arXiv subjects

Nitish Singh

Publications and source records attributed to Nitish Singh.

12 recordsLinked to original sources

arXiv2026

Coupled self-charging and self-propulsion of evaporating sessile droplets

Evaporating sessile water droplets universally deposit surface charges while simultaneously becoming self-charged---a phenomenon that remains under-appreciated despite its potentially important consequences. Here, we investigate one such consequence: droplet self-propulsion driven by self-charging. This self-propulsion arises spontaneously in the absence of external stimuli and produces self-avoiding trajectories that can displace droplets by many times their own diameter. Our results have important practical implications for processes requiring spatial precision, such as inkjet printing. The coupling between self-charging and self-propulsion produces mosaic surface charge patterns characterized by alternating polarities, which we explain with a physical model based on properties of the electric double layer.

cond-mat.soft↗

arXiv2026

Design and Development of a Lab Prototype of a Fiber-Based Integral Field Spectrograph

Integral Field Spectroscopy provides simultaneous spatial and spectral information, making it a powerful technique for studying both point like and extended astronomical sources. As part of our effort to develop a compact Integral Field Spectrograph (IFS) for optical astronomy, we have designed and realized a fiber based prototype with a resolving power of ~800 at the H-$\alpha$ wavelength. The front end optics, including the fore optics and lenslet based integral field unit (IFU), have been designed and optimized. In the present work, we focus on the design and laboratory verification of a fiber only IFU module consisting of 37 fibers arranged in a precise hexagonal geometry to match the lenslet pitch. The back end of the system forms a linear fiber slit that feeds a laboratory built spectrograph. The optical design was optimized using ZEMAX, and the system was experimentally tested using a Neon emission lamp and solar light. The measured spectra clearly show prominent features including the H-$\alpha$ lines, with dispersion and resolution consistent with theoretical predictions. Although lenslets are not yet integrated, this work establishes a validated methodology for accurate fiber alignment for IFS. This results form a crucial step toward the full implementation of a lenslet fiber based IFS for the 2.34 meter Vainu Bappu Telescope.

astro-ph.IM↗

arXiv2026

Optical design of a direct fibre-fed MO-IFS for the NLOT

The initial optical design and performance analysis of a dual-channel fibre-fed Multi-Object Integral Field spectrograph (Mo-IFS) being designed for a future National Large Optical/Infrared Telescope (NLOT) in India. The front end will be a moveable lenslet+fiber based integral field unit. The spectrograph is designed to directly accept an f/4 beam from the approximately 200 optical fibers, each with a 100 um core diameter without additional fore-optics. The instrument consists of two optimized spectral channels covering wavelength ranges of 0.32-0.62 um (blue channel) and 0.60-1.00 um (red channel). The optical design aims to achieve moderate spectral resolutions of approximately R ~ 2700 in the blue channel and R ~ 2500 in the red channel while maintaining high throughput over a broad spectral range. The spectrograph architecture includes a fiber-fed entrance slit, collimator optics, dichroic beam splitting system, dispersive elements, and dedicated camera optics for each channel. Zemax simulations were carried out to evaluate image quality, spot size distribution, spectral resolution, and detector sampling across the full wavelength range. The current work presents the initial optical configuration, design methodology, and expected performance of the instrument.

astro-ph.IM↗

arXiv2026

Design of a three-lens wide field corrector with aspherical surfaces for the 2.34-m VBT

We are developing a compact three-element Wide Field Corrector (WFC) with spherical and aspherical lenses for the 2.34 m Vainu Bappu Telescope (VBT) to enhance its field coverage for imaging and spectroscopic applications. The design consists of three optical elements, with at least one spherical lens movable to serve as an Atmospheric Dispersion Corrector (ADC), while the aspherical elements remain fixed to maintain optical stability. We are currently testing two design configurations, one with two spherical lenses and one aspherical lens, and another with two aspherical lenses and one spherical lens. The ADC is designed to correct atmospheric dispersion for zenith angles ranging from 0 degree to 60 degree. The system is optimized to operate over a wavelength range of 0.4 {\mu}m to 0.9 {\mu}m, targeting an effective field of view of about 0.5 degree. Considering the limited mechanical space available at the VBT prime focus, the design emphasizes compactness, ease of alignment, and manufacturability. The system achieves a mean D80 better than 0.3 arcsec and 0.23 arcsec for Design 1 and Design 2, respectively, at zenith, and maintains a mean D80 within 0.57 arcsec and 0.45 arcsec up to a zenith angle of 60 degree after atmospheric dispersion correction. Atmospheric dispersion at higher zenith angles (up to 60 degrees) is corrected using a movable lens element, enabling the system to preserve high image quality across the field.

