∧—型三能级系统的瞬态相干光谱：自由感应衰减现象

采用半经典理论中布洛赫方程方法研究相干脉冲与∧－型量子拍频三能级系统相互作用产生的自由感应衰减现象，讨论系统的弛豫对自由感应衰减信号场的影响。     

Λ-型三能级系统的瞬态相干光谱:自由感应衰减现象

采用半经典理论中布洛赫（Ｂｌｏｃｈ）方程方法研究相干脉冲与Λ－型量子拍频三能级系统相互作用产生的自由感应衰减现象，讨论系统的弛豫对自由感应衰减信号场的影响。     

铷原子蒸气中超精细基态的双光子相干操控

二能级量子体系的相干操控对于精密测量和量子信息处理非常重要,如原子钟、原子干涉仪和量子计算等。在实验上观察到相干微波-射频(MW-RF)场驱动下的铷原子超精细基态的双光子Rabi振荡现象。基于塞曼子能级之间热弛豫过程的标定和微波跃迁的测量,清晰地分辨出叠加有热弛豫过程的原子态布居相干振荡。实验测得并详细讨论了广义Rabi频率与中间态失谐和微波/射频功率的关系。当中间态失谐较大时,实验结果与等效二能级理论模型非常吻合;但当中间态失谐较小时,少量原子占据中间态造成实测的Rabi频率偏离理论值。这些结果为二能级量子系统的相干操控提供了有力的理论支持。     

多光子反Jaynes-Cummings模型中混合态二能级原子的粒子数反转特性

利用多光子反Jaynes-Cummings模型研究了单模相干光场与混合态二能级原子相互作用时,二能级原子的粒子数反转特性.讨论了相干光场的平均光子数、跃迁光子数以及原子初始态的混合度等参量对粒子数反转的影响.结果表明：二能级原子的粒子数反转有周期性的崩塌和回复现象. 相干光场平均光子数和跃迁光子数的增加会改变原子的回复周期和崩塌的衰减时间;随相干光场平均光子数的增加粒子数反转的振荡频率增大;随着混合度的减少回复的周期性和崩塌的衰减时间不发生变化但粒子数反转的振荡幅度增大.     

多光子反Jaynes-Cummings模型中混合态二能级原子的粒子数反转特性

利用多光子反Jaynes-Cummings模型研究了单模相干光场与混合态二能级原子相互作用时,二能级原子的粒子数反转特性.讨论了相干光场的平均光子数、跃迁光子数以及原子初始态的混合度等参量对粒子数反转的影响.结果表明：二能级原子的粒子数反转有周期性的崩塌和回复现象. 相干光场平均光子数和跃迁光子数的增加会改变原子的回复周期和崩塌的衰减时间;随相干光场平均光子数的增加粒子数反转的振荡频率增大;随着混合度的减少回复的周期性和崩塌的衰减时间不发生变化但粒子数反转的振荡幅度增大.     

超短激光脉冲操控双量子比特的量子态

用光与物质相互作用的半经典理论研究了超短激光脉冲与构成双量子比特的二体二能级体系的相互作用,对由不同的激发方式引起的独立双路(Independent Double Path-Ways,IDPW)和相干双路径(Coherent Double Path-Ways,CDPW)进行对比.研究结果表明,与单量子比特相比,双量子比特在超短激光脉冲的作用下,表现出更加丰富的相干性.这种相干特性与体系初态、失谐量和激发的路径都有关.同时,可以利用超短激光脉冲对双量子比特的相干激发,通过调节光脉冲面积来实现量子态的操控.     

GHZ类态原子体系与相干态光场相互作用的光场压缩特性

本文采用求解Schr dinger方程和数值计算的方法,研究了处于GHZ类态的三个全同二能级纠缠原子与相干态光场相互作用的光场压缩特性,结果表明:光场压缩量随时间的演化关系与三原子体系纠缠度和相干态光场的光场强度相关.在光场较弱、纠缠度较小时,可出现光场压缩现象;光场的增强或纠缠度的增加,都会使光场压缩现象消失.     

