K.N. Tripathi

Laser damage studies of silicon surfaces using ultra-short laser pulses

Laser-induced damage morphology using femtosecond laser pulses on Si surfaces is reported. Damage morphology shows the ablation of material. A magnified view of the ablated portion shows a periodic surface structure in the form of ripples. The spacing of these ripples was between 0.5 and 2 μm and increased, on increasing the power density or number of pulses, and finally broke into parts, leaving well-ordered grains of approximate diameter 5 μm. Also for 100 or larger number of pulses, an amorphous ring in the periphery was formed. The diameter of this ring increased, on increasing either the laser fluence or the number of pulses. The formation of ripples has been explained with the help of the hypothesis of Boson condensation proposed by Van Vechten (Solid State Commun 39 (1981) 1285).

Fabrication and characterization of polyvinyl- alcohol-based thin-film optical waveguides

Polyvinyl alcohol (PVA)-based thin film optical waveguides are fabricated and characterized. Various waveguide parameters, namely, refractive index, propagation losses and depth, birefringence, and polarization conversion properties of dip-coated thin film optical waveguides are evaluated. The effective refractive index of PVA film is found to be between 1.506 and 1.531 for various concentrations. The propagation losses are 2.35 to 0.30 dB/cm for various thicknesses. The propagation loss reduces when the thickness of the film is increased. The low propagation losses, birefringence insensitivity, high transmission in the aged and annealed waveguides, control over film refractive index, the good adhesiveness of the film and good compatibility with substrate show that PVA-based waveguides are promising candidates for the optoelectronic field.

Four layer polymeric mode polarization filter for integrated optics

We report on our systematic study of fabrication and characterization of four-layer polymer waveguides. Various optical properties (such as refractive index, birefringence, and propagation loss) of polycarbonate and polystyrene waveguides are presented. The thin film structure consisting of glass/polycarbonate/polystyrene/air has been used for demonstrating polarization filter action because the two polymers are quite different with respect to their optical anisotropy. Modal electric field plots for both TE and TM are generated to support the observed behavior. It is also observed that the four-layer lightguide exhibits relatively low loss values compared to the monolayer configuration.

Design and fabrication of multilayer metal-clad dielectric surface plasmon waveguide polarizers

Planar multilayer optical waveguides consisting of thin dielectric films with metal cladding have been investigated as TM-mode absorption polarizers. Two types of structure are analyzed. In the first one, the guided-mode losses were calculated as a function of the thicknesses of the buffer layer and the metal. Another polarizer, utilizing a MgF 2 /metal/MgF 2 overlayer, is discussed. It is observed that in this configuration TM absorption losses are enhanced and thus we get a better extinction ratio. The polarizer has also been fabricated and characterized on ion exchange optical planar waveguides with a MgF 2 /Ag/MgF 2 overlayer.

Laser-induced damage studies in GaAs

Abstract Laser-induced damage studies have been carried out on monocrystal GaAs at 1.06 μm wavelength as a function of pulse repetition rate in the nanosecond regime. The single shot observed laser damage threshold is 0.9 J/cm 2 . It has been found that the damage threshold decreases when the sample is irradiated with large number of pulses. However, the above effect is observed only when the repetition rate is higher than 1 Hz . Various laser damage mechanism theories have been discussed to explain the results.

Thermal and mechanical damage of GaAs in picosecond regime

Abstract An in-depth study of the single pulse and multiple pulse laser ( 35 ps , 10 Hz and 1064 nm ) damage for threshold fluence and greater fluence of GaAs 〈1 0 0〉 single crystal is presented. Damage which starts at a power 2×10 11 W / cm 2 in the form of pits occurs due to accumulation of laser induced microscopic defects. Effect of multiple pulse at first makes the pits more prominent in the form of Ga emission. Then the topmost layer is removed. If the number of pulses is further increased new pits are formed in the new surface (beneath the removed surface) and the above process is repeated. The thermal model is sufficient to explain this morphology. However, for larger fluences, a large cracking and fracture and the possibility of both Ga and As emission in different ratios suggest that mechanical damage is a dominant feature for higher fluences which arises due to generation of shock waves and rapid vaporization of material. Damage threshold has been calculated with the help of the thermal model given by Meyer et al. which is in good agreement with our experimental results.

Ripples and grain formation in GaAs surfaces exposed to ultrashort laser pulses

Abstract The damage morphology of GaAs 〈1 0 0〉 single crystal following femtosecond laser (wavelength 806 nm , pulse duration 110 fs , prf 10 Hz ) excitation was studied as a function of laser fluence and number of pulses. The threshold value for damage to occur in a GaAs surface in the present experiment was 1.3×10 14 W/cm 2 for a single pulse. The cooling rate for threshold fluence was calculated as 2.22×10 14 ° C / s . The damage occurred in the form of surface removal. Ripples and grains were formed in the removed surface. At higher fluences micron depth pits were also formed. The damage morphology was explained with the help of Boson-condensation hypothesis.

Effect of dye doping in poly(vinyl alcohol) waveguides

Dye-doped poly(vinyl alcohol) (PVA) waveguides have been fabricated and characterized. The waveguiding parameters of PVA film, for example the refractive index, thickness, birefringence, polarization conversion and propagation losses, were evaluated using the various concentration of dyes in polymeric solution. It has been shown that the dyes can affect the basic waveguiding parameters considerably. However, birefringence in PVA waveguides remains unaffected irrespective of any dyes and any concentration of dye used.

Partially and fully cured thin film photoresist waveguides for integrated optics

A systematic and detailed study of waveguide fabrication and characterization of a commercially available (from Shipley) photoresist is reported. Various waveguide properties such as refractive index, optical losses, waveguide thickness, dispersion and birefringence are reported. Effect of temperature of guide formation, curing (partial and full) and UV exposure on these parameters is also presented.

Dye-doped polymeric waveguides for integrated optics

We present a study of fabrication and characterization of methyl red doped polystyrene waveguides. Optical absorption and mode effective refractive index measurements were performed on a large number of waveguides fabricated on glass substrates prepared by a dip coating technique. It is observed that doping of azo dye increases the index of refraction of the waveguides. The losses, however, decrease initially for dye concentration up to 0.1% and thereafter increase rapidly.