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.     

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 interference in the presence of absorption

Experiments on two-photon interference are discussed in the case when there is absorption of all the modes participating in the process of spontaneous parametric down-conversion (SPDC) of light. The objects of investigation are 10-to 80-Å-thick ultrathin gold films deposited on fused-silica substrates. Conditions are determined under which the effect of absorption of the signal and pump waves on the interference pattern is small. It is shown that, under these conditions, the visibility of the interference pattern and the shape of the frequency-angular spectrum at the signal frequency are determined by the optical parameters of the medium at the idler frequency, which belongs to the near-infrared region.     

Two-photon absorption study of the large-sized nanocrystallites

The influence of size and shape for nanocrystallines (NC) of semiconducting materials (like GaN, ZnO, CdS, ZnS with sizes about 20-30 nm) incorporated into olygoetheracrylate polymer matrices on photoinduced optical properties, particularly the two-photon absorption (TPA), was investigated. We have shown that maximal values of the TPA diagonal tensor component was observed at low temperatures for the GaN NC (about 4.2 K), which corresponds to a NC concentration of 2.6-3.2 wt.% and size of about 24 nm. The shape and value of ellipticity play crucial roles in the observed non-linear optical phenomena. This contradicts the generally adopted model wherein the non-linear optical properties are mainly determined by averaged NC sizes. The observed temperature dependence is explained within a framework of anharmonic electron-phonon interaction in the large-sized nanocrystallites.     

Two-photon absorption in the vicinity of the exciton absorption in CdS

The two-photon absorption spectrum of CdS in the wavelength region of the free and bound exciton lines has been measured using a pulsed dye laser and a differential transmittance technique. An intense absorption band is observed which is possibly due to the interference of the contributions of different intermediate states to the interband two-photon absorption. No absorption is observed which can be attributed to the formation of free excitonic molecules.     

Two-photon excitation spectrum of benzonitrile vapour

The two-photon excitation spectrum of benzonitrile in the gas phase at about 1 torr has been recorded and analysed in the total energy region of 36300 to 40500 cm⁻¹ using both linearly and circularly polarised light from a Nd-YAG pumped dye laser. The two-photon spectrum consists of strong Franck Condon (fc) transitions bearing a good resemblance to the one-photon spectrum and also vibronic coupling (vc) transitions essentially arising out of the analogue of the benzeneν ₁₄. By a comparative study of thetp spectra of various other monosubstituted benzenes, it has been inferred that the strongtp spectrum of benzonitrile should be ascribed to a considerableπ charge delocalization over the phenyl and nitrile groups.     

Two-photon absorption of tetraphenylporphin free base

We study two-photon absorption (TPA) spectra for toluene solution of 5,10,15,20-tetraphenyl-21H,23H-porphin (H₂TPP) in B and Q bands regions and find the maximum TPA cross-section values of 25 and 1-6 GM in laser wavelength ranges, 730-790 and 1100-1400 nm, correspondingly. In the 730-790 nm range the spectrum is attributed to parity allowed two-photon transition into g parity state, positioned nearby B state. Much lower TPA cross-section of ∼1 GM is measured for the transition into pure electronic Q state and it is due to the contribution of only those low-symmetry H₂TPP confomers, where mutual orientation of the porphyrin plane and the four phenyl rings lifts the center of inversion. The intermediate values of TPA cross-section of ∼6 GM are observed for the transition into vibronic Q states and are explained by TPA-allowed transition into vibronic states, which can occur even for totally centrosymmetrical molecules. Measurement of two-photon polarization ratio, Ω=σ_circ/σ_lin, shows that for parity allowed g→g transition Ω =1.05±0.05 and for transitions into electronic and vibronic Q-states, Ω=0.62±0.06 and 0.79±0.1 (depending on wavelength), respectively. Quantum-chemical calculations of both u and g parity excited energy levels of H₂TPP molecule are performed by CNDO/S method and the results are in good agreement with the experimental data.     

Two-photon interband magneto absorption in InSb

Two-photon interband magnetoabsorption in InSb has been studied by measuring the resulting changes in the photoconductivity and free-hole absorption as a function of magnetic field and incident photon energy. In both the photoconductivity and transmission measurements, resonant structure with the peaks occuring at the same values of magnetic field was observed with the incident light polarized both parallel and perpendicular to the magnetic field. The results thus are not in agreement with the existing theories of two-photon magnetoabsorption. A modification to a perturbation theory approach is suggested that appears to explain the experimental observations.     

Two-photon absorption by rotating diatomic molecules

Explicit forms for the two-photon absorption cross section in linearly and circularly polarized light for rotating diatomic molecules have been developed. The ratio (σλλ/σ _cc ) of the cross sections for individual rotational transitions is precisely two-thirds for all branches of ΔΩ = 0, ±1, ±2 transitions except for the unique case—the Q branch of a ΔΩ=0 transition. In the latter case the ratio becomes J-dependent and may range from infinity to 4/1 (for high J) for different relative contributions to the absorption cross section of intermediate states having either the same or different electronic symmetry. These contributions are thus experimentally accessible in this favourable situation.     

Two-photon absorption spectroscopy of metastable hydrogen-like atoms

Recent experimental technology points to the possibility of measuring the 2S–3S transition in hydrogen-like atoms via two-photon absorption. The non-relativistic amplitude for this process is evaluated exactly and numerical values are given.     

Two-photon absorption of optical and x-ray quanta

Two-photon absorption of an optical (laser) and an x-ray quantum is considered within the context of the Born approximation. A simple expression relating the two-photon and one-photon photoionization absorption coefficients is obtained. If the upper state is one of the conduction bands of a solid, the two-photon absorption coefficient is shown to peak at points in the zone for which the curvature of the band is a maximum. Numerical estimates show that with a 100 kW Nd laser, the intensity of the two-photon absorption process is a few percent of that of the one-photon process.     

D-A型二噻吩并磷杂茂衍生物分子的双光子吸收性质

在密度泛函理论理论水平上,采用响应理论方法计算了一系列2,6-donor-acceptor 二噻吩并磷杂茂衍生物分子的单、双光子吸收性质. 理论计算结果表明该系列分子都具有较大的双光子吸收截面,并且分子中心的磷原子无论被氧化还是金属配合,其双光子吸收截面几乎保持不变. 此外,还考虑了溶剂效应对分子光学性质的影响,研究表明分子能级发生了溶剂化蓝移,并且分子的单光子和双光子吸收性质随着溶剂极性表现出非单调变化.