Photon Echo from Localized Excitons in Semiconductor Nanostructures

An overview on photon echo spectroscopy under resonant excitation of the exciton complexes in semiconductor nanostructures is presented. The use of four-wave-mixing technique with the pulsed excitation and heterodyne detection allowed us to measure the coherent response of the system with the picosecond time resolution. It is shown that, for resonant selective pulsed excitation of the localized exciton complexes, the coherent signal is represented by the photon echoes due to the inhomogeneous broadening of the optical transitions. In case of resonant excitation of the trions or donor-bound excitons, the Zeeman splitting of the resident electron ground state levels under the applied transverse magnetic field results in quantum beats of photon echo amplitude at the Larmor precession frequency. Application of magnetic field makes it possible to transfer coherently the optical excitation into the spin ensemble of the resident electrons and to observe a long-lived photon echo signal. The described technique can be used as a high-resolution spectroscopy of the energy splittings in the ground state of the system. Next, we consider the Rabi oscillations and their damping under excitation with intensive optical pulses for the excitons complexes with a different degree of localization. It is shown that damping of the echo signal with increase of the excitation pulse intensity is strongly manifested for excitons, while on trions and donor-bound excitons this effect is substantially weaker.     

Stimulated terahertz emission from intraexcitonic transitions in Cu2O

We report the first observation of stimulated emission of terahertz radiation from internal transitions of excitons. The far-infrared electromagnetic response of Cu2O is monitored via broadband terahertz pulses after ultrafast resonant excitation of three-dimensional 3p excitons. Stimulated emission from the 3p to the energetically lower 2s bound level occurs at a photon energy of 6.6 meV, with a cross section of approximately 10(-14) cm2. Simultaneous excitation of both exciton levels, in turn, drives quantum beats, which lead to efficient terahertz emission sharply peaked at the difference frequency.     

Effect of the electric current on the Casimir force between graphene sheets

The dependence of the thermal component of the Casimir force and Casimir friction between graphene sheets on the drift velocity of charge carriers in one of the sheets has been analyzed. It has been shown that the drift motion results in the measurable change in the thermal Casimir force owing to the Doppler effect. The thermal Casimir force, as well as Casimir friction, increases strongly in the case of resonant photon tunneling, when the energy of an emitted photon coincides with the excitation energy of an electron-hole pair. In the case of resonant photon tunneling, the dominant contribution to the Casimir friction even at temperatures above room temperature comes from quantum friction caused by quantum fluctuations. Quantum friction can be detected in an experiment on the friction drag between graphene sheets in a high electric field.     

Optical properties of ultrathin PbI2 microcrystallite in polymer

We have prepared PbI₂ microcrystallites embedded in polymer, which have a layer structure and are ultrathin crystals consisting of two to nine monolayers.

A very large energy shift of the exciton absorption band has been observed in these microcrystallites and interpreted in terms of size confinement of the translational motion of excitons in the c-direction perpendicular to the crystal surface. The simple effective mass approximation is broken down in 2, 3, 4 layer crystallites, because the crystal thickness is smaller compared to the exciton Bohr radius.

Secondly, in the Raman spectrum where the excitation energy is resonant to the exciton energy, there appears a new line in the energy region below 20cm⁻¹ which is characteristic of a ultrathin crystal. The Raman shift increases with decreasing the crystal thickness. This line is assigned as due to a longitudinal mode of a rigid-layer phonon.

Thirdly, the confinement of the internal motion is studied by measuring a diamagnetic shift of the exciton energy in the magnetic field up to 150 T and a bound exciton luminescence in the ultrathin microcrystallites. Much larger binding energy of the exciton compared to the bulk value is estimated. This fact suggests that the envelope function of the exciton shrinks not only by a strong spacial restriction in the c-direction but also by the dielectric screening from the surrounding polymer.

Fourthly, the exciton-phonon interaction is studied by the hole-burning measurement. From the temperature dependence of the hole width, it is suggested that the exciton is scattered by impurities or defects below 40 K and by a LO phonon above 40 K.

Finally, the hydrostatic pressure dependence of the exciton energy and the resonant Raman scattering in the energy region of the optical phonons have been measured and the size effect on the atomic bonding between I and Pb is discussed. It is concluded that covalent bonding between Pb and I atoms changes to ionic bonding in the ultrathin crystallites.     

