Soft Report - Dipartimento di Fisica - Sapienza

Optical Tweezing Laboratory – UdR RomaHolographic Optical Tweezer (HOT). A mesoscopicobject can be stably trapped in three dimensions by atightly focusing single laser beam. Computer-generatedholograms displayed on liquid crystal spatial lightmodulators (SLM) offer a convenient way of producinglarge three dimensional arrays of optical traps. Theability to dynamically manipulate matter at the mesoscaleopens the way to a wide range of applications inthe physical and biological sciences. We set up aholographic optical tweezers (HOT) apparatus which isshown in Figure. A TEM00 mode beam from a diodepumped, 532 nm, 2 W laser is expanded and reflectedoff a liquid crystal (45° twisted nematic) Spatial LightModulator. Novel techniques for the generation ofhighly optimized holograms have been developed. Thephase modulated wavefront is then imaged onto theexit pupil of a 100x NA 1.4 objective lens mounted in aninverted optical microscope. The same lens is used toimage trapped particles on a software controlled digitalCCD camera. We developed a set of tools to extract andprocess particle trajectories by digital video microscopy.We're working on applications of holographic opticalmicro-manipulation to micro-fluidics, statisticalmechanics, colloidal science and micro-rheology of softmaterials.Non Linear Optics Laboratory – UdR RomaZ-scan set-up The Z-scan is a sensitive andcommonly employed technique able to measureboth nonlinear absorpsion and refraction in solidsand liquids and is based essentially on refractiveindex modulation by laser irradiance on anonlinear material. In this technique a polarizedGaussian laser beam, propagating in the z-direction, is focused to a narrow waist by a lens.The sample is moved along the z-axis throughthe focal point and the transmitted intensity ismeasured, as a function of z-direction in the farfield using a photodiode behind a smallcalibrated pinhole. In this way the trasmittanseis sensible to wave phase shift variations due tothe refractive index gradient induced in anonlinear material. In our set up a CW pumpeddiode laser operates at power 10 mW andwavelenght 532 nm. The beam is focused bymeans of a 75 mm focal lenght lens giving a 20µm beam waist radius and a I 0=8. 10 6 w/m 2beam central intensity at the focus (z=0). Aphotodetector, at distance 310 mm from lensfocus, is used to probe the light power behind a2 mm aperture and the sample is scanned acrossthe focus with a 5 cm micrometer traslationstage. This apparatus allows to measurerefraction index variations due mostly to thermaland electostrictive effects with nonlinearrefraction index n 2 ∼ 10 -6 – 10 -10 cm 2 /W.25SOFT Scientific Report 2004-06