光场的线宽对无粒子数反转光放大增益的影响

针对拉曼驱动场下的四能级理论模型，讨论了实现无吸收、高折射率的条件，并分析了探测场及相干激发场的线宽对介质的折射率及增益的影响。计算结果表明，改变相干激发场 场强，或改变相干场的频率与原子能级间的失谐均可获得介质的无吸收，高折射率。激发扬线宽破坏了原子的相干性，使使介质的增闪下降并改变了折射率的大小，探测场的线宽使介质的探测增益谱及折射率谱的分辨率下降。     

The influence of EIT effect with double windows on electromagnetic characteristics of quasi-Λ-four-level atomic system

In this paper, the electromagnetic characteristics of quasi-Λ-four-level atomic system influenced by electromagnetically induced transparency (EIT) effect with double windows are investigated theoretically. It is shown that the permittivity can get two negative extreme values in transparent window extents and the permeability can realize negative value. Consequently, the atomic system can obtain the negative refraction in discontinuous probe detuning frequency ranges. EIT effect with double transparent windows would extend the tuning frequency range of realizing left handedness in atomic system. Furthermore, due to the strong magnetic transition, the real part of the refractive index and the absorption of atomic system can obtain negative values in the same certain frequency band. Such left-handed quasi-Λ-four-level atomic system may become an active medium.     

Phase control of probe response in an open ladder type system with spontaneously generated coherence

The control role of the relative phase between the probe and driving fields on the gain and dispersion in an open three-level ladder atomic system with incoherent pumping and spontaneously generated coherence is investigated. It is shown that by adjusting the value of the relative phase, the switching from lasing with inversion to lasing without inversion (LWI) can be realized; high index of the refraction with zero absorption can be achieved. It is also found that when the driving field is on resonance, the dispersion curve versus the probe detuning is very similar to the gain curve if the relative phase corresponding to the dispersion advances π/2 than that corresponding to the gain. LWI gain from the open system is obviously larger than that from corresponding closed system. When the incoherent pumping is absent, the open system can get LWI gain, moreover the gain is larger than that when the incoherent pumping presents for suitable value of the relative phase, but the corresponding closed system exhibits only absorption. In addition, manipulation effects of atomic exit and ratio of the injection rates on phase dependent gain are also analyzed.     

Effects of relative phase in an open ladder system without incoherent pumping

We studied effects of the relative phase between the probe and driving fields on the absorption and dispersion properties in an open three-level ladder system with spontaneously generated coherence but without incoherent pumping. It is shown that by the phase controlling, switching from absorption to lasing without inversion (LWI) and enhancing remarkablely LWI gain can be realized; large index of refraction with zero absorption and the electromagnetically induced transparency can be obtained. We also find that varying the atomic injection and exit rates has a considerable influence on the phase dependent-absorption property of the probe field, existent of the atomic injection and exit rates gives the necessary condition of the realization of LWI, getting LWI is impossible in the corresponding closed system without incoherent pumping.     

开放四能级系统中相对位相对探测响应的影响

利用具有自发辐射诱导相干开放四能级系统的定态数值解,从不同角度分析了探测场和驱动场之间的相对位相Φ对探测场增益(吸收)以及粒子数差的影响.研究结果表明：通过对相对位相的控制,可以实现探测场无反转增益和色散的周期性放大和减弱,并能得到无吸收高色散;Φ=α的色散曲线与Φ=α+π/2的增益曲线相似;在探测场共振条件下,适当调节相对位相,可以使无反转激光（LWI）增益达到最大.还研究了粒子注入和退出速率的变化对位相相关的LWI增益的调制作用.     

INFLUENCE OF DRIVING-FIELD PHASE FLUCTUATION ON ABSORPTION AND DISPERSION IN A SIMPLE THREE-LEVEL ATOMIC SYSTEM

The steady-state behavior of a simple three-level atomic system has been investi-gated by taking into account the effect of phase fluctuations in the driving field. For a monochromatic driving field, lasing with or without inversion can be established over a wide frequency range for the probe and driving fields. A large refractive index can also be generated even if the population inversion is positive. For a finite linewidth of the driving field, the refractive index enhancement tends to decrease. Furthermore, a behavior change from inversion laser to noninversion laser can occur as the driving field linewidth increases.     

