The radiation and absorption responses of atoms in strong laser fields for quantum probes are studied using the strong-field quantum optical method. It was shown that the emission and absorption are related, respectively, to the positive and negative frequencies of the current's autocorrelator, which can be separated into the coherent part from the expectation of the current operator and the incoherent part from its quantum fluctuations. While the current expectation determines the response to coherent-state probes, the current fluctuations have a significant impact on the response to Fock-state probes. Our studies extend the pump-probe scheme of strong-field physics into the quantum optical regime.
The radiation and absorption responses of atoms in strong laser fields for quantum probes are studied using the strong-field quantum optical method. It was shown that the emission and absorption are related, respectively, to the positive and negative frequencies of the current's autocorrelator, which can be separated into the coherent part from the expectation of the current operator and the incoherent part from its quantum fluctuations. While the current expectation determines the response to coherent-state probes, the current fluctuations have a significant impact on the response to Fock-state probes. Our studies extend the pump-probe scheme of strong-field physics into the quantum optical regime.
We develop a quantum optical description of radiation from a two-level system (TLS) in strong laser fields, which provides a clear insight into the final states of the TLS and the harmonics field. It is shown that there are two emission channels: the Rayleigh-like channel and the Raman-like channel, which correspond to the TLS ending up in the ground state and excited state after the emission, respectively. The numerical result shows that the harmonics are mainly produced by the Rayleigh-like channel. In addition, according to the coherence of emission among the emitters, the radiation is divided into coherent parts that result from the semi-classical dipole oscillation and incoherent parts that result from the quantum fluctuations of the dipole moment. In the weak field limits, the Rayleigh-like channel corresponds to the coherent parts, and the Raman-like channel corresponds to the incoherent parts. However, in strong laser fields, both channels contribute to coherent and incoherent radiation, and how much they contribute depends on the final excitation. By manipulating the laser field, we can make the Rayleigh-like channel produce either coherent or incoherent radiation.
Light scattered by a regular structure of atoms exhibits spatial interference signatures, similar to Young's classical double-slit experiment. The first-order interferences, however, are known to vanish for strong light intensities, where the incoherently fluctuating part of the emitted light dominates. Here, we show how to overcome these limitations to quantum interference in stronger laser fields, and how to recover the first-order interference in strong fields by a tailored electromagnetic vacuum with a suitable frequency dependence. We also discuss higher-order correlation functions of the scattered light, with applications, e.g., to lithography. In the second part, we study light propagation of a probe field pulse in closed-loop atomic systems. The closed interaction loop induces a sensitivity to the relative driving field phase, but in general prohibits a stationary steady state. In particular, the finite frequency width of the short probe pulse requires a time-dependent analysis beyond the so-called multiphoton resonance assumption. Using a Floquet decomposition, we identify the different contributions to the medium response, and demonstrate sub- and superluminal light propagation with small absorption or even gain, where a coupling field Rabi frequency allows to switch between sub- and superluminal light propagation.
The fundamental source of noise in laser and optical amplifier is spontaneous emission. However, it is possible to reduce this noise by preparing the gain medium (atoms) in coherent superposition of states. There are essentially two schemes for generating atomic coherence in optical amplifiers. \n \nFirst scheme involves the preparation of atoms in a coherent superposition of states before injecting them inside the cavity. In the other scheme atoms (initially pumped incoherently) are continuously driven by an external pump field inside the cavity. Almost all the studies concerning the quenching of spontaneous emission noise in optical amplifiers, based on the notion of atomic coherent superposition, ignore the finite bandwidth of the field that is responsible for generating coherence. In this dissertation, we have examined the effect of the finite bandwidth associated with the phase fluctuations of the driving field on a number of noise quenching schemes. \n \nIt is known that coherent preparation of atoms can, under certain conditions, lead to noise-free amplification and almost 50 percent phase squeezing in a two-photon correlated emission laser. We study the effect of phase fluctuations in the injected atoms on the phase noise of the field in these systems. We discuss the conditions under which squeezing is preserved in the two-photon linear amplifiers. We also show that, even for very large phase fluctuations in the injected atoms, almost 50 percent phase squeezing can be achieved. The corresponding gain is however significantly reduced. \n \nWe also study a system of phase-sensitive amplifier where atoms are prepared in a coherent superposition of states by a strong classical field. We study the role of pump phase fluctuations associated with the finite linewidth on the quantum statistical properties of the radiation field. A study of output field characteristics reveals that the presence of a weak pump field gives rise to equal variances in both the field quadratures which are independent of linewidth. On the other hand, the presence of strong pump field alters the output field characteristics with the generation of squeezing which is later on destroyed due to the presence of phase fluctuations
The absorption spectrum of a weak probe radiation by a two-level atom strongly driven by an intense radiation field is examined. Using a Green's function method developed for calculations of general multiphoton processes, the author obtains some earlier results in which the spectrum has regions (in the probe frequency domain) of amplification and regions of attenuation. Such a spectrum is shown explicitly to be due to a coherent interference effect arising from the interaction of the atom with the vacuum field. Furthermore, an equation that relates the area under the amplification part of the spectrum to that under the attenuation part is obtained. This indicates that amplification cannot occur if a very-wide-band probe beam is used instead of a narrow-band one. At high driving intensities the absorption spectrum is shown explicitly to be inversely proportional to the Rabi frequency and has a dispersion-like behaviour near the Rabi sidebands at driving resonance.
Pump-probe experiments are ubiquitous in atomic, molecular, and optical physics, where a strong pump field induces some change in a quantum system that is probed by a much weaker field. Often, high intensity of the pump field is at a premium with all other characteristics of the strong field subordinate. As a particular example, of relevance to atomic clocks and atomic magnetometers, a strong broad-linewidth laser optically pumps an atomic vapor, which is then probed by a much weaker narrow-linewidth laser. Though the broad-linewidth field suffers laser phase noise (PM) to amplitude noise (AM) conversion, the narrow-linewidth probe by itself produces a much quieter signal. Here, we consider the question of whether the noisy pump field maps its (PM-to-AM) absorption cross-section fluctuations onto the quiet probe field's transmission. Our results show that PM-to-AM noise does not transfer directly: atoms interacting with the pump field are instantaneously distinct from those interacting with the probe field. However, the broad-linewidth laser can influence the quiet field's transmission through the vapor due to optical pumping, resulting in fluctuations in the number density of atoms in the absorbing states. Nevertheless, there is a saving grace for this ``PM-to-AM induced optical pumping'' noise transfer: for very high noisy-field intensities (where optical pumping saturates) this type of noise on the probe field becomes negligible.
The spectral properties of an incoherently pumped one-atom laser operating in a regime of strong coupling are studied. The spectrum of the field outgoing from the cavity, the spectrum of resonance fluorescence of an atom, and the spectrum of the atom—field cross-correlation function are numerically calculated. Relations between the parameters of the system for which spectral lines corresponding to eigenstates of the Jaynes—Cummings Hamiltonian are resolved are found. A method for determining the widths of resonances from the spectrum of the atom—field cross-correlation function is proposed, and a procedure for measuring this function using spectroscopy of intensity fluctuations is described.