Prof. Soma obtained M.Sc. (Physics) in 1994 and Ph.D. (Physics) from the University of Hyderabad in 2000. He worked as a research fellow at the University of St. Andrews, Scotland, UK, from 2000 to 2003 and as a senior research fellow at the National University of Singapore from 2003 to 2004. He worked as an assistant professor at IIT Guwahati from 2004 to 2007 and joined the University of Hyderabad as a reader in 2007. He later became an Associate Professor in 2010 and Professor in 2013. He is now a Senior Professor at the University of Hyderabad.
RESEARCH SUPERVISION
- Successfully guided 14 Ph.D. students; 1 more are currently under supervision - Mentored numerous post-doctoral, M.Sc., undergraduate students
PUBLICATIONS
- 1 book and 17 book chapters - >450 papers in refereed journals and international conference proceedings - Presented >150 papers at various international/national conferences - >11600 citations (Google Scholar Citations), h-index of 58 - https://scholar.google.co.in/citations?user=drr_va8AAAAJ&hl=en&oi=ao
- Fellow SPIE - Fellow IAAM, 2023. - MRSI medal, 2022 - Fellow, National Academy of Sciences, India (NASI), 2021 - Recognized as the top 2% scientist in optics/photonics, Stanford University, 2020 - Fellow, Royal Society of Chemistry (FRSC), UK, 2020 - Fellow, Institute of Physics, London, UK, 2019 - PUBLONS TOP Peer Reviewer Award, 2018 and 2019 - Fellow, Telangana Academy of Sciences, 2017 - Chancellor’s Award, University of Hyderabad, 2016 - NASI-SCOPUS Award, Physics category, 2012 - Senior Member of prestigious organizations: OPTICA, SPIE, IEEE
EDITORIAL ROLES
- Associate Editor, Optics Letters - Associate Editor, Opto-electronic Advances - Editorial Board Member, Defence Science Journal, India - Associate Editor, RSC Advances - Member, Editorial Board for “Optics and Photonics” section of “Frontiers” journal
Publications (32)
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In this study, we have generated a femtosecond (fs), non-diffracting Bessel beam (800 nm, 1 kHz, 50 fs) of zeroth order through an axicon (IR range, 100, AR coated). We have performed laser ablation of a bimetallic alloy (50% gold, 50 % silver) in the air engaging the generated fs Bessel beam. The high-intensity Bessel beam-matter interaction resulted in the fabrication of exotic bimetallic nanostructures. Extensive field emission scanning electron microscope and atomic force microscopy characterizations were undertaken to study the nanoscale topographical formations. The fs Bessel beam-induced ablation on the alloy target, involving the beam profile imprint on a single surface spot, followed by overlapping two ablation zones, has been meticulously explored. The central lobe ablated area, along with concentric rings-ablated exotic patterns, were thoroughly investigated in the topographical characterization. In the case of the complete raster scan ablation, ladder-like periodic surface structures (with sub ~20 nm growths on the ladder steps) were observed. Energy-dispersive X-ray mapping was performed to confirm the elemental distribution in the nanostructured areas. Subsequently, these plasmonic nanostructures were utilized as surface-enhanced Raman scattering (SERS) platforms to detect traces of real-time explosives, ammonium nitrate (AN), and Tetryl (TL). The SERS spectra of AN depicted a signature Raman peak at 1043 cm-1, whereas TL exhibited a signature peak near 1353 cm-1. The lowest possible detected traces were 10 μM and 5 μM, for AN and TL, respectively.
