Ultra-short 230 fs laser pulses of 515 nm wavelength were tightly focused into 700 nm focal spots... more Ultra-short 230 fs laser pulses of 515 nm wavelength were tightly focused into 700 nm focal spots and utilised in opening ∼400 nm nano-holes in a Cr etch mask that was tens-of-nm thick. The ablation threshold was found to be 2.3 nJ/pulse, double that of plain silicon. Nano-holes irradiated with pulse energies below this threshold produced nano-disks, while higher energies produced nano-rings. Both these structures were not removed by either Cr or Si etch solutions. Subtle sub-1 nJ pulse energy control was harnessed to pattern large surface areas with controlled nano-alloying of Si and Cr. This work demonstrates vacuum-free large area patterning of nanolayers by alloying them at distinct locations with sub-diffraction resolution. Such metal masks with nano-hole opening can be used for formation of random patterns of nano-needles with sub-100 nm separation when applied to dry etching of Si.
Photoluminescence (PL) of modulation-doped Al0.22Ga0.78N/GaN heterostructures was investigated. T... more Photoluminescence (PL) of modulation-doped Al0.22Ga0.78N/GaN heterostructures was investigated. The PL peak related to recombination between the two-dimensional electron gases (2DEG) and photoexcited holes is located at 3.448 eV at 40 K, which is 45 meV below the free excitons (FE) emission in GaN. The peak can be observed at temperatures as high as 80 K. The intensity of the 2DEG PL peak is enhanced significantly by incorporating a thin Al0.12Ga0.88N layer into the GaN layer near the heterointerface to suppress the diffusion of photoexcited holes. The energy separation of the 2DEG peak and the GaN FE emission decreases with increasing temperature. Meanwhile, the 2DEG peak energy increases with increasing excitation intensity. These results are attributed to the screening effect of electrons on the bending of the conduction band at the heterointerface, which becomes stronger when temperature or excitation intensity is increased.
THz band-pass filters were fabricated by femtosecond-laser ablation of 25-μm-thick micro-foils of... more THz band-pass filters were fabricated by femtosecond-laser ablation of 25-μm-thick micro-foils of stainless steel and Kapton film, which were subsequently metal coated with a ∼70 nm film, closely matching the skin depth at the used THz spectral window. Their spectral performance was tested in transmission and reflection modes at the Australian Synchrotron’s THz beamline. A 25-μm-thick Kapton film performed as a Fabry–Pérot etalon with a free spectral range (FSR) of 119 cm−1, high finesse Fc≈17, and was tuneable over ∼10μm (at ∼5 THz band) with β=30∘ tilt. The structure of the THz beam focal region as extracted by the first mirror (slit) showed a complex dependence of polarisation, wavelength and position across the beam. This is important for polarisation-sensitive measurements (in both transmission and reflection) and requires normalisation at each orientation of linear polarisation.
ABSTRACT Optical investigations were carried out on undoped as well as on Cr-doped semi-insulatin... more ABSTRACT Optical investigations were carried out on undoped as well as on Cr-doped semi-insulating GaAs(100) wafers. The distribution of the non-equilibrium charge carrier grating erasure time τE across GaAs wafers has been investigated by means of picosecond laser-induced dynamic grating and picosecond-photoconductivity techniques. Additional photoluminescence data have been obtained to reveal the contamination of GaAs wafers. The influence of high temperature annealing conditions on the distribution of the deep level EL2 and on the τE mapping data has been studied. Also, feasibility of dynamic grating and picosecond-photoconductivity techniques for checking GaAs wafer quality is shown. Recombination through the deep levels EL2 and Cr are discussed.
Many polymer gels undergo reversible, discontinuous volume changes in response to changes in the ... more Many polymer gels undergo reversible, discontinuous volume changes in response to changes in the balance between repulsive intermolecular forces that act to expand the polymer network and attractive forces that act to shrink it. Repulsive forces are usually electrostatic or hydrophobic in nature, whereas attraction is mediated by hydrogen bonding or van der Waals interactions. The competition between these counteracting forces, and hence the gel volume, can thus be controlled by subtle changes in parameters such as pH (ref. 4), temperature, solvent composition or gel composition. Here we describe a more direct influence on this balance of forces, by showing that the radiation force generated by a focused laser beam induces reversible shrinkage in polymer gels. Control experiments confirm that the laser-induced volume phase transitions are due to radiation forces, rather than local heating, modifying the weak interactions in the gels, in agreement with previous observations of light-...
