High Spatial Frequency Laser Research Articles

Sub-100 nm high spatial frequency periodic structures driven by femtosecond laser induced desorption in GaAs

This paper presents a previously unreported mechanism for the formation of High Spatial Frequency Laser Induced Periodic Surface Structures (HSFL) in GaAs upon irradiation by femtosecond laser pulses (repetition rate = 1 kHz, τ = 150 fs, λ = 390 nm) that is driven by point defect diffusion, desorption of surface atoms, and roughening of the surface. The HSFL have trenches that are 100 nm deep, an average spatial period of 65 nm, and are completely below the original surface. Sub-100 nm periodicity with high depth to period aspect ratio has not been previously observed in GaAs. In the proposed mechanism, laser irradiation generates point defects that diffuse to the surface. Interstitials that reach the surface can be easily desorbed and the remaining vacancies coalesce into vacancy islands. This results in a rough surface, which can excite surface plasmon polaritons. Despite our observations of periodicity corresponding to SPPs, calculations done using the excited dielectric function indicate that SPPs should not be supported. This points to the need to incorporate the cumulative effects of laser irradiation in existing models. This paper also presents evidence that desorption can also occur during HSFL formation in GaAs when irradiated with 780 nm in vacuum. The HSFL have the similar spatial period as GaAs irradiated in air with 780 nm, but completely below the original surface.

Morphological Study of Nanostructures Induced by Direct Femtosecond Laser Ablation on Diamond.

High spatial frequency laser induced periodic surface structure (HSFL) morphology induced by femtosecond laser with 230 fs pulse duration, 250 kHz repetition rate at 1030 nm wavelength on CVD diamond surface is investigated and discussed. The spatial modification was characterized and analyzed by Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and 2D-Fast Fourier Transform (2D-FFT). We studied the effect of pulse number and laser power on the spatial development of nanostructures, and also deduced the impact of thermal accumulation effect on their morphology. A generalized plasmonic model has been used to follow the optical evolution of the irradiated surface and to determine the periodic value of the nanostructures. We suggest that non-thermal melting and plasmonic excitation are the main processes responsible for the formation of HSFL-type nanostructures.

Alignment of morphology during high spatial frequency periodic structure formation in GaAs

The interaction between multiple intense ultrashort laser pulses and solids is known to produce a regular nanoscale surface corrugation. A coupled mechanism has been identified that operates in a specific range of fluences in GaAs that exhibits transient loss of the imaginary part of the dielectric function and Χ2, which produces a unique corrugation known as high spatial frequency laser induced periodic surface structures (HSFL). The final structures have 180 nm periods, and their alignment perpendicular to the laser polarization is first observed in an intermediate morphology with correlation distances of 150 ± 40 nm. Quantum molecular dynamics simulations suggest that HSFL self-assembly is initiated when the intense laser field softens the interatomic binding potential, which leads to an ultrafast generation of point defects. The morphological evolution begins as self-interstitial diffusion, driven by stress relaxation, to the surface producing 1–2 nm tall islands. An ab initio calculation of excited electron concentration combined with a Drude-Lorentz model of the excited GaAs dielectric function is used to determine that the conditions for SPP coupling at HSFL formation fluences are both satisfied and occur at wavelengths that are imprinted into the observed surface morphologies. The evolution of these morphologies is explained as the interplay between surface plasmon polaritons that localize defect generation within the structures present on the previous laser exposure and stress relaxation driven defect diffusion.

Hierarchical microstructures with high spatial frequency laser induced periodic surface structures possessing different orientations created by femtosecond laser ablation of silicon in liquids