In this work, we report on the trace detection of an explosive molecule, picric acid (PA), and a dye molecule, malachite green (MG), using surface enhanced Raman scattering (SERS) technique. We have synthesized porous Silicon (PSi) by a simple electrochemical etching method and anisotropic gold nanostars (AuNSs) using chemical reduction of the gold salt (HAuCl4). Rough PSi acts as a suitable platform for generating SERS hotspots upon the addition of these AuNSs. The average particle size was found to be <50 nm with a strong absorption peak in the near infra-red (NIR) spectral region. PSi substrates along with AuNSs on its surface are used to explore their detection performance for PA and MG at different concentrations. Furthermore, we have compared the Raman signal intensities of Malachite Green (MG) on Si, PSi with and without Au NSs. Without Au NSs, bare PSi was found to exhibit a low Raman signal as compared to bare Si due to its hindering effect of an analyte molecule in the pore structures. However, this signal is enhanced by employing AuNSs onto the roughened porous surface. A portable Raman spectrometer (BWTEK) was used for all the SERS measurements with an excitation wavelength of 785 nm. We have achieved detection of MG at nanomolar (10-9 M) and PA at micromolar (10-6 M) concentrations using these hybrid SERS substrates. The enhancement factor was estimated to be in the range of 104-105. We believe that the optimization of porosity in PSi and sizes of AuNSs will improve the limit of detection further.
We have investigated the ultrafast third-order nonlinear optical (NLO) properties of a novel chalcone derivative, 3-(4- methoxyphenyl)-1-(4-nitrophenyl)prop-2-en-1-one (abbreviated as MNC) by Z-scan and degenerate four-wave mixing (DFWM) techniques using femtosecond Ti:Sapphire laser system (~70fs, 1 kHz, 800 nm). The molecular structure of the synthesized chalcone by Claisen-Schmidt condensation reaction was confirmed by FT-IR and 1H NMR spectroscopic techniques. The thermal stability was studied by thermogravimetric/ differential thermal analysis (TG/DTA) technique and melting point was found to be 177 °C. The linear absorption spectra suggest that the MNC chalcone is optically transparent in the Vis-NIR region. The open aperture Z-scan demonstrated two-photon absorption, evident from the reverse saturable absorption type mechanism, while the closed aperture Z-scan demonstrated a positive nonlinear refraction due to self-focusing effect. Further, the chalcone exhibited optical limiting (OL) and optical switching properties. The onset optical limiting threshold fluence was measured at 9.15 mJ/cm2 and the figures of merit for all-optical-switching were satisfied. From DFWM data we measured the magnitude of NLO coefficients, nonlinear response time and dephasing time. The third-order NLO susceptibility and molecular hyperpolarizability were calculated to be 1.39×10-14 esu and 6.89×10-34 esu, respectively, using Z-scan and 6.53×10-14 esu and 32.7×10-34 esu, respectively, using DFWM techniques. From both these techniques the magnitude of NLO coefficients were found to be in good agreement. The time-resolved DFWM studies revealed that the nonlinear response time of MNC was very short (~112 fs). These results indicate that the MNC chalcone is a potential material for optical limiting and all-optical-switching applications.
Herein, results from the investigation of ultrafast photophysical and third-order nonlinear optical properties of newly synthesized Zn phthalocyanine, namely [Zinc (II) 2,10,16,24tetrakis(2,6-dichloropyridin-3-yl) phthalocyanine, PyCl2PC] molecule are presented. The photophysical properties were studied using femtosecond transient absorption spectroscopy while the NLO properties were measured using the single beam Z-scan and degenerate four-wave mixing (DFWM) techniques. The transient absorption spectra were obtained at 400 nm photoexcitation and a white light continuum probed the corresponding dynamics from (440-780 nm) in the solution phase. The obtained transient absorption spectra were globally fitted using a kinetic model which yielded the different photophysical constants after photoexcitation such as (i) internal conversion from higher electronic excited state (Sn) to lower electronic state (S1) happening in 1 ps (ii) Vibrational relaxation (6.25 ps) occurring within the S1 states (III) Intersystem crossing (1.78 ns) (IV) relaxation from the triplet states to ground state (0.11 μs). Nonlinear absorption properties were measured at 800 nm wavelength utilizing ~70 fs, 1 kHz laser pulses in the solution phase. A large two-photon absorption coefficient (β) of ~8×10-13 cm/W was obtained and the corresponding cross-section was estimated to be 659 GM. Time-resolved degenerate four-wave-mixing measurements revealed a large magnitude and an ultrafast response of χ(3).
