288 research outputs found
Optimal Concentration of Light in Turbid Materials
In turbid materials it is impossible to concentrate light into a focus with
conventional optics. Recently it has been shown that the intensity on a dyed
probe inside a turbid material can be enhanced by spatially shaping the wave
front of light before it enters a turbid medium. Here we show that this
enhancement is due to concentration of light energy to a spot much smaller than
a wavelength. We focus light on a dyed probe sphere that is hidden under an
opaque layer. The light is optimally concentrated to a focus which does not
exceed the smallest focal area physically possible by more than 68%. A
comparison between the intensity enhancements of both the emission and
excitation light supports the conclusion of optimal light concentration.Comment: We corrected an ambiguous description of the focus size in our
abstract and text pointed out by an anonymous refere
Wavelength dependence of light diffusion in strongly scattering macroporous gallium phosphide
We present time-resolved measurements of light transport through strongly scattering macroporous gallium phosphide at various vacuum wavelengths between 705 nm and 855 nm. Within this range the transport mean free path is strongly wavelength dependent, whereas the observed energy velocity is shown to be independent of the wavelength. We conclude that microscopic resonances, which can strongly slow down the diffusion process in, e.g., granular TiO2, are absent in macroporous gallium phosphide in the wavelength region of concern
Exploiting disorder for perfect focusing
We demonstrate experimentally that disordered scattering can be used to
improve, rather than deteriorate, the focusing resolution of a lens. By using
wavefront shaping to compensate for scattering, light was focused to a spot as
small as one tenth of the diffraction limit of the lens. We show both
experimentally and theoretically that it is the scattering medium, rather than
the lens, that determines the width of the focus. Despite the disordered
propagation of the light, the profile of the focus was always exactly equal to
the theoretical best focus that we derived.Comment: 4 pages, 4 figure
Focusing and Compression of Ultrashort Pulses through Scattering Media
Light scattering in inhomogeneous media induces wavefront distortions which
pose an inherent limitation in many optical applications. Examples range from
microscopy and nanosurgery to astronomy. In recent years, ongoing efforts have
made the correction of spatial distortions possible by wavefront shaping
techniques. However, when ultrashort pulses are employed scattering induces
temporal distortions which hinder their use in nonlinear processes such as in
multiphoton microscopy and quantum control experiments. Here we show that
correction of both spatial and temporal distortions can be attained by
manipulating only the spatial degrees of freedom of the incident wavefront.
Moreover, by optimizing a nonlinear signal the refocused pulse can be shorter
than the input pulse. We demonstrate focusing of 100fs pulses through a 1mm
thick brain tissue, and 1000-fold enhancement of a localized two-photon
fluorescence signal. Our results open up new possibilities for optical
manipulation and nonlinear imaging in scattering media
Universal optimal transmission of light through disordered materials
We experimentally demonstrate increased transmission of light through
strongly scattering materials. Wavefront shaping is used to selectively couple
light to the open transport channels in the material, resulting in an increase
of up to 44% in the total transmission. The results for each of several
hundreds of experimental runs are in excellent quantitative agreement with
random matrix theory. Extrapolating our measurements to the limit of perfect
wavefront shaping, we find a universal transmission of 2/3, regardless of the
thickness of the sample.Comment: 10 pages, 4 figures. Accepted for publication in Phys. Rev. Let
Spatial amplitude and phase modulation using commercial twisted nematic LCDs
We present a method for full spatial phase and amplitude control of a laser
beam using a twisted nematic liquid crystal display combined with a spatial
filter. By spatial filtering we combine four neighboring pixels into one
superpixel. At each superpixel we are able to independently modulate the phase
and the amplitude of light. We demonstrate experimentally the independent phase
and amplitude modulation using this novel technique. Our technique does not
impose special requirements on the spatial light modulator and allows precise
control of fields even with imperfect modulators.Comment: 10 pages, 6 figure
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