论文标题
优化中红外量子级联激光频率梳子的广泛增益和高光学非线性
Optimization of broad gain and high optical nonlinearity of mid-infrared quantum cascade laser frequency combs
论文作者
论文摘要
中红外量子级联激光器（QCLS）是紧凑而有效的来源，非常适合分子光谱应用，例如双栓光谱。但是，尽管QCL频率梳子设备的积极开发多十年，但它们的带宽限制为$ 100 $ cm $^{ - 1} $，严重限制了它们用于多加仑，液体和固体传感的应用。即使已经提出了非常广泛的增益QCL，它们也无法改善梳子带宽，其主要局限性是随频率的增益和分散的变化。完全平坦的增益光谱可以减轻这种情况，因为分散体以及克服增益夹紧时损失所需的参数增益消失了。另一方面，梳子地层基于四波混合，这是三阶非线性过程，QCL在QCL中非常强。由于这些设备的子带性质，因此可以设计和增强这种非线性，以促进梳子形成。在这项工作中，我们介绍了具有宽阔和平坦的增益光谱的优化设计，其跨度高达220 cm $^{ - 1} $，并且比典型的界限到continuum QCL设计高达30倍的FWM非线性。该优化利用具有高预测能力的非quilibriumn Green的功能模型，并遵守增益和电流密度的约束，从而确保有效的设备。如此高的非线性与中等饱和的增益结合使用，可以允许QCL中的非经典光生产生。另一方面，将QCL梳子的光谱带宽加倍将是迈向复杂气体混合物和液体的高速光谱法的巨大步骤。
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.
