K. Hirao

Homogeneous and elongation-free 3D microfabrication by a femtosecond laser pulse and hologram

A new 3D microfabrication method has been developed, which uses only a single femtosecond laser pulse and a hologram. For the microfabrication inside transparent materials, the optical axial elongation of the fabricated structure is a major problem that has thus far limited design flexibility, especially for the direction along the optical axis. By controlling the light intensity distribution profile and using the adequate focal length of the hologram, this problem was solved and homogeneous and elongation-free 3D microfabrication was achieved.

Self-Assembled Nanostructures and Two-Plasmon Decay in Femtosecond Processing of Transparent Materials

Self-assembled nanostructures in transparent materials irradiated by ultrashort light pulses reveal two-dimensional periodicity. The mechanism of the phenomenon based on interference of bulk plasma waves excited via two plasmon parametric decay is proposed.

Ultrafast light blade

Anisotropic sensitivity of isotropic medium to femtosecond laser radiation is observed. The phenomenon is explained by unusual anisotropy at the interface produced by ultrashort light pulses with tilted front and referred as ultrafast light blade.

Internal modification in glass induced with a femtosecond laser

We proposed a research idea of “induced structure” which means spatially modified microstructure in glass by external intensive electromagnetic field. If we can control the variety, concentration and distribution of the induced structure, in glass, in particular, if we can space-selectively control the induced structure, we expect that novel optical functions will be achieved. We have systematically researched the realization of novel optical functions of glass by controlling the induced structures. In this paper, we review the phenomena, mechanisms, and the applications of refractive index change, space-selective manipulation of the valence state of active ions, space-selective precipitation and control of metal nanoparticles by using a femtosecond laser, and the femtosecond laserinduced polarization-dependent nanostructures. Glass was first produced more than 5000 years ago. Now, glass is widely used in our daily life. We can even say that there could be no highly civilized society without glass. Why is glass so widely used? Because glass is homogeneous, transparent and can be easily fabricated to various forms such as a bottle, a flat plate of large size or a fiber. Moreover, almost all elements such as rare-earth, transition metals and noble metals in the periodic table can be “stuffed” into glass. This results in glass that is splendidly colorful with various functions. It is well known that glass is one of the most important materials in optics, with uses in optical fibers, lenses, mirror substrates, and prisms. In all of these applications, however, glass is almost used as a passive medium. Since the development of integrated optics, glass is expected to be used in active functions as well, such as in light amplification, optical storage, ultrafast optical switching, and various modulations of light. In 1994, we proposed a basic research idea of “induced structure”. We paid attention to the fact that glass is metastable from the viewpoint of thermodynamics. A metastable state of glass is changed to other states in an external intensive electromagnetic field. If we can control the induced structure and the concentration, variety, and valence state of active ions in glass; in particular, if we can space-selectively control the induced structure in glass, we expect that novel optical functions of the glass will be achieved due to the quantum effect, surface effect, size effect and synergic effect of the induced structures. From the viewpoint of practical applications, we expect to obtain a glass with properties superior to corresponding single crystal. Based on this idea, we applied various external electromagnetic fields such as X-ray, ultra-violet light, electron beam and laser to make microscopic modifications to glass structure, and observed many interesting phenomena and discussed the promising applications of the observed phenomena. We selected a femtosecond laser as a powerful tool to make microscopic modifications to glass structure. Femtosecond laser has two apparent features compared with CW and long pulsed lasers: (1) elimination of the thermal effect due to extremely short energy deposition time, and (2) participation of various nonlinear processes enabled by highly localization of laser photons in both time and spatial domains. Due to the ultra-short light-matter interaction time and the high peak power, material processing with the femtosecond laser is generally characterized by the absence of heat diffusion and, consequently molten layers. The nature of ultra-short light-matter interaction permits femtosecond laser to overcome the diffraction limit. We started the systematic investigations on the femtosecond laser induced microstructures in glasses and applications in micro-optics at the end of 1994. The reason for using this laser is that the strength of its electric field in the focal point of the laser beam can reach 10 TW/cm, which is sufficient for inducing various nonlinear physicochemical reactions in materials by using a focusing lens, when the pulse width is 100 fs and the pulse energy is 1 μJ. The photo-induced reactions are expected to occur only near the focused part of the laser beam due to multiphoton processes. We QTuD4-3-INV

Anisotropic phenomena during direct writing with ultrashort light pulses in glass

New phenomena of light scattering and Cherenkov generation peaking in the plane of polarization during direct writing with ultrashort light pulses in glass are observed. The phenomena are interpreted in terms of angular distribution of photoelectrons, anisotropic index fluctuations and anisotropic diffusion of photoelectrons induced by intense light pulses.

Observation of anomalous light scattering in glass pumped by femtosecond laser

Summary form only given. It is well known that the scattering of polarized light in the plane of light polarization in an isotropic medium such as glass is always weaker compared to the orthogonal plane, since a dipole does not radiate in the direction of its axis. We report here the observation of a new phenomenon in glass pumped by intense laser radiation-the scattering of light, in particular luminescence, which peaks in the wrong plane-in the plane of light polarization (anomalous anisotropic light scattering).