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ABSTRACT: We show that coupling among multiple resonances can be conveniently
introduced and controlled by boundary wave scattering. We demonstrate this
principle in optical microcavities of quasi-circular shape, where the couplings
of multiple modes are determined by the scattering from different harmonic
boundary deformations. We analyze these couplings using a perturbation theory,
which gives an intuitive understanding of the first-order and higher-order
scattering processes. Different scattering paths between two boundary waves can
either enhance or reduce their coupling strength. The effect of controlled
multimode coupling is most pronounced in the direction of output from an open
cavity, which can cause a dramatic change of the external cavity field
distribution.
05/2013;
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ABSTRACT: Optical absorption is usually considered deleterious, something to avoid if
at all possible. We propose a broadband nanoabsorber that completely eliminates
the diffracting wave, resulting in a subwavelength enhancement of the field.
Broadband operation is made possible by engineering the dispersion of the
complex dielectric function. The local enhancement can be significantly
improved compared to the standard plane wave illumination of a metallic
nanoparticle. Our numerical simulation shows that an optical pulse as short as
6 fs can be focused to a 11 nm region. Not only the local field, but also its
gradient are greatly enhanced, pointing to applications in ultrafast nonlinear
spectroscopy, sensing and communication with deep-subwavelength resolution.
04/2013;
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ABSTRACT: Diffusion has been widely used to describe a random walk of particles or waves, and it re-quires only one parameter – the diffusion constant. For waves, however, diffusion is an ap-proximation that disregards the possibility of interference 1 . Anderson localization 2 , which manifests itself through a vanishing diffusion coefficient in an infinite system 3, 4 , originates from constructive interference of waves traveling in loop trajectories – pairs of time-reversed paths returning to the same point 5, 6 . In an open system of finite size, the return probabil-ity through such paths is reduced, particularly near the boundary where waves may escape. Based on this argument, the self-consistent theory of localization and the supersymmetric field theory predict 7–9 that the diffusion coefficient varies spatially inside the system. A di-rect experimental observation of this effect is a challenge because it requires monitoring wave transport inside the system. Here, we fabricate two-dimensional photonic random media and probe position-dependent diffusion inside the sample from the third dimension. By varying
the geometry of the system or the dissipation which also limits the size of loop trajectories,
we are able to control the renormalization of the diffusion coefficient. This work shows the
possibility of manipulating diffusion via the interplay of localization and dissipation.
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ABSTRACT: A standard multimode optical fiber can be used as a general purpose spectrometer after calibrating the wavelength dependent speckle patterns produced by interference between the guided modes of the fiber. A transmission matrix was used to store the calibration data and a robust algorithm was developed to reconstruct an arbitrary input spectrum in the presence of experimental noise. We demonstrate that a 20 meter long fiber can resolve two laser lines separated by only 8 pm. At the other extreme, we show that a 2 centimeter long fiber can measure a broadband continuous spectrum generated from a supercontinuum source. We investigate the effect of the fiber geometry on the spectral resolution and bandwidth, and also discuss the additional limitation on the bandwidth imposed by speckle contrast reduction when measuring dense spectra. Finally, we demonstrate a method to reduce the spectrum reconstruction error and increase the bandwidth by separately imaging the speckle patterns of orthogonal polarizations. The multimode fiber spectrometer is compact, lightweight, low cost, and provides high resolution with low loss.
Optics Express 03/2013; 21(5):6584-6600. · 3.59 Impact Factor
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ABSTRACT: We present a simple and robust approach to reduce laser speckle, which has limited the adoption of lasers in imaging and display applications. We use colloidal solutions that can quickly reduce speckle contrast due to the Brownian motion of the scattering particles. The high insertion loss associated with propagation through a colloidal solution was overcome by using white paint to cover the sides of the cuvette and an optical fiber to deliver the laser light deep into the colloidal solution, enabling transmission greater than 90%. The diffused laser output followed a Lambertian distribution and produced speckle contrast below 4% at an integration time of 129 μs. The ability for colloidal solutions to achieve fast speckle reduction without power consumption while maintaining high transmission, low cost, a compact size, and a long lifetime makes our approach useful for a wide range of laser imaging and projection applications.
