate two combs as the outputs absorbs a lot of interests [1-3]. In the practical applications, the stability and repeatability are important factors. The usage of polarization maintaining devices is effective to realize the stable dual comb lasers.

In recent years, early diagnosis is important to improve quality of our life. To realize the early diagnosis, measurement of ions in bloods with small volumes is one of an excellent option. In our group, a terahertz chemical microscopy (TCM) has been developed to measure ions in the small volume of solutions (1,2). TCM measures the electric potential change of ion selective membranes on a sensing plate. Generally, this kind of ion measurement method using the ion selective membranes requires a reference electrode inserted in the sample solutions, which limit the minimum volume of the sample solutions.

Terahertz (THz) plasmonic structures with surface plasmon polaritons (SPPs) based on periodic metal holes or slits have been investigated in the decade. THz electromagnetic waves can thus be trapped at the structural apertures with characteristic surface field, resembling the SPPs of normal metal. Therefore, THz plasmonic devices based on the surface-confined field can be applied for the optical sensing or spectral filter because of the response beyond the performance in free-space condition. In recent years, Fano resonances have been introduced as one important effect due to its superior characteristics with asymmetric and sharp spectral response [1-3]. Woven steel meshes were studied for the microwaves and THz waves due to the flexibility and deformable functions [4, 5]. The THz spectroscopic analysis on the meshes is revealed, but Fano resonance induced from the mesh structure has not found yet [4, 5]. In this paper, the metal wire woven mesh (MWWM) is experimentally investigated in the transmission spectrum of 0.1-1 THz and presented as a THz plasmonic structure with localized…

THz wave offers great potential for applications in security screening, nondestructive evaluation and high-speed wireless communication. It is expected that THz switches, attenuators, filters, polarization controllers and even routers will become available to handle THz waves in reconfigurable ways, just like their well-developed optical counterparts. Liquid crystal is a very promising candidate for the new platform of THz devices. However, the LC based THz components face significant challenges. Here we present a total solution for half waveplate with similar design in visible range and demonstrate various high-performance THz photonic devices for further amplitude, phase, polarization modulations and even complex wavefront controls, such as Q-plate and polarization gratings. The proposed solution may pave a simple and bright road towards various liquid crystal terahertz apparatuses.

The Terahertz (THz) frequency range refers to electromagnetic radiation from 100 GHz to 10 THz. The unique characteristics of THz waves have allowed the development of wide-ranging applications such as molecular spectroscopy, medical imaging, security scanning, nondestructive testing, information technology among many others [1]. The ability to realize efficient and compact THz devices will benefit the further development of THz technology both for basic research and applications.

The efficient creation and detection of spatial modes of light has become topical of late, driven by the need to increase photon bit-rates in classical and quantum communications. Here we put forward a new spatial mode detection technique based on the nonlinear optical process of SFG.

Phase change material Ge2Sb2Te5 (GST) has been utilized as optically active material to design and experimentally realize active silicon photonic devices, mainly owing to its self-holding bi-stability and large difference in optical properties between its amorphous and crystalline phases [1].

In recent years, the hydrogen was suggested as an alternate fuel source to produce clean and efficient green energy. Photoelectrochemical (PEC) water splitting is one of the green technologies for the hydrogen production [1].

Random lasers containing plasmonic nanoparticles (NPs) can exhibit speckle-free and tunable lasing emission. However, understanding of the physical mechanisms of the NPs enhancement that give rise to the emission with coherent feedback is poorly understood [1]. It has been suggested that these sharp emissions with linewidth less than 1.0 nm depend on several processes, including random variations of the refractive index of the gain medium, absorption induced localization, amplified extended modes and intrinsic quasimodes [2,3]. Better understanding of the lasing mechanisms would allow novel designs to balance losses and maximize emission for several applications, i.e. bio-imaging, photonics barcode and friend-foe identification [4].

We observed a phase change in methane hydrate, a potential next-generation energy resource, using terahertz time-domain spectroscopy (THz-TDS). In the temperature zone where methane hydrate should decompose, an increase in absorption and a shift in the time-domain waveform were observed, indicating mutation.

