Fundamental Investigations and Micromachining with Few-Cycle Pulses in the Volume of Wide-Bandgap Dielectrics

随着题为“宽带隙电介质体积中的少周期脉冲的基础研究和微加工”的初步研究计划的提出，在超短脉冲照射下等离子体形成以及少周期脉冲用于微加工的潜力等主题上取得了相当大的成就。通过记录和利用所谓的Brunel谐波的发射，研究了等离子体形成的基本原理。为了将这项工作放在固体中高次谐波产生的充满活力的领域中，我们已经证明，在我们的实验条件下，强场电离（SFI）的签名可以从所有其他伴随机制中分离出来。通过结合时间分辨Brunel谱和基于时域重叠关联的数值相位恢复算法，重建了强场电离导致的等离子体密度演化特征。这一重要结果为开展这些实验提供了理想的基础。根据这项更新建议，将从两个方向进行调查。首先，我们将探讨当激发波长改变时，SFI与碰撞电离之间的竞争。第二，等离子体的形成将在微加工的背景下进行研究。我们将着重讨论两种情况，钛宝石和钇铝石榴石的情况。研究几个周期的脉冲在微处理中的潜力同样提供了很好的结果。特别是，几个周期的脉冲，使直接激光写入体和表面波导上的原始熔融石英样品。表面波导表现出非常好的折射率对比度（接近0.01）。在这些波导上施加高折射率液体导致强模式泄漏，从而证明这些微结构对它们的环境的敏感性。这些成就为在集成生物传感方向上进行研究奠定了完美的基础。该更新提案旨在开发用于表面等离子体共振（SPR）传感的三维光子平台的直接激光印刷，其中等离子体共振由在激光诱导的表面波导中传播的电磁波激发。这种方法能够将SPR传感器的出色性能与集成光学器件提供的显著小型化程度相结合。

With the initial research proposal entitled 'Fundamental Investigations and Micromachining with Few-Cycle Pulses in the Volume of Wide-Bandgap Dielectrics', considerable achievements have been earned on the topics of plasma formation upon irradiation with ultrashort pulses as well as on the potential of few-cycle pulses for microprocessing.The fundamentals of plasma formation were studied by recording and exploiting the emission of the so-called Brunel harmonics. With the intent to place this work in the vibrant field of high harmonic generation in solids, we have demonstrated that in our experimental conditions, the signature of strong field ionization (SFI) could be isolated from all other concomittant mechanisms. By combining the time-resolved Brunel spectra with a numerical phase-retrieval algorithm based on time domain ptychography, the characteristic stepwise plasma density evolution due to strong field ionization was reconstructed. This important result provides an ideal basis to pursue these experiments. With this renewal proposal, investigations in two directions will be carried out. First, the competition between SFI and collisional ionization will be explored when the excitation wavelength changes. Second, plasma formation will be studied in the context of microprocessing. Two cases will be emphasized, the Ti:Sapphire scenario and the YAG scenario.Studying the potential of few-cycle pulses for microprocessing equally provided excellent results. In particular, few-cycle pulses enabled direct laser writing of bulk and surface waveguides on a pristine fused silica sample. The surface waveguides exhibited a very good refractive index contrast (close to 0.01). Applying a high refractive index liquid on these waveguides resulted in a strong mode leakage, thereby proving the sensitivity of these microstructures to their environment. These achievements set a perfect ground for pursuing research efforts in the direction of integrated biosensing. This renewal proposal aims at developing direct laser printing of three dimensional photonic platforms for surface plasmon resonance (SPR) sensing, where the plasmon resonance is excited by the electromagnetic waves propagating in the laser-induced surface waveguides. This approach enables combining the outstanding performance of SPR sensors with the remarkable degree of miniaturization offered by integrated optics devices.