Dynamic nuclear polarization in silicon microparticles

We report record high 29Si spin polarization obtained using dynamic nuclear polarization in microcrystalline silicon powder. Unpaired electrons in this silicon powder are due to dangling bonds in the amorphous region of this intrinsically heterogeneous sample. 29Si nuclei in the amorphous region become polarized by forced electron-nuclear spin flips driven by off-resonant microwave radiation while nuclei in the crystalline region are polarized by spin diffusion across crystalline boundaries. Hyperpolarized silicon microparticles have long T1 relaxation times and could be used as tracers for magnetic resonance imaging.     

Optical nuclear spin polarization in quantum dots

Hyperfine interaction between electron spin and randomly oriented nuclear spins is a key issue of electron coherence for quantum information/computation. We propose an efficient way to establish high polarization of nuclear spins and reduce the intrinsic nuclear spin fluctuations. Here, we polarize the nuclear spins in semiconductor quantum dot(QD) by the coherent population trapping(CPT) and the electric dipole spin resonance(EDSR) induced by optical fields and ac electric fields. By tuning the optical fields, we can obtain a powerful cooling background based on CPT for nuclear spin polarization. The EDSR can enhance the spin flip–flop rate which may increase the cooling efficiency. With the help of CPT and EDSR, an enhancement of 1300 times of the electron coherence time can be obtained after a 10-ns preparation time.     

Optical nuclear polarization — Targets and sources

The nuclear polarization of³He by optical pumping using discharge lamps is described, as well as attempts (both successful and unsuccessful) to make a polarized³He target. Recent experimental results with a polarized³He target are presented. Experiments to polarize³He using a laser as a pumping light source are described, and the prospects for producing a dense polarized³He target are indicated. The polarization of other nuclei, especially Xe and Kr, by spin transfer from laser polarized alkalis is described.On Leave of Abscence From, Department of Physics, University of Toronto, Toronto, Ontario, Canada, M5S 1A7     

P原子的光学模型势与核极化修正

基于SIC-X_α的较为严格的计算方法,对于Rydberg电子态的交换参数采用自洽场模型计算核极化,结果比文献[15[的计算方法要精确.另外文献[15]在计算P²³⁸U的能级跃迁时,模型还不够完善,存在着难以克服的弊端,计算结果也不可能准确.20世纪70年代初,Batty就进行强相互作用势模型的研究,经过二十几次不断探索与改进,在1997—2000年给出了光学模型势的正确形式.对Rydberg电子态的交换参数采用自洽场模型计算核极化,用以修正Batty光学模型势下的P²³⁸U能级跃迁,得到反质子原子的相应能级.结果与实验数据相当符合.连同π^-,K^-,∑^-,Ξ^-原子的情况支持了Batty光学模型势在描写核子间强相互作用的正确性,同时也表明计算核极化的方法是正确的,为反粒子原子及奇特原子的深入研究及应用提供了理论依据.     

Optical nuclear polarization for sensitivity enhancement of2H NMR single-crystal studies

A gain in detection sensitivity of more than three orders of magnitude is achieved in high-resolution solid-state²H nuclear magnetic resonance of monocrystalline fluorene-d₁₀ by applying optical nuclear polarization via excited triplet states of acridine-h₉ guest molecules. The sensitivity gain is utilized to measure the angular dependence (rotation pattern) of the²H nuclear magnetic resonance lines. In this way the principal values and orientations of all²H quadrupolar tensors are determined. Except for the methylene deuterons, all tensors belonging to the same molecule have one principal axis in common, namely the axis perpendicular to the molecular plane, showing that in the crystal lattice the fluorene molecule is in a planar configuration.     

Dynamic nuclear polarization and nuclear orientation in an antiferromagnet

Dynamic nuclear polarization is a well established technique which has been used to produce polarized targets for experiments in nuclear physics. This paper suggests experiments of a similar type but involving the nuclear magnetic resonance of two isotopes, one stable and the other radioactive. The substance is an antiferromagnet, dysprosium phosphate, at temperatures below the Néel point, where line widths are comparatively small. The effect may be detected through changes in the rate of gamma ray emission observed by a nuclear orientation experiment.     