High spatial frequency laser induced periodic surface structures (HSFLs) on silicon substrates are often developed on flat surfaces at low fluences near ablation threshold of 0.1 J/cm2, seldom on microstructures or microgrooves at relatively higher fluences above 1 J/cm2. This work aims to enrich the variety of HSFLs-containing hierarchical microstructures, by femtosecond laser (pulse duration: 457 fs, wavelength: 1045 nm, and repetition rate: 100 kHz) in liquids (water and acetone) at laser fluence of 1.7 J/cm2. The period of Si-HSFLs in the range of 110–200 nm is independent of the scanning speeds (0.1, 0.5, 1 and 2 mm/s), line intervals (5, 15 and 20 μm) of scanning lines and scanning directions (perpendicular or parallel to light polarization direction). It is interestingly found that besides normal HSFLs whose orientations are perpendicular to the direction of light polarization, both clockwise or anticlockwise randomly tilted HSFLs with a maximal deviation angle of 50° as compared to those of normal HSFLSs are found on the microstructures with height gradients. Raman spectra and SEM characterization jointly clarify that surface melting and nanocapillary waves play important roles in the formation of Si-HSFLs. The fact that no HSFLs are produced by laser ablation in air indicates that moderate melting facilitated with ultrafast liquid cooling is beneficial for the formation of HSFLs by LALs. On the basis of our findings and previous reports, a synergistic formation mechanism for HSFLs at high fluence was proposed and discussed, including thermal melting with the concomitance of ultrafast cooling in liquids, transformation of the molten layers into ripples and nanotips by surface plasmon polaritons (SPP) and second-harmonic generation (SHG), and modulation of Si-HSFLs direction by both nanocapillary waves and the localized electric field coming from the excited large Si particles.

Femtosecond laser-induced melting and shaping of indium nanostructures on silicon wafers

We study the modification of indium semi-spherical nanostructures with radii of around 175 nm on silicon wafers into linear microstructures more than 2 μm long in the direction of polarization of laser pulses (1.56 μm, 150 fs, up to 7.5 nJ and 30 000 laser pulses with 8 MHz repetition rate). The experimental results and a rudimentary analysis confirm that melting occurs from intense laser pulses. In short, we demonstrate that melting of the indium droplet followed by trapping in high spatial frequency laser induced periodic surface structures on a silicon substrate cause nanostructure modification. The understanding of the modification process, melting, and moving in the nano-grating structured field, pave the way to design nanostructures of arbitrary shapes at the sub-wavelength scale.

Low damage electrical modification of 4H-SiC via ultrafast laser irradiation

The electrical properties of 4H-SiC under ultrafast laser irradiation in the low fluence regime (<0.50 J/cm2) are presented. The appearance of high spatial frequency laser induced periodic surface structures is observed at a fluence near 0.25 J/cm2 and above, with variability in environments like in air, nitrogen, and a vacuum. In addition to the formation of periodic surface structures, ultrafast laser irradiation results in possible surface oxidation and amorphization of the material. Lateral conductance exhibits orders of magnitude increase, which is attributed to either surface conduction or modification of electrical contact properties, depending on the initial material conductivity. Schottky barrier formation on ultrafast laser irradiated 4H-SiC shows an increase in the barrier height, an increase in the ideality factor, and sub-bandgap photovoltaic responses, suggesting the formation of photo-active point defects. The results suggest that the ultrafast laser irradiation technique provides a means of engineering spatially localized structural and electronic modification of wide bandgap materials such as 4H-SiC with relatively low surface damage via low temperature processing.

Origin of femtosecond laser induced periodic nanostructure on diamond

We study the evolution of periodic nanostructures formed on the surface of diamond by femtosecond laser irradiation delivering 230 fs pulses at 1030 nm and 515 nm wavelengths with a repetition rate of 250 kHz. Using scanning electron microscopy, we observe a change in the periodicity of the nanostructures by varying the number of pulses overlapping in the laser focal volume. We simulate the evolution of the period of the high spatial frequency laser induced periodic surface structures at the two wavelengths as a function of number of pulses, accounting for the change in the optical properties of diamond via a generalized plasmonic model. We propose a hypothesis that describes the origin of the nanostructures and the principal role of plasmonic excitation in their formation during multipulse femtosecond laser irradiation.

Effect of scanning velocity on femtosecond laser-induced periodic surface structures on HgCdTe crystal

In this paper, a femtosecond laser line-scanning irradiation was used to induce the periodic surface microstructure on HgCdTe crystal. Low spatial frequency laser induced periodic surface structures of 650–770nm and high spatial frequency laser induced periodic surface structures of 152–246nm were respectively found with different scanning speeds. The evolution process from low spatial frequency laser induced periodic surface structures to high spatial frequency laser induced periodic surface structures is characterized by scanning electron microscope. Their spatial periods deduced by using a two-dimensional Fourier transformation partly agree with the predictions of the Sipe-Drude theory. Confocal micro-Raman spectral show that the atomic arrangement of induced low spatial frequency laser-induced structures are basically consistent with the crystal in the central area of laser-scanning line, however a new peak at 164cm−1 for the CdTe-like mode becomes evident due to the Hg vaporization when strong laser ablation happens. The obtained surface periodic ripples may have applications in fabricating advanced infrared detector.

