Published papers

We investigate multichannel dissociative ionization of ethanol in intense ultraviolet laser fields using a photoelectron-photoion coincidence momentum imaging technique. Product channels are clearly separated with detected photoions, and channel-specific photoelectron spectra exhibit multiple spectral components unambiguously assigned to four- and five-photon ionization to the electronic ground state and the electronically excited state. We measure kinetic energy distributions of fragment ions as a function of the energy of a correlated photoelectron to reveal how much energy is provided to the fragment recoil of fragment ions in the course of photoelectron emission and subsequent electronic excitation. Subsequent electronic excitation rather than photoelectron emission governs the internal energy of ethanol cations. The role of subsequent electronic excitation becomes more decisive as the laser intensity increases.

Visible lasers are sought for in a variety of applications. They are required in fields as diverse as medicine, materials processing, display and entertainment technology and many others. Moreover, in contrast to infrared lasers, they enable very simple and efficient access to the UV spectral range by a single frequency doubling step. Currently, the choice of direct visibly emitting lasers is limited: The ‘green gap’ prohibits the development of semiconductor lasers with emission in the green and yellow spectral range and only few laser active ions allow for efficient visible lasing. In particular trivalent praseodymium (Pr3+) and terbium (Tb3+) ions have been shown to be the most successful candidates for efficient high power visible solid-state lasers. Compared to semiconductor lasers, solid-state lasers also provide other advantages, e.g., in terms of energy storage in Q-switched operation as well as beam quality at high output power. In recent years, visibly emitting solid-state lasers have seen a revival enabled by the increasing commercial availability of GaN-based blue emitting pump diodes and an ever-increasing number of publications evidences the vivid research activities in this field. Still, due to the relatively short history of diode-pumped visible solid-state lasers, these are still in an early stage of their development and up to now only few direct visibly emitting solid-state lasers with comparably low output power are commercially available. However, we are convinced that visibly emitting solid-state lasers based on Pr3+ and Tb3+ have the potential for 100-W-class continuous wave output power levels as well as sub-ns pulse durations in Q-switched and sub-ps-pulse durations in mode-locked operation, which would qualify them to fulfil the requirements of most of the applications named above. In this work, we review the state of the art of continuous wave and pulsed visibly emitting solid-state lasers and amplifiers based on Pr3+ and Tb3+ as the active ion. After an introduction, we briefly review the spectroscopic properties of these two ions and their particularities for laser operation as well as the requirements for suitable host materials. In the third chapter, we present the state of the art in the field of continuous wave Pr3+-lasers emitting in the cyan-blue, green, orange, red, and deep-red spectral range based on fluoride, glass, and oxide host materials and discuss prospects for further power scaling. The fourth chapter is devoted to the current state of Tb3+-based continuous wave green and yellow emitting solid-state lasers. In the fifth and sixth chapter we give an overview over existing pulsed visibly emitting solid-state lasers in Q-switched and mode-locked operation mode, respectively. Finally, the seventh chapter is devoted to pulse amplifiers for ultrafast visible lasers before this review closes with a short conclusion.

Multi-photon holographic patterned illumination using temporal focusing (TF) pulses provides 10 μm multi-spots with an axial confinement of 5 μm, enabling parallel photo-stimulation of multi-target cells in three dimensions. However, interference fringes and speckles degrade the uniformity of the spots, causing spatio-temporal fluctuations in photoactivation. Here, we demonstrate fringe- and speckle-free holographic patterned illumination using multifocal time-multiplexed (TM) TF pulses even when the multi-spots overlap spatially. The uniform patterned illumination enabled the expansion of the field-of-view of TM-TF microscopy by more than fivefold and enhanced the axial resolution to 1.3 μm.

© 2018 The Japan Society of Applied Physics. We directly pumped a Ti:sapphire laser by combining 478 and 520 nm laser diodes to prevent the effect of absorption loss induced by the pump laser of shorter wavelengths (>450 nm). We obtain a continuous-wave output power of 660 mW at a total incident pump power of 3.15 W. We demonstrate mode locking using a semiconductor saturable absorber mirror, and 126 fs pulses were obtained at a repetition rate of 192 MHz. At the maximum pump power, the average output power is 315 mW. Shorter mode-locked pulses of 42 and 48 fs were respectively achieved by Kerr-lens mode locking with average output powers of 280 and 360 mW at a repetition rate of 117 MHz.