Applied Optics 02/2013; 52(6):1168-72. · 1.41 Impact Factor
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ABSTRACT: A standard multimode optical fiber can be used as a general purpose
spectrometer after calibrating the wavelength dependent speckle patterns
produced by interference between the guided modes of the fiber. A transmission
matrix was used to store the calibration data and a robust algorithm was
developed to reconstruct an arbitrary input spectrum in the presence of
experimental noise. We demonstrate that a 20 meter long fiber can resolve two
laser lines separated by only 8 pm. At the other extreme, we show that a 2
centimeter long fiber can measure a broadband continuous spectrum generated
from a supercontinuum source. We investigate the effect of the fiber geometry
on the spectral resolution and bandwidth, and also discuss the additional
limitation on the bandwidth imposed by speckle contrast reduction when
measuring dense spectra. Finally, we demonstrate a method to reduce the
spectrum reconstruction error and increase the bandwidth by separately imaging
the speckle patterns of orthogonal polarizations. The multimode fiber
spectrometer is compact, lightweight, low cost, and provides high resolution
with low loss.
02/2013;
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ABSTRACT: The performance of a coherent perfect absorber (time-reversed laser) is
limited by quantum and thermal noise. At zero temperature, the quantum shot
noise dominates the signal for frequencies close to the resonance frequency,
and both vanish exactly at the resonance frequency. We compute the sensitivity
of the absorbing cavity as a background-free detector, limited by finite signal
or detector bandwidth.
11/2012;
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ABSTRACT: We propose and demonstrate that a conventional multimode fiber can function as a high-resolution, low-loss spectrometer. The proposed spectrometer consists only of the fiber and a camera that images the speckle pattern generated by interference among the fiber modes. Although this speckle pattern is detrimental to many applications, it encodes information about the spectral content of the input signal, which can be recovered using calibration data. We achieve a spectral resolution of 0.15 nm over 25 nm bandwidth using 1 m long fiber, and 0.03 nm resolution over 5 nm bandwidth with a 5 m fiber. The insertion loss is less than 10%, and the signal-to-noise ratio in the reconstructed spectra is more than 1000.
Optics Letters 08/2012; 37(16):3384-6. · 3.40 Impact Factor
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ABSTRACT: We report a surprising observation that the output directionality from
wavelength-scale optical microcavities displays extreme sensitivity to
deformations of the cavity shape. A variation of the cavity boundary on the
order of ten thousandth of a wavelength may flip the output directions by 180
degrees. Our analysis based on a perturbation theory reveals that a tiny shape
variation can cause a strong mixing of nearly degenerate cavity resonances with
different angular momenta, and their interference determines the farfield
emission pattern. This work shows the possibility of utilizing
carefully-designed wavelength-scale microcavities for high-resolution detection
and sensing applications.
08/2012;
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ABSTRACT: Non-iridescent structural colours of feathers are a diverse and an important part of the phenotype of many birds. These colours are generally produced by three-dimensional, amorphous (or quasi-ordered) spongy β-keratin and air nanostructures found in the medullary cells of feather barbs. Two main classes of three-dimensional barb nanostructures are known, characterized by a tortuous network of air channels or a close packing of spheroidal air cavities. Using synchrotron small angle X-ray scattering (SAXS) and optical spectrophotometry, we characterized the nanostructure and optical function of 297 distinctly coloured feathers from 230 species belonging to 163 genera in 51 avian families. The SAXS data provided quantitative diagnoses of the channel- and sphere-type nanostructures, and confirmed the presence of a predominant, isotropic length scale of variation in refractive index that produces strong reinforcement of a narrow band of scattered wavelengths. The SAXS structural data identified a new class of rudimentary or weakly nanostructured feathers responsible for slate-grey, and blue-grey structural colours. SAXS structural data provided good predictions of the single-scattering peak of the optical reflectance of the feathers. The SAXS structural measurements of channel- and sphere-type nanostructures are also similar to experimental scattering data from synthetic soft matter systems that self-assemble by phase separation. These results further support the hypothesis that colour-producing protein and air nanostructures in feather barbs are probably self-assembled by arrested phase separation of polymerizing β-keratin from the cytoplasm of medullary cells. Such avian amorphous photonic nanostructures with isotropic optical properties may provide biomimetic inspiration for photonic technology.
