arXiv subjects

T. K. Sharma

Publications and source records attributed to T. K. Sharma.

8 recordsLinked to original sources

arXiv2026

NAIR-APREXIS: Enabling photonics-based instruments for long-baseline interferometry and integral-field spectroscopy

The NAIR project -- Novel Astronomical Instrumentation based on photonic light Reformatting -- aims at advancing photonic technologies for infrared long-baseline interferometry and precision spectroscopy. The rapid development of astrophotonics over the past decade has opened new pathways for astronomical instrumentation with unprecedented capabilities. We present results from NAIR that demonstrate the potential of the ultrafast-laser inscription (ULI) technique for fabricating remapping devices for a range of applications. We developed a single-mode integrated-optics astronomical K-band beam combiner, which we successfully tested on-sky, although using only one single baseline of the CHARA Array. Across several observing campaigns, the prototype exhibited excellent stability, achieving 1% precision on the interferometric visibilities and a total on-sky throughput >40%, with an achieved limiting magnitude of K~5 using the 1-m meter telescopes of CHARA and without external fringe tracking. We are also developing an integral field unit (IFU) designed for exoplanet detection and characterisation. This is due to be tested with MagAO-X in Chile in 2027. The IFU is based upon astrophotonic fiber technologies - two-photon-polymerized (TPP) lenslets, a custom multi-core fiber, and a ULI reformatter. We discuss our efforts to achieve contrasts of 1e-3 between adjacent spaxels whilst retaining throughput of >50%. Finally, we discuss the work we are doing developing the next generation of astrophotonic technologies, including TPP micro-dispersers designed for low resolving power, high transmission applications. We achieve R~30 in a sub-mm package, showing viability for future use. These results emphasize the versatility and simplicity of integrated photonic approaches as a major advance in optical technologies for astronomical instrumentation.

astro-ph.IM↗

arXiv2017

Fabrication of plasmonic surface relief gratings for the application of band-pass filter in UV-Visible spectral range

The measured experimental results of optical diffraction of 10, 5 and 3.4 micrometer period plasmonic surface relief grating are presented for the application of band-pass filter in visible spectral range. Conventional scanning electron microscopic (SEM) is used to fabricate the grating structures on the silver halide based film (substrate) by exposing the electron beam in raster scan fashion. Morphological characterization of the gratings is performed by atomic force microscopy (AFM) shows that the period, height and profile depends on the line per frame, beam spot, single line dwell time, beam current, and accelerating voltage of the electron beam. Optical transmission spectra of 10 micrometer period grating shows a well-defined localized surface plasmon resonance (LSPR) dip at ~366 nm wavelength corresponding to gelatin embedded silver nanoparticles of the grating structure. As the period of the grating reduces LSPR dip becomes prominent. The maximum first order diffraction efficiency (DE) and bandwidth for 10 micrometer period grating are observed as 4% and 400 nm in 350 nm to 800 nm wavelength range respectively. The DE and bandwidth are reduced up to 0.03% and 100 nm for 3.4 micrometer period grating. The profile of DE is significantly flat within the diffraction bandwidth for each of the gratings. An assessment of the particular role of LSPR absorption and varied grating period in the development of the profile of first order DE v/s wavelength are studied. Fabrication of such nano-scale structures in a large area using conventional SEM and silver halide based films may provide the simple and efficient technique for various optical devices applications.

physics.app-ph↗

arXiv2014

Development of high power quantum well lasers at RRCAT

We at RRCAT have recently developed high power laser diodes in the wavelength range of 740 to 1000 nm. A typical semiconductor laser structure is consisted of about 10 epilayers with different composition, thickness and doping values. For example, a laser diode operating at 0.8 micron has either GaAs or GaAsP quantum well as an active layer. The quantum well is sandwiched between AlGaAs wider bandgap waveguide and cladding layers. The complete laser structure is grown by metal organic vapour phase epitaxy technique and devices are fabricated through standard procedure using photolithography. We recently achieved about 5.3 Watt peak power at 853 nm. These laser diodes were tested under pulsed operation at room temperature for 500 nanosecond pulse duration with a duty cycle of 1:1000. Laser diode arrays consisting of 6-10 elements were also developed and tested for operation in pulsed mode at room temperature.

physics.optics↗

arXiv2013

Effect of Strain disorder on the magnetic glassy state in La5/8-yPryCa3/8MnO3 (y = 0.45) thin films