We present our initial experimental results from the LIBS studies of pyrazole, 1-nitropyrazole, 3-nitropyrazole, 3,4- dinitropyrazole and 1-methyl- 3,4,5 trinitro pyrazole recorded with femtosecond pulses and performed in argon atmosphere. CN molecular bands in three different spectral regions of 357 nm-360 nm, 384 nm-389 nm and 414 nm -423 nm, C2 swan bands near 460 nm-475 nm, 510 nm– 520 nm and 550 nm-565 nm were observed. The C peak at 247.82 nm, H peak at 656.2 nm have also been observed along with several peaks of O and N. CN/C2, CN/C, C2/C and C2/N ratios were measured from the average of 25 spectra obtained in argon. The effect of number of nitro groups on the atomic and molecular emission has been evaluated. A gate delay of 100 ns and a gate width of 800 ns were used for collecting the spectra.
Laser induced breakdown spectroscopy is an attractive and versatile spectroscopic technique employed successfully for the detection of hazardous substances. The specific advantages of using femtosecond (fs) pulses with LIBS technique include lower ablation threshold, reduced background Continuum emission. In addition to atomic peaks in plasma the molecular peaks (CN and C2) also play a significant role in classification of these samples. In the present work fs LIBS spectra were recorded from five different samples (RDX, HMX, NTO, ANTA, and DADNE) made in the form of pure pellets. Correlation statistics were used to discriminate the samples based on molecular, atomic ratios. This paper discusses, in detail, a simple correlation technique applied for the fs LIBS data for achieving classification.
We present our initial experimental results from the LIBS studies of pyrazole, 1-nitropyrazole, 3-nitropyrazole, 3,4- dinitropyrazole and 1-methyl- 3,4,5 trinitro pyrazole recorded with femtosecond pulses and performed in argon atmosphere. CN molecular bands in three different spectral regions of 357 nm-360 nm, 384 nm-389 nm and 414 nm -423 nm, C2 swan bands near 460 nm-475 nm, 510 nm– 520 nm and 550 nm-565 nm were observed. The C peak at 247.82 nm, H peak at 656.2 nm have also been observed along with several peaks of O and N. CN/C2, CN/C, C2/C and C2/N ratios were measured from the average of 25 spectra obtained in argon. The effect of number of nitro groups on the atomic and molecular emission has been evaluated. A gate delay of 100 ns and a gate width of 800 ns were used for collecting the spectra.
We investigated formation of defects in four polymers namely Poly (methylmethacrylate) [PMMA], Poly
dimethylsiloxane [PDMS], Polystyrene [PS], and Polyvinyl alcohol [PVA] and crystal media such as Lithium Niobate
[LiNbO3]. Spectroscopic studies of the femtosecond (fs) laser modified regions were systematically performed after
fabricating several gratings and micro-channels. We observed emission from the fs laser modified regions of these
polymers when excited at different wavelengths. Pristine polymers are not paramagnetic, but exhibited paramagnetic
behavior upon fs irradiation. LiNbO3 (LNB) crystal has not shown any defect formation upon laser irradiation. Confocal
micro-Raman studies were also performed to establish the formation of defects.
We have investigated femtosecond-laser-induced microstructures (on the surface and within the bulk), gratings, and craters in four different polymers: polymethyl methacrylate, polydimethylsiloxane, polystyrene, 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 modifications 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.
