Speaker
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Abstract:
Short, coherent pulses from infrared free electron lasers has been used extensively for studying population dynamics, and to a lesser extent coherence, of bound carrier states in semiconductors. Here we focus specifically on the hydrogen atom-like states of single donors and acceptors. Several challenges beset characterisation: the ladder of excited bound states is closely spaced in relation to the incident beam line width, and states lie very close to the conduction or valence band continuum. Measuring basic parameters such as the population life time $T_1$ is open to different interpretations. Coherent measurements such as photon echo and Ramsey spectroscopy offer a way to disambiguate this but require more complex setups than pump-probe. We demonstrate that by using a circularly polarized pump and a linearly polarized probe at ~10 THz we can harness the inherent optical dichroism of the excited Rydberg states to correctly attribute different decay paths of excited Rydberg states in phosphorus-doped silicon using only two beams, measuring optical Faraday rotations up to ±8°.
Access to circularly polarized IR FEL pulses allows us to harness spin selection rules to demonstrate optical pumping of hole spins in boron-doped silicon, analogous to spin preparation in atomic physics. We find a single dark state in the J=3/2 manifold of the hole’s orbital ground state that is not excited by circularly polarized light, and preferentially pump holes from the other Jz eigenstates into it. We recover two time scales: a fast (~30 psec) orbital decay and a slow (~1 nsec) spin decay. The measured decay and population transfer can be modelled with a rate equation and a few simple assumptions. We propose a realistic path to 99% spin initialization within hundreds of picoseconds, several orders of magnitude faster than current microwave initialization-based protocols.
| Scientific Topics | Quantum Science |
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