104 research outputs found
Optical phase conjugation with less than a photon per degree of freedom
We demonstrate experimentally that optical phase conjugation can be used to
focus light through strongly scattering media even when far less than a photon
per optical degree of freedom is detected. We found that the best achievable
intensity contrast is equal to the total number of detected photons, as long as
the resolution of the system is high enough. Our results demonstrate that phase
conjugation can be used even when the photon budget is extremely low, such as
in high-speed focusing through dynamic media, or imaging deep inside tissue
Controlling the propagation of light in disordered scattering media
This thesis describes experimental work on the use of wavefront shaping to
steer light through strongly scattering materials. We find that scattering does
not irreversibly scramble the incident wave. By shaping the incident wavefront,
we make opaque objects focus light as sharply as aberration free lenses. We use
feedback from a target behind, or in, an opaque object to shape the incident
wave. This way, light is focused through, or inside, opaque objects for the
first time ever.Comment: PhD thesis by I.M. Vellekoop. Thesis supervisors: A. Lagendijk and A.
P. Mosk. This work was performed at the Complex Photonic Systems (COPS)
group, Faculty of Science and Technology and MESA+ institute for
Nanotechnology, Univeristy of Twente. P.O. Box 217, 7500 AE Enschede, The
Netherlands. This work contains contributions by E. G. van Putte
Scattered light fluorescence microscopy: imaging through turbid layers
A major limitation of any type of microscope is the penetration depth in turbid tissue. Here, we demonstrate a fundamentally novel kind of fluorescence microscope that images through optically thick turbid layers. The microscope uses scattered light, rather than light propagating along a straight path, for imaging with subwavelength resolution. Our method uses constructive interference to focus scattered laser light through the turbid layer. Microscopic fluorescent structures behind the layer were imaged by raster scanning the focus
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
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
Model-based aberration corrected microscopy inside a glass tube
Microscope objectives achieve near diffraction-limited performance only when
used under the conditions they are designed for. In non-standard geometries,
such as thick cover slips or curved surfaces, severe aberrations arise,
inevitably impairing high-resolution imaging. Correcting such large aberrations
using standard adaptive optics can be challenging: existing solutions are
either not suited for strong aberrations, or require extensive feedback
measurements, consequently taking a significant portion of the photon budget.
We demonstrate that it is possible to pre-compute the corrections needed for
high-resolution imaging inside a glass tube based on a priori information only.
Our ray-tracing based method achieved over an order of magnitude increase in
image contrast without the need for a feedback signal.Comment: 9 pages, 3 figures, 1 table. Submitted to Optics Expres
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
Determination of the diffusion constant using phase-sensitive measurements
We apply a pulsed-light interferometer to measure both the intensity and the
phase of light that is transmitted through a strongly scattering disordered
material. From a single set of measurements we obtain the time-resolved
intensity, frequency correlations and statistical phase information
simultaneously. We compare several independent techniques of measuring the
diffusion constant for diffuse propagation of light. By comparing these
independent measurements, we obtain experimental proof of the consistency of
the diffusion model and corroborate phase statistics theory.Comment: 9 pages, 8 figures, submitted to Phys. Rev.
Speckle-scale focusing in the diffusive regime with time reversal of variance-encoded light (TROVE)
Focusing of light in the diffusive regime inside scattering media has long been considered impossible. Recently, this limitation has been overcome with time reversal of ultrasound-encoded light (TRUE), but the resolution of this approach is fundamentally limited by the large number of optical modes within the ultrasound focus. Here, we introduce a new approach, time reversal of variance-encoded light (TROVE), which demixes these spatial modes by variance encoding to break the resolution barrier imposed by the ultrasound. By encoding individual spatial modes inside the scattering sample with unique variances, we effectively uncouple the system resolution from the size of the ultrasound focus. This enables us to demonstrate optical focusing and imaging with diffuse light at an unprecedented, speckle-scale lateral resolution of ~5 µm
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