Sub-100-nm laser-induced periodic surface structures upon irradiation of titanium by Ti:sapphire femtosecond laser pulses in air

The formation of laser-induced periodic surface structures (LIPSS) on titanium upon irradiation with linearly polarized femtosecond (fs) laser pulses (τ=30 fs, λ=790 nm) in an air environment is studied experimentally and theoretically. In the experiments, the dependence on the laser fluence and the number of laser pulses per irradiation spot has been analyzed. For a moderate number of laser pulses (N<1000) and at fluences between ～0.09 and ～0.35 J/cm², predominantly low-spatial-frequency-LIPSS with periods between 400 nm and 800 nm are observed perpendicular to the polarization. In a narrow fluence range between 0.05 and 0.09 J/cm², high-spatial-frequency-LIPSS with sub-100-nm spatial periods (～λ/10) can be generated with an orientation parallel to the polarization (N=50). These experimental results are complemented by calculations based on a theoretical LIPSS model and compared to the present literature.     

Characteristics of laser and current pulses in a He–SrCl2 vapor laser

An experimental investigation on the characteristics of laser and current pulses in a He–SrCl₂ vapor laser is carried out. The temporal dependences of the discharge current pulse on the laser pulses at the 1.09 μm, ～3 μm and 6.45 μm lines in strontium atoms and ions are measured and analyzed under different laser output powers. It is found that all laser pulses appear at the falling edge of the current pulse and shift forward to the current pulse with increasing laser output power.     

Ultra-high-precision mid-IR spectrometer II: system description and spectroscopic performance

The development and spectroscopic performance evaluation of an ultra-sensitive, mid-IR spectrometer is reported. The laser system is based upon difference-frequency generation (DFG) at ～3.5 μm by mixing a DFB diode laser at 1562 nm and a DFB fiber laser at 1083 nm using a periodically poled LiNbO₃ crystal. DFG radiation was coupled to a 100?m optical path length astigmatic Herriott cell. Sensitive and selective spectroscopic detection of formaldehyde was performed with second-harmonic detection using Peltier-cooled HgCdTe detectors. By applying computer lock-ins, dual-beam optical noise subtraction, focus matching, thermal stabilization, active wavelength control, and advanced signal processing a sensitivity corresponding to an absorbance ～1.6×10^-7 is achieved for 260 s of averaging.     

Multi-beam second-harmonic generation in beta barium borate with a spatial light modulator and application to internal structuring in poly(methyl methacrylate)

Parallel beam frequency doubling of 170 fs, NIR pulses is demonstrated by placing a thin beta barium borate (BBO) nonlinear crystal after a spatial light modulator. Computer-generated holograms applied to the spatial light modulator create 18 parallel diffracted beams at the fundamental wavelength λ=775 nm, then frequency doubled to λ=387 nm and focussed inside the poly(methyl methacrylate) (PMMA) substrate for refractive index structuring. This procedure, demonstrated for the first time in PMMA, requires careful attention to phase matching of multiple beams and opens up dynamic parallel processing at UV wavelengths where nematic liquid crystal devices are more sensitive to optical damage. By overlapping filamentary modifications, an efficient, stable volume phase grating with dimensions 5×5×2.0 mm³ and pitch Λ=15 μm was fabricated in 18 minutes and reached a first-order diffraction efficiency of 70 % at the Bragg angle.     

Narrow band X-ray emission in the water-window spectral region from a laser heated gold copper mix-<Emphasis Type="Italic">Z</Emphasis> plasma

A laser plasma X-ray source of narrow spectral range in the water-window region, is reported using a 50–50 (atomic fraction) mixture of gold-copper mix-Z planar target. Plasma was produced using the second harmonic beam of an Nd:glass laser focused to an intensity ～10¹³ W/cm² on the target. The spectrum of the plasma radiation transmitted through a free-standing 0.4 μm aluminium/0.9 μm vanadium X-ray filter foil was measured to lie in the narrow-band of 24–26 Å. This provides a debris-free X-ray dose of 2–3 mJ/sr which can be used for single shot X-ray imaging of live biological samples.     

Detection of C2H2 and HCl using mid-infrared degenerate four-wave mixing with stable beam alignment: towards practical in situ sensing of trace molecular species

