Parama Pal

Diffractive Optical Analysis for Refractive Index Sensing using Transparent Phase Gratings.

We report the implementation of a micro-patterned, glass-based photonic sensing element that is capable of label-free biosensing. The diffractive optical analyzer is based on the differential response of diffracted orders to bulk as well as surface refractive index changes. The differential read-out suppresses signal drifts and enables time-resolved determination of refractive index changes in the sample cell. A remarkable feature of this device is that under appropriate conditions, the measurement sensitivity of the sensor can be enhanced by more than two orders of magnitude due to interference between multiply reflected diffracted orders. A noise-equivalent limit of detection (LoD) of 6 × 10−7 was achieved with this technique with scope for further improvement.

Imaging Flow Cytometry With Femtosecond Laser-Micromachined Glass Microfluidic Channels

Microfluidic/optofluidic microscopy is a versatile modality for imaging and analyzing properties of cells/particles while they are in flow. In this paper, we demonstrate the integration of fused silica microfluidics fabricated using femtosecond laser machining into optofluidic imaging systems. By using glass for the sample stage of our microscope, we have exploited its superior optical quality for imaging and bio-compatibility. By integrating these glass microfluidic devices into a custom-built bright field microscope, we have been able to image red blood cells in flow with high-throughputs and good fidelity. In addition, we also demonstrate imaging as well as detection of fluorescent beads with these microfluidic devices.

Slanted channel microfluidic chip for 3D fluorescence imaging of cells in flow

Three-dimensional cellular imaging techniques have become indispensable tools in biological research and medical diagnostics. Conventional 3D imaging approaches employ focal stack collection to image different planes of the cell. In this work, we present the design and fabrication of a slanted channel microfluidic chip for 3D fluorescence imaging of cells in flow. The approach employs slanted microfluidic channels fabricated in glass using ultrafast laser inscription. The slanted nature of the microfluidic channels ensures that samples come into and go out of focus, as they pass through the microscope imaging field of view. This novel approach enables the collection of focal stacks in a straight-forward and automated manner, even with off-the-shelf microscopes that are not equipped with any motorized translation/rotation sample stages. The presented approach not only simplifies conventional focal stack collection, but also enhances the capabilities of a regular widefield fluorescence microscope to match the features of a sophisticated confocal microscope. We demonstrate the retrieval of sectioned slices of microspheres and cells, with the use of computational algorithms to enhance the signal-to-noise ratio (SNR) in the collected raw images. The retrieved sectioned images have been used to visualize fluorescent microspheres and bovine sperm cell nucleus in 3D while using a regular widefield fluorescence microscope. We have been able to achieve sectioning of approximately 200 slices per cell, which corresponds to a spatial translation of ∼ 15 nm per slice along the optical axis of the microscope.

Quality monitoring of diesel exhaust fluid in vehicles using diffractive interference sensors

In this paper, we describe a simple design for an optical sensing device based on differential interferometry that can be deployed as an in-line sensor for monitoring the quality of diesel exhaust fluid inside engines of diesel vehicles. This sensor can precisely determine (to within ±1% and lower) the percentage of urea in diesel exhaust fluid (DEF), which is a critical reactant in the selective catalytic reduction (SCR) process for reducing harmful nitrous oxide emissions from diesel vehicles into the environment. The operating principle is based on diffraction of laser light from a regularly spaced microarray. Preliminary performance results indicate that our sensor can precisely determine the concentration of urea in the DEF to within ±0.0045%.

Diffractive interference optical analyzer (DiOPTER)

This report demonstrates a method for high-resolution refractometric measurements using, what we have termed as, a Diffractive Interference Optical Analyzer (DiOpter). The setup consists of a laser, polarizer, a transparent diffraction grating and Si-photodetectors. The sensor is based on the differential response of diffracted orders to bulk refractive index changes. In these setups, the differential read-out of the diffracted orders suppresses signal drifts and enables time-resolved determination of refractive index changes in the sample cell. A remarkable feature of this device is that under appropriate conditions, the measurement sensitivity of the sensor can be enhanced by more than two orders of magnitude due to interference between multiply reflected diffracted orders. A noise-equivalent limit of detection (LoD) of 6x10-7 RIU was achieved in glass. This work focuses on devices with integrated sample well, made on low-cost PDMS. As the detection methodology is experimentally straightforward, it can be used across a wide array of applications, ranging from detecting changes in surface adsorbates via binding reactions to estimating refractive index (and hence concentration) variations in bulk samples. An exciting prospect of this technique is the potential integration of this device to smartphones using a simple interface based on transmission mode configuration. In a transmission configuration, we were able to achieve an LoD of 4x10-4 RIU which is sufficient to explore several applications in food quality testing and related fields. We are envisioning the future of this platform as a personal handheld optical analyzer for applications ranging from environmental sensing to healthcare and quality testing of food products.

Optofluidic microscopy using femtosecond micromachined glass microfluidics

In this paper, we demonstrate the applicability of microfluidic devices fabricated using femtosecond laser machining in fused silica in optofluidic imaging systems. Using a custom-built bright field microscope, we have analyzed red blood cells flow through these microfluidic devices.

Nanomaterial-coated etched fiber Bragg grating sensors

Multi-parameter sensing in the form of sensor arrays functionalized with multiple receptors, is an approach for attaining selectivity in sensing. We have demonstrated a novel fiber sensor based on an etched Bragg grating whose core is coated with materials such as polyelectrolytes, carbon nanotubes, and polyallylamine-amino-carbon nanotubes, and can be used for detecting gases, pH, humidity, refractive index, proteins and other biomolecules. In this approach, the target molecules interact with the functionalized core of the etched FBG resulting in a change in the effective refractive index of the fiber core leading to a subsequent shift in the Bragg wavelength. The experimental data shows that the wavelength shift varies linearly with the concentration of the target analyte. Besides being reproducible and repeatable, the technique is fast, compact, and highly sensitive.