We present the experimental investigations on the filament characteristics of sharply focused fs pulses (800 nm, 45 fs, 1
kHz) in air. Pulses with input powers in 3-12.2 PCr range were focused using three different focusing geometries f/#10,
f/#15 and f/#20 corresponding to numerical apertures (NA) of 0.05, 0.033 and 0.025, respectively. The dynamics of
filaments were observed via direct imaging of the entire reaction zone. The length of the filament has decreased with
increasing NA from 0.025 to 0.05, while, the filament width has increased. For a given focusing geometry, the filament
length and width increased with increasing power. However with higher NA, the length and width were observed to
saturate at higher input powers. With the highest NA of 0.05 and higher input powers used in the current study, the
presence of coherently interacting multiple filaments either resulting in a fusion or exchange of power.
Phthalocyanines, Porphycenes, and Corroles are macromolecules with large number of delocalized π electrons. The
magnitude of response of these loosely bound electrons to short laser pulses determines their applicability in various
applications such as optical limiting, optical single processing etc. A meticulous understanding of their performance
using different pulses and at various wavelengths is indispensable to extract their accurate potential. Herein, we try to
compare and contrast the nonlinear optical performance of these molecules in the ns, ps, and fs time domains. The
nonlinear optical coefficients and figure of merits were estimated from the Z-scan data using different pulses over a
range of input wavelengths. Ultrafast excited state dynamics of these molecules were studied using the pump-probe and
degenerate four wave mixing techniques. A review of all the results obtained is presented.
Expanded porphyrins belong to the class of porphyrinoids, where the core of the porphyrin macrocycle is increased
either by incorporating additional pyrrolic units, or by increasing the number of bridging carbon atoms from more than
four, or a combination of both. The significance of these classes of compounds lies in their novel photophysical and
nonlinear optical properties. Superior nonlinear optical coefficients are usually observed for aromatic expanded
porphyrins with large number of π-electrons owing to their distinct structural features. In this regard, cyclo[8]pyrrole is
unique, owing to its large planar 30-π core macrocyclic ring in its diprotonated state. Here, all the eight pyrrole units are
directly linked to each other through their á-positions. Recently, we have synthesized, cyclo[4]naphthobipyrroles, a
unique class of cyclo[8]pyrroles, where alternate pyrrole units are fused with naphthalene moieties. This adds more
rigidity to the resultant cyclo[8]pyrrole while further extending the resultant π-conjugation. Herein, we present some of
our results from the picosecond nonlinear optical studies of a â-octa-isopropyl-cyclo[4]naphthobipyrrole. The nonlinear
optical coefficients were extracted from the Z-scan measurements. The values of two-photon cross-sections obtained for
these molecules were in the range of 104 GM.
We present the evolution of SCE associated with filaments due to the tilt of focusing lens under tight focusing
geometries. Transform limited femtosecond (fs) pulses (800 nm, 45 fs, 1 kHz repetition rate) were focused in ambient air
using three different focusing geometries f/#6, f/#7.5, and f/#12 corresponding to numerical apertures (NA) of 0.08, 0.06,
and 0.04, respectively. The focusing lens was tilted from zero up to 20 degrees. The filaments decayed into two shorter
parts through tilting of the lens and the separation between shorter filaments increased with increasing lens tilt, in tune
with earlier reports [Kamali et al., Opt. Commun. 282, 950-954 (2009)]. The separation between the filaments matched
well with the predicted distances due to astigmatism induced in loose focusing geometries. However the deviation
increased as we moved to the tighter focusing geometries. The SCE spectrum demonstrated an anomalous behaviour.
The SCE spectrum was suppressed at larger tilt angles of 12 - 20°. However at lower tilt angles, up to 8°, the SCE was
observed to be same to that measured without any tilt of the focusing lens. This behaviour is predominant with tighter
focusing geometries of f/#6 and f/#7.5, wherein the SCE was observed to be higher at 4° and 8° in comparison with that
observed at an angle of 0°. Systematic study of the focusing lens tilt on anomalous SCE spectra and filament
characteristics in the tight focusing geometry are presented.
