Maziar Afshar

Surface plasmon polariton model of high-spatial frequency laser-induced periodic surface structure generation in silicon

In recent years, high-spatial frequency laser-induced surfaces structures have been generated in a large variety of dielectrics. In silicon subwavelength ripples, some of which featured periodicities below 100 nm, were formed using ultrafast lasers. We demonstrate for Si(100) surfaces that generation of a dense electron-hole plasma in the focal spot of ultrashort-pulsed laser light followed by massive excitation of plasma waves provides an explanation for the formation of such high-spatial frequency surface structures. The applied Drude-like model includes carrier-carrier collisions and is in excellent agreement with the experimentally observed ripple period.

Sub-100 nm structuring of indium-tin-oxide thin films by sub-15 femtosecond pulsed near-infrared laser light

In magnetron sputtered indium-tin-oxide thin films of varying oxygen content, nanostructures were formed using tightly focused high-repetition rate near-infrared sub-15 femtosecond pulsed laser light. At radiant exposure well beyond the ablation threshold, cuts of 280-350 nm in width were generated. Illumination close to the ablation threshold resulted in periodic cuts of typically 20 nm in width at periodicities between 50 nm and 180 nm, as well as single sub-20 nm cuts. Subthreshold exposure, in combination with hydrochloric acid etching, yielded nanowires of 50 nm minimum lateral dimensions.

Sub-100 nm material processing and imaging with a sub-15 femtosecond laser scanning microscope

Low mean powers of 1–10 mW are sufficient for material nanoprocessing when using femtosecond laser microscopes. In particular, near infrared 12 fs laser pulses at peak TW/cm2 intensities, picojoule pulse energies, and 85 MHz repetition rate have been employed. Three-dimensional two-photon lithography as well as direct multiphoton ablation have been performed. Subwavelength sub-100 nm cuts have been realized in photoresists, silicon wafers, glass, polymers, metals, and biological targets. When reducing the mean power to the microwatt range, nondestructive two-photon imaging was performed with the same setup taking advantage of the broad laser emission spectrum. Multiphoton microscopes based on low-cost ultracompact sub-20 fs laser sources may become novel nonlinear optical tools for highly precise nanoprocessing and two-photon imaging.

Periodic nanostructures on Si(100) surfaces generated by high-repetition rate sub-15 fs pulsed near-infrared laser light

Nanoscale rifts and ripples at a periodicity of 130 nm were generated on Si(100) surfaces immersed in water using tightly focused 800 nm 12 fs pulsed 85 MHz laser light at subnanojoule pulse energies. At radiant exposure close to the ablation threshold rifts were typically 20-50 nm in width and 70 nm in depth running perpendicular to the laser polarization. On increase of the irradiance, the rifts broadened and formed periodic ripples, whereas at highest exposure, a random nanoporous surface topology emerged. Rift and ripple formation is explained by laser-induced standing surface plasma waves, which result in periodic variation of dissipation and ablation.

Multiphoton lithography and ITO structuring by high repetition-rate sub-15 femtosecond laser pulses

We report on experiments using a near-infrared Ti:Sapphire laser system based on a 85 MHz, sub-15 fs resonator. In the negative photoresist SU-8 multiphoton polymerization of 3D structures resulted in a minimum line width of approximately 80 nm at aspect ratios in excess of 50:1. The second part of our contribution deals with sub-wavelength nanostructuring and laser-annealing of thin indium-tin-oxide (ITO) films. The ablation experiments allowed for the generation of cuts of sub-100 nm width in periodic and single cut arrangements. Annealing resulted in a different phase of ITO which is more resistive against etching in HCl at room temperature. The dependence of cuts on scan parameters that affect the ITO film properties was investigated.

Nanoprocessing of glass and PMMA by means of near-infrared sub-15 femtosecond laser pulses

A near infrared sub-15 femtosecond laser scanning microscope was employed for structuring of bulk colored glass and polymethylmethacrylate (PMMA). The 400-mW Ti-Sapphire laser operates at 85 MHz with an M-shaped emission spectrum with maxima at 770 nm and 827 nm. Using a high numerical aperture objective light intensity of about 7 TW/cm2 at the focal plane can be reached. For PMMA a mean power of less than 17 mW, which corresponds to a pulse energy of 0.2 nJ, was sufficient for ablating material. Holes of a diameter of less than 170 nm were produced. Two-photon fluorescence measurements, which can be performed with the same microscope, reveal an extension of the focus length in the specimen, which is most likely caused by self-focusing effects. By applying the same power, the refractive index of the glass could be changed. Islands at the glass surface of a size of less than 100 nm have been produced.

