Polarization imaging camera with a waveplate array fabricated with a femtosecond laser inside silica glass.

In this study, we demonstrate a polarization imaging camera with a waveplate array of a silica glass fabricated by femtosecond (fs) laser direct writing. To use a waveplate array of silica glass for polarization imaging, non-uniformity of the transmittance and retardance in the waveplates must be considered. Therefore, we used a general method of polarization analysis with system matrices determined experimentally for all the units in the waveplate array. We found that a figure of merit based on the determinant of the system matrix could be applied to improve the accuracy of analysis and the robustness to the retardance dispersion for both the simulated and the fabricated waveplate array.

Condensation of Si-rich region inside soda-lime glass by parallel femtosecond laser irradiation.

Local melting and modulation of elemental distributions can be induced inside a glass by focusing femtosecond (fs) laser pulses at high repetition rate (>100 kHz). Using only a single beam of fs laser pulses, the shape of the molten region is ellipsoidal, so the induced elemental distributions are often circular and elongate in the laser propagation direction. In this study, we show that the elongation of the fs laser-induced elemental distributions inside a soda-lime glass could be suppressed by parallel fsing of 250 kHz and 1 kHz fs laser pulses. The thickness of a Si-rich region became about twice thinner than that of a single 250 kHz laser irradiation. Interestingly, the position of the Si-rich region depended on the relative positions between 1 kHz and 250 kHz photoexcited regions. The observation of glass melt during laser exposure showed that the vortex flow of glass melt occurred and it induced the formation of a Si-rich region. Based on the simulation of the transient temperature and viscosity distributions during laser exposure, we temporally interpreted the origin of the vortex flow of glass melt and the mechanism of the formation of the Si-rich region.

Photoacoustic microscopy using ultrashort pulses with two different pulse durations.

We propose photoacoustic microscopy using ultrashort pulses with two different pulse durations in the range from femtoseconds to picoseconds. The subtraction of images for longer-pulse excitation from those for shorter-pulse excitation extracts two-photon photoacoustic images effectively, based on observation that the intensity ratio of two-photon to one-photon absorption-induced photoacoustic signals depends on the pulse duration in the same manner as the intensity ratio of two-photon and one-photon fluorescence signals. Two-photon photoacoustic microscopy using this subtraction method enables precise observation of the cross-sections of silicone hollows filled with the mixture of one-photon and two-photon absorption solutions.

Modulation of laser induced-cracks inside a LiF single crystal by fs laser irradiation at multiple points.

Crack formations inside a LiF single crystal after femtosecond laser irradiation at multiple points were investigated. In the case of sequential laser irradiation at three points, the propagations of some cracks were prevented by the dislocation bands generated by the previous laser irradiation. On the other hand, in the case of simultaneous laser irradiation at three points with a spatial light modulator, cracks in all the <100> directions from the photoexcited regions were generated clearly, but the length of one crack depended on the distribution of laser irradiation positions. The simulation of elastic dynamics after fs laser irradiation at three points elucidated that the interference of laser induced stress waves depended on the distributions of the irradiation positions. We found that the constructive interference of stress waves at a crack tip should have prevented the crack from propagating further and the tensile stress by destructive interference of stress waves along a crack should have facilitated the propagation of the crack.

Shape control of elemental distributions inside a glass by simultaneous femtosecond laser irradiation at multiple spots.

The spatial distributions of elements in a glass can be modulated by irradiation with high repetition rate femtosecond laser pulses. However, the shape of the distribution is restricted to being axially symmetric about the laser beam axis due to the isotropic diffusion of photo-thermal energy. In this study, we describe a method to control the shape of the elemental distribution more flexibly by simultaneous irradiation at multiple spots using a spatial light modulator. The accumulation of thermal energy was induced by focusing 250 kHz fs laser pulses at a single spot inside an alumino-borosilicate glass, and the transient temperature distribution was modulated by focusing 1 kHz laser pulses at four spots in the same glass. The resulting modification was square-shaped. A simulation of the mean diffusion length of molten glass demonstrated that the transient diffusion of elements under heat accumulation and repeated temperature elevation at multiple spots caused the square shape of the distribution.

