We discuss the realization of atomic, molecular and optical physics experiments using microfabricated "chip" structures. We focus in particular on the potential for trapping ultra-cold polar molecules using such chip devices and consider the feasibility of several designs and applications.

Theoretical work has already established the existence of a light-induced torque acting on the centre of mass of an atom, ion or molecule immersed in twisted light, where the transition frequency is suitably detuned from that of the twisted light beam. The twisted beam carries l units of orbital angular momentum per photon, and the steady-state saturation form of the torque is also determined by the width of the upper state in the atomic transition. It has been shown that, to leading order, the transfer of orbital angular momentum can only occur between the twisted light and the centre of mass motion. We argue here that, for small linewidth, the full time-dependence of the torque is needed to account correctly for the dynamics of atoms in a twisted light beam. We outline the theoretical framework needed to derive this full time-dependence, applying the theory to the motion in a twisted light beam of Eu³⁺ ions, which possess a particularly narrow linewidth state. For relatively large linewidth, the steady-state forces and torque are appropriate, but the processes of cooling and trapping require the application of several suitably oriented twisted beams. The description of atomic motion in multiple twisted beams demands the application of special coordinate transformations. We show how to construct the appropriate transformation matrices to represent a twisted light beam propagating in an arbitrary direction, and we proceed to investigate the cooling and trapping of Mg⁺ ions in sets of pairs of counter-propagating twisted beams in two-dimensional and three-dimensional molasses configurations.

We study the optical fields that are produced when two Laguerre Gauss beams, each carrying an optical vortex, are superimposed collinearly. We find that the resulting beam contains new vortices. The number of vortices and their location depends on the charge of the vortices of the component beams and the relative intensity of the two beams. Our presentation focuses on the cases where the component Laguerre-Gauss beams are of order one and two.

Vectorial vortices obtained with quantized Pancharatnam-Berry phase optical elements (PBOEs) are presented. A vectorial vortex occurs around a point where a scalar vortex is centered in at least one of the scalar components of the vectorial wave field. PBOEs utilize the geometric phase that accompanies space-variant polarization manipulations to achieve a desired phase modification. The geometric phase is formed through the use of discrete computer-generated space-variant subwavelength dielectric gratings. By discretely controlling the local grating orientation, we could form complex vectorial fields. Propagation-invariant vectorial Bessel beams with linearly polarized axial symmetry were experimentally demonstrated. Moreover, a new class of vectorial vortices based on coherent addition of two orthogonal circular polarized Bessel beams of identical order, but with different propagation constant is presented. The transversely space-variant axially symmetric polarization distributions of these vectorial fields rotate as they propagate while still maintaining a propagation-invariant Bessel intensity distribution. The polarization properties were verified by both full space-variant polarization analysis and measurements. Rotating intensity patterns were also demonstrated by transmitting the vectorial vortices through a linear polarizer.

Light beams rotating about their axis can be created using rotating optical elements. We analyze the properties of rotating beams by expanding the mode function in eigenfunctions of angular momentum. Both the spin angular momentum, arising from the polarization, and orbital angular momentum, arising from the circulating phase gradient, are considered.

The OAM of light provides a new resource to explore quantum physics in a d-dimensional Hilbert space, beyond the two dimensional Hilbert space generated by the polarization state of the photons. The implementation of such a ddimensional quantum channel requires the generation of arbitrary engineered entangled states, thus controlling the OAM of the entangled photons is of paramount importance for many applications. Here we address the orbital angular momentum, i.e. the spatial shape, of photons generated in SPDC in non collinear geometries, when the interacting waves can exhibit Poynting vector walk off. The spatial shape depends on the interplay between the state ellipticity caused by the nonlinear geometry, and the Poynting vector walk off. The importance of both effects is dictated by the relationship between three characteristics lengths: the length of the nonlinear crystal, the walk off length and the non collinear length. The effects described here are relevant to current experiments, especially for the implementation of quantum information protocols based on spatially encoded information. Finally, the consideration of new geometries for SPDC, more specifically, highly non collinear configurations, will lead us to the discussion of the relationship between the OAM of the classical beam that pumps the nonlinear crystal, and the quantum OAM of the down converted photons. Regarding experimental measurements related to this issue, it is of great importance to make a clear distinction between the measurement of locally paraxial light beams in a suitable transverse frame, and the description of the global down conversion process, which is not necessarily paraxial.

