Zs. Bor

Group velocity dispersion in prisms and its application to pulse compression and travelling-wave excitation

Abstract In a beam propagating through a dispersive prism a delay of the pulse front occurs. A general expression relating the delay and the angular dispersion was found. Using a streak camera this delay was experimentally observed. In practical systems the delay varies from a few to several tens of picoseconds. A pulse compressor equivalent to the Treacy-compressor, based on this delay is described. A novel scheme for the excitation of a travelling-wave picosecond amplified spontaneous emission is also proposed.

Generation of diffraction-free beams for applications in optical microlithography

A new concept based on a Fabry–Perot interferometer for the generation of nondiffracting Bessel beams is described and proposed for potential applications in microlithography such as the fabrication of small isolated patterns. It was experimentally demonstrated that the depth of focus can be increased by a factor of about 2, and simultaneously the transverse resolution improved by a factor of 1.6, when using this technique to image contact holes. The properties of simultaneous imaging of two contact holes were also investigated. It was shown experimentally that, even in the most critical case (when the first diffraction rings overlap), undesirable interference effects between the adjacent contact holes can be eliminated by means of a phase shifting technique.

Absorbing film assisted laser induced forward transfer of fungi (Trichoderma conidia)

We present an investigation on absorbing film assisted laser induced forward transfer (AFA-LIFT) of fungus (Trichoderma) conidia. A KrF excimer laser beam [λ=248nm,FWHM=30ns (FWHM, full width at half maximum)] was directed through a quartz plate and focused onto its silver coated surface where conidia of the Trichoderma strain were uniformly spread. The laser fluence was varied in the range of 0–2600mJ∕cm2 and each laser pulse transferred a pixel of target material. The average irradiated area was 8×10−2mm2. After the transfer procedure, the yeast extract medium covered glass slide and the transferred conidia patterns were incubated for 20 h and then observed using an optical microscope. The transferred conidia pixels were germinated and the areas of the culture medium surfaces covered by the pixels were evaluated as a function of laser fluence. As the laser fluence was increased from 0 to 355mJ∕cm2 the transferred and germinated pixel area increased from 0 to 0.25mm2. Further increase in fluence resulted...

Broadband frequency doubler for femtosecond pulses

A highly efficient frequency doubler is developed for femtosecond pulses. Design aspects of the device are discussed. Doubling bandwidth calculations have shown, that with one millimeter long crystals pulses as short as 10 fs can be frequency doubled. Experimental results testing the doubler with 300 fs, 496 nm pulses are presented showing no spectral narrowing.

Group-delay measurement on laser mirrors by spectrally resolved white-light interferometry.

The frequency-dependent group delay of dielectric mirrors was measured by spectrally resolved white-light interferometry. Chirped mirrors and thin-film Gires-Tournois interferometers designed for dispersion control in a femtosecond Ti:sapphire laser oscillator-amplifier system were tested with a group-delay resolution of +/-0.2 fs and a spectral resolution of ~1 nm over the spectral range of 670-870 nm.

Sub‐100 nm lines produced by direct laser ablation in polyimide

Periodic line structures with a period of 167 nm and linewidths varying from 35 to 100 nm have been produced on polyimide by direct ablation with a KrF laser using an interferometric technique. Since ablation is a nonlinear process, the resolution can exceed that expected from the wavelength and numerical aperture of the system and the linewidth can be controlled by varying the laser fluence. This externally generated period of 167 nm prevents the spontaneous growth of periodic surface structures due to radiation remnants.

Laser-induced periodic surface structure formation on polyethylene-terephthalate

Abstract Periodic surface structures generated by linearly polarized ArF excimer laser (193 nm) were studied on polyethylene–terephthalate (PET), in a narrow fluence region (3–5 mJ/cm 2 ). Atomic force microscopy was used to follow the morphological changes caused by subsequent laser pulses on the same part of the surface. A homemade kinematic base plate ensured the reproducible positioning of the sample with respect to the AFM and the laser beam. The main subsequent phases of the Laser-Induced Periodic Surface Structure (LIPSS) formation were identified as: laser light scattering on the originally existing granules, formation of elliptical structures around the granules, and the transformation of ellipses into ripple-shape modulation. The LIPSS generating feedback mechanism was described: the incoming beam and the beams scattered on the granules and already existing LIPSS interfere and cause spatially modulated melting and crystallisation, which develops the structure. The dependence of the LIPSS period on the wavelength and on the angle of incidence was studied. The structures height and surface-roughness vs. the number of laser shots was measured. The experimental results are in good agreement with the theory of the self-organized diffraction gratings. It was proved experimentally that the LIPSS are formed by redistribution of the material on the surface, without significant material removal.

Distortion of femtosecond pulses in lenses. Wave optical description

Abstract The propagation and distortion of a 100 fs long pulse in a silica lens is described using wave optical theory. A fifty-fold increase of the pulse duration was calculated. The intensity distribution in the focal plane turned to be essentially different from the Airy pattern. Furthermore, the formation of a forerunner pulse is predicted. The forerunner is caused by the interference of the boundary waves generated by the aperture of the lens. Approximate analytical solutions for the diffraction of fs pulses are also given.

A novel pumping arrangement for tunable single picosecond pulse generation with a N2 laser pumped distributed feedback dye laser

A novel excitation scheme for a N2 laser pumped distributed feedback dye laser is described. The dye laser generates 80–100 ps pulses with a time-bandwidth product better than 0.6. Several tuning methods are proposed, and a number are investigated experimentally. Continuous tuning over a 50 A range without mode-hopping is demonstrated.

Amplification and measurement of single 1.6–3.5 ps Pulses generated by a distributed feedback dye laser

We have shown that distributed feedback dye lasers are capable of generating single transform-limited pulses of 1.6–3.5 ps duration when pumped by a mode-locked Nd: YAG laser. A short-pulse amplifier has been constructed in which the level of amplified spontaneous emission could be kept below 10−3 by properly taking into account the polarization anisotropy of the gain and by precise timing of the pump pulses. A sensitive autocorrelator is described which allows to measure pulse duration in a single shot.