Several machine learning (ML) classifiers were tested to automate the analysis of time-domain terahertz spectroscopy (THz-TDS) imaging, which is inherently the hyperspectral technique with often large datasets having the sliced image data in frequency and time domains. Among the tested ML classifiers for THz waveform recognitions, the random forest one was the best choice in terms of overall accuracy, speed, robustness, and user-friendliness (see Table 1 and Fig. 1). Combining these and our previously reported results in frequency domain [1], such ML techniques could have a great potential for THz-TDS analytical and non-destructive testing (NDT) applications.

Optical vortex beam exhibits a ring-shaped spatial profile and carries an orbital angular momentum (OAM), characterized by an azimuthal phase term, e−ilΦ%(l is a topological charge and Φ is the azimuthal angle) [1], and has been widely applied in a variety of fields, such as optical tweezing and manipulation, and material processing [2]. Also, flower-shaped modes, i.e. coherent superposition of optical vortices with different OAM states also receive much attention, because their unique properties provide potentially advanced applications, including optical trapping [3] and quantum optics [4]. The aforementioned applications strongly desire a versatility of wavelength and OAM state to optical vortex sources [5]. In this presentation, we report on the first demonstration of a tunable vortex laser with versatile flower-shaped modes based on an optical vortex pumped optical parametric oscillator (OPO) formed of a non-critical phasematching LiB3O5 (NCPM-LBO) crystal in a linear cavity configuration with a cavity length of 60 mm.

While graphene (G) is atomically thin, it has a strong influence on the surface potential when it is transferred on a metal film [1][2].

Optical vortex beams1) carry an annular spatial profile and a nonzero orbital angular momentum of lh (where l is the topological charge) per photon, owing to the phase singularity, and they have been investigated in various applications, such as optical trapping, optical communication, quantum optics, and super-resolution fluorescence microscopes. In particular, visible vortex sources with a selective handedness should be applied to nano/micro fabrication, for instance, the formation of chiral structured materials.

Trace detection of CO2, especially of the radioisotope 14C is of high importance in radio-dating, as well as environmental and biological tracer applications, where isotope-labeled samples need to be investigated. We have previously demonstrated our capability to perform detection of 14C using laser absorption based on cavity ring down spectroscopy (CRDS) at abundance levels lower than feasible with standard liquid scintillation counters [1,2].

The approaches to investigate spin dynamics and properties using the terahertz pulse and terahertz techniques have been studied actively recently [1-15]. Especially, the usage of the magnetic field component of the high intensity terahertz (THz) pulses [1,16-18] is recognized as a powerful tool for investigating spin dynamics via magnetic dipole interaction. Up to now, we have reported some studies using THz magnetic pulse excitation such as ultrafast spin spectroscopy, the circularly polarized THz wave emission, tunable THz absorber and wave plate, and macroscopic magnetic domain control in the nano-magnet e -Fe2O3 and the weak ferromagnet rare-earth orthoferrites [1-15].

Three-dimensional (3D) holographic display system which is issued on next generation displays, and its realization is a challenging task. Recently, electronics and telecommunications research institute (ETRI) and our team proposed the 360-degree table-top holographic display system, which is one of the next generation 3D holographic display systems through the Giga-Korea Project. This system consists of 4-f off-axis system, a mirage mirror, digital micro-mirror devices (DMDs) which sequentially load computer generated holograms (CGHs) at 360 degrees, and a mechanical motor synchronized with the DMDs [1]. Also, the non-mechanical table-top system without motors is being researched for safer and more practical system.

THz quantum cascade lasers (QCLs) are promising as compact and high output power semiconductor THz sources over sub-Watt [1] to Watt-order peak power [2] with narrow bandwidth. Based on the bottle neck of limited operation temperature around 200 K [3], practical applications of THz-QCLs are still quite limited caused by a requirement of the large cooling system and operated at low duty cycle with small average output power. Operation temperature and output power are the most critical parameters to be used at real THz applications. We reported an improvement of optical gain of THz-QCL by reducing a horizontal current leakage from upper lasing level to neighbor high energy levels under the guidance of theoretical calculations based on non-equilibrium Green’s function (NEGF) method [1]. We design the different Al composition AlGaAs barriers-wells in short period structures in order to keep the lasing energy levels with artificially miss-alignment the coupling of upper lasing level to neighbor high energy levels. Dramatic increase of optical gain especially temperature over 200K was…

