Excitonic Molecules In CuCl Research Articles

Direct Two-Photon Creation of Excitonic Molecule in CuCl

The probability of direct excitation of excitonic molecule in the two-pho- ton absorption process is calculated with an Hylleraas-Ore type biexciton envelope, which is variationally optimized. In the second order of perturba- tion, because of the resonance effect, only the exciton .r5 intermediate state is taken into account. The band structure at band extrema due to spin—orbit interaction is assumed. The transition probability of two-photon absorption is expressed by matrix element between the free exciton and the biexciton envelopes. The obtained probability of two-photon absorption in CuCl is about three orders of magnitude smaller when compared to that obtained by Hauamura.

Two-photon generation of excitonic molecules in CuCl: An exactly solvable bipolariton model and high-precision experiments.

The conventional giant oscillator strength model of the two-photon (\ensuremath{\gamma}) generation of an excitonic molecule (m) attributes this process to the x-\ensuremath{\gamma} optical conversion (where x is the exciton) i.e, to the scheme \ensuremath{\gamma}+\ensuremath{\gamma}\ensuremath{\rightarrow}x+\ensuremath{\gamma}\ensuremath{\rightarrow}m. Recenty [A.L. Ivanov and H. Haug, Phys. Rev. B 48, 1490 (1993)], it was argued that a more adequate description can be done within the bipolariton model which follows the other scenario: \ensuremath{\gamma}+\ensuremath{\gamma}\ensuremath{\rightarrow}x+x\ensuremath{\rightarrow}m, where the Coulombic resonant coupling of the x components of the two interacting polaritons gives rise to the m formation. In the present work, we develop and analyze for the deuteron potential of an x-x interaction an exactly solvable bipolariton model of the two-\ensuremath{\gamma} m generation. This model treats an m optical creation in terms of the polariton-polariton resonant scattering and includes both the x-y polariton coupling and the x-x attraction beyond a low-order perturbation theory. The bipolariton model and the giant oscillator strength model give different descriptions of the third-order m nonlinear optical susceptibility ${\mathrm{\ensuremath{\chi}}}^{(3)}$ and of the two-\ensuremath{\gamma} m absorption. With the high-precision measurements in CuCl of the m radiative width ${\mathrm{\ensuremath{\Gamma}}}^{\mathit{m}}$, ${\mathrm{\ensuremath{\chi}}}^{(3)}$, and the two-\ensuremath{\gamma} m absorption coefficient ${\mathit{K}}^{(2)}$, we make a systematic comparison between the experiments and the two models, which allows us to unambiguously discriminate both models in favor of the bipolariton one.

Optical Nonlinearity Due to the Excitonic System in CuCl

AbstractThe optical nonlinearity associated with the giant two‐photon absorption, GTA, of excitonic molecules in CuCl is discussed. The metamorphoses of the four‐wave mixing spectra due to the generation of the exciton grating are demonstrated by the observation of their polarization characteristics. The propagation characteristics of the excitation around the GTA resonance are observed by micro‐Raman spectroscopy. The two‐photon Raman scattering light is used as a probe to monitor the spatial character of the excitation. The propagation characteristics show a drastic change in the very narrow spectral range around the GTA resonance energy.

A Comment on Non-Linear Depolarization Effects under the Giant Two-Photon Excitation of Excitonic Molecules in CuCl

Photo-induced depolarization effect of linearly polarized beam in the giant two-photon absorption (GTA) band of excitonic molecules was examined in CuCl with the use of non-linear ellipsometry. After the careful elimination of the instrumental optical anisotropy, no depolarization was observed under the excitation of 1 MW/cm 2 . The stability of the linearly polarized state was also studied by measuring the polarization character for a slightly elliptical beam and a partially polarized beam. Growth of the depolarization was found in both cases and thus the linear polarization is concluded to be an unstable state in the GTA resonance.

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 Γ1 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.

