Abstract
We present an experimental investigation of optical spin orientation in colloidal CdS quantum dots (QDs) by a femtosecond laser pulse at room temperature. The spin carrier and its spin-generation process are clarified. Firstly, the observed spin signals of CdS QDs in time-resolved Faraday rotation measurements are shown to belong to electron carriers, by comparing the spin dephasing dynamics and Landé g factor between CdS QDs and bulk materials. Secondly, spin dynamics unaffected by the faster carrier recombination suggests that the spin-polarized electrons are not photoexcited but resident in the dots. Moreover, hole spins should dephase very fast compared with electron spins, otherwise the trion (two electrons with opposite spin orientations and one hole) recombination process will affect the resident electron spin signals. The electron spin is generated in a short time of which the excitation light is absorbed and the resident electron is excited to trion states, i.e., of pulse durations. Due to fast hole spin dephasing, trion recombination gives null spin signals, and the subsequent electron spin dynamics is controlled by its intrinsic mechanisms.
Highlights
The spin states of semiconductor quantum dots carry prospects in realization of solid-state qubits for quantum-information processing and computation (QIPC) [1,2]
The signals oscillate as a result of spin precession in the magnetic field, with a frequency being equal to Larmor precession frequency, ωL gμ B B /
By comparing the spin dephasing dynamics and Landé g factor between CdS quantum dots (QDs) and bulk materials, it is concluded that the observed spin signals of CdS QDs belong to electron carriers
Summary
The spin states of semiconductor quantum dots carry prospects in realization of solid-state qubits for quantum-information processing and computation (QIPC) [1,2]. As quantum error correction (QEC) schemes require >104 operations within the decoherence time, the state initialization speed must be much faster than the quantum state decoherence rate [3] In this regard, ultrashort laser pulses have advantages in fast spin generation and control, as well as convenient detection of transient spin states [3,4,5,6,7,8,9]. We experimentally judge that the spin relaxation time of the hole in trion complexes (two electrons and one hole) should be much faster than the trion recombination time Based on these experimental results, the spin generation process will be illustrated in detail
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