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Physical Review Letters

Biexciton and Triexciton States in Quantum Dots in the Weak Confinement Regime

Michio Ikezawa, Yasuaki Masumoto, T. Takagahara, Selvakumar V. Nair

Published Nov 3, 1997• 46 citations
Publisher

Abstract

Biexciton and triexciton states in CuCl quantum dots were studied by means of time-resolved size-selective pump-and-probe technique. A clear induced absorption band is observed on the high-energy side of the excitation photon energy. The new induced absorption is assigned to the transition from the exciton ground state to one of the weakly correlated exciton pair states which were theoretically predicted to exist and to play an important role in nonlinear optical processes. Its pump energy dependence and temporal evolution strongly support this assignment. Under high-density or two-color excitation condition, a triexciton state in quantum dots is observed for the first time.

DOI
10.1103/physrevlett.79.3522

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Journal of Physics Conference Series•May 1, 2019•3 citations
Biexciton and triexciton states in CuCl quantum dots were studied by means of time-resolved size-selective pump-and-probe technique. A clear induced absorption band is observed on the high-energy side of the excitation photon energy. The new induced absorption is assigned to the transition from the exciton ground state to one of the weakly correlated exciton pair states which were theoretically predicted to exist and to play an important role in nonlinear optical processes. Its pump energy dependence and temporal evolution strongly support this assignment. Under high-density or two-color excitation condition, a triexciton state in quantum dots is observed for the first time.
We present a thorough investigation of the different relaxation channels opened in a strongly interacting system of excitonic quasiparticles in CuCl for density regimes ranging up to 50 MW/cm2{\mathrm{cm}}^{2}cm2. A spectrally narrow pump beam consisting of 30-ps pulses is tuned around one-half the biexciton energy. It creates an initial distribution of biexcitons through resonant two-photon absorption. A spectrally broad probe beam made of pulses that also have 30-ps duration monitors the excitation-induced variation of the transmission spectrum of CuCl samples near the Z3{\mathit{Z}}_{3}Z3​ exciton resonance. Coherent processes corresponding to parametric emissions of various orders are observed. Absorption and gain bands that reflect the evolution of biexciton and exciton populations on their respective dispersion curves are followed as functions of the time delay between pump and probe pulses.
We measured the optical transitions due to the internal energy levels of an exciton and biexciton in a CuCl single crystal using pump-probe spectroscopy. The transient absorption bands due to the transitions from the 1s1s1s to 2p2p2p and from the 1s1s1s to 3p3p3p exciton states were observed at 3 K, which is consistent with their reported energies. Simultaneously, the transient absorption peak due to the biexciton was observed, which corresponds to a transition from the lowest state (composed of two 1s1s1s excitons) to higher states (composed of 1s1s1s and 2p2p2p excitons). The value of the observed biexciton peak energy was reasonable considering the hydrogen molecule orbitals and the electron-to-hole effective mass ratio. In addition, the transient absorption peaks were broadened at 77 K, because of the increase in the homogeneous width of the 2p2p2p exciton state. The transient absorption spectrum was almost completely determined by this width. Our findings are of importance with regard to the optical phenomena in the infrared region related to the quantum coherence of excitons and biexcitons in semiconductors.
Transient optical nonlinearities of semiconductor quantum dots in the strongly confined regime are studied, in which the exciton, biexciton and magnetic field effects are taken into account. The photo induced band-to-band electronic transitions are considered from both the heavy-hole (hh) and light-hole (lh) valence bands to the lowest (1s) exciton state below the fundamental absorption edge. The time dependent perturbation technique for a three-level system, under the near band gap resonant excitation regime, has been employed; with the incorporation of the relaxation and dephasing mechanisms phenomenologically. The induced polarization is calculated from where the real and imaginary parts of the third-order nonlinear optical susceptibility, responsible for the occurrence of nonlinear refraction and nonlinear absorption respectively, have been derived. The temporal and detuning spectra of nonlinear refraction and absorption; and their dependence on density of biexcitons and a moderately strong magnetic field, have been analyzed. It is observed that the nonlinear refraction and nonlinear absorption enhance with increasing magnetic field in small CdS quantum dots.
We report on large optical nonlinear effects induced by coherent interactions between excitons and biexcitons in quantum dots. The absorption spectra in an exciton–biexciton system are measured in InGaAs quantum dots by using single dot spectroscopy. The spectra exhibit unusual shapes that have a transparent region under a strong excitation condition. A pump–probe measurement ensures that the exciton state is erased by a quantum interference effect with exciton and biexciton photoabsorption processes. These results suggest that optical functions such as electromagnetically induced transparency are feasible with excitons and biexcitons in quantum dots.
Abstract We investigated the influence of excitation processes of a biexciton on the generation of superfluorescence, which is pulsed radiation from coherently coupled two-level systems, from biexcitons in CuCl quantum dots. Two excitation processes of biexcitons were examined: resonant two-photon excitation of biexcitons, and resonant one-photon excitation of excitons. The excitation density dependence of the time-resolved photoluminescence was measured. The results showed that for one-photon excitation, a stronger peak intensity, longer delay time and wider pulse width were observed compared to resonant two-photon excitation. The high density of the excited dots in the one-photon excitation process results in the strong pulse, while an initial population of excitons results in a suppression of coherent coupling.