Phase control in an open Λ-type system with spontaneously generated coherence

This paper investigates the control role of the relative phase between the probe and driving fields on the gain, dispersion and populations in an open A system with spontaneously generated coherence （SGC）. It shows that by adjusting the value of the relative phase, a change from lasing with inversion to lasing without inversion can be realized; the values and frequency spectrum regions of the inversionless gain and dispersion can be obviously varied; high refractive index with zero absorption and electromagnetically induced transparency can be achieved. It is also found that when the driving field is resonant, the shapes of the dispersion and the gain curves versus the probe detuning are very similar if the relative phase of the dispersion lags π/2 than that of the gain, however for the off-resonant driving field the similarity will disappear; the gain, dispersion and populations are periodical functions of the relative phase, the modulation period is always 2π; the contribution of SGC to the inversionless gain and dispersion is much larger than that of the dynamically induced coherence.     

Refractive index enhancement with vanishing absorption in an atomic vapor

We report a proof-of-principle experiment where the refractive index of an atomic vapor is enhanced while maintaining vanishing absorption of the beam. The key idea is to drive alkali atoms in a vapor with appropriate control lasers and induce a gain resonance and an absorption resonance for a probe beam in a two-photon Raman configuration. The strength and the position of these two resonances can be manipulated by changing the parameters of the control lasers. By using the interference between these two resonances, we obtain an enhanced refractive index without an increase in the absorption.     

Ground-state pump-probe spectroscopy

The absorption/gain spectrum of a weak probe field is theoretically analyzed for an atomic three-level Λ system driven by a strong pump field and perturbed by collisions with buffer-gas atoms. It is assumed that both pump and probe drive the same transition. When the dark state is sufficiently long lived, the probe spectrum exhibits an extremely narrow (possibly subnatural-width) resonance. It is shown that coherence-preserving collisions drastically (both qualitatively and quantitatively) change the probe absorption spectrum even at a low collision frequency, when Dicke narrowing is negligible. Strong coherence effects on the resonance structure at low gas pressures is explained by a change in refractive index due to coherence-preserving collisions for particles in the resonant velocity group.     

四能级Tripod-型原子系统中量子相干导致的交叉Kerr非线性效应

计算了四能级Tripod-型原子系统中探针光极化率随其频率失谐量的变化曲线.结果表明,当触发光作用于该系统的一个共振跃迁能级时,可使探针光的吸收和色散在其电磁感应透明(Electromagnetically Induced Transparency,EIT)窗口(由耦合光产生)处发生显著变化.随着触发光Rabi频率的增加,探针光在EIT窗口的吸收显著降低,色散显著增加.这种由触发光引起的探针光极化率的变化对应着三阶Kerr非线性光学效应,这一效应在偏振量子相位门中有着潜在的应用价值.     

Er$lt;sup$gt;3+$lt;/sup$gt;：YAlO$lt;sub$gt;3$lt;/sub$gt;晶体中Λ型四能级系统的量子相干左手性

用相干量子干涉和电磁感应透明（Electromagnetically induced transparency,EIT） 效应在Er³⁺：YAlO₃晶体中计算得到了Thommen,Mandel和Kastel对原子气提出的Λ型四能级系统的负折射率.计算结果得到的负折射率对应的频带宽度约为1 MHz,比先前报道的10³ Hz量级要宽得多.通常而言对应于折射/吸收比|Re［n］/lm［n］|≈1,而计算所得结果是|Re［n］/lm［n］|为4.6,表明吸收被EIT效应很好地抑制.Im［n］出现的负值可能与Er³⁺的⁴I_13/2→⁴I_15/2跃迁的受激发射有关.由此可知,稀土离子掺杂材料具有丰富的能级和各种不同的电磁跃迁,也是电磁感应负折射率材料中出色的应用材料的候选者. 关键词： 稀土掺杂晶体 量子干涉 电磁感应透明 负折射率材料     

Electromagnetically induced negative refraction in an atomic system with spontaneously generated coherence

We propose a new scheme for realizing left-handed electromagnetic properties in a coherently driven four-level system. At the presence of spontaneously generated coherence and the local field effect, both dielectric permittivity and magnetic permeability may have negative real parts of the same order in a small frequency band. Thus, one can achieve the negative refractive index and then manipulate it by modulating the coherent field or changing the atomic density. With incoherent pumping applied to have more remarkable spontaneously generated coherence, one can further obtain a wider frequency band of negative refraction and simultaneously reduce the accompanied absorption.     

