Precision laser ablation of dielectrics in the 10-fs regime

Received: 4 January 1999 / Accepted: 5 January 1999     

Characterization of particulates accompanying laser ablation of pressed polytetrafluorethylene (PTFE) targets

We present observations of sub-micron- to micron-sized particles generated by high fluence (≈2 J/cm²) 248-nm laser ablation of pressed polytetrafluorethylene (PTFE) targets in air at atmospheric pressure. The original target material was hydrostatically compressed ≈7 μm PTFE powder, sintered at 275 °C. Collected ejecta due to laser irradiation consists of four basic particle morphologies ranging from small particles 50–200 nm in diameter to larger particles ≈10 μm in diameter. Many particles formed in air carry electric charge. Using charged electrodes we are able to collect charged particles to determine relative numbers of ± charge. We observe roughly equal numbers of positively and negatively charged particles except for the largest particles which were predominantly negative. For a range of particle sizes we are able to measure the sign and magnitude of this charge with a Millikan-oil-drop technique and determine surface charge densities. The implications of these observations with respect to pulsed laser deposition of PTFE thin films and coatings are discussed. Received: 15 January 1999 / Accepted: 18 January 1999 / Published online: 7 April 1999     

Efficient, kHz repetition rate, gain-switched Cr:forsterite laser

We have developed a gain-switched room-temperature Cr:forsterite laser operating at repetition rates of between 1 and 34 kHz, and pumped by a continuous wave, Q-switched Nd:YAG laser. With optimised output coupling, an output pulse energy of 52 μJ was measured at 1.5 kHz repetition rate, corresponding to 11% efficiency and 13% slope efficiency. Threshold pulse energy was 53 μJ. Output power of 370 mW was obtained at 10 kHz repetition rate and 4.4 W pump power. Water cooling was not required for repetition rates up to 10 kHz. In a tunable, folded resonator, the Cr:forsterite wavelength tuned between 1173 and 1338 nm. This laser operated with maximum pulse energy of 34 μJ, efficiency of 13%, and power of 307 mW. The laser output was close to diffraction-limited with M² of 1.2. Received: 6 January 1999 / Published online: 29 July 1999     

Investigation of bulk laser damage in KDP crystal as a function of laser irradiation direction, polarization, and wavelength

Bulk laser-induced damage in KDP crystal was measured using a single-shot 1-ns pulse Nd:YAG laser in a transverse and longitudinal single mode. It is found that the damage threshold of KDP single crystal depends on the laser irradiation direction, polarization direction and laser wavelength. The damage threshold in the direction of c axis is about two times higher than that of in the a(b) axis at 0.532 and 1.064 μm wavelength. This result is consistent with the mechanical strength tests for various directions of KDP crystal. Received: 19 February 1999 / Revised version: 3 September 1999 / Published online: 27 January 2000     

Reduced absorption of neon-like bromine X-ray laser radiation in helium

We have measured the absorption of the 19.47-nm neon like bromine (J=2–1) X-ray laser line in low-pressure helium. The experiment was motivated by the coincidence of this line with the low-absorption wing of an autoionizing transition in helium. We observe that, with 1 mbar of helium, the continuum background and another bromine X-ray laser line at 19.82 nm are strongly reduced, enhancing the relative strength of the 19.47-nm laser line. Increasing the helium pressure to 1.5 mbar makes the continuum virtually disappear, resulting in an almost monochromatic emission of the X-ray laser line. An estimate of the absorption cross section for the 19.47-nm line is given as ≈3.9×10^-19 cm² and for the nearby continuum as 0.9–1.3×10^-18 cm². Received: 8 March 1999 / Revised version: 26 April 1999 / Published online: 11 August 1999     

Application of electron paramagnetic resonance to the study of Cu2+ ions in Cu-Na ion-exchanged glasses

The copper ion-exchange process in two different silicate glasses is experimentally studied by means of electron paramagnetic resonance analysis. Differences in the environment of divalent copper ions are found depending on the substrate. The Cu²⁺ concentration in-depth profile is determined and compared to the total copper diffusion profile as determined from secondary ion mass spectrometry. Received: 12 February 1998 / Accepted: 5 January 1999 / Published online: 7 April 1999     

Micro-lens arrays generated by UV laser irradiation of doped PMMA

Micro-lenses with well-defined optical parameters are generated on polymethylmethacrylate (PMMA) substrates doped with diphenyltriazene (DPT) by controlled use of a swelling effect generated under conditions of subablative excimer laser illumination. The surface profiles depend on the laser spot size and energy density. A sensitively balanced combination of matrix softening, substrate volume expansion due to photochemical nitrogen release, and surface tension is responsible for the final shape of the lenses. Complete arrays of identical lenses with 15 μm diameters and a focal length of 30 μm are produced by irradiation of (0.25 wt. %) DPT-PMMA with a single laser pulse at a wavelength of 308 nm and a fluence of 3 J/cm². It is shown experimentally and theoretically that appropriate volume expansion is possible without introducing internal light scattering due to the formation of small bubbles. Received: 7 April 1999 / Accepted: 8 April 1999 / Published online: 5 May 1999     

