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Coherent transmission of light through a cold atomic cloud

Chang Chi Kwong

Published Jan 1, 2017• 1 citation
Publisher

Abstract

We study the coherent transmission of light through cold strontium atomic clouds, using the narrow intercombination line. In the first part of the thesis, the cooperative emission in the coherent transmission is studied experimentally. By applying fast changes to the incident probe field, flashes of light are observed in the forward direction. The peak intensity of the flashes can be much larger than the incident intensity. These flashes are called superflashes. Due to cooperative effect, the time scale of the flashes are shortened with respect to the single scatterer lifetime. By packing these flashes, we generate a pulse train with short repetition time, where the dynamics can almost be fully governed by cooperative effect. The effect of atomic density on the coherent transmission of light is also studied theoretically and numerically. The effective refractive index of the cloud is calculated up to the second order in atomic density. It is eventually validated with a numerical simulation of the coherent light transmission. The second part of the thesis focuses on the effect of magnetic field on the coherent transmission of light. The strontium intercombination line has a sensitivity in the milligauss regime, which is the typical value of stray magnetic field in the lab. An active magnetic field control system is implemented to cancel the low frequency and AC 50~Hz stray magnetic field. Using this control system, we study the nonlinear magneto-optical rotation (NMOR) effect due to a high intensity light field transmitting through a dilute and thin cloud of strontium atom. Interestingly, at sufficiently high cloud temperature, the NMOR effect can have a higher sensitivity to the magnetic field, compared to its linear counterpart at the same cloud temperature.