三能级闭环λ原子系统中吸收和色散的相位依赖特性

在微波场闭合的三能级Lambda系统中计算了弱探测场的吸收和色散特性,发现闭合原子系统中的量子干涉导致弱探测场的吸收和色散特性依赖于施加场的相对位相。通过调节相对位相,获得了大的无反转增益和零吸收的高折射率。在三光子非共振的情况下,探测增益和吸收呈现随时间的振荡行为。     

Anisotropy of electromagnetically induced left-handedness in atomic three-level media based upon bianisotropic polarizabilities and tensor character

A three-level atomic system, configured as either a gaseous medium or a solid state material, with a driving field establishing a Rabi frequency of control, is tested by a probe field. The medium has bianisotropic microscopic polarizability and magnetizability, from which the permittivity and permeability tensors are derived. Non-isotropy and polarization dependence for left-handedness (negative index of refraction) is demonstrated through examination of tensor components in the detuning frequency spectrum. These results have important implications for use in optical or electronic devices.     

Y型四能级原子系统对探测场的吸收和色散

运用数值模拟的方法计算了Y型四能级原子系统在弱场条件下对探测场的吸收和色散.结果发 现，通过调制能级间附加的外场强度，该系统呈现出电磁感应透明特性，出现增益区并伴有 较大负折射率的色散效应.同时还发现，当外加场的拉比频率相位发生改变时，系统对探测 光场的吸收和色散将随之变化. 关键词： 电磁感应透明 自发辐射 量子干涉     

The linear optical properties in quantum dots under a single coupling field

Using the density matrix method, the linear optical properties of truncated pyramid quantum dots have been investigated when an additional coupling field is introduced. The absorption coefficients and refractive index changes are strongly affected by the amplitude of the coupling field and the phase difference between the probe and coupling fields. The numerical results indicate that negative absorption and transparency can be obtained when both of the probe and coupling fields are in resonance with the quantum dots.     

Negative refraction observed in a metallic ferromagnet in the gigahertz frequency range

It is generally believed that nature does not provide materials with negative refraction. Here we demonstrate experimentally that such materials do exist at least at GHz frequencies: ferromagnetic metals reveal a negative refraction index close to the frequency of the ferromagnetic resonance. The experimental realization utilizes a colossal magnetoresistance manganite La(2/3)Ca(1/3)MnO(3) as an example. In this material the negative refractive index can be achieved even at room temperature using external magnetic fields.     

Large refractive index with vanishing absorption in optical fibers

We propose and demonstrate a novel scheme for realizing large refractive index with vanishing absorption in a three-level quantum system consisting of the energy levels of Er³⁺-doped ZrF₄–BaF₂–LaF₃–AlF₃–NaF (ZBLAN) optical fiber. It is found that the refractive index of the probe field can be controlled via the coherent coupling field and incoherent pumping field. The switching from negative refractive index to positive refractive index and manipulation of the value of the index of refraction while maintaining vanishing absorption of the probe field can be achieved via tuning the detuning of the coherent coupling field. Our scheme may provide some new possibility for technological applications in fiber communication.     

Conditional phase-shifter in refractive index enhancement scheme

We propose a conditional phase-shifter between two weak laser beams in an alkali vapor. We begin by enhancing the refractive index of a weak, far-detuned probe beam while maintaining vanishing absorption. The enhancement is a result of interfering an absorptive and an amplifying Raman resonance in a four-level atomic system. Then, a conditional switching beam ac stark shifts the narrow Raman resonances, modifying the phase accumulation experienced by the probe beam. We find that the scheme works with acceptable fidelity at the tens of photons level, and that the energy requirement can be reduced further at the expense of noise.     

Theoretical calculations on the NaS and NaS+ radicals: electronic structure, spectroscopy and spin-orbit couplings

We propose a closed three-level V-type system with an incoherent pumping which achieves a high index of refraction without absorption via spontaneously generated coherence. The system has two near-degenerate excited levels. We derive full analytical formulas for the index of refraction and the gain (absorption) coefficient. We found that the proper choice of the incoherent pumping, the relative phase of the two coherent fields, the Rabi frequencies, the probe detuning and the angle between the two transitions leads to the enhancement of the refractive index without absorption.