Akihiro Takita

Variable holographic femtosecond laser processing by use of a spatial light modulator

We propose a holographic femtosecond laser processing system capable of parallel, arbitrary, and variable patterning. These features are achieved by introducing a spatial light modulator displaying a hologram into the femtosecond laser processing system. We demonstrate the variable parallel processing of a glass sample.

Sparse-exposure technique in holographic two-photon polymerization

Holographic two-photon polymerization is based on a high-speed, low-loss parallel laser irradiation technique inside photosensitive materials using a computer-generated hologram displayed on a liquid crystal spatial light modulator. We demonstrated a sparse exposure technique combining parallel exposure and scanning exposure to improve the fabrication throughput and to achieve simultaneous fabrication of linear structures with different widths. We also demonstrated fabrication of space-variant structures by changing a CGH, as well as parallel fabrication of voxel structures with single femtosecond laser pulse irradiation.

Time-resolved interferometry of femtosecond-laser-induced processes under tight focusing and close-to-optical breakdown inside borosilicate glass

We use an interferometric time-resolved observation of a femtosecond-laser pulse (800 nm/45 fs) interaction with glass from 100 fs to 10 ns at spatial lateral resolution down to the wavelength of the pulse. The phase and amplitude images reveal sequence of events after the irradiation of a single ultra-short laser pulse at close-to-threshold intensity when permanent refractive index changes occur. The proposed method is applicable to characterization of the processes induced by tightly focused fs-laser pulses during three-dimensional structuring of glasses and crystals for fundamental studies and optical applications. Generation of carriers, thermal expansion, generation and propagation of shockwaves, and formation of refractive index changes are experimentally observed and resolved in time and space with the highest resolution. Quantitative estimations of the threshold energies of different processes are achieved. The threshold energy of carrier generation is found the same as that of shockwave generation while the threshold energy of refractive index changes was by 40% higher. Application potential of the method is discussed.

Time-resolved axial-view of the dielectric breakdown under tight focusing in glass

We present time-resolved studies of the dielectric breakdown using tightly focused femtosecond (fs) laser pulses in glass. Axial evolution of the breakdown and material modifications have been retrieved over the time span from 0 to 1 ns with a 50 fs resolution and ∼ 1 μm spatial resolution using interferometric pump-probe technique. It is shown that even at pulse power slightly above critical Pcr ≃ 1 MW/pulse, the filamentation was limited at tight focusing and the central focal region with resolidified glass was localised axially within ∼ 10 μm; it can be used for the waveguide recording. Mechanisms of light-matter interaction at tight focusing and application potential are discussed. The electron-ion scattering time, τe–i ≃ 1.1 fs, for the glass at electron concentration ne ≃ (4–5)×1020 cm−3 was determined within Drude approximation.

Interference Measurement of Superposition of Laser-Induced Shock Waves in Water

Time-resolved phase distributions of laser-induced breakdowns and the subsequent shock waves generated by two pulses in water were observed by a pump–probe interference microscopy. When the two pulses overlapped temporally, the optical interference of the two pulses resulted in a fluctuation in pulse energy inducing breakdown. However, the effect of the optical interference could be avoided by delaying one pulse by a time longer than the pulse duration. When shock waves emitted from two breakdown points overlapped, the pressure in the overlapping region was the sum of the pressures of the individual shock waves, showing the possibility of shock synthesis, such as pressure enhancement and wavefront shaping, by inducing multiple breakdowns.

Three-dimensional optical memory using a human fingernail

We realized optical data storage in a human fingernail. A structural change is recorded by irradiating a focused femtosecond laser pulse and is read out with fluorescent observation by making use of an increased fluorescence intensity. The shape of the structural changes drastically depends on the irradiated pulse energy. The fluorescence spectrum of the structure coincided with the auto-fluorescence spectra of a fingernail and a heated fingernail. It is suggested that the increased fluorescence is most likely caused by a local denaturation of the keratin protein by the femtosecond laser pulse irradiation. We demonstrate that the increased fluorescence effect is useful for reading out three-dimensionally recorded data.

Optical bit recording in a human fingernail

Optical bit recording in a human fingernail is demonstrated by using a femtosecond laser processing technique. A bit is formed in a fingernail by irradiation of a focused femtosecond laser pulse. We have investigated the dependence of the irradiation pulse energy on the diameter and length of the formed bit. The bit diameter and length are theoretically explained based on a Gaussian beam irradiation model. Furthermore, the achievable recording density is estimated from the measured bit diameter and bit length. We have found that the pulse energy required to form a bit depends on the recording depth and the surface condition of the fingernail.

Smart street light system with energy saving function based on the sensor network

Our project for developing a smart street light system is reviewed. In this project, the street light system, in which lights on when needed and light-off when not needed. Currently, in the whole world, enormous electric energy is consumed by the street lamps, which are automatically turn on when it becomes dark and automatically turn off when it becomes bright. This is the huge waste of energy in the whole world and should be changed. Our smart street light system consists of a LED light, a brightness sensor, a motion sensor and a short-distance communication network. The lights turn on before pedestrians and vehicles come and turn off or reduce power when there is no one. It will be difficult for pedestrians and drivers of vehicles to distinguish our smart street lamps and the conventional street lights, since our street lamps all turn on before they come. The present status and the future prospects of our smart start light project will be reviewed.

Processing Structures on Human Fingernail Surfaces Using a Focused Near-Infrared Femtosecond Laser Pulse

We investigated the processing of a human fingernail surface using a tightly focused femtosecond laser pulse. The processed structure in the fingernail surface is strongly dependent on the focus position and irradiation energy of the single laser pulse. We observed a ring, a simple pit, a small pit with a surrounding uplift, an irregular jagged surface, and a swell containing a void, depending on the focus position. We also observed a sudden change in the size of the processed structure according to the irradiation pulse energy. From a linear theoretical estimation based on the diffraction of the laser beam, we found that the sudden change is primarily due to the diffraction pattern generated by the circular aperture of the objective lens. We also describe the processing features by comparing the structures processed in a fingernail with those processed in glass.

Photo-acoustic sub-micrometer modifications of glass by pair of femtosecond laser pulses

We present time-resolved studies of glass densification created by an acoustic phenomenon: collision of the transverse and longitudinal sound waves inside glass. Localization of the permanent denisfied region has a lateral cross section ∼ 0.4 μm and is approximately half of the wavelength of femtosecond laser pulses which were used to generate breakdown and launched shock waves inside glass. Controlled time delay between two closely spaced irradiation spots reveals dynamics and relaxation (electronic, thermal, stress) of glass after excitation. The observed phenomenon is important for femtosecond direct laser writing and recording of waveguide couplers using multiple beams.