Reza Ghadiri

Microassembly of complex and three-dimensional microstructures using holographic optical tweezers

In this paper we investigate a flexible method for the fabrication of complex microstructures using binding microparticles. Utilizing optical forces, micro-objects are caught, positioned and used as building blocks to form defined structures, analogous to assembling processes in the macroscopic world. Durable linkage between the individual particles is realized using biomolecules with high affinities applied as particle coatings. Planar structures can be assembled employing optical manipulation as well as three-dimensional patterns by stacking the generated layers. Even the properties of the generated structures can be locally designed as desired by using building blocks from diverse materials exhibiting different properties. This method benefits from its simplicity and the potential extensibility of the fabricated structure at any time of the experiment.

Using Laser Microfabrication to Write Conductive Polymer/SWNTs Nanocomposites

We present a novel laser microfabrication method to generate structures on the basis of a new class of functional photosensitive composites. particular, the focus lies Inon the development of conductive composites by incorporatingSWNTs into the matrix of polymers thus forming highly conductive nanocomposites. Conductive microstructures have been produced with the ultra-high resolution technology based on laser direct writing (e.g. single-, two-photon polymerization) using polymer/SWNTs nanocomposites. This technology opens new prospects for the realization of novel MEMS and MOEMS with increased functionality, integration, and higher level of miniaturization.

Laser direct writing of high refractive index polymer/TiO2 nanocomposites

This work reports the preparation of polymer/TiO2 nanocomposite by adding TiO2 nanoparticles to the polymer matrices. TiO2 nanoparticles can be effectively dispersed into the polymer. The refractive index of the nanocomposites can be tuned by increasing the concentration of TiO2 nanoparticles. The prepared samples exhibit excellent optical transparency in the Vis-NIR region, i.e. at two-photon polymerization (TPP) processing wavelength, and can be used to write threedimensional structures by means of TPP. Structures with high refractive index have been produced with the novel ultrahigh resolution technology based on TPP processing of polymer/TiO2 nanocomposites.

Optical tweezers in microassembly

Integrated hybrid MEMS require new micromanipulation devices in assembly processes. Although absolute forces are restricted optical tweezers are promising tools with unique advantages. Recent developments in beam shaping allow the control of a large number of different particles. Optical manipulation can also be used to assemble tiny structures by a generative process. Any type of particle, primarily coated with high-affinity biomolecules, can be applied as building blocks to form complex structures. By moving the particle into the requested orientation by holographic optical tweezers complex parts become possible. Also, shape-complimentary preforms can be fabricated with 2-photon-polymerization (2PP) and utilized to assemble the desired structure. Finally, microvalves and motors in lab-on-a-chip systems can be optically fabricated and also driven by optical forces.

Microfabrication by optical tweezers

A new method to fabricate microstructures built by polymer microparticles using a bottom-up technique is presented. The microstructures find broad application in micro-fluidics technology, photonics and tissue-engineering. The handling of the particles is realized by a holographic optical tweezers setup, ensuring the precise allocation of the particles to the desired structure. A biochemical technique ensures that the structure remains stable independent of the laser source. We show that with this method complex two-dimensional durable structures can be assembled and cannot be separated by optical forces. The structures are extendable during the entire fabrication process and can be linked to further particles and structures as desired.

Influence of multiple particles in optical tweezers on the trapping efficiency

Since the early times of Arthur Ashkins groundbreaking experiments on optical tweezers, a great number of theoretical works was dedicated to this subject. Most of them treated the optical trapping of single spherical or elliptical particles. In the last years optical tweezers have become more and more a tool for assembling three dimensional structures using single microspheres as building blocks. Since all structures and particles inside the light beams influence the properties of the traps, we investigated theoretically the influence of additional single particles and particle arrays on the properties of optical traps. For this reason a geometrical optics based model is used with the inherent flexibility to be applied for various shapes and particle numbers.

