Sudeep Mandal

Nanoscale optofluidic sensor arrays

In this paper we introduce Nanoscale Optofluidic Sensor Arrays (NOSAs), which are an optofluidic architecture for performing highly parallel, label free detection of biomolecular interactions in aqueous environments. The architecture is based on the use of arrays of 1D photonic crystal resonators which are evanescently coupled to a single bus waveguide. Each resonator has a slightly different cavity spacing and is shown to independently shift its resonant peak in response to changes in refractive index in the region surrounding its cavity. We demonstrate through numerical simulation that by confining biomolecular binding to this region, limits of detection on the order of tens of attograms (ag) are possible. Experimental results demonstrate a refractive index (RI) detection limit of 7 x 10(-5) for this device. While other techniques such as SPR possess a equivalent RI detection limit, the advantage of this architecture lies in its potential for low mass limit of detection which is enabled by confining the size of the probed surface area.

Nanomanipulation Using Silicon Photonic Crystal Resonators

Optical tweezers have enabled a number of microscale processes such as single cell handling, flow-cytometry, directed assembly, and optical chromatography. To extend this functionality to the nanoscale, a number of near-field approaches have been developed that yield much higher optical forces by confining light to subwavelength volumes. At present, these techniques are limited in both the complexity and precision with which handling can be performed. Here, we present a new class of nanoscale optical trap exploiting optical resonance in one-dimensional silicon photonic crystals. The trapping of 48 nm and 62 nm dielectric nanoparticles is demonstrated along with the ability to transport, trap, and manipulate larger nanoparticles by simultaneously exploiting the propagating nature of the light in a coupling waveguide and its stationary nature within the resonator. Field amplification within the resonator is shown to produce a trap several orders of magnitude stronger than conventional tweezers and an order of magnitude stiffer than other near-field techniques. Our approach lays the groundwork for a new class of optical trapping platforms that could eventually enable complex all-optical single molecule manipulation and directed assembly of nanoscale material.

Optofluidic transport in liquid core waveguiding structures

Here the authors introduce a method to achieve optofluidically based particle transport using liquid core waveguiding structures. Optically driven transport of 3μm polystyrene particles through a liquid core photonic crystal fiber is demonstrated and the resulting velocity distribution is characterized. The authors also show that dielectric particles can form highly concentrated bands within the liquid core with negligible transport based dispersion. They anticipate that this approach could lay the groundwork for an innovative class of optically driven particle concentration and separation devices.

Comparison of silicon photonic crystal resonator designs for optical trapping of nanomaterials

The use of silicon photonic devices for optical manipulation has recently enabled the direct handling of objects like nucleic acids and nanoparticles that are much smaller than could previously be trapped using traditional laser tweezers. The ability to manipulate even smaller matter however requires the development of photonic structures with even stronger trapping potentials. In this work we investigate theoretically several photonic crystal resonator designs and characterize the achievable trapping stiffness and trapping potential depth (sometimes referred to as the trapping stability). Two effects are shown to increase these trapping parameters: field enhancement in the resonator and strong field containment. We find trapping stiffness as high as 22.3 pN nm(-1) for 100 nm polystyrene beads as well as potential depth of 51,000 k(B)T at T = 300 K, for one Watt of power input to the bus waveguide. Under the same conditions for 70 nm polystyrene beads, we find a stiffness of 69 pN nm(-1) and a potential depth of 177,000 k(B)T. Our calculations suggest that with input power of 10 mW we could trap particles as small as 7.7 nm diameter with a trapping depth of 500 k(B)T. We expect these traps to eventually enable the manipulation of small matter such as single proteins, carbon nanotubes and metallic nanoparticles.

Biopatterning for label-free detection

We present a biopatterning technique suitable for applications which demand a high degree of surface cleanliness, such as immobilization of biological recognition elements onto label-free biosensors. In the case of label-free biosensing, the mechanism of signal transduction is based on surface bound matter, making them highly sensitive to surface contamination including residues left during the biopatterning process. In this communication we introduce a simple, rapid processing step that removes 98% of the residues that often remain after standard parylene lift-off patterning. Residue-free parylene biopatterning is combined with microfluidics to localize biomolecule immobilization onto the sensing region and to enable multiplexed biopatterning. We demonstrate the applicability of this method to multiplexed label-free detection platforms by patterning nucleic acid capture probes corresponding to the four different serotypes of Dengue virus onto parallel 1D photonic crystal resonator sensors. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) are used to quantify surface cleanliness and uniformity. In addition to label-free biosensors, this technique is well suited to other nanobiotechnology patterning applications which demand a pristine, residue-free surface, such as immobilization of enzymes, antibodies, growth factors, or cell cultures.

Optically Resonant Nanophotonic Devices for Label-Free Biomolecular Detection

Optical devices, such as surface plasmon resonance chips and waveguide-based Mach–Zehnder interferometers, have long been successfully used as label-free biomolecular sensors. Recently, however, there has been increased interest in developing new approaches to biomolecular detection that can improve on the limit of detection, specificity, and multiplexibility of these early devices and address emerging challenges in pathogen detection, disease diagnosis, and drug discovery. As we describe in this chapter, planar optically resonant nanophotonic devices (such as ring resonators, whispering gallery modes, and photonic crystal cavities) are one method that shows promise in significantly advancing the technology. Here we first provide a short review of these devices focusing on a handful of approaches illustrative of the state of the art. We then frame the major challenge to improving the technology as being the ability to provide simultaneously spatial localization of the electromagnetic energy and biomolecular binding events. We then introduce our “Nanoscale Optofluidic Sensor Arrays” which represents our approach to addressing this challenge. It is demonstrated how these devices serve to enable multiplexed detection while localizing the electromagnetic energy to a volume as small as a cubic wavelength. Challenges involved in the targeted immobilization of biomolecules over such a small area are discussed and our solutions presented. In general, we have tried to write this chapter with the novice in mind, providing details on the fabrication and immobilization methods that we have used and how one might adapt our approach to their designs.

Optofluidic Transport: Optical Waveguides as Microfluidic “Train Tracks”

In this work we demonstrate optofluidic [1] transport and trapping of dielectric particles using the intense electromagnetic energy in the evanescent field of optically excited microphotonic waveguides. A conceptual overview of the transport mechanism is presented along with the detailed optofluidic theory which describes the transport. Experimental results for straight and curved waveguides are presented along with details of our experimental technique. The final section details our recent work on developing a stability condition for this form of transport.© 2007 ASME

Nanomanipulation using silicon nitride photonic crystal resonators

Silicon nitride photonic crystal resonators are designed for manipulating nanomaterials in water using 1064-nm laser. The material of the resonator and the operating wavelength were chosen to minimize thermal heating in the cavity.

Design and experimental demonstration of optical resonators for nanotweezing

Resonant silicon photonics has recently enabled the direct optical tweezing of nano-objects on chip. Here we present a comprehensive evaluation of different resonator designs and demonstrate one with a stiffness of 22.3 pN nm-1 W-1.

Nanoscale Optofluidic Sensor Arrays for Dengue virus detection

Here we present nanoscale optofluidic sensor arrays (NOSAs) for Dengue virus detection, which is an optofluidic architecture for performing label free, highly parallel, detections of biomolecular interactions in aqueous environments with potential for a very low mass limit of detection.