Axially phase-matched parametric four- and six-wave mixing in potassium vapor

Systematic experimental observations of axially phase-matched parametric four- and sixwave mixing processes have been made in potassium vapor pumped on or near the 4S–6S two-photon resonance. An IR up-conversion with a beta barium borate crystal was used for the detection of IR emissions. All the predicted processes have been observed. It is found that the excitation spectra of the emissions for those processes observed in this work are quite different from the excitation spectra of similar processes in sodium vapor. A dye-dependence of some emissions generated by the axially phase-matched parametric six-wave mixing processes has been found by using different dyes in the dye laser. The reason for the dye-dependence is discussed.     

Amplification of radiation in atomic vapor induced by a linearly polarized laser radiation

We present the results of spectroscopic and polarization studies of dilute rubidium vapor exposed to a single-frequency linearly polarized diode laser radiation in a spectral range of atomic D₂ line. We report the origin of a circularly polarized radiation on V-type transitions of ⁸⁷Rb F _g = 2 → F _e = 3 and ⁸⁵Rb F _g = 3 → F _e = 4, and amplification of this radiation in backward direction caused by a partial population inversion among magnetic sublevels of the ground and excited levels. This is confirmed experimentally by high directivity of backward radiation, absence in its spectrum of ⁸⁵Rb F _g = 2 → F _e = 1 (Λ-type) radiation, as well as by different nature of intensity dependences of backward and fluorescence radiations.     

Experimental observation of four-photon entanglement from parametric down-conversion

We observe polarization entanglement between four photons produced from a single down-conversion source. The nonclassical correlations between the measurement results violate a generalized Bell inequality for four qubits. The characteristic properties and its easy generation with high interferometric contrast make the observed four-photon state well suited for implementing advanced quantum communication schemes such as multiparty quantum key distribution, secret sharing, and telecloning.     

Spin dynamics of magnetic resonance with parametric modulation in a potassium vapor cell

A typical magnetic-resonance scheme employs a static bias magnetic field and an orthogonal driving magnetic field oscillating at the Larmor frequency, at which the atomic polarization precesses around the static magnetic field. Here we demonstrate both theoretically and experimentally the variations of the resonance condition and the spin precession dynamics resulting from the parametric modulation of the bias field. We show that the driving magnetic field with the frequency detuned by different harmonics of the parametric modulation frequency can lead to resonance as well. Also, a series of frequency sidebands centered at the driving frequency and spaced by the parametric modulation frequency can be observed in the precession of the atomic polarization. We further show that the resonant amplitudes of the sidebands can be controlled by varying the ratio between the amplitude and the frequency of the parametric modulation. These effects could be used in different atomic magnetometry applications.     

Studies of tunable four-photon parametric fluorescence of crystals in the visible region

Tunable four-photon parametric fluorescence, which is valuable for investigating material nonlinearities and parametric devices, was studied theoretically and experimentally.Theoretical analysis leads to the conclusion that the emitted power depends directly on the focusing of the pumping beam, unlike the process of three-photon parametric fluorescence which is independent of the focusing of the pumping beam. The calculation of the phasematching conditions for nonlinear crystals such as KDP, ADP, TiO₂ and CdS shows that collinear phase-matching can be realised over the visible region using a ruby laser for pumping. Experimentally, tunable emission was observed in the visible range from 4200 Å to 5300 Å with KDP and ADP crystals pumped by a Q-switched ruby laser.     

钾蒸汽中双光子激发产生2~3∏_g—α~3∑_u~+扩散带受激辐射

本文报道了在K_2-K中通过双光子共振激发原子或双光子激发分子产生2~3∏_(?)—a~3∑_u~+扩散带辐射的实验结果,并对激发机制以及扩散带辐射随激发能量和温度的变化进行了讨论.     

Observation of spontaneous parametric four-photon scattering in aqueous solutions of electrolytes

We have observed spontaneous parametric four-photon scattering in electrolytic solutions (alkali halides and alkaline earth halides in water). The spectral, temporal, and spatial intensity distributions of the scattered light have been measured. Using a Q-switched ruby laser as a pump, and a CuSO₄-solution as a filter we observed signal wavelengths in the range from 350–550 nm with a maximum intensity at 480 nm for all of the investigated solutions. This radiation is emitted into a cone with a half-angle of roughly 60 mrad. In comparison to pure water the scattered radiation in electrolytes has a much higher intensity. Most of the experimental results confirm predictions of the theory of spontaneous four-photon scattering.Parts of this paper were reported at the 2nd. International Conference Laser und ihre Anwendungen, Dresden, 1973.     

Generation of picosecond light continua by parametric four-photon interactions in liquids and solids

Light continua extending from the i.r. to the u.v. are generated with picosecond laser pulses in liquids and solids. It is demonstrated that the light spectra in water are produced by stimulated parametric fourphoton interactions, which are resonantly enhanced by single and difference frequency resonances of the non-linear susceptibility ⁽³⁾. A theory of the various parametric four-photon interaction processes is developed and the resonant structure of ⁽³⁾ is discussed. The experimental results are compared with theoretical calculations. The generation of light continua by other nonlinear optical processes is briefly discussed.     

