Spectral technique for measuring hyperfine structure of atoms

A new spectral technique for measuring the hyperfine structure of atoms is reported. A divergent atomic beam and a divergent laser beam are crossed. Because of the Doppler effect, the hyperfine structure of atomic levels will be directly displayed in the interaction region in the form of spatially resolved fluorescence arc bands. By measuring the spatial-fluorescence intensity distribution, it is possible to obtain the hyperfine splittings of atomic levels. Basic principles and experimental results are given.

Energy transfer between Cr3+ and Ni2+ in transparent silicate glass ceramics containing Cr3+/Ni2+ co-doped ZnAl2O4 nanocrystals

The emission intensity of Ni2+ at 1200 nm in transparent ZnO-Al2O3-SiO2 glass ceramics containing ZnAl2O4 nanocrystals is improved approximately 8 times by Cr3+ codoping with 532 nm excitation. This enhanced emission could be attributed to an efficient energy transfer from Cr3+ to Ni2+, which is confirmed by time-resolved emission spectra. The energy transfer efficiency is estimated to be 57% and the energy transfer mechanism is also discussed. (C) 2008 Optical Society of America.

Enhanced near-infrared emission from Ni2+ in Cr3+ Ni2+ codoped transparent glass ceramics

Transparent Li2O-Ga2O3-SiO2 glass ceramics containing Cr3+/Ni2+ codoped LiGa5O8 nanocrystals were synthesized. The steady state emission spectra indicated that the near-infrared emission intensity of Ni2+ at 1300 nm in Cr3+/Ni2+ codoped glass ceramics was enhanced up to about 7.3 times compared with that in Ni2+ single-doped glass ceramics with 532 nm excitation. This enhancement in emission intensity was due to efficient energy transfer from Cr3+ to Ni2+, which was confirmed by time-resolved emission spectra. The energy transfer efficiency was estimated to be 85% and the energy transfer mechanism was discussed. (C) 2008 American Institute of Physics.

