192 research outputs found
Perspectives and recent advances in super-resolution spectroscopy: Stochastic and disordered-based approaches
Spectroscopic applications are characterized by the constant effort to combine high spectral resolution with large bandwidth. A trade-off typically exists between these two aspects, but the recent development of super-resolved spectroscopy techniques is bringing new opportunities into this field. This is particularly relevant for all applications where compact and cost-effective instruments are needed such as in sensing, quality control, environmental monitoring, or biometric authentication, to name a few. These unconventional approaches exploit several strategies for spectral investigation, taking advantage of concepts such as sparse sampling, artificial intelligence, or post-processing reconstruction algorithms. In this Perspective, we discuss the main strengths and weaknesses of these methods, tracing promising future directions for their further development and widespread adoption. Published under an exclusive license by AIP Publishing
Free-form Light Actuators - Fabrication and Control of Actuation in Microscopic Scale
Liquid crystalline elastomers (LCEs) are smart materials capable of reversible shape-change in response to external stimuli, and have attracted researchers' attention in many fields. Most of the studies focused on macroscopic LCE structures (films, fibers) and their miniaturization is still in its infancy. Recently developed lithography techniques, e.g., mask exposure and replica molding, only allow for creating 2D structures on LCE thin films. Direct laser writing (DLW) opens access to truly 3D fabrication in the microscopic scale. However, controlling the actuation topology and dynamics at the same length scale remains a challenge.
In this paper we report on a method to control the liquid crystal (LC) molecular alignment in the LCE microstructures of arbitrary three-dimensional shape. This was made possible by a combination of direct laser writing for both the LCE structures as well as for micrograting patterns inducing local LC alignment. Several types of grating patterns were used to introduce different LC alignments, which can be subsequently patterned into the LCE structures. This protocol allows one to obtain LCE microstructures with engineered alignments able to perform multiple opto-mechanical actuation, thus being capable of multiple functionalities. Applications can be foreseen in the fields of tunable photonics, micro-robotics, lab-on-chip technology and others
Photon statistics of a random laser
A general relationship is presented between the statistics of thermal
radiation from a random medium and its scattering matrix S. Familiar results
for black-body radiation are recovered in the limit S to 0. The mean photocount
is proportional to the trace of 1-SS^dagger, in accordance with Kirchhoff's law
relating emissivity and absorptivity. Higher moments of the photocount
distribution are related to traces of powers of 1-SS^dagger, a generalization
of Kirchhoff's law. The theory can be applied to a random amplifying medium (or
"random laser") below the laser threshold, by evaluating the Bose-Einstein
function at a negative temperature. Anomalously large fluctuations are
predicted in the photocount upon approaching the laser threshold, as a
consequence of overlapping cavity modes with a broad distribution of spectral
widths.Comment: 26 pages, including 9 figure
Statistical Signatures of Photon Localization
The realization that electron localization in disordered systems (Anderson
localization) is ultimately a wave phenomenon has led to the suggestion that
photons could be similarly localized by disorder. This conjecture attracted
wide interest because the differences between photons and electrons - in their
interactions, spin statistics, and methods of injection and detection - may
open a new realm of optical and microwave phenomena, and allow a detailed study
of the Anderson localization transition undisturbed by the Coulomb interaction.
To date, claims of three-dimensional photon localization have been based on
observations of the exponential decay of the electromagnetic wave as it
propagates through the disordered medium. But these reports have come under
close scrutiny because of the possibility that the decay observed may be due to
residual absorption, and because absorption itself may suppress localization.
Here we show that the extent of photon localization can be determined by a
different approach - measurement of the relative size of fluctuations of
certain transmission quantities. The variance of relative fluctuations
accurately reflects the extent of localization, even in the presence of
absorption. Using this approach, we demonstrate photon localization in both
weakly and strongly scattering quasi-one-dimensional dielectric samples and in
periodic metallic wire meshes containing metallic scatterers, while ruling it
out in three-dimensional mixtures of aluminum spheres.Comment: 5 pages, including 4 figure
Resonance optimization of polychromatic light in disordered structures
Disorder offers rich possibilities for manipulating the phase and intensity of light and designing photonic devices for various applications including random lasers, light storage, and speckle-free imaging. Disorder-based optical systems can be implemented in one-dimensional structures based on random or pseudo-random alternating layers with different refractive indices. Such structures can be treated as sequences of scatterers, in which spatial light localization is characterized by random sets of spectral transmission resonances, each accompanied by a relatively high-intensity concentration. The control and manipulation of resonances is the key element in designing disorder-based photonic systems. In this work, we introduce a method of controlling disorder-induced resonances by using the established non-trivial interconnection between the symmetry of bi-directional light propagation properties and the features of the resonant transmissions. Considering a fiber with resonant Bragg gratings as an example, the mechanism of enhancing or suppressing the resonant transmission of polychromatic light and the effectiveness of the method have been demonstrated both theoretically and experimentally. The proposed algorithm of controlling disorder-induced resonances is general and applicable to classical waves and quantum particles, for disordered systems both with and without gain
