Narayana Rao Desai

Fabrication and optical characterization of microstructures in poly(methylmethacrylate) and poly(dimethylsiloxane) using femto- second pulses for photonic and microfluidic applications

We fabricated several microstructures, such as buried gratings, surface gratings, surface microcraters, and microchannels, in bulk poly(methylmethacrylate) (PMMA) and poly(dimethylsiloxane) (PDMS) using the femtosecond (fs) direct writing technique. A methodical study of the diffraction efficiency (DE) of the achieved gratings was performed as a function of scanning speed, energy, and focal spot size in both PMMA and PDMS. An optimized set of writing parameters has been identified for achieving efficient gratings in both cases. The highest DE recorded in a PDMS grating was ∼10% and ∼34% in a PMMA grating obtained with an 0.65 NA (40X) objective with a single scan. Spectroscopic techniques, including Raman, UV-visible, electron spin resonance (ESR), and physical techniques, such as laser confocal and scanning electron microscopy (SEM), were employed to examine the fs laser-modified regions in an attempt to understand the mechanism responsible for physical changes at the focal volume. Raman spectra collected from the modified regions of PMMA indicated bond softening or stress-related mechanisms responsible for structural changes. We have also observed emission from the fs-modified regions of PMMA and PDMS. An ESR spectrum, recorded a few days after irradiation, from the fs laser-modified regions in PMMA did not reveal any signature of free radicals. However, fs-modified PDMS regions exhibited a single peak in the ESR signal. The probable rationale for the behavior of the ESR spectra in PMMA and PDMS are discussed in the light of free radical formation after fs irradiation. Microchannels within the bulk and surface of PMMA were achieved as well. Microcraters on the surfaces of PMMA and PDMS were also accomplished, and the variation of structure properties with diverse writing conditions has been studied.

Experimental investigation of photonic band gap influence on enhancement of Raman-scattering in metal-dielectric colloidal crystals

A simple chemical technique is implemented to fabricate a metal-dielectric colloidal crystal structure (MDCS) in order to enhance the otherwise weak Raman signals by combining the effects of localized surface plasmon resonance (LSPR) enhancement due to gold nanoparticles, precise field confinement of dielectric and air bands in the periodic dielectric structure and field enhancements at the photonic band gap (PBG) edges. The higher density of electromagnetic modes (DOS) near these band edges is explained as due to the reduced group velocity at the photonic band gap edges. Intense electric field strength due to the excitation of high DOS at the edges of PBG of MDCS and the LSPR excitation through field confinement in the dielectric medium of MDCS are employed to study the Raman-scattering signals of adsorbed benzenethiol (BT) molecule on the MDCS. Large enhancement for the Raman signal in MDCS in comparison to the Raman spectra observed for BT molecule dispersed on sputtered gold film shows the effectivene...

Femtosecond-laser direct writing in polymers and potential applications in microfluidics and memory devices

We have investigated femtosecond-laser-induced microstruc- tures (on the surface and within the bulk), gratings, and craters in four different polymers: polymethyl methacrylate, polydimethylsiloxane, poly- styrene, and polyvinyl alcohol. The structures were achieved using a Ti:sapphire laser delivering 100-fs pulses at 800 nm with a repetition rate of 1 kHz and a maximum pulse energy of 1 mJ. Local chemical mod- ifications leading to the formation of optical centers and peroxide radicals were studied using ultraviolet-visible absorption and emission, confocal micro-Raman and electron spin resonance spectroscopic techniques. Potential applications of these structures in microfluidics, waveguides, and memory-based devices are demonstrated.

Spectral anomalies due to temporal correlation in a white-light interferometer.

We present what we believe to be the first experimental demonstration of anomalous spectral behavior such as spectral shifts and spectral switches due to temporal correlation around the intensity minima in a white-light interferometer. Unusual behavior in the number of spectral fringes, measured within the source bandwidth, as a function of path delay between the interfering beams is also reported. Experimental observations match well with the spectra calculated by using the interference law in the spectral domain.

Optical properties of germanium nanoparticles synthesized by pulsed laser ablation in acetone

Germanium (Ge) nanoparticles (NPs) are synthesized by means of pulsed laser ablation of bulk germanium target immersed in acetone with ns laser pulses at different pulse energies. The fabricated NPs are characterized by employing different techniques such as UV-visible absorption spectroscopy, photoluminescence, micro-Raman spectroscopy, transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM). The mean size of the Ge NPs is found to vary from few nm to 40 nm with the increase in laser pulse energy. Shift in the position of the absorption spectra is observed and also the photoluminescence peak shift is observed due to quantum confinement effects. High resolution TEM combined with micro-Raman spectroscopy confirms the crystalline nature of the generated germanium nanoparticles. The formation of various sizes of germanium NPs at different laser pulse energies is evident from the asymmetry in the Raman spectra and the shift in its peak position towards the lower wavenumber side. The FESEM micrographs confirm the formation of germanium micro/nanostructures at the laser ablated position of the bulk germanium. In particular, the measured NP sizes from the micro-Raman phonon quantum confinement model are found in good agreement with TEM measurements of Ge NPs.