细胞色素b6f蛋白复合体中叶绿素 a的性质和功能研究

细胞色素b6f蛋白复合体（Cytochrome b6f complex, Cyt b6f）是光合膜上参与光合作用原初反应过程的主要膜蛋白超分子复合体之一。莱茵衣藻和嗜热蓝细菌的Cyt b6f三维晶体结构均显示，每Cyt b6f单体分子含有1分子Chlorophyll a (Chl a )，充分肯定了Chl a 是Cyt b6f天然成分的观点(Kurisu et al，2003;Stroebel et al，2003)。研究表明不同来源的Cyt b6f中Chla单线激发态寿命(或荧光寿命)并不一样，多数的研究结果认为Cyt b6f中Chla单线激发态寿命只有200ps左右，但是也有Cyt b6f中Chla单线激发态寿命为～600ps的报道;而甲醇中游离Chl a 的单线激发态寿命为4ns左右。针对Cyt b6f中Chla单线激发态寿命快速淬灭的现象，Dashdorj 等(2005)根据晶体结构推测Cyt b6f中Chla单线激发态和邻近的Cyt b6亚基上Tyr105残基发生电子交换传递，从而快速淬灭Chla单线激发态，减少了三线态Chl a和单线态氧的产生，并且认为这是Cyt b6f保护自身不受单线态氧破坏的一种机制，但是这一推测缺乏有力的证据。另外，Cyt b6f中Chla的功能仍然未知。本文以菠菜Cyt b6f为对象，结合多种实验手段，测定了菠菜Cyt b6f中Chl a单线激发态寿命，并对复合体中Chl a 单线激发态淬灭的机理进行了深入研究。此外，我们还对复合体中Chl a 可能的功能进行了初步地探讨。获得了如下的结果： 1．针对不同来源的Cyt b6f中Chla单线激发态寿命（或荧光寿命）测定结果不同的报道，仔细分析了其中的原因，发现除了样品来源的差异外，使用不同的去垢剂可能是一个不可忽视的因素。在实验中，不同的研究者分别采用了十二烷基麦芽糖苷（n-Dodecyl β-D-maltoside，DDM，）和八烷基葡萄糖苷（n-Octyl β-D-glucopyranoside，β－OG)作为溶解样品的去垢剂。因此，本文借助稳态吸收和稳态荧光光谱、瞬态光散射技术，CD光谱和亚皮秒时间分辨吸收光谱等技术，分别研究了这两种去垢剂对Cyt b6f结构和功能的不同影响。结果表明，DDM去垢剂能使Cyt b6f处于较好的分散体系中，其中血红素和Chl a分子处于特定的蛋白环境中，不会导致Cyt b6f变性;而β－OG去垢剂会使Cyt b6f产生聚合现象，其中的血红素和Chl a与蛋白环境的相互作用减弱，和DDM相比其电子传递活性显著降低，Chl a单线激发态寿命延长，Chl a更容易被光破坏。通过这一工作，我们优化和确定了Cyt b6f的溶解条件，为下面的研究工作打下了良好的基础。 2．利用Tyr的特异性修饰剂p-Nitrobenzenesulfonyl Fluoride（NBSF）对Cyt b6f样品进行特性修饰，经原子吸收谱、荧光谱、CD谱、质谱等方法对修饰后的样品进行鉴定，并结合时间分辨飞秒吸收光谱技术，测得修饰后的样品在660nm激发下Chl a 单线激发态寿命延长，从而在实验上提供了Tyr与淬灭Chla单线激发态有关的证据。但是对Cyt b6f 中Chl a瞬态吸收图谱仔细研究表明，菠菜Cyt b6f 中Chl a单线激发态快速淬灭过程中并没有发现Tyr与Chl a 之间发生电子传递时所形成的Chla•¯。以上结果表明，Cyt b6f 中Chl a单线激发态快速的淬灭确实和邻近的Tyr105有关，但是与Dashdorj 等提出的Chla单线激发态和Tyr105残基发生了电子交换传递从而淬灭Chla单线激发态这一想法不符，关于这一问题值得进一步深入研究。 3．红光和绿光对Cyt b6f 照射，Cyt f可以被还原，并且红光比绿光更容易使Cyt f 还原，暗示Cyt f 的还原与Chl a 的关系密切，有可能是Chl a 被激发后，其被激发的电子传给Cyt f。对这一现象的进一步研究表明，Cyt b6f在光照条件下，Cyt f 的还原与体系内C10－PQ库的氧化还原状况相关，氧化型的C10－PQ可能介导电子从Chla传向Cyt f。由于在体内质体醌库的氧化还原状态往往决定Cyt b6f 的氧化还原状态，而通过对Cyt b6f不同氧化和还原状态的吸收谱和荧光谱的研究表明，氧化态和还原态的Cyt b6f 中，Chl a 与蛋白的结合状态是有差异的。这些差异可能暗示着Chl a 分子在行使其功能时与复合体的氧化还原状态是有关系的。通过以上的结果，对Cyt b6f中Chl a 可能的功能进行了假设。 4．此外，我们还发现Cyt b6f具有明显的过氧化物酶活性。在0.1 mmol/L乙酸钠缓冲液，pH3.6，25℃，[H2O2] <40mmol/L的条件下，其催化反应的速度常数为kobs＝26±1.2M•s-1;对H2O2的Km 值为50mmol/L，对guaiacol的Km 值为2mmol/L;反应的最适pH为3.6，最适离子强度为0.1，最适温度为35℃。推测Cyt b6f的这种过氧化物酶活性可能是在胁迫环境下使Cyt b6f中的血红素和Chl a 分子免受H2O2的破坏。

Ultrafast photo-induced turning of magnetization and its relaxation dynamics in GaMnAs

We report that, by linearly polarized pumping of different wavelengths, Kerr transients appear at zero magnetic field only in the case when GaMnAs samples are initialized at 3 K by first applying a 0.8 Tesla field and then returning to zero field. We find that, instead of magnetization precession, the near-band gap excitation induces a coherent out-of-plane turning of magnetization, which shows very long relaxation dynamics with no precession. When photon energy increases, the peak value of the Kerr transient increases, but it decays rapidly to the original slow transient seen under the near-band-gap excitation.