Efficient ultrafast laser writing with elliptical polarization.

Photosensitivity in nature is commonly associated with stronger light absorption. It is also believed that artificial optical anisotropy to be the strongest when created by light with linear polarization. Contrary to intuition, ultrafast laser direct writing with elliptical polarization in silica glass, while nonlinear absorption is about 2.5 times weaker, results in form birefringence about twice that of linearly polarized light. Moreover, a larger concentration of anisotropic nanopores created by elliptically polarized light pulses is observed. The phenomenon is interpreted in terms of enhanced interaction of circularly polarized light with a network of randomly oriented bonds and hole polarons in silica glass, as well as efficient tunneling ionization produced by circular polarization. Applications to multiplexed optical data storage and birefringence patterning in silica glass are demonstrated.

Three-dimensional temperature distribution and modification mechanism in glass during ultrafast laser irradiation at high repetition rates.

We experimentally determined the three-dimensional temperature distribution and modification mechanism in a soda-lime-silicate glass under irradiation of ultrafast laser pulses at high repetition rates by analyzing the relationship between the morphology of the modification and ambient temperature. In contrast to previous studies, we consider the temperature dependence of thermophysical properties and the nonlinear effect on the absorbed energy distribution along the beam propagation axis in carrying out analyses. The optical absorptivity evaluated with the temperature distribution is approximately 80% and at most 3.5% smaller than that evaluated by the transmission loss measurement. The temperature distribution and the strain distribution indicate that visco-elastic deformation and material flow play important roles in the laser-induced modification inside a glass.

Localized control of light-matter interactions by using nanoscale asymmetric TiO2.

This paper reports an asymmetry structure-mediated route for highly localized control of light-matter interactions by using tapered TiO(2). We demonstrate for the first time that the growth habit of Ag nanostructures on tapered TiO(2) can be tuned by controllable photolysis. Site-selective anchoring of Ag nanoparticles or nanowires on tapered TiO(2) can be achieved by simply changing the external light. We further show that the obtained tapered TiO(2)-Ag hetero-nanostructures present excellent light-trapping ability over a wide range of wavelengths which is considered to originate from the unique synergistic effects of graded waveguiding and plasmonic light trapping. This improved photon-management capability renders the prepared substrate a very promising candidate for optical sensing application. For this purpose, an enhanced sensitivity for trace detection is confirmed. These findings open up promising avenues for tailoring of light-matter interactions which are of special interest for studying controllable photolysis activation processes and diverse applications such as nanostructure growth, trace detection, photocatalysis and solar cells.

Observation of laser-induced stress waves and mechanism of structural changes inside rock-salt crystals.

The structural changes inside rock-salt crystals after femtosecond (fs) laser irradiation are investigated using a microscopic pump-probe technique and an elastic simulation. The pump-probe imaging shows that a squircle-shaped stress wave is generated after the fs laser irradiation as a result of the relaxation of thermal stress in the photoexcited region. Pump-probe crossed-Nicols imaging and elastic simulation elucidate that shear stresses and tensile stresses are concentrated in specific regions during the propagation of the stress wave. The shear stresses and tensile stresses observed in this study can explain the characteristic laser-induced structural changes inside rock-salt crystals.

Photosensitivity control of an isotropic medium through polarization of light pulses with tilted intensity front.

We present the first experimental evidence of anisotropic photosensitivity of an isotropic homogeneous medium under uniform illumination. Our experiments reveal fundamentally new type of light induced anisotropy originated from the hidden asymmetry of pulsed light beam with a finite tilt of intensity front. We anticipate that the observed phenomenon, which enables employing mutual orientation of a light polarization plane and pulse front tilt to control interaction of matter with ultrashort light pulses, will open new opportunities in material processing.

Improved phase hologram design for generating symmetric light spots and its application for laser writing of waveguides.

