Sub-10-fs pulse generation from 10 nJ Yb-fiber laser with cascaded nonlinear pulse compression.

We demonstrate cascaded nonlinear pulse compression of a Yb-doped fiber laser. The system is based on two pulse compression stages with bare single-mode fiber (SMF) and ultra-high NA (UHNA) fibers combined with two pairs of chirped mirrors. The 10 nJ, 110 fs input pulses are compressed down to 9.1 fs at 90 MHz, revealing a broadband spectrum from 800 nm to 1350 nm. This technique provides a simple approach to sub-10-fs compact Yb-doped fiber lasers for a variety of applications.

Probing thermal dissipation dimensionality to laser ablation in the pulse duration range from 300 fs to 1 µs.

We propose a quantitative method to determine the thermal dissipation dimensionality to laser ablation. We derived an analytical expression for the melting condition due to a single pulse for arbitrary spot diameters and pulse durations, which explicitly contains the dimensionality of the thermal diffusion process. As a demonstration, we compared the analytical expression with multi-shot ablation thresholds measured over pulse durations of more than six orders of magnitude for copper. The result shows that the thermal dissipation processes dominate for pulse durations longer than 5 ps, while nonthermal processes begin to dominate for shorter pulse durations.

Coincidence measurements of two quantum-correlated photon pairs widely separated in the frequency domain.

Quantum correlation is a key concept characterizing the properties of quantum light sources and is important for developing quantum applications with superior performance. In particular, it enables photon pairs that are widely separated in the frequency domain, one in the visible region, the other in the infrared region, to be used for quantum infrared sensing without direct detection of infrared photons. Here, simultaneous multiwavelength and broadband phase matching in a nonlinear crystal could provide versatile photon-pairs source for broadband infrared quantum sensing. This paper describes direct generation and detection of two quantum-correlated photon pairs produced via simultaneous phase-matched processes in periodic crystals. These simultaneous photon pairs provide a correlated state with two frequency modes in a single pass. To confirm the correlation, we constructed an infrared-photon counting system with two repetition-synchronized fiber lasers. We performed coincidence measurements between two pairs, 980 nm and 3810 nm, and 1013 nm and 3390 nm, which yielded coincidence-to-accidental ratios of 6.2 and 6.5, respectively. We believe that our novel correlated light source with two separate pairs in the visible and infrared region complements a wide-range of multi-dimensional quantum infrared processing applications.

Real-time high-spectral-resolution mid-infrared spectroscopy with a signal-to-noise ratio of ten thousand.

We developed a mid-infrared spectroscopy system with high spectral resolution and a high signal-to-noise ratio using an extremely high-order germanium immersion grating. The spectroscopic system covers wavelengths from 3 to 5 µm and has a spectral resolution of 1 GHz with a single-shot bandwidth of 2 THz. We proposed a method of improving the signal-to-noise ratio and achieved a ratio of over 3000 with a data acquisition rate of 125 Hz in the presence of fluctuations in the light source and environment. A signal-to-noise ratio of 10,000 was achieved with 0.1-s integration for 100-µW mid-infrared light.

Ultrafast Excited State Dynamics of Forward and Reverse trans-cis Photoisomerization of Red-Light-Absorbing Indigo Derivatives.

Femtosecond time-resolved transient absorption (TRTA) spectroscopy was carried out to investigate the ultrafast excited state dynamics of both trans → cis and trans ← cis photoisomerization of red-light-absorbing indigo derivatives. For N,N'-bis(tert-butyloxycarbonylmethyl)indigo (tBOMI), the excited state lifetime of the trans-form was measured to be 41 ps while that of the cis-form was as short as 730 fs in acetonitrile (Acn). The excited state lifetime of trans-N,N'-dimethylindigo (DMI) in Acn was also measured to be as short as 10 ps. These values are much shorter than those of the blue-light-absorbing trans-forms of indigo derivatives such as N,N'-diacetylindigo (DAI) and thioindigo (ThI). The chromophore of indigo is composed of two pairs of electron donor and acceptor substituents conjugated in the shape of a letter "H" (so-called "H-chromophore"), although DFT and TDDFT calculations suggest that the charge transfer (CT) character is not very significant. Nevertheless, when a weak CT within the H-chromophore is promoted, the absorption band shifts to longer wavelengths and the excited state lifetime shortens. For the photoisomerization of DAI and ThI, a relatively long excited state lifetime is required for the photoisomerization, while for tBOMI and DMI, a vibrationally hot ground state that overcomes the energy barrier in the ground state is produced by rapid nonradiative decay through conical intersection. In the case of cis-tBOMI, the repulsion between the two adjacent negatively charged carbonyl groups and the weakening of the central C═C double bond in the S1 state twist the molecule, shorten the excited state lifetime, and increase the quantum yield of the trans ← cis photoisomerization.