astro-ph.IM↗

arXiv2025

Bipolar surface charging by evaporating water droplets

Surface charging is a ubiquitous phenomenon with important consequences. On one hand, surface charging underpins emerging technologies such as triboelectric nanogenerators; on the other, uncontrolled charging can damage delicate nanostructures and devices. Despite its significance, surface charging by evaporating water droplets remains poorly understood. Here, using Kelvin Probe Force Microscopy, we spatially resolve the surface-charge patterns from evaporating droplets and propose a physical model that quantitatively explains the origin of bipolar charging.

cond-mat.soft↗

arXiv2025

Design and Fabrication of a lightweight three-lens corrector system for the 2.34-m Vainu Bappu Telescope

The Vainu Bappu Telescope (VBT) is a 2.34-m reflector, primarily supported on-axis field of view, offering high-resolution and low-to-medium resolution spectroscopic observations in its prime and Cassegrain configurations. This study presents the design and fabrication of a compact, lightweight, three-element wide-field corrector (WFC) utilizing three spherical lenses to cover a polychromatic wavelength range over a 30$'$ FoV at prime focus. The WFC design was optimized using ZEMAX, ensuring precision in aberrations, tolerances, and atmospheric dispersion. The fabricated lenses met stringent tolerances, with a $\pm$1 mm deviation in radius of curvature and $\pm$2 mm deviation in center thickness. A mechanical mount was developed to integrate all the WFC lenses, and wavefront error testing for the WFC system was performed using ZYGO interferometry, yielding a Wavefront Error of 0.05 $λ$. Laboratory performance tests were designed and conducted using a dedicated setup with achromatic lenses and 100 $μm$ fiber-coupled polychromatic light source showed a deviation of 0.1 pixel on-axis and 0.5 pixel at the extreme off-axis field compared to the ZEMAX design, demonstrating that the optical performance of WFC is with minimal aberrations across the entire FoV. The successful integration of the WFC at the VBT prime focus will increase the FoV, enabling the multi-fiber, multi-spectrograph setup in 30$'$ field that will facilitate both OMR and Echelle spectrograph to be used on the same night along with the addition of new multi-object spectrograph and an integral field unit instrument. This will mark a significant upgrade for the VBT, broadening its research potential, and expanding its observational versatility.

astro-ph.IM↗

arXiv2024

Fine-grained large-scale content recommendations for MSX sellers

One of the most critical tasks of Microsoft sellers is to meticulously track and nurture potential business opportunities through proactive engagement and tailored solutions. Recommender systems play a central role to help sellers achieve their goals. In this paper, we present a content recommendation model which surfaces various types of content (technical documentation, comparison with competitor products, customer success stories etc.) that sellers can share with their customers or use for their own self-learning. The model operates at the opportunity level which is the lowest possible granularity and the most relevant one for sellers. It is based on semantic matching between metadata from the contents and carefully selected attributes of the opportunities. Considering the volume of seller-managed opportunities in organizations such as Microsoft, we show how to perform efficient semantic matching over a very large number of opportunity-content combinations. The main challenge is to ensure that the top-5 relevant contents for each opportunity are recommended out of a total of $\approx 40,000$ published contents. We achieve this target through an extensive comparison of different model architectures and feature selection. Finally, we further examine the quality of the recommendations in a quantitative manner using a combination of human domain experts as well as by using the recently proposed "LLM as a judge" framework.

cs.IR↗

arXiv2024

Design for Enabling Echelle and OMR Spectrograph Observations for Point and Extended Sources on the Same Night at VBT

The 2.34m Vainu Bappu Telescope (VBT) is a reflecting telescope that operates in two modes, prime focus and cassegrain focus, and is equipped with two instruments. In prime focus mode, the telescope has the F-number of f/3.25, and the High-Resolution Echelle Spectrograph (HRES) is employed through optical fiber. On the other hand, in cassegrain focus mode, the F-number is f/13, and the OMR Spectrograph (OMRS) is mounted for low and medium-resolution spectroscopy. Currently, the VBT faces a limitation: either the OMRS or the HRES can be used due to the switch in the heavy secondary mirror. To overcome this, we present a novel method enabling the OMRS to operate from prime mode alongside the HRES. The fiber setup for OMRS is optimized with a 25-lenslet + fiber-based Integral Field Unit (IFU) capable of observing both point and extended sources. The optimized lenslet, fiber, and fore optics design is undergoing lab testing. Our approach allows seamless operation of both spectrographs on the same night, enhancing the observational capabilities of astronomical studies with VBT.

