Kavya Mohan

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Light-sheet Lithography for Generating Micro/Nano-Structures

Nanolithography using counter-propagating light-sheets

We propose a laser interference nano-lithography technique for fabrication of nano-structures. Th... more We propose a laser interference nano-lithography technique for fabrication of nano-structures. This is inspired by a 2pi-illumination system that consists of two cylindrical lens arranged face-to-face at a distance 2f with a common geometrical focus. When illuminated by a coherent light, this results in the super-position of two counter-propagating light-sheets. The interference gives rise to nano-bump structures along the optical axis. This technique overcomes the existing point-scanning techniques and paves the way for mass production of nano-structures. Study shows the structures with a feature size of 60 nm and an inter-nanobump separation of 180 nm. Proposed technique may find applications in plasmonics, nanophotonics and nanobiology.

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Experimental realization of sub-micron patterning using counter-propagating interfering lightsheets (iCLASS)

OSA Continuum

A light-sheet-based plane-selective sub-micron patterning technique is proposed to fabricate 1D s... more A light-sheet-based plane-selective sub-micron patterning technique is proposed to fabricate 1D sub-micron patterns with specificity and selectivity. The proposed technique is termed as, interfering coherent light-sheet assisted structure synthesis (iCLASS). The technique uses specialized 2π optical illumination geometry to expose the photoresist film. Counter-propagating light sheets (satisfying phase-matching condition) produce 1D interference pattern with feature size below the diffraction limit. A conventional S1813 photoresist coated on a cleaned glass substrate is exposed to the light-sheets pattern and subsequently, the photoresist film is developed to imprint the sub-micron pattern. AFM study confirms imprinted 1D pattern with a periodicity and feature-size of approximately, λ/2. Analysis show that the light-dose interaction-time (τexp, τdev) plays crucial role in determining the periodicity of 1D sub-micron pattern.

Note: Multi-sheet light enables optical interference lithography

The Review of scientific instruments, 2018

We propose and demonstrate a modified spatial filter-based single-shot lithography technique for ... more We propose and demonstrate a modified spatial filter-based single-shot lithography technique for fabricating an array of microfluidic channels. This is achieved by illuminating the photopolymer specimen with a multiple light sheet (MLS) pattern. Modified spatial filtering is employed in a cylindrical lens system to generate the MLS pattern. The transmission window [the difference (α - β) angle] of the spatial filter determines the characteristics of the pattern and the fabricated microfluidic channel array. After exposing to a negative photoresist (DPHPA monomer with rose bengal as the photoinitiator), this gives rise to an array of micro-fluidic channels (post development process). We studied the effect of micro-channel geometry (channel width, inter-channel separation, and aspect ratio) for varying exposure times that show near-linear dependence. The results show that the fabricated array has 7 prominent channels with an individual channel width and inter-channel separation of app...

Light-sheet based lithography technique for patterning an array of microfluidic channels

Microscopy Research and Technique, 2017

We propose a Light-sheet laser interference lithography technique for fabricating periodic microf... more We propose a Light-sheet laser interference lithography technique for fabricating periodic microfluidic channels. This technique uses multiple light-sheet illumination pattern that is generated using a spatial filter at the back-aperture of the cylindrical lens. Specially designed spatial filter is used that give rise to a periodic pattern at the focal plane which is essentially a 1D Fourier transform of the spatial filter transfer function. One-dimensional focusing property of the cylindrical lens result in the generation of line shaped channel geometry. To design microfluidic channels, the illumination pattern is exposed to the glass substrate coated with a photopolymer sensitized to 532 nm and subsequently developed using standard chemical protocols. Experimentally, the 1D periodic channel structure has an approximate width and periodicity of approximately 11.25 microns. Light-sheets based lithography technique offer a fast and single-shot process to generate microfluidic channels.

Note: Experimental observation of nano-channel pattern in light sheet laser interference nanolithography system

The Review of scientific instruments, 2016

We experimentally observed nano-channel-like pattern in a light-sheet based interference nanolith... more We experimentally observed nano-channel-like pattern in a light-sheet based interference nanolithography system. The optical system created nano-channel-like patterned illumination. Coherent counter-propagating light sheets are made to interfere at and near geometrical focus along the propagation z-axis. This results in the formation of nano-channel-like pattern (of size ≈ 300 nm and inter-channel periodicity of ≈337.5 nm) inside the sample due to constructive and destructive interference. In addition, the technique has the ability to generate large area patterning using larger light-sheets. Exciting applications are in the broad field of nanotechnology (nano-electronics and nano-fluidics).