Journal of The Royal Society Interface 05/2012; 9(75):2563-80. · 4.40 Impact Factor
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ABSTRACT: We show theoretically that coherent light can be completely absorbed and transferred to surface plasmons in a two- or three-dimensional metallic nanostructure by exciting it with the time-reversed mode of the corresponding surface plasmon laser ("spaser"). The narrow-band perfect absorption is a generalization and application of the concept of critical coupling to a nanocavity with surface plasmon resonances. Perfect coupling of light to nanostructures has potential applications to nanoscale probing as well as background-free spectroscopy and ultrasensitive detection or sensing.
Physical Review Letters 05/2012; 108(18):186805. · 7.37 Impact Factor
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ABSTRACT: The local chiral symmetry between clockwise (CW) and counter-clockwise (CCW)
propagating light in a deformed microcavity can be broken by wave optics
effects, which become significant as the cavity size approaches the wavelength.
We show that the spatial separation of the CW and CCW ray orbits underlying the
high quality factor resonant modes results in unidirectional emission in free
space. In the presence of a waveguide, evanescent coupling also becomes
directional, and the output direction can be varied by selecting the coupling
position along the cavity boundary. Our results demonstrate that the local
chirality can be utilized to control the output directionality and enhance the
collection efficiency of emission from ultrasmall resonators.
01/2012;
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ABSTRACT: We present a numerical study of the structural properties, photonic density of states and bandedge modes of Vogel spiral arrays of dielectric cylinders in air. Specifically, we systematically investigate different types of Vogel spirals obtained by the modulation of the divergence angle parameter above and below the golden angle value (≈137.507°). We found that these arrays exhibit large fluctuations in the distribution of neighboring particles characterized by multifractal singularity spectra and pair correlation functions that can be tuned between amorphous and random structures. We also show that the rich structural complexity of Vogel spirals results in a multifractal photonic mode density and isotropic bandedge modes with distinctive spatial localization character. Vogel spiral structures offer the opportunity to create novel photonic devices that leverage radially localized and isotropic bandedge modes to enhance light-matter coupling, such as optical sensors, light sources, concentrators, and broadband optical couplers.
Optics Express 01/2012; 20(3):3015-3033. · 3.59 Impact Factor
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ABSTRACT: We present a numerical study on photonic bandgap and band edge modes in the golden-angle spiral array of air cylinders in dielectric media. Despite the lack of long-range translational and rotational order, there is a large PBG for the TE polarized light. Due to spatial inhomogeneity in the air hole spacing, the band edge modes are spatially localized by Bragg scattering from the parastichies in the spiral structure. They have discrete angular momenta that originate from different families of the parastichies whose numbers correspond to the Fibonacci numbers. The unique structural characteristics of the golden-angle spiral lead to distinctive features of the band edge modes that are absent in both photonic crystals and quasicrystals.
Optics Express 11/2011; 19(24):23631-42. · 3.59 Impact Factor
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ABSTRACT: Structural colors are generated by scattering of light by variations in tissue nanostructure. They are widespread among animals and have been studied most extensively in butterflies and moths (Lepidoptera), which exhibit the widest diversity of photonic nanostructures, resultant colors, and visual effects of any extant organism. The evolution of structural coloration in lepidopterans, however, is poorly understood. Existing hypotheses based on phylogenetic and/or structural data are controversial and do not incorporate data from fossils. Here we report the first example of structurally colored scales in fossil lepidopterans; specimens are from the 47-million-year-old Messel oil shale (Germany). The preserved colors are generated by a multilayer reflector comprised of a stack of perforated laminae in the scale lumen; differently colored scales differ in their ultrastructure. The original colors were altered during fossilization but are reconstructed based upon preserved ultrastructural detail. The dorsal surface of the forewings was a yellow-green color that probably served as a dual-purpose defensive signal, i.e. aposematic during feeding and cryptic at rest. This visual signal was enhanced by suppression of iridescence (change in hue with viewing angle) achieved via two separate optical mechanisms: extensive perforation, and concave distortion, of the multilayer reflector. The fossils provide the first evidence, to our knowledge, for the function of structural color in fossils and demonstrate the feasibility of reconstructing color in non-metallic lepidopteran fossils. Plastic scale developmental processes and complex optical mechanisms for interspecific signaling had clearly evolved in lepidopterans by the mid-Eocene.