Relaxation Process of Excitonic Molecules in CuCl under the Two-Photon Resonant Excitation. II. Transverse Relaxation

Under the off-resonant excitation at the slightly higher energy side of the giant two-photon absorption band (GTA) for the direct generation of excitonic molecules (EM) in CuCl, new narrow emission bands designated as X T and X L have been found in the energy region of the M T and M L broad bands, respectively. Their photon energies increase with the decrease of the pump photon energy from the higher-energy side of the resonance and finally, at the on-resonant excitation, their bands merge into M T 0 and M L 0 bands, respectively, previously reported by Mita et al. Based on the detailed studies on these bands, it is found that there exists a certain kind of transverse relaxation process which acts on the EM just after their generation by the GTA and brings about the X emission as a hot luminescence.

Relaxation Process of Excitonic Molecules in CuCl under the Two-Photon Resonant Excitation. I. Mutual Collision of Excitonic Molecules

Line shapes of the giant two-photon absorption (GTA) band for the direct generation of excitonic molecules (EM) and the subsequent secondary emission of the EM resonantly generated have been studied in CuCl at various pump powers with the use of a high resolution spectroscopic method. The mutual collision of the EM, the rate of which is nearly proportional to their density, is found to be the main origin for causing the broadening of the GTA band, the decrease of the Raman yield and the rapid increase of the EM luminescence. The collision rate increases up to 10 12 /sec for the pump power of ∼1 MW/cm 2 . With the use of pump and probe method further evidence is obtained for the effectiveness of the mutual collision of the EM.

Optical phase conjugation based on the giant two-photon resonance of the excitonic molecule in CuCl

Optical phase conjugation resulting from the two-photon coherence due to the excitonic molecule resonance in CuCl has been observed and studied as a function of frequency, polarization, and pump intensity. Reconstruction of the polarization of an aberrated probe beam is demonstrated.

Bose-Einstein statistical properties and condensation of excitonic molecules in CuCl

A comprehensive investigation of the optical properties of excitonic molecules in CuCl films is presented and discussed. The experimental work includes resonant two-photon absorption (TPA), observations of the resonantly excited biexciton emission, and optical pump-and-probe studies. The collision broadening of the TPA implies a collision rate \ensuremath{\sim}${10}^{\ensuremath{-}12}$ sec for the biexcitons, which is sufficiently rapid to establish a quasithermal equilibrium for the particles on a time scale that is short in comparison with the biexciton lifetime and the duration of the laser pump pulse. The biexciton luminescence line shapes obtained with resonant $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}=0$ two-photon excitation have been fitted using a Bose-Einstein thermal distribution of biexcitons. The chemical potential obtained from the fits goes to zero as the biexciton density is increased at a fixed lattice temperature or if the lattice temperature is decreased at fixed density. The sharp luminescence features that are observed at high densities and low temperatures with single-beam, two-photon excitation are interpreted as resulting from a Bose-Einstein---condensed state of the biexcitons at twice the wave vector of the laser pumping photons. Further evidence for a condensed state is obtained from optical-probe measurements in which a small number of probe biexcitons is injected into the sample, with and without the presence of a condensed state produced by an intense laser pump beam. The luminescence of these injected probe biexcitons is modified in the presence of the pump beam such that it exhibits the sharp features of the emission from the condensed state. Measurements of optical gain and stimulated emission in the presence of the pump beam show that the redistribution of the probe luminescence does not result from those processes, but must involve a preferential redistribution of the probe particles in momentum space via collisions. Such a redistribution is expected when additional particles are added to a condensed system of bosons.

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.

Determination of diffusion coefficients of an exciton and excitonic molecule in CuCl by picosecond transient grating spectroscopy

We have made the first successful direct observation of the diffusion coefficients of an exciton and excitonic molecule in CuCl by tunable picosecond transient grating spectroscopy under the resonant two-photon excitation of the excitonic molecule. The diffusion coefficients of the exciton and the excitonic molecule are 330 \ifmmode\pm\else\textpm\fi{} 200 and 45 \ifmmode\pm\else\textpm\fi{} 10 ${\mathrm{cm}}^{2}$/sec at 1.8 K, respectively. The main part of this difference between the diffusion coefficients of the exciton and excitonic molecule is explained by the difference of the size of the exciton and excitonic molecule.