Recently we synthesized 3,8,13,18-tetrachloro-2,7,12,17-tetramethoxyporphyrin and its metallo-derivatives [1]. The
free-base molecule is unique owing to the presence of an electron donating methoxy group and electron withdrawing
chloro group on the adjacent β- positions of each pyrrole moiety. We could synthesize these molecules through two
different routes; the first route provided pure isomer, albeit in low yield, whereas the second route provided mixture of
isomers with higher yield [1]. Herein we report the third-order nonlinear optical properties of these porphyrins obtained
from Z-scan measurements using ~40 fs, 800 nm pulses. Open aperture data confirmed the presence of saturable
absorption whereas the closed aperture data indicated a positive nonlinearity. We have compared the data of the pure
isomer with that of the mixture of isomers.
KEYWORDS: Absorption, Picosecond phenomena, Nonlinear optics, Femtosecond phenomena, Molecules, Saturable absorption, Transmittance, Optical limiting, System on a chip, Thin films
We present our results of nonlinear optical properties of Tritolyl Corrole (TTC) and Triphenyl Corrole (TPC) studied in
the form of solution using Z-scan technique with 660 nm, ~2 picosecond (ps) pulses and 800 nm, ~40 femtosecond (fs)
pulses excitation. Picosecond open-aperture Z-scan data revealed these molecules exhibited strong saturable absorption.
These molecules possessed negative nonlinear refractive index (n2). The estimated value of n2 was 6×10-15 cm2/W and
8×10-15 cm2/W for TPC and TTC, respectively. We have recently reported NLO properties of Corroles with 800 nm
excitation where they exhibited strong two-photon absorption (2PA) at higher intensities and effective two-photon
absorption at lower intensities in the ps regime. Femtosecond open aperture Z-scan studies indicated the presence of
strong saturable absorption with effective nonlinear absorption coefficients (β) of ~0.8×10-13 cm/W and ~2.7×10-13
cm/W for TPC and TTC, respectively. We have also estimated the sign and magnitude of real part of third order
nonlinearity through the closed aperture scans. We discuss the nonlinear optical performance of these organic molecules.
In this paper we report the fabrication of nanoparticles and nanostructures through the interaction of ultrashort (~40
fs) and short (~2 ps) laser pulses with bulk Aluminum immersed in various liquid media of different polarity
[chloroform which is polar, carbon tetrachloride which is non-polar, water which is polar, dichloromethane (DCM)
which is polar, and Cyclohexane which is non-polar] using the laser ablation technique. Except water and
Cyclohexane, other media showed yellow coloration after ablation took place indicating formation of nanoparticles
in the solution in both fs and ps domains. The coloration of the laser exposed portion in the Al substrate was golden
yellow and its closer view depicted micro-grating (~1-2 μm) and nano-ripple (period 330 nm) formation depending
on the focal conditions. The investigation of polarization dependence on the ablation was performed for water
media. Depending on the ablation threshold, we observed micron sized structures and nano-ripples on the surface.
As the rate of ablation depends on the position of the focus on the Al substrate and beam waist parameters, we have
studied the liquid level dependence of ablation with different water levels on the Al substrate and we compared
these patterns obtained below, near, and above the ablation thresholds of the sample. Field Emission-Scanning
Electron Microscope (FE-SEM), UV-Vis absorption spectra, Electron Diffraction Pattern and Transmission Electron
Microscope (TEM) were used for the characterization and comparison of products in both domains.
From the initial observation of self-channeling of high-peak power femtosecond (fs) laser pulses in air, propagation of
intense ultrashort laser pulses in different media has become one of the most investigated research areas. The
supercontinuum emission (SCE), a spectral manifestation of the spatio-temporal modifications experienced by a
propagating ultrashort laser pulse in a nonlinear medium, has many practical applications. However, the extent of blue
shift of SCE is reported to be constant due to the phenomenon of "intensity clamping". To further explore the recently
observed regime of filamentation without intensity clamping, we measured the evolution of spectral blue shift of SCE
resulting from the propagation of fs pulses (800 nm, 40 fs, 1 kHz) in distilled water under different focusing geometries.
