Generation And Control Of Electromagnetically Induced Double And Tirple Photonic Bandgaps

Atomic coherence effects based on the interaction between light and matter caneffectively change the polarization, absorption, dispersion and refractive index of themedium. It’s one of the hottest areas of the researches on quantum optics and physicalin recent years. we can precisely manipulate the corresponding optical characteristicsof the atomic system via laser-induced atomic interference effects, which can give riseto many interesting phenomena, such as EIT, slow light, stationary light pulses and soon. In recent years, to achieve the more flexible control of the optical signal,laser-induced atomic coherence has been extended from the spatially homogeneouspattern into the spatially periodic pattern so that dynamic photonic band gaps can betheoretically foreseen and experimentally observed. In this thesis, based onelectromagnetically induced transparency, we study the double and thriple PBGstructure induced by a SW field.First, we study the steady optical response of a four-level ultracold atomic systemdriven by a traveling-wave field and a standing-wave field, which is shown in FIG1.Theoretical calculations show that two well-developed photonic band gaps with reflec-tivities of about95%can be generated on the probe resonance in specific parameters.The pulse will be fully reflected if its all frequencies fall into the band gaps（see in FIG 2）. However, if we block a component of the standing wave field, the two band gapswill be changed into two high transmission regions. By analyzing the impact ofcoupling fields parameters on two band gaps, we find that the stronger the twocoupling fields are, the larger the frequency range of the PBG becomes, and thefarther the frequency positions become from the resonance point. At the same time,we can adjust the positions of PBGs by changing the detuning of the two couplingfields. In other words, both photonic bandgaps depend critically on the external pumpcontrol. Further more, we explain the formation of the two well developed PBGs: Inthe two EIT windows created by the traveling and standing wave, the reflective indexvaries periodically, leading to Bragg scattering of an incident probe light beam.Because of the constructive interference of different Bragg reflection componentwhich is formed when the phase difference is an integer multiple of2π, the probebeam is absolutely reflected in the medium.Second, we investigate dynamical evolution for three photonic band gaps in thefive-level system. We extend dynamically induced PBGs system to a five-level coldatomic system driven by a standing-wave and two traveling-wave fields in FIG3. Byanalyzing the optical response of this system, we can see that in the specificparameters, three optically tunable photonic band gaps in three different spectralregions will be found as the one shown in FIG4. Then we study the influence ofcoupling field parameters on three PBGs in order to determine the conditions for perfect bandgaps. We find that the left band gap is determined by the intensities ofthree coupling fields, the right band gap is decided byΩ_c2andΩ_s, and thedetuning of the standing-wave has a conclusive effect on the middle band gap.Meanwhile, our triple band-gap structure will reduce to single or doubleband-gap structure by modulating the coupling field, the standing wave, and theatomic density. In addition, turning off the reverse component of the standing wavefield, three band gaps disappear, three high transmission EIT windows turn up forstrong coupling fields eliminating the absorption on the probe resonance. All thesemake our scheme more flexible control of light signal in the optical network.FIG4Photonic band-gap structures （a, b） and reflection and transmission spectra （c, d） Taking advantage of tunable double and triple PBGs, we can devise an efficientscheme for multichannel all optical routing of weak light signals. By dynamicallymodulating the SW and TW fields, we can well manipulate the bandgaps so that twoor three weak light signals having fixed but different frequencies can either passthrough or rebound from an atomic sample with little loss and deformation. Weanticipate that this new findings of PBGs be instructive to devise novel photonicdevice, e.g. all-optical switching and routing、optic filter、oscillector and so on.