光拍法测光速实验中的几个误解

利用光拍法测光速的实验中主要存在3个误解.笔者结合实验教学,从光拍频波的原理、光拍频波的形成和传播以及光拍频波的接收和光路调节3个方面,对光拍法测光速实验中易产生误解的内容进行了剖析并给予澄清.     

关于探照灯照射的光斑速度

光速(真空中)是自然界物体运动或信息及能量传递的极限,如果不注意这个条件,一般地谈速度,那么,找寻超光速的现象并不是难事.有些教科书上时常以探照灯光斑移动的速度来阐述这种超光速现象,演算得到光斑移动的速度为v=hωcos2θ.本文指出,此结果是错的!对正确结果做的定量计算表明:光斑速度虽仍然能超光速,但正确结果的超光速条件远比错误结果的超光速复杂,而且物理含义也更加丰富.     

Deformation in the light Br isotopes

The nuclear ground state spins of the odd-A Br nuclides^{75, 77, 79}Br with Z=35 are all 3/2⁻. Nilsson orbital calculations show that the 35^th proton occupies the f_5/2 [301]3/2⁻ orbital for ε<0.20 and the p_3/2 [312]3/2⁻ orbital at larger deformations. The magnetic moments of these two states differ by a factor of two, giving clear evidence for the magnitude of the ground state deformation. Low Temperature Nuclear Orientation measurements made in Oxford and Bonn on^{76, 77g}BrFe prepared at ISOLDE, and on line at the SERC Daresbury Laboratory on^{72, 74m, 75}BrFe, have yielded the magnetic moments of these isotopes, using a two non-zero field model with magnetic hyperfine fields of +81.38(6) T and 26(2) T. The spin of the⁷³Br ground state is also deduced. An interpretation of the ground state configurations of these isotopes is given.     

Gravitation in the light cone gauge

The formulation of gravitation theory in the light cone gauge is studied. After a brief discussion of Yang- Mills theory for purposes of illustration, tensor and scalartensor gravitation are investigated. We show that if the gauge conditions are properly chosen the constrained components of the metric tensor can be explicitly solved for by quadrature, so that the field theory can be reformulated entirely in terms of the physical transverse fields. It is also shown that the light cone gauge is useful for finding wave solutions of classical field equations. Occasional reference is made to dual models, primarily to explain our motivation, but familiarity with them is not required for an understanding of this paper.     

对单轴晶体中光的性质和双折射问题的几点讨论

指出了关于光在单轴晶体中的传播、偏振及双折射问题中的几个模糊概念或论点;证明了在单轴晶体中属同一波矢的两个线偏振光光矢量的偏振方向也是相互垂直的,而且此两光的主平面是相互重合的;分析了在双折射教学中尚存的不足之处和个别欠妥的提法;指出在讨论双折射问题时必须加入适当的基础理论,而仅用定性作图法将对理解掌握该部分内容带来某些弊端．     

探照灯光斑移动现象的再讨论

指出用传统的"平行线"模型和光速有限来解释光斑移动现象,当将情况推广到极限时,会出现一些局限,并提出了一种新的与实际情况更符合的"同心圆"模型,由此可以解释光斑移动现象.     

Driving light pulses with light in two-level media

A two-level medium, described by the Maxwell-Bloch system, is engraved by establishing a standing cavity wave with a linearly polarized electromagnetic field that drives the medium on both ends. A light pulse, polarized along the other direction, then scatters the medium and couples to the cavity standing wave by means of the population inversion density variations. We demonstrate that control of the applied amplitudes of the grating field allows one to stop the light pulse and to make it move backward (eventually to drive it freely). A simplified limit model of the Maxwell-Bloch system with variable boundary driving is obtained as a discrete nonlinear Schr?dinger equation with tunable external potential. It reproduces qualitatively the dynamics of the driven light pulse.     

Slow and fast light in liquid crystal light valves

We show that fast and slow light results from multiple scatterings in a liquid crystal light valve, where nondegenerate two-wave mixing occurs in the Raman-Nath regime of optical diffraction. The large nonlinear response and dispersive characteristics of the liquid crystals allow us to obtain group velocities as slow as less than 0.2 mm/s, which is attractive for the realization of ultrahigh precision interferometers and metrology measurements.     

Nuclear structure in the light of QCD

The aim of this note is to discuss whether conventional nuclear theory may be constructed as a part of general gauge theory.Presented at the symposium Mesons and Light Nuclei, Bechyn, Czechoslovakia, May 27–June 1, 1985.     

Beyond-limit light focusing in the intermediate zone

We experimentally verify that a new nanolens of a designed plasmonic aperture can focus visible light to a single line with its width smaller than the limit of half the wavelength in the intermediate zone. The experimental measurement indicates that while the near field plays a role to increase the spot size in the near zone, it is negligible at the beyond-limit focused region; i.e., the focused light is dominated by the radiative fields. The image taken by the optical microscope shows that the fields focused have propagated to the far zone. Besides being of academic interest, the nanolens capable in achieving a lower diffraction limit in the intermediate zone is important for application possibilities.     

Shape coexistence in the light Po isotopes

The systematics of the even light Po isotopes (N ⩽ 126) are studied in the framework of the Particle-Core Model. The strong perturbation of the systematics in the very light isotopes is interpreted as arising from the interaction between regular and intruder structures. Results of Potential Energy Surface (PES) calculations and predictions of the Pairing Vibration Model support this interpretation. The mixing between the regular and intruder structures is studied within the IBM-2 and in a simple two-state mixing picture. Matrix elements of the interaction and their spin dependence are extracted. The ‘reconstructed systematics’ show the coexistence of a spherical structure, which varies little with the neutron number, with an intruder band, strongly lowered in energy as the neutron number approaches midshell. The crossing of the two configurations takes place over a few isotopes; the intruder band becomes the ground-state configuration in ¹⁹²Po.     

