Dan Cojoc

Wave front engineering for microscopy of living cells.

A new method to perform simultaneously three dimensional optical sectioning and optical manipulation is presented. The system combines a multi trap optical tweezers with a video microscope to enable axial scanning of living cells while maintaining the trapping configuration at a fixed position. This is achieved compensating the axial movement of the objective by shaping the wave front of the trapping beam with properly diffractive optical elements displayed on a computer controlled spatial light modulator. Our method has been validated in three different experimental configurations. In the first, we decouple the position of a trapping plane from the axial movements of the objective and perform optical sectioning of a circle of beads kept on a fixed plane. In a second experiment, we extend the method to living cell microscopy by showing that mechanical constraints can be applied on the dorsal surface of a cell whilst performing its fluorescence optical sectioning. In the third experiment, we trapped beads in a three dimensional geometry and perform, always through the same objective, an axial scan of the volume delimited by the beads.

Design and fabrication of on-fiber diffractive elements for fiber-waveguide coupling by means of e-beam lithography

The aim of this paper is to demonstrate that efficient fiber-waveguide optical coupling can be achieved using a multilevel phase diffractive element (PDE) fabricated directly on the top of the fiber by means of e-beam lithography. The diffractive phase element is calculated to focus and reshape the gaussian symmetric beam exiting a single-mode fiber into a desired asymmetric intensity distribution at the waveguide input plane. Phase modulation is obtained by multilevel profiling a polymeric material coated on the top of the fiber by means of a specific fabrication process including e-beam lithography and chemical etching. Experimental results obtained for fiber-waveguide coupling with a 20-µm diameter diffractive element are also presented.

LILIT beamline for soft and deep X-ray lithography at Elettra

In this paper we present the beamline for X-ray lithography installed at ELETTRA (Trieste, Italy). The peculiarity of the beamline design consists mainly in its wide lithographic window. This is achieved by combining high-pass filters (Beryllium windows or other suitable material filters) with low-pass filters (mirrors at increasing angles of incidence). The design allows to change the spectral range of interest continuously, from the soft (around 1.5 keV) where we can achieve the highest lithographic resolution, to hard X-ray region (higher than 10 keV) where sensitive materials of thickness of tens of microns can be exposed.

Optical Micro-Manipulation Using Laguerre-Gaussian Beams

In this work we investigate the features of single-ringed Laguerre-Gaussian (LG) beams, often referred to as optical vortices, for laser trapping and micro-manipulation experiments that can not be performed using Gaussian beams. LG beams, exhibiting “doughnut”-like transversal intensity distributions and carrying orbital angular momentum (OAM) about their axis, greatly extend the capabilities of laser tweezers. LG beams can be obtained by converting the Gaussian beam generated by a common laser source, by means of phase-only diffractive optical elements (DOEs). We present a trapping system based on DOEs implemented on a liquid crystal display. Trapping of small dielectric high-index particles on the “doughnut” profile is demonstrated. Orbital angular momentum transfer to trapped particles, which are caused to rotate, is studied as a function of the doughnut radius. Moreover, low-index particles, that would be rejected by a conventional Gaussian beam, are trapped in the zero intensity region of the doughnut. Finally, trapping of low-index particles with multi-LG beams, obtained by means of DOEs, is achieved.

Design and fabrication of diffractive optical elements for optical tweezer arrays by means of e-beam lithography

Optical tweezer arrays generated by diffractive optical elements can extend the capabilities to manipulate and organize microscopic particles into complex structures, to sort them intelligently and to study collective behavior in many-body systems materials. We describe new design methods, based on iterative algorithms, to calculate phase-only diffractive optical elements able to generate planar and three-dimensional arrays of tweezers. Experimental results showing the validity of the design approach are presented for an array of eight gaussian spots.

