Balpreet Singh Ahluwalia

Surface transport and stable trapping of particles and cells by an optical waveguide loop

Waveguide trapping has emerged as a useful technique for parallel and planar transport of particles and biological cells and can be integrated with lab-on-a-chip applications. However, particles trapped on waveguides are continuously propelled forward along the surface of the waveguide. This limits the practical usability of the waveguide trapping technique with other functions (e.g. analysis, imaging) that require particles to be stationary during diagnosis. In this paper, an optical waveguide loop with an intentional gap at the centre is proposed to hold propelled particles and cells. The waveguide acts as a conveyor belt to transport and deliver the particles/cells towards the gap. At the gap, the diverging light fields hold the particles at a fixed position. The proposed waveguide design is numerically studied and experimentally implemented. The optical forces on the particle at the gap are calculated using the finite element method. Experimentally, the method is used to transport and trap micro-particles and red blood cells at the gap with varying separations. The waveguides are only 180 nm thick and thus could be integrated with other functions on the chip, e.g. microfluidics or optical detection, to make an on-chip system for single cell analysis and to study the interaction between cells.

Optical trapping and propulsion of red blood cells on waveguide surfaces

We have studied optical trapping and propulsion of red blood cells in the evanescent field of optical waveguides. Cell propulsion is found to be highly dependent on the biological medium and serum proteins the cells are submerged in. Waveguides made of tantalum pentoxide are shown to be efficient for cell propulsion. An optical propulsion velocity of up to 
6 µm/s on a waveguide with a width of ~6 µm is reported. Stable optical trapping and propulsion of cells during transverse flow is also reported.

Design and fabrication of a double-axicon for generation of tailorable self-imaged three-dimensional intensity voids

We propose a new design for fabrication of a highly power-efficient double axicon to generate self-imaged three-dimensional intensity voids along the propagation of a beam. The conventional conical structure of an axicon is modified and shaped like an axiconlike structure with a double-gradient surface profile. The gradient conical surfaces generate Bessel beams with varying radial wave vectors that are superimposed and interfere to generate a sequence of three-dimensional intensity voids. The proposed element was fabricated using electron-beam lithography, and experimental verification of the design is reported.

Fabrication of Submicrometer High Refractive Index Tantalum Pentoxide Waveguides for Optical Propulsion of Microparticles

Design, fabrication, and optimization of tantalum pentoxide (Ta2O5 ) waveguides to obtain low-loss guidance at a wavelength of 1070 nm are reported. The high-refractive index contrast (Deltan ~ 0.65, compared to silicon oxide) of Ta2O5 allows strong confinement of light in waveguides of submicrometer thickness (200 nm), with enhanced intensity in the evanescent field. We have employed the strong evanescent field from the waveguide to propel micro-particles with higher velocity than previously reported. An optical propelling velocity of 50 mum/s was obtained for 8-mum polystyrene particles with guided power of only 20 mW.

Fabrication of efficient microaxicon by direct electron-beam lithography for long nondiffracting distance of Bessel beams for optical manipulation

We demonstrate a fabrication technique in the realization of microaxicon by single-step processing via electron-beam lithography. Microaxicon is used for the generation of propagation-invariant Bessel beams which find tremendous applications in optical trapping. The proposed technique is a simple, reliable, and reproducible method for the production of high-quality Bessel beams with long propagation-invariant distances, in our configuration, in excess of 20cm. Such Bessel beams with long nondiffracting distances are essential for optical tweezers systems in many cases.

The generation of an array of nondiffracting beams by a single composite computer generated hologram

Nondiffracting beams have generated great interest recently owing to their potential applications and unique properties. In this paper, we propose and validate a simple and effective method to generate arrays of various nondiffracting modes using a composite computer-generated hologram, which comprises N × N holograms each generating an individual nondiffracting beam. Employing the composite hologram approach, we can modulate individual nondiffracting beam in an array by varying the location, dimension, and phase information coded on each hologram. We experimentally generated regular arrays of Bessel beams, customized-shaped arrays of Bessel beams, and arrays of self-imaged optical bottle beams. The interference among the beams in the arrays was found to be weak within the nondiffracting distance.

Optical steering of high and low index microparticles by manipulating an off-axis optical vortex

We demonstrate the use of a rotating off-axis optical phase singularity, generated through an intentional misalignment of a high optical efficiency spiral phase plate (SPP), to optically steer both high and low index microparticles trapped within the optical beam in a controlled manner. This intentional misalignment of the SPP creates an asymmetrical intensity beam pattern due to the optical vortex being displaced from the centre of the beam propagation axis. By using this optical beam pattern, we propose that a cell can be trapped by an optical beam of matching beam diameter while its internal structure can be manipulated by the off-axis optical vortex.

High-power efficient multiple optical vortices in a single beam generated by a kinoform-type spiral phase plate

We propose using a solitary kinoform-type spiral phase plate structure to generate an array of vortices located in a single beam. Kinoform-type spiral surfaces allow each wavelength component of the phase modulation value to be wrapped back to its 2 pi equivalent for optical vortices of high charge. This allows the surface-relief profiles of high-charge vortices to be microfabricated with the same physical height as spiral phase plates of unity-charged optical vortices. The m-charged optical vortex obtained interacts with the inherent coherent background, which changes the propagation dynamics of the optical vortex and splits the initial m charge into /m/ unity-charged optical vortices within the same beam. Compared to a hologram, a multistart spiral phase plate is more efficient in the use of available spatial frequencies and beam energy and also is computationally less demanding. Furthermore, using microfabrication techniques will allow for greater achievable tolerances in terms of smaller feature sizes.

Micromanipulation of high and low indices microparticles using a microfabricated double axicon

The technique of transferring the momentum of optical potential landscapes to control the kinetics of the microscopic particles has recently gained considerable interest. In this paper, we report the optical micromanipulations of high and low indices particles using an optical trapping system integrated with a micron-sized double axicon. A double axicon is used to generate a self-imaged bottle beam, a propagation invariant beam. The transverse intensity profile of the self-imaged bottle beam oscillates along the propagation axis embedding three-dimensional intensity-null points, which are unique to conventional beams used in tweezers-like Gaussian, Laguerre-Gaussian, and Bessel beams. By imaging different portions of a self-imaged bottle beam, the same tweezers system can easily be modified for trapping applications of high and low indices microparticles. Furthermore, the self-reconstruction property of a self-imaged bottle beam is numerically studied and the minimum self-reconstruction distance of an obs...

Optical transport, lifting and trapping of micro-particles by planar waveguides

Optical waveguides can be used to trap and transport micro-particles. The particles are held close to the waveguide surface by the evanescent field and propelled forward. We propose a new technique to lift and trap particles above the surface of the waveguides. This is made possible by a gap between two opposing, planar waveguides. The field emitted from each of the waveguide ends diverge fast, away from the substrate and into the cover-medium. By combining two fields propagating at an angle upwards and coming from opposite sides of a gap, particles can be stably lifted and trapped at the crossing of the two fields. Thus, particles are transported by waveguides leading to a gap, where they are lifted away from the substrate and trapped. The experiments are supported by numerical simulations of the forces on the micro-particles. Fluorescence imaging is used to track the particles in 3D with a precision of 50 nm.