High-frequency dynamic nuclear polarization in the nuclear rotating frame

A proton dynamic nuclear polarization (DNP) NMR signal enhancement (epsilon) close to thermal equilibrium, epsilon = 0.89, has been obtained at high field (B(0) = 5 T, nu(epr) = 139.5 GHz) using 15 mM trityl radical in a 40:60 water/glycerol frozen solution at 11 K. The electron-nuclear polarization transfer is performed in the nuclear rotating frame with microwave irradiation during a nuclear spin-lock pulse. The growth of the signal enhancement is governed by the rotating frame nuclear spin-lattice relaxation time (T(1rho)), which is four orders of magnitude shorter than the nuclear spin-lattice relaxation time (T(1n)). Due to the rapid polarization transfer in the nuclear rotating frame the experiment can be recycled at a rate of 1/T(1rho) and is not limited by the much slower lab frame nuclear spin-lattice relaxation rate (1/T(1n)). The increased repetition rate allowed in the nuclear rotating frame provides an effective enhancement per unit time(1/2) of epsilon(t) = 197. The nuclear rotating frame-DNP experiment does not require high microwave power; significant signal enhancements were obtained with a low-power (20 mW) Gunn diode microwave source and no microwave resonant structure. The symmetric trityl radical used as the polarization source is water-soluble and has a narrow EPR linewidth of 10 G at 139.5 GHz making it an ideal polarization source for high-field DNP/NMR studies of biological systems.     

Nonequilibrium polarization of triplet centers in silicon

The Liouville equation is considered for the density operator of a triplet center with phenomenological population and decay constants (K₊ and K_-). In the case of anisotropy of K- with respect to the internal magnetic axis of the triplet, the spin polarization and intensities of the EPR lines of the triplet center in a strong magnetic field are calculated. On this basis, an interpretation of experiments on Si-Si centers in irradiated silicon is given. Quantum-chemical calculations of the model of the center in the form of a cluster OSi₄H₁₂ are performed, taking account of the spin-orbital interaction, the anisotropy of which agrees with the EPR spectra of Si-Si centers in silicon.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 97–101, June, 1989.     

Dynamic nuclear polarization and nuclear orientation in terbium vanadate

Dynamic nuclear polarization is a well established technique, which has been used to produce polarized targets for experiments in nuclear physics. This paper suggests new experiments, involving the nuclear magnetic resonance of two isotopes, one stable, the other radioactive, in an antiferromagnet, terbium vanadate. At temperatures well below the Néel point, the line widths should be comparatively small. Resonance may be detected through changes in the rate of gamma-ray emission observed by a nuclear orientation experiment.     

Single polarization transmission in pedestal-supported silicon waveguides

In this Letter, properties of a pedestal-supported silicon waveguide are investigated, showing that it supports single polarization transmission. The pedestal is fabricated easily through a wet-etching process on strip waveguides. Theoretical analysis shows that this property is due to the leakage of quasi-TM mode when the pedestal width is small. A polarization extinction ratio larger than 20 dB at 1550 nm is measured in the pedestal waveguide sample, demonstrating single polarization transmission property experimentally. Thanks to its large single polarization transmission bandwidth, robustness in fabrication tolerance, and simple fabrication process, pedestal waveguides will have potential applications as simple silicon-integrated polarizers.     

Nuclear relaxation in dynamic nuclear polarization

The behavior of nuclear polarization in a substance, i.e., a solution of the complex HMBA(Cr^V)Na⁺ in 1,2-propylene glycol used in polarized nuclear targets is experimentally investigated by magnetic spectroscopic methods under conditions of dynamic nuclear polarization at hv_S/kT=≈1.5−3.2. Nuclear polarization is measured and analyzed as a function of time at different values of the saturating microwave signal and temperature. It is shown that the process of decreasing the nuclear polarization involving free nuclear relaxation is described by a nonmonoexponential law with two damping decrements, which determine the time of reaching equilibrium between the Zeeman nuclear subsystem, the dipole-dipole pool, and the lattice. Specific features of dynamic processes proceeding in the electronic-nuclear system of the substance investigated are discussed. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 3, pp. 363–366, May–June, 1997.     

