343 research outputs found
Superpixel-based spatial amplitude and phase modulation using a digital micromirror device
We present a superpixel method for full spatial phase and amplitude control
of a light beam using a digital micromirror device (DMD) combined with a
spatial filter. We combine square regions of nearby micromirrors into
superpixels by low pass filtering in a Fourier plane of the DMD. At each
superpixel we are able to independently modulate the phase and the amplitude of
light, while retaining a high resolution and the very high speed of a DMD. The
method achieves a measured fidelity for a target field with fully
independent phase and amplitude at a resolution of pixels per
diffraction limited spot. For the LG orbital angular momentum mode the
calculated fidelity is , using DMD pixels. The
superpixel method reduces the errors when compared to the state of the art Lee
holography method for these test fields by and , with a comparable
light efficiency of around . Our control software is publicly available.Comment: 9 pages, 6 figure
Optimal control of light propagation through multiple-scattering media in the presence of noise
We study the control of coherent light propagation through
multiple-scattering media in the presence of measurement noise. In our
experiments, we use a two-step optimization procedure to find the optimal
incident wavefront. We conclude that the degree of optimal control of coherent
light propagation through a multiple-scattering medium is only determined by
the number of photoelectrons detected per single speckle spot. The prediction
of our model agrees well with the experimental results. Our results offer
opportunities for imaging applications through scattering media such as
biological tissue in the shot noise limit
Light propagation and emission in complex photonic media
We provide an introduction to complex photonic media, that is, composite
materials with spatial inhomogeneities that are distributed over length scales
comparable to or smaller than the wavelength of light. This blossoming field is
firmly rooted in condensed matter physics, in optics, and in materials science.
Many stimulating analogies exist with other wave phenomena such as sound and
seismology, X-rays, neutrons. The field has a rich history, which has led to
many applications in lighting, novel lasers, light harvesting, microscopy, and
bio optics. We provide a brief overview of complex photonic media with
different classes of spatial order, varying from completely random to
long-periodically ordered structures, quasi crystalline and aperiodic
structures, and arrays of cavities. In addition to shaping optical waves by
suitable photonic nanostructures, the realization is quickly arising that the
spatial shaping of optical wavefronts with spatial light modulators
dramatically increases the number of control parameters. As a result, it is
becoming possible for instance to literally see through completely opaque
complex media. We discuss a unified view of complex photonic media by means of
a photonic interaction strength parameter. This parameter gauges the
interaction of light with any complex photonic medium, and allows to compare
complex media from different classes for similar applications.Comment: 8 pages, 2 figures, Light Localisation and Lasing: Random and
Quasi-Random Photonic Structures, Eds. M. Ghulinyan and L. Pavesi, (Cambridge
Univ. Press, Cambridge, 2015) Ch. 1, p.
Gain narrowing in few-atom systems
Using a density matrix approach, we study the simplest systems that display
both gain and feedback: clusters of 2 to 5 atoms, one of which is pumped. The
other atoms supply feedback through multiple scattering of light. We show that,
if the atoms are in each other's near-field, the system exhibits large gain
narrowing and spectral mode redistribution. The observed phenomena are more
pronounced if the feedback is enhanced. Our system is to our knowledge the
simplest exactly solvable microscopic system which shows the approach to laser
oscillation
Pathlengths of open channels in multiple scattering media
We report optical measurements of the spectral width of open transmission
channels in a three-dimensional diffusive medium. The light transmission
through a sample is enhanced by efficiently coupling to open transmission
channels using repeated digital optical phase conjugation. The spectral
properties are investigated by enhancing the transmission, fixing the incident
wavefront and scanning the wavelength of the laser. We measure the transmitted
field to extract the field correlation function and the enhancement of the
total transmission. We find that optimizing the total transmission leads to a
significant increase in the frequency width of the field correlation function.
Additionally we find that the enhanced transmission persists over an even
larger frequency bandwidth. This result shows open channels in the diffusive
regime are spectrally much wider than previous measurements in the localized
regime suggest
Design of a 3D photonic band gap cavity in a diamond-like inverse woodpile photonic crystal
We theoretically investigate the design of cavities in a three-dimensional
(3D) inverse woodpile photonic crystal. This class of cubic diamond-like
crystals has a very broad photonic band gap and consists of two perpendicular
arrays of pores with a rectangular structure. The point defect that acts as a
cavity is centred on the intersection of two intersecting perpendicular pores
with a radius that differs from the ones in the bulk of the crystal. We have
performed supercell bandstructure calculations with up to
unit cells. We find that up to five isolated and dispersionless bands appear
within the 3D photonic band gap. For each isolated band, the electric-field
energy is localized in a volume centred on the point defect, hence the point
defect acts as a 3D photonic band gap cavity. The mode volume of the cavities
resonances is as small as 0.8 (resonance wavelength cubed),
indicating a strong confinement of the light. By varying the radius of the
defect pores we found that only donor-like resonances appear for smaller defect
radius, whereas no acceptor-like resonances appear for greater defect radius.
From a 3D plot of the distribution of the electric-field energy density we
conclude that peaks of energy found in sharp edges situated at the point
defect, similar to how electrons collect at such features. This is different
from what is observed for cavities in non-inverted woodpile structures. Since
inverse woodpile crystals can be fabricated from silicon by CMOS-compatible
means, we project that single cavities and even cavity arrays can be realized,
for wavelength ranges compatible with telecommunication windows in the near
infrared.Comment: 11 figure
Local density of optical states in the band gap of a finite photonic crystal
We study the local density of states (LDOS) in a finite photonic crystal, in
particular in the frequency range of the band gap. We propose a new point of
view on the band gap, which we consider to be the result of vacuum fluctuations
in free space that tunnel in the forbidden range in the crystal. As a result,
we arrive at a model for the LDOS that is in two major items modified compared
to the well-known expression for infinite crystals. Firstly, we modify the
Dirac delta functions to become Lorentzians with a width set by the crystal
size. Secondly, building on characterization of the fields versus frequency and
position we calculated the fields in the band gap. We start from the fields at
the band edges, interpolated in space and position, and incorporating the
exponential damping in the band gap. We compare our proposed model to exact
calculations in one dimension using the transfer matrix method and find very
good agreement. Notably, we find that in finite crystals, the LDOS depends on
frequency, on position, and on crystal size, in stark contrast to the
well-known results for infinite crystals.Comment: 22 pages, 8 figure
Intrinsic fluctuations in random lasers
We present a quantitative experimental and theoretical study of shot-to-shot
intensity fluctuations in the emitted light of a random laser. A model that
clarifies these intrinsic fluctuations is developed. We describe the output
versus input power graphs of the random laser with an effective spontaneous
emission factor (beta factor).Comment: accepted by Phys. Rev. A. submitted; 7 pages, 5 figure
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