Internal reflection mode scanning near-field optical microscopy with the tetrahedral tip on metallic samples

An internal reflection mode is introduced for scanning near-field optical microscopy (SNOM) with the tetrahedral tip. A beam of light is coupled into the tip and the light specularly reflected out of the tip is detected as a photosignal for SNOM. An auxiliary STM mode is used to control the distance during the scanning process and to record the topography of the sample simultaneously with the SNOM image. Images were obtained of different metallic samples which show a contrast in the order of 10% of the total reflected photosignal. In images of metallic samples an inverted contrast is consistently obtained compared to images previously obtained of comparable samples in a transmission mode. The contrast shows a pronounced dependence on the polarization of the incident beam with respect to the orientation of the edges of the tip. In the case of gold surfaces, the photosignal as a function of distance between the tip and the surface shows a pronounced peak in the near-field range of 0–20 nm which is tentatively attributed to the excitation of surface plasmons on the gold surface. The pronounced near-field effects and the strong contrast in the near-field images and the resolution well below 50 nm are an indication of a highly efficient coupling of the incident beam to a local excitation of the tip apex which is essential for the function of the tip as a probe for SNOM. Received: 17 May 1999 / Accepted: 18 May 1999 / Published online: 21 October 1999     

Diode-pumped erbium-ytterbium-glass laser passively Q-switched with a PbS semiconductor quantum-dot doped glass

Q-switched and cw operation of different diode-pumped erbium-ytterbium doped glasses at 1.5 μm has been studied in a compact microlaser setup. For Q-switching we used a novel PbS semiconductor quantum-dot doped glass which offers low saturation intensity compared with typical absorbers used and a fast time response. The cw laser delivered output powers of 35 mW with slope efficiencies of 16%. In Q-switched operation pulse energies of 1 μJ at repetition rates of 1–2 kHz and pulse durations of about 30–50 ns, depending on absorber thickness were obtained. Received: 5 June 2000 / Revised version: 29 June 2000 / Published online: 13 September 2000     

Calculation of the temperature and thermal expansion of a STM tip heated by a short laser pulse

A mathematical model for the calculation of the temperature field in a scanning tunneling microscope (STM) tip under laser illumination is developed. The duration of the laser pulse is a few nanoseconds or shorter. A Gaussian distribution of the laser light intensity in time and space is assumed. Two different mechanisms of tip heating are taken into account: 1. due to an enhanced electric field on the tip; 2. due to heating of the side surface of the tip by the focused spot of laser light. An average tip temperature is calculated using the heat conductivity equation. The enhanced electric field on the tip is calculated by the method of boundary integral equations. Received: 20 August 2002 / Revised version: 4 December 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-2551/962-490, E-mail: sklein@fh-muenster.de     

Effects of a prepulse on laser-induced EUV radiation conversion efficiency

The influence of prepulses on the conversion efficiency (CE) of laser radiation into 13 nm extreme ultraviolet (EUV) radiation, resulting from lithium-like oxygen ions from O20 μm water droplets, was investigated. The laser pulse durations in the experiment ranged from 200 fs to 120 ps. Applying prepulses preceding the main pulse at various delays of up to 11 ns, it was shown that the CE increases differently for each measured pulse duration. The strongest dependence on the introduction of a prepulse was observed for 2 ps laser pulses with a 20 mJ laser pulse energy. The EUV CE was improved by a factor of 15 by the introduction of a prepulse. Calculations on the atomic physics of oxygen ions and simulations of the laser–plasma interaction revealed the influence of the prepulse on the EUV yield. Received: 25 October 2002 / Published online: 22 January 2003 RID="*" ID="*"Corresponding author. Fax: +49-3641/947-202, E-mail: duesterer@ioq.uni-jena.de     

Low-threshold field electron emission of Si micro-tip arrays produced by laser ablation