We report on the theoretical and experimental demonstration of a geometrical representation for nondiffracting beams with orbital angular momentum. This representation is a SU(2) structure equivalent to the Poincare sphere for the polarization states of light, which describes the different states of light beams possessing orbital angular momentum in terms of nondiffracting beams. We have also investigated unitary transformations within our geometrical representation using linear optical elements, equivalent to a polarization state rotation on the Poincare sphere. A new class of nondiffracting beams is also suggested.

We use holographic optical tweezers to create and monitor the liquid flow within a micro-fluidic device. Using the tweezers to both trap and spin micron-sized beads within a 10-20 micron wide channel creates a fluid flow of the order of 200 cubic microns/sec. We also use the optical tweezers to measure the fluid flow by trapping and releasing probe particles that are imaged with high temporal and spatial resolution. Using the multi-trap capability of the holographic optical tweezers we measure the transverse fluid velocity at many positions simultaneously with an accuracy of better than 1 micron/sec. Such studies are highly pertinent to lab-on-chip systems for various applications and studies within the biosciences.

We demonstrate the use of multiple optical traps for driving various microfabricated silica structures in liquid host medium. Multiple counterpropagating-beam traps are formed using a generalized phase contrast (GPC) -based optical trapping system. A combination of UV-lithography and reactive-ion etching (RIE) is employed to fabricate the microtools whose design includes having multiple appendages with rounded endings by which optical traps hold and actuate them. Experiments show the collective and user-coordinated utility of multiple beams for driving microstructured objects whose future integration may lead to optically controlled micromachineries.

The response of the biological cells to optical manipulation in the bio-microfluidic devices is strongly influenced by the flow and motion inertia. There is a variety of microfluidic architectures in which both the cell-fluid interaction and the optical field are driving forces for segregation and manipulation of the cells. We developed a computational tool for analysis/optimization of these devices. The tool consists of two parts: an optical force library generator and the computational fluid dynamics solver with coupled optical force field. The optical force library can be computed for spherical and non-spherical objects of rotational symmetry and for complex optical fields. The basic idea of our method is to a) represent an incident optical field at the biological cell location as an angular spectrum of plane waves; b) compute the scattered field, being a coherent superposition of the scattered fields coming from each of the incident plane waves, with the powerful T-matrix method used to compute the amplitude matrix; c) use the incident and computed scattered fields to build a spatial map of optical forces exerted on biological cells at different locations in the optical beam coordinate system, and d) apply the library of optical forces to compute laser beam manipulation in microfluidic devices. The position and intensity of the optical field in the microfluidic device may be dynamic, thus optical forces in microfluidic device are based on the instantaneous relative location of the cell in the beam coordinate system. The cell is simulated by the macroparticle that undergoes mutual interactions with the fluid. We will present the exemplary applications of the code.

Since optical tweezers trapped microspheres can be used as an ultrasensitive force measurements technique, the knowledge of its theoretical description is of utmost importance. However, even the description of the incident electromagnetic fields under very tight focusing, typical of the optical trap, is not yet a closed problem. Therefore it is important to experimentally obtain whole accurate curves of the force as a function of wavelength, polarization and incident beam 3D position with respect to the center of the microsphere. Theoretical models for optical forces such as the Generalized Lorenz-Mie theory, can then be applied to the precisely evaluated experimental results. Using a dual trap in an upright standard optical microscope, one to keep the particle at the equilibrium position and the other to disturb it we have been able to obtain these force curves as a function of x, y and z position, incident beam polarization and also wavelength. Further investigation of optical forces was conducted for wavelengths in and out Mie resonances of the dielectric microspherical cavities for both TM and TE modes.

