Laser-induced microexplosion confined in a bulk of silica: formation of nanovoids

We report on the nanovoid formation inside synthetic silica, viosil, by single femtosecond pulses of 30–100nJ energy, 800nm wavelength, and 180fs duration. It is demonstrated that the void is formed as a result of shock and rarefaction waves at pulse power much lower than the threshold of self-focusing. The shock-compressed region around the nanovoid is demonstrated to have higher chemical reactivity. This was used to reveal the extent of the shock-compressed region by wet etching. Application potential of nanostructuring of dielectrics is discussed

Modification of refractive index by a single femtosecond pulse confined inside a bulk of a photorefractive crystal

We demonstrate that the interaction of intense femtosecond pulse with photorefractive crystal at conditions close to the optical-breakdown threshold differs drastically from that of long pulse and cw illumination. Our theoretical estimations show that the high number density of excited electrons modifies the dielectric function leading to the transient negative change in the refractive index, Δn/ n0 ∼- 10-2 that vanishes on nanosecond time scale. Moreover, the high-frequency laser field, two orders of magnitude larger than the field of spontaneous polarization, prevents the stationary charge distribution during the pulse. The diffusion and recombination of charge carriers continues over a nanosecond time scale, after the end of the pulse. The main driving force for the current after the pulse is the field of spontaneous polarization in the ferroelectric medium: the current terminates when the field of charge separation balances this field. We show here that the stationary modification of refractive index according to this model is then independent of the polarization of the pump light beam, in agreement with experiments, and saturates at Δn 10-3 in semiquantitative fit to the experimental data

Microfabrication of Three-Dimensional Structures in Polymer and Glass by Femtosecond Pulses

We report three-dimensional laser microfabrication, which enables microstructuring of materials on the scale of 0.2-1 micrometers. The two different types of microfabrication demonstrated and discussed in this work are based on holographic recording, and light-induced damage in transparent dielectric materials. Both techniques use nonlinear optical excitation of materials by ultrashort laser pulses (duration < 1 ps).Comment: This is a proceedings paper of bi-lateral Conf. (Republics of China & Lithuania) on Optoelectronics and Magnetic Materials, Taipei, May 25-26, 2002.

Plasmon modes in single gold nanodiscs

Optical properties of single gold nanodiscs were studied by scanning near-field optical microscopy. Near-field transmission spectra of a single nanodisc exhibited multiple plasmon resonances in the visible to near-infrared region. Near-field transmission images observed at these resonance wavelengths show wavy spatial features depending on the wavelength of observation. To clarify physical pictures of the images, theoretical simulations based on spatial correlation between electromagnetic fundamental modes inside and outside of the disc were performed. Simulated images reproduced the observed spatial structures excited in the disc. Mode-analysis of the simulated images indicates that the spatial features observed in the transmission images originate mainly from a few fundamental plasmon modes of the disc

Laser matter interaction in the bulk of transparent dielectrics: Confined micro-explosion

We present here the experimental and theoretical studies of drastic transformations induced by a single powerful femtosecond laser pulse tightly focused inside a transparent dielectric, that lead to void formation in the bulk. We show that the laser pulse energy absorbed within a volume of less than 1μm3 creates the conditions with pressure and temperature range comparable to that formed by an exploding nuclear bomb. At the laser intensity above 6 × 1012 W/cm2 the material within this volume is rapidly atomized, ionized, and converted into a tiny super-hot cloud of expanding plasma. The expanding plasma generates strong shock and rarefaction waves which result in the formation of a void. Our modelling indicates that unique states of matter can be created using a standard table-top laser in well-controlled laboratory conditions. This state of matter has temperatures 105 K, heating rate up to the 1018 K/s, and pressure more than 100 times the strength of any solid. The laser-affected sites in the bulk were detected ("read") by generation of white continuum using probe femtosecond pulses at much lower laser intensity of 1010 W/cm 2 - 1011 W/cm2. Post-examination of voids with an electron microscope revealed a typical size of the void ranges from 200 to 500 nm. These studies will find application for the design of 3D optical memory devices and for formation of photonic band-gap crystals

Verification of scenario for substorm ignition in the M-I coupling region

第2回極域科学シンポジウム/第35回極域宙空圏シンポジウム 11月16日（水） 統計数理研究所 セミナー室

Initial results of Husafell solar radio spectrograph

Observing the moon surface and subsurface materials using various radio frequencies is very important for investigating the physical properties of the moon. In particular, the frequency dependence of the dielectric constant of surface and subsurface materials provides information on the density profile. Because the dielectric constant is identified by measuring the reflectivity of the radio waves, we attempted to observe direct solar radio bursts in Iceland and reflected solar radio bursts in Iitate simultaneously. A new solar radio spectrograph to observe solar radio bursts has been installed at Husafell station in Iceland. The spectrograph covers two frequency bands in the ranges of 18MHz to 38MHz and 190MHz to 350MHz. Since September 2004, several successful observations have been made: 30 events of Type-I, -II, -III, and -IV bursts have been found in data obtained between September 2004 and August 2005. The flux density of the solar radio bursts detected in this study was within the range of 10 to 100s.f.u. We previously confirmed that when strong solar burst phenomena occur in the UHF range, the reflected wave signal from the moon surface can be detected using the Iitate Planetary Radio Telescope, installed in Japan