Low-threshold field electron emission (FEE) is reported for periodic arrays of micro-tips produced by laser ablation of Si wafers. The best samples show emission at threshold fields as low as 4–5 V/μm for n-type Si substrates and of 1–2 V/μm for p-doped Si substrates, as measured with a flat-screen technique. Auger electron spectroscopy and X-ray electron spectroscopy reveal island-like deviation of the SiO₂ stoichiometry on the tip surfaces, with lateral dimensions of less than 100 nm. Microscopic studies using a special field-emission STM show that the emission originates from well-conducting regions of sub-micron size. The experimental data suggest FEE from the tip arrays by a geometric field enhancement of both the individual micro-tip and the narrow conducting channels in the tip body. Received: 3 May 2002 / Accepted: 1 July 2002 / Published online: 28 October 2002 RID="*" ID="*"Corresponding author. Fax: +7-095/135-82-34, E-mail: shafeev@kapella.gpi.ru     

Time-resolved STM light emission from an evaporated Au film

STM (scanning tunneling microscope) light emission from an evaporated Au film irradiated by pico second laser pulses has been investigated for various bias voltages of the STM. The observed emission consists of two components. The first component that has the same duration as the incident laser pulse is excited by laser induced tunneling electrons. The second component that has a longer duration than that of the laser pulse is emitted from surface polariton plasmons excited non-linearly by the laser pulse.     

Optical near-field distribution in an asymmetrically illuminated tip–sample system for laser/STM nanopatterning

In surface nano-patterning using an atomic force microscope (AFM) tip in scanning tunnelling microscopy (STM) mode and illuminated by a laser, two controversial physical mechanisms exist in the literature: the field-enhancement (FE) model and the thermal-induced mechanical contact (TMC) model. Due to the presence of evanescent waves in the optical near-field, the exact calculation of the field distribution of the tip–sample system in micro/nano scales becomes complicated. There is a lack of understanding of the asymmetrically illuminated tip–sample system. In this paper, full 3D finite-difference time-domain (FDTD) analysis was carried out to investigate the field distribution in different tip–sample systems. The effects of different tip/sample materials (either dielectric or plasmonic material), the gap distance, and laser incidence angles on the field distribution/enhancement have been studied. For the first time, we have demonstrated two new effects which are helpful in distinguishing the controversial mechanisms: (1) on the sample surface, the field peak position has a shift away from the tip-axis at large angles of incidence, and (2) the field enhancement could depend strongly on the horizontal component (perpendicular to tip-axis) of the incident wave instead of the vertical component (along tip-axis). The optimal incident angle is around 30° for the maximum field under the tip. The existence of field-distribution nodes on the 3D tip surface that leads to the in-homogenous heating of the tip is also predicted. PACS 81.16.Mk; 61.80.Ba; 81.16.Rf; 81.65.Cf     

Controlled two-pulse generation in Q-switch mode from a single laser using Q-modulator with frustrated total internal reflection

In this work a cavity design for double-pulse generation in Q-switched mode from a single laser is proposed, based on the construction of a second laser channel using a FTIR (frustrated total internal reflection) Q-modulator. A time interval between the two pulses from 500 ns to 120 μs is obtained in a Nd:YAG laser. A comparison with other methods and cavity designs for double-pulse generation is presented. The case when this technique is applied on a tunable laser with metastable upper laser level (Cr:LiCAF, Cr:LiSAF, Alexandrite, Co:MgF₂ or other) is also considered. Even though the method presented in the paper does not rely only on the cavity configuration proposed, some advantages can be obtained – both polarization and wavelength-independent tuning without polarization and wavelength restrictions in combination with the possibility of different wavelengths and polarizations in each pulse. Moreover, by using an active medium generating wavelengths around and up to 3 μm, the Pockels-cell-Q-switch optical transmission problems can be avoided. Received: 9 May 2001 / Revised version: 2 August 2001 / Published online: 15 October 2001     

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     

Simple generation of tunable near-infrared picosecond dye-laser pulses using spectro-temporal selection