On the rotational states in light nuclei

Some properties of the projected Hartree-Fock functions which describe the ground state bands in ⁸Be, ¹²C and ²⁰Ne, and the excited 8p-8h band in ¹⁶O are studied in order to answer the question whether these states correspond to real rotational motion in these light nuclei.     

Background light measurements in the deep ocean

Summary Ambient light intensities in the ocean at depths between 1500 m and 4700 m near Hawaii Island were measured around the one photoelectron level with 5″ diameter hemispherical photomultipliers. Measurements of count rates above variable thresholds were carried out in ship-suspended and bottom-tethered configurations. The ship-suspended rates show considerable fluctuation and their mean value decreases with depth approximately as exp [−x(m)/877]. The bottomtethered rates are about an order of magnitude lower than the ship-suspended rates and show little fluctuation. The calibration of our instrument indicates an absolute flux at 4700 m depth based on the bottom-tethered measurement of 218 ₋₆₀ ⁺²⁰ photons/cm²·s, which is consistent with calculated intensities due to β-decay electrons from⁴⁰K. The difference in the two cases is attributed to bioluminescence due to environmental stimulation.

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Riassunto Sono state misurate le intensità di luce ambiente nell'oceano a profondità tra 1500 m e 4700 m vicino alle isole Hawaii intorno al livello ad un fotoelettrone con fotomoltiplicatori emisferici di 5″ di diametro. Sono state effettuate le misure delle frequenze di conteggio oltre alle soglie delle variabili in configurazioni appese alla nave e ancorate al fondo. Le frequenze delle configurazioni appese alla nave mostrano una fluttuazione considerevole e il loro valore medio decresce con la profondità come exp [−x(m)/877]. Le frequenze delle configurazioni ancorate al fondo sono circa di un ordine di grandezza inferiore di quelle delle configurazioni appese alla nave e mostrano una piccola fluttuazione. La calibrazione del nostro strumento indica un flusso assoluto a 4700 m di profondità basato sulla misura delle configurazioni ancorate al fondo di 218 ₋₆₀ ⁺²⁰ fotoni/cm²·s, che è consistente con le intensità calcolate dovute al decadimento β degli elettroni da⁴⁰K. Si attribuisce la differenza nei due casi alla bioluminescenza dovuta alla stimolazione ambientale.

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Резюме Измеряются интенсивности свечения окружающей среды в океане на глубинах от 1500 м до 4700 м вблизи Гавайских островов с помощью 5-дюймовых полусферических фотоумножителей. йзмерения скоростей счета выше изменяющихся порогов были проведены в случае подвешенном на корабле и в случае прикрелления ко дну. Интенсивности счета в подвешенном состояиии обнаруживают значительные флуктуации и их средняя величина уменьшается с глубиной приблизительно как ехп [−x(m)/877]. Интенсивности счета в случае прикрепления ко дну примерно на порядок меньше интенсивностей в подвешенном состоянии и обнаруживаут малые флуктуации. Градуировка нашей аппаратуры даст на основе измерений в случае прикрепления ко дну величину абсолутного потока на глубине 4700 м, равную 218 ₋₆₀ ⁺²⁰ фотонов/см² с, которая согласуется с вычисленными интенсивностями, обусловленными электронами β-распада от⁴⁰К. Различия, обнаруженные в двух случаях, приписываются биолюминесценции, связанной с возбуждением окружающеи среды.

     

PandaX limits on the light dark matter with a light mediator in the singlet extension of MSSM

Using the latest PandaX limits on the light dark matter(DM) with a light mediator, we check their implication on the parameter space of the general singlet extension of MSSM(without Z_3 symmetry), which can have a sufficient DM self-interaction to solve the small-scale structure problem. We find that the PandaX limits can tightly constrain the parameter space, depending on the coupling λ between the singlet and doublet Higgs fields. For the singlet extension of MSSM with Z_3 symmetry, the so-called NMSSM, we also demonstrate the PandaX constraints on its parameter space, which gives a light DM with the correct relic density but without sufficient self-interaction to solve the small-scale structure problem. We find that in NMSSM, the GeV dark matter with a sub-GeV mediator is tightly constrained.     

An approach to remove the background light based on linearly polarized light in the inter-satellite optical communications

In the inter-satellite optical communications, background light is an important factor that worsens the acquisition performance and tracking precision of the system. In general, optical filters are applied in eliminating background light. But the bandwidth of the optical filter is a bottleneck for improving the performance of the optical system further. We propose a new method for decreasing background light, in which linearly polarized light is employed as the beacon light. The theoretical model of acquisition probability is derived. Contrastive analysis is performed, among the system with the pure optical filter, the system using the linearly polarized light and the system with the circularly polarized light. Numerical simulations and experimental verification lead to the conclusion that acquisition probability of the system with linearly polarized light is greater than that of the other systems, and this approach is effective. This work can benefit the design of inter-satellite optical communication system.     

Generation of quadrature-squeezed light during the propagation of a light wave in a birefringent fiber

It is shown that generation of quadrature-squeezed states of a vector electromagnetic field in which quantum fluctuations in one of the quadrature components are smaller than in the coherent state can occur in cubically nonlinear media (birefringent fibers) with efficient energy transfer between the polarization modes of the field. It is shown that for certain distributions of the initial total power between modes, light with suppressed quantum fluctuations in both polarization modes of the vector field is formed at the fiber exit. The optimal conditions for obtaining quadrature-squeezed light are determined. New analytical expressions are obtained for the degree of squeezing in the two polarization modes in the case when there is no energy transfer between the polarization components of the field propagating in the fiber (eigenmodes of two-wave mixing).