中红外量子级联laser频率梳宽增益高非线性的优化
Optimization of broad gain and high optical nonlinearity of mid-infrared quantum cascade laser frequency combs
摘要：
中红外量子级联激光器（QCL）是紧凑高效的光源，非常适合分子光谱应用，如双梳光谱。然而，尽管QCL频率梳装置的积极发展超过十年，但其带宽限于约$100$ cm $^{-1}$，严重限制了其用于多气体、液体和固体感测的应用。尽管已经提出了非常宽的增益QCL，但这些QCL不能改善梳状带宽，其主要限制是增益和色散随频率的变化。一个完全平坦的增益谱将缓解这一点，因为色散以及克服增益箝位时的损耗所需的参量增益都消失了。另一方面，梳的形成依赖于四波混频，这是一个三阶非线性过程，在QCL中非常强。由于这些器件的子带性质，可以设计和增强这种非线性，以促进梳形成。在这项工作中，我们提出了优化的设计，宽和平顶增益谱跨度高达220厘米$^{-1}$，以及高达30倍强的FWM非线性比一个典型的绑定到连续QCL设计。该优化利用具有高预测能力的非平衡格林函数模型，并遵守增益和电流密度的约束，确保高效的设备。这种高非线性与适度的可饱和增益相结合，可以允许在QCL中产生非经典光。另一方面，将QCL梳的光谱带宽加倍将是向复杂气体混合物和液体的高速光谱学迈出的一大步。
英文摘要：
Mid-infrared Quantum Cascade Lasers (QCLs) are compact and efficient sources ideal for molecular spectroscopy applications, such as dual-comb spectroscopy. However, despite over a decade of active developments of QCL frequency comb devices, their bandwidth is limited to around $100$ cm$^{-1}$, severely limiting their application for multi-gas, liquid, and solid sensing. Even though very broad gain QCLs have been presented, these were not able to improve the comb bandwidth, whose main limitations are variations of the gain and dispersion with frequency. A perfectly flat gain spectrum would mitigate this, as the dispersion as well as the parametric gain necessary to overcome the losses at gain clamping, vanishes. On the other hand, comb formation rests on four-wave mixing, a third-order nonlinear process, which is very strong in QCLs. Due to the subband nature of these devices, this nonlinearity can be designed and enhanced in order to facilitate comb formation. In this work, we present optimised designs with broad and flat-top gain spectra spanning as much as 220 cm$^{-1}$, as well as up to 30 times stronger FWM nonlinearity than a typical bound-to-continuum QCL design. The optimisation utilises a nonequilibriumn Green's function model with high predictive power, and obeys constraints on gain and current density, ensuring efficient devices. Such high nonlinearity in combination with a moderate, saturable gain, could allow for non-classical light generation in QCLs. On the other hand, doubling the spectral bandwidth of QCL combs would be a large step towards high-speed spectroscopy of complex gas mixtures and liquids.
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