Wavefront control of subcycle vortex pulses via carrier-envelope-phase tailoring.

The carrier-envelope phase (CEP) of an ultrashort laser pulse is becoming more crucial to specify the temporal characteristic of the pulse's electric field when the pulse duration becomes shorter and attains the subcycle regime; here, the pulse duration of the intensity envelope is shorter than one cycle period of the carrier field oscillation. When this subcycle pulse involves a structured wavefront as is contained in an optical vortex (OV) pulse, the CEP has an impact on not only the temporal but also the spatial characteristics owing to the spatiotemporal coupling in the structured optical pulse. However, the direct observation of the spatial effect of the CEP control has not yet been demonstrated.

100-mJ class, sub-two-cycle, carrier-envelope phase-stable dual-chirped optical parametric amplification.

Based on dual-chirped optical parametric amplification (DC-OPA) and type-I BiBO (BiBO) crystals, the generation of >100 mJ, 10.4 fs, 10 Hz, carrier-envelope phase (CEP)-stable laser pulses, which are centered at 1.7 µm, was demonstrated producing a peak power of 10 TW.

Carrier-envelope phase control of synthesized waveforms with two acousto-optic programmable dispersive filters.

We demonstrate the scanning and control of the carrier-envelope phases (CEPs) of two adjacent spectral components totally spanning more than one-octave in the short-wave infrared (SWIR) wavelength region by operating two individual acousto-optic programmable dispersive filters (AOPDFs) applied to each of the two spectral components. The total CEP shift of the synthesized sub-cycle pulse composed of the two spectral components is controlled with simultaneous scans of the two CEPs. The resultant error of the controlled CEP was 642 mrad, so that this technique is useful for searching zero CEP of the synthesized pulse with the maximum field amplitude.

Adaptive optics with spatio-temporal lock-in detection for temporal focusing microscopy.

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.

Opening a new route to multiport coherent XUV sources via intracavity high-order harmonic generation.

High-order harmonic generation (HHG) is currently utilized for developing compact table-top radiation sources to provide highly coherent extreme ultraviolet (XUV) and soft X-ray pulses; however, the low repetition rate of fundamental lasers, which is typically in the multi-kHz range, restricts the area of application for such HHG-based radiation sources. Here, we demonstrate a novel method for realizing a MHz-repetition-rate coherent XUV light source by utilizing intracavity HHG in a mode-locked oscillator with an Yb:YAG thin disk laser medium and a 100-m-long ring cavity. We have successfully implemented HHG by introducing two different rare gases into two separate foci and picking up each HH beam.

Optical parametric amplification of sub-cycle shortwave infrared pulses.

Few-cycle short-wave infrared (SWIR) pulses are useful tools for research on strong-field physics and nonlinear optics. Here we demonstrate the amplification of sub-cycle pulses in the SWIR region by using a cascaded BBO-based optical parametric amplifier (OPA) chain. By virtue of the tailored wavelength of the pump pulse of 708 nm, we successfully obtained a gain bandwidth of more than one octave for a BBO crystal.

Fully stabilized multi-TW optical waveform synthesizer: Toward gigawatt isolated attosecond pulses.

A stable 50-mJ three-channel optical waveform synthesizer is demonstrated and used to reproducibly generate a high-order harmonic supercontinuum in the soft x-ray region. This synthesizer is composed of pump pulses from a 10-Hz repetition-rate Ti:sapphire pump laser and signal and idler pulses from an infrared two-stage optical parametric amplifier driven by this pump laser. With full active stabilization of all relative time delays, relative phases, and the carrier-envelope phase, a shot-to-shot stable intense continuum harmonic spectrum is obtained around 60 eV with pulse energy above 0.

Optimization of a multi-TW few-cycle 1.7-µm source based on Type-I BBO dual-chirped optical parametric amplification.

This paper presents the optimization of a dual-chirped optical parametric amplification (DC-OPA) scheme for producing an ultrafast intense infrared (IR) pulse. By employing a total energy of 0.77 J Ti:sapphire pump laser and type-I BBO crystals, an IR pulse energy at the center wavelength of 1.

Selective Resonance Photoionization of Odd Mass Zirconium Isotopes Towards Efficient Separation of Radioactive Waste.

