Microresonator Kerr frequency combs with high conversion efficiency

Microresonator‐based Kerr frequency comb (microcomb) generation can potentially revolutionize a variety of applications ranging from telecommunications to optical frequency synthesis. However, phase‐locked microcombs have generally had low conversion efficiency limited to a few percent. Here we report experimental results that achieve conversion efficiency ( on‐chip comb power excluding the pump) in the fiber telecommunication band with broadband mode‐locked dark‐pulse combs. We present a general analysis on the efficiency which is applicable to any phase‐locked microcomb state. The effective coupling condition for the pump as well as the duty cycle of localized time‐domain structures play a key role in determining the conversion efficiency. Our observation of high efficiency comb states is relevant for applications such as optical communications which require high power per comb line.

     

Multiple frequency conversion via atomic spin coherence of storing a light pulse

We experimentally demonstrate multiple frequency conversion via atomic spin coherence of storing a light pulse in a doped solid.The essence of this multiple frequency conversion is four-wave mixing based on stored atomic spin coherence.Through electromagnetically induced transparency,an input probe pulse is stored into atomic spin coherence by modulating the intensity of the control field.By using two different control fields to interact with the coherently prepared medium,the stored atomic spin coherence can be transformed into three different information channels.Multiple frequency conversion is implemented efficiently by manipulating the spectra of the control fields to scatter atomic spin coherence.This multiple frequency conversion is expected to have potential applications in information processing and communication network.     

Theory of cascade tripler of optical frequency with high conversion efficiency

The regimes of high conversion efficiency of cascade triplers of optical radiation are theoretically investigated. A conversion efficiency of 100% for plane waves can be reached. Analytical expressions for the wave amplitudes in this case are given. The conversion efficiencies for a laser beam with Gaussian distribution of the amplitude in space and time are numerically computed. The maximum efficiency for such a beam is about 50%. Some experimental situations are discussed.     

On-demand superradiant conversion of atomic spin gratings into single photons with high efficiency

We create quantized spin gratings by single-photon detection and convert them on demand into photons with retrieval efficiencies exceeding 40% (80%) for single (a few) quanta. We show that the collective conversion process, proceeding via superradiant emission into a moderate-finesse optical resonator, requires phase matching. The storage time of 3 micros in the cold-atom sample, as well as the peak retrieval efficiency, are likely limited by Doppler decoherence of the entangled state.     

Entanglement in a three spin system controlled by electric and magnetic fields

We study the effect of electric field and magnetic flux on spin entanglement in an artificial triangular molecule built of coherently coupled quantum dots. In a subspace of doublet states an explicit relation of concurrence with spin correlation functions and chirality is presented. The electric field modifies superexchange correlations and shifts many-electron levels (the Stark effect), as well as changing spin correlations. For some specific orientation of the electric field one can observe monogamy, for which one of the spins is separated from two others. Moreover, the Stark effect manifests itself in a different spin entanglement for small and strong electric fields. The role of magnetic flux is opposite: it leads to circulation of spin supercurrents and spin delocalization.     

High X-ray conversion efficiency with target irradiation by a frequency tripled Nd : Glass laser

High conversion efficiency of laser energy into X-rays from a laser irradiated target is of great interest for a variety of dynamical (pulsed) studies, e.g.: radiography of laser-imploded targets, structure determination by diffraction and absorption fine-structure, and X-ray laser pumping. We report here on a frequency tripled Nd : glass laser used to irradiate targets of various materials at ～5 x 10¹⁴W/cm². We find conversion efficiencies of between 1% and 0.1% (with respect to the incident laser energy) for individual X-ray lines between 1.8 and 7.8 keV. These efficiencies are more than an order of magnitude higher than whose achieved with 1.06 μm lasers.     

