Tsuneyuki Ozaki

Frequency domain optical parametric amplification

General restrictions arising from gain-narrowing and phase-matching are circumvented by employing parametric amplification in the frequency rather than the time domain. Frequency-domain OPA has been used for amplifying few-cycle pulses and for high gain amplification.

Excitation of a high-Q subradiant resonance mode in mirrored single-gap asymmetric split ring resonator terahertz metamaterials

We propose a mirrored arrangement of asymmetric single split ring resonators (ASRs) that dramatically enhances the quality factor of the inductive-capacitive resonance. In a regular non-mirrored arrangement, the surface current modes are all oriented in phase. Hence, light scattered by individual ASRs interferes constructively. In contrast, the proposed configuration sustains surface currents that are oppositely oriented for neighboring ASRs, in turn leading to the cancellation of the net dipole moment accompanied by destructive interference of the scattered fields. The proposed arrangement holds promise to suppress radiation losses in terahertz, microwave and infrared plasmonic metamaterials.

Excitation of multiple trapped-eigenmodes in terahertz metamolecule lattices

We report on the excitation of sharp Fano-like resonances in lattices of metamolecules composed of two differing types of metaatoms. The proposed structures exhibit modes originating from the individual metaatoms as well as a very sharp mode from the collective excitation of the metamolecule lattice as a whole. Next-generation thin film sensors (e.g., for bio/chemical hazard detectors) could especially benefit from such artificial materials. Having multiple modes at different spectral positions enables the characterization of dispersive materials, while the high Q-factors of the eigenmodes lead to a very high sensitivity.

Membrane metamaterial resonators with a sharp resonance: A comprehensive study towards practical terahertz filters and sensors

We investigate the resonant properties of high quality-factor membrane-based metamaterial resonators functioning in the terahertz regime. A number of factors, including the resonator geometry, dielectric loss, and most importantly the membrane thickness are found to extensively influence the resonance strength and quality factor of the sharp resonance. Further studies on the membrane thickness-dependent-sensitivity for sensing applications reveal that high quality-factor membrane metamaterials with a moderate thickness ranging from 10 to 50 μm are the most promising option towards developing realistic integrated terahertz filters and sensors.

Terahertz Faraday rotation in a magnetic liquid: High magneto-optical figure of merit and broadband operation in a ferrofluid

We report on the demonstration of a high figure of merit (FOM) Faraday rotation in a liquid in the terahertz (THz) regime. Using a ferrofluid, a high broadband rotation (11 mrad/mm) is experimentally demonstrated in the frequency range of 0.2–0.9 THz at room temperature. Given the low absorption of the liquid, a high magneto-optical figure of merit (5-16 rad.cm/T) is obtained.

Toward High-Power Terahertz Emitters Using Large Aperture ZnSe Photoconductive Antennas

We investigate the generation of free-space terahertz (THz) radiation from photoconductive antennas using single crystal and polycrystalline ZnSe substrates. The photoconductive antennas have been excited above (400 nm) and below (800 nm) the bandgap. We investigate the dependence of the THz electric field radiated from biased ZnSe emitters on the applied bias field and on the incident optical fluence for bias fields as high as 26 kV/cm and for optical fluences of 0.005-3 mJ cm-2. The saturation fluence is observed to be significantly different both above and below bandgap excitation. These results show that, in comparison with GaAs substrates, ZnSe has strong potential as a high-power THz emitter.

Intense terahertz generation at low frequencies using an interdigitated ZnSe large aperture photoconductive antenna

We demonstrate the generation of intense THz pulses at low frequencies, and THz pulse shaping, using a ZnSe interdigitated large aperture photoconductive antenna. We have experimentally measured a THz pulse energy of 3.6 ±0.8 μJ, corresponding to a calculated peak THz electric field of 143 ± 17 kV/cm. We also used a binary phase mask instead of a traditional shadow mask with our interdigitated photoconductive antenna, which allows us to generate THz field profiles that range from a symmetric single-cycle THz pulse to an asymmetric half-cycle THz pulse.

High-order harmonic generation from a solid surface plasma by using a picosecond laser

Investigations have been performed on harmonic generation from solid-vacuum interfaces using a prepulse-free 2.2-ps 1054-nm laser pulse at maximum intensities of /spl sim/1/spl times/10/sup 15/ W/spl middot/cm/sup -2/. The harmonic intensities were strongest for p-polarized pump and dropped to about 50% in the case of s-polarized pumps. The ratio I/sub 4w/I/sub 2w/ between the intensities of the fourth and second harmonics was 0.25 at I/spl lambda//sup 2/=6/spl times/10/sup 14/ W/spl middot/cm/sup -2//spl middot//spl mu/m/sup 2/, which is relatively large when compared with the value predicted by simulations. The harmonics are collimated in the specular direction, with a transverse mode and divergence comparable to that of the driving laser.

Nd:glass and Ti:sapphire hybrid short pulse laser system

Abstract A Nd:glass and Ti:sapphire hybrid laser system capable of producing 0.2-J/1-ps pulses has been developed. Gain narrowing is not observed after the 10 3 times amplification by the Nd:glass amplifier, which shows the potentiality of producing 1-J/1-pspulses from a Ti:sapphirc and Nd:glass hybrid laser system.

Time-Resolved Terahertz Spectroscopy of Free Carrier Nonlinear Dynamics in Semiconductors

Nonlinear dynamics of free carriers in direct bandgap semiconductors at terahertz (THz) frequencies is studied using the intense few-cycle source available at the Advanced Laser Light Source (ALLS). Techniques such as Z-scan and optical-pump/THz-probe are employed to explore nonlinear interactions in an n-doped InGaAs thin film and a photoexcited GaAs sample, respectively. The physical mechanism that gives rise to such interactions is found to be intervalley scattering. A simple Drude-based mathematical model that incorporates the intervalley scattering process is developed and agrees well with the THz response of free carriers in semiconductors.