Diffusion Raman | EPFL Graph Search

La diffusion Raman, ou effet Raman, est un phénomène optique découvert indépendamment en 1928 par les physiciens Chandrashekhara Venkata Râman et Leonid Mandelstam. Cet effet consiste en la diffusion inélastique d'un photon, c'est-à-dire le phénomène physique par lequel un milieu peut modifier légèrement la fréquence de la lumière qui y circule. Ce décalage en fréquence correspond à un échange d'énergie entre le rayon lumineux et le milieu. Cet effet physique fut prédit par Adolf Smekal en 1923. Cet échange peut avoir plusieurs causes : vibrations du cristal ou de la molécule, excitations magnétiques... La mesure de ce décalage permet de remonter à certaines propriétés du milieu. On parle alors de Spectroscopie Raman. Cette technique est largement répandue dans l'industrie et la recherche depuis l'apparition du laser. Dans le cas particulier où la diffusion est due à des ondes acoustiques, on parle de diffusion Brillouin. Historique En 1922, le physicien indien Chandras

Non-classical photon-phonon correlations at room temperature

Santiago Tarrago Velez

With the development of quantum optics, photon correlations acquired a prominent role as a tool to test our understanding of physics, and played a key role in verifying the validity of quantum mechanics. The spatial and temporal correlations in a light field also reveal information about its origin, and allow us to probe the nature of the physical systems interacting with it. Additionally, with the advent of quantum technologies, they have acquired technological relevance, as they are expected to play an important role in quantum communication and quantum information processing.This thesis develops techniques that combine spontaneous Raman scattering with Time Correlated Single Photon Counting, and uses them to study the quantum mechanical nature of high frequency vibrations in crystals and molecules. We demonstrate photon bunching in the Stokes and anti-Stokes fields scattered from two ultrafast laser pulses, and use their cross-correlation to measure the 3.9 ps decay time of the optical phonon in diamond. We then employ this method to measure molecular vibrations in CS2, where we are able to excite the respective vibrational modes of the two isotopic species present in the sample in a coherent superposition, and observe quantum beating between the two signals. Stokes scattering, when combined with a projective measurement, leads to a well defined quantum state. We demonstrate this by measuring the second order correlation function of the anti-Stokes field conditional on detecting one or more photons in the Stokes field, which allows us to observe a phonon modeâs transition form a thermal state into the first excited Fock state, and measure its decay over the characteristic phonon lifetime. Finally, we use this technique to prepare a highly entangled photon-phonon state, which violates a Bell-type inequality. We measure S = 2.360 Â± 0.025, violating the CHSH inequality, compatible with the non-locality of the state.The techniques we developed open the door to the study of a broad range of physical systems, where spectroscopic information is obtained with the preparation of specific quantum states. They also hold potential for future technological use, and promote vibrational Raman scattering to a resource in nonlinear quantum optics -- where it used to be considered as a source of noise instead.

EPFL2021

3D Reconstruction of Optical Diffraction Tomography Based on a Neural Network Model

Morteza Hasani Shoreh

Optical tomography has been widely investigated for biomedical imaging applications. In recent years, it has been combined with digital holography and has been employed to produce high quality images of phase objects such as cells. In this Thesis, we look into some of the newest optical Diffraction Tomography (DT) based techniques to solve Three-Dimensional (3D) reconstruction problems and discuss and compare some of the leading ideas and papers. Then we propose a neural-network-based algorithm to solve this problem and apply it on both synthetic and biological samples. Conventional phase tomography with coherent light and off axis recording is performed. The Beam Propagation Method (BPM) is used to model scattering and each x-y plane is modeled by a layer of neurons in the BPM. The network's output (simulated data) is compared to the experimental measurements and the error is used for correcting the weights of the neurons (the refractive indices of the nodes) using standard error back-propagation techniques. The proposed algorithm is detailed and investigated. Then, we look into resolution-conserving regularization and discuss a method for selecting regularizing parameters. In addition, the local minima and phase unwrapping problems are discussed and ways of avoiding them are investigated. It is shown that the proposed learning tomography (LT) achieves better performance than other techniques such as, DT especially when insufficient number or incomplete set of measurements is available. We also explore the role of regularization in obtaining higher fidelity images without losing resolution. It is experimentally shown that due to overcoming multiple scattering, the LT reconstruction greatly outperforms the DT when the sample contains two or more layers of cells or beads. Then, reconstruction using intensity measurements is investigated. 3D reconstruction of a live cell during apoptosis is presented in a time-lapse format. At the end, we present a final comparison with leading papers and commercially available systems. It is shown that -compared to other existing algorithms- the results of the proposed method have better quality. In particular, parasitic granular structures and the missing cone artifact are improved. Overall, the perspectives of our approach are pretty rich for high-resolution tomographic imaging in a range of practical applications.

EPFL2018

Investigation of photoinduced effects in plasmonic nanocavities

Aqeel Ahmed

Light matter interaction can be boosted by several orders of magnitude by tailoring the photonic environment, thus enabling a wide range of applications. One particular example are plasmonic nanostructures that support localized surface plasmon polariton (LSPP) leading to enhancement and confinement of incident electromagnetic fields. These novel properties provide access to a large number of optically driven effects within matter placed in the vicinity of such nanostructures. In this thesis, we investigate the effects of optical excitation on gold nanostructures in the presence of Raman active molecules. This is accomplished by fabricating nanoparticle on mirror (NPoM) plasmonic nanostructures where a gold nanoparticle is placed at a fixed distance from a smooth gold film using molecular monolayers. As the nanoparticle and mirror are placed only a few nanometers apart, optical excitation leads to formation of hybridized LSPP modes due to coupling between the plasmons within the nanoparticle and the mirror. This plasmonic coupling also results in confinement and subsequent enhancement of the incident light within the gap and is extremely sensitive to structural parameters of the NPoM. The creation of a homogeneous gap between the nanoparticle and mirror is quite challenging due the roughness of the mirror. Moreover, due to the large number of NPoM nanostructures present on the sample it is quite difficult to locate a specific NPoM. To bypass these difficulties atomically smooth and patterned substrates were developed using template stripping combined with conventional photolithography techniques. Next, in order to characterize the optical response of the NPoMs, a custom optical setup was built. The setup was primarily designed to investigate the plasmonic properties of the NPoM using elastic scattering spectroscopy along with surface enhanced Raman scattering (SERS) from the molecules within the gap. The combination of these two techniques was later used to investigate the effect of laser power on the NPoMs. It was discovered that the scattering spectra of the NPoMs changed irreversibly after laser exposure even with a few ÎŒW/ÎŒm2 incident intensities. Moreover, the plasmonic response of the NPoMs fabricated with well organized molecular monolayer changed much faster than the NPoMs prepared with disorganized ones. This phenomenon was also investigated in NPoMs prepared using a dielectric gap layer, by changing the gap conductivity, and by changing the nanoparticle shape. The experimental results combined with simulations suggest a decrease in gap height due to the reorientation of the molecular monolayer under optical excitation.

EPFL2021

Non-classical photon-phonon correlations at room temperature

Santiago Tarrago Velez

EPFL2021

Investigation of photoinduced effects in plasmonic nanocavities

Aqeel Ahmed

EPFL2021

3D Reconstruction of Optical Diffraction Tomography Based on a Neural Network Model

Morteza Hasani Shoreh

EPFL2018