We describe all-optical transistor action in photonic band gap materials doped with active atoms. In the presence of a photonic band gap (PBG) material, a coherent laser beam with the frequency slightly detuned from the resonant atomic transition frequency can drive a collection of two-level atoms to an almost totally inverted state, a phenomenon strictly forbidden in ordinary vacuum. By changing the laser field intensity in the neighborhood of a threshold value, it is possible to drive the atomic system through a transition from states in which the atoms populate preferentially the ground level to almost totally inverted states. In this process, the atomic system switches from a passive medium (highly absorptive) to a active medium (highly amplifying). The switching action in a PBG material is not associated with operating near a narrow cavity resonance with conventional trade-off between switching time and switching threshold intensity. Rather it is associated with an abrupt discontinuity in the engineered broad band electromagnetic density of states of the PBG material. We demonstrate all-optical transistor action in PBG materials by analyzing the absorption spectrum of a second probe laser beam and we show that the probe beam experience a substantial differential gain by slight intensity modulations in the control laser field.
We describe all-optical transistor action in photonic band gap materials doped with active atoms. In the presence of a photonic band gap (PBG) material, a coherent laser beam with the frequency slightly detuned from the resonant atomic transition frequency can drive a collection of two-level atoms to an almost totally inverted state, a phenomenon strictly forbidden in ordinary vacuum. By changing the laser field intensity in the neighborhood of a threshold value, it is possible to drive the atomic system through a transition from states in which the atoms populate preferentially the ground level to almost totally inverted states. In this process, the atomic system switches from a passive medium (highly absorptive) to a active medium (highly amplifying). The switching action in a PBG material is not associated with operating near a narrow cavity resonance with conventional trade-off between switching time and switching threshold intensity. Rather it is associated with an abrupt discontinuity in the engineered broad band electromagnetic density of states of the PBG material. We demonstrate all-optical transistor action in PBG materials by analyzing the absorption spectrum of a second probe laser beam and we show that the probe beam experience a substantial differential gain by slight intensity modulations in the control laser field.
We describe all-optical transistor action in photonic bandgap (PBG) materials doped with active atoms and analyse the advantages of this system over other all-optical transistor proposals. In the presence of a PBG material, a coherent laser beam with the frequency slightly detuned from the resonant atomic transition frequency can drive a collection of two-level atoms to an almost totally inverted state, a phenomenon strictly forbidden in ordinary vacuum. By varying the laser field intensity in the neighbourhood of a threshold value, it is possible to drive the atomic system through a transition from states in which the atoms populate preferentially the ground level to almost totally inverted states. In this process, the atomic system switches from a passive medium (highly absorptive) to a active medium (highly amplifying). The large differential gain exhibited by the atomic medium is very robust with respect to nonradiative relaxation and dephasing mechanisms. The switching action in a PBG material is not associated with operation near a narrow cavity resonance with conventional trade-off between switching time and switching threshold intensity. Rather it is associated with an abrupt discontinuity in the engineered broad-band electromagnetic density of states of the PBG material. We demonstrate all-optical transistor action in PBG materials by analysing the absorption spectrum of a second probe laser beam and we show that the probe beam experiences a substantial differential gain by slight intensity modulations in the control laser field. Under certain conditions, the fluctuations in the number of totally inverted atoms that contribute to the amplification process are strongly diminished (the statistics of the excited atoms becomes sub-Poissonian), which, in turn, determines a very low-noise regime of amplification.
In this thesis we demonstrate that the photonic band edge facilitates fundamental switching effects for resonant two-level atomic systems. In a photonic crystal, a coherent laser beam can drive a two-level atom to inverted atomic states, a phenomenon forbidden in ordinary vacuum. In this process, the atomic system switches from a passive medium to a gain medium as a function of the driving field intensity. We show that the large differential gain exhibited by the atomic medium is robust against non-radiative relaxation and dephasing mechanisms. This switching effect exhibits collective enhancement for a large number of two-level atoms within a cubic wavelength. In this case, the ensemble of atoms sharply switches to a highly inverted state at a very well defined threshold value of the laser field intensity. We next demonstrate that a probe laser field of prescribed wavelength experiences a substantial differential gain by slight modulations in the control laser field. We suggest that a doped photonic band gap material may, in this sense, be used as an all-optical micro-transistor. We analyze the effects of inhomogeneous line broadening on the probe beam spectrum and demonstrate that further electromagnetic density of states engineering combined with photo-bleaching techniques can be used to both offset the deleterious effects of inhomogeneous broadening and to enlarge the switching bandwidth. We investigate the prospects and challenges in designing microstructures with characteristics required by our proposal. We further show that the switching power and the response time of an all-optical transistor can be dramatically reduced by including collective effects, which also provide a natural mechanism of broadening of the gain spectrum. Finally, we develop an exact multi-photon scattering theory of resonance fluorescence in frequency-dependent photonic reservoirs. We show that, in the limit of weak driving field, the scattering cross section becomes strongly non-Lorentzian, reflecting the non exponential character of the atomic decay. As the driving field intensity increases, the scattered field acquires new features, consistent with predictions of a dynamical decoupling of the atomic system from the photonic reservoir.