The spectral and temporal processes in broadband pulsed-laser emissions of near-infrared dyes generated from low-Q and short laser cavities have been investigated with a rate equation model extended to a multiwavelength analysis. A very fast spectral evolution is proved, and this leads to the intrinsic presence of a single short pulse on the short-wavelength wing of the broadband IR dye laser spectrum. From the results, the direct generation of picosecond (≈90 ps)infrared dye-laser pulses adjustable in the 700–850 nm spectral range is successfully demonstrated with spectro-temporal selection, in a compact and simple device, using a nitrogen laser (337.1 nm, 8 ns) or a Q-switched ruby laser (694.3 nm, 25 ns) as a pumping source. Received: 6 October 1998 / Revised version: 14 April 1999 / Published online: 11 August 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     

Laser ablation and micropatterning of thin TiN coatings

Laser ablation of thin TiN films deposited on steel substrates has been studied under wide-range variation of irradiation conditions (pulsewidth, wavelength, energy density and spot size). It has been demonstrated that both picosecond (150–300 ps) and nanosecond (5–9 ns) laser pulses were suitable for controllable ablation and microstructuring of a 1-μm-thick TiN film unlike longer 150-ns pulses. The ablation rate was found to be practically independent of the wavelength (270–1078 nm) and pulsewidth (150 ps–9 ns), but it increased substantially when the size of a laser spot was reduced from 15–60 μm to 3 μm. The laser ablation technique was applied to produce microstructures in the thin TiN films consisting of microcraters with a typical size of 3–5 μm in diameter and depth less than 1 μm. Tests of lubricated sliding of the laser-structured TiN films against a steel ball showed that the durability of lubricated sliding increased by 25% as compared to that of the original TiN film. Received: 28 July 1999 / Accepted: 17 April 2000 / Published online: 20 September 2000     

Hard tissue ablation with a free running Er:YAG and a Q-switched CO2 laser: a comparative study

The Er:YAG and the CO₂ laser are competitors in the field of hard tissue ablation. The use of Er:YAG lasers (2.94 μm, pulse length _L of 100 to 200 μs) show smaller areas of thermal defects then ‘‘superpulsed’’ CO₂ lasers with pulse lengths of approximately 100 μs. Only the development of a Q-switched CO₂ laser (9.6 μm, τ_L=250 ns) allowed for similar results. In this paper new results for the Er:YAG and the Q-switched CO₂ laser under the influence of water spray will be presented. Several parameters are of special interest for these investigations: the specific ablation energy, which shows a minimum for the CO₂ laser at an energy density of 9 J/cm ² and a broad shallow minimum in the range of 10 to 70 J/cm² for the Er:YAG laser, and comparison of the cut-shape and depth. Surface effects and cutting velocity are discussed based on SEM pictures. Received: 19 July 2000 / Revised version: 1 November 2000 / Published online: 30 November 2000     

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     

Comparative time-resolved study of solid-state and liquid ablation of polyethylene-glycol 1000: temperature, viscosity and surface tension dependence

Comparative study of solid and liquid phase ablation on the same sample by time-resolved investigations is presented in this paper. Polyethylene-glycol (PEG) 1000 having relatively low melting point (35 °C) was used in our experiments. By varying the sample temperature in the 20–80 °C range we could study the ablation mechanism in both solid and liquid (below and above the melting point) state of matter. An ArF excimer laser (λ=193 nm, FWHM=20 ns) was used for ablation at 1.95 J/cm² fluence. Ablation processes were observed by transmission fast photographic arrangement. It was demonstrated that plasma development and expansion (primer ablation, in 0–50 ns time range), formation and propagation parameters of shock wave and contact front did not depend on sample temperature and state of matter. The secondary material ejection (between 1–100 μs) showed a strong temperature dependence. Material ejection in the case of solid target occurred in the form of dense material cloud, and in the form of splashing for liquid (molten) sample. The ejection velocity of splashed jets depended on the sample temperature, significantly. This can be due to the change of molten PEG 1000 viscosity. Received: 1 November 1999 / Accepted: 17 April 2000 / Published online: 5 July 2000     