We achieve a considerable improvement in proposed schemes for the selective photoionization of odd mass zirconium isotopes. The technique implements intermediate-state alignment for isotope-selective laser excitation by broadband pulsed lasers, which incorporates the spectroscopic selection rules for the absorption of polarized light. The improvement includes newly found intermediate levels, where J = 0 character as a third excited-state intermediate, in cooperation with four-step photoexcitation (J = 2-1-1-0 scheme).

3D Biomimetic Chips for Cancer Cell Migration in Nanometer-Sized Spaces Using "Ship-in-a-Bottle" Femtosecond Laser Processing.

Cancer cells undergo dramatic morphology changes when migrating in confined spaces narrower than their diameter during metastasis, and thus it is necessary to understand the deformation mechanism and associated molecular events in order to study tumor progression. To this end, we propose a new biochip with three-dimensional (3D) polymer nanostructures in a closed glass microfluidic chip. "Ship-in-a-bottle" femtosecond laser processing is an exclusive technique to flexibly create 3D small details in biochips.

Towards a petawatt-class few-cycle infrared laser system via dual-chirped optical parametric amplification.

Expansion of the wavelength range for an ultrafast laser is an important ingredient for extending its range of applications. Conventionally, optical parametric amplification (OPA) has been employed to expand the laser wavelength to the infrared (IR) region. However, the achievable pulse energy and peak power have been limited to the mJ and the GW level, respectively.

Interferometric temporal focusing microscopy using three-photon excitation fluorescence.

Super-resolution microscopy has become a powerful tool for biological research. However, its spatial resolution and imaging depth are limited, largely due to background light. Interferometric temporal focusing (ITF) microscopy, which combines structured illumination microscopy and three-photon excitation fluorescence microscopy, can overcome these limitations.

Temporal focusing microscopy using three-photon excitation fluorescence with a 92-fs Yb-fiber chirped pulse amplifier.

Temporal focusing (TF) microscopy is a wide-field two-photon excitation fluorescence (2PEF) microscopy technique, the optical sectioning capability of which is lower than that of point-scanning 2PEF microscopy. Here we demonstrate TF microscopy using three-photon excitation fluorescence (3PEF), which enhances the optical sectioning capability. As an excitation light source for the 3PEF, we developed an Yb-fiber chirped pulse amplifier, which produces 92-fs 9.

Controllable alignment of elongated microorganisms in 3D microspace using electrofluidic devices manufactured by hybrid femtosecond laser microfabrication.

This paper presents a simple technique to fabricate new electrofluidic devices for the three-dimensional (3D) manipulation of microorganisms by hybrid subtractive and additive femtosecond (fs) laser microfabrication (fs laser-assisted wet etching of glass followed by water-assisted fs laser modification combined with electroless metal plating). The technique enables the formation of patterned metal electrodes in arbitrary regions in closed glass microfluidic channels, which can spatially and temporally control the direction of electric fields in 3D microfluidic environments. The fabricated electrofluidic devices were applied to nanoaquariums to demonstrate the 3D electro-orientation of (an elongated unicellular microorganism) in microfluidics with high controllability and reliability.

Real-time broadband terahertz spectroscopic imaging by using a high-sensitivity terahertz camera.

Terahertz (THz) imaging has a strong potential for applications because many molecules have fingerprint spectra in this frequency region. Spectroscopic imaging in the THz region is a promising technique to fully exploit this characteristic. However, the performance of conventional techniques is restricted by the requirement of multidimensional scanning, which implies an image data acquisition time of several minutes.

Sub-10-fs control of dissociation pathways in the hydrogen molecular ion with a few-pulse attosecond pulse train.

The control of the electronic states of a hydrogen molecular ion by photoexcitation is considerably difficult because it requires multiple sub-10 fs light pulses in the extreme ultraviolet (XUV) wavelength region with a sufficiently high intensity. Here, we demonstrate the control of the dissociation pathway originating from the 2pσu electronic state against that originating from the 2pπu electronic state in a hydrogen molecular ion by using a pair of attosecond pulse trains in the XUV wavelength region with a train-envelope duration of ∼4 fs. The switching time from the peak to the valley in the oscillation caused by the vibrational wavepacket motion in the 1sσg ground electronic state is only 8 fs.