Summary form only given. Unlike semiconductors which facilitate the coherent propagation of electrons, photonic band gap (PBG) materials execute their novel functions through the coherent localization of photons. When a PBG material is doped with impurity atoms which have an electronic transition that lies within the gap, spontaneous emission of light from the atom is inhibited. Inside the gap, the photon forms a bound state to the atom. Outside the gap, radiative dynamics in the colored vacuum is highly nonMarkovian. I discuss the influence of these memory effects on laser action. When spontaneous emission is absent, the next order radiative effect (resonance dipole-dipole interaction between atoms) must be incorporated, leading to anomalous nonlinear optical effects which occur at a much lower threshold than in ordinary vacuum. I describe the collective switching of two-level atoms near a photonic band edge, by external laser field, from a passive stare to one exhibiting population inversion. This effect is forbidden in ordinary vacuum. Finally, I discuss the prospects for a phase sensitive, single (3-level) atom quantum memory device, onto which information may be written by an external laser pulse. By scattering polarized photons (flying qubit) this system may execute certain quantum logic operations.
Summary form only given. Photonic band gap (PBG) materials are a new class of dielectrics which represent a major new frontier in quantum, nonlinear, and classical optics. They also represent a new technology platform for the optical networking industry. Unlike semiconductors which facilitate the coherent propagation of electrons, PBG materials execute their novel functions through the coherent localization of photons. When a PBG material is doped with impurity atoms (or other two-level systems) which have an electronic transition that lies within the gap, spontaneous emission of light from the atom is inhibited. Instead, the photon forms a bound state to the atom. This has dramatic consequences for collective light emission from a large number of atoms and for the interaction of such atoms with an external laser field. I describe the design of an all optical micro-transistor based on collective switching of two-level atoms near a photonic band edge, by an external laser field. I discuss tunable PBG materials whose band structure can be modulated by means of an external field. This effect enables the steering of light in a manner similar to the steering of electrons by an electric field in a semiconductor.
We describe the spectral characteristics of the radiation scattered by two-level atoms (quantum dots) driven by a strong external field, and coupled to a photonic crystal radiation reservoir. We show that in the presence of strong variations with the frequency of the photonic reservoir density of states, the atomic, Mollow, sideband components of the scattered intensity can be strongly modified. Consequently, a weak optical probe field experiences a substantial differential gain in response to slight variations in the intensity of an optical driving field. We suggest that these effects may be of relevance to all-optical transistor action in photonic crystals. Using a specific photonic crystal heterostructure, we suggest that an all-optical microtransistor based on photonic crystals may operate at less than 100nW switching threshold power. Collective N-atom effects substantially enhance this optical switching effect. Near the switching threshold intensity, collective effects are manifest in the N2 scaling of the intensity spectrum (reminiscent of superradiance). Above and below this critical region, the gain spectrum widens (linearly with N). This correspondingly reduces the switching time scales of the atomic system in response to external fields. Furthermore, the quantum degree of second-order coherence exhibits unusual features. Scattered photons display a variable degree of antibunching as function of driving laser field intensity and the photonic density of states discontinuity. We analyze the effects of the inhomogeneous atomic line broadening on the amplification process. We show, using suitable photonic density of states engineering, that it is possible to select a narrow spectral range around the central frequency of the atomic frequency distribution over which amplification and switching occur. This is done either by spectral decoupling of the active elements from the electromagnetic field (through the introduction of band gaps at specified spectral locations) or through incoherent pumping to selectively saturate atoms outside the spectral region chosen for amplification.
A method of dynamic control of absorption and dispersion of a two-level quantum system (atoms, ions or quantum dots) doping a photonic band gap (PBG) material via variation of the intensity and frequency of an external laser field is proposed. The frequency of an optical transition should be inside a PBG and located near or at a photonic band edge. In this case the laser field 'dresses' the quantum system (Mollow splitting) while the decay rates of the dressed states become very different due to a photonic band edge and depend on the form of spectral density of electromagnetic modes as well as intensity and frequency of the laser field. This enables us to control absorption and dispersion of a signal laser field, which is near resonant to the quantum transition of a dopant.