偶奇qs相干态的非经典特性

利用数值计算方法研究了ｑ和ｓ畸变对双参数形变偶奇ｑｓ相干态量子统计特性的委现偶奇ｑｓ相干态与通常的偶奇相干态有着显著不同的量子统计性质，压缩效应和反聚束效应均可在一定条件下的两种双参数畸变态中出现，偶奇ｑｓ相干态和通常偶奇相干态的差异随ｑ值偏离１越大和ｓ的取值越小而越大。     

劣腔内二能级原子的双光子共振荧光光谱

研究了劣腔内二能级原子与单模腔场的双光子相互作用，腔内原子受外加固相干光场驱动，并向一般真空态自发辐射，通过腔镜内腔内注入压缩真空态光场，使腔模向压缩真空态光场衰减，本文利用劣腔近似条件得到原子约化密度算符主方程，在此基础上讨论了二能级原子的双光子共振荧光光谱。     

两纠缠原子与相干态光场相互作用的动力学

采用时间演化算符和数值计算方法,研究了两全同二能级纠缠原子与相干态光场相互作用的动力学,结果表明:原子布居和偶极压缩特性与两原子体系纠缠度和相干态光场强度相关联.     

Optical coherent transient effects in a magnetized semiconductor quantum wire

Effects of an external magnetic field on free induction decay (FID) and the Stark effect in GaAs/GaAlAs quantum wires have been investigated analytically. Our results show that both FID and the Stark effect become enhanced due to the presence of a magnetic field. We have also seen that the magnetic field plays an important role in wider wires while in thinner wires, the geometric confinement dominates over the magnetic effects. The results are found to be in good qualitative agreement with that available in the literature.     

Artifacts introduced by zero order phase correction in proton NMR spectroscopy and a method of elimination by phase filtering

In in-vivo applications of proton NMR spectroscopic imaging, an oscillatory "ringing" artifact has been observed in some of the spectra. The source of this artifact was found to be the presence of a harmonic "beating" effect in the amplitude of the water reference free induction decay (FID) which was used for zero order phase correction for B0 inhomogeneity and eddy current compensation. The source of the beats was found to be the presence of distinct populations of spins resonating at slightly different frequencies. When the common method of zero order phase correction was implemented using such an FID, the resulting phase-corrected, water-suppressed spectra displayed ringing. Examination of the unwrapped phase correction angle revealed unexpected jumps in phase at points in time corresponding to nodes in the amplitude of the FID. Low-pass filtering of the phase correction angle of the reference FID was found to smooth out these unanticipated phase jumps. When used as a reference for phase correction, the filtered phase information gave a phase-corrected, water-suppressed spectrum free from ringing.     

Eigenmodes in the long-time behavior of a coupled spin system measured with nuclear magnetic resonance

The many-body quantum dynamics of dipolar coupled nuclear spins I=1/2 on an otherwise isolated cubic lattice are studied with nuclear magnetic resonance. By increasing the signal-to-noise ratio by 2 orders of magnitude compared with previous reports for the free induction decay (FID) of (19)F in CaF(2) we obtain new insight into its long-time behavior. We confirm that the tail of the FID is an exponentially decaying cosine, but our measurements reveal a second decay mode with comparable frequency but twice the decay constant. This result is in agreement with a recent theoretical prediction for the FID in terms of eigenvalues for the time evolution of chaotic many-body quantum systems.     

Frequency dependence of EPR signal intensity, 248 MHz to 1.4 GHz

The electron paramagnetic resonance pulsed free induction decay (FID) of a degassed solution of a triaryl methyl radical, methyl tris(8-carboxy-2,2,6,6-tetramethyl(-d3)-benzo[1,2-d:4,5-d']bis(1,3)dithiol-4-yl) tripotassium salt, 0.2 mM in H2O, was measured at VHF (247.5 MHz) and L-band (1.40 GHz). The calculated and observed FID signal amplitudes (in millivolts) agreed within 1 and 6%, and the ratio of the normalized FID signals at the two frequencies agreed within 5%. The FID decay time constant was 2.7 micros at both frequencies.     

Phase relaxation in disordered nuclear paramagnets

A new method for the construction of main correlation functions of nuclear paramagnets has been developed on the basis of the combination of the projection technique and cumulant expansions. It gives satisfactory results for the free induction decay and resonance line shape function in magnetically concentrated crystals. Polarization transfer in magnetically dilute crystals has been taken into account. It significantly retards a decrease in the FID at times longer than the phase relaxation time T ₂. The FID remains mainly monotonic in contrast to magnetically concentrated crystals, where the FID oscillates. The results have been compared to the existing experimental data.     

