Two-photon physics

A brief discussion is given on the status of two-photon exchange contributions to elastic electron-nucleon scattering. The apparent discrepancy in the extraction of elastic nucleon form factors between unpolarized Rosenbluth and polarization transfer experiments is discussed, as well as the understanding of this puzzle in terms of two-photon exchange corrections. It is also discussed how the imaginary part of the two-photon exchange amplitude can be accessed from the beam normal spin asymmetry in elastic electron-nucleon scattering. Correspondence: M. Vanderhaeghen, Thomas Jefferson National Accelerator Facility, Newport News, VA 23606, USA     

Two-photon fourth-order polarimetery

We demonstrate a fourth-order polarimeter that employs two-photon detection to measure the fourth-order polarization coherency matrix (FOCM) of an optical signal. We measure the FOCM of picosecond pulses subjected to polarization mode dispersion (PMD). The fourth-order polarimeter distinguishes different PMD states even when the state of polarization, including the degree of polarization, is constant.     

双光子吸收光谱

本文报道以皮秒脉冲激光抽运的光参量放大器为宽带可调谐激发光源，直接测量非线性光学材料的双光子吸收谱的方法。光参量放大器不但可调谐谱带很宽，而且可用一对尼科尔棱镜很容易地实现全波段功率足够强和相等，使此方法的测量结果准确可靠。用此方法研究了一个新有机化合物的双光子吸收谱。实验结果表明所测的新有机化合物在很宽的波长范围内呈现出很大的双光子吸收截面。     

Two-photon resonance fluorescence

The excitation spectrum of a two-level atom interacting with a strong electromagnetic field is considered when the atomic transition frequency is nearly twice the frequency of the laser field. The spectral function consists of four Lorentzian lines describing: the central line peaked near the two-photon resonance, two sidebands peaked at the high and low frequency regions respectively and the one-photon frequency mode. In the limit of high photon densities, the low frequency photon mode is induced by the pump field which removes the singularity occurring at the two-photon resonance frequency.     

Two-photon conical emission

A two-photon resonantly enhanced four-wave mixing (FWM) process leading to the conical emission of two new frequency components has been observed in atomic sodium vapor. A dye laser tuned close to the 3s → 3d two-photon allowed transition produces broad-band emission near the frequencies of the 3d → 3p and 3p → 3s transitions. This radiation is emitted in the forward direction in the form of cones surrounding the transmitted laser beam. The dependence of the cone angle on the emission wavelength and atomic number density is in excellent agreement with the predictions of a model that ascribes the origin of the conical emission to a phase-matched four-wave mixing process.     

Two-photon synchrotron emission

A second-order effect of perturbation theory — two-photon synchrotron emission — is discussed for an ultrarelativistic electron in a constant and uniform magnetic field of strength H H₀ = 4.41 · 10¹³ Oe on the basis of the exact solutions of Dirac' s equation for this field. An equation is derived for the spectral-angular distribution (for both photons) of the probability of the process in question, taking into account the spin states and polarizations of the photons.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 46–52, September, 1976.     

Two-photon light shift

We have made the first observation of a two-photon light shift: a shift of a two-photon resonance due to a third, non-resonant light field. The effect is observed in cross-saturation spectroscopy of neon pumped by a standing-wave saturator field, to which the neon is rendered transparent. A novel high-intensity theory gives a satisfactory fit to the spectrum.     

Two-photon multiterm nonadiabatic transitions

The resonance Raman scattering phenomenon via the predissociated state is described in the four crossing terms A-(1, 2)-B model when terms 1 and 2 strongly interact. The Green's function closed representation for the coupled intermediate electronic terms 1 and 2 is obtained. The transition amplitude I _{A-(1, 2)-B} is found in the WKB approximation. The resonance Raman scattering via the predissociated ³Π_Ou ⁺ state of the interhalogen molecules is treated in detail.     

