Sergei Kühn

Loss-based optical trap for on-chip particle analysis.

Optical traps have become widespread tools for studying biological objects on the micro and nanoscale. However, conventional laser tweezers and traps rely on bulk optics and are not compatible with current trends in optofluidic miniaturization. Here, we report a new type of particle trap that relies on propagation loss in confined modes in liquid-core optical waveguides to trap particles. Using silica beads and E. coli bacteria, we demonstrate unique key capabilities of this trap. These include single particle trapping with micron-scale accuracy at arbitrary positions over waveguide lengths of several millimeters, definition of multiple independent particle traps in a single waveguide, and combination of optical trapping with single particle fluorescence analysis. The exclusive use of a two-dimensional network of planar waveguides strongly reduces experimental complexity and defines a new paradigm for on-chip particle control and analysis.

Ultrasensitive Qβ phage analysis using fluorescence correlation spectroscopy on an optofluidic chip

We demonstrate detection and analysis of the Qbeta bacteriophage on the single virus level using an integrated optofluidic biosensor. Individual Qbeta phages with masses on the order of attograms were sensed and analyzed on a silicon chip in their natural liquid environment without the need for virus immobilization. The diffusion coefficient of the viruses was extracted from the fluorescence signal by means of fluorescence correlation spectroscopy (FCS) and found to be 15.90+/-1.50 microm(2)/s in excellent agreement with previously published values. The aggregation and disintegration of the phage were also observed. Virus flow velocities determined by FCS were in the 60-300 microm/s range. This study suggests considerable potential for an inexpensive and portable sensor capable of discrimination between viruses of different sizes.

Hollow-core waveguide characterization by optically induced particle transport

We introduce a method for optical characterization of hollow-core optical waveguides. Radiation pressure exerted by the waveguide modes on dielectric microspheres is used to analyze salient properties such as propagation loss and waveguide mode profiles. These quantities were measured for quasi-single-mode and multimode propagation in on-chip liquid-filled hollow-core antiresonant reflecting optical waveguides. Excellent agreement with analytical and numerical models is found, demonstrating that optically induced particle transport provides a simple, inexpensive, and nondestructive alternative to other characterization methods.

Improving solid to hollow core transmission for integrated ARROW waveguides

Optical sensing platforms based on anti-resonant reflecting optical waveguides (ARROWs) with hollow cores have been used for bioanalysis and atomic spectroscopy. These integrated platforms require that hollow waveguides interface with standard solid waveguides on the substrate to couple light into and out of test media. Previous designs required light at these interfaces to pass through the anti-resonant layers.We present a new ARROW design which coats the top and sides of the hollow core with only SiO2, allowing for high interface transmission between solid and hollow waveguides. The improvement in interface transmission with this design is demonstrated experimentally and increases from 35% to 79%. Given these parameters, higher optical throughputs are possible using single SiO2 coatings when hollow waveguides are shorter than 5.8 mm.

Multi-mode mitigation in an optofluidic chip for particle manipulation and sensing

A new waveguide design for an optofluidic chip is presented. It mitigates multi-mode behavior in solid and liquid-core waveguides by increasing fundamental mode coupling to 82% and 95%, respectively. Additionally, we demonstrate a six-fold improvement in lateral confinement of optically guided dielectric microparticles and double the detection efficiency of fluorescent particles.

Particle manipulation with integrated optofluidic traps

On an integrated optofluidic waveguide platform we implement two complementary types of traps, in a transverse and a novel axial geometry. The traps are characterized and used to manipulate and study biomaterial.

New chip miniaturizes particle manipulation, detection

The laser-based optical trap has been instrumental in a broad range of investigations of small particles in liquids, gases, and vacuum.1, 2 The field has enabled, for example, manipulation of bacteria and viruses, cooling and trapping of atoms, and precise measurements of bioparticle properties.2 However, optical trapping instruments have traditionally been bulky, involving many optical elements (e.g., microscopes). Recently, waveguidebased, on-chip optical manipulation has emerged as an inexpensive and compact alternative.3–10 We have conducted unique optical manipulation, trapping, and sensing applications using an optofluidic chip based on liquid-core antiresonant-reflecting optical waveguide (ARROW) technology.10 The low refractive index core is surrounded by specific highindex dielectric layers that enable light and liquid to be confined in the same volume: see Figure 1(a). This key feature allows fluorescence detection with single-molecule-level sensitivity.10 We developed an integrated optofluidic platform to interface liquidand solid-core ARROWs (see Figure 1) to detect single bioparticles on a chip.10 In addition, due to the small waveguide cross sections (typically 12 5 m2), the quasi-single-mode optical fields have high intensities and can manipulate small particles (100–2000nm in diameter). The response of a particle under irradiance of a given color is determined not only by its physical properties but also the beam’s optical power and spatial area. We can exploit this sensitive particle behavior to extract information about an optical system. For example, a particle’s optically induced trajectory within a liquid-core ARROW can be used to characterize the waveguide loss (the loss of light from the core).5 The main advantages of this technique are its simplicity and nondestructive nature. Introducing a counter-propagating beam (see Figure 1, trapping beam) makes it possible to optically trap particles by Figure 1. Scanning electron microscope images of (a) hollowand (b) solid-core waveguides with a guided-light output overlaid (bar 12 m). (c) An optofluidic platform with integrated hollowand solid-core waveguides.

Fluorescence correlation spectroscopy of single molecules on an optofluidic chip

We review our recent progress in bringing fluorescent correlation spectroscopy (FCS) of single molecules on a silicon optofluidic platform. Starting from basic concepts and applications of FCS we move to a description of our integrated optofluidic device, briefly outlining the physics behind its function and relevant geometrical characteristics. We then derive an FCS theoretical model for our sensor geometry, which we subsequently apply to the examination of molecular properties of single fluorophores and bioparticles. The model allows us to extract the diffusion coefficient, translational velocity and local concentration of particles in question. We conclude with future directions of this research.

Single virus detection using integrated optofluidics

Detection and analysis of single enterobacteria phage Qbeta nucleocapsids on an integrated optofluidic chip are presented. Diffusion coefficient and flow velocity of the capsids were measured to be 16.4plusmn0.5 mum2/s and 60-225 mum/s respectively.

Planar optofluidics for single molecule analysis

The recent development of liquid-core antiresonant reflecting optical (ARROW) waveguides for integrated optofluidic devices is reviewed. Advanced fluorescence detection techniques and single bioparticle detection on a chip are presented, and an outlook for planar integrated optofludics for novel biomedical instrumentation is given.