Anisotropy in two-photon direct creation of biexcitons in CuBr cubic crystals

The polarization dependence of two-photon absorption is considered in the case of CuBr crystals. Geometrical selection rules for direct creation of Γ₁, Γ₃ and Γ₅ biexcitons are provided. Using the fact that the absorption to Γ₃ and Γ₅ final states exhibits anisotropy with respect to the crystal axes, several geometrical situations for separating experimentally biexciton lines are discussed.     

Two-pulse two-photon optical nutation of biexcitons in semiconductors

The specific features of two-photon nutation in a system of coherent biexcitons in CuCl-type semiconductors are investigated. It is shown that nutation is a process of periodic transformation of photon pairs into biexcitons and vice versa, depending on the parameters of the system. The possibility of exercising the phase control of optical nutation is predicted.     

Light mixing induced by the giant two-photon absorption of excitonic molecules in CuCl

Three-wave mixing of light has been observed in connection with a giant two-photon absorption generating excitonic molecules. The efficiency of mixing of light for energies 2Ω₁ - Ω₂ and 2Ω₂ - Ω₁ is enhanced remarkably when Ω₁ and/or Ω₂ is varied across the giant two-photon absorption band-region. Moreover, the mixed lights with energies 3Ω₁ - 2Ω₂ and 4Ω₁ - 3Ω₂ have also been detected.     

Exciton-polaritons by two-photon absorption in semiconductors

The creation of excitonic polariton states by two-photon absorption is theoretically investigated. A semi-classical approach is adopted to compute the two-photon transition probability to polariton states through an intermediate exciton state. The numerical results in CuCl show two peaks corresponding to the longitudinal exciton and transverse polariton states, respectively. These results are in good agreement with the experiment.     

Photochromism induced by infrared two-photon absorption

An IR laser of 940 nm wavelength induced photochromic reaction in acrylate that contained both spirobenzopyran and rare-earth doped oxide (Gd(2)O(2)S:YbEr). The rare-earth elements were excited by two 940 nm photons and emitted a 550 nm photon, which caused photochromic isomerization of spirobenzopyran. This acrylate turned to its original orange color by either thermal relaxation or ultraviolet irradiation, and was bleached again by IR laser irradiation.     

Photorefractive polymers sensitized by two-photon absorption

We demonstrate the recording of holograms and their nondestructive readout in a photorefractive polymer, using two-photon absorption. Sensitivity is provided by the excitation of the electroactive chromophore with femtosecond pulses, followed by charge injection into the photoconducting poly(N -vinylcarbazole) matrix. The holograms can be fully erased with a pulsed laser source but are insensitive to cw laser beams with the same wavelength. Studies of the field and intensity dependence of the diffraction efficiency indicate that the holograms are formed through the photorefractive effect.     

Direct two-photon creation of a biexciton

The nonlinear absorption coefficient for the two-photon creation of biexciton in CuCl is calculated in the approximation of one intermediate, optically active excitonic state. The two-band model with realistic band structure at the band extrema is considered. The exciton-biexciton dipole transition momentum matrix element is expressed by the functional of the exciton and biexciton envelopes. In the numerical calculations the envelope of the Hylleraas-Ore type, modified by Brinkman et al., is used. Our calculated value of the absorption coefficient is about two orders of magnitude smaller than that estimated previously by Hanamura, and agrees better with the experimental value reported by Gale and Mysyrowicz.     

Angle- and polarization dependences of the two-photon Raman scattering via biexcitons in CdS

Two-photon Raman scattering via virtually excited biexcitons is reported for CdS, in different scattering configurations and polarizations. If the crystallographic $\text{c}$-axis is perpendicular to the scattering plane, the results correspond to those found in cubic CuCl¹, whereas for $\text{c}$ in the scattering plane, the influence of mixed-mode polaritons in the final state is obvious.     

Specific features of two-photon optical nutation in a system of biexcitons in semiconductors

Specific features of two-photon nutation in a system of coherent biexcitons in CuCl-type semiconductors are studied. It is shown that, depending on the parameters of the system, nutation represents a process of periodic conversion of photon pairs into biexcitons and vice versa. The possibility of phase control of optical nutation is predicted.     

Excitonic molecule in semiconductors by two-photon absorption

Direct creation of bi-exciton states by two-photon absorption in direct gap semiconductors is investigated theoretically. A numerical application to the case of CuCl shows that the two-photon absorption coefficient for bi-excitonic transitions is larger than that for two-photon interband transitions by three orders of magnitude. It becomes comparable to that for one-photon excitonic transitions for available laser intensities. The main contribution to this enhancement of the absorption coefficient for the transitions to the bi-exciton states is found to be from the resonance effect.     

Ultrafast measurement of two-photon absorption by loss modulation

We demonstrate a direct and sensitive technique for measuring two-photon absorption (TPA). An intensity-modulated femtosecond laser beam passes through a sample exhibiting TPA. A TPA signal at twice the modulation frequency is then generated and subsequently measured by a lock-in amplifier. The absolute TPA cross section of Rhodamine 6G at 800nm is found to be (15.3+/-2.0)x10(-50) cm(4) s/photon and agrees well with previously published results obtained with much higher intensity [J.Chem.Phys.112, 9201 (2000)]. Our method may be especially useful in measuring nonlinear absorptions of nonfluorescent materials.     

Selection rules for hybridization

Selection rules for the hybridization of the crystal-field levels of a magnetic ion are investigated by group theoretical methods. They appear only in a few cases. In absence of strict selection rules the possibility for drastic quantitative differences in the hybridization of different levels is discussed.This work was supported by the Deutsche Forschungsgemeinschaft through SFB 125 (Aachen-Jülich-Köln)     

Nonlinear propagation of light in semiconductors in the presence of two-photon excitation of biexcitons

The steady-and nonsteady-state passage of light through a ring resonator in the presence of two-photon excitation of biexcitons from the ground state of the crystal is studied. The values of the parameters for which complicated nonlinear temporal formations are possible in the system are found. The possibility of observing the predicted effects experimentally is discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 1999–2002 (November 1998)     

Adaptive pulse compression by two-photon absorption in semiconductors

We investigate the adaptive optimization of broadband laser pulses, using a closed-loop learning algorithm in which the merit function is derived from two-photon absorption in semiconductors. Photoluminescence experiments with CdS thin films and photocurrent measurements of a GaAsP photodiode have been performed. The experimental data demonstrate that reliable and accurate pulse compression to the bandwidth limit can be achieved, unperturbed by nontrivial phase effects. Therefore two-photon absorption proves to be an easy-to-implement alternative to second-harmonic generation for the compression of broadband laser pulses.     

Exciton molecule in semiconductors by phonon-assisted two-photon absorption

Direct creation of bi-excitonic states by photon-assisted two-photon absorption in indirect-gap semiconductors is investigated theoretically. The symmetry of the indirect bi-exciton states and of the phonon used are given for the case of Ge. A numerical application to the case of Si shows that the indirect two-photon absorption coefficient for bi-excitonic α²_I (bi-ex) transitions is several orders of magnitude larger than both indirect two-photon interband, α²_I (band), and excitonic, α²_I (exc), transitions. It becomes smaller than both indirect one-photon interband, α¹_I (band), and excitonic, α¹_I (exc), transitions for available laser intensities. The essential contribution to this enhancement of α²_I (bi-ex) is found to be from the resonance effect in the first process and from both the resonance effect and matrix elements included in the second process.