Ultra-short 230 fs laser pulses of 515 nm wavelength were tightly focused into 700 nm focal spots... more Ultra-short 230 fs laser pulses of 515 nm wavelength were tightly focused into 700 nm focal spots and utilised in opening ∼400 nm nano-holes in a Cr etch mask that was tens-of-nm thick. The ablation threshold was found to be 2.3 nJ/pulse, double that of plain silicon. Nano-holes irradiated with pulse energies below this threshold produced nano-disks, while higher energies produced nano-rings. Both these structures were not removed by either Cr or Si etch solutions. Subtle sub-1 nJ pulse energy control was harnessed to pattern large surface areas with controlled nano-alloying of Si and Cr. This work demonstrates vacuum-free large area patterning of nanolayers by alloying them at distinct locations with sub-diffraction resolution. Such metal masks with nano-hole opening can be used for formation of random patterns of nano-needles with sub-100 nm separation when applied to dry etching of Si.
Photoluminescence (PL) of modulation-doped Al0.22Ga0.78N/GaN heterostructures was investigated. T... more Photoluminescence (PL) of modulation-doped Al0.22Ga0.78N/GaN heterostructures was investigated. The PL peak related to recombination between the two-dimensional electron gases (2DEG) and photoexcited holes is located at 3.448 eV at 40 K, which is 45 meV below the free excitons (FE) emission in GaN. The peak can be observed at temperatures as high as 80 K. The intensity of the 2DEG PL peak is enhanced significantly by incorporating a thin Al0.12Ga0.88N layer into the GaN layer near the heterointerface to suppress the diffusion of photoexcited holes. The energy separation of the 2DEG peak and the GaN FE emission decreases with increasing temperature. Meanwhile, the 2DEG peak energy increases with increasing excitation intensity. These results are attributed to the screening effect of electrons on the bending of the conduction band at the heterointerface, which becomes stronger when temperature or excitation intensity is increased.
THz band-pass filters were fabricated by femtosecond-laser ablation of 25-μm-thick micro-foils of... more THz band-pass filters were fabricated by femtosecond-laser ablation of 25-μm-thick micro-foils of stainless steel and Kapton film, which were subsequently metal coated with a ∼70 nm film, closely matching the skin depth at the used THz spectral window. Their spectral performance was tested in transmission and reflection modes at the Australian Synchrotron’s THz beamline. A 25-μm-thick Kapton film performed as a Fabry–Pérot etalon with a free spectral range (FSR) of 119 cm−1, high finesse Fc≈17, and was tuneable over ∼10μm (at ∼5 THz band) with β=30∘ tilt. The structure of the THz beam focal region as extracted by the first mirror (slit) showed a complex dependence of polarisation, wavelength and position across the beam. This is important for polarisation-sensitive measurements (in both transmission and reflection) and requires normalisation at each orientation of linear polarisation.
ABSTRACT Optical investigations were carried out on undoped as well as on Cr-doped semi-insulatin... more ABSTRACT Optical investigations were carried out on undoped as well as on Cr-doped semi-insulating GaAs(100) wafers. The distribution of the non-equilibrium charge carrier grating erasure time τE across GaAs wafers has been investigated by means of picosecond laser-induced dynamic grating and picosecond-photoconductivity techniques. Additional photoluminescence data have been obtained to reveal the contamination of GaAs wafers. The influence of high temperature annealing conditions on the distribution of the deep level EL2 and on the τE mapping data has been studied. Also, feasibility of dynamic grating and picosecond-photoconductivity techniques for checking GaAs wafer quality is shown. Recombination through the deep levels EL2 and Cr are discussed.
Many polymer gels undergo reversible, discontinuous volume changes in response to changes in the ... more Many polymer gels undergo reversible, discontinuous volume changes in response to changes in the balance between repulsive intermolecular forces that act to expand the polymer network and attractive forces that act to shrink it. Repulsive forces are usually electrostatic or hydrophobic in nature, whereas attraction is mediated by hydrogen bonding or van der Waals interactions. The competition between these counteracting forces, and hence the gel volume, can thus be controlled by subtle changes in parameters such as pH (ref. 4), temperature, solvent composition or gel composition. Here we describe a more direct influence on this balance of forces, by showing that the radiation force generated by a focused laser beam induces reversible shrinkage in polymer gels. Control experiments confirm that the laser-induced volume phase transitions are due to radiation forces, rather than local heating, modifying the weak interactions in the gels, in agreement with previous observations of light-...
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Papers by S. Juodkazis