Non-thermal ablation of neural tissue with femtosecond laser pulses

2 ; for 100 fs pulses from the same laser the experimental threshold was at 1.5 J/cm². Histopathological examinations and scanning electron micrographs confirm the high quality of the excisions. No sign of significant thermal damage was observed. Received: 29 January 1997/Revised version: 14 July 1997     

Laser-induced breakdown spectroscopy for on-line control of laser cleaning of sandstone and stained glass

Laser-induced breakdown spectroscopy (LIBS) was used to control the cleaning process of polluted sandstone and medieval stained glass. A combination of a KrF Excimer and LIBS was used to clean areas of the artwork. The spectroscopic study of the plasma emission was used to determine the elemental composition of the crust and the underlying material. The on-line implementation of the spectroscopic technique LIBS to the cleaning process provides important information about the optimal cleaning parameters of artworks from glass and stone in order to avoid over-cleaning. Received: 26 January 1999 / Accepted: 22 April 1999 / Published online: 7 July 1999     

The effect of doping and processing conditions on the optical performance of Rh:BaTiO3

We demonstrate the advantages of using high levels of rhodium (2000–3200 ppm) to dope barium titanate for achieving finite absorption coefficients (0.36 cm^-1), high two-beam coupling gain (11.5 cm^-1), and acceptable response time (7 s) at 1.06 μm. We also report on the mass spectroscopy measurements on Rh:BaTiO₃ samples indicating a small segregation coefficient for rhodium (below 0.01) and the presence of a relatively large concentration (6000 ppm) of unintentionally added strontium. Received: 20 November 1998 / Revised version: 25 January 1999 / Published online: 12 April 1999     

4-W and 23-ps pulses from a lamp-pumped Nd:YAG laser passively mode-locked by polarization switching in a KTP crystal

Passive mode-locking of a cw lamp-pumped Nd:YAG laser using nonlinear polarization switching in a type-II SHG crystal is reported. Light pulses with more than 5 W of average power and pulse duration shorter than 25 ps have been obtained at 1064 nm. Received: 29 January 1999 / Revised version: 24 March 1999 / Published online: 1 July 1999     

Oxidation-induced stoichiometric and morphological change of oxide films on stainless-steel surfaces

The oxidation-induced stoichiometric and morphological changes of the oxide film on a stainless-steel surface are observed by X-ray photoelectron spectroscopy and atomic force microscopy for annealing temperatures in the range 400–500 °C in oxygen partial pressures of 10^-9 to 10^-4 Torr With increasing the temperature, a significant shift occurs in the Cr 2p_3/2 binding energy towards higher energies, indicating a change in the oxidation state of chromium. It is found that at 450 °C lower oxygen partial pressures favor the formation of a smooth, pure chromium oxide. At a low oxygen pressure the oxide formed mainly consists of chromium oxide that shows a markedly smooth surface with no distinct grains, whereas at a high pressure the oxide formed mainly consists of iron oxide with distinct grains. Received: 27 January 1999 / Accepted: 18 March 1999 / Published online: 16 September 1999     

Simultaneous measurement of pure-optical and thermo-optical nonlinearities induced by high-repetition-rate, femtosecond laser pulses: application to CS2

A simple experimental technique is presented capable of separating the contribution of purely optical Kerr effects from that of thermo-optical effects in the nonlinear response of materials under high-repetition-rate laser irradiation. The technique has been realized by combining the single-beam Z-scan method with the single-beam thermal lens measurement method. We demonstrate this technique by analysing the nonlinear response at 770 nm of CS₂ which exhibits cumulative thermal effects when irradiated by very intense femtosecond laser pulses at a 76-MHz repetition rate. Received: 3 November 1998 / Revised version: 4 January 1999 / Published online: 2 June 1999     

Amorphous hydrogenated carbon synthesized by pulsed laser deposition from cyclohexane

It is demonstrated that a liquid hydrocarbon precursor, cyclohexane, is appropriate for laser-induced carbon deposition. Amorphous hydrogenated carbon films (a-C:H) were deposited by KrF excimer laser irradiation of single-crystal silicon surface immersed under cyclohexane. The technique is simple and easy to operate. IR absorption spectra of the deposited films confirmed the presence of carbon in the diamond phase. Raman and XPS studies showed diamond-like character of the deposited films. Moreover, these two studies provided strong evidence that laser fluence played an important role in the formation of DLC bondings and the quality of the deposited films. Received: 15 September 1998 / Accepted: 5 January 1999 / Published online: 5 May 1999     