The efficiency of SCE from tight focusing (f/6) geometry was always higher than the loose focusing (f/12) geometry for
both linear and circular polarized pulses. The blue edge of the SCE spectrum (λmin) was found to be blue shifted for f/6
focusing conditions compared to f/12 focusing geometry. The lower bound of the intensity deposited in the medium
measured from the self-emission from the filament demonstrated the existence of intensities ~ 6x1013 Wcm-2, far beyond
the clamping intensities achieved erstwhile.
We have investigated femtosecond laser induced microstructures, gratings, and craters in four
different polymers: poly methyl methacrylate (PMMA), poly dimethyl siloxane (PDMS),
polystyrene (PS) and poly vinyl alcohol (PVA) using Ti:sapphire laser delivering 800 nm, 100
femtosecond (fs) pulses at 1 kHz repetition rate with a maximum pulse energy of 1 mJ. Local
chemical modifications leading to the formation of optical centers and peroxide radicals which
were studied using UV-Visible absorption and emission, confocal micro-Raman and Electron
Spin Resonance (ESR) spectroscopic techniques.
We present some of our initial experimental results from laser induced breakdown spectroscopy (LIBS) studies of few
high energy materials such as a simple match stick (MS) and BKNO3 (BPN), and ammonium perchlorate (AP) using
nanosecond (ns), picosecond (ps), and femtosecond (fs) pulses. The characteristic peaks of each sample in different time
domains are analyzed. The merits and de-merits of ultrashort pulses in LIBS experiments for discrimination of high
energy materials are highlighted.
We have fabricated straight line structures and Y-couplers in X-cut lithium niobate crystals using femtosecond laser
pulses. A systematic characterization study was performed initially to determine the effects of pulse energy on feature
size. The optimal parameters were determined from experiments and simulations obtained using a two dimensional split
step beam propagation method. Later the waveguides and couplers were fabricated using these optimized parameters.
We present our results on the physical and optical characterization of these structures.
We have investigated the nonlinear optical properties, optical limiting thresholds, and figures of merits for five
different phthalocyanine thin films, achieved through doping in PMMA, using the Z-scan technique at 800 nm with 2 ps
laser pulses. From the open-aperture Z-scan data we established that these molecules exhibit strong two photon
absorption (2PA) with the nonlinear coefficients in the range of 15-200 cm/GW. We have also estimated the sign and
magnitude of real part of third order nonlinearity through the closed aperture scans. Preliminary femtosecond pump-probe
data suggests that the lifetimes of excited states are in the sub-100 ps regime for all the molecules in film form.
Our studies provide tangible evidence that these phthalocyanines are potential candidates for multi-photon imaging and
optical limiting applications.
We present a comprehensive review of the nonlinear optical (NLO) properties of various phthalocyanines
studied by our group over the last few years. The NLO coefficients obtained in the continuous wave (cw), nanosecond
(ns), picosecond (ps), and femtosecond (fs) regimes are summarized and important conclusions drawn from these studies
are highlighted. Wherever possible the figures of merit in different pulse domains are evaluated and discussed for
possible applications in the field of photonics. Various schemes to identify and exploit the potential of these molecules
are proposed. Necessary measures required for the realization of practical devices out of these molecules are delineated.
The performance of these molecules vis-à-vis other phthalocyanines and related compounds is evaluated.
Herein we present some of our initial experimental results obtained from the laser induced breakdown
spectroscopic (LIBS) measurements of RDX and HMX using nanosecond (ns), picosecond (ps), and femtosecond (fs)
laser pulses acquired without gating and delay. RDX and HMX were mixed with KBr and pellets were prepared for the
spectroscopic studies. Nanosecond pulses at 532 nm, ps/fs pulses at 800 nm were used for the experiments. The spectra
were collected using Ocean Optics 4000/Maya spectrometer using a UV transmitting, 400 μm core diameter fiber in one
case and a combination of lenses to collect the light from plasma in the second case. Several features were observed in
the spectra exclusive for each pulse domain. The differences/similarities in the spectra collected using different pulses
are presented.