Limited-view light sheet fluorescence microscopy for three dimensional volume imaging

Applied Physics Letters, 2015

We propose and demonstrate a limited-view light sheet microscopy (LV-LSM) for three dimensional (... more We propose and demonstrate a limited-view light sheet microscopy (LV-LSM) for three dimensional (3D) volume imaging. Realizing that longer and frequent image acquisition results in significant photobleaching, we have taken limited angular views (18 views) of the macroscopic specimen and integrated with maximum likelihood (ML) technique for reconstructing high quality 3D volume images. Existing variants of light-sheet microscopy require both rotation and translation with a total of approximately 10-fold more views to render a 3D volume image. Comparatively, LV-LSM technique reduces data acquisition time and consequently minimizes light-exposure by many-folds. Since ML is a post-processing technique and highly parallelizable, this does not cost precious imaging time. Results show noise-free and high contrast volume images when compared to the state-of-the-art selective plane illumination microscopy.

Nanolithography using counter-propagating light-sheets

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Spatial filter based light-sheet laser interference technique for three-dimensional nanolithography

Applied Physics Letters, 2015

ABSTRACT We propose a laser interference technique for the fabrication of 3D nano-structures. Thi... more ABSTRACT We propose a laser interference technique for the fabrication of 3D nano-structures. This is possible with the introduction of specialized spatial filter in a 2π cylindrical lens system (consists of two opposing cylindrical lens sharing a common geometrical focus). The spatial filter at the back-aperture of a cylindrical lens gives rise to multiple light-sheet patterns. Two such interfering counter-propagating light-sheet pattern result in periodic 3D nano-pillar structure. This technique overcomes the existing slow point-by-point scanning, and has the ability to pattern selectively over a large volume. The proposed technique allows large-scale fabrication of periodic structures. Computational study shows a field-of-view (patterning volume) of approximately 12.2 mm3 with the pillar-size of 80 nm and inter-pillar separation of 180 nm. Applications are in nano-waveguides, 3D nano-electronics, photonic crystals, and optical microscopy.

$Research paper thumbnail of Efficient generation of diffraction-limited multi-sheet pattern for biological imaging$

Efficient generation of diffraction-limited multi-sheet pattern for biological imaging

Optics Letters, 2015

We demonstrate a new technique to generate multiple light-sheets for fluorescence microscopy. Thi... more We demonstrate a new technique to generate multiple light-sheets for fluorescence microscopy. This is possible by illuminating the cylindrical lens using multiple copies of Gaussian beams. A diffraction grating placed just before the cylindrical lens splits the incident Gaussian beam into multiple beams traveling at different angles. Subsequently, this gives rise to diffraction-limited light-sheets after the Gaussian beams pass through the combined cylindrical lens-objective sub-system. Direct measurement of field at and around the focus of objective lens shows multi-sheet pattern with an average thickness of 7.5 μm and inter-sheet separation of 380 μm. Employing an independent orthogonal detection sub-system, we successfully imaged fluorescently-coated yeast cells (≈4 μm) encaged in agarose gel-matrix. Such a diffraction-limited sheet-pattern equipped with dedicated detection system may find immediate applications in the field of optical microscopy and fluorescence imaging.

High resolution light-sheet based high-throughput imaging cytometry system enables visualization of intra-cellular organelles

AIP Advances, 2014

Visualization of intracellular organelles is achieved using a newly developed high throughput ima... more Visualization of intracellular organelles is achieved using a newly developed high throughput imaging cytometry system. This system interrogates the microfluidic channel using a sheet of light rather than the existing point-based scanning techniques. The advantages of the developed system are many, including, single-shot scanning of specimens flowing through the microfluidic channel at flow rate ranging from microto nano-lit./min. Moreover, this opens-up in-vivo imaging of sub-cellular structures and simultaneous cell counting in an imaging cytometry system. We recorded a maximum count of 2400 cells/min at a flow-rate of 700 nl/min, and simultaneous visualization of fluorescently-labeled mitochondrial network in HeLa cells during flow. The developed imaging cytometry system may find immediate application in biotechnology, fluorescence microscopy and nano-medicine.

