Junji Yumoto

Size dependence of optical nonlinearity of CdSSe microcrystallites doped in glass

This paper investigates the size dependence of the effective nonlinear cross section σeff and the carrier recombination time on the size of CdS0.12Se0.88 microcrystallites with average radii of 10, 30, 50, and 100 A by degenerate four‐wave mixing (DFWM) experiments. The decay curves of the DFWM signal as a function of the probe delay time have fast and slow components. As the microcrystallite size decreases, the fast component becomes dominant and a carrier recombination time of 2 ps was observed in the 10‐A microcrystallites. The diffraction efficiency of DFWM is almost the same for all microcrystallites, that is, σeff has a weak size dependence. Smaller microcrystallites show smaller magnitudes of the third‐order nonlinear susceptibility ‖χ(3)‖, which is calculated from the measured effective nonlinear cross section and the carrier recombination time.

Microcrystallite size dependence of absorption and photoluminescence spectra in CdSxSe1−x‐doped glass

Absorption and photoluminescence peak shifts due to the quantum size effect are observed in CdSxSe1−x microcrystallites with average radii ranging from several angstroms to 100 A. For microcrystallite radii between 15 and 100 A, the observed peak shifts can be described using an effective mass of 0.46m0 (m0 is the free‐electron mass), which is 4.6 times as large as the reduced mass in CdS0.12Se0.88. When the radius is reduced to less than 15 A, the effective mass, which is estimated from the experimental results, increases. The discrepancy between the theoretical prediction and the obtained results is discussed.

Broadband difference frequency generation around phase-match singularity

It is shown that a wide tunability range of greater than 500 nm can be obtained in the 2μm band in an 18 mm long periodically poled lithium niobate chip with a single quasiphase-matching grating period at a constant temperature. This letter demonstrates the difference frequency generation bandwidth. A tunable bandwidth of over 130 nm is experimentally confirmed in the 2μm region. This broad bandwidth can be realized by means of the phase-match curve distortion caused by the phase-match singularity. The wavelength conversion temperature dependence is also shown for several quasiphase-matching grating periods.

Broadband quasi-phase-matched second-harmonic generation in a nonlinear photonic crystal

We analyze the properties of quasi-phase-matched second-harmonic generation (SHG) from a defect waveguide in photonic-crystal (PhC) slabs embedded in a periodically χ(2) inverted material with a collinear beam configuration. We show that by controlling the material dispersion with the structural dispersion of a defect waveguide in PhC slabs of infinite height, it is possible to realize a much wider frequency range for quasi-phase-matched SHG than without the PhC structure. Also, taking the fabrication of actual devices into consideration, we examine the case for PhC slabs of finite height and show that, although guided modes are index confined in the vertical direction (while they are confined by the PhC structure in the horizontal direction), the effect that the PhC structure has on the broadening of the frequency range remains and that the broadened range can be comparable to that for the PhC slabs of infinite height if appropriate structural parameters are taken.

Ultrafast photonic interfaces for storage networking using serial-to-parallel and parallel-to-serial conversion

We propose novel ultrafast photonic interfaces for use in storage networking based on all-optical serial-to-parallel and photonic parallel-to-serial conversion. We confirm their operation with 40-Gbit/s 16-bit optical data using compact modules and a potential bandwidth of over 100 Gbit/s. We realize a photonic random access memory (RAM) by sandwiching a CMOS RAM with these two interfaces and achieve the storage and read-out of 40-Gbit/s 16-bit optical data. We also discuss the advantages of the interfaces and their possible applications to storage networking such as the real-time remote back-up of huge quantities of data, disaster recovery and video communication systems.

Linking energy density with Morphology in laser grooving of sapphire

With the maturing of laser technologies, laser processing is presenting itself as an attractive alternative to traditional machining techniques due to its free-form, non-contact nature. It is believed to be especially relevant for hard and brittle materials, such as sapphire. However, due to the complex nature of the physical processes involved, especially for transparent materials, optimization of laser processing procedures often requires tedious amounts of trial-and-error work. In such regards, scaling relationships prove to be an invaluable resource from both a basic science and application standpoint, as they not only reveal physical relationships which aid in the development of theoretical models, but also reduce the experimental trial space for optimization.