In earlier work, it has been established that laser-induced coupling between a pair of nanoparticles can enable the generation of novel patterns, entirely determined and controlled by the frequency, intensity, and polarization of the optical input. In this paper, the detailed spatial disposition about the beam axis is determined for two-, three- and four-nanoparticle systems irradiated by a Laguerre-Gaussian (LG) laser mode. The range-dependent laser induced energy shift is identified by the employment of a quantum electrodynamical description, calculations are performed to determine the distribution of absolute minima as a function of the topological charge, and the results are graphically displayed. This analysis illustrates a number of interesting features, including the fact that on increasing the LG beam's topological charge the particles increasingly cluster, i.e. the order of the structure is significantly raised - also the number of minima for which the particles can be trapped is enhanced. Finally, it is shown that similar principles apply to other kinds of radially structured optical modes.

The observation of an unusual light-induced agglomeration phenomenon that occurs besides the trapping of the gold nanoparticles aggregates (GNAs) has been observed. The observed agglomerate has a 60-100 μm donut-shaped metal microstructure with the rate of formation dependent on the laser power used. In this paper, the forces involved and the mechanism of this further agglomeration phenomenon are analyzed in detail. The observed trapping can partially be explained by a model including the optical radiation force and radiometric force. However, the lightinduced agglomeration cannot be explained by optical trapping alone as the size of the agglomerate is much greater than the waist of the Gaussion beam used in the optical trapping. Hydrodynamic drag force induced by the laser heating is also considered to play a role. Besides these forces, the mechanism of light-induced agglomeration is attributed to ion detachment from the surface of the nanoparticles/aggregates due to light illumination or heating. This is supported by the observation of reversible conductivity changes in the nanoparticle/aggregate solution upon laser illumination or direct heating. Light-induced agglomeration can be useful in the design and fabrication of microstructures from nanomaterials for various device applications.

We report on a light-induced isothermal transition of a polymer film from an isotropic solid to an anisotropic liquid state in which the degree of mechanical anisotropy can be controlled by light. Whereas during irradiation by circular polarized light the film behaves as an isotropic viscoelastic fluid, it displays considerable fluidity only in the direction parallel to the light field vector under linear polarized light, demonstrating thus extraordinarily anisotropic mechanical properties. The fluidisation phenomenon is related to photoinduced motion of azobenzene-functionalised molecular units, which can be effectively activated only when their transition dipole moments are oriented close to the direction of the light polarization. Along with the phenomenological finding, our work allows us to make a substantial step in understanding the mass transport effect in azobenzene containing systems under conditions of far- and near-field illumination.

The past decade has seen a rapidly developing interest in the response of subwavelength-structured surfaces to optical excitation. Many studies have interpreted the optical coupling to the surface in terms of surface plasmon polaritons, but recently another approach involving diffraction of surface evanescent waves, the Composite Diffractive Evanescent Wave (CDEW) model has been proposed. We present here a series of measurements on very simple one-dimensional (1-D) subwavelength structures with the aim of testing key properties of the surface waves predicted by the CDEW model.

Space-variant polarization manipulation of enhanced omnidirectional thermal emission in a narrow spectral peak is presented. The emission is attributed to surface phonon-polariton excitation from space-variant subwavelength SiO₂ gratings. Polarization manipulation was obtained by discretely controlling the local orientation of the grating. We experimentally demonstrated thermal emission in an axially symmetric polarization distribution. We show that by coupling surface phonon-polaritons to a propagating field, large anisotropy of the emissivity is obtained within a narrow spectral range. We experimentally demonstrate this effect by fabricating a space-variant subwavelength grating on a SiO₂ substrate to encrypt an image in the polarization state of a thermal radiation field. Theoretical calculations based on rigorous coupled-wave analysis are presented along with experimental results.

In the following paper we show that near-field optical manipulation can be greatly increased through the use of cavity enhanced evanescent fields. This approach utilises a resonant dielectric structure and a prism coupler to produce Fabry- Perot like cavity modes at a dielectric-fluid interface, which can be utilised in optical manipulation. Using this structure we show a ten-times increase in the optical interaction of micrometer-sized colloids compared with the standard evanescent wave configuration. In addition, stable accumulation and ordering of large-scale arrays of colloids is demonstrated using two counter propagating cavity enhanced evanescent fields. We believe that this technique has considerable scope for promoting the role of near-field optical manipulation at the nanometer scale.