Optical tweezers as manufacturing and characterization tool in microfluidics

Pumping and mixing of small volumes of liquid samples are basic processes in microfluidic applications. Among the number of different principles for active transportation of the fluids microrotors have been investigated from the beginning. The main challenge in microrotors, however, has been the driving principle. In this work a new approach for a very simple magnetic driving principle has been realized. More precisely, we take advantage of optical grippers to fabricate various microrotors and introduce an optical force method to characterize the fluid flow generated by rotating the structures through magnetic actuation. The microrotors are built of silica and magnetic microspheres which are initially coated with Streptavidin or Biotin molecules. Holographic optical tweezers (HOT) are used to trap, to position, and to assemble the microspheres with the chemical interaction of the biomolecules leading to a stable binding. Using this technique, complex designs of microrotors can be realized. The magnetic response of the magnetic microspheres enables the rotation and control of the structures through an external magnetic field. The generated fluid flow around the microrotor is measured optically by inserting a probe particle next to the rotor. While the probe particle is trapped by optical forces the flow force leads to a displacement of the particle from the trapping position. This displacement is directly related to the flow velocity and can be measured and calibrated. Variations of the microrotor design and rotating speed lead to characteristic flow fields.

Plasmonic improvement of microcavity biomedical sensor spectroscopic characteristics

New opportunity to improve a sensetivity of a label-free biomolecule detection in sensing systems based on microcavity evanescent wave optical sensors has been recently found and is being under intensive development. Novel technique based on combination of optical resonance on microring structures with plasmon resonance. Recently developed tools based on neural network data processing can realize real-time identification of biological agents. So combining advantages of plasmon enhancing optical microcavity resonance with identification tools can give a new platform for ulta sensitive label-free biomedical sensor. Our developed technique used standard glass and polymer microspheres as sensetive elements. They are fixed in the solution flow by adhesive layer on the surface being in the field of evanescence wave. Sensitive layer have been treated by gold nanoparticel (GN) solution. Another technique used thin film gold layers deposited on the substrate below adhesive. The light from a tuneable diode laser is coupled into the microsphere through a prism and was sharply focussed on the single microsphere. Images were recorded by CMOS camera. Normalized by free spectral range resonance shift of whispering gallery mode (WGM) and a relative efficiency of their excitation were used as input data for biomolecule classification. Both biomolecules and NP injection was obtained caused WGM spectra modification. But after NP treatment spectral shift and intensity of WGM resonances in biomolecule solutions increased. WGM resonances in microspheres fixed on substrate with gold layer with optimized layer thickness in biomolecule solutions also had higher intensity and spectra modification then without gold layer.

Ex-situ preparation of high-conductive polymer/SWNTs nanocomposites for structure fabrication

This paper reports ex-situ preparation of conductive polymer/single-walled carbon nanotubes (SWNTs) nanocomposites by adding high conductive SWNTs to the polymer matrix. Sonication methods were used to disperse the SWNTs in the polymer. The conductivity of the nanocomposites is tuned by increasing the concentration of SWNTs. Furthermore, we present two-photon polymerization (2PP) method to fabricate structures on the basis of conductive photosensitive composites. The conductive structures were successfully generated by means of 2PP effect induced by a femtosecond laser.

Optical micro-assembling of non-spherical particles

Holographic optical tweezers have been developed for the manipulation of polymeric microparticles or biological cells with almost circular shape. As is well known, spherical particles can be trapped and controlled by optical tweezers and assembled with an additional light modulator application. Complementary building blocks, which are used in the following experiments, are generated by a two-photon-polymerization process in micrometer range and are not equipped with spherical trapping points. The possibilities of manufacturing arbitrary building blocks within the 2PP process and the potential of HOTs lead to the idea of combining manufacturing techniques with manipulation processes in a bottomup operation. In this work we present an experimental setup with an integrated fiber laser for holographic optical trapping of non-spherical building blocks. Furthermore experimental requirements which permit trapping will be illustrated.