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Chinese Physics Letters

Transparency and Strong Gain Without Population Inversion in Photonic Bandgap Crystals

杜春光, 胡正峰

Published Jan 1, 2002• 0 citations
OpenAlex

Abstract

Without using the weak-field approximation,we investigate the transient properties of a Λ-type atom with one transition near resonant with a photonic bandgap edge.Whatever the initial state of the atom is,the atom can become transparent to a probe field,and strong gain without population inversion is also possible if the atom has been pre-excited.The defect mode formed by atomic doping can have a strong effect on the absorption properties of the atom.

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Without using the weak-field approximation,we investigate the transient properties of a Λ-type atom with one transition near resonant with a photonic bandgap edge.Whatever the initial state of the atom is,the atom can become transparent to a probe field,and strong gain without population inversion is also possible if the atom has been pre-excited.The defect mode formed by atomic doping can have a strong effect on the absorption properties of the atom.
We analytically and numerically investigate the response of an open Λ-type atomic system to a probe laser field, where one of the atomic transition is coupled with a double-band radiation reservoir and high- Q defect modes formed by atomic doping. This system can be transparent to a weak probe field being two-photon resonant with the frequency of the defect modes or one of the two band edges, and the high- Q defect modes can even induce an atom–field coherent population trapping (CPT) state without involving the excited state of the atom, in spite of the background decay of the excited state of the atom. The steady-state properties of this CPT are independent of the smoothness feature of the density of modes (DOM) at band edges. However, the CPT never occurs in the absence of any defect even if the DOM is singular at both band edges. We also find that the background decay of the second ground state of the atom will destroy the CPT and increase the probe absorption, but the atomic coherence induced by the defect modes is more robust than that induced by the band edges. The transient properties of the CPT are also studied.
We study the spontaneous emission, absorption, and dispersion properties of a \ensuremath{\Lambda}-type atom where one transition interacts near resonantly with a double-band photonic crystal. Assuming an isotropic dispersion relation near the band edges, we show that two distinct coherent phenomena can occur. First the spontaneous emission spectrum of the adjacent free-space transition obtains ``dark lines'' (zeros in the spectrum). Second, the atom can become transparent to a probe laser field coupling to the adjacent free-space transition.
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.
We investigate spontaneous emission and quantum interference effects involving a three-level atom in the vicinity of a photonic band edge beyond the weak-driving-field approximation. We consider two different three-level atoms, each subject to a probe field from the ground state and each embedded within a different photonic crystal (PhC). The first atom has the two excited states separated by a dipole transition in the optical frequency range, with this frequency being close to the surrounding PhC's band edge. The probe field couples the ground state and the highest excited state, and is well outside the PhC band gap. If a coupling field is applied between the two upper levels, electromagnetically induced transparency (EIT) may occur, depending on the position of the band edge. The second atom has the two upper levels each dipole-coupled to the ground state and close enough that the emissions from each can coherently interfere. This atom is embedded within a PhC whose band edge lies near the lower of the two excited states, and a probe field is applied that lies just beyond this band edge. This atom exhibits a quantum interference phenomenon related to EIT called field-induced transparency, again depending on the position of themore » band edge relative to the lower excited state.« less