Coordinate-transformed filter for shift-invariant and scale-invariant pattern recognition

A variable radial coordinate transformation of the phase-only filter (POF) that is dependent on the energys angular distribution of the target spectrum is used to perform shift- and scale-invariant pattern recognition. The POF of a basic size target and the cumulative energy of its angular distribution are calculated. The filter function is then transformed by means of stretching along the radial coordinate so that the same energy contribution to the correlation peak is provided for any size target. The maximum ratio for recognizing scaled objects is 1:1.5. Computer simulations and optical experiments showing the performances of the filter are presented.

Electronic and centre of mass transitions driven by Laguerre–Gaussian beams

We derive the interaction Hamiltonian of a Laguerre–Gaussian beam with a simple atomic system, under the assumption of a small spread of the centre of mass wavefunction in comparison with the waist of the Laguerre–Gaussian beam and taking into account the centre of mass motion of the atomic system. Using the properties of regular spherical harmonics the internal and centre of mass dynamical variables are separated without making any multipolar expansion. The features of angular momentum exchange process assisted by Laguerre–Gaussian beams and the influence of their winding number on the selection rules and transition probability of internal and centre of mass motion are discussed.

Nano-optical elements fabricated by e-beam and X-ray lithography

In this paper we report results obtained in the design and fabrication of diffractive optical elements (DOEs) with minimum feature size down to tens of nanometers by the use of e-beam and x-ray lithography. The DOEs are patterned using e-beam lithography and replicated by x-ray lithography. Since in our days there is an increased interest for extreme ultraviolet and x-ray microscopy our work has been focused toward the fabrication of DOEs mainly for these applications. Different types of zone plates (ZPs) were fabricated for x-ray beam focusing: high resolution ZPs for high resolution beam focusing, multilevel phase ZPs to increase the diffraction efficiency in the desired order and high aspect ratio ZPs for hard x-rays. Recently we have extended the concept of the ZPs to a more general category of DOEs which beside simple focusing can perform new optical functions in the range of x-rays. In particular, the intensity of the beam after the DOE can be distributed with almost complete freedom. We have designed and fabricated DOEs that focus the beam in an array of spots disposed either in plane or along the optical axis. This type of DOEs has been tested successfully in x-ray differential interference contrast microscopy. The possibility to introduce a specified phase shift between the generated spots is demonstrated in this paper by preliminary results obtained from computer simulations and experiments performed in visible light.

Multiple optical tweezers for micro Raman spectroscopy

The goal of our study is to develop a setup that combines multi-trapping and manipulation with micro Raman spectroscopy of microns size particles. Multiple trapping, in 2D or 3D (two or three dimensional) configurations, is obtained in an inverted microscope scheme by shaping the trapping beam (1064 nm) with diffractive optical elements implemented on a spatial light modulator (SLM). Manipulation of multiple particles, directly trapped by the beam, can be achieved using the dynamic displaying of the SLM. Indirect trapping and manipulation of the sample can be obtained surrounding it with trapped micro beads that are manipulated by the optical tweezers. Laser light is not directly focused on the sample but is distributed on the beads and therefore the photo-induced damaging of biological samples is reduced. This technique offers also other advantages: the sample can be kept in a stable position during the spectroscopic investigation or can be moved in x-y-z to get spatial resolved information in a scanning mode measurement and the shape of a deformable sample can be changed in a controlled manner during the measurement. Samples excitation and Raman signal collection are accomplished with a separate laser beam (514.5 nm) in a non-inverted microscope coupled with the spectrometer. Some experimental results showing multi trapping and indirect manipulation of human red blood cells are presented and discussed.

Shaping X-rays by diffractive coded nano-optics

In this paper we report results obtained in the fabrication and use of novel coded diffractive nano-optics that, beyond focusing, can perform new optical functions. In particular, the intensity of light in the space beyond the optical elements can be redistributed with almost complete freedom. These novel X-ray optical elements have been tested and found to perform multi-focusing in single or multiple focal planes and beam shaping of a generic monochromatic beam into a desired continuous geometrical pattern. Already available extreme ultraviolet and X-ray sources are suitable as ideal sources for such diffractive optical elements. Their new optical functions have been tested in differential interference contrast microscopy and we suggest their use also in maskless lithography and chemical vapour deposition induced by extreme ultraviolet and X-ray radiation.