Enzo M. Di Fabrizio

Differential interference contrast x-ray microscopy with twin zone plates

X-ray imaging in differential interference contrast (DIC) with submicrometer optical resolution was performed by using a twin zone plate (TZP) setup generating focal spots closely spaced within the TZP spatial resolution of 160 nm. Optical path differences introduced by the sample are recorded by a CCD camera in a standard full-field imaging and by an aperture photodiode in a standard scanning transmission x-ray microscope. Applying this x-ray DIC technique, we demonstrate for both the full-field imaging and scanning x-ray microscope methods a drastic increase in image contrast (approximately 20×) for a low-absorbing specimen, similar to the Nomarski DIC method for visible-light microscopy.

Orbital angular momentum of inhomogeneous electromagnetic field produced by polarized optical beams

It is shown that the polarization singularities of vector field are connected with the presence or absence of the angular momentum of electromagnetic field. In the vicinity of C-point the orbital angular momentum is observed. Direction of influence of such orbital angular momentum is defined by the sign of topological charge of vibration phase in this area. Spin angular momentum of a field vanishes on the s-contour. At the same time similar momentum appears if the resulting field is formed by the waves with different wavelengths.

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.

Spherical-based approach to design diffractive optical elements

A novel method to design diffractive optical elements, based on the spherical wave propagation and superposition, is presented in this paper. Underlying theory, practical considerations, results and limitations of this method are presented based on computer simulation experiments. Diffraction patterns produced by such diffractive optical elements are shown for binary intensity objects described either by the Cartesian coordinates of their non-zero points or by image format files. The method by itself stands as an effective method to calculate phase diffractive elements for free space optical interconnection between an array of point sources and an array of point detectors, but is shown to be also useful to generate binary intensity objects. The optimization of the phase function which describes the phase diffractive element is also demonstrated by means of a micro-genetic algorithm.

Novel diffractive optics for x-ray beam shaping

In our days, there is an increased interest for extreme ultraviolet and x-ray microscopy, which is mainly due to the availability of nearly ideal optical sources for diffractive optics. Synchrotrons of the latest generation and free electron lasers (in the near future) are sources that can produce x-ray beams with low divergence, whose wavelength can be tuned over a range of several keV and whose spectrum can be monochromatised within a band pass Δλ/λ< 10-4. In this paper we present the design, fabrication and use of novel diffractive optical elements that, beyond simple focusing, can perform new optical functions in the range of soft X-rays: multi-focusing in single or multiple focal planes and beam shaping of a generic monochromatic beam into a desired continuous geometrical pattern. The design is based on scalar diffraction approaches using iterative or direct algorithms to calculate the optical function. Diffractive optical elements with 100x100 microns size and 100 nanometers resolution have been fabricated using e-beam lithography and their optical functions have been tested in differential interference contrast microscopy. We suggest their use also in mask-less lithography and chemical vapor deposition induced by extreme ultraviolet and x-ray radiation.

Dynamic multiple beads manipulation on x-y-z directions

The use of diffractive optical elements (DOEs) for multiple trapping of dielectric micro-spheres immersed in a fluid has been demonstrated recently. When the DOEs are implemented on a spatial light modulator (SLM), the trapped particles can be independently moved by changing the configuration of the DOE. In this paper we demonstrate phase DOEs implemented on an optical addressable SLM to move an array of trapped particles in a volume of about 20x20x6 μm. Experimental results show the usefulness of this technique for particle micromanipulation in biology.

Advantages and disadvantages in using oil-immersed microscope objectives for optical trapping

Since the first demonstrations of optical trapping, both theoretical and experimental parts of this technique evolved. With all this, the main problem when trapping in the Mie regime is due to the limited numerical aperture a microscope objective has. In literature one finds characterizations of “classical” microscope lens or, at most, water immersed ones. In this paper we are analysing the forces generated in an optical tweezers setup using oil immersed microscope objective and having as entrapped particles water-immersed silica beads. Using such a set-up, we can take advantage of the numerical aperture an oil-immersed objective can reach. This numerical aperture can have a value as high as 1.4. From Roosens 1 and Ashkins 2 formulas, we calculated the forces involved in our experiment. We observed that the entrapping range on the optical path axis is larger and asymmetric. This generates the possibility to build optical catapult and optical tweezers in the same time, changing only the distance from the sample to the entrapment point. One of the disadvantages of optical trapping in these conditions is that the focus point and the entrapment one can be different. This fact generates the need of using a second microscope for inspecting the entrapped particle so the optical setup is more complicated. To our belief, this set-up for optical tweezers can have big advantages in the field of optical trapping mainly due to the not so strict trapping spatial conditions.

Diffractive optical elements: design and fabrication at TASC-INFM

Diffractive optical elements (DOE’s) can be used to manipulate the amplitude, the phase and the polarization of light beams. They can operate over a wide range of wavelength from UV to x-ray radiation. Interest in DOE’s has grown rapidly in recent years since they are smaller and lower in size and weight than conventional optical elements and in addition allow optical functions impossible to reach with other refractive or reflective optical elements. In this paper we report results obtained in DOE’s design and fabrication at the LILIT Nanolithography beam line developed at the national laboratory TASC-INFM. Among the design methods we have used, the phase retrieval iterative algorithm approach is presented in more detail here. There are also presented aspects involved in the fabrication of high-resolution zone plates for focusing soft x-rays. The fabrication process is based on e-beam lithography and etching techniques similar to those used in the microelectronic technology, allowing structures sized down to few tens of nanometers. Experimental results are presented for some applications of our DOE’s: laser beam shaping, optical tweezers array generation and x-ray microscopy.

Zone plate for x-ray applications

High resolution and high efficiency Zone Plate for X-rays in the energy range of 300 eV and 12 KeV fabricated by means of electron beam and X-ray lithography are presented. Regarding the high resolution regime zone plate with 40 nm outermost zone and thickness of 0.2 micrometer are shown. For high efficiency performances, multilevel zone plate and continuous profile were fabricated to provide an increase of efficiency at the first diffraction order and to suppress higher ones. The combination of the two lithography allows a powerful design flexibility at several energy regimes.

Multipurpose experimental station for soft x-ray microscopy on BACH beamline at Elettra

A new experimental station for soft x-ray microscopy is under construction at BACH beamline, at Elettra Synchrotron Radiation Facility (Italy). This station will be devoted both to scanning transmission x-ray microscopy (STXM) and photoemission microscopy (SPEM), with spatial resolution of about 50 nm. A Fresnel Zone Plate (FZP) will provide the micro-focusing of the beam delivered by the monochromator of BACH. The photon beam features are high resolving power (30000-5000 in the 40-1500 eV range), high flux (more than 1011 photons/s after the exit slit) and the possibility to select the light polarization. The experimental chamber will host several photon and electron detectors which should provide spatially resolved information of the bulk and surface composition. The expected acquisition times are of the order of the seconds for STXM and less than 1 minute for SPEM. The branch line hosting this station will start from the exit slit of the BACH monochromator. A toroidal mirror will focus the exit slit-spot on a pinhole which will be the source for the following FZP. With a 10x10 micrometers 2 pinhole it will be possible to obtain a spot of about 50x50 nm2 with enough flux (from 108 to 109 ph/sec) to perform microscopic experiments with polarized radiation. In this paper we present the optical scheme of the instrument as well as the foreseen performances in terms of resolution and flux.