An experimental study about the influence of well thickness on the electroluminescence of InGaN/GaN multiple quantum wells

The influence of well thickness on the electroluminescence (EL) of InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition is investigated. It is found that the peak wavelength of EL increases with the increase of well thickness when the latter is located in the range of 3.0-5.1 nm. The redshift is mainly attributed to the quantum confined Stark effect (QCSE). As a contrast, it is found that the EL intensity of InGaN/GaN MQWs increases with the increase of well thickness in spite of QCSE. The result of X-ray diffraction demonstrates that the interface become smoother with the increase of well thickness and suggests that the reduced interface roughness can be an important factor leading to the increase of EL intensity of InGaN/GaN MQWs. (C) 2009 Elsevier B.V. All rights reserved.

Tuning photoluminescence of single InAs quantum dot by electric field

By using photoluminescence (PL) and time-resolved PL spectra, the optical properties of single InAs quantum dot (QD) embedded in the p-1-n structure have been studied under an applied electric field With the increasing of electric field, the exciton lifetime increases due to the Stark effect. We noticed that the decrease or quenching of PL intensity with increasing the electric field is mainly due to the decrease of the carriers captured by QD.

Dynamics of photo-enhanced magneto-crystalline anisotropy in diluted ferromagnetic GaMnAs

By chopping a pump beam in conventional time-resolved Kerr rotation (TRKR) experiments and measuring the time evolution of M-shaped "major" hysteresis loops of magnetic linear dichroism (Delta MLD = MLDpump-on MLDpump-off), the differential MLD signal in the presence and the absence of the pump beam, we studied the dynamics of photo-enhanced magneto-crystalline anisotropy, and found that its very long recovering time (much longer than 13 ns) might reflect the nature of the coherent coupling between photo-excited holes and localized spins in the d shell of manganese.

IMPROVED PEAK POWER METHOD FOR MEASURING FREQUENCY RESPONSES OF PHOTODETECTORS IN A SELF-HETERODYNE SYSTEM

An improved peak power method for measuring frequency responses of photodetectors in a self-heterodyne system consisting of a distributed Bragg reflector laser is proposed. The time-resolved spectrum technique is used to measure the peak power of the beat signal and the intrinsic linewidth of heat signal for calibration. The experimental results show that the impact of the thermal-induced frequency drift, which is the main reason for producing an error in measurement by conventional peak power method and spectrum power method, can be removed.

Effects of external magnetic field on the effective g factor of (Ga,Mn)As

With the help of time resolved magneto-optic Kerr rotation measurements, the optically induced spin precession in heavily doped diluted magnetic semiconductor Ga0.937Mn0.063 As was observed. It was found that the effective g factor increases with increasing magnetic field, which is attributed to the magnetic-field-induced increase of the density of the non-localized holes. Those free holes will couple with the localized magnetic ions by p-d interactions, leading to the formation of spontaneous magnetization in Ga0.937Mn0.063As, which in turn to the enhancement of the effective g factor.

Optical study of lateral carrier transfer in (In,Ga)As/GaAs quantum-dot chains

We have studied the lateral carrier transfer in a specially designed quantum dot chain structure by means of time-resolved photoluminescence (PL) and polarization PL. The PL decay time increases with temperature, following the T-1/2 law for the typical one-dimensional quantum system. The decay time depends strongly on the emission energy: it decreases as the photon energy increases. Moreover, a strong polarization anisotropy is observed. These results are attributed to the efficient lateral transfer of carriers along the chain direction. (c) 2008 American Institute of Physics.