Temporal focusing (TF) microscopy has a trade-off between temporal resolution, optical sectioning capability, and field of view. To improve this trade-off, we demonstrate time-multiplexed multi-line temporal focusing (TM-ML-TF) microscopy, which combines the time-multiplexed TF technique with multi-line focusing using a digital micromirror device. Compared with those for TF microscopy, the optical sectioning capability and the signal-to-background ratio were improved by factors of 2.5 and 4.17, respectively. In addition, by applying post-processing for structured illumination microscopy to TM-ML-TF microscopy, the optical sectioning capability and the signal-to-background ratio were further enhanced by factors of 1.2 and 4.63, respectively.

We experimentally investigate crystals doped with tetravalent chromium or divalent cobalt as saturable absorbers for passive Q-switching of visible solid-state lasers. The recovery time of the ground-state and excited-state absorption cross sections are determined by pump-probe and Z-scan measurements, respectively. We provide saturation intensities, useful wavelength ranges of the investigated materials, and advices to realize passive Q-switching of visible lasers using these crystals as saturable absorbers.

Wavefront distortion in temporal focusing microscopy (TFM) results in a distorted temporal profile of the excitation pulses owing to spatio-temporal coupling. Since the pulse duration is dramatically changed in the excitation volume, it is difficult to correct the temporal profile for a thick sample. Here, we demonstrate adaptive optics (AO) correction in a thick sample. We apply structured illumination microscopy (SIM) to an AO correction in wide-field TFM to decrease the change in the pulse duration in the signal detection volume. The AO correction with SIM was very successful in a thick sample for which AO correction with TFM failed.

As a method of quantum pulse gating in a quantum simulator, we experimentally demonstrate arbitrary mixing of multimode quantum states prepared in the frequency domain.

希ガス中空ファイバで広帯域化された超広帯域フェムト秒レーザーの短波長帯と長波長帯を周波数チャープさせて時間遅延をつけて重畳することで、type-I　BBO結晶で差周波発生を行うことで、中心周波数およびバンド幅可変の中赤外レーザー発生が可能であることを実験から明らかにし、さらにOPAで増幅し分光応用可能な出力レベルを確保できることを実証した。

© OSA 2018 We present a detailed characterization of Cr 4+ and Co 2+ -doped oxides as saturable absorbers for the visible spectral region. The first visibly emitting Q-switched laser utilizing a Co 2+ :Gd3Ga5O12 saturable absorber is demonstrated.

© 2018 Optical Society of America. We demonstrate the high-power continuous-wave operation of a Pr3:YLF laser end pumped by blue laser diodes. As the pump source, we used four 5 W single-emitter blue laser diodes and a >20 W fiber-coupled module. In single-emitter-diode pumping, we obtained pump-limited output powers of 6.7 and 3.7 W at 640 and 607 nm, respectively. We successfully suppressed thermal aberration and obtained an output power of 3.4 W at a slope efficiency of 25% at 640 nm with good beam quality in the fiber-coupled module pumping.

© 2018, Springer International Publishing AG. Isolated attosecond pulse (IAP) generation with pulse energy of a few μJ has been reported in the 40-nm wavelength region. For diffraction-limited focusing on the IAP, we are developing a Schwarzschild objective made of two-curved multilayer mirrors. To generate intense light fields with a maximum intensity of over 10 16 W/cm 2 on the objective focus, we designed, fabricated, and tested multilayer mirrors for practical high reflectivity in the 40-nm wavelength region.

© OSA 2018 We demonstrate a continuous-wave Pr 3+ :YLF laser pumped by blue laser diodes, and obtain the output power of 6.7 and 3.7 W at 640 and 607 nm, respectively. The Pr 3+ :YLF laser is extended to passive Q-switching by utilizing a Co 2+ :MgAl2O4 spinel saturable absorber and its intracavity second harmonic generation at DUV.

© 2020 Optical Society of America We demonstrated the regenerative amplification of picosecond laser pulses generated by a gain-switched laser diode at 640 nm with a Pr:YLF crystal that was continuously pumped by a multimode blue laser diode. A 0.7-pJ seed pulse energy sufficiently suppressed the self-oscillation in the amplifier. The amplified pulse energy reached 33 µJ at a repetition rate of 10 kHz. The spatial beam quality was nearly TEM00. We also demonstrated second- and third-harmonic generation and obtained 320- and 213-nm pulse energies of 18 and 0.83 µJ at 10 kHz.