ITO nanowires for gas-sensor applications

In this work we have realized ITO nanowires with typical dimensions of 700 nm width and 200 μm length. They were fabricated by using a novel approach of laser writing in a sputtered indium tin oxide (ITO) film by using a high-repetition rate near-infrared Ti:sapphire laser system based on a 85 MHz, sub-10 fs resonator. These nanowires were characterized electrically and tested as resistive gas sensors with self-heating capability. For this purpose they were exposed to NO2 concentrations in the ppm range within synthetic air, showing a clear increase of resistance. At ambient temperature the sensor exhibits an integrating behavior with relatively long relaxation times. It was shown that the relaxation times can be shortened by exploiting the self-heating capability of this sensor. The self heating effect was studied by FEM simulations.

Three-dimensional polymer nanostructures for applications in cell biology generated by high-repetition rate sub-15 fs near-infrared laser pulses

In recent years two-photon photopolymerization has emerged as a novel and extremely powerful technique of three-dimensional nanostructure formation. Complex-shaped structures can be generated using appropriate beam steering or nanopositioning systems. Here, we report on the fabrication of three-dimensional arrangements made of biocompatible polymer material, which can be used as templates for cell growth. Using three-dimensional cell cages as cell culture substrates is advantageous, as cells may develop in a more natural environment as compared to conventional planar growth methods. The two-photon fabrication experiments were carried out on a commercial microscope setup. Sub-15 fs pulsed Ti:Sapphire laser light (centre wavelength 800 nm, bandwidth 120 nm, repetition rate 85 MHz) was focused into the polymer material by a high-numerical aperture oil immersion objective. Due to the high peak intensities picojoule pulse energies in the focal spot are sufficient to polymerize the material at sub-100 nm structural element dimensions. Therefore, cell cages of sophisticated architecture can be constructed involving very fine features which take into account the specific needs of various types of cells. Ultimately, our research aims at three-dimensional assemblies of photopolymerized structural elements involving sub-100 nm features, which provide cell culture substrates far superior to those currently existing.

Generation of laser-induced periodic surface structures in indium-tin-oxide thin films and two-photon lithography of ma-N photoresist by sub-15 femtosecond laser microscopy for liquid crystal cell application

Indium-tin-oxide (ITO) is a widely used electrode material for liquid crystal cell applications because of its transparency in the visible spectral range and its high electrical conductivity. Important examples of applications are displays and optical phase modulators. We report on subwavelength periodic structuring and precise laser cutting of 150 nm thick indium-tin-oxide films on glass substrates, which were deposited by magnetron reactive DC-sputtering from an indiumtin target in a low-pressure oxygen atmosphere. In order to obtain nanostructured electrodes laser-induced periodic surface structures with a period of approximately 100 nm were generated using tightly focused high-repetition rate sub-15 femtosecond pulsed Ti:sapphire laser light, which was scanned across the sample by galvanometric mirrors. Three-dimensional spacers were produced by multiphoton photopolymerization in ma-N 2410 negative-tone photoresist spin-coated on top of the ITO layers. The nanostructured electrodes were aligned in parallel to set up an electrically switchable nematic liquid crystal cell.

Nanoscale laser writing of Indium-Tin-Oxide nanowires

In this study we report on sub-wavelength nanostructuring of sputtered Indium-Tin-Oxide (ITO) films using a high-repetition rate near-infrared Ti:Sapphire laser system based on a 85 MHz, sub-10 fs resonator. Our experiments demonstrate that cuts as small as 20 nm in width can be generated by ablation. ITO nanowires ranging in size down to 50 nm were produced by laser writing at radiant exposure below the ablation threshold followed by etching in hydrochloric acid. The dependence of the minimum structure size on irradiation and material parameters as well as the electrical properties of the nanowires were investigated.