Two-photon processes at Belle

We review recent measurements of pure neutral final state production in the two-photon processes,γγ→π0π0 and ηπ0 at the Belle experiment.In the lower invariant mass region,properties of light scalar mesons are measured by partial wave analysis.In the higher energy region the total and differential cross section are compared with QCD models.     

Two-photon spectroscopy in AgCl

Two-photon absorption constant β⁽²⁾ has been measured in the indirect gap of AgCl between 4 and 4.4 eV. The absorption constant has been derived from the excitation spectrum of the recombination luminescence. The absolute value of β⁽²⁾ is in fair agreement with that calculated for a two-photon phonon-assisted transition.     

Two-photon optically pumped laser

At high temperature, the transmission peaks of a Fabry-Perot cavity filled with rubidium show sharp structures when the frequency of the laser is close to the 5S_{$\text{1}\text{2}$} - 5D_{$\text{5}\text{2}$} two-photon transition. These structures correspond to a laser emission between 5D_{$\text{5}\text{2}$} and 5P_{$\text{3}\text{2}$}. The wavelength of this laser emission (7759 Å) is smaller than the exciting wavelength (7779 Å).     

Two-photon processes at Belle

We review recent measurements of pure neutral final state production in the two-photon processes, γγ→π^0π^0 and ηπ^0 at the Belle experiment. In the lower invariant mass region, properties of light scalar mesons are measured by partial wave analysis. In the higher energy region the total and differential cross section are compared with QCD models.     

Two-photon amplification on cascade-transitions

Stimulated 2-photon emission was observed in a continuous wave 3-level cascade He-Ne laser by measuring the power tuning curve of the 2.40-μm line for various detunings of the 3.39-μm line as a parameter. The directional anisotropy of the 2-photon gain was detected with a ringlaser configuration. Qualitative agreement between experiment and a generalized rate equation treatment of the 3-level system is demonstrated.     

Two-photon absorption in hexagonal-CdS

The “composite sample technique” has been used to investigate the two-photon absorption coefficient (β) of CdS at 3.91 eV. The absolute value of β and its optical anisotropy have been studied for samples with different carrier densities. The present results may provide a new interpretation of experiments on CdS and other semiconductors reported in the literature.     

Two-photon interactions at LEP

Experimental results of investigation of two-photon interactions at LEP (CERN) are presented. Experimental conditions provided a possibility of investigation of the whole set of two-photon interactions from the production of lepton pairs in collisions of quasi-real photons to interactions of high-virtual photons.     

Two-photon spectroscopy of ytterbium

Two-photon laser spectra of the Yb vapor have been obtained. Transitions to highly excited 4f¹⁴ 6sns¹S₀ and 4f¹⁴ 6snd ¹D₂ states are seen in direct two-photon excitation. Hybrid resonances involving 4f¹⁴ 6s6p ¹P⁰₁ and 4f¹⁴ 5d6s ³D₂ intermediate states lead to transitions to 4f¹⁴ 6sns¹S₀, 4f¹⁴ 6snp ³P⁰_2,1 and 4f¹⁴ 6snd ¹D₂ levels.     

Two-photon fluorescence isotropic-single-objective microscopy

Two-photon excitation provides efficient optical sectioning in three-dimensional fluorescence microscopy, independently of a confocal detection. In two-photon laser-scanning microscopy, the image resolution is governed by the volume of the excitation light spot, which is obtained by focusing the incident laser beam through the objective lens of the microscope. The light spot being strongly elongated along the optical axis, the axial resolution is much lower than the transverse one. In this Letter we show that it is possible to strongly reduce the axial size of the excitation spot by shaping the incident beam and using a mirror in place of a standard glass slide to support the sample. Provided that the contribution of sidelobes can be removed through deconvolution procedures, this approach should allow us to achieve similar axial and lateral resolution.     

Two-photon absorption in CuCl

The two-photon absorption spectrum of CuCl at 4.2°K is studied in the energy range of the excitonic lines. The observed lines are suggested to be due to the creation of excitons in the 1S, 2P and 3P states and also to the simultaneous creation of 2P and 3P excitons and one or more longitudinal optical phonons.