Abdollah Hassanzadeh

Waveguide evanescent field fluorescence microscopy: theoretical investigation of optical pressure on a cell

The optical force of an evanescent field is useful for trapping individual proteins and molecules. We theoretically investigate the optical pressure exerted on an attached and well-spread cell on a waveguide in the vicinity of an evanescent field. To do this, the cell is modeled as a three-planar-layer (membrane-cytoplasm-membrane). These layers and a gap of water are used as a stratified cover medium for the waveguide. Then, the mode equation of a slab waveguide with a simple and semi-infinite cover medium is modified and solved graphically. The effects of the cell cytoplasm and membrane thicknesses and their refractive indices and the sample-waveguide separation distance on the optical pressure are numerically studied. The results show that the optical pressure increases when the cytoplasm thickness increases, but decreases when the cell membrane thickness is increased. Furthermore, the optical pressure does not significantly change when the sample film thickness increases to more than 0.2 of a wavelength. The optical pressure decreases when the waveguide-sample distance for a specific mode is increased, but can be attractive or repulsive depending on both the cytoplasm refractive index and the mode number.

Graphene based resonance structure to enhance the optical pressure between two planar surfaces.

To enhance the optical pressure on a thin dielectric sample, a resonance structure using graphene layers coated over a metal film on a high index prism sputtered with MgF2 was theoretically analyzed. The number of graphene layers and the thicknesses of metal and MgF2 films were optimized to achieve the highest optical pressure on the sample. Effects of three different types of metals on the optical pressure were investigated numerically. In addition, simulations were carried out for samples with various thicknesses. Our numerical results show that the optical pressure increased by more than five orders of magnitude compared to the conventional metal-film-base resonance structure. The highest optical pressure was obtained for 10 layers of graphene deposited on 29-nm thick Au film and 650 nm thickness of MgF2 at 633nm wavelength, The proposed graphene based resonance structure can open new possibilities for optical tweezers, nanomechnical devices and surface plasmon based sensing and imaging techniques.

Multiexposure laser interference lithography

Abstract. Nanopatterns resulting from two-beam interference in single, double, and multiexposures were simulated. Several patterns were experimentally fabricated to compare with the simulated patterns, allowing judgment of the quality of the simulation tool. Experimental and simulation results were consistent for single and double exposures. Photoresist nanofibers attached and detached from the substrate were fabricated with few changes in the development process. Results show that by increasing the number of exposures, a wide variety of patterns with very fine structures and sophisticated geometries can be generated. Fresnel-lens type structures are formed when the number of exposures is increased. These Fresnel-like patterns might have potential use in obtaining radial and azimuthal polarizations and optical vortices in addition to areas such as security patterns and diffractive optical elements.

Rigorous electromagnetic theory for waveguide evanescent field fluorescence microscopy

Recently, waveguide evanescent field fluorescence (WEFF) microscopy was introduced and used to image and analyze cell-substrate contacts. Here, we establish a comprehensive electromagnetic theory in a seven-layer structure as a model for a typical waveguide-cell structure appropriate for WEFF microscopy and apply it to quantify cell-waveguide separation distances. First, electromagnetic fields at the various layers of a model waveguide-cell system are derived. Then, we obtain the dispersion relation or characteristic equation for TE modes with a stratified media as a cover. Waveguides supporting a defined number of modes are theoretically designed for conventional, reverse, and symmetric waveguide structures and then various waveguide parameters and the penetration depths of the evanescent fields are obtained. We show that the penetration depth of the evanescent field in a three-layer waveguide-cell structure is always lower than that of a seven-layer structure. Using the derived electromagnetic fields, the background and the excited fluorescence in the waveguide-cell gap, filled with water-soluble fluorophores, are analytically formulated. The effect of the waveguide structures on the fluorescence and the background are investigated for various modes. Numerical results are presented for the background and the stimulated fluorescence as functions of the water gap width for various waveguide structures, which can be used to find the water gap width. The results indicate that the background and excited fluorescence increase by increasing the penetration depth of the evanescent field. In addition, we show that for various guided modes of a conventional waveguide, the electric fields in the cell membrane and the cytoplasm are evanescent and they do not depend on the waveguide structure and the mode number. However, for the reverse symmetry and symmetric waveguide structures, the waves are sinusoidal in the cell membrane and the cytoplasm for the highest-order modes.

