Classical soft graviton theorem rewritten

Classical soft graviton theorem gives the gravitational wave-form at future null infinity at late retarded time u for a general classical scattering. The large u expansion has three known universal terms: the constant term, the term proportional to 1/u and the term proportional to ln u/u(2), whose coefficients are determined solely in terms of the momenta of incoming and the outgoing hard particles, including the momenta carried by outgoing gravitational and electromagnetic radiation produced during scattering. For the constant term, also known as the memory effect, the dependence on the momenta carried away by the final state radiation / massless particles is known as non-linear memory or null memory. It was shown earlier that for the coefficient of the 1/u term the dependence on the momenta of the final state massless particles / radiation cancels and the result can be written solely in terms of the momenta of the incoming particles / radiation and the final state massive particles. In this note we show that the same result holds for the coefficient of the ln u/u(2) term. Our result implies that for scattering of massless particles the coefficients of the 1/u and ln u/u(2) terms are determined solely by the incoming momenta, even if the particles coalesce to form a black hole and massless radiation. We use our result to compute the low frequency flux of gravitational radiation from the collision of massless particles at large impact parameter.

Coherence properties of microcavity polaritons

Stefan Kundermann

This thesis presents the coherence properties of polaritons in semiconductor microcavities. Semiconductor microcavities are microstructures in which the exciton ground state of a semiconductor quantum well is coupled to a photonic mode of a microresonator. The strong coupling mixes the character of excitons and photons, giving rise to the lower and upper polariton branches, quasiparticles with an unusual energetic dispersion relation due to the extreme mass difference between exciton and photon. Particularly special is the dispersion of the lower polariton, which forms a dip in the 2-dimensional k-space around the lowest energy state with zero in-plane momentum. In this dip, which can be seen as a trap in momentum space, the polaritons are efficiently isolated from dephasing mechanisms involving phonons. Polaritons can be resonantly excited at desired points on the polariton dispersion by shining on the microcavity laser light at the appropriate angle and wavelength. Polaritons can interact and scatter pairwise with each other conserving energy and in plane momentum k, a process similar to parametric scattering of photons in a nonlinear crystal. One polariton from a pump reservoir scatters down to the signal state at k = 0 (corresponding to normal incidence) and a second takes away the excess energy and momentum of the first and scatters up to the idler position ({kP, kP} → {0, 2kP }). This process can be stimulated by a small amount of signal polaritons injected with a probe laser beam at normal incidence. Here the coherence properties of the polariton parametric scattering have been investigated using spectroscopy techniques sensitive to the optical phase, for example coherent control with phase-locked femtosecond probe pulses. Just above the threshold for the stimulated parametric scattering, the parametric amplification process is given by the linear superposition of the individual amplification processes of each probe pulse. The emission of signal, pump, and idler can be controlled by tuning the relative phase of the 150fs-long probe pulses, which are separated by a few picoseconds in time. Experiments are presented that deal with the real-time dynamics of the parametric scattering in the spontaneous and the stimulated regime. It is shown, that in the spontaneous regime the scattering is started by a small amount of polaritons which have relaxed to the band bottom by emitting phonons. In the regime where polariton scattering is stimulated by an external probe, the rise of the signal intensity is delayed with respect to the arrival time of both pump and probe, a feature that can be attributed to the complex phase-matching mechanism for the parametric scattering. In the second part of the thesis, the spontaneous build up of a macroscopic coherence in a CdTe microcavity under non-resonant laser excitation is analysed. The build up of a long-range spatial coherence easily exceeding the thermal wavelength of the polaritons is shown. This is the hallmark of Bose-Einstein condensation and the proof of a macroscopic wavefunction. Experimental data on the statistical distribution of the polaritons in time, the polarisation of the non-linear emission, and the quantum transition from a thermal to a coherent state1 confirm that Bose-Einstein condensation of microcavity polaritons has been observed. We regard these observations as the first bullet-proof evidence for spontaneous Bose-Einstein condensation in a solid state system, a phenomenon that has been the subject to many investigations and controversies during the past four decades. ------------------------------ 1 The data about the statistical distribution, the polarisation, and the transition from a thermal to a coherent state is by courtesy of Jacek Kasprzak of the University of Grenoble.

EPFL2006

Comparison of scattering and reflection SFG: a question of phase-matching

Hilton Barbosa De Aguiar, Kailash Chandra Jena, Sylvie Roke, Rüdiger Scheu

We present a comparison between sum frequency scattering (SFS) and reflection mode sum frequency generation (R-SFG). We have used scattering theory to describe both scattering experiments as well as reflection mode experiments. The interfacial vibrational spectrum of nanoscopic oil droplets dispersed in water was probed with SFS as well as with R-SFG. Spectra recorded in phase-matched R-SFG mode and spectra recorded with SFS from the same sample are different, which shows that different interfaces are measured. Scattering spectra at different scattering angles agree with nonlinear light scattering theory. We further present experiments with polymer films aimed at quantifying the comparative strength of R-SFG and SFS experiments.

2012

Optical microscopy for imaging through scattering media

Xin Yang

Optical microscopy has been widely used in the life sciences and biological research for several decades. It possesses many advantages over x-ray radiography, computed tomography, ultrasound imaging and magnetic resonance imaging, in terms of being low cost, high resolution, harmless and capable of multiple contrast mechanism, etc. However, the weakness of optical microscopy is the inability to perform deep tissue imaging. Owing to the short wavelength of optical waves, it experiences much more scattering than longer waves in inhomogeneous materials such as biological tissues. Scattering scrambles the wavefront and limits the penetration depth of optical microscopy to only hundreds of microns. We explore in this thesis, novel optical approaches to image through scattering media. There are two main categories of the state of art techniques: imaging with ballistic photons, which requires separation of ballistic photons from the sneak or multiple scattered ones; and imaging with scattered photons, which relies on manipulation of the scattered photons according to the specific scattering properties. We primarily focus on the second category in this thesis. Taking advantage of the reversibility of light, phase conjugation has a long history in aberration compensation. A deterministic wave is incident on a scattering medium and results in a random scattered field. If we record the full information of the scattered field (amplitude and phase) and generate a phase conjugated replica of it, the phase conjugated wave can follow the same path and reconstruct the original deterministic wave after passing back through the scattering medium. In this work, phase conjugation is achieved using digital method, called digital phase conjugation. The scattered field is recorded through digital holography, and a spatial light modulator is applied to construct the phase conjugated wave. Second harmonic radiation imaging probes are used as the original light source, rendering the generation of a focus behind the scattering medium. The memory effect, which indicates the correlation between speckles with different incident angles, is exploited to move the focus around and obtain three-dimensional scanning microscopy behind scattering medium. Speckle scanning microscopy is the second method demonstrated in this thesis for imaging through turbid media. In contrast to digital phase conjugation, which requires access from both sides of the scatterers to measure the scattered field, in speckle scanning microscopy, excitation and collection can be achieved from the same side. The specific scattering events are not taken into account, instead, the statistic property of the scatterers plays a major role. This technique makes full use of two facts: 1. The statistically averaged autocorrelation of speckle fields approaches a delta function; 2. When a speckle pattern scans across a fluorescence object, the collected fluorescence intensity is proportional to the convolution of the speckle field and the object. Linking these two facts with delicate calculation, we can eventually minimize the influence of the speckle, and reconstruct two-dimensional images of objects behind turbid medium. Encouraged by the success of two-dimensional speckle scanning microscopy, we investigate more advanced possibilities by simulation, including three-dimensional speckle scanning and reference based speckle scanning techniques. [...]