循环跃迁的Δ 型三能级系统中的无反转激光

研究了在引入非相干抽运的情况下,循环跃迁的Δ型共振三能级系统中产生的无反转激光.利用主方程的方法, 导出了缀饰态中强相干场极限下布居数项和相干项的稳态近似解析解以及产生无反转激光所需要满足的条件;并利用数值模拟的方法讨论了布居数分布、 系统增益对探测场和相干场拉比频率的依赖性以及随时间的演化规律. 结果表明:无论各个参量取何值,共振Δ型三能级系统总是处于无粒子数反转的状态; 只要探测场或相干场之中有一个为强场,便可产生无反转激光; 当其中任意一个场远强于另一个场时,系统的无反转增益将不再依赖于任何一个场的拉比频率.     

Population inversion and IR amplification induced by intersubband electron transitions and resonant Auger processes in quantum wells

A mechanism for electron population inversion and mid-IR amplification is proposed for the case of the current or optical injection of electron-hole pairs into the undoped region of a heterojunction with quantum wells. The presence of an upper long-lived size-quantization level and resonant Auger recombination in the well are crucial features of the mechanism. A long electron lifetime at the upper level or a relatively low probability of electron scattering to the other subbands is achieved by choosing the shape of the well and its parameters in such a way as to provide weak overlap between the upper level and two lower level electronic wave functions. Resonant Auger recombination plays a positive role. It stabilizes the electron and hole concentrations at lower levels and makes a substantial contribution to the excitation of the upper electronic level and the population inversion. The degree of population inversion and the gain coefficient are estimated.     

Universal relation between hall conductivity and the damping constant of edge magnetoplasma resonances

The spectra of edge magnetoplasma excitations in two-dimensional (2D) electron disks have been analyzed by the method of optical detection of resonant microwave absorption. The magnetic dispersion of an edge magnetoplasmon in samples with a high 2D electron density is found to be poorly reproduced by existing theoretical models. Analysis of the magnetic-field dependence of the linewidth of resonant microwave absorption for samples with various 2D electron densities shows that the inverse width of the main mode of resonant microwave absorption is universally proportional to the Hall resistance of 2D electrons.     

失谐对开放的四能级系统无粒子数反转激光的影响

提出开放的四能级双驱动场无反转激光系统的理论模型,由电偶极和旋转波近似得到其密度矩阵方程,讨论无反转激光产生的物理机制,利用数值计算结果分析探测场和驱动场失谐对系统无反转激光增益和粒子数差的影响.     

Robust quantum dot exciton generation via adiabatic passage with frequency-swept optical pulses

The energy states in semiconductor quantum dots are discrete as in atoms, and quantum states can be coherently controlled with resonant laser pulses. Long coherence times allow the observation of Rabi flopping of a single dipole transition in a solid state device, for which occupancy of the upper state depends sensitively on the dipole moment and the excitation laser power. We report on the robust population inversion in a single quantum dot using an optical technique that exploits rapid adiabatic passage from the ground to an excited state through excitation with laser pulses whose frequency is swept through the resonance. This observation in photoluminescence experiments is made possible by introducing a novel optical detection scheme for the resonant electron hole pair (exciton) generation.     

Carrier kinetics and population inversion in Landau level system in cascade GaAs/AlGaAs quantum well structures

The carrier distribution over Landau levels was studied in resonant tunneling GaAs/AlGaAs quantum well structures under tunneling pumping of the upper subband. The numerical calculations of the Landau levels population for various values of pumping intensity (tunneling time), magnetic field and the structure doping were carried out. The effect of various scattering mechanisms, as two-electron (electron–electron scattering) as single-electron (acoustic phonon and interface roughness scattering) ones on level population was studied. The population inversion between the zeroth Landau level of the upper subband and the first Landau level of the lowest subband was shown to exist in wide range of the magnetic field strength thus providing the possibility of wide range tunable stimulated terahertz emission.     

Strong field effects in the evolution of a two-level system

Evolution of a two-level system (TLS) in a strong resonant quantum field is considered beyond the rotating wave approximation (RWA). An analytical expression for the atomic population inversion that approximates numerical results with high accuracy is deduced. It allows us to describe new qualitative peculiarities of the TLS evolution in the strong field: i) suppression of the ‘collapse-revival’ effect, and ii) change of the spectrum and form of the population oscillations.     