Ultrafast laser ablation simulator using deep neural networks.

Laser-based material removal, or ablation, using ultrafast pulses enables precision micro-scale processing of almost any material for a wide range of applications and is likely to play a pivotal role in providing mass customization capabilities in future manufacturing. However, optimization of the processing parameters can currently take several weeks because of the absence of an appropriate simulator. The difficulties in realizing such a simulator lie in the multi-scale nature of the relevant processes and the high nonlinearity and irreversibility of these processes, which can differ substantially depending on the target material. Here we show that an ultrafast laser ablation simulator can be realized using deep neural networks. The simulator can calculate the three-dimensional structure after irradiation by multiple laser pulses at arbitrary positions and with arbitrary pulse energies, and we applied the simulator to a variety of materials, including dielectrics, semiconductors, and an organic polymer. The simulator successfully predicted their depth profiles after irradiation by a number of pulses, even though the neural networks were trained using single-shot datasets. Our results indicate that deep neural networks trained with single-shot experiments are able to address physics with irreversibility and chaoticity that cannot be accessed using conventional repetitive experiments.

Autonomous parameter optimization for femtosecond laser micro-drilling.

There is a strong need for a highly efficient method to find the optimal conditions to achieve a desired result in laser processing, oftentimes from a multidimensional parameter space. In this study, we adopted Bayesian optimization as an efficient statistical optimization method robust to the inherent variations observed in typical laser processing results. Specifically, the intensity and spatial beam profile of a femtosecond laser processing system were optimized according to results obtained from an in situ optical microscope observation. In this way, we show that the optimum set of parameters to achieve a desired shape can be obtained autonomously and more than an order of magnitude faster than with a simple grid-search.

Neural-network-assisted in situ processing monitoring by speckle pattern observation.

We propose a method to monitor the progress of laser processing using laser speckle patterns. Laser grooving and percussion drilling were performed using femtosecond laser pulses. The speckle patterns from a processing point were monitored with a high-speed camera and analyzed with a deep neural network. The deep neural network enabled us to extract multiple information from the speckle pattern without a need for analytical formulation. The trained neural network was able to predict the ablation depth with an uncertainty of 2 µm, as well as the material under processing, which will be useful for composite material processing.

Piezo-electric transducer actuated mirror with a servo bandwidth beyond 500 kHz.

We demonstrate a novel system that uses a piezoelectric transducer (PZT)-actuated mirror for laser stabilization. A combination of a simple mechanical design and electronic circuits is used to realize an ultra-flat frequency response, which enables an effective feedback bandwidth of 500 kHz. The PZT also performed well when used in a mode-locked laser with a GHz repetition rate, to which it is difficult to apply an electro-optic modulator (EOM).

Raman-assisted broadband mode-locked laser.

The pulse duration that is available from femtosecond mode-locked lasers is limited by the emission bandwidth of the laser crystals used. Considerable efforts have been made to broaden the emission gain bandwidth in these lasers over the past five decades. To break through this limitation, intracavity spectral broadening is required. Here, we propose a new spectral broadening method inside the mode-locked cavity based on use of stimulated Raman scattering and demonstrate significant pulse shortening using this method. We configured Kerr-lens mode-locked lasers based on Yb:CaGdAlO4, Yb:KY(WO4)2 and Yb:Y2O3 materials and achieved significant spectral broadening that exceeds the emission bandwidth. The spectral broadening in the Yb:CaGdAlO4 oscillator shortens the pulse duration to 22 fs, which is a one-third of the duration of our unbroadened mode-locked pulse. The results presented here indicate that Raman-assisted spectral broadening can break the limitations of the emission gain bandwidth and shorten the duration of pulses from femtosecond mode-locked lasers.

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.

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.

Ultrafast carrier dynamics in graphene under a high electric field.

We investigated ultrafast carrier dynamics in graphene with near-infrared transient absorption measurement after intense half-cycle terahertz pulse excitation. The terahertz electric field efficiently drives the carriers, inducing large transparency in the near-infrared region. Theoretical calculations using the Boltzmann transport equation quantitatively reproduce the experimental findings. This good agreement suggests that the intense terahertz field should promote a remarkable impact ionization process and increase the carrier density.