High frequency limit of spectroscopy

TL;DR

This paper introduces a new high-frequency spectroscopic technique called NLHFPS, which leverages nonlinear responses in quantum systems to achieve high surface sensitivity and overcome traditional selection rule constraints.

Contribution

The paper develops a theoretical framework for high-frequency limit responses and proposes NLHFPS, a novel spectroscopy method with enhanced surface sensitivity and broad applicability.

Findings

Perfect self-cancellation of linear response at high frequencies

Surface-sensitive excitation spectrum in jellium models

Potential for nanoscience characterization

Abstract

We consider an arbitrary quantum mechanical system, initially in its ground-state, exposed to a time-dependent electromagnetic pulse with a carrier frequency $\omega_0$ and a slowly varying envelope of finite duration. By working out a solution to the time-dependent Schr\"odinger equation in the high-$\omega_0$ limit, we find that, to the leading order in $\omega_0^{-1}$, a perfect self-cancellation of the system's linear response occurs as the pulse switches off. Surprisingly, the system's observables are, nonetheless, describable in terms of a combination of its linear density response function and nonlinear functions of the electric field. An analysis of jellium slab and jellium sphere models reveals a very high surface sensitivity of the considered setup, producing a richer excitation spectrum than accessible within the conventional linear response regime. On this basis, we propose a new spectroscopic technique, which we provisionally name the Nonlinear High-Frequency Pulsed Spectroscopy (NLHFPS). Combining the advantages of the extraordinary surface sensitivity, the absence of constraints by the traditional dipole selection rules, and the clarity of theoretical interpretation utilizing the linear response time-dependent density functional theory, NLHFPS has a potential to evolve into a powerful characterization method for nanoscience and nanotechnology.