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Scientific Report 2007-2009
Laboratories and Facilities of the Department of Physics
L10. Holographic Micromanipulation and Microscopy Lab
A mesoscopic object can be stably trapped in three dimensions by a
tightly focused single laser beam. Computer-generated holograms displayed
on liquid crystal spatial light modulators (SLM) offer a convenient
way of producing large three dimensional arrays of dynamic optical
traps. The ability to dynamically manipulate matter at the meso-scale
opens the way to a wide range of applications in the physical and biological
sciences. In our holographic optical tweezers (HOT) setup a TEM00
mode beam from a diode pumped, 532 nm, 2 W laser is expanded and
reflected off a liquid crystal Spatial Light Modulator. Highly optimized
holograms are generated in real time using custom parallel code running
on state of the art Graphic Processing Units. The phase modulated
wavefront is then focused onto a tiny trapping hologram by a 100x NA
1.4 objective lens mounted in an inverted optical microscope. The same
lens is used to image trapped particles on a software controlled digital
CMOS camera. 2D particle trajectories can be tracked by digital video
microscopy with a spatial resolution of about 10 nm and up to 1 kHz
framerate. A second setup combines HOT with Digital Holographic microscopy
(DHM). The recorded hologram is a complex interference pattern
produced by the propagation of a coherent laser beam through a
thick sample. Numerical processing allows to obtain from a single shot
hologram a full volumetric reconstruction with nanometer resolution.
We are working on applications of holographic trapping and imaging to
micro-fluidics, statistical mechanics, colloidal science and microbiology.
Figure 1: Holographic optical tweezers setup.
http://glass.phys.uniroma1.it/dileonardo/
Related research activities: C30.
L11. Photon Correlation Lab
In the last years the Photon Correlation laboratory has been engaged in experimental researches in Soft Matter. In
particular the ageing phemonenon and the transitions towards arrested states both of gel and glass nature have been
investigated.
The laboratory in Roma is equipped with two different
photoncorrelation set-up running independently. Conventional
Photon Correlation Spectroscopy set-up: a He-Ne laser (λ=632.8
nm) of 10 mW focused on the centre of a vat mounted on a
goniometer. The temperature of the sample, sit in the centre
of the vat, is controlled by a cooler-heater (HAAKE K35). The
scattered light is focused, selected by a pinhole and revealed by
a multimode fiber and a photomultiplier detector. A commercial
ALV-5000 logarithmic correlator computes the autocorrelation
functions. Measurements can be performed at various scattering
vectors (moving the collecting arm and so varying the collecting
angle) and in a correlation time window between 1 µs and 10 s.
Advanced photon correlation spectroscopy set-up: a He-Ne laser
of 35 mW is sent on a polarizing maintaining single mode fiber
and is focused on the centre of a vat mounted on a goniometer.
The temperature of the sample, sit in the centre of the vat,
is controlled by a cooler-heater (HAAKE FUZZYSTARC35).
A lens-collimator system couples the scattered intensity with
a single mode fiber connected to a photodiode detector and a
Figure 2: Photon correlation set-up.
home made software provides a logarithmic correlation of the
data. By means of the use of single mode fiber the coherence
factor reaches the ideal value of 1 and therefore autocorrelation functions with a very high signal to noise ratio are obtained.
Measurements at various scattering vectors (varying the collecting angle) and in a time correlation window between 1 µs
and 2 s can be performed.
Related research activities: C8.
Sapienza Università di Roma 184 Dipartimento di Fisica