Hiroto Kuroda

Generation of broadband noise-like pulse from Yb-doped fiber laser ring cavity

We have demonstrated a generation of the noise-like pulse (NLP) with broadband spectrum in a nonlinear polarized evolution-based passive mode-locked Yb-doped fiber (YDF) ring laser. At the cavity dispersion of near zero, the NLP with spectrum bandwidth up to 131 nm (FWHM) was obtained at a central wavelength of 1070 nm with output power of 136 mW and 80 MHz repetition rate. To our best knowledge, this spectrum bandwidth of NLP is the broadest among the reported YDF lasers. The autocorrelation function of pulse contained the short (30 fs) and long (4.6 ps) components. This short coherence light source is well suited for the optical coherent tomography used for ophthalmology at a wavelength of ∼1000  nm.

Quasi-phase-matching-induced enhancement of high-order harmonics during two-colour pump of multi-jet plasmas

The first demonstration of quasi-phase-matching during high-order harmonic generation (HHG) of laser radiation in multi-jet plasmas produced on the silver and chromium targets under the conditions of two-colour pump (804 and 402 nm) is presented. We analyse the enhancement of the groups of odd and even harmonics along the plateau range and compare the harmonic yields using a different number of plasma jets. The comparison of extended line plasma and multi-jet plasma shows the advanced properties of the latter formation for the efficient HHG using the two-colour pump. The 25× and 13× enhancement factors of quasi-phase-matched harmonics were obtained in the cases of the two-colour and single-colour pumps of multi-jet plasmas.

Low- and high-order harmonic generation in the extended plasmas produced by laser ablation of zinc and manganese targets

The systematic studies of the harmonic generation of ultrashort laser pulses in the 5-mm-long Zn and Mn plasmas (i.e., application of nanosecond, picosecond, and femtosecond pulses for ablation, comparison of harmonic generation from atomic, ionic, and cluster-contained species of plasma, variation of plasma length, two-color pump of plasmas, etc.) are presented. The conversion efficiency of the 11th–19th harmonics generated in the Zn plasma was ∼5 × 10−5. The role of the ionic resonances of Zn near the 9th and 10th harmonics on the enhancement of harmonics is discussed. The enhancement of harmonics was also analyzed using the two-color pump of extended plasmas, which showed similar intensities of the odd and even harmonics along the whole range of generation. The harmonics up to the 107th order were demonstrated in the case of manganese plasma. The comparison of harmonic generation in the 5-mm-long and commonly used short (≤0.5 mm) plasma plumes showed the advanced properties of extended media.

Correlation between Lamina Cribrosa Tilt Angles, Myopia and Glaucoma Using OCT with a Wide Bandwidth Femtosecond Mode-Locked Laser

Purpose To measure horizontal and vertical lamina cribrosa (LC) tilt angles and investigate associated factors using prototype optical coherence tomography (OCT) with a broad wavelength laser light source. Design Cross sectional study. Methods Twenty-eight no glaucoma eyes (from 15 subjects) and 25 glaucoma eyes (from 14 patients) were enrolled. A total of 300 optic nerve head B-scans were obtained in 10 µm steps and the inner edge of Bruchs membrane opening (BMO) was identified as the reference plane. The vertical and horizontal angles between BMO line and approximate the best-fitting line for the surface of the LC were measured and potential associated factors were estimated with univariate and multivariate logistic regression analyses. Results The median (interquartile range) horizontal and vertical tilt angles were 7.10 (2.43–11.45) degrees and 4.15 (2.60–6.85) degrees in eyes without glaucoma and 8.50 (4.40–14.10) degrees and 9.30 (6.90–14.15) degrees in glaucoma eyes, respectively. The refractive errors had a statistically significant association with horizontal LC tilt angles (coefficients, −1.53 per diopter) and glaucoma had a significant correlation with vertical tilt angles (coefficients, 6.56) using multiple logistic regression analysis (p<0.001). Conclusions OCT allowed evaluation of the internal tilting of the LC compared with the BMO. The horizontal internal LC tilt angle was correlated with refractive errors, corresponding to myopic physiological changes, and vertical internal LC tilt was correlated with glaucoma, corresponding to glaucomatous pathological changes. These parameters have important implications for investigation of the correlation between myopia, glaucoma and LC morphological features.

High-order harmonic enhancement using the quasi-phase-matching in a laser plasma

The quasi-phase-matching of the high-order harmonics generating in the multiple plasma jets produced on the surfaces of perforated Ag, V, and Mn targets is demonstrated. The 20× and 30× enhancement factors of quasi-phase-matched harmonics of 804 nm radiation in the range of the 35th harmonic in the cases of silver and vanadium plasmas were achieved.

High-order harmonic generation during propagation of femtosecond pulses through the laser-produced plasmas of semiconductors

The generation of high-order harmonics of the ultrashort pulses of Ti:sapphire laser in the plasma plumes produced during laser ablation of various semiconductors (Te, Se, Si, As, Sb, and Ge) is reported. Application of two-color pump allowed the generation of enhanced odd and even harmonics. The resonance-induced enhancement of single harmonics was observed in the Ge, Se, Sb, As, and Te plasmas. The enhanced 35th harmonic obtained from selenium plasma was twelve times stronger than the neighboring harmonics. We also demonstrate the quasi-phase-matching of the harmonics generated in the As plasma.

Resonance-enhanced harmonic generation in nanoparticle-containing plasmas

We demonstrate the enhancement of a single harmonic of ultrashort pulses in nanoparticle-containing plasmas. The application of three laser-produced nanoparticle-containing plasmas (In2O3, Sn and Mn2O3) allowed the efficient generation of lower-order harmonics and the resonance enhancement of single (13th, 17th and 33rd) harmonics. The comparison of the harmonic spectra obtained using the plasmas produced on the bulk and nanoparticle-containing surfaces showed that similar enhancement of single harmonics occurs in the case of strong excitation of the latter targets, while weaker ablation of nanoparticles led to generation of featureless lower-order harmonic spectra.

Morphology of laser-produced carbon nanoparticle plasmas and high-order harmonic generation of ultrashort pulses in clustered media

We analyse the morphology of the laser plasmas produced on surfaces containing various carbon clusters (nanotubes, nanofibres, fullerenes, and nanoparticles). It is shown that the presence of C5–C25 nanoparticles in the laser plasmas coincides with the enhancement of harmonics in the spectral range of 40–80 nm. We suggest that these nanoparticles, rather than larger clusters, play a decisive role in the observed growth of harmonic yield.

Application of mid-infrared pulses for quasi-phase-matching of high-order harmonics in silver plasma

We demonstrate the quasi-phase-matching of a group of harmonics generated in Ag multi-jet plasma using tunable pulses in the region of 1160 - 1540 nm and their second harmonic emission. The numerical treatment of this effect includes microscopic description of the harmonic generation, propagation of the pump pulse, and the propagation of the generated harmonics. We obtained more than 30-fold growth of harmonics at the conditions of quasi-phase-matching in the region of 35 nm using eight-jet plasma compared with the case of imperforated plasma.

Resonance enhancement of harmonics in laser-produced Zn II and Zn III containing plasmas using tunable mid-infrared pulses

We demonstrate the generation of enhanced tuneable harmonics of the mid-infrared (1250−1400 nm), 65 fs pulses in laser-produced zinc plasma in the regions of Zn II and Z III autoionizing states (77 and 68 nm, respectively). The role of singly and doubly ionized zinc in the modification of harmonic spectra was analyzed by variation of plasma formation conditions. Microprocesses and the propagation effect are also discussed to describe the narrowing of enhanced emission spectra.