Abstract
We performed and studied cascaded third-harmonic generation (THG) in a quasi-periodically poled KTP (QPPKTP) crystal allowing simultaneous phase-matching of the two cascading steps ω + ω → 2ω and 2ω + ω → 3ω. The phase-matching was achieved at the fundamental wavelength λω = 1587 nm when the QPPKTP crystal was heated to 95°C. The energy conversion efficiency reached 40% in the picosecond regime for a fundamental energy of 20 µJ that corresponds to an intensity of 1.5 GW/cm2. It is the highest value of THG efficiency ever reported to the best of our knowledge. The modeling in the case of the depleted pump regime accurately described the experiments.
Highlights
Extending the bandwidth of laser sources is a huge field of application of parametric nonlinear optics starting from the pioneering work on Second-harmonic Generation (SHG) [1] and Optical Parametric Oscillators (OPO) [2]
Third-Harmonic Generation (THG) is of strong interest in this frame of frequency conversion since it is often used as a pump source of OPO [3], but it is at the heart of nuclear reactions [4] and optical processing [5] for example
Quantum optics is another area of interest for THG since it can lead to the generation of squeezed light [6,7]
Summary
Extending the bandwidth of laser sources is a huge field of application of parametric nonlinear optics starting from the pioneering work on Second-harmonic Generation (SHG) [1] and Optical Parametric Oscillators (OPO) [2]. While third-order susceptibility is present in all materials, it is usually very small, causing conversion efficiency of THG to be low and requires very high pump intensities Another approach is by cascading two processes driven by the second-order susceptibility χ(2) : ω + ω → 2ω, which corresponds to SHG, and THG it-self , i.e. 2ω + ω → 3ω creating the third harmonic. Our objective is to go beyond these pioneering works by developing a model of THG in the depleted pump regime and by experimentally optimizing the conversion efficiency, in particular in the visible around 0.5 μm This last point is of prime importance in the more general framework of the generation of new quantum states of light, where the exact reverse of cascaded THG, i.e. 3ω → ω + 2ω coupled with 2ω → ω + ω, is a way to produce three highly correlated photons at ω [17]. Since TPG and THG exhibit the same phase-matching properties, the optimization of one leads to the optimization of the other
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