Makoto Kuwata

Two-Photon Polarization Spectroscopy in CuCl

Highly sensitive momentum-space spectroscopy, named two-photon polarization spectroscopy (TPPS), is presented in the Z 3 exciton resonant region of CuCl. This spectroscopy is based on the induced optical anisotropy associated with the formation of \( \varGamma_{1}\) excitonic molecules (EMs). By this method the following facts are confirmed: the upper branch polaritons (UBPs) and the lower branch polaritons (LBPs) coexist in the wide energy range above the longitudinal exciton energy; the conversion of UBP into LBP occurs in the reflection at the sample boundary; the EM relaxation depends on the wave number in the large wave number region. From the quantitative analysis of the spectra, it has been found that the damping of the Z 3 exciton depends strongly on its energy in the L-T gap region. The prospects of the ABC problem are also discussed.

Optical nonlinearities in the exciton resonant region studied by polarization spectroscopy

Abstract The results of the measurements on photo-induced reflectivity change in the exciton resonant region in crystalline solids are reported. To achieve high sensitivity, a reflection-type polarization spectroscopy is developed and applied to the lowest exciton resonant region in CuCl and red-HgI 2 . In both materials, a very sharp Rayleigh-type structure, whose line width reflects the longitudinal relaxation of the exciton state, is observed for the first time; the χ (3) is estimated to be of the order of 10 -5 esu. In anthracene, unsually large optical nonlinearity is observed near the surface exciton resonances. According to the analysis based on a continuum model, we obtain a value of effective χ (3) of the order of 1 esu.

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.

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.

Self-Induced Polarization Rotation Effect of an Elliptically Polarized Beam in CuCl

The two-photon self-induced polarization rotation effect of an elliptically polarized laser beam is demonstrated experimentally in the giant two-photon absorption (GTA) region of \(\varGamma_{1}\) excitonic molecule in CuCl. The rotation angle of the principal axsis is found to be proportional to the incident beam intensity and sin 2ψ where ψ is its ellipticity angle. On the basis of a simplified four-level model, these characters are expressed quantitatively by the difference between the non-linear susceptibilities for the right- and the left-circularly polarized components of the elliptical polarized beam. The non-linear dispersion curve associated with the GTA resonance is determined.

Giant optical nonlinearity due to biexcitons in CuCl

A lower limit for the value of χ(3) associated with the two‐photon resonance of the Γ1 biexcitons in CuCl at 1.6 K is found to be 5.4×10−5 esu. The upper bound of the two‐photon resonance width is ΔE=0.02 meV. Broadening of the resonance occurs for light intensities I0>103 W/cm2.A lower limit for the value of χ(3) associated with the two‐photon resonance of the Γ1 biexcitons in CuCl at 1.6 K is found to be 5.4×10−5 esu. The upper bound of the two‐photon resonance width is ΔE=0.02 meV. Broadening of the resonance occurs for light intensities I0>103 W/cm2.

Picosecond time-resolved photoelectric correlation measurement with a photon-counting streak camera

Abstract An essentially new method of picosecond time-resolved photoelectric correlation measurement with a newly developed photon-counting streak camera (Hamamatsu C2909) is proposed. By this method temporal and spatial coherence of a quantized radiation field can be measured simultaneously. Statistical properties of a single-mode HeNe laser field (632.8 nm) have been examined quantitatively. The Poisson distribution of the laser field has been observed down to the time domain of about 10 ps.

Non-Linear Ellipsometer for the Measurement of Coherent and Incoherent Polarization Change in Optically Non-Linear Medium

A microcomputer-aided ellipsometer with a spinning quarter-wave plate and an analyzer was developed for the simultaneous determination of all the Stokes parameters. This system is quite good for measurements of a photo-induced non-linear polarization change, where a low-repetition pulsed laser is used as the light source. We applied this system for measurements of the photo-induced optical activity associated with the two-photon formation of ?1 excitonic molecules in CuCl. The coherent polarization change of a linearly polarized probe beam due to two-photon dispersion was clearly observed under irradiation by a circularly polarized excitation beam of 0.5 MW/cm2, and a strong depolarization was found near the two-photon resonance. We ascribe the origin of this depolarization to a random modulation caused by an intensity fluctuation in the pump beam.