Large Area One‐Step Fabrication of Three‐Level Multiple‐Scaled Micro and Nanostructured Nickel Sleeves for Roll‐to‐Roll Hot Embossing

Direct laser interference patterning enabled the fabrication of three level multiple‐scaled microstructures on nickel surfaces using infrared picosecond laser pulses. While the largest spatial period (1.5 to 5.7 µm) could be controlled by adjusting the angle between the interfering laser beams and the laser wavelength, low and high spatial frequency laser induced periodic surface structures with pitches of approximately 800 nm and 160 nm, respectively, were also observed. Using a set of optimized processing parameters, large area Ni‐sleeves were treated. These sleeves were later employed in preliminarily investigations of roll‐to‐roll high throughput hot‐embossing of polymer foils.

High spatial frequency laser induced periodic surface structure formation in germanium by mid-IR femtosecond pulses

Formation of high spatial frequency laser induced periodic surface structures (HSFL) in germanium by 90 fs mid-IR pulses at a 1 kHz repetition rate with wavelengths between λ = 2 and 3.6 μm was studied with varying angle of incidence and polarization. The period of these structures varied from λ/3 to λ/8. A modified surface-scattering model including Drude excitation and the optical Kerr effect explains the spatial period scaling of HSFL across the mid-IR wavelengths. Transmission electron microscopy shows the presence of a 30 nm amorphous layer above the structure of crystalline germanium. Various mechanisms including two photon absorption and defect-induced amorphization are discussed as probable causes for the formation of this layer.

Periodic surface structure bifurcation induced by ultrafast laser generated point defect diffusion in GaAs

The formation of high spatial frequency laser induced periodic surface structures (HSFL) with period <0.3 λ in GaAs after irradiation with femtosecond laser pulses in air is studied. We have identified a point defect generation mechanism that operates in a specific range of fluences in semiconductors between the band-gap closure and ultrafast-melt thresholds that produces vacancy/interstitial pairs. Stress relaxation, via diffusing defects, forms the 350–400 nm tall and ∼90 nm wide structures through a bifurcation process of lower spatial frequency surface structures. The resulting HSFL are predominately epitaxial single crystals and retain the original GaAs stoichiometry.

Femtosecond Ablation of Silicon in Acetone: Tunable Photoluminescence from Generated Nanoparticles and Fabrication of Surface Nanostructures

Silicon (Si) nanoparticles (NPs) and self-organized high spatial frequency laser (HSFL) induced periodic surface structures were fabricated by means of femtosecond ablation of bulk Si target in acetone. The ablation was performed with ∼40 fs (fwhm) pulses and different input energies of ∼500, ∼200, ∼150, ∼100, ∼50, and ∼10 μJ. Fabricated NPs and nanostructures (NSs) were characterized by UV–visible absorption spectroscopy, photoluminescence (PL) spectroscopy, Raman spectroscopy, transmission electron microscopy, and field emission scanning electron microscopy. The average sizes of the NPs were estimated to be in the 4–135 nm range. From the PL studies of Si NPs of different sizes, we have observed a size-dependent shift toward blue spectral region. We could tune the observed PL peak in the spectral range of 440–515 nm. The crystalline and amorphous nature of the Si nanoparticles and nanostructures was investigated using selected area electron diffraction and Raman spectra. Complex refractive index, conduc...

Fine grain of the neural representation of human spatial vision

It is widely held that in human spatial vision the visual scene is initially processed through visual filters, each of which is responsive to narrow ranges of image spatial frequencies. The physiological basis of these filters are thought to be cortical neurons with receptive fields of different sizes. The grain of the neural representation of spatial vision is much finer than had been supposed. Using laser interferometry, which effectively bypasses the demodulation of the optics of the eye, we measured discrimination of, and adaptation to, high spatial frequency laser interference fringe patterns. Spatial frequency discrimination was good right up to the visual resolution limit (average Weber fractions of 0.13 at 50 c/deg). Both contrast and spatial frequency matches made after adapting to extremely fine interference fringes strongly suggested that there existed even finer, relatively unadapted, filters (mechanisms with small receptive fields). The smallest cortical receptive fields processing spatial information in human vision are so small that they can possess receptive field centers hardly wider than single cone photoreceptors.