Non-Markovian dynamics in a dense atomic vapor

Two- and three-pulse photon echo spectroscopy in a dense potassium vapor reveals a non-Markovian correlation function of frequency fluctuations. Through comparison with calculations using an exciton picture a slowly-decaying component of the correlation function is attributed to long-range resonant interactions. A time-resolved photon echo experiment shows the photon-echo-like behavior at short timescales.     

Resonant light-controlled self-assembly of ordered nanostructures

The possibility of light-controllable formation of heterogeneous nanostructures containing resonant metallic and semiconductor nanoparticles is considered. Interaction energy between light-induced dipole polarization of nanoparticles at modest light intensity can be much more than the thermal motion energy. The configuration of self-assembled nanostructure can be controlled by the frequency and polarization of the light.     

Resonant stokes and anti-stokes Raman scattering of light in CdSe/ZnSe nanostructures

Raman scattering spectra of CdSe/ZnSe multilayer nanostructures with a CdSe insert of a nominal thickness of 2.1 monolayers were studied. A heavy dependence of the intensity and of the frequency position of the multiphonon Stokes and anti-Stokes LO bands on the exciting photon energy was detected. The results obtained are interpreted as a resonance with various exciton transitions in the CdSe insert and barrier ZnSe layers. A difference between the Stokes and anti-Stokes frequencies of LO bands observed as the resonance conditions are varied confirms the inhomogeneous nature of the photoluminescence band of CdSe quantum dots.     

Effect of dielectric confinement on optical properties of colloidal nanostructures

We review the effects caused by a large difference in the dielectric constants of a semiconductor and its surrounding in colloidal semiconductor nanostructures (NSs) with various shapes, e.g., nanocrystals, nanorods, and nanoplatelets. The difference increases the electron–hole interaction and consequently the exciton binding energy and its oscillator transition strength. On the other hand, this difference reduces the electric field of a photon penetrating the NS (the phenomenon is called the local field effect) and reduces the photon coupling to an exciton. We show that the polarization properties of the individual colloidal NSs as well as of their randomly oriented ensemble are determined both by the anisotropy of the local field effect and by the symmetry of the exciton states participating in optical transitions. The calculations explain the temperature and time dependences of the degree of linear polarization measured in an ensemble of CdSe nanocrystals.     

Exciton-phonon interaction in mixed silver halides: Resonant Raman scattering vs photoluminescence

An investigation of resonant Raman scattering in mixed crystals of AgBr:Cl at 1.8 K shows that the zero-phonon and LO phonon-assisted exciton luminescence excited in the free indirect exciton absorption, exhibits an anomalous dependence on the exciton photon energy E_L. Close to the exciton gap, the bands show a Raman-like behaviour with their peaks at constant energetic distance from E_L. As E_L is tuned further into the absorption, the bands gradually develop into normal photoluminescence. The effect is explained by taking into account exciton relaxation via scattering by long-wavelength acoustic phonons, a process which is strongly energy dependent. In addition, resonant Raman scattering observed for excitation in the zero-phonon absorption suggests study for the first time of the mode behaviour of certain off-zone center phonons in this system.     

Scattering spectra in orthorhombic indium bromide under resonant excitation

Emission spectra in indirect gap InBr are measured at liquid helium temperature under Ar⁺ laser excitation. They consist of many weak lines up to 20 with LO phonon energy (16.4 meV) spacing. They shift with incident laser lines. Even-numbered lines are stronger than odd-numbered lines. They show resonant enhancements when photon energy of scattered light is near the direct exciton energy, 2.33 eV.     

Correlated exciton relaxation in Poly(3-hexylthiophene)

Two-color 3 pulse photon echo peak shift (2C-3PEPS) measurements on poly(3-hexylthiophene) (3PHT) demonstrate that spectral regions in the photoluminescence remain correlated with the excitation, despite large differences in energy (>0.5 eV). The observations are explained in terms of exciton-phonon coupling that is dominated by only two motions: one high frequency bond stretch and a low frequency torsional motion. Numerical simulations of the 2C-3PEPS are shown to be consistent with the experimental observations. The results demonstrate that initial intramolecular exciton relaxation in P3HT is not primarily a stochastic process, but is driven by strong, selective exciton-phonon coupling to torsional motions.     