DOI
10.32657/10356/69897

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We study the coherent transmission of light through cold strontium atomic clouds, using the narrow intercombination line. In the first part of the thesis, the cooperative emission in the coherent transmission is studied experimentally. By applying fast changes to the incident probe field, flashes of light are observed in the forward direction. The peak intensity of the flashes can be much larger than the incident intensity. These flashes are called superflashes. Due to cooperative effect, the time scale of the flashes are shortened with respect to the single scatterer lifetime. By packing these flashes, we generate a pulse train with short repetition time, where the dynamics can almost be fully governed by cooperative effect. The effect of atomic density on the coherent transmission of light is also studied theoretically and numerically. The effective refractive index of the cloud is calculated up to the second order in atomic density. It is eventually validated with a numerical simulation of the coherent light transmission. The second part of the thesis focuses on the effect of magnetic field on the coherent transmission of light. The strontium intercombination line has a sensitivity in the milligauss regime, which is the typical value of stray magnetic field in the lab. An active magnetic field control system is implemented to cancel the low frequency and AC 50~Hz stray magnetic field. Using this control system, we study the nonlinear magneto-optical rotation (NMOR) effect due to a high intensity light field transmitting through a dilute and thin cloud of strontium atom. Interestingly, at sufficiently high cloud temperature, the NMOR effect can have a higher sensitivity to the magnetic field, compared to its linear counterpart at the same cloud temperature.
An optically thick cold atomic cloud emits a coherent flash of light in the forward direction when the phase of an incident probe field is abruptly changed. Because of cooperativity, the duration of this phenomena can be much shorter than the excited lifetime of a single atom. Repeating periodically the abrupt phase jump, we generate a train of pulses with short repetition time, high intensity contrast, and high efficiency. In this regime, the emission is fully governed by cooperativity even if the cloud is dilute.
The problem of the interaction of N atoms with a laser beam and vacuum modes can give rise to many interesting phenomena concerning the spontaneous emission of light and its propagation in the medium. The cooperative effects, for example, such as superadiance and subradiance, are effects related to the coherence created between the atoms when a photon is emitted spontaneously by a single excited atom. Superradiance can be defined as the enhancement of the spontaneous emission due to constructive interference of the scattered light. Its counterpart, subradiance, is the trapping of some remaining light due to destructive interference. In cold atoms, some previous theoretical works predict and characterize these two cooperative effects in a large and diluted atomic cloud, in the regime of low intensities and large detunings of the incident laser. The theoretical model is a coupled-dipole model for two-level atoms driven by a low-intensity field and in the scalar approach. The experiment consists in measuring the super- and subradiant decay rates from the temporal emitted intensity after the switch off of the incident laser in the steady state. Our experimental setup consists in a magneto-optical trap of rubidium 87 atoms at large resonant optical thicknesses. A probe beam excites the atoms close to the D2 line. The intensity emitted is detected by a single photon detector with no afterpulsing and a calibration procedure allows us to determine the resonant optical thickness of the cloud and its temperature. In this work, we report the experimental observation of super- and subradiance in a large cloud of cold atoms. For subradiance, the main result is the linear evolution of the characteristic time with the resonant optical thickness of the cloud and its independence of the detuning. For superradiance, we observe superradiance out of the forward direction. We verify the validity of our interpretations with the predictions of the coupled-dipole model. Finally, we discuss the interplay of subradiance and radiation trapping, as well as theoretical predictions for: a setup of a phased cloud, to control the subradiant amplitude emission; and temperature effects, where subradiance is shown to be robust in a large range of temperatures.
The objective of this thesis is to expand our understanding of collective light-scattering phenomena, which manifest themselves in the strong suppression of radiation from the scattering medium. In particular, we are interested in subradiance, a collective effect in spontaneous emission which, as a result of the dissipative coupling of atoms through common electromagnetic modes, creates long-lived excitation modes in an ensemble of cold atoms, with lifetimes much longer than those of individual, isolated atoms. Previously, subradiance had been well-characterized for dilute cold-atom scattering media and the low-intensity limit of the driving laser (linear-optics regime). In this thesis, we study this collective phenomenon beyond the linear-optics and dilute regimes.To study experimentally subradiance beyond the linear-optics regime, we varied the saturation parameter of the excitation laser up to relatively large values and we have observed the increase of the subradiant population with increasing saturation parameter. More importantly, we have identified the super-linear scaling of the subradiant population beyond the linear-optics regime, due to a process similar to optical pumping via multi-excitation superradiant states. This study showed that the increase in the intensity of the driving laser beyond the linear-optics regime can overcome the weak coupling of subradiant modes with the external world.Moreover, we have numerically studied subradiance using the vectorial coupled-dipole model, with a focus on understanding the effect of the near-field dipole-dipole interaction. For that, we extensively compared the vectorial and scalar (which does not contain near-field interaction terms) models and studied the subradiant lifetime as a function of the density and optical thickness of the cold-atom cloud. We observed that the near-field terms are detrimental for subradiance, which was seen in the reduction of the subradiant lifetimes with increasing density. This detrimental character of near-field terms can be interpreted as being due to an effective inhomogeneous broadening induced by the near-field interaction, which reduces the subradiant lifetimes as the density increases.The detrimental character of near-field interaction has also been previously numerically identified for Anderson localization of light waves (Anderson localization is a coherent multiple-scattering wave phenomenon in a strongly scattering medium that results in a cessation of a diffusive wave transport; it also yields long-lived modes): the scalar coupled-dipole model predicts Anderson localization of light waves, but not the vectorial one. However, adding a strong magnetic field allows one to recover localized modes for vectorial light. In this context, we numerically studied a possible signature of Anderson localization of light in cold atoms, based on the measurement of intensity fluctuations of the scattered light. We have shown that at the transition to the localized regime, the fluctuations are enhanced. In particular, for vectorial light, we have demonstrated the absence and recovery of enhanced intensity fluctuations without and with the applied magnetic field, which further corroborates that the intensity fluctuations could serve as an experimental way to probe the localized states.
A cold atomic cloud is a versatile object, because it offers many handles to control and tune its properties. This facilitates studies of its behavior in various circumstances, such as sample temperature, size and density, composition, dimensionality and coherence time. The range of possible experiments is constrained by the specifications of the atomic species used. In this thesis presents the work done in the experiment for laser cooling of strontium atoms, focusing on its stability, which should provide cold and ultracold samples for the study of collective effects in light scattering. From the initial apparatus, innumerous changes were performed. The vacuum system got improved and now reached lower ultra high vacuum due to the pre-baking done to its parts and adding a titanium-sublimation stage. The quadrupole trap were improved by the design and construction of a new pair of coils. The stability of the blue, green and red laser systems and the loss prevention of laser light were improved, giving rise to a robust apparatus. Another important point is the development of homemade devices to reduce the costs and to be used as a monitor of different parts of an cold atoms experiment. From this homemade devices, we could demonstrate a dramatic linewidth narrowing by injection lock of an low cost 461 nm diode laser and its application to our strontium experiment. In the end, this improved experimental apparatus made possible the study of a new scattering effect, the mirror assisted coherent back-scattering (mCBS).