Quantum wire heterostructure for optoelectronic applications

Quantum wire (QWR) heterostructures suitable for optoelectronic applications should meet a number of requirements, including defect free interfaces, large subband separation, long carrier lifetime, efficient carrier capture. The structural and opticl properties of GaAs/AlGaAs and InGaAs/GaAs quantum wire (QWR) heterostructures grown by organometallic chemical vapor deposition on nonplanr substrates, which satisfy many of these criteria, are described. These crescent-shaped QWRs are formed in situ during epitaxial growth resulting in virtually defect free interfaces. Effective wire widths as small as 10nm have been achieved, corresponding to electron subband separations greater than K_BT at room temperature. The enhanced density of states at the QWR subbands manifests itself in higher optical absorption and emission as visualized in photoluminescence (PL), PL excitation, amplified spontaneous emission and lasing spectra of these structures. Effective carrier capture into the wires via connected quantum well regions, which is important for enhancing the otherwise extremely small capture cross section of these wires, has also been observed. Room temperature operation of GaAs/AlGaAs and strained InGaAs/GaAs QWR lasers with threshold currents as low as 0.6mA has been demonstrated.     

Stranski–Krastanow growth of multilayer In(Ga)As/GaAs QDs on Germanium substrate

We have described Stranski–Krastanow growth of multilayer In(Ga)As/GaAs QDs on Ge substrate by MBE. The growth technique includes deposition of a thin germanium buffer layer followed by migration-enhanced epitaxy (MEE) grown GaAs layer at 350°C. The MEE layer was overgrown by a thin low-temperature (475°C) grown GaAs layer with a subsequent deposition of a thick GaAs layer grown at 590°C. The sample was characterized by AFM, cross-sectional TEM and temperature-dependent PL measurements. The AFM shows dense formation of QDs with no undulation in the wetting layer. The XTEM image confirms that the sample is free from structural defects. The 8 K PL emission exhibits a 1051 nm peak, which is similar to the control sample consisting of In(Ga)As/GaAs QDs grown on GaAs substrate, but the observed emission intensity is lower. The similar slopes of Arrhenius plot of the integrated PL intensity for the as-grown QD sample grown on Ge substrate as well as for a reference QD sample grown on GaAs substrate are found to be identical, indicating a similar carrier emission process for both the samples. This in turn indicates coherent formation of QDs on Ge substrate. We presume due to the accumulated strain associated with the self-assembled growth of nanostructures on Ge that nonradiative recombination centers are introduced in the GaAs barrier in between the QD layers, which in turn degrades the overall optical quality of the sample.     

Gaussian approximation in the theory of MR signal formation in the presence of structure-specific magnetic field inhomogeneities

A detailed theoretical analysis of the free induction decay (FID) and spin echo (SE) MR signal formation in the presence of mesoscopic structure-specific magnetic field inhomogeneities is developed in the framework of the Gaussian phase distribution approximation. The theory takes into account diffusion of nuclear spins in inhomogeneous magnetic fields created by arbitrarily shaped magnetized objects with permeable boundaries. In the short-time limit the FID signal decays quadratically with time and depends on the objects' geometry only through the volume fraction, whereas the SE signal decays as 5/2 power of time with the coefficient depending on both the volume fraction of the magnetized objects and their surface-to-volume ratio. In the motional narrowing regime, the FID and SE signals for objects of finite size decay mono-exponentially; a simple general expression is obtained for the relaxation rate constant deltaR2. In the case of infinitely long cylinders in the motional narrowing regime the theory predicts non-exponential signal decay lnS approximately -tlnt in accordance with previous results. For specific geometries of the objects (spheres and infinitely long cylinders) exact analytical expressions for the FID and SE signals are given. The theory can be applied, for instance, to biological systems where mesoscopic magnetic field inhomogeneities are induced by deoxygenated red blood cells, capillary network, contrast agents, etc.     

Selective excitation in pulsed EPR of a spin-correlated triplet-radical pair

Experiments are described in which a low-amplitude microwave pulse excites only one out of three allowed transitions of the quinone radical (Q(A)(-)) in a spin-correlated triplet-radical pair 3PQ(A)(-) of the bacterial photosynthetic reaction center. A second high-amplitude pulse produces a FID whose temporal shape is strongly modulated with frequencies determined by electron-electron dipolar interaction in the pair. The FID is detected in both the in-phase and the out-of-phase channels. The out-of-phase FID is a result of switching off the magnetic dipolar interaction between 3P and Q(A)(-) due to decay of 3P during the time interval between the two pulses. Refocusing of FID by an additional non-selective pulse allows a dead-time free measurement of this modulation. The influence of the dead-time problem on the distance determination is discussed.     

Influence of screening on deformation potential and piezoelectric scattering in doped semiconductors

Mobility relations are derived for deformation potential and piezoelectric scattering accounting for the screening by free charge carriers. Estimations of the screening influence on the carrier mobility in n-type GaN and p-type GaAs are made. It is shown that particularly in p-type GaAs the screening effect cannot be neglected in analysing the hole mobility even at high carrier concentrations.