Giant two-photon absorption due to excitonic molecule

The direct excitation of excitonic molecule due to the two-photon absorption process is shown to be strongly enhanced because of two effects of the resonance and the giant oscillator strength as known for the bound exciton. Then it is pointed out that the existence of the excitonic molecule can be confirmed also by this two-photon absorption spectroscopy. We discuss also the property of the excitonic molecules highly excited by this method.     

Single exciton-to-excitonic molecule optical conversion in CdS

The induced absorption due to the optical conversion of a single exciton to an excitonic molecule, which is the reverse process of excitonic molecule luminescence, is observed in CdS. The theoretical prediction that the transition probability of this optical conversion is considerably enhanced by the giant oscillator strength effect is experimentally confirmed.     

The dispersion curves of the excitonic polariton and the excitonic molecule in CdS and their interaction

By means of the two-photon Raman scattering (TPRS) process we investigate the dispersion relation of the excitonic polariton in the energetic regions around the A-exciton resonance and near half the bi-exciton energy in CdS. In a high resolution experiment an anomaly is observed due to two-polariton transitions to the excitonic molecule (biexciton) state. This anomaly is explained on the basis of a previously developed theory of the intensity dependent dielectric function. Excitation spectroscopy of the TPRS-lines yields information about the damping of the excitonic molecule. Luminescence assisted two polariton spectroscopy (LATS) experiments are performed to determine the eigenenergy of the biexciton as well as its dispersion curve.     

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.     

Stress-induced stimulated emission from excitonic molecule in CdS

Stimulated emission has been observed at the M line (2.543 eV) in optically excited CdS crystals under an applied uniaxial stress perpendicular to the C axis at 4.2 K. This emission is strongly polarized with an electric vector parallel to the applied stress. The observed polarization characteristics are successfully accounted for by assuming a radiative annihilation of an excitonic molecule, providing an experimental evidence for ascribing this emission to the excitonic molecule in CdS.     

Picosecond decay kinetics of CdS luminescence studied by a streak camera

Using a picosecond laser and a streak camera we have observed the time dependence of the luminescence intensity of free excitons, bound excitons, and excitonic molecules in CdS. The observed kinetics show that the P band is due to bimolecular emission from free excitons and that bound excitons are generated from free excitons through monomelecular process and excitonic molecule through bimolecular process.     

Pico-second time-resolved luminescence spectra of excitonic molecules in CuCl

Transient luminescence behavior of excitonic molecules in CuCl is studied at 4.2 K with a time resolution of ～ 10 ps. The lifetime of the excitonic molecule state is determined to be several hundred pico-seconds. The spectral shape and the time characteristics of the luminescence intensity depend on excitation wavelength and also on the direction of observation. This is explained by the effect of the stimulated emission process.     

The strict upper limit for dissociation energy of an “anisotropic” excitonic molecule

The strict upper limit for dissociation energy is determined in the case of the model Hamiltonian of Wannier-Mott biexcitons with anisotropic effective masses. This value is estimated for a number of materials. For a large anisotropy the upper limit of the dissociation energy for the production of two single excitons from an excitonic molecule may reach the value of the order of one excitonic rydberg.     

The excitonic molecule emission in the uniaxially stressed Si with the different electron valley degeneracy

The excitonic molecule emission spectra in Si uniaxially stressed along <100> and <110> directions have been investigated. The binding energy of the excitonic molecules is found to be weakly dependent of space orientation of the electron valleys.     

Picosecond studies of highly excited CdS

Results on picosecond luminescence and excite-and-probe transmission as well as transient grating measurements for highly excited CdS measured at a bath temperature of 5 K will be presented. The luminescence and optical gain both due to electron-hole plasma and excitonic molecule recombination are observed. The electron-hole plasma decays very fast by bimolecular recombination of electrons and holes in the plasma and diffusion of the carrier toward the low density regions, and transforms into excitons and excitonic molecules within 100–200 ps. The possibility of electron-hole liquid formation is definitely excluded. The exciton and excitonic molecule decay rather slowly and govern the optical properties for times longer than 200 ps.     

Two-photon echo method to determine the transverse relaxation time of excitonic molecules

A nonlinear optical method to determine the transverse relaxation time of excitonic molecules by means of two-photon echo is proposed. In the case of two-photon transition such as excitonic molecule one needs three pulses to obtain an echo signal. When the wave-vector and frequency of the three successive pulses are denoted by k₁,ω₁, k₂,ω₂ and k₃,ω₃, the two-photon echo can be observed in the direction of 4k₂-2k₁-k₃ with frequency 4ω₂-2ω₁-ω₃. Tuning both ω₁ and ω₂ to be two-photon resonant with excitonic molecule, we can satisfy the phase-matching condition rather easily for appropriate values of ω₃ due to the large dispersion of excitonic polaritons. From the correlation trace of the two-photon echo we can determine directly the transverse relaxation time of excitonic molecules.     