Amplification assisted optical parametric oscillator in the mid-infrared region

We demonstrate a high efficiency mid-infrared laser source based on optical parametric oscillator (OPO) assisted by an intracavity optical parametric amplification (OPA). The OPA-assisted-OPO scheme was realized in one piece of commensurable dual-periodic superlattice in which the signal light generated from the OPO process serves as the pump light for the OPA process. A maximum output power of 508 mW at 3.92 μm was achieved under a pump power of 2.85 W at 1.064 μm. The pump-to-idler conversion efficiency is 17.8% and the slope efficiency is 23.8%, and the enhancements of them are 58.9% and 67.6%, respectively, comparing with the standard OPO scheme.     

Quasi-rotational spectral structure of optical radiation due to stimulated Raman scattering and parametric processes in high-pressure hydrogen

Received: 20 June 1997/Revised version: 26 January 1998     

Amplification of noise in the quantum theory of parametric processes

A quantum mechanical treatment is given of the acousto-optical parametric amplification processes in dielectric crystals when the signal acoustical and idler light waves are amplified in the presence of intensive optical pumping. The approximate Heisenberg equations of motion are found and solved for the creation and annihilation operators of signal and idler modes with due regard for the interaction of these modes with other light and vibratory modes of the crystal (“the thermostat”). It is shown that the thermostat influence results in noise and attenuation effects. These persistent noises are also amplified. If the pumping is not far above the threshold the persistent noises are more important than the initial ones.     

Influence of cross-phase modulation and four-photon parametric mixing on the relative motion of optical pulses

Two mechanisms of self-confinement of wave packets on different frequencies in a monomode optical fibre are investigated. It is shown that the equations describing the processes possess a Hamiltonian structure. A new class of asymptotically free soliton solution is found. Two spectral bandwidths for self-confinement of the optical pulses are determined.     

Collision induced resonant four-photon parametric generation as a method of nonlinear spectroscopy

Generation of the combination tone ω_v = 2ω₀ ? ω_μ in the case where the nonlinear process is possible only in the presence of coherent reorienting collisions, is considered. A simple measurement method of the cross-section difference between “orientation” and “alignment” destruction is proposed.     

基于自发参量下转换源二阶激发过程产生四光子超纠缠态

目前,多光子纠缠态的制备大多通过线性光学器件演化自发参量下转换一阶激发过程产生的纠缠光子对得到.本文考虑由自发参量下转换源二阶激发产生四个不可区分的纠缠光子制备四光子超纠缠态的情况.通过几组分束器、半波片和偏振分束器等线性光学器件设计量子线路演化四光子系统,结合四模符合探测,可得到同时具有偏振纠缠和空间纠缠的四光子超纠缠态.     

Noncollinear phase-matched four-photon mixing of CO2 laser radiation in germanium

Noncollinear four photon mixing of two TEA CO₂ laser beams in germanium at room temperature has been used to obtain phase-matched generation of step tunable radiation in the 8.7 μm region which is of interest for the uranium isotope separation. Using an 8.3 cm long crystal of germanium, peak output power of ≈10kW (corresponding to 1 mJ per pulse) was obtained at 8.7 μm with 3 MW peak input power from each of the two CO₂ lasers operating at 9.6 μm and 10.6 μm.     

Theoretical investigation of noncollinear phase-matched parametric four-photon amplification of ultrashort light pulses in isotropic media

The amplification of light signals (angular frequency _S in some isotropic media (D₂O, fused silica, and Schott type SF10 glasses) by noncollinear phase-matched parametric four-photon interaction ₁+_2S+₁ is studied theoretically. Computer simulations are carried out for fundamental and second-harmonic pump pulses of a mode-locked Nd: glass laser. Degenerate interaction (wavelength ₁=₂=1054nm or 527 nm) and nondegenerate interaction (₁=1054nm, ₂=527 nm are considered. Characteristic phase-matching parameters and gain parameters versus wavelength are determined. Limitations by spectral bandwidth, optical absorption, optical damage, self-phase modulation, self-focusing and stimulated Raman scattering are analysed.     

Bistability of the refractive index due to parametric resonance in crystals

It is shown that the nonlinear refractive index of a crystal interacting with a biharmonic optical field exhibits bistability due to the process of parametric resonant generation of the coherent overtone vibration.     

Influence of cross phase modulation and four-photon parametric mixing on the relative motion of optical pulses

Two mechanisms of self-confinement of wave packets on different carrying frequencies in monomode optical fibre are investigated. It is shown that the equations describing the processes possess a Hamiltonian structure. A new class of asymptotically free soliton solutions are found. Two spectral bandwidths for self-confinement of the optical pulses are determined.     

Contribution to the theory of parametric generation of cyclotron radiation

A linear theory of the cyclotron parametric instability in systems which are classical analogues of quantum lasers without inversion is developed. The cyclotron interaction of different types of modulated electron beams with a bichromatic field, produced by waves propagating at an angle with respect to a constant magnetic field, is investigated. It is shown that simultaneous amplification of two parametrically coupled modes with different frequencies and positive energy is possible in this system with modulation of the active and reactive components of the susceptibility of an electronic ensemble. The results obtained are important from the standpoint of the general theory of radiation processes in electron beams and plasma and for the advancement of microwave electronics.