激光线宽对单次通过PPMgO:LN晶体倍频效率的影响

通过单次穿过PPMgO:LN晶体产生了2.06 W的780 nm可调谐的连续倍频光. 采用1560 nm的分布反馈式(DFB)半导体激光器、光栅外腔半导体激光器(ECDL)和分布反馈式掺铒光纤激光器(DFB-EDFL)分别作为掺铒光纤放大器(EDFA)的注入光源, 所用的EDFA具有保持窄线宽的功能, 因此可以忽略它对基波线宽的展宽. 研究了激光线宽对单次通过PPMgO:LN 晶体的倍频效率的影响. 控制三台激光器各自注入EDFA的功率一致, 同时也保持EDFA 的输出功率. 在基波功率为12.42 W 时, 使用DFB半导体激光器注入EDFA时得到了1.36 W的780 nm倍频光输出, 转换效率为11.0%; 使用ECDL作为种子源时得到了1.78 W 的780 nm倍频光输出, 转换效率为14.3%; 使用DFB-EDFL作为种子源时得到了2.06 W的780 nm倍频光输出, 转换效率为16.6%. 测得三台种子激光器的线宽分别为1.2 MHz (DFB), 200 kHz (ECDL)和600 Hz (DFB-EDFL). 线宽越窄, 倍频效率越高, 实验结果与理论分析一致.     

Exploring the limits of polarization transfer efficiency in homonuclear three spin systems

The limits of polarization transfer efficiency are explored for systems consisting of three isotropically coupled spins 1/2 in the absence of relaxation. An idealized free evolution and control Hamiltonian is studied, which provides an upper limit of transfer efficiency (in terms of transfer amplitude and transfer time) for realistic homonuclear spin systems with arbitrary Heisenberg-type coupling constants J12, J13, and J23. It is shown that optimal control based pulse sequences have significantly improved transfer efficiencies compared to conventional transfer schemes. An experimental demonstration of optimal polarization transfer is given for the case of the carbon spin system of fully 13C labelled alanine at 62.5 MHz Larmor frequency.     

Current to frequency conversion in a Josephson circuit

The voltage oscillations which occur in an ideally current-biased Josephson junction were proposed to make a current standard for metrology. We demonstrate similar oscillations in a more complex Josephson circuit derived from the Cooper pair box: the quantronium. When a constant current I is injected in the gate capacitor of this device, oscillations develop at the frequency f(B)=I/2e, with e the electron charge. We detect these oscillations through the sidebands induced at multiples of f(B) in the spectrum of a microwave signal reflected on the circuit, up to currents I exceeding 100 pA. We discuss the potential interest of this current-to-frequency conversion experiment for metrology.     

The effect of electron spin on plasma frequency of a narrow-gap semiconductor in quantizing magnetic fields

The effect of the electron spin on the plasma frequency of a narrow-gap semiconductor is studied including nonparabolicity of the band structure. The effect of spin is found to enhance the plasma frequency, particularly for high magnetic fields.     

Switching of a spin valve with three magnetic layers

A spin valve with two pinned ferromagnetic layers sandwiching a free ferromagnetic layer with a thickness smaller than the spin diffusion length in the same layer and than the domain wall thickness is considered. The instability conditions are determined for various mutual orientations of the magnetization of the layers. The possibility of a considerable decrease in the instability threshold due to joint action of spin-polarized current and an external magnetic field is indicated. It is shown that in addition to collinear states, a nonequilibrium noncollinear state can exist, into which the system is switched after exceeding the instability threshold.     

Dynamic behaviour of a beam subjected to a force of time-dependent frequency

It is a well-known fact that, for an ideal system in which damping is ignored, the deflection of a beam becomes infinite for the case where the constant frequency of a steady-state external force is equal to the critical frequency of the beam. In this paper, the external force is assumed to be proportional to $\text{sin}\text{(}\text{at}{}^2\text{2}\text{+}\text{bt}{}^3\text{3}\text{)}$ with respect to time, where a and b are constants, and the effect of time-dependent frequency on the dynamic behaviour of the beam is investigated. Integrations involved in the theoretical results are carried out by Simpson's rule. From the results of the theoretical analysis, it is evident that the maximum values of the dynamic deflection are much affected by the time-dependence of the frequency of the external force when it passes through the critical frequency, increasing or decreasing.     

