Quantum Dial for High-Harmonic Generation

TL;DR

This work demonstrates that high-harmonic generation (HHG) can be controlled with a weak classical drive perturbed by a much weaker bright squeezed vacuum (BSV), enabling nondestructive tuning of spectra, emission angles, and ionization in two-dimensional MoS$_2$. A two-time response-function framework captures the memoryful quantum perturbation from BSV, revealing distinct ring-like responses around the $K$ and $K'$ valleys and enabling valley-selective control. Key findings include up to three orders of magnitude ionization control via BSV energy and center frequency, non-Markovian dynamics introduced by the quantum light, and ultrafast, polarization-dependent switching of harmonic emissions. The results suggest practical, low-damage nonlinear spectroscopy and chip-scale quantum-optical diagnostics compatible with high-repetition-rate table-top lasers, with potential applications in valleytronics and ultrafast optoelectronics.

Abstract

High-harmonic generation (HHG) is a highly nonlinear optical process that typically requires an intense laser to trigger emissions at integer multiples of the driving field frequency. However, the strong fields required for conventional HHG inevitably perturb the system, limiting its use as a nondestructive spectroscopic probe. Recent advances in bright squeezed vacuum (BSV) sources have created opportunities to drive HHG with quantum fields alone. In this work, we demonstrate a regime in which the light-matter interactions can be controlled and tuned using a weak classical field, whose pulse energy is two orders of magnitude lower than that in standard HHG-perturbed by an even weaker quantum field such as BSV. This approach opens new avenues for nonlinear spectroscopy of materials while substantially suppressing strong laser-induced damage, distortions, and heating. We show that a BSV pulse containing less than 5% of the classical driving energy can act as an 'optical dial', allowing tuning of the nonlinear emission spectrum, emission angular dependence, and ionization.

Figures (4)

• Figure 1: Panel a illustrates the concept of controlling harmonic emissions by a weak external perturbation. Panel b and c show the response functions of the (ii) Thermal $\mathcal{R}_T(t_2-t_1)$ and (iii) BSV $\mathcal{R}_S(t_2,t_1)$, as listed. Unlike $\mathcal{R}_T(t_2-t_1)$, which is only dependent on $t_2-t_1$, the response function of BSV $\mathcal{R}_S(t_2,t_1)$ depends on two time variables, and is therefore presented as a two-dimensional distribution in panel d.
• Figure 2: Panel a shows the MoS$_2$ lattice structure in real space, and panel b displays its band structure in momentum space. The harmonic spectra of the three cases are presented in c. Panels d–l present the momentum-space spectral distributions of the selected harmonic orders. The amplitude is represented by brightness, while the phase is encoded by color. From left to right, the columns correspond to cases (i), (ii), and (iii), while from top to bottom, the rows show the 3rd, 4th, and 5th harmonics, respectively. The Brillouin zone is outlined by a white hexagon, with the $K$ and $K^\prime$ points located at its corners.
• Figure 3: The ionization enhancement as a function of BSV pulse energy $U$ and center frequency $\omega_s$ is presented. The ionization of case (iii) BSV and case (i) is denoted by $n_\text{BSV}$ and $n_0=7\times10^{-6}$ respectively. The white star marks the BSV parameters used elsewhere throughout this work, corresponding to a center frequency matched to the driving laser $\omega_s=\omega_0$ and the BSV energy $8$ nJ, which yields a peak fluctuation amplitude of $10^8\text{V}/\text{m}$.
• Figure 4: Panel a shows the angular dependence of the harmonic emissions when the BSV rotates together with the driving field ($\phi=\phi_\text{BSV}$). Panel b presents the angular dependence of the emission when the polarisation of the BSV is fixed along the $x$ axis ($\phi_\text{BSV}=0$), and only the driving field is rotated.