342 research outputs found
Dissipative phase transition in systems with two-photon drive and nonlinear dissipation near the critical point
We study dissipative phase transition near the critical point for a system
with two-photon driving and nonlinear dissipation. The proposed mean-field
theory, which explicitly takes into account quantum fluctuations, allowed us to
describe properly the evolution dynamics of the system and to demonstrate the
new effects in the steady-state. We show that the presence of quantum
fluctuations leads to a power-law dependence of the anomalous average at the
phase transition point, with which the critical exponent is associated. Also,
we investigate the effect of the quantum fluctuations on the critical point
renormalization and demonstrate the existence of a two-photon pump threshold.
It is noteworthy that the obtained results are in a good agreement with the
numerical simulations.Comment: 7 pages, 4 figure
Modelling rotational failure in confined geometries using DLO
Discontinuity layout optimisation (DLO) is a generally applicable numerical limit analysis procedure that can be used to identify critical plastic collapse mechanisms in engineering problems. Considering the modelling of in-plane failure, the authors have previously presented a formulation capable of identifying rotational failure mechanisms in non-dilating media. However, the formulation presented did not explicitly address cases involving confined geometries, where curved slip lines could potentially intersect boundaries. In this paper, methods are outlined which permit efficient modelling of such cases. Details of the kinematic and equilibrium formulations are provided, which are then verified through application to various geotechnical and structural mechanics problems. It is shown that results of high accuracy can be obtained, both in terms of the predicted collapse load and the corresponding failure mechanism
Laser beams with conical refraction patterns
Laser beams with cone-refracted output from the plane mirror is demonstrated for the first time in lasers based on intracavity conical refraction (CR) phenomenon. Transverse profile of such lasers comprises a crescent ring of CR-like distribution, where any opposite points are of orthogonal linear polarizations. We confirm the existence of such mode of CR lasers by two observations. First, cascaded CR in reflection geometry has been demonstrated for first time and it provides experimental prove that a light beam passed along optic axis of a biaxial crystal, reflected back from a plane mirror and passed again through the crystal is restored. Second, CR cavity mode with CR-like pattern through the plane mirror is experimentally and theoretically demonstrated for the first time
QCL active region overheat in pulsed mode: effects of non-equilibrium heat dissipation on laser performance
Quantum cascade lasers are of high interest in the scientific community due
to unique applications utilizing the emission in mid-IR range. The possible
designs of QCL are quite limited and require careful engineering to overcome
some crucial disadvantages. One of them is an active region (ARn) overheat,
that significantly affects the laser characteristics in the pulsed operation
mode. In this work we consider the effects related to the non-equilibrium
temperature distribution, when thermal resistance formalism is irrelevant. We
employ the heat equation and discuss the possible limitations and structural
features stemming from the chemical composition of the AR. We show that the
presence of alloys in the ARn structure fundamentally limits the heat
dissipation in pulsed and CW regimes due to their low thermal conductivity.
