arXiv subjects

Nikhil Jayakumar

Publications and source records attributed to Nikhil Jayakumar.

7 recordsLinked to original sources

arXiv2026

Piezoelectric MEMS Phase Modulator for Silicon Nitride Platform in the Visible Spectrum

Active photonic integrated circuits (PICs) in the visible spectrum are essential for on-chip applications, requiring low-loss waveguides with broad transparency and efficient, low-power phase modulation. Here, we demonstrate a compact, ultra-low-power phase modulator based on a silicon nitride (Si$_3$N$_4$) waveguide integrated with thin-film lead zirconate titanate (PZT) that actuates a bridge-type MEMS. The suspended actuator exploits PZT's strong piezoelectric effect to induce mechanically driven phase shifts, enabling efficient modulation in a Mach--Zehnder interferometer. For 3~mm and 5~mm modulators, phase shifts of $1.45\pi$ and $2.5\pi$ are achieved at 10~V, corresponding to a scalability metric ($V_{\pi}\cdot L$) of 2.25~V$\cdot$cm at 635~nm. This represents an order-of-magnitude improvement in scalability over stress-optic PZT modulators. The devices also exhibit ultralow power consumption ($\sim 12\,\mathrm{nW}$), $\sim 5\,\mathrm{ms}$ rise time, and optical loss $< 0.75\,\mathrm{dB/cm}$. Furthermore, we demonstrate on-chip beam shaping.

physics.optics↗

arXiv2023

Label-free incoherent super-resolution optical microscopy

The photo-kinetics of fluorescent molecules have enabled the circumvention of far-field optical diffraction-limit. Despite its enormous potential, the necessity to label the sample may adversely influence the delicate biology under investigation. Thus, continued development efforts are needed to surpass the far-field label-free diffraction barrier. The coherence of the detected light in label-free mode hinders the application of existing super-resolution methods based on incoherent fluorescence imaging. In this article, we present the physics and propose a methodology to circumvent this challenge by exploiting the photoluminescence of silicon nitride waveguides for near-field illumination of unlabeled samples. The technique is abbreviated EPSLON, Evanescently decaying Photoluminescence Scattering enables Label-free Optical Nanoscopy. We demonstrate that such an illumination has properties that mimic the photo-kinetics of nano-sized fluorescent molecules. This allows for developing a label-free incoherent system that is linear in intensity, and stable with time thereby permitting the application of techniques like structured illumination microscopy (SIM) and intensity-fluctuation-based optical nanoscopy (IFON) in label-free mode to circumvent the diffraction limit.

physics.optics↗

arXiv2021

Label-free chip-based evanescent light scattering super-resolution and superior-contrast optical microscopy (cELS)

Chip-based Evanescent Light Scattering (cELS) utilizes the multiple modes of a high-index contrast optical waveguide for near-field illumination of unlabeled samples, thereby repositioning the highest spatial frequencies of the sample into the far-field. The multiple modes scattering off the sample with different phase differences is engineered to have random spatial distributions within the integration time of the camera, mitigating the coherent speckle noise. This enables label-free superior-contrast imaging of weakly scattering nanosized specimens such as extra-cellular vesicles (EVs) and liposomes, dynamics of living HeLa cells etc. The article explains and validates experimentally the physics behind cELS by demonstrating a multi-moded straight waveguide as a partially coherent light source. For isotropic super-resolution, spatially incoherent light engineered via multiple-arms waveguide chip and intensity-fluctuation based algorithms are used. The proof-of-concept results are demonstrated on 100 nm polystyrene beads and resolution improvement of close to 2X is shown. cELS also realizes (2-10)X more contrast as opposed to conventional imaging techniques. In addition, cELS platform is miniaturized and enables large field-of-view imaging compared to state of the art label-free techniques. cELS holds a potential for label-free super-resolution imaging of nanosized biological specimens at high-throughput.

physics.optics↗

arXiv2021

Demystifying speckle field quantitative phase microscopy

Quantitative phase microscopy (QPM) has found significant applications in the field of biomedical imaging which works on the principle of interferometry. The theory behind achieving interference in QPM with conventional light sources such as white light and lasers is very well developed. Recently, the use of dynamic speckle illumination (DSI) in QPM has attracted attention due to its advantages over conventional light sources such as high spatial phase sensitivity, single shot, scalable field of view (FOV) and resolution. However, the understanding behind obtaining interference fringes in QPM with DSI has not been convincingly covered previously. This imposes a constraint on obtaining interference fringes in QPM using DSI and limits its widespread penetration in the field of biomedical imaging. The present article provides the basic understanding of DSI through both simulation and experiments that is essential to build interference optical microscopy systems such as QPM, digital holographic microscopy and optical coherence tomography. Using the developed theory of DSI we demonstrate its capabilities of using non-identical objective lenses in both arms of the interference microscopy without degrading the interference fringe contrast and providing the flexibility to use user-defined microscope objective lens. It is also demonstrated that the interference fringes are not washed out over a large range of optical path difference (OPD) between the object and the reference arm providing competitive edge over low temporal coherence light sources. The theory and explanation developed here would enable wider penetration of DSI based QPM for applications in biology and material sciences.