The improved method for calculation of a phase hologram and its application to laser writing of waveguides with a spatial light modulator are presented. It was found that the amplitude and phase distributions of light spots generated by a phase hologram can be distorted compared to those of a focused single beam. The distortion of light spots could be reduced by adding a simple constraint, in which light intensities around a light spot should be as small as possible, to the conventional calculation method of a phase hologram. It was also demonstrated that the improved calculation method can be considered essential for laser writing of waveguides.

Formation mechanism of element distribution in glass under femtosecond laser irradiation.

We report on the formation mechanism of element distribution in glass under high-repetition-rate femtosecond laser irradiation. We simultaneously focused two beams of femtosecond laser pulses inside a glass and confirmed the formation of characteristically shaped element distributions. The results of the numerical simulation in which we considered concentration- and temperature-gradient-driven diffusions were in excellent qualitative agreement with the experimental results, indicating that the main driving force is the sharp temperature gradient. Since the composition of a glass affects its refractive index, absorption, and luminescence property, the results in this study provide a framework to fabricate a functional optical device such as optical circuits with a high-repetition-rate femtosecond laser.

Fabrication and characterization of silicon antireflection structures for infrared rays using a femtosecond laser.

We focus on IR sensors with lower reflection for the wavelength around 10 µm, strongly awaited for detecting human bodies. A concave structure was designed as a more suitable reflection-free structure for IR light, and an optical system with a femtosecond laser was employed for verification of the effectiveness of the structure. The microstructures prepared through this process were fabricated and optically measured using SEM, FT-IR, and Raman spectroscopy. The measurement revealed that good reflection-free structures were realized for IR sensors with lower reflection for the wavelength of around 10 µm.

Intermetallic magnetic nanoparticle precipitation by femtosecond laser fragmentation in liquid.

Intermetallic Nd(2)Fe(14)B nanoparticles with an average diameter of 30 nm, which are smaller than a theoretical single magnetic domain size of 220 nm, were successfully prepared by the femtosecond laser fragmentation in liquid. The self-passivating amorphous carbon layer resulting from the decomposition of the surrounding solvent prevents the Nd(2)Fe(14)B nanoparticle from aggregation and oxidation. The coercivity of Nd(2)Fe(14)B nanoparticle increases with increase of the laser irradiation time, despite the reduction of crystallinity.

Silicon precipitation via photoinduced reaction using femtosecond laser.

Silicon precipitation inside a glass is an important technique for silicon photonics. We successfully precipitated silicon inside silicate glasses containing an Al metal film using femtosecond laser irradiation. First, the Al-inserted sandwiched glass was fabricated by the direct bonding method. The results of a tensile test indicated that the adhesive strength of the sandwich structure reached approximately 4 MPa. Next, femtosecond laser pulses were focused at the Al/glass interface in the sandwich structure. A transmission electron microscopy photograph at the focus of the laser showed that the Al particles were dispersed into the glass substrate to a depth of approximately 2 microm from the initial Al layer. In addition, Raman spectra indicated that silicon had formed at the interface between the glass and Al film after the laser irradiation. The morphology or the particle size of the precipitated silicon was successfully modified by changing the repetition rate or the pulse energy of the laser.

Simultaneous tailoring of phase evolution and dopant distribution in the glassy phase for controllable luminescence.

Construction of an active composite with multicolor visible and broadband near-infrared luminescence is of great technological importance for various applications, including three-dimensional (3D) display, broadband telecommunication, and tunable lasers. The major challenge is the effective management of energy transfer between different dopants in composite. Here we present an in situ strategy for controlling energy transfer between multiple active centers via simultaneous tailoring of the evolution of phases and the distribution of dopants in the glassy phase. We show that the orderly precipitation of Ga(2)O(3) and LaF(3) nanocrystals and the selective incorporation of Ni(2+) and Er(3+) into them can be achieved. The obtained composite shows unique multicolor visible and broadband near-infrared emission. Possible mechanisms for the selective doping phenomenon are proposed, based on thorough structural and optical characterizations and crystal-field calculation results. Moreover, the strategy can be successfully extended to accomplish space-selective control of multicolor luminescence by employing the modulated stimulation field. The results suggest that the strategy could be applied to fabricate a multifunctional light source with a broad range of important host/activator combinations and to construct various types of three-dimensional active microstructures.