Direct observation of depolarization shift of the intersubband resonance

We have studied the intersubband resonance of GaAs/AlGaAs multi-quantum-well systems by comparing photon drag and absorption spectra obtained by in-plane photocurrent and photoconduction measurements. The peak absorption at room temperature is found to be blueshifted from the photon drag resonance by as much as 33 cm(-1). We argue that this difference gives directly the depolarization shift, since the resonant photon drag current is driven by the Doppler effect, which is a k-vector dependent single particle process.     

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.     

Exciton optical absorption coefficients and refractive index changes in a strained InAs/GaAs quantum wire: The effect of the magnetic field

Magnetic field induced exciton binding energy is investigated in a strained InAs/GaAs quantum wire within the framework of single band effective mass approximation. The strain contribution to the potential is determined through deformation potentials. The interband emission energy of strained InAs/GaAs wire is investigated in the influence of magnetic field with the various structural parameters. Magnetic field induced photoionization cross section of the exciton is studied. The total optical absorption and the refractive index changes as a function of normalized photon energy between the ground and the first excited state in the presence of magnetic field are analyzed. The optical absorption coefficients and the refractive index changes strongly depend on the incident optical intensity and the magnetic field. The occurred blueshift of the resonant peak due to the magnetic field will give the information about the variation of two energy levels in the quantum well wire. The optical absorption coefficients and the refractive index changes are strongly dependent on the incident optical intensity and the magnetic field.     

Kinetics of an exciton-trion system in shallow GaAs/AlGaAs quantum wells

The formation of three-particle charged exciton complexes (trions) in shallow GaAs/AlGaAs quantum wells in the temperature range 1.7–15 K has been investigated by luminescence spectroscopy and resonant light scattering. The effect of the photon energy and the intensity of additional above-barrier illumination on the trion formation kinetics has been analyzed. It is established that, upon intrawell excitation, illumination leads to the formation of trions when the light photon energy corresponds to the regions of effective formation of trions in the photoluminescence excitation spectra. It is shown that, with an increase in the illumination level, the trion concentration first increases and then reaches a plateau since the quantum well acquires an electric charge whose field equalizes the electron and hole capture rates.     

Resonant light delay in GaN with ballistic and diffusive propagation

We report on a strong delay in light propagation through bulk GaN, detected by time-of-flight spectroscopy. The delay increases resonantly as the photon energy approaches the energy of a neutral-donor bound exciton (BX), resulting in a velocity of light as low as 2100 km/s. In the close vicinity of the BX resonance, the transmitted light contains both ballistic and diffusive components. This phenomenon is quantitatively explained in terms of optical dispersion in a medium where resonant light scattering by the BX resonance takes place in addition to the polariton propagation.     

The effect of center-of-mass motion on photon statistics

We analyze the photon statistics of a weakly driven cavity quantum electrodynamics system and discuss the effects of photon blockade and photon-induced tunneling by effectively utilizing instead of avoiding the center-of-mass motion of a two-level atom trapped in the cavity. With the resonant interaction between atom, photon and phonon, it is shown that the bunching and anti-bunching of photons can occur with properly driving frequency. Our study shows the influence of the imperfect cooling of atom on the blockade and provides an attempt to take advantage of the center-of-mass motion.     

Optical switching of electron transport in a waveguide-QED system

Electron switching in waveguides coupled to a photon cavity is found to be strongly influenced by the photon energy and polarization. Therefore, the charge dynamics in the system is investigated in two different regimes, for off-resonant and resonant photon fields. In the off-resonant photon field, the photon energy is smaller than the energy spacing between the first two lowest subbands of the waveguide system, the charge splits between the waveguides implementing a $\sqrt{\mathrm{NOT}}$-quantum logic gate action. In the resonant photon field, the charge is totally switched from one waveguide to the other due to the appearance of photon replica states of the first subband in the second subband region instigating a quantum-NOT transition. In addition, the importance of the photon polarization to control the charge motion in the waveguide system is demonstrated. The idea of charge switching in electronic circuits may serve to built quantum bits.     

Theory of resonant Raman enhancement of a forbidden phonon in Cu2O

A theoretical calculation is given of the frequency dependence of resonant enhancement of Raman scattering by the normally forbidden 109 cm^?1 (Γ12-) optic phonon for incident frequencies near the 1s Yellow exciton energy. The theoretical results are in good agreement with recent experiments. This gives support to a quadrupole-dipole mechanism with Yellow-Blue exciton scattering in the intermediate state.