The conversion efficiency of a piezoelectric quartz crystal: Relation between input electrical power and frequency

The dependence of the conversion efficiency of a piezoelectric transducer on its frequency and the input electrical power has been examined and it is found that the efficiency is dependent not on the input power but on frequency and the nature of the liquid     

From free fields to AdS: The case of particles with spin

It is shown that one-loop diagrams with three external lines in the usual conformal field theory in D dimensions are equal to a finite sum of tree diagrams with one vertex in theory with the AdS _{D + 1} metric. Some terms of the dual Lagrangian that are responsible for interaction are calculated. Spin-0, spin-1/2, and spin-1 particles are considered.     

Compensation of phase-to-amplitude modulation conversion in a complete frequency conversion system with an all-fiber system

We report on an experimental demonstration of linear precompensation of nonlinear phase and amplitude transfer functions. We show the effective compensation with a linear all-fiber system of phase-to-amplitude modulation conversion due to a complete frequency conversion system including plane gratings and a nonlinear crystal.     

Local fields with the wrong connection between spin and statistics

It is proved that there exist free field operators which satisfy local commutativity and which transform according to certain unitary representations of the homogeneous Lorentz group. The fields satisfy axioms similar to the Wightman axioms, and give rise to local algebras of observables obeying postulates similar to those suggested byHaag. They describe a tower of particles with spins 1/2, 1 1/2, 2 1/2, ..., but commute at space-like separation, giving rise to Bose statistics for the particles. This shows that the well-known theorem on spin and statistics cannot be extended to general theories of local observables; it also shows that the assumptions made in S-matrix theory do not hold for theS-matrix of a theory of interacting infinite fields.The research reported in this document has been sponsored in part by the Air Force Office ofScientific Research OAR through the European Office Aerospace Research U.S. Air Force.     

Measurement of electron spin lifetime and optical orientation efficiency in germanium using electrical detection of radio frequency modulated spin polarization

We propose and demonstrate a technique for electrical detection of polarized spins in semiconductors in zero applied magnetic fields. Spin polarization is generated by optical injection using circularly polarized light which is modulated rapidly using an electro-optic cell. The modulated spin polarization generates a weak time-varying magnetic field which is detected by a sensitive radio-frequency coil. Using a calibrated pickup coil and amplification electronics, clear signals were obtained for bulk GaAs and Ge samples from which an optical spin orientation efficiency of 4.8% could be determined for Ge at 1342 nm excitation wavelength. In the presence of a small external magnetic field, the signal decayed according to the Hanle effect, from which a spin lifetime of 4.6±1.0 ns for electrons in bulk Ge at 127 K was extracted.     

Terahertz generation based on parametric conversion: from saturation of conversion efficiency to back conversion

By stacking alternatively rotated gallium phosphide (GaP) plates, the maximum photon conversion efficiency of 40% for the terahertz (THz) generation based on difference-frequency generation has been achieved. The corresponding peak power generated inside the four GaP plates approaches 4 kW. As the number of plates is increased from four to five, the THz output power is significantly decreased, due to back parametric conversion.     

Stability of a system of three degrees of freedom subjected to a circulatory force

The free vibrations of a uniformly accelerating linear elastic, non-dissipative system of three degrees of freedom, carrying a tip mass and subjected to a constant circulatory thrust applied at a free end, are investigated for the purpose of determining the influence of the orientation of the thrust on the system's state of stability. A dimensionless bi-cubic frequency equation whose coefficients depend upon the tip mass parameter, the thrust parameter, and the tangency coefficient, α, is solved and eigencurves are plotted for several values of α. These eigencurves reveal that, for a certain range of small values of α, the system becomes unstable by divergence for any positive thrust, no matter how small. There also exists a range of α for which the system is first unstable by divergence, secondly stable, and finally unstable by flutter as the magnitude of the thrust is increased. For α sufficiently large, the system is stable for a certain range of values of the thrust, but eventually it becomes unstable by flutter as the thrust is increased beyond its critical value.