Nonlinear properties of gated graphene in a strong electromagnetic field

We develop a microscopic theory of a strong electromagnetic field interaction with gated bilayer graphene. Quantum kinetic equations for density matrix are obtained using a tight binding approach within second quantized Hamiltonian in an intense laser field. We show that adiabatically changing the gate potentials with time may produce (at resonant photon energy) a full inversion of the electron population with high density between valence and conduction bands. In the linear regime, excitonic absorption of an electromagnetic radiation in a graphene monolayer with opened energy gap is also studied.     

用低杂波驱动电流的等离子体电导率

本文用Liu的低杂波驱动电流模型,导出了弱场近似下低杂波驱动电流的等离子体电导率。结果表明,电导率对磁场B_0,电子密度n_0,以及电子温度T_e非常敏感。而这些参量决定着共振电子的数目和速度。     

Room temperature intra-band lasing in quantum dot arrays placed in high photon density cavities—a theoretical study

Intersubband (intra-band) transitions are very attractive forlong wavelength lasers due to the high degree of tailoring possible in the emission spectra. In general, if intra-band population inversion is to be created in a conduction band quantum well by carrier injection at the barrier energy, it is necessary that the electron non-radiative intra-band energy relaxation times are long. Additionally, the extraction time for the electron from the lower state should be short. In a bipolar device studied here, this means the bandedge electron-hole recombination times should be short.The use of sub-two-dimensional (2D) structures (quantum dots) allows us to increase the intra-band energy relaxation time from about a picosecond for bulk or quasi-2D systems to several hundred picoseconds at room temperatures. Also, by placing these structures in a cavity with a high photon number, it is possible to decrease the bandedge electron-hole recombination times through stimulated emission. Our studies show that strong population inversion and lasing under d.c. conditions is possible at room temperature in such systems.     

Chemistry in Lasers. IX. Reexamination of the Einstein Coefficients and Two Level Population Inversion

From a re-examination of the Einstein coefficients in terms of current theory the common assumption that population inversion cannot be maintained in a two level system by optical pumping is shown to be erroneous. The nonisotropy of the radiative ensemble in an optical resonant cavity, absent from the original derivation by Einstein, is shown to be of decisive importance.     

Sculpting the vacuum in a photonic band gap micro-chip

We describe the engineering of the electromagnetic vacuum in a 2D–3D photonic bandgap (PBG) hetero-structure. This facilitates the development of novel active devices and the observation of novel quantum electrodynamic phenomena. We consider a specific architecture suitable as an all-optical micro-transistor capable of novel ultra-fast response with low switching power requirements. This relies on a unique collective atomic switching and population inversion achieved by coherent resonant pumping in a suitably engineered vacuum. Specific waveguide architectures within the 3D PBG micro-chip provide local density-of-states (LDOS) peaks near their cutoff frequency. These provide “building blocks” for electromagnetic vacuum engineering without recourse to conventional high Q-factor micro-cavities. For the all-optical micro-transistor, a fork shape LDOS within the micro-chip is desirable, using trimodal waveguide architecture. We delineate the functional robustness of these architectures to disorder caused by manufacturing errors within the PBG micro-chip.     

Control of two-dimensional electron population in a semiconductor quantum well

We investigate the two-dimensional (2D) electron population in a semiconductor quantum well. It is found that, due to the position-dependent quantum interference, the 2D spatial distribution of electron population can be easily controlled via adjusting the system parameters. Thus, our scheme shows the underlying probability for the applications in solid-state optoelectronics.     

Fermi-edge singularity in the vicinity of the resonant scattering condition

Fermi-edge absorption theory predicting the spectrum A(ω) ∝ ω(-2δ(0)/π+δ(0)92)/π2) relies on the assumption that scattering phase δ(0) is frequency independent. The dependence of δ(0) on ω becomes crucial near the resonant condition, where the phase changes abruptly by π. In this limit, because of the finite time spent by electron on a resonant level, the scattering is dynamic. We incorporate the finite time delay into the theory, solve the Dyson equation with a modified kernel, and find that, near the resonance, A(ω) behaves as ω(-3/4)|lnω|. Scattering off the core hole becomes resonant in 1D and 2D in the presence of an empty subband above the Fermi level; then a deep hole splits off a level from the bottom of this subband. Fermi-edge absorption in the regime when resonant level transforms into a Kondo peak is discussed.     

