Chandra S.R. Nathala

Experimental study of fs-laser induced sub-100-nm periodic surface structures on titanium.

In this work the formation of laser-induced periodic surface structures (LIPSS) on a titanium surface upon irradiation by linearly polarized femtosecond (fs) laser pulses with a repetition rate of 1 kHz in air environment was studied experimentally. In particular, the dependence of high-spatial-frequency-LIPSS (HSFL) characteristics on various laser parameters: fluence, pulse number, wavelength (800 nm and 400 nm), pulse duration (10 fs - 550 fs), and polarization was studied in detail. In comparison with low-spatial-frequency-LIPSS (LSFL), the HSFL emerge at a much lower fluence with orientation perpendicular to the ridges of the LSFL. It was observed that these two types of LIPSS demonstrate different fluence, shot number and wavelength dependencies, which suggest their origin is different. Therefore, the HSFL formation mechanism cannot be described by the widely accepted interference model developed for describing LSFL formation.

Nanoscale surface boiling in sub-threshold damage and above-threshold spallation of bulk aluminum and gold by single femtosecond laser pulses

The sub and near-threshold topographic signatures of the spallation of nanometer-thick melt layers during single-shot femtosecond laser ablation of bulk aluminum and gold were experimentally observed for the first time, using scanning electron microscopy with high spatial resolution. The novel ablative nanofeatures—sub-threshold boiling nanopits, the partially detached ultrathin solidified melt layer, and the lamellar surface structure under the layer along the spallative crater border, as well as the foam-like nanostructure of the crater bottom—indicate the boiling origin of the spallation threshold, rather than the thermomechanical rupture of the molten surface layer in a propagating unloading wave. These ablative surface nanofeatures were also revealed in trenches of single-shot, near-wavelength normal interference ripples on the aluminum surface, indicating their spallative nature and being promising for biosensing applications.

Dynamic polarization flip in nanoripples on photoexcited Ti surface near its surface plasmon resonance.

Both normal and abnormal sub-100-nanometer ripples (wavenumber ∼10  μm(-1)) were separately observed on Ti surfaces excited by linearly polarized IR femtosecond laser pulses at lower and higher fluences. Numerical modeling of dispersion curves for surface plasmon-polaritons on the photoexcited Ti surfaces demonstrates its surface plasmon resonance with the peak wavenumber ∼8  μm(-1) spectrally tuned by prompt surface optical response, prompt surface charging, and pre-oxidation, with normal/abnormal nanoripples appearing at its red/blue shoulders, respectively.

Effect of fluence and ambient environment on the surface and structural modification of femtosecond laser irradiated Ti

Under certain conditions, ultrafast pulsed laser interaction with matter leads to the formation of self-organized conical as well as periodic surface structures (commonly reffered to as, laser induced periodic surface structures, LIPSS). The purpose of the present investigations is to explore the effect of fsec laser fluence and ambient environments (Vacuum & O2) on the formation of LIPSS and conical structures on the Ti surface. The surface morphology was investigated by scanning electron microscope (SEM). The ablation threshold with single and multiple (N = 100) shots and the existence of an incubation effect was demonstrated by SEM investigations for both the vacuum and the O2 environment. The phase analysis and chemical composition of the exposed targets were performed by x-ray diffraction (XRD) and energy dispersive x-ray spectroscopy (EDS), respectively. SEM investigations reveal the formation of LIPSS (nano & micro). FFT d-spacing calculations illustrate the dependence of periodicity on the fluence and ambient environment. The periodicity of nano-scale LIPSS is higher in the case of irradiation under vacuum conditions as compared to O2. Furthermore, the O2 environment reduces the ablation threshold. XRD data reveal that for the O2 environment, new phases (oxides of Ti) are formed. EDS analysis exhibits that after irradiation under vacuum conditions, the percentage of impurity element (Al) is reduced. The irradiation in the O2 environment results in 15% atomic diffusion of oxygen.

Study on contribution of the asymmetric stress to the birefringence induced by an ultrashort single laser pulse inside fused silica glass

Stress induced birefringence due to asymmetry in axial and radial directions that is generated because of the interaction of ultrashort laser pulses with a transparent material is numerically studied. The coupled equations of nonlinear Schrodinger and plasma density evolution are solved to calculate the deposited energy density and initial temperature profile. Fouriers heat equation and the equations related to the thermo-elasto plastic model are solved to calculate the temperature evolution and distribution of induced displacement inside the material, respectively. Finally, by numerically calculating the distribution of the induced refractive index changes experienced by both axially and radially probe beams, induced stress birefringence is calculated for different characteristics of writing pulses. Furthermore, the induced stress birefringence is experimentally realized, and the effect of the energy of the writing pulse is investigated. To know how the induced refractive index changes and birefringence...