© 2019 Optical Society of America We demonstrated regenerative laser pulse amplification at 640 nm for the first time, to the best of our knowledge, with a mode-locked Pr3-doped LiYF4 (Pr3:YLF) oscillator as a picosecond seed pulse. A regenerative amplifier with a Pr3:YLF crystal was continuously pumped by a multi-mode InGaN diode laser. At an absorbed pump power of 3.1 W, we obtained amplified pulse energy of 13 μJ at 10 kHz with an excellent spatial beam quality of M 2∼1.1, demonstrated second-harmonic generation, and obtained a 320 nm pulse energy of 5.9 μJ.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement. We demonstrated the power scaling of a passively Q-switched Pr3+:LiYF4 (YLF) laser at 523, 607, and 640 nm with a Co2+:MgAl2O4 (MALO) saturable absorber and analyzed the experimental results with a numerical simulation based on rate equations. A maximum pulse energy of 33.5 µJ was obtained with a pulse width of 30.9 ns and a repetition rate of 64.0 kHz at 640 nm. We demonstrated ultraviolet pulse generation at 261 and 320 nm by intracavity frequency doubling and obtained 63.3-ns pulses with a pulse energy of 7.0 µJ and a repetition rate of 64.6 kHz at 320 nm as well as 356-ns 261-nm pulses with a pulse energy of 0.2 µJ at a repetition rate of 82.0 kHz. To the best of our knowledge, a 261-nm pulse is the shortest wavelength laser directly generated from the intracavity second-harmonic generation of passively Q-switched lasers.

© 2019 Optical Society of America We report the construction of diagnostics for ultrafast surface plasmon polariton (SPP) pulses that evolve spatiotemporally in femtosecond and nanometer scales. We constructed two types of scanning near-field optical microscopes (SNOMs) and verified that the temporal waveform of ultrafast SPP pulses can be measured by combining spectral interferometry (SI). In the illumination-collection (I-C) mode SNOM, which uses a single fiber probe to excite samples and collect optical responses, a lock-in detection scheme using a lock-in camera detects SI fringes even for extremely weak signal light pulses. With this I-C SI-SNOM scheme, we measured the complex plasmon response functions of gold (Au) nanorods on Ge2Sb2Te5 thin film, both in the crystal and amorphous phases. For a dual-probe SNOM (DSNOM), a dual-band modulation technique was introduced to independently control the probe-sample and probe–probe distances. With the DSNOM and by employing femtosecond SPP pulse excitation, we successfully measured the temporal waveform of an ultrafast SPP pulse that is propagating on an Au thin layer.

© 2020 OSA - The Optical Society. All rights reserved. We investigated whether Pr:YAlO3 and Pr:Y3Al5O12 (YAG) can work as gain media for high-power visible lasers and replace trivalent praseodymium (Pr)-doped fluoride crystals, with particular focus on thermal loading resistivity. Pr:YAlO3 exhibits a high laser gain at 747 nm, and we obtained a maximum output power of 1.2 W and a slope efficiency of 26.7% with high-power GaN laser diode pumping. Excited state absorption and large phonon energy hinder laser oscillation of Pr:YAG at room temperature. We obtained 616 nm laser oscillation of Pr:YAG at 40 K. Furthermore, we achieved a visible laser with Pr:YAG ceramics for the first time. The maximum output power is ∼30 mW with a slope efficiency of ∼0.7%.

We used ultrafast surface plasmon polariton (SPP) pulses that were focused into several tens of nm at the apex of a tapered metal tip to induce photoreduction of graphene oxide (GO), and we successfully fabricated nano graphene stripes with a minimum width of ∼200 nm. GO was reduced using about 10 shots of SPP pulse irradiation. We evaluated the GO reduction with selective in situ coherent anti-Stokes Raman scattering measurements using spectrally focused SPP pulses. 10

We experimentally demonstrate programmable multimode phase-sensitive amplification multiplexed in the frequency domain for flexible control of parallelly generated squeezed states. We utilize the unique phase-matching condition of a type-II periodically poled potassium titanyl phosphate (PPKTP) crystal and pulse shaping technique to fully control the frequency-domain parallel generation of squeezed states in the optical telecommunication band. We experimentally verify that the independent programmability of phase-sensitive optical parametric amplification (OPA) for the modes corresponding to different frequency bands can be achieved by shaping the pump laser pulse from optical parametric gain measurements using a coherent probe light generated by a degenerate synchronously pumped optical parametric oscillator.