Polarization controlled deep sub-wavelength periodic features written by femtosecond laser on nanodiamond thin film surface

Deep sub-wavelength (Λ/λ = ∼0.22) periodic features are induced uniformly on a nanodiamond (ND) thin film surface using femtosecond (fs) laser irradiation (pulse duration = ∼110 fs and central wavelength of ∼800 nm). The topography of the surface features is controlled by the laser polarization. Orientation of features is perpendicular to laser polarization. Periodicity (spatial periodicity of < λ/4) of the surface features is less than the laser wavelength. This work gives an experimental proof of polarization controlled surface plasmon-fs laser coupling mechanism prompting the interaction between fs laser and solid matter (here ND thin film) which in turn is resulting in the periodic surface features. Scanning electron microscopy in conjunction with micro Raman scattering, X-ray diffraction, and atomic force microscopy are carried out to extract surface morphology and phase information of the laser irradiated regions. This work demonstrates an easy and efficient surface fabrication technique.

Direct Writing in Polymers with Femtosecond Laser Pulses: Physics and Applications

Nonlinear optical phenomena in the optical spectral range followed by the invention of laser in early 1960’s directed the generation of optical pulses using Q switching and mode locking techniques. Ultrafast lasers with extremely short pulse duration (<100 fs) opened a new avenue towards fabrication of integrated photonic and signal processing devices in a variety of transparent materials. A new approach for the local modification of transparent materials through nonlinear optical processes has been investigated due to extraordinarily high peak intensities of short pulses. A variety of materials including metals, dielectrics, polymers, and semiconductors have been successfully processed by the use of fs pulses [1-14]. Bulk refractive index change in transparent materials is found to be useful in applications of waveguides. Various applications resulting from fs laser writing of different materials, especially in polymers, have been successfully demonstrated in the fields of micro-fluidics, bio-photonics, and photonics etc. [15-32]. The minimal damage arising from the generation of stress waves, thermal conduction, or melting has proved to be one of the main responsible mechanisms for various applications demonstrated using fs laser micromachining. In the present chapter we discuss the formation of free radicals and defects which are responsible for emission in polymer systems. The impact of fs lasers pulses causing minimal damage can be utilized to fabricate emissive micro-craters, especially in polymers. These emissive microcraters find prospective applications in memory based devices.

Silver nanoclusters decked diamond thin film as a substrate for surface-enhanced Raman scattering

Modified diamond thin film surfaces are proven candidates for bio-sensoric applications. A typical polycrystalline diamond thin film surface decked with silver (Ag) nanoclusters is found to be suitable for surface-enhanced Raman scattering (SERS). Simple and time effective procedures are used to fabricate Ag nanoclusters/diamond/silicon structure. Standard Rhodamine 6G probe molecules are used to access the SERS activity of the test surface. The observed SERS activity is explained in terms of predominant electromagnetic enhancement mechanism. This work is expected to benefit SERS based bio-sensing with diamond as the core sensing material.

Optical characteristics of different Bragg planes of 3D polystyrene photonic crystals in the LU and LK path of the first Brillouin zone of close packed fcc structure with large band gap depth and steeper band edges

We report the angle dependent transmission spectral characteristics of 331 nm diameter polystyrene (PS) photonic crystals (PhCs) with large band gap depth and steeper band edges which allow one to identify different Bragg planes promptly. The spectral characteristics recorded in the clock-wise (CW) and anti clock-wise (ACW) directions follow the LK and LU paths in the first Brillouin zone (FBZ) of the face centered cubic (fcc) structure. We obtained 25% of the band gap depth for (220) plane in the LK path, which is the highest value reported till now to the best of our knowledge. Interestingly we observed that (200) plane features for lower angles and (111) plane features for higher angles in the LU path while in the LK path the dips follow the (111) plane fit completely from lower angles to higher angle of incidence and the possible reasons are discussed.

Optical and biomedical properties of eco-friendly metal nanostructures synthesized using Trigonella foenum-graecum leaf extract

Biogenic, eco-friendly, reliable, and cost-effective metal nanostructures were synthesized using Trigonella foenum-graecum leaf extract, which acted as reducing and stabilizing agent with no additional chemical surfactants. Nanoparticle formation was confirmed from UV–Vis absorption, XRD, and FTIR spectra. Morphology and crystallinity of spherical silver nanoparticles (Ag NPs) and gold nanoparticles (Au NTs) were further confirmed from TEM analysis. Luminescence enhancement and quenching of lanthanide ion complexes were observed and found to be dependent on metal nanostructure concentration. The factors relay on the energy transfers between nanostructures and rare-earth ions (RE). The nonlinear optical studies of Ag NPs and Au NTs were studied using Z-scan and degenerate four wave mixing (DFWM) techniques with picosecond and femtosecond lasers. The surface-enhanced Raman scattering (SERS) of the test molecule, rhodamine 6G, was studied to check the performance of Ag NPs and Au NTs in enhancing the Raman scattering signals. In vitro cytotoxic effect of the Ag NPs and Au NTs has been checked against A549 lung cancer cell lines and the efficiency of toxicity depended on the dosage. These experiments demonstrate that the nanostructures are ideal candidates for applications in optics and biology as PL based devices with appropriate concentrations of RE ions, optical switches, SERS substrates, and therapeutic agents towards cancer cell lines.