SEM and Raman spectroscopy analyses of laser-induced periodic surface structures grown by ethanol-assisted femtosecond laser ablation of chromium

The effect of fluence and pulse duration on the growth of nanostructures on chromium (Cr) surfaces has been investigated upon irradiation of femtosecond (fs) laser pulses in a liquid confined environment of ethanol. In order to explore the effect of fluence, targets were exposed to 1000 pulses at various peak fluences ranging from 4.7 to 11.8 J cm–2 for pulse duration of ∼25 fs. In order to explore the effect of pulse duration, targets were exposed to fs laser pulses of various pulse durations ranging from 25 to 100 fs, for a constant fluence of 11.8 J cm–2. Surface morphology and structural transformations have been analyzed by scanning electron microscopy and Raman spectroscopy, respectively. After laser irradiation, disordered sputtered surface with intense melting and cracking is obtained at the central ablated areas, which are augmented with increasing laser fluence due to enhanced thermal effects. At the peripheral ablated areas, where local fluence is approximately in the range of 1.4–4 mJ cm–2, very well-defined laser-induced periodic surface structures (LIPSS) with periodicity ranging from 270 to 370 nm along with dot-like structures are formed. As far as the pulse duration is concerned, a significant effect on the surface modification of Cr has been revealed. In the central ablated areas, for the shortest pulse duration (25 fs), only melting has been observed. However, LIPSS with dot-like structures and droplets have been grown for longer pulse durations. The periodicity of LIPSS increases and density of dot-like structures decreases with increasing pulse duration. The chemical and structural modifications of irradiated Cr have been revealed by Raman spectroscopy. It confirms the formation of new bands of chromium oxides and enol complexes or Cr-carbonyl compounds. The peak intensities of identified bands are dependent upon laser fluence and pulse duration.

Morphological characterization of chitosan biopolymer thin films modified via fs irradiation and its potential application as functional surfaces in regenerative medicine

The creation of microporous surface modification of chitosan thin films irradiated by ultrashort laser pulses are studied. For this purpose, chitosan substrates were treated by using an amplified Ti:sapphire laser system at 800 nm central wavelength with 30 fs and 150 fs pulse duration and repetition rate 1 kHz and 50 Hz, respectively. Formation of surface modifications for both cases (30 fs and 150 fs) after femtosecond laser irradiation were observed. The threshold values for single-pulse (N = 1) and multi-pulse (N > 1) modification were evaluated by studying the linear relationship between the squared crater diameter D2 and the logarithm of the laser fluence (F) for N = 1, 2, 5, 10, 20, 30 and 50 number of laser pulses. The coefficient of incubation ξ, a major parameter in the process of surface modification and ablation of materials also was calculated for multi - pulse fluence threshold estimation by power - law relationship Fth (N) = Fth (1) Nξ-1, where N is the number of applied laser pulses. The surface properties of chitosan based thin films before and after femtosecond laser irradiation were investigated. The aim of this work is to determine the optimal morphological characteristics of the created structures for tailoring of protein adsorption and cell behavior.

Femtosecond laser micropatterning of chitosan thin films for surface functionalization

The possibility to control surface properties of materials and to tailor behaviour of cells and biomolecules are the basic requirements in the development of a new generation biomaterials for applications in tissue engineering. Surface patterning on micro and nano-scale is critical to distinguish the effects of cell shape, focal adhesion, and ligand input for cell functions. Recently, much attention has been paid to laser-assisted micro and nanofabrication technologies to pattern surfaces with different topographies for providing valuable inside on cell-substrate junction [1]. Laser modification by pulses in the femtosecond time domain, provide a quality of modification of thin films of biopolymers that is unobtainable with longer pulses in the range of nanoseconds [2].

Parabolic-like nanoantennas fabrication by femtosecond laser pulses for strong-field plasmonics

We have demonstrated for the first time that an array of nanoantennas on an aluminum surface, fabricated using a double-pulse femtosecond laser irradiation scheme, results in a 28-fold enhancement of electron photoemission yield, driven by a third femtosecond laser pulse. The numerical electrodynamic modeling indicates that the electron emission is increased owing to instant local electromagnetic field enhancement near the nanoantenna, contributed by both the tips “lightning rod” effect and the focusing effect of the pit as a microreflector in parabolic nanoantenna.