Superbroadened distribution of excitonic polaritons in CuCl

Superbroadened distribution of excitonic polaritons injected by intense laser beam is observed around the two-photon resonance of excitonic molecules in CuCl. By measuring the induced absorption from the injected excitonic polariton state to the excitonic molecule state, the energy distribution of excitonic polaritons is derived. The origin of this broadening is attributed to four-polariton parametric scattering. This phenomenon gives clear explanation to the excitation intensity-dependent broadening of two-photon absorption of excitonic molecules in CuCl.

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

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 (EM's) 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 EM's.

High intensity effect on two-photon resonant, coherent Raman scattering via excitonic molecules in CuCl and CuBr

Two-phonon resonant, coherent Raman scattering (four-photon degenerate coherent scattering) via excitonic molecules has been studied as a function of intensity of incident light in CuCl and CuBr. Observed line-shaped of coherent scattering spectra broadens as excitation intensity is raised. This broadening is found to be closely correlated with that of the two-photon absorption. These results are qualitatively explained by taking account of the excitation intensity-dependence of χ (3), but cannot be explained by the autoinization model.

Nonequilibrium Bose condensation — Excitonic molecules in CuCl

It is shown that the previously proposed theory of the nonequilibrium Bose-Einstein condensation of an initially prepared system of Bose particles describes in a quantitative way recent experimental results [2] on the dense system of excitonic molecules in CuCl.

Two-Photon Resonant, Coherent Raman Scattering via Excitonic Molecules in CuCl

The three-wave mixing process has been studied as functions of incident photon energy and also of the geometrical configuration when incident photon energy lies around the two-photon resonance of excitonic molecule. Experiments indicate that the intensity of the third order nonlinear light beam is enhanced when the phase matching condition derived from the dispersion of excitonic polariton holds or when incident photon energy is equal to the two-photon resonance of excitonic molecule. The relation between this three-wave mixing process and two-photon resonant Raman scattering so far observed is discussed. It is shown that this three-wave mixing process is regarded as the two-photon resonant, coherent Raman scattering via excitonic molecules.

Phonon Interaction of Excitonic Molecule in CuCl

The line shapes of the giant two-photon absorption band for the direct generation of excitonic molecules and the molecule luminescence in CuCl have been studied in a temperature region between 6 K and 100 K. The interaction between excitonic molecules and acoustical phonons is found to play an important role for the relaxation mechanism of excitonic molecules at high temperature. For temperatures higher than ∼50 K, the multi-phonon scattering process becomes predominant, and it causes the asymmetry in line shapes of the two-photon absorption band and the relevant two-photon resonant Raman lines.

Secondary emissions under two-photon resonance excitation of excitonic molecules in CuCl

The origin of the narrow secondary emission lines under two-photon just- resonance excitation of the excitonic molecule in CuCl is discussed. Luminescence due to cold, but not Bose-condensed excitonic molecules is concluded to be dominant. Recent experiments to discriminate between Raman scattering and luminescence by picosecond spectroscopy on these emissions are criticized.

Resonant Raman Scattering and Luminescence due to Excitonic Molecule

Competitive behavior of two channels of resonant Raman scattering and luminescence is discussed for the giant two-photon excitation of excitonic molecules in CuCl. The stochastic theory of intermediate state interaction is applied to describe the time-integrated and time-resolved emission spectra of the excitonic molecule excited resonantly by a short light pulse. Particularly as for the time-resolved spectrum, the frequency-time uncertainty due to the finite observation time is incorporated in the present theory. Our theory explains the observed characteristics of the emission spectra, and the relaxation constants of the excitonic molecule and the exciton in the final state are determined from comparison between the observed and calculated spectra.

Phonon‐Assisted Two‐Photon Absorption of the Excitonic Molecule in CuCl I. Bare‐Exciton Approach

AbstractPhonon‐assisted two‐photon absorption of biexcitonic levels in CuCl is considered including damping of the nearly resonant excitonic intermediate state. A pronounced structure of the line shape due to resonance effects, threshold effects, and structure in the Fourier transform of the envelope of the excitonic molecule is predicted. A detailed discussion of both temperature and damping effects is given. Large cross sections are predicted. The most favourable energetic range to observe the effects derived here is the low energy tail of the line, on the left of the Γ⅕ exeiton.