Mechanism of Single-Cycle THz Pulse Generation and X-ray Emission: Water-Flow Irradiated by Two Ultra-Short Laser Pulses.

The interaction of two subsequent ultra-short sub-milli-Joule laser pulses with a thin water flow results in an emission of a strong single-cycle THz pulse associated with enhanced soft X-ray emission. In this paper, a chain of processes produced in this interaction is analyzed and compared with other THz generation studies. It is demonstrated that the enhanced THz and X-ray emissions are produced by an energetic electron beam accelerated in the interaction of a main laser pulse with liquid water ejected from the surface by the pre-pulse.

Computational broadband imaging with laser-driven sequential light source arrays on a water film.

Imaging and computational processing fusion technologies have expanded the wavelength range that can be visualized. However, it is still challenging to realize a system that can image a wide range of wavelengths, including non-visible regions, in a single system. Here, we propose a broadband imaging system based on femtosecond-laser-driven sequential light source arrays.

Shockwave-based THz emission in air.

THz emission in air under the irradiation of a pair of tightly-focused femtosecond laser pulses (800nm, 35fs) with nanosecond time delay and micro-meter spatial offsets is studied with polarization-sensitive THz time-domain spectroscopy and time-resolved imaging. The pre-pulse irradiation induces air-breakdown at its focus, which results in the expansion of shockwave front traveling outward. When the main pulse irradiates such shockwave front far from the pre-pulse focus with nanosecond delay, THz emission intensity was enhanced up to ∼13-times and its linear polarization was aligned along the line between the two focus positions of the pre- and the main pulses which is parallel to the expansion direction of the shockwave front.

Feasibility of using bimetallic Au-Ag nanoparticles for organic light-emitting devices.

Matching the resonant wavelength of plasmonic nanoparticles (NPs) and the emission band of organic materials is critical for achieving optimal plasmon-enhanced luminescence in organic light-emitting devices (OLEDs). However, the spectral matching is often unsatisfactory because the interior architecture of OLEDs limits the dimensions of the NPs to support the desired wavelength adjustment. In this article, we proposed a design strategy via AuAg alloy NPs to enable resonance tuning while preserving the size of the NP to suit the OLED design requirements.

Surface plasmon-enhanced solution-processed phosphorescent organic light-emitting diodes by incorporating gold nanoparticles.

Organic light-emitting diodes (OLEDs) have attracted increasing attention due to their superiority as high quality displays and energy-saving lighting. However, improving the efficiency of solution-processed devices especially based on blue emitter remains a challenge. Excitation of surface plasmons on metallic nanoparticles has potential for increasing the absorption and emission from optoelectronic devices.

Enhancement of X-ray emission from nanocolloidal gold suspensions under double-pulse excitation.

Enhancement of X-ray emission was observed from a micro-jet of a nano-colloidal gold suspension in air under double-pulse excitation of ultrashort (40 fs) near-IR laser pulses. Temporal and spatial overlaps between the pre-pulse and the main pulse were optimized for the highest X-ray emission. The maximum X-ray intensity was obtained at a 1-7 ns delay of the main pulse irradiation after the pre-pulse irradiation with the micro-jet position shifted along the laser beam propagation.

Dual THz Wave and X-ray Generation from a Water Film under Femtosecond Laser Excitation.

Simultaneous emission of the THz wave and hard X-ray from thin water free-flow was induced by the irradiation of tightly-focused femtosecond laser pulses (35 fs, 800 nm, 500 Hz) in air. Intensity measurements of the THz wave and X-ray were carried out at the same time with time-domain spectroscopy (TDS) based on electro-optic sampling with a ZnTe(110) crystal and a Geiger counter, respectively. Intensity profiles of the THz wave and X-ray emission as a function of the solution flow position along the incident laser axis at the laser focus show that the profile width of the THz wave is broader than that of the X-ray.

Dynamic position shifts of X-ray emission from a water film induced by a pair of time-delayed femtosecond laser pulses.

Femtosecond double-pulsed laser excitation of a water film in air showed enhancements of X-ray intensity as compared with single pulse irradiation. The position of the highest yield of X-rays strongly depends on temporal separation between the pre-pulse and the main-pulse (energy ratios where ∼ 1 : 10). The strongest X-ray emission was observed at 10-15 ns delay of the main-pulse.

Photoacoustic signal enhancements from gold nano-colloidal suspensions excited by a pair of time-delayed femtosecond pulses.

Photoacoustic signal enhancements were observed with a pair of time-delayed femtosecond pulses upon excitation of gold nanosphere colloidal suspension. A systematic experimental investigation of photoacoustic intensity within the delay time, Δt = 0 to 15 ns, was carried out. The results revealed a significant enhancement factor of ∼2 when the pre-pulse energy is 20-30% of the total energy.

Regenerated volume gratings in PMMA after femtosecond laser writing.