PLoS Biology 11/2011; 9(11):e1001200. · 11.45 Impact Factor
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ABSTRACT: Many imaging applications require increasingly bright illumination sources,
motivating the replacement of conventional thermal light sources with light
emitting diodes (LEDs), superluminescent diodes (SLDs) and lasers. Despite
their brightness, lasers and SLDs are poorly suited for full-field imaging
applications because their high spatial coherence leads to coherent artifacts
known as speckle that corrupt image formation. We recently demonstrated that
random lasers can be engineered to provide low spatial coherence. Here, we
exploit the low spatial coherence of specifically-designed random lasers to
perform speckle-free full-field imaging in the setting of significant optical
scattering. We quantitatively demonstrate that images generated with random
laser illumination exhibit higher resolution than images generated with
spatially coherent illumination. By providing intense laser illumination
without the drawback of coherent artifacts, random lasers are well suited for a
host of full-field imaging applications from full-field microscopy to digital
light projector systems.
10/2011;
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ABSTRACT: We demonstrate uni-directional evanescent coupling of lasing emission from a
wavelength-scale deformed microdisk to a waveguide. This is attributed to the
Goos-H\"anchen shift and Fresnel filtering effect that result in a spatial
separation of the clockwise (CW) and counter-clockwise (CCW) propagating ray
orbits. By placing the waveguide tangentially at different locations to the
cavity boundary, we may selectively couple the CW (CCW) wave out, leaving the
CCW (CW) wave inside the cavity, which also reduces the spatial hole burning
effect. The device geometry is optimized with a full-wave simulation tool, and
the lasing behavior and directional coupling are confirmed experimentally.
10/2011;
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ABSTRACT: Structural colours, the most intense, reflective and pure colours in nature, are generated when light is scattered by complex nanostructures. Metallic structural colours are widespread among modern insects and can be preserved in their fossil counterparts, but it is unclear whether the colours have been altered during fossilization, and whether the absence of colours is always real. To resolve these issues, we investigated fossil beetles from five Cenozoic biotas. Metallic colours in these specimens are generated by an epicuticular multi-layer reflector; the fidelity of its preservation correlates with that of other key cuticular ultrastructures. Where these other ultrastructures are well preserved in non-metallic fossil specimens, we can infer that the original cuticle lacked a multi-layer reflector; its absence in the fossil is not a preservational artefact. Reconstructions of the original colours of the fossils based on the structure of the multi-layer reflector show that the preserved colours are offset systematically to longer wavelengths; this probably reflects alteration of the refractive index of the epicuticle during fossilization. These findings will allow the former presence, and original hue, of metallic structural colours to be identified in diverse fossil insects, thus providing critical evidence of the evolution of structural colour in this group.
Proceedings of the Royal Society B: Biological Sciences 09/2011; 279(1731):1114-21. · 5.41 Impact Factor
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ABSTRACT: We present a numerical study on photonic bandgap and bandedge modes in the
golden-angle spiral array of air cylinders in dielectric media. Despite the
lack of long-range translational and rotational order, there is a large PBG for
the TE polarized light. Due to spatial inhomogeneity in the air hole spacing,
the bandedge modes are spatially localized by Bragg scattering from the
parastichies in the spiral structure. They have discrete angular momenta that
originate from different families of the parastichies whose numbers correspond
to the Fibonacci numbers. The unique structural characteristics of the
golden-angle spiral lead to distinctive features of the bandedge modes that are
absent in both photonic crystals and quasicrystals.
09/2011;
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ABSTRACT: We demonstrated lasing in two-dimensional trivalent network structures with short-range order. Despite the lack of translational and rotational symmetries, such structures possess a large isotropic photonic bandgap. Different from those of a photonic crystal, the band-edge modes are spatially localized and have high quality factor.
Optics Letters 09/2011; 36(18):3560-2. · 3.40 Impact Factor
