S. K. Das

Femtosecond laser-induced periodic surface structures revisited: A comparative study on ZnO

Laser-induced periodic surface structures (LIPSS) (ripples) with different spatial characteristics have been observed after irradiation of single-crystalline zinc oxide surfaces with multiple linearly polarized femtosecond pulses (150–200 fs, 800 nm) in air. For normal incident laser radiation, low spatial frequency LIPSS (LSFL) with a period (630–730 nm) close to the wavelength and an orientation perpendicular to the laser polarization have been found in the fluence range between ∼0.7 and ∼0.8 J/cm2 and predominantly for pulse numbers up to N=100. For lower fluences (0.5–0.7 J/cm2), a sharp transition from the LSFL features toward the formation of high spatial frequency LIPSS (HSFL) appears at any given pulse number below N=100. The HSFL are always parallel to the LSFL, exhibit spatial periods between 200 and 280 nm, and completely substitute the LSFL for pulse numbers N>100. Additionally, the influence of the angle of incidence has been studied experimentally for both LIPSS types revealing a different b...

Formation of laser-induced periodic surface structures on fused silica upon multiple cross-polarized double-femtosecond-laser-pulse irradiation sequences

The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of fused silica with multiple irradiation sequences consisting of five Ti:sapphire femtosecond (fs) laser pulse pairs (150 fs, 800 nm) is studied experimentally. A Michelson interferometer is used to generate near-equal-energy double-pulse sequences with a temporal pulse delay from −20 to +20 ps between the cross-polarized individual fs-laser pulses (∼0.2 ps resolution). The results of multiple double-pulse irradiation sequences are characterized by means of Scanning Electron and Scanning Force Microscopy. Specifically in the sub-ps delay domain striking differences in the surface morphologies can be observed, indicating the importance of the laser-induced free-electron plasma in the conduction band of the solids for the formation of LIPSS.

Ultrashort highly localized wavepackets.

The recently introduced concept of radially non-oscillating, temporally stable ultrashort-pulsed Bessel-like beams we referred to as needle beams is generalized to a particular class of highly localized wavepackets (HLWs). Spatio-temporally quasi-nondiffracting pulses propagating along extended zones are shaped from Ti:sapphire oscillator radiation with a spatial light modulator and characterized with spatially resolved second order autocorrelation. Few-cycle wavepackets tailored to resemble circular disks, rings and bars of light represent the closest approximation of linear-optical light bullets known so far. By combining multiple HLWs, complex pulsed nondiffracting patterns are obtained.

Second harmonic generation of femtosecond pulses using ZnO nanorods grown by chemical bath deposition with drop casted seed layer

We report efficient second harmonic generation (SHG) of femtosecond (fs) pulses using ZnO nanorods grown by chemical bath deposition (CBD) method with drop-casted seed layer. The SHG behavior of the nanorods are tested using an amplified Ti:sapphire fs laser of pulse duration of 100fs at 800nm. The SHG signal from the ZnO nanorods is found to be of very high intensity as detected by a low cost, compact spectrometer. In a comparative study, the SHG signal from ZnO nanorods grown over seed layer is found to be 12 times higher than the SHG signal observed from the ZnO nanorods grown on substrate without any seed layer. The efficient SHG in former case is due to the growth of high density, well oriented nanorods whereas the lower signal in the later case is due to growth of low density, randomly oriented nanorods. The polarization dependence behavior of the SHG signal is studied both experimentally and theoretically.

Noncollinear autocorrelation with radially symmetric nondiffracting beams

Fringe-resolved noncollinear autocorrelation extracts information about the pulse duration of ultrashort optical signals from analyzing the intensity envelope of fringes. By detecting nonlinear autocorrelation functions after frequency conversion, even an evaluation of temporal asymmetry and frequency chirp are enabled. Here we report on a modified approach based on replacing crossed plane waves by Bessel-like beams. In comparison to the conventional method, appropriate mathematical transforms have to be applied. The method is simple and single-shot capable and takes advantage of specific advantages of pseudo-nondiffracting beams. First proof-of-principle experiments with few-femtosecond pulse durations were performed and compared to simulations. In multishot operation regime, the implementation of phase-shifting procedures by spatial light modulators promises considerable improvements of the time resolution analogous to the known principle of phase-shift interferometry.

Scattering-controlled femtosecond-laser induced nanostructuring of TiO2 thin films

The formation of laser induced periodic surface structures (LIPSS) is to a large extent of self-organizing nature and in its early stages essentially influenced by optical scattering. The evolution of related mechanisms, however, has still to be studied in detail and strongly depends on materials and laser parameters. Excitation with highly intense ultrashort pulses leads to the creation of nanoripple structures with periods far below the fundamental wavelength because of opening multiphoton excitation channels. Because of the drastically reduced spatial scale of such laser induced periodic nanostructures (LIPNS), a particular influence of scattering is expected in this special case. Here we report on first investigations of femtosecond-laser induced nanostructuring of sputtered titanium dioxide (TiO2) layers in comparison to bulk material. The crucial role of the optical film quality for the morphology of the resulting LIPNS was worked out. Typical periods of nanoripples were found to be within the range of 80-180 nm for an excitation wavelength of 800 nm. Unlike our previously reported results on bulk TiO2, LIPNS in thin films appeared preferentially at low pulse numbers (N=5-20). This observation was explained by a higher number of scattering centers caused by the thin film structure and interfaces. The basic assumptions are further supported by supplementary experiments with polished and unpolished surfaces of bulk TiO2 single crystals.