Present study reveals that the free energy landscape of the La5/8-yPryCa3/8MnO3 (LPCMO) system could be modified by elastic strain interaction in the epitaxial thin films. Epitaxial LPCMO thin films of various thicknesses are grown on LaAlO3 substrate by pulsed laser deposition. With increasing thickness, by virtue of island growth morphology, strain disorder is invoked in thin films during strain relaxation process. The length-scale of phase separation is found to be highly correlated with strain disorder. Magneto-transport measurements demonstrate that coherent strain stabilizes charge ordered insulating phase while strain disorder stabilizes metallic phase. Resistivity under cooling and heating in unequal field (CHUF) protocol exhibits lower value of freezing temperature for strain disordered films compared to bulk system. Raman spectroscopy reveals that the charge ordered insulating and ferromagnetic metallic phases are structurally dissimilar and possess monoclinic and rhombohedral like symmetries respectively. Interfaces between two phases strongly influence low temperature glassy metastable state resulting in different phase separation states in the LPCMO thin films.

cond-mat.str-el↗

arXiv2013

Modification in structural, dielectric and magnetic properties of La and Nd co-substituted epitaxial BiFeO3 thin films

The influence of La and Nd co-substitution on the structural and magnetic properties of BiFeO3 (BFO) thin films was examined. Epitaxial thin films of pure and, La and Nd co-doped BFO on the SrRuO3 buffered single crystal SrTiO3 (001) substrate were deposited using pulsed laser deposition. The structural change in co doped La and Nd BFO thin films which was caused by the changes of force constant in the crystal lattice induced by ionic radii mismatch was investigated. Raman spectroscopy studies manifest the structural change in doped BFO films from rhombohedral to monoclinic distorted phase which is induced by the co substitution of La and Nd. Room temperature magnetic hysteresis curves indicated that saturation magnetization is enhanced in the doped film with saturation magnetization of ~20 emu/cm3. The dielectric and magnetic properties are effectively improved in BLNFO films compared to pure BFO thin films.

cond-mat.mtrl-sci↗

arXiv2012

Origin of Periodic Modulations in the Transient Reflectivity Signal at Cryogenic Temperatures

Periodic modulations that appear in the low-temperature transient reflectivity signal of a GaAsP/AlGaAs single quantum well is studied. Similar anomalous oscillations are also observed in layered manganite [K. Kouyama et.al. J. Phys. Soc. Jpn. 76:123702(1-3), 2007]. We show that such periodic modulations are caused by changes in the linear reflectivity of the sample during transient reflectivity measurements. Studied carried out on reflectivity of different materials under identical conditions shows that these modulations on the true transient reflectivity signal are caused by condensation of residual gases on the surface of quantum well. Methods to obtain reliable transient reflectivity data are also described.

physics.optics↗

arXiv2011

Strain-driven light polarization switching in deep ultraviolet nitride emitters

Residual strain plays a critical role in determining the crystalline quality of nitride epitaxial layers and in modifying their band structure; this often leads to several interesting physical phenomena. It is found, for example, that compressive strain in AlxGa1-xN layers grown on AlyGa1-yN (x<y) templates results in an anti-crossing of the valence bands at considerably much higher Al composition than expected. This happens even in the presence of large and negative crystal field splitting energy for AlxGa1-xN layers. A judicious magnitude of the compressive strain can support vertical light emission (out of the c-plane) from AlxGa1-xN quantum wells up to x\approx 0.80, which is desirable for the development of deep ultraviolet light-emitting diodes designed to operate below 250nm with transverse electric polarization characteristics.

cond-mat.mtrl-sci↗

arXiv1999

Observation of Type I/II Transition in GaAs/InGaP Heterostructure by C-V Profiling

It is known in the literature that InGaP alloy synthesized within a certain range of growth temperature (520 - 720 degree Centrigrade) shows Cu-Pt crystal ordering. This ordering reduces the band gap energy . It was predicted that the ordering induced modification of energy levels of InGaP lowers its conduction band edge, which could change a type I band alignment at a GaAs/InGaP heterojunction to type II . Here we report the first observation of this ordering related type I/II transition for a GaAs/InGaP heterojunction by C-V profiling experiments done at room temperature .

cond-mat.mtrl-sci↗