Jeremy J. Baumberg

Coherent control of exciton density and spin

We demonstrate femtosecond coherent control of excitons in quantum wells with phase-locked pairs of 100 fs infrared pulses. Copolarized pump pulses allow coherent control of exciton density and coherent destruction of excitons within a few hundred femtoseconds of their creation. This technique thus promises to avoid speed penalties in devices associated with long-lived persistent carrier populations. Cross-polarized pump pulses allow coherent control of spin dynamics and conversion of unpolarized excitons into spin polarized ones. Carrier density and spin are determined, respectively, from the differential reflection and from the Faraday rotation of a third probe pulse. The experimental results are in good agreement with calculations based on the semiconductor Bloch equations.

Reduced threshold in pierced semiconductor microdisk lasers

GaAs microdisk lasers with holes pierced through the disk surface are investigated for their threshold characteristics. Disks are fabricated with either a single hole or two diametrically opposite holes at various distances from the disk outer edge. Even though the disk area is reduced by only 1%, we find that the lasing threshold for a disk with one hole is reduced by up to 50% compared to a disk with no hole. We attribute this reduction to the perturbation of nonlasing modes, which decreases the amplification of spontaneous emission in these modes and makes more carriers available to contribute to lasing.

Femtosecond dynamics in semiconductor lasers: Dark pulse formation

We track the dynamic response of GaAs quantum-well semiconductor diode lasers after injection of a femtosecond pulse, using ultrafast upconversion. The continuous-wave emission shows gain dynamics and relaxation oscillations on timescales of 1–100 ps. The recirculating femtosecond pulse evolves into an ultrafast “dark pulse” in the wake of subpicosecond oscillations.

Ultrafast coherent carrier control in quantum wells

We report the coherent control of excitons in quantum wells with pairs of 150-fs infrared pulses. Oscillations in the exciton density versus inter-pump-pulse separation, which correspond to the resonant transition energy, yield new insight into coherent exciton dynamics when their phase is analyzed. We demonstrate that these oscillations are caused when the second laser pulse destroys or enhances excitons that were created by the first pulse. The coherent carrier destruction generates ultrafast exciton dynamics that are not limited by the linewidth of the optical transition.

Ultrafast Coherent Carrier Control in Quantum Wells

A fast scanning ultrafast phase-dependent reflectivity technique is used to measure the coherent properties of multiple quantum wells at carrier densities below 10 cm . Signal to noise ratios in the reflected intensity >80 dB can be achieved allowing underlying coherences to be measured at time delays beyond 30 ps. An unexpected 1.5 ps rise time of the coherence is resolved before a nonexponential decay and ascribed to interference of polaritons. By time-resolving the pair of pulses reflected off the sample, a complete history of energy flow in the quantum wells can be identified. Both spontaneous and stimulated coherent reemission are observed in good agreement with models based on the Bloch equations.

Femtosecond pulse shaping for coherent carrier control

Abstract We investigate coherent control of exciton dynamics in semiconductor quantum wells with Fourier-domain shaped pulses. Off-resonant optical nutation is demonstrated after excitation of quantum wells with two phase-locked pulses of different energy. The effect can be modeled with semiconductor Bloch equations. Resonant excitation of multiple quantum wells with a square pulse is shown to induce a rise in differential reflectivity only after the end of the pulse.

Optical coherence in semiconductor quantum structures

Abstract Work in the last year has revealed new coherent behaviours of coupled modes of excitons and light, including the unexpeted influence from disorder. New experiments studying the scattered light from semiconductors demonstrate our incomplete understanding of localisation in quantum wells. Semiconductor coherence now enables directional control of eletron currents.

Ultrafast Coherent Spin Torques In Magnetic Quantum Wells

Photoinjecting spin-polarized carriers into magnetic semiconductor heterostructures induces a coherent magnetic impulse with which we demonstrate the first time-domain all-optical electron-spin-resonance applied to sub-monolayer magnetic planes.

Femtosecond Coherent Spin Control in Quantum Wells

We demonstrate the coherent control of spin in quantum wells with phase-locked pairs of perpendicularly polarised 100 fs pulses. These pulses induce strong interference in the electron spin polarisation, which shows up in Faraday rotation. The interference oscillations show quantum beating which differs strongly from the case of pulses with parallel polarisation.

Femtosecond Dynamics in Semiconductor Lasers: "Dark Pulse" Formation

We track the dynamic response of GaAs quantum-well semiconductor diode lasers after injection of a femtosecond pulse, using ultrafast upconversion. The continuous-wave emission shows gain dynamics and relaxation oscillations on timescales of 1–100 ps. The recirculating femtosecond pulse evolves into an ultrafast ‘‘dark pulse’’ in the wake of subpicosecond oscillations.