A stable and convenient optical system to realize the forward phase-matching geometry for degenerate four-wave mixing (DFWM) is demonstrated in the mid-infrared spectral region by measuring DFWM signals generated in acetylene (C₂H₂) and hydrogen chloride (HCl) molecules by probing the fundamental ro-vibrational transitions. IR laser pulses tunable from 2900 cm^?1 to 3350 cm^?1 with a 0.025 cm^?1 linewidth were obtained using a laser system composed of an injection seeded Nd:YAG laser, a dye laser, and a frequency mixing unit. At room temperature and atmospheric pressure, a detection limit of 35 ppm (～ 9.5×10¹⁴ molecules/cm³) for C₂H₂ was achieved in a gas flow of a C₂H₂/N₂ mixture by scanning the P(11) line of the (010(11)⁰)–(0000⁰0⁰) band. The detection limit of the HCl molecule was measured to be 25 ppm (～6.8×10¹⁴ molecules/cm³) in the same environment by probing the R(4) line. The dependences of signal intensities on molecular concentrations and laser pulse energies were demonstrated using C₂H₂ as the target species. The variations of the signal line shapes with changes in the buffer gas pressures and laser intensities were recorded and analyzed. The experimental setup demonstrated in this work facilitates the practical implementation of in situ, sensitive molecular species sensing with species-specific, spatial and temporal resolution in the spectral region of 2.7–3.3 μm (3000–3700 in cm^?1), where various molecular species important in combustion have absorption bands.     

Extended homogeneous nanoripple formation during interaction of high-intensity few-cycle pulses with a moving silicon wafer

We report the study of extended nanoripple structures formed during the interaction of high-intensity 3.5 fs pulses with a moving silicon wafer. The optimization of laser intensity (8×10¹³ W?cm^?2) and sample moving velocity (1 mm?s^?1) allowed the formation of long strips (～5 mm) of homogeneous nanoripples along the whole area of laser ablation. The comparison of nanoripples produced on the silicon surfaces by few- and multi-cycle pulses is presented. We find that few-cycle pulses produce sharp and more homogenous structures than multi-cycle pulses.     

Characteristics of laser-produced Ge ion fluxes used for modification of semiconductor materials

This paper describes the laser generation of Ge ion fluxes and their application to the modification of semiconductor materials by ion implantation. The Ge ions were produced by ablating solid targets using the PALS high-power iodine laser system at the PALS Research Centre in Prague, operating at its third harmonic frequency (438 nm wavelength) and producing 0.4 ns pulses with energy up to 0.25 kJ (intensity≤10¹⁵ W/cm²). The goal of these investigations was optimisation of the implantation of low and medium energy laser-generated Ge ion fluxes and they were carried out as part of the project PALS000929. Recently, a new repetitive pulse laser system at IPPLM in Warsaw, with a wavelength of 1.06 μm, energy of ～0.8 J in a 3.5 ns-pulse, repetition rate of up to 10 Hz, and intensity on target of up to 10¹¹ W/cm², has also been employed to produce Ge ions by irradiating solid targets. The laser-generated ions were investigated with diagnostics based on the time-of-flight method: various ion collectors and an electrostatic ion-energy analyzer. The Ge ion fluxes were implanted into Si and SiO₂ substrates located at distances of 10–30 cm from the target. The SiO₂ films were prepared on single crystal Si substrates and were implanted with Ge ions with different properties. The properties of the Ge-implanted layers, in particular, the depth distributions of implanted Ge ions, were characterised using Rutherford backscattering and other material surface diagnostic methods.     

Thermal characterization of a liquid resin for 3D printing using photothermal techniques

Thermal properties of a liquid resin were studied by thermal lens spectrometry (TLS) and open photoacoustic cell (OPC), respectively. In the case of the TLS technique, the two mismatched mode experimental configuration was used with a He–Ne laser, as a probe beam and an Argon laser was used as the excitation source. The characteristic time constant of the transient thermal lens was obtained by fitting the theoretical expression to the experimental data in order to obtain the thermal diffusivity (α) of the resin. On the other hand, the sample thermal effusivity (e) was obtained by using the OPC technique. In this technique, an Argon laser was used as the excitation source and was operated at 514 nm with an output power of 30 mW. From the obtained thermal diffusivity (α) and thermal effusivity (e) values, the thermal conductivity (k) and specific heat capacity per unit volume (ρc) of resin were calculated through the relationships k = e(α)^1/2 and ρc = e/(α)^1/2. The obtained thermal parameters were compared with the thermal parameters of the literature. To our knowledge, the thermal characterization of resin has not been reported until now. The present study has applications in laser stereo-lithography to manufacture 3D printing pieces.     

Si homojunction structured near-infrared laser based on a phonon-assisted process

We fabricated several near-infrared Si laser devices (wavelength ～1300 nm) showing continuous-wave oscillation at room temperature by using a phonon-assisted process induced by dressed photons. Their optical resonators were formed of ridge waveguides with a width of 10 μm and a thickness of 2 μm, with two cleaved facets, and the resonator lengths were 250–1000 μm. The oscillation threshold currents of these Si lasers were 50–60 mA. From near-field and far-field images of the optical radiation pattern, we observed the high directivity which is characteristic of a laser beam. Typical values of the threshold current density for laser oscillation, the ratio of powers in the TE polarization and TM polarization during oscillation, the optical output power at a current of 60 mA, and the external differential quantum efficiency were 1.1–2.0 kA/cm², 8:1, 50 μW, and 1 %, respectively.     