Mode-locked laser can produce stably a pulse train with a broadband spectrum to facilitate a precision frequency metrology. In contrast, with an overdrive in a cavity, an oscillator operates a chaotic, and noise-like pulse although seemingly regular pulse train. Furthermore, a conventional spectrometer, which consists of grating and line sensor, would be difficult to distinguish between mode-locked operation and noise-like pulse. Thanks to the single-shot spectrum measurement of a photonic time stretch spectroscopy [1] , the shot-to-shot spectrum is enabling to capture with long record length in real time. With this single shot spectral measurement, a dissipative soliton, soliton molecule, soliton rain, and optical rogue wave, was observed recently.

InGaN/GaN multiple quantum wells (MQWs) are widely studied for light emitting diodes (LEDs) and laser diodes (LDs). However, the lattice mismatch and miscibility between GaN and InN, the diffusion and segregation of indium in MQW due to high-temperature growth, change the potential distribution in MQW, results in degradation of the optical properties of devices. Therefore, if a nondestructive evaluation method of the potential change in the MQW structure can be realized, it can greatly contribute to the improvement of LDs and LEDs with MQWs. We have been conducting research using laser-induced terahertz (THz) emission as an evaluation method for semiconductor surfaces and interfaces [1, 2]. In this study, we observed THz emissions from InGaN/GaN MQWs excited by a femtosecond (fs) laser and investigated generation mechanism of THz waves based on detailed characterization of the THz emissions with the aim of developing it as an evaluation method of MQW in the future.

Metallic spintronic heterostructures consisting of ultrathin ferromagnetic (FM) and non-ferromagnetic (NM) layers emit THz radiation through a distinct and efficient THz generation mechanism, which arises from the conversion of spin current generated in the FM layers into transverse charge current in the NM layers [1, 2]. We have previously demonstrated that an optimized bilayer of 2-nm Fe and 3-nm Pt epitaxial grown on 500-μm MgO substrate is a remarkably versatile spintronic THz emitter, as it exhibited fairly the same THz emission efficiency at 780-nm and 1550-nm pulsed excitation wavelengths [2]. In this work, we investigated spintronic THz generation when the Fe/Pt bilayer is epitaxial grown on 500-μm Si substrate. As with the Fe/Pt on MgO, the THz emission properties of the Fe/Pt on Si were probed by standard THz time-domain emission spectroscopy using a mode-locked femtosecond (fs) fiber laser source (pulse duration: ~100-fs; repetition rate: ~100 MHz; central wavelengths: ~780 nm, ~1550 nm).

In birefringent fiber lasers, a new type of optical solitons involving two orthogonal polarization components were found, which are termed as vector solitons [1]. Due to the potential applications in pulse shaping and microwave photonics, many optical methods were proposed to synthesize Gaussian monocycle pulses (GMPs) and Gaussian doublet pulses (GDPs), which have the shapes of the first and second derivatives of Gaussian functions respectively [2]. In this paper, we demonstrate that GMPs and GDPs can be generated in a ring-cavity erbium-doped fiber laser (EDFL) with polarization-locked vector solitons. We further employ qualitative analysis for the interactions of system parameters and their combined effects on the pulse shaping, by solving the coupled complex Ginzburg-Landau equations (CGLEs) with sech- and tanh-based functions.

Conventional ghost imaging aims at reconstructing any given object by means of spatially correlated photons. Our work highlights the importance of the selection of the quantum states that are used for such reconstructions, showing how the object can be rotated, or even doubled, by properly engineer the spatial correlations each of the photons possess.

The effects of squeezing have been of great importance in quantum computing [1], as evidenced in the studies of squeezed light presented in the literature [2]. The present work discusses on the possibility of enhancing squeezing using nonlinear coupler utilizing different operating frequencies of the input modes. In our two-channel model, one of the channels contains the fundamental mode, whereas the other one confines the next two higher-order modes. Here we examine numerically the effect of frequency mismatch on the dynamics of the generated squeezed light.

Weak measurement for orbital angular momentum (OAM)-induced spatial shifts of vortex beams are still confusing. The beam shift will be amplified by selecting the appropriate pre-selected and post-selected states. Here the maximum shifts are theoretically predicted and experimentally verified with optimal preselection and postselection states.