Imaging a tunneling ionization front by using a schlieren method

A schlieren method was used to generate time-resolved images of the tunneling ionization front produced when an ultrashort high-power laser pulse irradiates He gas. By superimposing sequential schlieren images, we obtained information about the laser propagation and found that the ionization front propagated farther with decreasing density of the target gas. Ray-tracing suggested that this density dependence is a result of the spatial distribution of the laser intensity. Received: 20 May 1999 / Revised version: 19 August 1999 / Published online: 27 January 2000     

Field-induced transfer of lithium in a scanning tunneling microscope

Field-induced transfer of Li atoms from a [110]-oriented W tip to a Pt sample is realized in the scanning tunneling microscope (STM) under UHV conditions, using a working principle of the new solid state Li surface diffusion metal ion source (Li-SDMIS), in which the supply of Li to the apex of the tip occurs via surface diffusion. By applying 5–10 μs voltage pulses to the Li-covered W tip placed within tunneling range, single Li hillocks (400 × 400 Ų) were formed on the chosen area of the Pt(110) sample in a well-rep way. The pulse magnitude necessary for Li emission displays a distinct threshold character. A comparative analysis of the energetics of Li field desorption in the STM and Li-FDM (lithium field desorption microscope) modes indicates chemically-assisted field desorption of Li as the field-induced transfer mechanism.     

Experimental investigation of pulse diodes side-pumped 2 μm Tm:YAG lasers

The free running and Q-switched operation of 2 μm Tm:YAG lasers side-pumped by pulse laser diodes were reported. In the free running mode the maximum output energies were 102.5 mJ at 1 Hz and 94.6 mJ at 10 Hz, respectively. With an acousto-optic modulator in the laser resonator, 21 mJ 2 μm Q-switched pulse was obtained, with a pulse width of about 330 ns. The dependences of the output energy and the efficiency on the laser resonator parameters were investigated.     

Bistability of single 1,5 cyclooctadiene molecules on Si(001)

The adsorption and current-induced bistability of single 1,5 cyclooctadiene molecules on Si(001) were studied in ultrahigh vacuum by low-temperature scanning tunneling microscopy (STM). After a dosage of ≈0.05 L at room temperature followed by cooling to the measuring temperature of 7 K, we find that the cyclic alkene molecule preferably adsorbs in the bridge structure with both C=C double bonds reacting with two adjacent Si dimers via [2+2] cycloaddition reaction. The time-dependent current measured upon tunneling through the adsorbed molecule at fixed STM tip height displays a switching between two current levels with the same mean residence time in each level. Higher bias and/or reduced tip height—and therefore higher current—increase the switching rate, suggesting that the reversible switching is due to inelastic electron tunneling. The observed bistability is interpreted as a dynamic interconversion between two degenerate conformations of the adsorbed molecule.     

High-power picosecond LiB3O5 optical parametric oscillators tunable in the blue spectral range

The paper reports on an experimental investigation and numerical analysis of noncritically and critically phasematched LiB₃O₅ (LBO) optical parametric oscillators (OPOs) synchronously pumped by the third harmonic of a cw diode-pumped mode-locked Nd:YVO₄ oscillator–amplifier system. The laser system generates 9.0 W of 355-nm mode-locked radiation with a pulse duration of 7.5 ps and a repetition rate of 84 MHz. The LBO OPO, synchronously pumped by the 355-nm pulses, generates a signal wave tunable in the blue spectral range 457–479 nm. With a power of up to 5.0 W at 462 nm and 1.7 W at 1535 nm the conversion efficiency is 74%. The OPO is characterized experimentally by measuring the output power (and its dependence on the pump power, the transmission of the output coupler and the resonator length) and the pulse properties (such as pulse duration and spectral width). Also the beam quality of the resonant and nonresonant waves is investigated. The measured results are compared with the predictions of a numerical analysis for Gaussian laser and OPO beams. In addition to the blue-signal output visible-red 629-nm radiation is generated by sum-frequency mixing of the 1.535-μm infrared idler wave with the residual 1.064-μm laser radiation. A power of 1.25 W of 1.535-μm idler radiation and 5.7 W of 1.064-μm laser light generated a red 629-nm output power of 2.25 W. Received: 2 February 2000 / Revised version: 28 July 2000 / Published online: 22 November 2000