An adaptive mid-infrared ultrashort pulse source for applications in coherent control

Abstract

An adaptive mid-infrared (MIR) ultrashort pulse source is investigated for application to the coherent control of molecules. The MIR regime will allow access to vibrational modes of common organic bonds, and ultrashort pulse durations should enable the required interaction to occur before the energy is redistributed throughout the molecule. By using the molecular system as part of an adaptive learning loop, one can deliver the desired MIR pulse without the need for prior lengthy calculations to solve the Hamiltonian.

The adaptive MIR pulse shaper is presented as a feasibility study in this thesis. It involves shaping a near-infrared (NIR) pulse using a spatial light modulator in a phase-only pulse shaper. The shaped NIR pulse is then transferred to the MIR via a synchronously pumped optical parametric oscillator (SPOPO), which is consequently measured using a nonlinear detector whose signal is used as the feedback parameter to be optimised. Using a global optimisation algorithm, initial experiments demonstrated adaptive MIR pulse shaping, achieving pulse compression and double pulse generation.

The transfer of the pulse shape from the NIR to the MIR in the SPOPO however, is non-trivial and is discussed in detail, both numerically and experimentally, in this thesis. The results show that parameters such as the signal pulse bandwidth, temporal walk-off of the interacting pulses, signal gain, pump depletion, and group velocity dispersion should be considered when high fidelity transfer is required. It is also shown that, for an SPOPO based on periodically poled LiNbO₃ high-fidelity transfer is possible for wavelengths centred around 3.5µm with a tunability of ±0.5µm.

The investigation then progresses to the femtosecond regime where the demonstration of coherent control experiments becomes more accessible. Using a fibre-based chirped pulse amplification system, which is an attractive pump source for the SPOPO, adaptive pulse shaping is demonstrated, showing significant improvement in the quality of the 500 fs source at high pulse energies of 65µJ, as a result of the learning loop.

Thus the individual components to make the adaptive MIR ultrashort pulse shaping system have all been demonstrated; namely the adaptive shaping of MIR pulses via an SPOPO, the high-fidelity transfer of NIR pump pulses to the MIR in an SPOPO, and the femtosecond NIR pump source.

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Identifiers

ORCID for D.P. Shepherd: orcid.org/0000-0002-4561-8184

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