Allen Yang

Optical manipulation of nanoparticles and biomolecules in sub-wavelength slot waveguides

The ability to manipulate nanoscopic matter precisely is critical for the development of active nanosystems. Optical tweezers are excellent tools for transporting particles ranging in size from several micrometres to a few hundred nanometres. Manipulation of dielectric objects with much smaller diameters, however, requires stronger optical confinement and higher intensities than can be provided by these diffraction-limited systems. Here we present an approach to optofluidic transport that overcomes these limitations, using sub-wavelength liquid-core slot waveguides. The technique simultaneously makes use of near-field optical forces to confine matter inside the waveguide and scattering/adsorption forces to transport it. The ability of the slot waveguide to condense the accessible electromagnetic energy to scales as small as 60 nm allows us also to overcome the fundamental diffraction problem. We apply the approach here to the trapping and transport of 75-nm dielectric nanoparticles and λ-DNA molecules. Because trapping occurs along a line, rather than at a point as with traditional point traps, the method provides the ability to handle extended biomolecules directly. We also carry out a detailed numerical analysis that relates the near-field optical forces to release kinetics. We believe that the architecture demonstrated here will help to bridge the gap between optical manipulation and nanofluidics.

Optofluidic trapping and transport on solid core waveguides within a microfluidic device

In this work we demonstrate an integrated microfluidic/photonic architecture for performing dynamic optofluidic trapping and transport of particles in the evanescent field of solid core waveguides. Our architecture consists of SU-8 polymer waveguides combined with soft lithography defined poly(dimethylsiloxane) (PDMS) microfluidic channels. The forces exerted by the evanescent field result in both the attraction of particles to the waveguide surface and propulsion in the direction of optical propagation both perpendicular and opposite to the direction of pressure-driven flow. Velocities as high as 28 mum/s were achieved for 3 mum diameter polystyrene spheres with an estimated 53.5 mW of guided optical power at the trapping location. The particle-size dependence of the optical forces in such devices is also characterized.

Optofluidic ring resonator switch for optical particle transport

In this work, we demonstrate an optofluidic switch using a microring resonator architecture to direct particles trapped in the evanescent field of a solid-core waveguide. When excited at the resonant wavelength, light inserted into the bus waveguide becomes amplified within the ring structure. The resulting high optical intensities in the evanescent field of the ring generate a gradient force that diverts particles trapped on the bus to the ring portion of the device. We show that this increase in optical energy translates to an increase of 250% in the radiation pressure induced steady-state velocity of particles trapped on the ring. We also characterize the switching fraction of the device, showing that 80% of particles are diverted onto the ring when the device is at an on-resonance state. The optofluidic switch we present here demonstrates the versatility in exploiting planar optical devices for integrated particle manipulation applications.

Forces and Transport Velocities for a Particle in a Slot Waveguide

Optofluidic transport seeks to exploit the high-intensity electromagnetic energy in waveguiding structures to manipulate nanoscopic matter using radiation pressure and optical trapping forces. In this paper, we present an analysis of optical trapping and transport of sub-100 nm polystyrene and gold nanoparticles in silicon slot waveguides. This study focuses on the effect of particle size, particle refractive index, and slot waveguide geometry on trapping stability and the resulting transport speed. Our results indicate that stable trapping and transport can be achieved for objects as small as 10 or 20 nm in diameter with as much as a 100 fold enhancement in trapping stiffness over the state of the art.

Stability analysis of optofluidic transport on solid-core waveguiding structures

Optofluidic transport involves the use of electromagnetic energy to transport nanoparticles through the exploitation of scattering, adsorption and gradient (polarization) based forces. This paper presents a new approach to stability analysis for a system of broad applicability to such transport, namely the optical trapping of dielectric particles in the evanescent field of low index (polymer) and high index (silicon) solid-core waveguide structures integrated with microfluidics. Three-dimensional finite element based simulations are used to determine the electromagnetic and hydrodynamic field variables for the system of interest. The net force acting on particles is determined through evaluation of the full Maxwell and flow shear stress tensors, and a trapping stability number is obtained by comparing the work required to remove a particle from the waveguide with available random thermal energy. These forces are correlated to controllable experimental parameters such as particle size, fluid velocity, and channel height, and a series of trapping stability diagrams is produced which detail the conditions under which optofluidic transport is possible.

Gel-based optical waveguides with live cell encapsulation and integrated microfluidics.

In this Letter, we demonstrate a biocompatible microscale optical device fabricated from agarose hydrogel that allows for encapsulation of cells inside an optical waveguide. This allows for better interaction between the light in the waveguide and biology, since it can interact with the direct optical mode rather than the evanescent field. We characterize the optical properties of the waveguide and further incorporate a microfluidic channel over the optical structure, thus developing an integrated optofluidic system fabricated entirely from agarose gel.

Optical trapping platform based on highly confining silicon waveguiding structures with microfluidics

We demonstrate strongly enhanced optical trapping forces on sub-micron-diameter dielectric spheres within a pressure-driven microfluidic flow of several hundred mum/s using the evanescent field of the light in silicon waveguides.

Super-Gaussian pumping profiles for solid state lasers

For a diode end-pumped solid state laser, the pump beam is described as a Gaussian profile initially and then as a flat-top profile. In fact, the fiber-coupled pump beam is more accurately fit by a super-Gaussian beam. With the same beam waist, different beam profiles can be obtained with different m. It is a Gaussian profile if m = 1; and it is a flat-top profile if m is infinite. The super-Gaussian beam should be normalized before it is used to characterize the thermal and optical properties of a solid state laser. With this normalized super-Gaussian pump beam, the corresponding analysis on thermal temperature, thermal radial and tangential stresses, refractive index changes, OPD, birefringence, depolarization loss, and rate equations are modeled more accurately. At the meantime, as the progress of diode laser technology, powerful and narrow line width diodes are becoming available. It is possible to directly pump nonlinear crystals such as KTP, LBO for second harmonic generation using diode laser. Nonlinear conversion efficiency is discussed. Thermal properties for a super-Gaussian beam (with different m) on nonlinear crystal are analyzed using finite element analysis.

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

Optofluidics: Fluidics Enabling Optics and Optics Enabling Fluidics

Optical devices which incorporate liquids as a fundamental part of the structure can be traced at least as far back as the 18th century where rotating pools of mercury were proposed as a simple technique to create smooth mirrors for use in reflecting telescopes. Modern microfluidic and nanofluidics has enabled the development of a present day equivalent of such devices centered on the marriage of fluidics and optics which we refer to as “Optofluidics.” In this review paper we will present an overview of our approach to the development of three different optofluidic devices. In the first of these we will demonstrate how the fusion of novel nanophotonic structures with micro- and nanofluidic networks can be used to perform ultrasensitive, label free biomolecular analysis. This will be done in the context of our newly developed devices for screening of Dengue and Influenza virus RNA. For the second class of device I will discuss and demonstrate how optical forces (scattering, adsorption and polarization) in solid and liquid core nanophotonic structures can be used to drive novel microfluidic processes. Some of the advanced analytical, numerical and experimental techniques used to investigate and design these systems will be discussed as well as issues relating to integration and their fabrication.Copyright