Self-regeneration of volume gratings recorded inside polymethyl methacrylate (PMMA) after 70-100 days is demonstrated. First, volume gratings were made inside PMMA by femtosecond laser writing. The diffraction efficiency of the gratings reached the maximum-was regenerated-following an initial slow decrease within the first several days after the fabrication.

Femtosecond laser-induced hard X-ray generation in air from a solution flow of Au nano-sphere suspension using an automatic positioning system.

Femtosecond laser-induced hard X-ray generation in air from a 100-µm-thick solution film of distilled water or Au nano-sphere suspension was carried out by using a newly-developed automatic positioning system with 1-µm precision. By positioning the solution film for the highest X-ray intensity, the optimum position shifted upstream as the laser power increased due to breakdown. Optimized positioning allowed us to control X-ray intensity with high fidelity.

MHz-ultrasound generation by chirped femtosecond laser pulses from gold nano-colloidal suspensions.

Strong absorption of femtosecond laser pulses in Au nano-colloidal suspensions was used to generate coherent ultrasound signals at 1-20 MHz frequency range. The most efficient ultrasound generation was observed at negative chirp values and was proportional to the pulse duration. Maximization of a dimensionless factor A ≡ αctp defined as the ratio of pulse duration tp and the time required for sound at speed c to cross the optical energy deposition length (an inverse of the absorption coefficient α) given by 1/(αc).

Enhanced photoacoustics from gold nano-colloidal suspensions under femtosecond laser excitation.

Enhanced photoacoustic (PA) intensity from gold nanosphere and nanorod colloidal suspensions in water under tightly-focused femtosecond pulsed laser irradiation was systematically investigated. PA signal amplitudes were measured by ultrasound transducers at frequencies of 5, 10, and 25 MHz. The experimental results revealed a linear-dependence of the relative photoacoustic amplitude on the laser power and the mechanism was attributed to non-radiative relaxation dynamics of surface plasmon oscillations.

Polarization-independent guided-mode resonance filter with cross-integrated waveguide resonators.

A cavity-resonator-integrated guided-mode resonance filter (CRIGF) has been proposed and investigated in order to realize high-efficiency narrowband reflection with a small aperture. The CRIGF consists of a grating coupler integrated in a cavity resonator constructed by a pair of distributed Bragg reflectors on a thin-film waveguide. This time, orthogonally crossed integration of two CRIGFs was demonstrated in order to obtain polarization-independent reflection spectrum.

Cavity-resonator-integrated guided-mode resonance filter for aperture miniaturization.

A guided-mode resonance filter integrated in a waveguide cavity resonator constructed by two distributed Bragg reflectors is designed and fabricated for miniaturization of aperture size. Reflection efficiency of >90% and wavelength selectivity of 0.4 nm are predicted in the designed SiO(2)-based filter with 50-μm aperture by a numerical calculation using the finite-difference time-domain method.

Water expansion dynamics after pulsed IR laser heating.

A nanosecond pulsed IR (1.9 microm) laser rapidly heated water, in an open vessel, to temperatures well below the boiling point. The subsequent dynamics of volume expansion were monitored using time-resolved interferometry in order to measure the increase in the water level in the heated area.

Chirp effect in hard X-ray generation from liquid target when irradiated by femtosecond pulses.

The chirp effect on a X-ray emission intensity from a CsCl aqueous solution jet irradiated by femtosecond pulses was systematically studied. The p-polarized chirped pulses were more efficient as compared with the shortest pulses determined by the spectral bandwidth. The negatively-chirped pulses of approximately 240 fs duration produced up to 10 times larger X-ray intensity as compared with the transform-limited 160 fs pulses.

Density changes accompanying wave propagation in the cerium-catalyzed Belousov-Zhabotinsky reaction.

Refractive index measurement using an interferometric imaging system and observation of chemical wave shapes were carried out during chemical wave propagation of a cerium-catalyzed Belousov-Zhabotinsky (BZ) reaction. Densities increased as chemical waves propagated in samples without NaBr, and decreased in samples with NaBr. Concentration changes of malonic acid, bromomalonic acid, and BrO3- were estimated from Raman spectral measurements in a stirred batch BZ reaction, and these also exhibited differences between samples with and without NaBr.

Dynamics of Re(2,2'-bipyridine)(CO)3Cl MLCT formation and decay after picosecond pulsed X-ray excitation and femtosecond UV excitation.

The dynamics of Re(2,2'-bipyridine)(CO)3Cl MLCT state formation and decay were determined after femtosecond UV laser excitation and picosecond pulsed X-ray excitation, in an N,N-dimethylformamide (DMF) solution as well as in its solid form. At room temperature, after UV excitation, this MLCT excited state emits both in DMF solution and in the solid form. Transient absorption spectra were measured in solution at various delay times following excitation by a 160 fs, 390 nm laser pulse.