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Annual Meeting Optical Society of America

Femtosecond coherent absorption changes in CdSSe quantum dot-doped glasses

Kenneth W. DeLong, Junji Yumoto, Naoshi Uesugi

Published Jan 1, 1991• 0 citations
Publisher

Abstract

We report what we believe to be the first observation of coherent transient absorption changes in glasses doped with quantum dots of CdS0.12Se0.88. These observations were made at room temperature with on-resonant laser pumping. We observe a transient bleaching on the high energy side of the pump center wavelength, and a transient absorption increase on the low energy side. The transient effects last for a time about equal to the pump pulse duration, 80 fsec, after which real carrier generation masks the effects. The results can be qualitatively explained by a spectral shift in the direction predicted by the optical Stark effect. This was a pump-probe experiment where the 622-nm pump center wavelength was on the long wavelength side of the lowest energy transition in the quantum dots but still well within the absorption band. The radii of the microcrystallites ranged from 20 to 40 Å, and T2 in similar samples was estimated to be 60 fsec or less. Although the laser pulsewidth is longer than the dephasing time in the sample, we were still able to see coherent effects.

DOI
10.1364/oam.1991.wt4

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•May 1, 1990•0 citations
We report what we believe to be the first observation of coherent transient absorption changes in glasses doped with quantum dots of CdS0.12Se0.88. These observations were made at room temperature with on-resonant laser pumping. We observe a transient bleaching on the high energy side of the pump center wavelength, and a transient absorption increase on the low energy side. The transient effects last for a time about equal to the pump pulse duration, 80 fsec, after which real carrier generation masks the effects. The results can be qualitatively explained by a spectral shift in the direction predicted by the optical Stark effect. This was a pump-probe experiment where the 622-nm pump center wavelength was on the long wavelength side of the lowest energy transition in the quantum dots but still well within the absorption band. The radii of the microcrystallites ranged from 20 to 40 Å, and T2 in similar samples was estimated to be 60 fsec or less. Although the laser pulsewidth is longer than the dephasing time in the sample, we were still able to see coherent effects.
Coherent transient effects in CdSSe microcrystallites are observed by a pump-probe measurement. The probe transmission shows oscillatory structures in the temporal and spectral domains when the probe reaches the sample before the pump. This oscillation is attributed to the photon echo propagating in the direction of the probe. The A-B splitting of the valence sublevel in CdSSe microcrystallites hidden in the wide inhomogeneous broadening can be resolved using the polarization dependence of the transitions from A and B valence sublevels to the conduction sublevel.
The optical Stark effect of excitons in semiconductors, where the exciton resonance is blue shifted by an off-resonance light beam, has been actively studied. Here we report on the observation of a very fast recovery of the exciton bleaching for an off-resonance excitation condition in a quantum well sample at room temperature. The good agreement between the experimental results and our numerical calculations of the time-resolved absorption spectra suggests a coherent transient population buildup and decay occurring on the time scale of the pump pulse.
Using laser pulses of duration comparable to semiconductor relaxation and dephasing times, the coherent phenomena common to two-level systems have been demonstrated in II-VI and III-V semiconductors. These light-matter interactions are characterized by electrons and holes in their initially excited states and/or a macroscopic polarization that persists after the excitation. A nonthermal carrier distribution was observed in bulk CdSe, demonstrating the extremely fast energy relaxation possible when carrier-LO-phonon scattering is included. Quantum-confined CdSe microcrystallites were then employed to limit the available decay avenues. The one and two-pair transitions were characterized, and using spectral hole-burning, energy relaxation was shown to be substantially slower than in bulk. Dipole dephasing remained very rapid. Persistent macroscopic polarization was demonstrated in semiconductors through pump and probe experiments conducted on time scales faster than the inverse linewidth of the exciton. In the first set of experiments, it was found that the exciton optical Stark shift deviates from that predicted by a steady-state theory. The shift is smaller than expected and lasts longer than the pump pulse. Both effects are explained by free evolution of the exciton's polarization. Transient exciton bleaching is observed and shown to be a result of adiabatic following. In a second study of coherent polarization, spectral oscillations at negative delay times were studied. Oscillations were measured in a wide variety of semiconductors, temporally preceding several different pump-probe phenomena. By comparison with a semiclassical theory, it was shown that spectral oscillations in the spectral region of the exciton are evidence of persistent exciton polarization. Oscillations in the band are due to four wave mixing of the pump and probe beams.
In this thesis, the coherent interaction of single semiconductor quantum dots and ultrafast optical pulses is studied. Under certain conditions, localized exciton transitions in quantum dots can be seen as semi-isolated two-level systems. While this description is sufficient for the explanation of some observations in coherent experiments, it is sometimes necessary to explicitly consider coupling of the discreet quantum states confined to the dot with the environment. We start out from simple, classical examples of coherent spectroscopy and then turn towards experiments where the interaction with the vicinity of the dot becomes an important factor. First, a novel method for transient differential reflectivity spectroscopy of single quantum systems is introduced. It is a pure far-field optical technique which does not require any sophisticated sample preparation steps which makes it applicable to a broad range of structures. Pump pulses excite the sample structure and probe pulses read out the pump-induced changes in the system after a variable delay time. In the case of a single dipole, the signal is given in the form of the spectral inteferogram between the backscattered wave from the particle and the probe light which is reflected at the sample surface. This form of homodyne detection amplifies the weak scattered wave from the particle and thus makes this kind of spectroscopy for single quantum dots feasible. In the remainder of this thesis our spectroscopic method is applied to either characterize the coherent properties of single quantum dots, to prepare and read-out a desired quantum state or to deliberately manipulate them. Coherence times and oscillator strengths are determined for localized exciton transitions. Arbitrary population states can be written by driving coherent population oscillations using resonant pulses, while entangled superpositions of two exciton states in a single dot are investigated by quantum beats on transient differential spectra. We finally exploit the interaction between the dot and a nearby absorbing layer to switch the dot's absorption spectrum on ultrafast timescales via light-induced transient electric fields.
We report the first experimental study of the optical Stark effect in single semiconductor quantum dots (QD). For below band gap excitation, two-color pump-probe spectra show dispersive line shapes caused by a light-induced blueshift of the excitonic resonance. The line shape depends strongly on the excitation field strength and is determined by the pump-induced phase shift of the coherent QD polarization. Transient spectral oscillations can be understood as rotations of the QD polarization phase with negligible population change. Ultrafast control of the QD polarization is demonstrated.