Evanescent field interferometric optical tweezers with rotational symmetric patterns

A simple configuration to create optical lattices with N-fold rotational symmetry using the interference of N evanescent fields is proposed. The potential energy of a Rayleigh particle in the generated patterns (optical lattices) is investigated. The results show that by adjusting the relative phases and polarization states of the incident beams, an enormous range of optical lattices can be obtained. When the number of interfering beams is increased, circular spots beyond the diffraction limit can be created. For s-polarization, the intensity landscape has a dark-centered distribution at the origin encircled by a very small bright circular ring whose radius is 120 nm. This intensity distribution acts as a hollow beam and can be used for efficient trapping of low-index particles. We believe that the resulting lattices can find applications in the arrangement of particles, creation of optical lattices, and efficient trapping of very small Rayleigh particles.

Multibeam interferometric optical tweezers

Abstract. Using the interference of N collimated laser beams, optical lattices with N-fold rotational symmetry are generated over the interface of two semi-infinite dielectric media. The interaction of small dielectric particles with these interference patterns is investigated using Rayleigh approximation. The polarization state of the interfering beams considerably influences the interference patterns and potential landscapes. Therefore, both parallel and perpendicular polarized interfering beams are considered and the corresponding potential profiles are compared and analyzed. We also study how the number of interfering waves, incident and azimuth angles, and initial phases of the incident beams influence optical lattices and potential profiles. It is found that the ring-shaped patterns with good confinement properties can be achieved by increasing the number of incident beams. In addition, by increasing the number of incident beams one can make an optical trap with sharper intensity gradient and deeper potential well, which is an advantage for trapping small Rayleigh particles. The lattices resulting from the interference of N incident waves with different incident angles are also investigated. Furthermore, the effects of changing the azimuth angles between two adjacent incident wave vectors on the intensity patterns are studied. The proposed configuration and the numerical results can find numerous applications in particle arrangement, particle sorting, and the creation of quasicrystals. We believe that interference approaches have many potential capabilities for molding light wavefronts and creating multiple traps with sophisticated patterns.

Waveguide-based dielectric resonance structure: a theoretical analysis to change cell–substrate separation by a repulsive optical force

A waveguide-based dielectric resonance structure is introduced to enhance the optical pressure on a well-spread and attached cell. To calculate the change in cell–substrate separation a three-layered dielectric film, which is considered as a model for a well-spread and attached cell to its substrate, is connected to the substrate by springs. Each spring represents a single adhesion bound. The enhanced optical pressure on the sample, the changes in the cell–substrate separation distance, and strain on the cell are found. The obtained results are compared with those of both total internal reflection and interference reflection microscopes. Then, the penetration depth of the evanescent field and the enhancement factor for various modes are obtained. The results show that the enhancement factor and the optical pressure in the proposed resonance structure are 3 orders of magnitude higher than the conventional structure and the penetration depth of the evanescent wave is increased by 30 percent. We show that a measurable change in the cell–substrate distance (around 6 nm) can occur under the applied optical force. If this waveguide-based resonance structure is used in a waveguide evanescent field microscopy setup it is possible to simultaneously image and apply an effective optical pressure on cells and also to reduce the imaging time. Furthermore, there is no metal in the resonance structure to be worried about the heating and damaging biological samples.

Graphene-based multilayer resonance structure to enhance the optical pressure on a Mie particle

Abstract. We theoretically investigate the optical force exerted on a Mie dielectric particle in the evanescent field of a graphene-based resonance multilayer structure using the arbitrary beam theory and the theory of multilayer films. The resonance structure consists of several thin films including a dielectric film (MgF2), a metal film (silver or gold), and several graphene layers which are located on a prism base. The effects of the metal film thickness and the number of graphene layers on the optical force are numerically investigated. The thickness of the metal layer and the number of graphene layers are optimized to reach the highest optical force. The numerical results show that an optimized composition of graphene and gold leads to a higher optical force compared to that of the graphene and silver. The optical force was enhanced resonantly by four orders of magnitude for the resonance structure containing graphene and a gold film and by three orders of magnitude for the structure containing graphene and a silver film compared to other similar resonance structures. We hope that the results presented in this paper can provide an excellent means of improving the optical manipulation of particles and enable the provision of effective optical tweezers, micromotors, and microaccelelators.

Variable angle total internal reflection fluorescence microscopy in s-polarization: a new approach to quantify cell-substrate distances in contacts

Total internal reflection fluorescence microscopy is an evanescent based fluorescence microscope providing a selective visualization of cell-substrate contacts without interference from other, deeper cellular regions. Total internal reflection fluorescence microscope is used extensively to visualize cell-substrate contacts. However, quantifying these contacts - in particular the measurement of cell-substrate distances - has not been performed often. In order to quantify the cellsubstrate distances we have developed a new theoretical method which is based on a change in the penetration depth of the evanescent field by tuning the angle of incidence slightly above the angle of total internal reflection for s-polarized light. This is simpler and much more accurate in comparison to the few existing approaches.