Photoresponsive Polymer-Based Biomimetic Contractile Units as Building Block for Artificial Muscles
Loss of muscular mechanical function occurs in several diseases affecting millions of people worldwide, including heart failure, stroke, and neuromuscular disorders. To date, no medical or surgical treatments can restore muscular contractility, and the development of artificial muscles is of extreme interest. Mimicking biological muscles, which are optimized systems displaying quick reaction times, is not trivial; only few examples are reported, mainly focused on the use of biomimetic smart materials. Among them, liquid crystalline elastomers (LCEs) can be biocompatible, show contraction parameters comparable to those of native striated muscles, and are able to effectively potentiate cardiac contraction in vitro. To go further and develop in vivo implantable devices, the integration of the stimulation system with the LCE material represents an essential step. Here, a light-stimulated biomimetic contractile unit (BCU), combining ultra-thin photoresponsive LCE films and mini-LED (mLED) matrixes is described. BCU performance (in terms of extent and kinetics of contractile force and shortening) can be fine-tuned by modulating both mLED light power and spatial stimulation patterns, allowing to reproduce mechanical dynamics of native muscles. These results pave the way for the development of novel LCE-based contraction assist devices for cardiac, skeletal, or smooth muscle support by assembling multiple BCUs
Broadband random optoelectronic oscillator
[EN] Random scattering of light in transmission media has attracted a great deal of attention in the field of photonics over the past few decades. An optoelectronic oscillator (OEO) is a microwave photonic system offering unbeatable features for the generation of microwave oscillations with ultra-low phase noise. Here, we combine the unique features of random scattering and OEO technologies by proposing an OEO structure based on random distributed feedback. Thanks to the random distribution of Rayleigh scattering caused by inhomogeneities within the glass structure of the fiber, we demonstrate the generation of ultra-wideband (up to 40¿GHz from DC) random microwave signals in an open cavity OEO. The generated signals enjoy random characteristics, and their frequencies are not limited by a fixed cavity length figure. The proposed device has potential in many fields such as random bit generation, radar systems, electronic interference and countermeasures, and telecommunications.Thanks N. Shi and Y. Yang for comments and discussion. This work was supported by the National Key Research and Development Program of China under 2018YFB2201902 and the National Natural Science Foundation of China under 61925505. This work was also partly supported by the National Key Research and Development Program of China under 2018YFB2201901, 2018YFB2201903, and the National Natural Science Foundation of China under 61535012 and 61705217.Ge, Z.; Hao, T.; Capmany Francoy, J.; Li, W.; Zhu, N.; Li, M. (2020). Broadband random optoelectronic oscillator. Nature Communications. 11(1):1-8. https://doi.org/10.1038/s41467-020-19596-xS18111Feng, S., Kane, C., Lee, P. A. & Stone, A. D. Correlations and fluctuations of coherent wave transmission through disordered media. Phys. Rev. Lett. 61, 834 (1988).Wiersma, D. 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Bright-white beetle scales optimise multiple scattering of light
This is the final version of the article. Available from the publisher via the DOI in this record.Error in funder statement in this article is corrected in http://hdl.handle.net/10871/22212Whiteness arises from diffuse and broadband reflection of light typically achieved through optical scattering in randomly structured media. In contrast to structural colour due to coherent scattering, white appearance generally requires a relatively thick system comprising randomly positioned high refractive-index scattering centres. Here, we show that the exceptionally bright white appearance of Cyphochilus and Lepidiota stigma beetles arises from a remarkably optimised anisotropy of intra-scale chitin networks, which act as a dense scattering media. Using time-resolved measurements, we show that light propagating in the scales of the beetles undergoes pronounced multiple scattering that is associated with the lowest transport mean free path reported to date for low-refractive-index systems. Our light transport investigation unveil high level of optimisation that achieves high-brightness white in a thin low-mass-per-unit-area anisotropic disordered nanostructure.We wish to thank R. Blumenfeld, T. Svensson, R. Savo and K. Vynck for fruitful discussions,
B.D. Wilts for the comments on the manuscript and J. Aizenberg for support in the SEM
measurements. The research leading to these results has received funding from the
European Research Council under the European Union’s Seventh Framework Programme
(FP7/2007–2013)/ERC grant agreement n [291349] and USAF grant FA9550-10-1-002
ERRATUM: Bright-white beetle scales optimise multiple scattering of light.
Original article available via doi:10.1038/srep06075Erratum: Scientific Reports 4, Article number: 6075 (2014); Published: 15 August 2014; Updated: 19 December 2014
doi:10.1038/srep06075. This Article contains an error in the Acknowledgements section
Optical parametric oscillation with distributed feedback in cold atoms
There is currently a strong interest in mirrorless lasing systems, in which
the electromagnetic feedback is provided either by disorder (multiple
scattering in the gain medium) or by order (multiple Bragg reflection). These
mechanisms correspond, respectively, to random lasers and photonic crystal
lasers. The crossover regime between order and disorder, or correlated
disorder, has also been investigated with some success. Here, we report
one-dimensional photonic-crystal lasing (that is, distributed feedback lasing)
with a cold atom cloud that simultaneously provides both gain and feedback. The
atoms are trapped in a one-dimensional lattice, producing a density modulation
that creates a strong Bragg reflection with a small angle of incidence. Pumping
the atoms with auxiliary beams induces four-wave mixing, which provides
parametric gain. The combination of both ingredients generates a mirrorless
parametric oscillation with a conical output emission, the apex angle of which
is tunable with the lattice periodicity
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