Robust plasmonic substrates

We report on resonant infrared laser ablation of polystyrene using single 8 ps pulses at a wavelength of 3.31 μm generated by a MgO:PPLN optical parametric amplifier pumped by a Nd:YLF laser. We determined the single-pulse ablation threshold to be 0.46 J/cm², about a factor of five smaller than in previous free-electron-laser studies. Time-resolved imaging of the laser–target interaction reveals that the detailed dynamics of the ablation process begin with thermal expansion of a large volume of hot material from which a less dense plume of polymeric material evaporates. This plume disappears on a time scale of 0.75 μs and the hot polymer material recedes back into the crater from which it was expelled. Subsequently, and on a much longer time scale, structural alterations in the ablation crater continue to evolve for at least another millisecond. Our results suggest that single picosecond pulses are effective for the ablation of polymers and exhibit dynamics similar to those observed in studies using a free-electron laser.     

Diode-pumped orthogonally polarized dual-wavelength Nd3+:LaBO2MoO4 laser

Polarized spectroscopic properties related to 1.07 μm laser operation of a 1.8 at.% Nd³⁺:LaBO₂MoO₄ crystal grown by the Czochralski method were investigated at room temperature. Using a 2.2-mm-thick, Z-cut Nd³⁺:LaBO₂MoO₄ crystal as gain medium, orthogonally polarized dual-wavelength laser at 1,068 and 1,074 nm was first realized in a plano-concave resonator end-pumped by a quasi-continuous-wave 795 nm diode laser. A total output peak power of 1.2 W with slope efficiency of 26 % around 1.07 μm was obtained. The influences of resonator length and pump power on output laser wavelength were also investigated.     

Laser pulses designed in time by adaptive hydrodynamic modeling for optimizing ultra-fast laser–metal interactions

Determining optimal temporal pulse shapes is an essential aspect for controlling the nature and the energetic characteristics of the ablation products following laser irradiation of materials on ultra-fast scales. In this respect, adaptive feedback loops based on temporal pulse manipulation have been inserted into a hydrodynamic code. The procedure enables us to reach the theoretical maximal temperature at a certain energy input. Several regimes have been considered with fluences ranging from the ablation threshold (F _th=0.34 J/cm²) up to 10 J/cm², proposing an optimal coupling for laser–solid and laser–plasma interactions in these fluence regimes. We determine shapes of optimal pulses on ultra-short and short scales (up to 42 ps) and forecast optimized interaction scenarios with fundamental control factors difficult to access experimentally. Simulations performed on aluminum reveal that ultra-short pulses are the natural better solution for localizing energy in space and time for F～F _th. For higher fluences, pulses spread over tens of picoseconds and ended by a final peak enable a better impulsive coupling with the nascent plasma, optimizing its maximal temperature.     

Polycrystalline GaSb films prepared by the coevaporation technique

Gallium antimonide (GaSb) films were deposited onto fused silica and n-Si (100) substrates by coevaporating Ga and Sb from appropriate evaporation sources. The films were polycrystalline in nature. The size and the shape of the grains varied with the change in the substrate temperature during deposition. The average surface roughness of the films was estimated to be 10 nm. Grain boundary trap states varied between 2×10¹² and 2.2×10¹² cm^?2 while barrier height at the grain boundaries varied between 0.09 eV and 0.10 eV for films deposited at higher temperatures. Stress in the films decreased for films deposited at higher temperatures. XPS studies indicated two strong peaks located at ～543 eV and ～1121 eV for Sb 3d_3/2 and Ga 2p_3/2 core-level spectra, respectively. The PL spectra measured at 300 K was dominated by a strong peak located ～0.55 eV followed by two low intensity peaks ～0.63 eV and 0.67 eV. A typical n-Si/GaSb photovoltaic cell fabricated here indicated V _oc～311 mV and J～29.45 mA/cm², the density of donors (N _d)～3.87×10¹⁵ cm^?3, built in potential (V _bi)～0.48 V and carrier life time (τ)～28.5 ms. Impedance spectroscopy measurements indicated a dielectric relaxation time ～100 μs.     

Radiation effects on poly(methyl methacrylate) induced by pulsed laser irradiations

Poly(methyl methacrylate) (PMMA) was irradiated using a medical UV-ArF excimer laser operating at the fundamental wavelength of 193 nm. Characterized by a beam diameter of 1.8 mm and energy of 180 mJ with a Gaussian energy profile, it operates in a single mode or at 30 Hz repetition rate. Mechanical profilometry was carried out on ablation craters in order to study the rugosity and the ablation yield in the various operative conditions. Optical transmission and reflection measurements at six wavelengths were conducted in order to characterize the optical properties of the irradiated surfaces. Measured crater depths in PMMA were lower with respect to the forecasted ones in corneal tissue, while the lateral crater aperture was maintained. The rugosity produced at the crater bottom after irradiation was about 0.3 μm, and the ablation yield was about 10¹⁵ molecules/laser pulse, while etching depth and diameter show a roughly linear dependence on the number of laser shots. These experiments constitute a base for deeper clinical investigations.     

