Ultrafast Laser Source Tunable Around 920 nm for Quantum Applications

Quantum technologies and applications is a recent, revolutionary and fast-growing domain. Single photon sources are crucial for quantum applications. Among the various mechanisms, GaAs-based semiconductor quantum dots (QD) are promising single-photon sources [1]. These particular QDs are excited by Ti:Sa oscillators delivering high repetition rate trains of picosecond transform-limited pulses at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(920\pm 20)$</tex> nm with sub-nanojoule energy [2]. To increase compactness and portability, the development of custom-design excitation sources, based e.g. on fiber lasers, is in an active stage. Here, we propose a scheme based on second-harmonic generation (SHG) of pulses at 1840 nm. We followed a similar approach in a previous work [3], where we produced femtosecond pulses at 920 nm for two-photon microscopy from a soliton self-frequency shifted source at 1840 nm. However, for efficient generation of single photons in QDs, the pulses must be narrow linewidth (< 1 nm) corresponding to transform-limited duration longer than 1 ps. To this aim, we exploit SHG in a regime of large group-velocity mismatch (GVM) between the fundamental frequency (FF) and its second harmonic (SH). This way we achieve efficient conversion (~50%), while producing transform-limited SH pulses whose duration exceeds that of the fundamental radiation by a factor of 40.