We present our results on the stoichiometric analysis of ammonium nitrate (AN) and ammonium Perchlorate
(AP) studied using laser induced breakdown spectroscopy (LIBS) with nanosecond pulses. The LIBS spectra collected
for AP and AN, without any gating and using a high resolution spectrometer, exhibited characteristic lines corresponding
to O, N, H, C, and K. The Oxygen line at 777.38 nm and three Nitrogen lines (N1, N2, N3) at 742.54 nm, 744.64 nm,
747.12 nm were used for evaluating the Oxygen/Nitrogen ratios. The intensities were calculated using area under the
peaks and normalized to their respective transition probabilities and statistical weights. The O/N1 ratios estimated from
the LIBS spectra were ~4.94 and ~5.11 for AP and O/N3 ratios were ~1.64 and ~1.47 for AN obtained from two
independent measurements. The intensity ratios show good agreement with the actual stoichiometric ratios - four for AP
and one for AN.
Herein we present our results from the picosecond and nanosecond nonlinear optical studies of two novel
phthalocyanines {[(SO3Na)4CuPc] and [(SO3Na)4NiPc]} using the Z-scan technique. Open aperture Z-scan data
revealed that the picosecond nonlinear absorption was dominated by three-photon absorption while in the nanosecond
domain reverse saturable absorption prevailed. Closed aperture data with nanosecond pulses indicated strong thermal,
negative nonlinearity while picosecond excitation demonstrated positive nonlinearity. The nonlinearity in CuPc was
higher than in NiPc in both the time domains. The nonlinear coefficients extracted from the fits to experimental data
were large compared to some of the recently reported works on similar molecules.
We present our results on the characterization of ultrafast excited state dynamics of two phthalocyanines in thin film
form studied using femtosecond pump-probe technique. One was a symmetric Zinc phthalocyanine (SPc) while the
other was an unsymmetrical Zinc phthalocyanine (USPc). The femtosecond (fs) pulses at 800 nm were characterized
using a single shot autocorrelator. The pump probe measurements were carried out with 590/610 nm pulses emanating
from an optical parametric amplifier. SPc demonstrated an excited lifetime of ~30 ps while the ASPc exhibited ~60 ps
lifetime. The potential applications of these molecules are discussed briefly.
Herein we report our experimental results on nonlinear optical properties of (H2)2SnPc (I), Sn(OH)2Pc (II), and
Sn(Cl)2Pc (III) studied using Z-scan technique with 800 nm, 100 fsec pulses, and 633 nm continuous wave (cw) laser
excitation. Femtosecond open-aperture Z-scan data revealed these molecules exhibited strong 3PA coefficient (α3). The
estimated values of α3 were ~4.0×10-5, ~2.0×10-5 cm3/GW2, and ~1.5×10-5 cm3/GW2 for I, II, and III respectively
obtained after deducting the solvent contribution. Closed aperture data recorded with femtosecond pulses revealed
positive nonlinearity for all the molecules. We also observed large nonlinear response in the cw regime at 633 nm.
Closed aperture scans performed with 633 nm indicated strong negative nonlinearity while open aperture scans depicted
mixed response. The performance of these alkyl phthalocyanines in various time domains vis-à-vis recently reported
phthalocyanines is discussed in detail.
We present some of our results on the femtosecond laser direct writing and characterization of micro-gratings in
Baccarat glass. Gratings were inscribed with amplified 800 nm, ~100 femtosecond pulses at 1 kHz repetition rate. The
change in refractive index of the modified region was estimated from grating efficiency measurements and was found to
be ~10-3. Micro-Raman studies demonstrated an increase in the intensity of the band near 596 cm-1 in the laser irradiated region clearly indicating an increase in the refractive index. Micro-Raman mapping of the grating showed a periodic
variation of the band intensity further confirming the formation of grating. Structures with sub wavelength dimensions
(<800 nm) were achieved with shaping of the input pulses using a rectangular slit. Waveguides were inscribed by optimizing parameters like slit width, focusing conditions, translation speed etc. We shall present our results on the physical, spectroscopic and optical characterization of these structures.