About investigations of excitonic molecules in direct semiconductors

The method of studying the excitonic molecules in direct semiconductors is proposed. The exciton luminescence in the LO-phonon sideband is shown to be suppressed relative to that of the molecule. This demonstrates the fact that the luminescence of the molecules should be more clearly observed in the LO-phonon sideband. The time to form the molecule from two excitons is estimated.     

An interpretation of the correlation between upper and lower polaritons in terms of four polariton parametric effect via excitonic molecules

Recently Kuwata and Nagasawa observed the correlation between the upper and lower branch polaritons in the experiments of two-photon polarization rotation via the excitonic molecule level in CuCl crystals. An interpretation will be presented that the correlation is not intrinsic but is caused by the change of state of the sample due to the four polariton parametric scattering under a rather strong excitation. This interpretation can explain the decrease of the signal of two-photon polarization rotation and the reflectivity if an appropriate amount of the population change is assumed to occur under the two-photon resonance of excitonic molecules.     

Heteroexcitonic molecule

The excitonic molecule formed by a band edge exciton and an exciton associated with a deeper band state has been discovered for the first time. Such a biexciton has been named the heteroexcitonic molecule (HEM).     

Induced absorption and gain from high density excitons in CdS

Induced absorption and gain in CdS at 1.8 K has been investigated under excitation densities up to 10 MW cm^?2. The absorption and gain below the free exciton energy is governed by exciton interactions and optical conversion of excitons into excitonic molecules. At the highest density, induced transparency due to excitonic molecule recombinations is observed. E_M=5.1002 eV is determined.     

Stimulated two-photon emission from excitonic molecules in ZnO

The M-band emission in ZnO at 1.7 K is investigated by tuning the excitation light through the A-B exciton region. Externally stimulated two-photon emission from excitonic molecules is observed when the pump photon energy is resonant with the upper B-polariton. The experiments suggest two excitonic molecule levels separated by 4.6 meV and with a ground state energy E_M = 6.7394 eV.     

Complement to theory of Raman scattering from adsorbed molecule on semiconductor

The Raman polarizability of a molecule adsorbed on a semiconductor is calculated when it couples to excitonic and interband excitations in the substrate. It is found that a difference in the electronic properties of each excitation manifests itself in the excitation profile of the Raman intensity of the adsorbed molecule, i.e., the former produces a sharp peak below the onset of interband excitation, whereas the latter gives rise to weaker but more broadened structure over a wide range of the spectrum.     

Power-law-distributed dark states are the main pathway for photobleaching of single organic molecules

We exploit the strong excitonic coupling in a superradiant trimer molecule to distinguish between long-lived collective dark states and photobleaching events. The population and depopulation kinetics of the dark states in a single molecule follow power-law statistics over 5 orders of magnitude in time. This result is consistent with the formation of a radical unit via electron tunneling to a time-varying distribution of trapping sites in the surrounding polymer matrix. We furthermore demonstrate that this radicalization process forms the dominant pathway for molecular photobleaching.     

Carbon nanotubes as substrates for molecular spiropyran-based switches

We present a joint theory-experiment study investigating the excitonic absorption of spiropyran-functionalized carbon nanotubes. The functionalization is promising for engineering switches on a molecular level, since spiropyrans can be reversibly switched between two different conformations, inducing a distinguishable and measurable change of optical transition energies in the substrate nanotube. Here, we address the question of whether an optical read-out of such a molecular switch is possible. Combining density matrix and density functional theory, we first calculate the excitonic absorption of pristine and functionalized nanotubes. Depending on the switching state of the attached molecule, we observe a red-shift of transition energies by about 15?meV due to the coupling of excitons with the molecular dipole moment. Then we perform experiments measuring the absorption spectrum of functionalized carbon nanotubes for both conformations of the spiropyran molecule. We find good qualitative agreement between the theoretically predicted and experimentally measured red-shift, confirming the possibility for an optical read-out of the nanotube-based molecular switch.     

Bipolariton in quantum wells

Il Nuovo Cimento D - We develop the bipolariton concept for a quasi-2D excitonic molecule (m) in quantum wells (QWLs). The bipolariton wave equation, which includes both the exciton-exciton (x-x)...