astro-ph.IM↗

arXiv2024

Theoretical Analysis of Fiber Arrangements for Improved Coupling Efficiency in the VBT High-Resolution Echelle Spectrograph

In fiber-based spectroscopy within telescopes, a prevailing limitation has been the necessity to align the fiber diameter with the telescope's seeing conditions, often characterized by the Full Width at Half Maximum of the point spread function. This alignment constraint captures around 50 \% of the incoming flux from any point source. Furthermore, the challenge is compounded when high-resolution spectroscopy is in play, as it often demands a minute slit width, further exacerbating flux loss. The essence of this paper lies in a comprehensive exploration, accomplished through theoretical simulations, of strategies aimed at enhancing the coupling efficiency of high-resolution spectrographs. The primary objective is to bolster the flux capture without compromising the critical aspect of spectral resolution. This research endeavors to unlock the potential for more effective utilization of high-resolution spectrographs to study celestial objects.

astro-ph.IM↗

arXiv2024

A case study of Generative AI in MSX Sales Copilot: Improving seller productivity with a real-time question-answering system for content recommendation

In this paper, we design a real-time question-answering system specifically targeted for helping sellers get relevant material/documentation they can share live with their customers or refer to during a call. Taking the Seismic content repository as a relatively large scale example of a diverse dataset of sales material, we demonstrate how LLM embeddings of sellers' queries can be matched with the relevant content. We achieve this by engineering prompts in an elaborate fashion that makes use of the rich set of meta-features available for documents and sellers. Using a bi-encoder with cross-encoder re-ranker architecture, we show how the solution returns the most relevant content recommendations in just a few seconds even for large datasets. Our recommender system is deployed as an AML endpoint for real-time inferencing and has been integrated into a Copilot interface that is now deployed in the production version of the Dynamics CRM, known as MSX, used daily by Microsoft sellers.

cs.IR↗

arXiv2023

Enhancement of the Rate of Surface Reactions by Elasto-capillary Effect

Rate of a reaction is enhanced by increase in temperature, partial pressure or the concentration of reactants, and/or via use of a catalyst or an enzyme. Whereas the former increases the probability of collision rate between different molecules, the latter provides an alternate reaction path that diminishes the activation energy barrier for reaction, both effects increase the reaction rate. The deformability of the substrate on which a reaction occurs is not known to affect this rate. In contrast, by carrying out reduction of Gold and Silver salt on soft, crosslinked layers of poly(dimethylsiloxane) (PDMS), we show here that surface tension driven deformation of the solid too can increase the reaction rate. The PDMS contains SiH groups as organosilanes which reduces the salt thereby producing the corresponding metallic nano-particles. When an aqueous solution of the salt is dispensed as a sessile drop on a sufficiently soft PDMS layer, nano-particles get generated at ~3 times the rate over which when the salt solution is dispensed as a pool on the same surface. Unlike the pool, the drop has a three-phase contact line, where it pulls up the solid forming a ridge. By balancing the resultant surface stresses at the vicinity of this ridge, we have estimated the excess surface free energy associated with it. We show that this excess energy can diminish the activation energy barrier and increase the reaction rate to the extent that would have been achieved by increasing the reaction temperature by over 30o C.

cond-mat.soft↗

arXiv2021

Hierarchically Rough Surface used as Rewritable and Reprintable paper

We describe here the design and fabrication of a rewritable and reprintable paper as an alternative to conventional papers which are generally used for writing and printing purposes. This paper is prepared by casting a crosslinkable silicone on a porous template, leading to hierarchical random patterns having dimensions ranging from tens of nanometer to hundreds of micron. On the resultant rough surface, ink particles of wide ranging sizes and different surface chemical characteristics adhere via physical forces. Unlike conventional papers, in which particles get trapped inside the porous network of the cellulose fibers, here the ink particles remain accessible for cleaning via gentle shear with a rough and wet substrate. It is possible to write on this paper by different commercially available pens and to print using different printers. In almost all cases, the ink particles adhere to the paper surface strongly enough that they can not be dislodged by gentle shearing, yet adhesion can be decreased by inserting a liquid between the particles and the surface. Because of the resultant reversibility in adhesion, the paper remains usable over 200 cycles. This reusable paper is expected to be a viable alternative to the conventional papers that we use today.

physics.app-ph↗