Improvement of polarization extinction in silicon waveguide devices

The SOI based waveguide devices are found to be highly polarization sensitive. Unwanted polarization excitations can be attenuated by integrating a TE- or TM-pass polarizer. A large attenuation of TM-polarized light has been observed when a thin film of metal is coated on the top of silicon rib waveguide, while TE-polarized light remains almost unaffected. The attenuation of TM-polarized light is attributed to the plasmonic absorption of the evanescent field in the metal cladding. Typically, with an Al cladding of thickness ～ 100 nm and a length of 1 mm on top of a single-mode (λ ～ 1550 nm) SOI rib waveguide structure, TE vs TM extinction ratio of ～ 15 dB has been obtained. Integrating such waveguide polarizers in a directional coupler and MZI based DWDM channel interleaver, we have also achieved an improvement in polarization extinction by ～ 15 dB.     

Estimates of the spontaneous polarization in silicon carbide

Various approaches to calculating the spontaneous polarization P _sp for different polytypes of silicon carbide are discussed. Our estimates combined with data reported by other authors reveal a considerable scatter in the values of P _sp for the 2H-SiC polytype (from ?1.11 to ?4.32 × 10^?2 C/m²). The need for further studies is stressed.     

铷原子蒸汽中的光偏振旋转效应

对铷原子(⁸⁷Rb)蒸汽中的法拉第旋转、光学偏振自旋转以及二者的叠加旋转效应进行了理论和实验研究.对三种情况下旋转现象建立了简单而有效的理论模型. 实验中把铷原子泡置于自行设计的磁屏蔽腔内, 以屏蔽地磁场的影响.实验选择⁸⁷Rb F=2→F'=3能级D2跃迁线并采用零多普勒光谱 实验结构消除多普勒展宽对实验光谱的影响. 实验中分别观测到了三种旋转现象,实验结果与理论模拟结果非常符合.     

Thermally induced nuclear dipolar polarization in powders

Photon spin symmetry conversion of methyl groups undergoing tunnelling rotation in powder samples at low temperature is accompanied by induced proton dipolar polarization.     

Dynamic nuclear polarization in electron spin echo

It is shown that under the action of a proper microwave pulse sequence the equilibrium polarization of the electron spin may be transferred dynamically to the longitudinal nuclear magnetization which will oscillate due to the nuclear spin precession around the effective fields relating to differnt electron quantum number manifolds. These oscillations may be measured directly in the radiofrequency band. Analytical formulae are obtained for the case when all the nuclei coupled to an unpaired electron have spins of 1/2.     

Dynamic nuclear polarization in pulsed ENDOR experiments

Properly prepared pulse sequences of microwave and radio frequency have been employed to investigate the effect of polarization transfer from the polarized photo excited triplet state of pentacene in p-terphenyl crystals to the surrounding protons in pulsed ENDOR experiments. The ENDOR signal, measured as the change of electron spin echo (ESE) amplitude, is affected by the mode of RF pulses. When B0 parallelx (the long molecular axis), the ESE amplitude of the high-field transition of the triplet state changes from the maximum positive to zero with a pi RF pulse, and to the maximum negative with a 2pi pulse, while that of the low-field transition changes from nearly zero to the maximum negative as the RF pulse width increases. The effect is attributed to the strong electron spin polarization produced in the creation of the photoexcited triplet state and the subsequent efficient electron- nuclear polarization transfer process.     

Optical absorption in silicon doping superlattices

The optical absorption beyond the fundamental edge in moderate to large period silicon doping superlattices of various designs is studied theoretically. At 1.3 μm, the largest absorption coefficient found is 0.2 cm⁻¹. Only moderate external biases are required to reach maximum absorption.