Effect of electron-electron scattering on spin dephasing in a high-mobility low-density two-dimensional electron gas

By utilizing time-resolved Kerr rotation techniques, we have investigated the spin dynamics of a high-mobility low density two-dimensional electron gas in a GaAs/Al0.35Ga0.65As heterostructure in the dependence on temperature from 1.5 to 30 K. It is found that the spin relaxation/dephasing time under a magnetic field of 0.5 T exhibits a maximum of 3.12 ns around 14 K, which is superimposed on an increasing background with rising temperature. The appearance of the maximum is ascribed to that at the temperature where the crossover from the degenerate to the nondegenerate regime takes place, electron-electron Coulomb scattering becomes strongest, and thus inhomogeneous precession broadening due to the D'yakonov-Perel' mechanism becomes weakest. These results agree with the recent theoretical predictions [J. Zhou et al., Phys. Rev. B 15, 045305 (2007)], which verify the importance of electron-electron Coulomb scattering to electron spin relaxation/dephasing.

Carrier spin relaxation in (Ga, Mn) As at room temperature

In this paper, the excitation energy density dependence of carrier spin relaxation is studied at room temperature for the as-grown and annealed (Ga, Mn) As samples using femtosecond time-resolved pump-probe Kerr spectroscopy. It is found that spin relaxation lifetime of electrons lengthens with increasing excitation energy density for both samples, and the annealed ( Ga, Mn) As has shorter carrier recombination and electron spin relaxation lifetimes as well as larger Kerr rotation angle than the as-grown ( Ga. Mn) As under the same excitation condition. which shows that DP mechanism is dominant in the spin relaxation process for ( Ga, Mn)As at room temperature. The enhanced ultrafast Kerr effect in the annealed (Ga,Mn)As shows the potential application of the annealed ( Ga, Mn) As in ultrafast all-optical spin switches, and also provides a further evidence for the p-d exchange mechanism of the ferromagnetic origin of (Ga, Mn) As.

Ultrafast carrier dynamics in undoped and p-doped InAs/GaAs quantum dots characterized by pump-probe reflection measurements

We investigate the dependence of the differential reflection on the structure parameters of quantum dot (QD) heterostructures in pump-probe reflection measurements by both numerical simulations based on the finite-difference time-domain technique and theoretical calculations based on the theory of dielectric films. It is revealed that the value and sign of the differential reflection strongly depend on the thickness of the cap layer and the QD layer. In addition, a comparison between the carrier dynamics in undoped and p-doped InAs/GaAs QDs is carried out by pump-probe reflection measurements. The carrier capture time from the GaAs barrier into the InAs wetting layer and that from the InAs wetting layer into the InAs QDs are extracted by appropriately fitting differential reflection spectra. Moreover, the dependence of the carrier dynamics on the injected carrier density is identified. A detailed analysis of the carrier dynamics in the undoped and p-doped QDs based on the differential reflection spectra is presented, and its difference with that derived from the time-resolved photoluminescence is discussed. (C) 2008 American Institute of Physics.

Temperature dependence of effective g factor in diluted magnetic semiconductor (Ga,Mn)As

Time resolved magneto-optic Kerr rotation measurements of optically induced spin quantum beats are performed on heavily doped bulk (Ga,Mn)As diluted magnetic semiconductors (DMS). An effective g-factor of about 0.2-0.3 over a wide range of temperature for both as-grown and annealed (Ga,Mn)As samples is obtained. A larger effective g-factor at lower temperature and an increase of the spin relaxation with increasing in-plane magnetic field are observed and attributed to the stronger p-d exchange interaction between holes and the localized magnetic ion spins, leading to a larger Zeeman splitting and heavy-hole-light-hole mixing. An abnormal dip structure of the g-factor in the vicinity of the Curie temperature suggests that the mean-field model is insufficient to describe the interactions and dynamics of spins in DMS because it neglects the short-range spin correlation effect. (c) 2008 American Institute of Physics.