Conical third-harmonic generation of optical vortex through ultrashort laser filamentation in air.

We experimentally generate third-harmonic (TH) vortex beams in air by the filamentation of femtosecond pulses produced in a lab-built Ti:sapphire chirped pulse amplifier. The generated TH beam profile is shown to evolve with increasing pump energy. At a sufficiently high pump energy, we observe a conical TH emission of the fundamental vortex and confirm that the conical radiation follows the conservation law for orbital angular momentum.

Wide-range narrowband multilayer mirror for selecting a single-order harmonic in the photon energy range of 40-70 eV.

An experimental demonstration of a wide-range narrowband multilayer mirror for selecting a single-order high-harmonic (HH) beam from multiple-order harmonics in the photon energy range between 40 eV and 70 eV was carried out. This extreme ultraviolet (XUV) mirror, based on a pair of Zr and AlSi multilayers, has a reflectivity of 20-35% and contrast of more than 7 with respect to neighboring HHs at angles of incidence from 10 to 56.9 degrees, assuming HHs pumped at 1.

Indirect high-bandwidth stabilization of carrier-envelope phase of a high-energy, low-repetition-rate laser.

We demonstrate a method of stabilizing the carrier-envelope phase (CEP) of low-repetition-rate, high-energy femtosecond laser systems such as TW-PW class lasers. A relatively weak high-repetition-rate (~1 kHz) reference pulse copropagates with a low-repetition-rate (10 Hz) high-energy pulse, which are s- and p-polarized, respectively. Using a Brewster angle window, the reference pulse is separated after the power amplifier and used for feedback to stabilize its CEP.

Direct observation of an attosecond electron wave packet in a nitrogen molecule.

Capturing electron motion in a molecule is the basis of understanding or steering chemical reactions. Nonlinear Fourier transform spectroscopy using an attosecond-pump/attosecond-probe technique is used to observe an attosecond electron wave packet in a nitrogen molecule in real time. The 500-as electronic motion between two bound electronic states in a nitrogen molecule is captured by measuring the fragment ions with the same kinetic energy generated in sequential two-photon dissociative ionization processes.

High-energy infrared femtosecond pulses generated by dual-chirped optical parametric amplification.

We demonstrate high-energy infrared femtosecond pulse generation by a dual-chirped optical parametric amplification (DC-OPA) scheme [Opt. Express19, 7190 (2011)]. By employing a 100 mJ pump laser, a signal pulse energy exceeding 20 mJ at a wavelength of 1.

Carrier-envelope phase stabilization of a 16 TW, 10 Hz Ti:sapphire laser.

We propose and demonstrate a simple and promising method for stabilizing the carrier-envelope phase (CEP) of a high-energy ultrashort pulse laser operating at a low repetition rate. The method was successfully applied to a Ti:sapphire laser operating at 10 Hz with 400 mJ pulse energy and 25 fs pulse duration (16 TW). The laser system consists of a 1 kHz front-end preamplifier and a 10 Hz back-end power amplifier.

Two-dimensional spatiotemporal focusing of femtosecond pulses and its applications in microscopy.

We demonstrate and theoretically analyze the two-dimensional spatiotemporal focusing of femtosecond pulses by utilizing a two-dimensional spectral disperser. Compared with spatiotemporal focusing with a diffraction grating, it can achieve widefield illumination with better sectioning ability for a multiphoton excitation process. By utilizing paraxial approximation, our analytical method improves the axial confinement ability and identifies that the free spectra range (FSR) of the two-dimensional spectral disperser affects the out-of-focus multiphoton excitation intensity due to the temporal self-imaging effect.

Settling time of a vibrational wavepacket in ionization.

The vibrational wavepacket of a diatomic molecular ion at the time of ionization is usually considered to be generated on the basis of the Franck-Condon principle. According to this principle, the amplitude of each vibrational wavefunction in the wavepacket is given by the overlap integral between each vibrational wavefunction and the ground vibrational wavefunction in the neutral molecule, and hence, the amplitude should be a real number, or equivalently, a complex number the phase of which is equal to zero. Here we report the observation of a non-trivial phase modulation of the amplitudes of vibrational wavefunctions in a wavepacket generated in the ground electronic state of a H₂⁺ molecular ion at the time of ionization.