Physics

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Landau Seminar Series 2017-18

Wednesday 29 November 2017 - 14.00, N1.12 Haslegrave Building

Wednesday 22 November 2017 - 14.00, SMB104 Stewart Mason Building

Wednesday 08 November 2017 - 14.00, SMB104 Stewart Mason Building

Wednesday 25 October 2017 - 14.00, N1.12 Haslegrave Building

Spatial filtering of radiation from terahertz wire lasers, by Dr Ekaterina Orlova, Institute of Microwaves and Photonics, School of Electrical and Electronics Engineering, University of Leeds and Institute for Physics of Microstructures RAS, Russia University of Leeds

Abstract: Terahertz lasers are based on quantum cascade semiconductor hetero-structures (THz QCL-s) are attractive for numerous applications due to their compact size, high output power, ability to control the radiation spectrum by methods of band-gap engineering, and the possibility of continuous generation regime. However, broad application of such lasers is hindered due to peculiarities of their beam profile. Typical waveguides of THz QCL-s have a wire geometry with sub-wavelength transverse dimensions and the length much larger than the radiation wavelength (wire lasers). Sub-wavelength transverse size of laser waveguides leads to high radiation divergence. Additionally, strong intensity modulations accompanied by rapid phase shifts have been observed in the far field of such lasers [1]. It was shown that the far-field pattern of wire lasers is formed by the interference of radiation from the longitudinal distribution of sources along the laser waveguide [2]. Directive emission from THz QCL wire lasers has been achieved experimentally using gratings with appropriate periodicity, acting as a discrete array of phased sources [3]. However, this approach is not robust. Indeed, the modes producing directive beam exhibit higher radiation losses leading to an increase of the radiation threshold and to suppression of directive emission by competing modes. Here we review the methods of improvement of the beam profile of wire lasers based on spatial filtering of wire laser radiation by external optical elements. These methods include application of specially designed phase plates [5], formation of a limited divergence beam as an image of a wire laser placed along the axis of a spherical lens [6], and separation of one of the image field maxima with a diaphragm [7].

[1] A. J. L. Adam, et al., Appl. Phys. Lett., 88, 151105, (2006).

[2] E. E. Orlova, et al., Phys. Rev. Lett., 96, 173904 (2006).

[3] M. I. Amanti, et al., Nature Photonics, 3, 586, (2010)

[5] E. E. Orlova, Phys. Scr. T162, 014006 (2014).

[6] E. E. Orlova, J. N. Hovenier, P.J. de Visser, J. R. Gao, Phys. Rev. A, 91, 51802(R) (2015).

[7] E. E. Orlova, et al. Laser Physics Letters 14, 045001 (2017). 

Wednesday 18 October 2017 - 14.00, N.1.12 Haselgrave Building

Nonequilibrium dynamics and thermalisation of closed quantum systems of cold bosons, by Dr Anna Posazhennikova, Royal Holloway

Abstract: If and how an isolated quantum system thermalizes despite its unitary time evolution is a long-standing, open problem of many-body physics. We consider a Bose-Einstein condensate (BEC) trapped in a double-well potential with an initial population imbalance. We find that the system thermalizes under certain conditions. With respect to the non equilibrium dynamics we identify three regimes. After an initial regime of undamped Josephson oscillations, the subsystem of incoherent excitations or quasiparticles (QP) becomes strongly coupled to the BEC subsystem by means of a dynamically generated, parametric resonance. When the energy stored in the QP system reaches its maximum, the number of QPs becomes effectively constant, and the system enters a quasi-hydrodynamicregime where the two subsystems are weakly coupled. In this final regime the BEC acts as a grand-canonical heat reservoir for the QP system (and vice versa), resulting in thermalization. We term this mechanism dynamical bath generation (DBG). We discuss applications of our mechanism to optical lattices and realistic double-well potentials.  

Thursday 12 October 2017 - 11.00, WAV011 (Wavy Top Building)

Ferroic superglasses, by Prof. Wolfgang Kleemann, Physics Department, University Duisburg-Essen, Germany

Abstract: 'Ferroic superglasses' have entered mesoscopic solid state physics in different versions - as matrix isolated nanoparticular ‘superspin glasses’ [1] and as defect-activated intrinsic ‘ferroic glasses’ [2], respectively. Both undergo glassy dynamic criticality as T approaches Tg and non-ergodicity at T < Tg. While superspin glasses largely resemble atomic spin glasses [3], ferroic nanoregional glass states are proposed to emerge from random field-controlled nanoclustering and subsequent polar glass formation. Well-known examples are ferroelectric relaxors like PbMg1/3Nb2/3/O3 [4] and ferroelastic martensites like Ti48.5Ni51.5 [5]. Current approaches to modelling will be discussed.

 

[1] C. Djurberg, P. Svedlindh, P. Nordblad, M. F. Hansen, F. Bødker, and S. Mørup, Phys. Rev. Lett. 79 (1997) 5154.

[2] X. Ren, Y. Wang, K. Otsuka, P. Lloveras, T. Castán, M. Porta, A. Planes, and A. Saxena, MRS Bull. 34 (2009) 838.

[3] S. Bedanta and W. Kleemann, J. Phys. D: Appl. Phys. 42 (2009) 013001.

[4] W. Kleemann and J. Dec, Phys. Rev. B 94 (2016) 174203.

[5] S. Sarkar, X. Ren, and K. Otsuka, Phys. Rev. Lett. 95 (2005) 205702.

Wednesday 4 October 2017 - 11.00, SCH.0.13 (Schofield Building)

Wigner function reconstruction of experimental three-qubit GHZ and W states, by Professor Todd Tilma, Tokyo Institute of Technology

Abstract: Assessing the quality of a multi-qubit state is a multi-faceted problem, It involves considering disparate aspects, and, while some of these are collective indicators such as purity and degree of entanglement, others look more in detail at the form of the correlations between subsystems. Obtaining a visualisation of these details would give at least a qualitative indication, but the density matrix, with its large number of parameters, hardly offers insights. Here we show how relying on the Wigner function of a multipartite system provides rich information on the state, both at an illustrative level, by choosing proper slicing on the phase space, and at a quantitative one, by introducing new figures of merit for the state quality. We test this concept on a GHZ and W state produced in a photonic architecture making use of the multiple degrees of freedom of two photons.