We present some of our results on the femtosecond laser direct writing and characterization of micro-structures in
silicate, FoturanTM, and tellurite glasses. Structures with different sizes were fabricated with varying input energy and
spatially modified pulse using a slit. Various characterization techniques including fluorescence spectroscopy, micro-
Raman spectroscopy, and laser confocal microscopy were employed to analyze the structural and physical modifications
at focal volume resulting in the change of refractive index (RI). The RI change due to material modification was estimated using diffraction from a continuous wave laser beam and is presented in this work. The results obtained are analyzed vis-a-vis the recent work in similar glasses and the applications of such structures in the fields of photonics.
We present our results of nonlinear optical properties of 2(3), 9(10), 16(17), 23(24) tetra tert-butyl
phthalocyanine (pc1) and 2(3), 9(10),16(17), 23(24) tetra tert-butyl Zinc phthalocyanine (pc2) studied in solution using
Z-scan technique with 800 nm, 100 fsec pulses, 532 nm, 6 nsec pulses and 633 nm continuous wave (cw) laser
excitation. Femtosecond open-aperture Z-scan data revealed these molecules exhibited strong 3PA coefficient (α3). The
estimated value of α3 was ~9.1 × 10-5 and ~9.5 × 10-5 cm3/GW2 for pc1 and pc2 respectively. Nanosecond open aperture Z-scan studies indicated the presence of strong nonlinear absorption with effective coefficients (α2) of ~310 and ~420 cm/GW for pc1 and pc2 respectively which are at least two orders higher than the recently reported phthalocyanines.
These phthalocyanines also exhibited strong optical limiting properties with nsec excitation with recorded limiting
thresholds of ~0.45 J/cm2. We observed large nonlinear response in the cw regime at 633 nm. The performance of
these alkyl phthalocyanines in various time domains vis-à-vis recently reported phthalocyanines is discussed in detail.
Proton beam writing is a lithographic technique that can be used to fabricate microstructures in a variety of materials including PMMA, SU-8 and FoturanTM. The technique utilizes a highly focused mega-electron volt beam of protons to direct write latent images into a material which are subsequently developed to form
structures. Furthermore, the energetic protons can also be used to modify the refractive index of the material at a precise depth by using the end of range damage. In this paper we apply the proton beam writing technique to the fabrication of a lab-on-a-chip device that integrates buried waveguides with microfluidic channels. We have chosen to use FoturanTM photostructurable glass for the device because both direct patterning and refractive index modification is possible with MeV protons.
Proton beam writing is a new direct-write micromachining technique capable of producing 3-dimensional (3-D), high aspect ratio micro-structures with straight and smooth sidewalls. It uses a focused sub-micron beam of 2.0 MeV protons to direct-write on a suitable polymer, such as the photoresists: poly-methylmethacrylate (PMMA) and SU-8, a negative tone photoresist from MicroChem. In this paper, we report on the application of proton beam writing to fabricate low-loss passive polymer waveguide structures such as symmetric y-branching waveguides in SU-8. SU-8 channel waveguides are fabricated by first direct-writing the pattern using a proton beam and subsequently chemically developing the latent image formed. A UV-cured resin, Norland Optical Adhesive 88 (NOA-88) is used as the cladding layer. Being a direct-write technique, proton beam writing offers us great flexibility to fabricate waveguides of arbitrary patterns and this is an asset that can be applied to the rapid prototyping of optical circuits. With all its unique characteristics, proton beam writing is an excellent technique for waveguide fabrication.
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