Thick THz metamaterials fabricated by 3D printer for THz high-pass filter application

Terahertz (THz) technologies have been gaining a great deal of interests, but current THz optics is still facing various challenges in their functionality and performance. Metamaterials, which are artificial structures consisting of many periodical units smaller than the wavelength of electromagnetic waves, are attracting attention as a new tool for the development of THz optics because the optical properties of metamaterials can be controlled by appropriate structures. However, lithography techniques conventionally used to make metamaterials can only make quasi-two-dimensional thin-film structures with thicknesses much smaller than the wavelength of THz waves (typically several hundred micrometer). This is a serious limiting factor, as the three dimensionality of the structure is important to extend the functionality of metamaterials as optical components [1]. Recently, the 3D printer has gained attention as a means to fabricate 3D metamaterial because there is, in principle, no limitation to achievable thickness [2]. However, the typical resolution necessary to make THz metamaterial is smaller than several hundred micrometers, and this is still challenging for conventional metal 3D printers. In order to overcome this difficulty, we have developed a new 3D printing technology which can make metallic structures with sufficient resolution for making THz metamaterial. As a demonstration to show the advantage of our technology, we made a “thick” THz metamaterial with 1-mm thickness, which is larger than the typical THz wavelength. This device works as high-pass filter with a sharp cut-off frequency because of the thick characteristic. This presented 3D printing technology, enables to make 3D THz metamaterials without thicknesses limitation, and should bring innovation to the field of development of THz optics.

Extended solid-state three-step model for high-harmonic generation from periodic crystals

Solid-state materials have recently emerged as a new stage for strong-field physics. Since the first observation of high-harmonic generation (HHG) from solids by Ghimire et al. [1], its mechanism is under intensive discussion.

Fabrication of low loss waveguide using fundamental light of Yb-based femtosecond laser (Conference Presentation)

Laser direct writing of optical devices and circuits is attracted attention because of its ability of three-dimensional fabrication without any mask[1]. Recently, Yb-fiber or solid-state laser has been commonly used for fabrication in addition to traditional Ti:S laser. However, it is reported that waveguide cannot be fabricated in fused silica by using the fundamental light from Yb-based femtosecond laser[2]. Some groups reported on waveguide fabrication by using second-harmonic beam of such lasers[3], but wavelength conversion using nonlinear process has drawbacks such as destabilization of laser power and beam deformation by walk off. In this study, we investigated fabrication of low-loss waveguide in fused silica by using the fundamental beam (1030nm) from an Yb solid-state femtosecond laser with a pulse duration of 250 fs. The NA of focusing objective lens was 0.42. The fabricated waveguide was made to have a circular cross-section by shaping laser beam with a slit[4]. We fixed repetition rate to 150 kHz, and identified appropriate scan speed and pulse energy for fabrication of low loss waveguide. Waveguide fabricated with appropriate condition had a propagation loss of 0.2 dB/cm, and this is the first report on optical waveguides in a fused silica fabricated by femto-second laser pulses at a wavelength of 1030nm. [1]K. M. Davis, et. al., Opt. Lett 21, 1729(1996) [2]J. Canning, et. al., Opt. Mater. Express 1, 998(2011) [3]L. Shah, et. al., Opt. Express 13, 1999(2005) [4]M. Ams, et. al., Opt. Express 13, 5676(2005)

Coherent Photon Technology: Science to Innovation

Coherent photon technology, especially, coherent addition and Coherent Photon Ring Technology for generation of short pulses with a high peak intensity are presented. Some activities named “Innovative Center for Coherent Photon Technology” is also described and it is aimed at the technological transfer of cutting edge knowledge to industry is also included.