Also the QCL post-growths affects the thermal properties of a device only in
(near)CW mode while it is absolutely invaluable in the pulsed mod
Partially coherent conical refraction promises new counter-intuitive phenomena
In this paper, we extend the paraxial conical refraction model to the case of the partially coherent light using the unified optical coherence theory. We demonstrate the decomposition of conical refraction correlation functions into well-known conical refraction coherent modes for a Gaussian Schell-model source. Assuming randomness of the electrical field phase of the input beam, we reformulated and significantly simplified the rigorous conical refraction theory. This approach allows us to consider the propagation of light through a conical refraction crystal in exactly the same way as in the classical case of coherent radiation. Having this in hand, we derive analytically the conical refraction intensity both in the focal plane and in the far field, which allows us to explain and rigorously justify earlier experimental findings and predict new phenomena. The last include the counterintuitive effect of narrowing of the conical refraction ring width, disappearance of the dark Poggendorffâs ring in the Lloydâs plane, and shift of Raman spots for the low-coherent conical refraction light. We also demonstrate a universal power-law dependence of conical refraction cones coherence degree on the input correlation length and diffraction-free propagation of the low-coherent conical refraction light in the far field
Conical refraction with low-coherence light sources
We report on conical refraction (CR) with low-coherence light sources, such as light-emitting diodes and decoherentized HeNe laser radiation, and demonstrate different CR patterns. In our experiments, a variation of the pinhole sizes from 25 to 100 ”m and the distances to pinhole from 50 to 5 cm reduced spatial coherence of radiation that resulted in the disappearance of the dark Poggendorffâs ring in the Lloydâs plane. This is attributed to the interference nature of the Lloydâs distribution and found to be in excellent agreement with the paraxial dual-cone model of conical refraction
3D laser nano-printing on fibre paves the way for super-focusing of multimode laser radiation
Multimode high-power laser diodes suffer from inefficient beam focusing, leading to a focal spot 10â100 times greater than the diffraction limit. This inevitably restricts their wider use in âdirect-diodeâ applications in materials processing and biomedical photonics. We report here a âsuper-focusingâ characteristic for laser diodes, where the exploitation of self-interference of modes enables a significant reduction of the focal spot size. This is achieved by employing a conical microlens fabricated on the tip of a multimode optical fibre using 3D laser nano-printing (also known as multi-photon lithography). When refracted by the conical surface, the modes of the fibre-coupled laser beam self-interfere and form an elongated narrow focus, usually referred to as a âneedleâ beam. The multiphoton lithography technique allows the realisation of almost any optical element on a fibre tip, thus providing the most suitable interface for free-space applications of multimode fibre-delivered laser beams. In addition, we demonstrate the optical trapping of microscopic objects with a super-focused multimode laser diode beam thus rising new opportunities within the applications sector where lab-on-chip configurations can be exploited. Most importantly, the demonstrated super-focusing approach opens up new avenues for the âdirect-diodeâ applications in material processing and 3D printing, where both high power and tight focusing is required
Di-chromatic InGaN based color tuneable monolithic LED with high color rendering index
We demonstrate a phosphor free, dichromatic GaN-based monolithic white LED with vertically stacked green and blue emitting multiple quantum wells. The optimal thickness of GaN barrier layer between green and blue quantum wells used is 8 nm. This device can be tuned over a wide range of correlated color temperature (CCT) to achieve warm white (CCT = 3600 K) to cool white (CCT = 13,000 K) emission by current modulation from 2.3 A/cm2 to 12.9 A/cm2. It is also demonstrated for the first time that a color rendering index (CRI) as high as 67 can be achieved with such a dichromatic source. The observed CCT and CRI tunability is associated with the spectral power evolution due to the pumping-induced carrier redistribution
Optical trapping with superfocused high-M2 laser diode beam
Many applications of high-power laser diodes demand tight focusing. This is often not possible due to the multimode nature of semiconductor laser radiation possessing beam propagation parameter M2 values in double-digits. We propose a method of 'interference' superfocusing of high-M2 diode laser beams with a technique developed for the generation of Bessel beams based on the employment of an axicon fabricated on the tip of a 100 ÎŒm diameter optical fiber with highprecision direct laser writing. Using axicons with apex angle 140Âș and rounded tip area as small as 10 ÎŒm diameter, we demonstrate 2-4 ÎŒm diameter focused laser 'needle' beams with approximately 20 ÎŒm propagation length generated from multimode diode laser with beam propagation parameter M2=18 and emission wavelength of 960 nm. This is a few-fold reduction compared to the minimal focal spot size of 11 ÎŒm that could be achieved if focused by an 'ideal' lens of unity numerical aperture. The same technique using a 160Âș axicon allowed us to demonstrate few-ÎŒm-wide laser 'needle' beams with nearly 100 ÎŒm propagation length with which to demonstrate optical trapping of 5-6 ÎŒm rat blood red cells in a water-heparin solution. Our results indicate the good potential of superfocused diode laser beams for applications relating to optical trapping and manipulation of microscopic objects including living biological objects with aspirations towards subsequent novel lab-on-chip configurations
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