Highly periodic laser-induced nanostructures on thin Ti and Cu foils for potential application in laser ion acceleration

The feasibility of femtosecond laser-induced periodic nanostructures on thin Ti and Cu foils (thickness down to 1 μm) is demonstrated. At pulse durations of 120 fs and a wavelength of 400 nm, periods of 61 nm to 320 nm were obtained. Particle-in-cell simulations of laser ion acceleration processes with such nanostructured targets indicate their potential for high energy particle physics applications. In particular, a measurable enhancement of the proton cut-off energy and a significant enhancement of the number of accelerated particles compared to non- or weakly structured targets of same thickness and material are expected.

MEMS axicons for nondiffracting line shaping of ultrashort pulses

For a growing number of applications in nonlinear spectroscopy, micro- and nano-machining, optical data processing, metrology or medicine, an adaptive shaping of ultrashort pulsed, ultrabroadband laser beams into propagation-invariant linear focal zones (light blades) is required. One example is the femtosecond laser high-speed large area nanostructuring with moving substrates and cylindrical optics we reported about recently. Classical microoptical systems, however, distort the temporal pulse structure of few cycle pulses by diffraction and dispersion. The temporal pulse transfer can be improved with innovative types of reflective MEMS axicons based on two integrated rectangular mirrors, tilted by a piezoelectric bending actuator. In contrast to pixelated liquid-crystal-on-silicon (LCoS) based devices, cutoff frequencies in multi-kilohertz range, a purely reflective setup and continuous profiles with larger phase shift are realized which enable for shaping extended propagation-invariant zones at a faster and more robust operation. Additionally, a fixed phase offset can be part of the structure. Here, the performance of a prototype of linear mechanically tunable MEMS axicon is demonstrated by generating a pseudo-nondiffracting line focus of variable diameter and depth extension from a femtosecond laser pulse. The temporal transfer of 6-fs pulses of a Ti:sapphire laser oscillator is characterized with spectral phase interferometry for direct electric-field reconstruction (SPIDER) and spatially resolved nonlinear autocorrelation. Spatial and temporal self-reconstruction properties were studied. The application of the flexible focus to the excitation of plasmon-polaritons and the self-organized formation of coherently linked deep sub-wavelength laser-induced periodic surface structures (LIPSS) in semiconductors and dielectrics is reported.

Femtosecond-Laser Induced Periodic Surface Structures for Surface Enhanced Raman Spectroscopy of Biomolecules

Nanostructured metal surfaces can be used as high-efficiency substrates for surface-enhanced Raman spectroscopy (SERS). For this purpose, laser induced periodic surface structures (LIPSS) were generated by illuminating silver and copper substrates with pulses of a 1-kHz Ti:sapphire femtosecond laser and second harmonic generation (SHG). We report on the details of the structuring experiments and the application of the metallic nanogratings to the SERS of selected types of biomolecules like DNA (herring sperm) and protein molecules (egg albumen) (Messaoudi et al. Proc. SPIE 8972: 8972-17, 2014). Maximum enhancement was detected for Ag substrates processed with at the SHG wavelength of 400 nm and structure periods at a comparable scale. In contrast to that, long-period ripples induced at the fundamental wavelength of 800 nm were found to show the highest enhancement for copper substrates. As a possible explanation we assume an additional significant influence of nanoparticles deposited on the surface. A structuring speed in the range of 0.125 mm2/s was obtained. The surface quality was characterized by field emission scanning electron microscopy and Raman spectroscopy. As a probably significant factor for the functionality, the chemical modifications of the metal structures were studied as well. The analysis of the vibration spectra indicates that the generation of LIPSS in air leads to thin oxidation layers which may improve the biocompatibility of the nanostructures. The better insight into the complex mechanisms of laser-induced nanostructure formation is expected to further stimulate neighboring fields of applications of functionalized surfaces like photocatalysis, photovoltaics or biomedicine.

Temporal self-reconstruction of few-cycle nondiffracting wavepackets

The temporal self-reconstruction of pulsed Bessel-like needle beams was studied. Arrays of nondiffracting sub-7-fs needle beams were shaped from Ti:sapphire oscillator pulses by programming multiple axicons in a phase-only spatial light modulator. Defined distortions in the time domain were induced by local spectral filtering. By differently shading parts of selected sub-beams, the self-reconstruction was analyzed under variable conditions. Pulse duration maps were measured with two-dimensional second order autocorrelation based on the Shack-Hartmann sensor principle of wavefront division. Completely distorted pulses were found to have a pulse duration of > 13 fs whereas partially distorted sub-beams returned to pulse durations close to the initial ones. Specific applications are proposed.