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MRT Letter: Two-photon excitation-based 2pi Light-sheet system for nano-lithography

Microscopy Research and Technique, 2014

We propose two-photon excitation-based light-sheet technique for nano-lithography. The system con... more We propose two-photon excitation-based light-sheet technique for nano-lithography. The system consists of 2π-configured cylindrical lens system with a common geometrical focus. Upon superposition, the phase-matched counter-propagating light-sheets result in the generation of identical and equi spaced nano-bump pattern. Study shows a feature size of as small as few tens of nanometers with a inter-bump distance of few hundred nanometers. This technique overcomes some of the limitations of existing nano-lithography techniques, thereby, may pave the way for mass-production of nano-structures. Potential applications can also be found in optical microscopy, plasmonics, and nano-electronics.

Generation of multiple sheets of light using spatial-filtering technique

Optics Letters, 2014

We develop an optical system for generating multiple light sheets. This is enabled by employing a... more We develop an optical system for generating multiple light sheets. This is enabled by employing a special class of spatial filters in a cylindrical lens geometry. The proposed binary filter placed at the back aperture of the cylindrical lens results in the generation of a periodic transverse pattern extending along the z axis (i.e., multiple light sheets). Experimental results confirm the generation of multiple light sheets of thickness 6.6 μm with an intersheet spacing of 13.4 μm. The proposed imaging technique may facilitate three-dimensional imaging in nano-optics, fluorescence microscopy, and nanobiology.

Three Dimensional Fluorescence Imaging Using Multiple Light-Sheet Microscopy

PLoS ONE, 2014

We developed a multiple light-sheet microscopy (MLSM) system capable of 3D fluorescence imaging. ... more We developed a multiple light-sheet microscopy (MLSM) system capable of 3D fluorescence imaging. Employing spatial filter in the excitation arm of a SPIM system, we successfully generated multiple light-sheets. This improves upon the existing SPIM system and is capable of 3D volume imaging by simultaneously illuminating multiple planes in the sample. Theta detection geometry is employed for data acquisition from multiple specimen layers. This detection scheme inherits many advantages including, background reduction, cross-talk free fluorescence detection and high-resolution at long working distance. Using this technique, we generated 5 equi-intense light-sheets of thickness approximately 7:5 mm with an inter-sheet separation of 15 mm. Moreover, the light-sheets generated by MLSM is found to be 2 times thinner than the state-of-art SPIM system. Imaging of fluorescently coated yeast cells of size 4+1 mm (encaged in Agarose gel-matrix) is achieved. Proposed imaging technique may accelerate the field of fluorescence microscopy, cell biology and biophotonics.

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MRT letter: Experimental verification of vectorial theory to determine field at the geometrical focus of a cylindrical lens

Microscopy Research and Technique, 2014

We provide experimental evidence supporting the vectorial theory for determining electric field a... more We provide experimental evidence supporting the vectorial theory for determining electric field at and near the geometrical focus of a cylindrical lens. This theory provides precise distribution of field and its polarization effects. Experimental results show a close match (≈ 95% using χ(2)-test) with the simulation results (obtained using vectorial theory). Light-sheet generated both at low and high NA cylindrical lens shows the importance of vectorial theory for further development of light-sheet techniques. Potential applications are in planar imaging systems (such as, SPIM, IML-SPIM, imaging cytometry) and spectroscopy.

Light sheet based imaging flow cytometry on a microfluidic platform

Microscopy Research and Technique, 2013

We propose a light sheet based imaging flow cytometry technique for simultaneous counting and ima... more We propose a light sheet based imaging flow cytometry technique for simultaneous counting and imaging of cells on a microfluidic platform. Light sheet covers the entire microfluidic channel and thus omits the necessity of flow focusing and point scanning based technology. Another advantage lies in the orthogonal detection geometry that totally cuts-off the incident light, thereby substantially reducing the background in the detection. Compared to the existing state-of-art techniques the proposed technique shows marked improvement. Using fluorescently-coated Saccharomyces cerevisiae cells we have recorded cell counting with throughput as high as 2,090 cells/min in the low flow rate regime and were able to image the individual cells on-the-go. Overall, the proposed system is cost-effective and simple in channel geometry with the advantage of efficient counting in operational regime of low laminar flow. This technique may advance the emerging field of microfluidic based cytometry for applications in nanomedicine and point of care diagnostics.

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