physics.optics↗

arXiv2020

High throughput spatially sensitive single-shot quantitative phase microscopy

High space-bandwidth product with high spatial phase sensitivity is indispensable for a single-shot quantitative phase microscopy (QPM) system. It opens avenue for widespread applications of QPM in the field of biomedical imaging. Temporally low coherence length light sources are generally implemented to achieve high spatial phase sensitivity in QPM at the cost of either reduced temporal resolution or smaller field of view (FOV). On the contrary, high temporal coherence light sources like lasers are capable of exploiting the full FOV of the QPM systems at the expense of less spatial phase sensitivity. In the present work, we employed pseudo-thermal light source (PTLS) in QPM which overcomes the limitations of conventional light sources. The capabilities of PTLS over conventional light sources are systematically studied and demonstrated on various test objects like USAF resolution chart and thin optical waveguide (height ~ 8 nm). The spatial phase sensitivity of QPM in case of PTLS is measured to be equivalent to that for white light source. The high-speed and large FOV capabilities of PTLS based QPM is demonstrated by high-speed imaging of live sperm cells that is limited by the camera speed and by imaging extra-ordinary large FOV phase imaging on histopathology placenta tissue samples.

physics.optics↗

arXiv2020

On-chip TIRF nanoscopy by applying Haar wavelet kernel analysis on intensity fluctuations induced by chip illumination

Photonic-chip based TIRF illumination has been used to demonstrate several on-chip optical nanoscopy methods. The sample is illuminated by the evanescent field generated by the electromagnetic wave modes guided inside the optical waveguide. In addition to the photokinetics of the fluorophores, the waveguide modes can be further exploited for introducing controlled intensity fluctuations for exploitation by techniques such as super-resolution optical fluctuation imaging (SOFI). However, the problem of non-uniform illumination pattern generated by the modes contribute to artifacts in the reconstructed image. To alleviate this problem, we propose to perform Haar wavelet kernel (HAWK) analysis on the original image stack prior to the application of (SOFI). HAWK produces a computational image stack with higher spatio-temporal sparsity than the original stack. In the case of multimoded non-uniform illumination patterns, HAWK processing bre aks the mode pattern while introducing spatio-temporal sparsity, thereby differentially affecting the non-uniformity of the illumination. Consequently, this assists nanoscopy methods such as SOFI to better support super-resolution, which is otherwise compromised due to spatial correlation of the mode patterns in the raw image. Furthermore, applying HAWK prior to SOFI alleviates the problem of artifacts due to non-uniform illumination without degrading temporal resolution. Our experimental results demonstrate resolution enhancement as well as reduction in artifacts through the combination of HAWK and SOFI.

physics.optics↗

arXiv2020

A transparent waveguide chip for versatile TIRF-based microscopy and nanoscopy

Total internal reflection fluorescence microscopy (TIRF) has enabled low-background, live-cell friendly imaging of cell surfaces and other thin samples thanks to the shallow penetration of the evanescent light field into the sample. The implementation of TIRF on optical waveguide chips (c-TIRF) has overcome historical limitations on the magnification and field of view (FOV) compared to lens-based TIRF, and further allows the light to be guided in complicated patterns that can be used for advanced imaging techniques or selective stimulation of the sample. However, the opacity of the chips themselves has thus far precluded their use on inverted microscopes and complicated sample preparation and handling. In this work, we introduce a new platform for c-TIRF imaging based on a transparent substrate, which is fully compatible with sample handling and imaging procedures commonly used with a standard #1.5 glass coverslip, and is fabricated using standard complementary metal-oxide-semiconductor (CMOS) techniques, which can easily be scaled up for mass production. We demonstrate its performance on synthetic and biological samples using both upright and inverted microscopes, and show how it can be extended to super-resolution applications, achieving a resolution of 116 nm using super resolution radial fluctuations (SRRF). These new chips retain the scalable FOV of opaque chip-based TIRF and the high axial resolution of TIRF, and have the versatility to be used with many different objective lenses, microscopy methods, and handling techniques. We thus see c-TIRF as a technology primed for widespread adoption, increasing both TIRF's accessibility to users and the range of applications that can benefit from it.

physics.app-ph↗