Narrow bandwidth operation of high-power broad-area diode laser using cascaded phase-conjugate injection locking

A broad-area laser is injection-locked by another broad-area laser that is also injection-locked by a single-mode diode laser. Two double-phase conjugate mirrors of photorefractive BaTaO₃ are used to couple the master laser beams to the first slave laser, and the first slave laser output to the second slave laser. One of the double-phase conjugate mirrors is built up with the beams from two broad-area lasers. Two slave lasers are oscillating in single longitudinal mode at 808.5 nm and the spectral width is the same as that of the master laser. Final single-mode output power from the second slave broad-area laser is 840 mW, which is limited by the power of the injection beam. This work verifies the possibility of the multi-stage cascaded injection locking of high-power diode lasers with phase-conjugate injection. Received: 18 November 1998 / Revised version: 29 January 1999 / Published online: 7 April 1999     

Thermal effect on multiphonon-assisted anti-Stokes excited upconversion fluorescence emission in Yb3+-sensitized Er3+-doped optical fiber

The temperature effect upon infrared-to-visible frequency upconversion fluorescence emission in Yb³⁺-sensitized Er³⁺-doped germanosilicate optical fibers excited with cw radiation at 1.064 μm is investigated. The experimental results revealed an eightfold enhancement in the visible upconversion emission intensity as the fiber temperature was increased from 17 °C to 180 °C. The fluorescence emission enhancement is attributed to the temperature-dependent multiphonon-assisted anti-Stokes excitation process of the ytterbium sensitizer. A theoretical approach that takes into account a sensitizer absorption cross-section, which depends on the phonon occupation number, has proven to agree very well with the experimental data Received: 6 April 1999 / Revised version: 27 August 1999 / Published online: 27 January 2000     

Dynamics of helical-wave emission in a fiber-coupled diode end-pumped solid-state laser

We have generated TEM_0,l ^* modes in an end-pumped microchip laser using a standard fiber-coupled diode. A rich set of dynamic behaviors, such as periodic and quasi-periodic self-modulation, chaotic pulsing and frequency locking was observed in the generated TEM_0,l ^* modes. Experimental results confirm the theoretical predictions that the locking occurs as a subcritical bifurcation and that a region of coexisting locked and unlocked states exists. Received: 14 November 2000 / Revised version: 22 January 2001 / Published online: 23 May 2001     

Prompt electron emission and collisional ionization of ambient gas during pulsed laser ablation of silver

17 cm^-3 and it is found that the density increases as a function of distance away from the target. Dependence of probe current on laser intensity shows plasma shielding at high laser intensities. Received: 5 January 1998/Accepted: 3 July 1998     

Time-resolved current response of a nanosecond laser pulse illuminated STM tip

Time-resolved dependence of the transient current through a ns laser pulse illuminated scanning tunneling microscope (STM) tip/sample gap in tunneling mode and out of tunneling range is presented. A self-designed fast STM-preamplifier (bandwidth 35 MHz) allows one to resolve the fine structure of the transient signal as well as the observation of some effects that are undetectable by using conventional low-band preamplifiers. The dependence of the threshold laser pulse intensity, which corresponds to the beginning of electron emission from tip (in non-tunneling mode), as a function of the tip/sample distance was investigated. At tip/sample distances from tunnel contact up to approximately 1 μm a linear dependence is found. This behavior is in good agreement with the theory for field enhancement in a STM tip/sample system. In tunneling mode a ns (fast component) as well as a μs (slow component) current response was found as a result of the laser pulse illumination. These data suggest the tip bending to be an important factor in clarifying the thermal/mechanical mechanism of laser-assisted surface nanomodification. Received: 4 May 1998 / Accepted: 29 January 1999 / Published online: 28 April 1999     

Operation characteristics of the Au-II 690-nm laser transition in a segmented hollow-cathode discharge

Single-mode operation at the Au-II 690-nm transition was obtained in a segmented hollow-cathode discharge laser without the use of any additional frequency-selective device. The pressure of the helium buffer gas, which is responsible for the significant homogeneous broadening of the laser line, was varied between 10 and 20 mbar. The discharge was excited with rectangular current pulses (up to 3 A) six times exceeding the threshold value. The time dependence of the laser output during the 1-ms-long discharge pulses is explained on the basis of the temperature and pressure changes in the tube. The highest small-signal gain at optimal discharge conditions was 11%m⁻¹. Received: 1 July 1999 / Revised version: 4 November 1999 / Published online: 23 February 2000