Electron bunching in a Penning trap and accelerating process for CO_2 gas mixture active medium

In PASER(particle acceleration by stimulated emission of radiation),in the presence of an active medium incorporated in a Penning trap,moving electrons can become bunched,and as they get enough energy,they escape the trap forming an optical injector.These bunched electrons can enter the next PASER section filled with the same active medium to be accelerated.In this paper,electron dynamics in the presence of a gas mixture active medium incorporated in a Penning trap is analyzed by developing an idealized ID model.We evaluate the energy exchange occurring as the train of electrons traverses into the next PASER section.The results show that the oscillating electrons can be bunched at the resonant frequency of the active medium.The influence of the trapped time and population inversion are analyzed,showing that the longer the electrons are trapped,the more energy from the medium the accelerated electrons get,and with the increase of population inversion,the decelerated electrons are virtually unchanged but the accelerated electrons more than double their peak energy values.The simulation results show that the gas active medium needs a lower population inversion to bunch the electrons compared to a solid active medium,so the experimental conditions can easily be achieved.     

Theoretical estimates of optimal parameters for quantum cascade lasers

The quantum cascade (QC) laser is a new light source which is based on one type of carrier (electrons) making transitions between energy levels created by quantum confinement. In this paper, focusing on the working conditions which a QC laser should satisfy, we have discussed the subband lifespans in QC laser active regions. The results show that the population inversion condition can be achieved by resonant tunneling associated with an optical phonon, and this population inversion can be facilitated by the short escaping time of electrons from one active region to the neighboring active region. Our calculations also show that the lifespans of levels 3 and 2 are dominated by the phonon scattering time, and the escaping time from one active region to the next active region is determined by the thickness of exit barrier and the proper design of the miniband between the active regions.     

Effect of Intrinsic Decoherence on Nonclassical Effects of the Nondegenerate Bimodal Multiquanta JCM

We study the nondegenerate multiquanta Jaynes-Cummings model governed by Milburn equation. This models the decoherence of a quantum system as it evolves through intrinsic mechanisms beyond conventional quantum mechanics governed by the Schrödinger equation. We find an exact solution of this equation and apply it to investigate the effects of the intrinsic decoherence on nonclassical effects of the system, such as collapses and revivals of the population inversion and squeezing of the radiation field, for the resonant and the off-resonant cases when the particle (atom or trapped ion) is taken to be prepared initially in a coherent superposition state.     

Imaging transport resonances in the quantum Hall effect

We use a scanning capacitance probe to image transport in the quantum Hall system. Applying a dc bias voltage to the tip induces a ring-shaped incompressible strip (IS) in the 2D electron system (2DES) that moves with the tip. At certain tip positions, short-range disorder in the 2DES creates a quantum dot island in the IS. These islands enable resonant tunneling across the IS, enhancing its conductance by more than 4 orders of magnitude. The images provide a quantitative measure of disorder and suggest resonant tunneling as the primary mechanism for transport across ISs.     

Nonperturbative coherent population trapping: an analytic model

Coherent population trapping is shown to occur in a driven symmetric double-well potential in the strong-field regime. The system parameters have been chosen to reproduce the 0(-) <--> 3(+) transition of the inversion mode of the ammonia molecule. For a molecule initially prepared in its lower doublet we find that, under certain circumstances, the 3(+) level remains unpopulated, and this occurs in spite of the fact that the laser field is resonant with the 0(-) <--> 3(+) transition and intense enough so as to strongly mix the 0(+) and 0(-) ground states. This counterintuitive result constitutes a coherent population trapping phenomenon of nonperturbative origin which cannot be accounted for with the usual models.     

Amplification of a weak circularly polarized light signal in a multilevel atomic medium

Amplification of a weak circularly polarized light signal in a multilevel atomic medium excited by a strong linearly polarized resonant radiation is studied. A strong optical pumping may lead to the population inversion between magnetic sublevels of hyperfine components of the ground and excited states. Solution to the self-consistent system of density matrix equations and Maxwell equations for propagation of the weak signal allows analyzing optimum conditions of amplification.