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The very foundation of IIT Hyderabad is based on&nbsp;<a href="https://iith.ac.in/research/">research and innovation</a>. The vibrant research culture is evident from the number of patents and publications that IITH produces consistently every year. IITH offers&nbsp;<a href="https://iith.ac.in/academics/programmes-offered/">graduate programs</a>&nbsp;at both a masters, and a doctoral level, in several diverse areas. There are separate programs for technology, design, science, and liberal arts. The&nbsp;<a href="https://iith.ac.in/academics/post-graduate/">MTech program</a>&nbsp;is offered in various modes and durations to cater to the ever-growing need of postgraduate level professionals in the country.

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IITH creates a unique holistic ecosystem for education that offers interactive learning, a very flexible academic structure, cutting-edge research, strong industry collaboration, and entrepreneurship. This is an environment which enables students and faculty to translate their dreams into realities.

IIT Hyderabad

In this paper, we present a study of depolarization of a supercontinuum across its spectral range as a function of the femtosecond laser pump intensity for an anisotropic crystalline condensed medium, potassium-dihydrogen-phosphate (KDP) crystal, and compare our results with commonly used supercontinuum generation (SCG) materials, namely borosilicate glass Schott (BK-7) glass (representing isotropic amorphous condensed media) and Ba F2 (isotropic crystalline condensed media). Our results show that at higher input powers, depolarization in the continuum increases for BK-7, Ba F2, and along the direction of the optic axis of the KDP crystal. However, in the case of KDP crystal, we observe that the depolarization properties are strongly dependent on (i) the plane of polarization of incident light and (ii) the orientation of the crystal with respect to the incident light. Our studies also confirm that one can achieve SCG in a KDP crystal that maintains the same state of input polarization even at high input intensities when proper orientation of the crystal is used. © 2008 The American Physical Society.

Department of Physics

World University root node

Physical Review A - Atomic, Molecular, and Optical Physics

Depolarization properties of the femtosecond supercontinuum generated in condensed media

We present some of our results on the femtosecond laser direct writing and characterization of micro-structures in silicate, Foturan™, 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, microRaman 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-à-vis the recent work in similar glasses and the applications of such structures in the fields of photonics.

Proceedings of SPIE - The International Society for Optical Engineering

Inscription and characterization of micro-structures in silicate, FOTURAN™ and tellurite glasses by femtosecond laser direct writing

A systematic experimental study on the polarization properties of supercontinuum generation (SCG) in a noncentrosymmetric crystal (KDP) is reported. Our studies show that depolarization of the SCG is dependent on the plane of polarization of incident light as well as on the orientation of the crystal, allowing a better control of the polarization properties. © 2008 Optical Society of America.

Optics Letters

Control of the polarization properties of the supercontinuum generation in a noncentrosymmetric crystal

Role of the donor groups connected to the oxyallyl central moiety (suggested to be a powerful acceptor) has been emphasized in the literature for tuning the absorption maxima and obtaining large NLO activity in croconates and squaraine dyes which are oxyallyl derivatives. Here we have prepared a series of new croconate dye model molecules with aniline and substituted anilines as strong donors to the oxyallyl ring. Using experimental and theoretical techniques like UV-visible spectra, degenerate four wave mixing (DFWM), density functional theory, time-dependent density functional theory, and symmetry adopted cluster-configuration interaction (SAC-CI), we have characterized linear absorption, nonresonant third order optical nonlinearity, charge transfer, and excited states of these molecules. We find that these molecules have smaller singlet diradical character and the absorption is not in the near-infrared (NIR), as other reported croconates, but in the 440-480 nm range. There is a larger charge transfer (CT) from the side groups to the central ring in diese croconates evident from SAC-CI calculations compared to CT in NIR croconates. This clearly supports the model that the NIR absorption in oxyallyl derivatives is correlated to the singlet diradical character and not to the donor capacity. The γ values determined by DFWM experiments show large nonresonant values of -2.4 to -5.3 × 10-32 esu, which is larger than that of similar squaraines suggesting that the larger oxyallyl ring size and diradical character in croconates play the major role and not the donor groups. We conclude that with a noncentrosymmetric structure, tunable absorption (visible-NIR), and larger γ values, diese less studied croconate dyes are more interesting and will have a major role to play than the widely reported centrosymmetric squaraines as molecular materials. © 2008 American Chemical Society.

Journal of Physical Chemistry C

Linear and nonlinear optical properties of mesoionic oxyallyl derivatives: Enhanced non-resonant third order optical nonlinearity in croconate dyes

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 (&lt;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.

Micro-Raman mapping of micro-gratings in 'BACCARAT' glass directly written using femtosecond laser

In this article we report our experimental results on non collinear four-wave mixing (FWM) at air-dielectric interfaces. We generated FWM signal from the two interfaces of a 1mm thick fused silica slide with air using two 800 nm, 100 fs pump pulses in non-collinear pump-probe type geometry. We observe that there is a maximum peak shift of ∼1.5 nm in FWM signals corresponding to the two interfaces of the same silica slide. Further, we find that the intensity of FWM signal observed at air to silica interface is less than that from silica to air interface. We briefly elaborate on this observation with systematic study of FWM in fused silica. © 2008 Optical Society of America.

