Eiichi Matsubara

Generation of 2.6 fs optical pulses using induced-phase modulation in a gas-filled hollow fiber

We demonstrate the first pulse compression of ultrabroadband white-light continuum generated using both induced- and self-phase modulations in an Ar-gas-filled hollow fiber. By feedback chirp compensation with a liquid crystal spatial light modulator and a modified spectral interferometry for direct electric-field reconstruction, 2.6-fs, 1.4-GW, 1.3-cycle transform-limited pulses are generated in the visible to near-infrared region.

Ultrabroadband coherent electric field from far infrared to 200 THz using air plasma induced by 10 fs pulses

We generated an ultrabroadband infrared pulse ranging from far infrared to 200 THz through a plasma by focusing a hollow-fiber compressed intense 10-fs pulse and its second harmonic in air. We coherently detected the signal up to 100 THz with electro-optic sampling and clarified its drastic dependence on the orientation of the second harmonic crystal in a range of 100–200 THz with an HgCdTe detector. From these, we confirmed the whole frequency components originated from the AC biased plasma and were phase locked. This result opens the possibility of a pump-probe spectroscopy which covers the whole infrared range.

Generation of ultrashort optical pulses using multiple coherent anti-Stokes Raman scattering in a crystal at room temperature

We demonstrate Fourier synthesis of multiple coherent anti-Stokes Raman scattering signals in a LiNbO3 crystal at room temperature. The signals up to the 20th order (470–800nm) were generated by two crossing femtosecond Ti:sapphire laser pulses. Angle dispersion of the signals was compensated into one white-continuum beam by modifying a conventional 4f configuration. Spectral phase of the signal was measured by spectral phase interferometry for direct electric-field reconstruction. Isolated pulses with 25fs duration at 1kHz were generated only by appropriately aligning the angle-dispersion compensator. This result opens the possibility of the generation of subfemtosecond pulses in the visible region.

High-efficiency terahertz pulse generation via optical rectification by suppressing stimulated Raman scattering process

We demonstrate high-efficiency terahertz pulse generation via optical rectification in LiNbO3. We avoid the spectral broadening of an excitation pulse via the stimulated Raman scattering process by the bandwidth control of the excitation pulse and achieve the highest efficiency of 0.21% for energy conversion into THz pulse.

Generation of ultrashort optical pulses in the 10 fs regime using multicolor Raman sidebands in KTaO3.

We demonstrated Fourier synthesis of a new type of multiple coherent anti-Stokes Raman scatterings in KTaO(3) crystal. The signals with a wavelength range of 500-750 nm were generated by two femtosecond pulses whose frequency difference was set to be a two-phonon Raman frequency. Angle dispersion of the signals was compensated into one white-continuum beam by spherical mirrors and a prism. The spectral phase, measured by spectral phase interferometry for direct electric-field reconstruction, was compensated for by carefully aligning the angle and the position of the optical components so that 13 fs isolated pulses were generated. This result shows the robustness of our scheme to produce ultrashort optical pulses.

Dynamical Symmetry Breaking Induced by Ultrashort Laser Pulses in KTaO3

Raman selection rules were observed to be violated dynamically under resonant and coherent pumping of the Raman-active two-phonon mode in the cubic perovskite-type oxide KTaO 3 . In this study, two...

Generation and Detection of THz Pulses With a Bandwidth Extending Beyond 4 THz Using a Subpicosecond Yb-Doped Fiber Laser System

We experimentally demonstrate the generation and detection of broadband THz pulses with a bandwidth extending beyond 4 THz by using a 1.04- μm Yb-doped fiber laser with a pulse duration of 0.48 ps. Owing to higher order nonlinear processes (such as the cascaded χ(2) process and χ(3) process) in LiNbO3 crystals, THz pulses can be emitted efficiently with a bandwidth extending beyond the boundaries of the spectral width of the excitation pulse. By compressing divided sampling pulses to as short as 80 fs, we detected the THz pulse in the full spectral range, which could not be resolved with the original pulse. The precise control of the nonlinear effects and the compression of the sampling pulses compensated for the disadvantage of using a 1- μm fiber laser. This work will contribute to the progress of nonlinear THz spectroscopy.

Intrinsic carrier multiplication efficiency in bulk Si crystals evaluated by optical-pump/terahertz-probe spectroscopy

We estimated the carrier multiplication efficiency in the most common solar-cell material, Si, by using optical-pump/terahertz-probe spectroscopy. Through close analysis of time-resolved data, we extracted the exact number of photoexcited carriers from the sheet carrier density 10 ps after photoexcitation, excluding the influences of spatial diffusion and surface recombination in the time domain. For incident photon energies greater than 4.0 eV, we observed enhanced internal quantum efficiency due to carrier multiplication. The evaluated value of internal quantum efficiency agrees well with the results of photocurrent measurements. This optical method allows us to estimate the carrier multiplication and surface recombination of carriers quantitatively, which are crucial for the design of the solar cells.

Coherent transitions between the shallow acceptor levels in germanium using intense THz pulses

Nonlinear transmission spectroscopy was performed on a doped Ge:Ga semiconductor using intense THz pulses with different cycle numbers. When single-cycle pulses were used, non-perturbative phenomena, such as the ionization of shallow impurities, competed with the conventional coherent transition, whereas the coherent transition was dominant when multi-cycle pulses were used.

Sensitive monitoring of photocarrier densities in the active layer of a photovoltaic device with time-resolved terahertz reflection spectroscopy

We demonstrate the sensitive measurement of photocarriers in an active layer of a GaAs-based photovoltaic device using time-resolved terahertz reflection spectroscopy. We found that the reflection dip caused by Fabry-Perot interference is strongly affected by the carrier profile in the active layer of the p-i-n structure. The experimental results show that this method is suitable for quantitative evaluation of carrier dynamics in active layers of solar cells under operating conditions.