Shuntaro Tani

High-speed 100 MHz strain monitor using fiber Bragg grating and optical filter for magnetostriction measurements under ultrahigh magnetic fields

A high-speed 100 MHz strain monitor using a fiber Bragg grating, an optical filter, and a mode-locked optical fiber laser has been devised, whose resolution is ΔL/L∼10-4. The strain monitor is sufficiently fast and robust for the magnetostriction measurements of materials under ultrahigh magnetic fields generated with destructive pulse magnets, where the sweep rate of the magnetic field is in the range of 10-100 T/μs. As a working example, the magnetostriction of LaCoO3 was measured at room temperature, 115 K, and 7 ∼ 4.2 K up to a maximum magnetic field of 150 T. The smooth dependence on the squared magnetic field and the first-order transition were observed at 115 K and 7 ∼ 4.2 K, respectively, reflecting the field-induced spin state evolution.

High repetition pump-and-probe photoemission spectroscopy based on a compact fiber laser system

The paper describes a time-resolved photoemission (TRPES) apparatus equipped with a Yb-doped fiber laser system delivering 1.2-eV pump and 5.9-eV probe pulses at the repetition rate of 95 MHz. Time and energy resolutions are 11.3 meV and ∼310 fs, respectively, the latter is estimated by performing TRPES on a highly oriented pyrolytic graphite (HOPG). The high repetition rate is suited for achieving high signal-to-noise ratio in TRPES spectra, thereby facilitating investigations of ultrafast electronic dynamics in the low pump fluence (p) region. TRPES of polycrystalline bismuth (Bi) at p as low as 30 nJ/mm2 is demonstrated. The laser source is compact and is docked to an existing TRPES apparatus based on a 250-kHz Ti:sapphire laser system. The 95-MHz system is less prone to space-charge broadening effects compared to the 250-kHz system, which we explicitly show in a systematic probe-power dependency of the Fermi cutoff of polycrystalline gold. We also describe that the TRPES response of an oriented Bi(111)/HOPG sample is useful for fine-tuning the spatial overlap of the pump and probe beams even when p is as low as 30 nJ/mm2.

Beyond 500-kHz bandwidth piezo-electric transducers for GHz-comb applications

Lasers with high optical frequency stability have played a significant role in numerous applications including metrology and spectroscopy. A laser with better frequency stability allows shorter integration time and higher accuracy. To achieve this, one of the most crucial factors is the stability of the cavity length of the laser. The length of the cavity is always disturbed by environmental noises such as acoustic or vibrational ones, and we often utilize cavity length modulators to compensate it. Piezo-electric transducers (PZTs) and electro-optic modulators (EOMs) are commonly used for this purpose. PZTs typically work up to several kHz ranges, whereas EOMs have much broader feedback bandwidth (more than several hundred kHz) and lead to much higher stability. However, we cannot apply EOMs in many cases, such as laser cavities with high-power, high-quality factor or broadband spectrum, because EOMs are used in transmissive configurations. PZTs with a mirror can be used in such cases thanks to their reflective configurations. Therefore PZTs with broad feedback bandwidth have been in great demand. The bandwidth of a PZT is restricted by mechanical resonances lying around 10 kHz to several hundred kHz, which are caused by vibration coupling with a mirror mount. To damp these resonances, various structures of the mirror mount were demonstrated [1]. However, resonances lying over the 200-kHz region have not been damped, and the resulting feedback bandwidth has not exceeded 200 kHz as well.

Observing laser ablation dynamics with sub-picosecond temporal resolution

Laser ablation is one of the most fundamental processes in laser processing, and the understanding of its dynamics is of key importance for controlling and manipulating the outcome. In this study, we propose a novel way of observing the dynamics in the time domain using an electro-optic sampling technique. We found that an electromagnetic field was emitted during the laser ablation process and that the amplitude of the emission was closely correlated with the ablated volume. From the temporal profile of the electromagnetic field, we analyzed the motion of charged particles with subpicosecond temporal resolution. The proposed method can provide new access to observing laser ablation dynamics and thus open a new way to optimize the laser processing.

Magneto-optic modulator for high bandwidth cavity length stabilization

We propose a novel magneto-optical approach for the repetition frequency stabilization of optical frequency combs. We developed a Yb:fiber mode-locked laser with a fiber-based magneto-optic modulator used to stabilize one of the longitudinal modes to an optical reference with sub-hundred mrad residual phase noise. This modulator does not induce mechanical resonances and as such has the potential to achieve much broader feedback bandwidths than conventional modulators used for cavity length stabilization.

Magneto-optical modulator for precise cavity length control

We propose a novel magneto-optic approach for high-bandwidth phase stabilization of the cavity length of fiber-based frequency combs. Stabilizing the mode-locked laser to an optical reference resulted in sub-hundred mrad residual phase noise.

Multi-GHz mode-locked lasers and comb-resolved spectroscopy

A femtosecond optical frequency comb with a maximum repetition frequency of 15 GHz was developed and successfully applied to comb-resolved spectroscopy, as well as the manipulation of acoustic phonons in a silica fiber.