Fulltext

Optics Express

Four wave mixing at air-dielectric interfaces with a femtosecond laser excitation

Zinc oxide (ZnO) is recognised as a potential II-VI photonic semiconductor and wavelength-scale ordered patterning of such material helps favourably in tailoring the photonic properties. Here we present two novel approaches for fabricating ZnO photonic structures, namely, via a synthetic route and via electrochemical deposition. We demonstrate fabrication of well-ordered mesa and microphotonic structures from self-assembly of template-assisted electrochemical deposition. We have explored various aspects of the fabrication techniques for achieving an optimized performance. Several optical, electrical and structural techniques are used to highlight the potential utility of these ZnO photonic structures. Our results suggest that these structures show promise in many novel photonic applications. © 2007 Elsevier B.V. All rights reserved.

Materials Letters

Fabrication and optoelectronic characterisation of ZnO photonic structures

We present here an enhanced broadband supercontinuum generation in a potassium di-hydrogen phosphate (KDP) crystal. The enhancement in the bandwidth of the white light is obtained towards the shorter wavelength regime (<400 nm) by employing supercontinuum generation and sum frequency generation in tandem. The tunability in the blue region of the spectrum with angle is demonstrated. The bandwidth of supercontinuum achieved spans from 350 nm to 1300 nm. Further, we show the excellent polarization maintenance of continuum generated in KDP in comparison to that generated in water and BK-7 glass. © Springer-Verlag 2007.

Applied Physics B: Lasers and Optics

Broadband supercontinuum generation in a single potassium di-hydrogen phosphate (KDP) crystal achieved in tandem with sum frequency generation

We report on the synthesis, crystal structure and optical characterization of chalcone derivatives developed for second-order nonlinear optics. The investigation of a series of five chalcone derivatives with the second harmonic generation powder test according to Kurtz and Perry revealed that these chalcones show efficient second-order nonlinear activity. Among them, high-quality single crystals of 3-Br-4′-methoxychalcone (3BMC) were grown by solvent evaporation solution growth technique. Grown crystals were characterized by X-ray powder diffraction (XRD), laser damage threshold, UV-vis-NIR and refractive index measurement studies. Infrared spectroscopy, thermogravimetric analysis and differential thermal analysis measurements were performed to study the molecular vibration and thermal behavior of 3BMC crystal. Thermal analysis does not show any structural phase transition. © 2007 Elsevier B.V. All rights reserved.

Journal of Crystal Growth

Second harmonic generation and crystal growth of new chalcone derivatives

We report our results on nonlinear optical properties of 2(3), 9(10), 16(17), 23(24) tetra tert-butyl phthalocyanine and 2(3), 9(10), 16(17), 23(24) tetra tert-butyl Zinc phthalocyanine studied using Z-scan technique with 800 nm femtosecond and 532 nm nanosecond pulses. Nonlinear absorption behavior in both femtosecond and nanosecond domains was studied in detail. We observed three-photon absorption with femtosecond laser excitation and strong reverse saturable absorption with nanosecond pulse excitation. We have also evaluated the sign and magnitude of the third-order nonlinearity. © 2007 Elsevier B.V. All rights reserved.

Chemical Physics Letters

Femtosecond and nanosecond nonlinear optical properties of alkyl phthalocyanines studied using Z-scan technique

The nonlinear optical absorption and switching properties of sol-gel derived of Au nanoparticle doped SiO 2-TiO 2 sol-gel films having different Au-surface plasmon absorption positions are reported in this paper. The Au nanoparticles are embedded in SiO 2 and SiO2-TiO 2mixed glassy film matrices with different refractive index values. To study the nonlinear absorption properties, lasers with three different wavelengths are used. The optical switching behavior is studied by using the pump-probe technique with 532 nm as the excitation wavelength. Ground state conduction band, surface plasmon band, and the free carrier band are taken as three level model to explain theoretically the obtained RSA and SA behaviors. Copyright © 2006 American Scientific Publishers All rights reserved.

Journal of Nanoscience and Nanotechnology

Nonlinear optical absorption and switching properties of gold nanoparticle doped SiO 2-TiO 2 sol-gel films

A systematic study of nonlinear absorption and scattering properties of 4.5 nm sized CdS nanocrystals dispersed in dimethylformamide with varying concentrations is reported in this paper. The study is undertaken by performing open aperture Z -scan measurements with 532 nm nanosecond and 790 nm femtosecond pulse excitations. A modified Z -scan experimental setup is used to collect both nonlinear absorption and nonlinear scattering at different forward angles simultaneously. Optical limiting studies of these nanocrystals show that nonlinear scattering (NLS) is comparable to the two-photon absorption (TPA). Both the TPA and NLS coefficients are estimated by theoretical fit of the observed open aperture Z -scan curves. The variation of NLS coefficients with concentration is discussed in detail, and we estimated the thermo-optic coefficient of CdS nanocrystals. The nonlinear refractive index and limiting threshold (I12) of CdS nanocrystals at different concentrations (at 532 nm) are also estimated and reported. Further, we report our observation of the formation of diffraction fringes in the transmitted beam at high intensities. © 2006 American Institute of Physics.