Influence of ordering change on the optical and thermal properties of inflation polyethylene films

Changes of thermal diffusivity inside femtosecond laser-structured volumes as small as few percent were reliably determined (with standard deviation less than 1%) with miniaturized sensors. An increase of thermal diffusivity of a crystalline high-density polyethylene (HDPE) inflation films by 10-20% from the measured (1.16 ± 0.01) × 10⁻⁷ m² s⁻¹ value in regions not structured by femtosecond laser pulses is considerably larger than that of non-crystalline polymers, 0-3%. The origin of the change of thermal diffusivity are interplay between the laser induced disordering, voids’ formation, compaction, and changes in molecular orientation. It is shown that laser structuring can be used to modify thermal and optical properties. The birefringence and infrared spectroscopy with thermal imaging of CH₂ vibrations are confirming inter-relation between structural, optical, and thermal properties of the laser-structured crystalline HDPE inflation films. Birefringence modulation as high as Δn ∼ ± 1 × 10⁻³ is achieved with grating structures.     

Preparation and comparative structural properties of porous SnO2 microrods and submicrorods

A facile two-step approach has been used for the synthesis of porous SnO₂ rods: the initial room-temperature precipitation of precursor SnC₂O₄ and its subsequent thermal decomposition at 550 °C. Both the as-obtained porous SnO₂ microrods (length ～10.0?±?3.5 μm, diameter ～1.1?±?0.4 μm) and submicrorods (length ～5.8?±?1.9 μm, diameter ～0.4?±?0.1 μm) are the crystalline mixtures of major tetragonal and minor orthorhombic crystal phases, showing a tetragonal fraction of 84.7 and 87.0 %, respectively. When applied as a lithium-ion battery anode, the porous submicrorods (specific surface area ～13.6 m² g^?1) can deliver an initial discharge capacity of 1,730.7 mAh g^?1 with a high coulombic efficiency of 61.6 % and show the 50th discharge capacity of 662.8 mAh g^?1 at 160 mA g^?1 within a narrow potential range of 10.0 mV to 2.0 V. Similarly, even the anode of porous microrods (specific surface area ～11.8 m² g^?1) can still exhibit an initial discharge capacity of 1,661.1 mAh g^?1 at 160 mA g^?1 with a coulombic efficiency of 60.9 %. Regardless of the polymorphic nature, the acquired porosity may only alleviate the huge volume change of anodes for the first cycle; thus, the structural parameters of average size and specific surface area can be feasibly associated with the enhanced lithium storage capability. Anyway, these indicate a facile oxalate precursor method for the controlling synthesis and high performance of rodlike SnO₂ for lithium-ion batteries.     

Third-order nonlinear optical response in transparent solids using ultrashort laser pulses

The third-order optical nonlinearity, χ ⁽³⁾, is measured in transparent glasses (BK7 and fused silica) and crystals (BaF₂ and quartz) using 36-fs, 800-nm laser pulses and the optical Kerr gate (OKE) technique; values are found to lie in the range 1.3–1.7×10^-14 esu, in accordance with theoretical estimates. We probe the purely electronic response to the incident ultrashort laser pulse in fused silica and BK7 glass. In BaF₂ and quartz, apart from the electronic response we also observe contribution from the nuclear response to the incident ultrashort pulses. We observe oscillatory modulations that persist for ～400 fs. The response of the media (glasses and crystals) to ultrashort pulses is also measured using two-beam self-diffraction; the diffraction efficiency in the first-order grating is measured to be in the range of 0.06–0.13 %. Third harmonic generation due to self-phase matching in the transient grating geometry is measured as a function of temporal delay between the two incident ultrashort pulses, yielding the autocorrelation signal.     

Acetylene detection based on diode laser QEPAS: combined wavelength and residual amplitude modulation

Quartz-enhanced photoacoustic spectroscopy (QEPAS) is demonstrated for acetylene detection at atmospheric pressure and room temperature with a fiber-coupled distributed feedback (DFB) diode laser operating at ~1.53 μm. An efficient approach for gas concentration calibration is demonstrated. The effect of residual amplitude modulation on the performance of wavelength modulated QEPAS is investigated theoretically and experimentally. With optimized spectrophone parameters and modulation depth, a minimum detectable limit (1σ) of ~2 part-per-million volume (ppmv) was achieved with an 8.44-mW diode laser, which corresponds to a normalized noise equivalent coefficient (1σ) of 6.16 × 10^?8 cm^?1 W/Hz^1/2.