Fabrication of three-dimensional 1 x 4 splitter waveguides inside a glass substrate with spatially phase modulated laser beam.

Multiple light spots can be generated by modulating the spatial phase distribution of laser beam with a spatial light modulator (SLM). In this paper, we demonstrate the fabrication of three-dimensional 1 x 4 splitter waveguides inside a glass by focusing multiple light spots of femtosecond (fs) laser pulses, which can be controlled by switching spatial phase distributions on an SLM. In the conventional fs laser writing technique, a highly precise positioning of a substrate is essential for fabricating a branched waveguide in a splitter. Using the technique proposed in this paper, a continuously branched waveguide can be produced easily by translating a glass substrate only one time; therefore this technique can eliminate the need for a high precision in positioning of a substrate and save a fabrication time.

Ultralow-loss geometric phase and polarization shaping by ultrafast laser writing in silica glass.

Polarization and geometric phase shaping via a space-variant anisotropy has attracted considerable interest for fabrication of flat optical elements and generation of vector beams with applications in various areas of science and technology. Among the methods for anisotropy patterning, imprinting of self-assembled nanograting structures in silica glass by femtosecond laser writing is promising for the fabrication of space-variant birefringent optics with high thermal and chemical durability and high optical damage threshold. However, a drawback is the optical loss due to the light scattering by nanograting structures, which has limited the application. Here, we report a new type of ultrafast laser-induced modification in silica glass, which consists of randomly distributed nanopores elongated in the direction perpendicular to the polarization, providing controllable birefringent structures with transmittance as high as 99% in the visible and near-infrared ranges and >90% in the UV range down to 330 nm. The observed anisotropic nanoporous silica structures are fundamentally different from the femtosecond laser-induced nanogratings and conventional nanoporous silica. A mechanism of nanocavitation via interstitial oxygen generation mediated by multiphoton and avanlanche defect ionization is proposed. We demonstrate ultralow-loss geometrical phase optical elements, including geometrical phase prism and lens, and a vector beam convertor in silica glass.

Directionally controlled 3D ferroelectric single crystal growth in LaBGeO5 glass by femtosecond laser irradiation.

Laser-fabrication of complex, highly oriented three-dimensional ferroelectric single crystal architecture with straight lines and bends is demonstrated in lanthanum borogermanate model glass using a high repetition rate femtosecond laser. Scanning micro-Raman microscopy shows that the c-axis of the ferroelectric crystal is aligned with the writing direction even after bending. A gradual rather than an abrupt transition is observed for the changing lattice orientation through bends up to approximately 14 degrees. Thus the single crystal character of the line is preserved along the bend through lattice straining rather than formation of a grain boundary.

Ultrashort pulse manipulation of ZnO nanowire growth.

A procedure of femtosecond pulse laser irradiation was incorporated into the synthesis of zinc oxide (ZnO) nanowires in aqueous solutions to investigate the photo-initiated heterogeneous nucleation induced by the irradiation and the associated nanowire growth. Elongated ZnO nanowires with smooth planes and end tips were successfully grown following the irradiation process and subsequent hydrothermal treatments in a catalyst-free environment, compared to aggregated flower-like nanostructures with porous and rough surfaces, grown from homogeneous nucleation without laser irradiation. Studies using femtosecond laser systems at 1 kHz and 250 kHz repetition rates show that the pulse energy is critical in the heterogeneous nucleation process for the growth of ZnO nanowires. A minimum threshold pulse energy, 200 microJ/pulse for the 1 kHz system and 2.4 microJ/pulse for 250 kHz, is observed beyond which well-defined and individually separated nanowires were grown. Thermal effect caused by the 250 kHz repetition rate provides a counter-balance to the low pulse energy required for the growth process. XRD analysis of the nanowires reveals a hexagonal structure while photoluminescence shows emission at about 385 nm. The overall results show that the pulse energy is critical for heterogeneous nucleation while the irradiation duration affects the density of nucleation, which together with the hydrothermal treatment temperature influence the growth rate and thus the morphology of the nanowires.