N. Nagasawa

Radiative and Two-Photon Resonance Raman Processes Associated with Excitonic Molecules in CuCl

Giant two-photon absorption spectrum for the direct generation of excitonic molecules is measured with using a frequency tunable dye laser, and absorption peak is found at 3.1870 eV. Emission spectrum with excitation at the giant two-photon absorption band depends on the spectral width, \(\varDelta\varOmega_{1}\), of excitation light. In the case of \(\varDelta\varOmega_{1}{=}0.25\) meV, the two-photon resonance Raman scattering is found to be predominant. The Raman process involves an excitonic molecule and a longitudinal exciton as the intermediate and final states, respectively. On the other hand, for the case of \(\varDelta\varOmega_{1}{=}2.3\) meV, the emission spectrum shows a very sharp line at 3.1649 eV, which has been previously ascribed to be due to the Bose-Einstein condensation of excitonic molecules. Discussions are made on the dependence of emission spectrum upon the energy band-width of excitation light.

Anomalous wave interference at Z3-exciton resonance of CuCl

Abstract The transmission and the reflection spectra of a thin CuCl single crystal of 0.15μ thickness have been measured in the Z 3 -exciton resonance region at 1.6K by using a weak dye-laser light as a light source. Well resolved interference fringes have been obtained over the exciton resonance. In the higher energy region than the longitudinal excitons energy, the separation of adjacent fringes cannot be explained by interference of the lower branch polariton waves (LBP) or the upper branch polariton waves (UBP). These structures have been explained by the mutual interference effect between the UBP and LBP waves, anomalous waves. This has been confirmed by the measurements of two-photon absorption due to the excitonic molecule via respective polariton states.

Revived interest on yellow-exciton series in Cu2C: An experimental aspect

Abstract The yellow-exciton series in natural single crystals of Cu2O are studied by one- and two-photon spectroscopy in high precision at 2K. The energies of the p-exciton series are resolved up to 12p state in one-photon absorption spectra, while those of the (s-d) excitons are resolved up to 7(s-d) states in two-photon spectra. The band gap energy is evaluated to be 2.1722eV. The different effective Rydberg constants are obtained to be 0.0981 ± 0.0005eV for p-series, 0.1083±0.0138eV and 0.1066±0.0014eV for (s-d) series, respectively.

Renormalization of polaritons due to the formation of excitonic molecules in CuCl; an experimental aspect

Abstract The modulation of the real part of the non-linear refractive index (Δn) associated with the giant two-photon absorption (GTA) band of the excitonic molecules (EMs) has been measured in CuCl at 1.6K. This has been measured by the observation of the polarization rotation effects due to the induced optical anistropy between the right circularly polarized (σ + ) and the left circularly polarized (σ - ) components of the linearly polarized probe team of Ω 1 under the strong irradiation of the σ - beam of Ω 2 fixed around the GTA band at 3.1861 eV. The maximum amount of Δn has been 2.9×10 -4 at 3.1920 eV under the maximum excitation level when Ω 2 has been fixed at 3.1796 eV. The experimental results have been discussed in terms of the renormalization of the polaritons due to the real and virtual formation of EMs.

Band gap energy and binding energies of Z3-excitions in CuCl

Linear and nonlinear spectroscopical studies on s- and p-series of the Z3 -excitons in CuCl are carried out at 2K to evaluate the band gap energy and the binding energies of the Z3-excitons. Resonance energies of the 2p- and 3p-excitons are precisely determined and the resonance of the 4s-exciton is newly observed. It is found that the energy of each s- and p-exciton band converges to 3. 3990 ± 0,0005eV, which corresponds to the band gap energy of CuCl Effective Rydberg constants derived from a simple hydrogenic model are estimated to be 130. 6 ± 2. 9meV and 109. 1 ± 3. 3meV for s- and p-series, respectively.

Phase-Conjugation by Four-Wave Mixing in CuCl: Resonance Enhancement by Exciton and Excitonic Molecule and Its Polarization Dependence

The phase-conjugation in crystalline CuCl has been studied at 1.6 K. It has been found that the two-photon resonance of the \(\varGamma_{1}\)-excitonic molecule and the one-photon resonance of the \(\varGamma_{5}\)-exciton contribute to the generation of the phase-conjugated wave. Each contribution has been separated spectroscopically by adjusting the polarization configuration of the incident fields adequately and has been compared qualitatively with the theory. The efficiency of the phase-conjugated reflection has become maximum at around the two-photon resonance of the excitonic molecule, 3.186 eV and has been ∼1% at the maximum.

A study on the optical nonlinearity due to the Z3 and Z1,2 excitons of CuCl by the reflected four-wave mixed light generation

Abstract We have demonstrated the generation of the reflected four-wave mixed light in the energy region from 3.185 eV to 3.320 eV including the highly absorptive region due to the Z 3 and Z 1,2 exciton bands of CuCl at 1.6 K. The mixed light signal is concluded to be due to the one-photon resonant contribution of the respective exciton level, from its polarization characteristics. The corresponding third order non-linear susceptibility is estimated from the data by the correction with the Fresnel factors which are calculated on the basis of the Pekars Additional Boundary Condition.

A new experimental aspect of exciton polaritons in spatially dispersive region of CuCl

Abstract The two-photon polarization (TPP) spectra associated with the two-photon resonance of the excitonic molecule (EM) state via the upper or the lower exciton polariton branch are studied in the spatially dispersive region of CuCl at 1.6K under the resonant pumping of the respective polariton states to the EMs state by a strong laser beam. The selective annihilation of the exciton polaritons of one branch induces the prominent diminution of the TPP signal corresponding to the exciton polaritons of the other branch. The reflectivity of the incident light at the same energy that these polaritons have is also diminished remarkably by the pumping. A feature of the exciton polariton as a coherent coupled state composed of the exterior photons and the interior polaritons is discussed considering the role of the boundary.

Polarization rotation effects associated with the two-photon transition of Γ1-excitonic molecules in CuCl

Abstract Two-photon polarization rotation effects have been observed around the two-photon resonance of the Γ 1 -excitonic molecule, EM, in CuCl. It has been found that the effects originate from the rotation of the polarization plane of a linearly polarized weak probe beam induced by the irradiation of a strong circularly polarized excitation beam when the sum of the energies of both beams resonates to the EM level at 6.3722 eV. The effects have been observed in a wide range over 10 meV around the EM resonance. The observation of the effects offers a sensitive method to detect the modulation in the complex refractive index due to the two-photon transition of the EMs. It is also pointed out that compact optically-controlled fast-switching devices are assured by making use of these effects.

A Comment on the Broadening of the Excitonic Molecule Band in CuCl

The broadening of the excitonic molecule band in CuCl is studied from the giant two-photon absorption (GTA) measurements by a weak linearly polarized probe light beam under the simultaneous excitation around the same GTA by another strong pumping light beam of the orthogonal polarization to the probe beam. The width of the GTA line monitored by the probe light remains almost constant less than 0.3 meV under the strong irradiation of the pumping light. It is pointed out that the broadened width of the GTA band which have been observed by the strong one monochromatic beam experiments is not associated with the mutual collision between the really produced excitonic molecules.