Floquet Engineering of the Berry Curvature Dipole and Nonlinear Hall Response in Two-Dimensional Materials

Concetti Chiave

This research paper demonstrates a novel method for significantly enhancing the Berry curvature dipole (BCD) and nonlinear Hall response in two-dimensional materials using Floquet engineering, specifically through optical driving and quench protocols.

Sintesi

Qin, F., Chen, R., & Lee, C. H. (2024). Light-enhanced nonlinear Hall effect. arXiv preprint arXiv:2401.18038v4.

This study aims to address the challenge of experimentally accessing and controlling the Berry curvature dipole (BCD) and its associated nonlinear Hall response in materials with broken inversion symmetry. The authors propose a novel approach using optical driving and quench protocols to enhance and manipulate the BCD for practical applications.

Approfondimenti chiave tratti da

Light-enhanced nonlinear Hall effect

by Fang Qin, Ru... alle arxiv.org 11-14-2024

https://arxiv.org/pdf/2401.18038.pdf

Domande più approfondite

How might the presence of disorder or impurities in real materials affect the efficiency and controllability of the proposed Floquet engineering approach for the BCD?

Disorder and impurities can significantly impact the efficiency and controllability of Floquet engineering for the Berry Curvature Dipole (BCD) in real materials. Here's how:

Scattering and Relaxation Time: Disorder introduces scattering centers that limit the electron mobility and reduce the relaxation time (τ). As seen in the expression for the nonlinear Hall response [Eq. (8) in the context], a shorter relaxation time diminishes the overall magnitude of the BCD response. This reduction stems from the fact that electrons have less time to accumulate the anomalous velocity contribution arising from the Berry curvature dipole before being scattered.

Broadening of Berry Curvature Features: Impurities can locally break the crystal symmetry, leading to spatial variations in the Berry curvature. This inhomogeneity can smear out the sharp features in the Berry curvature, including the peak near the topological transition where the BCD is enhanced. Consequently, the sensitivity of the BCD to the driving light amplitude (A0) or quench durations (T1, T2) would be reduced.

Emergent Phenomena: While detrimental in some cases, disorder can also lead to new phases and phenomena not present in pristine systems. For instance, disorder-induced localization can interplay with the Floquet driving, potentially resulting in novel topological phases characterized by quantized responses. Additionally, impurities can act as scattering centers that enhance the nonlinear optical response, as observed in some materials.

Experimental Considerations: From an experimental perspective, minimizing disorder is crucial for realizing the full potential of Floquet-engineered BCD enhancement. This involves using high-quality samples with low impurity concentrations and employing fabrication techniques that minimize defects.
In summary, while disorder poses challenges to the efficiency and controllability of Floquet engineering for the BCD, it also presents opportunities for exploring new physics. Careful material selection and experimental techniques are essential for mitigating the adverse effects of disorder and harnessing its potential benefits.

Could the proposed method of enhancing the BCD be applied to other physical systems beyond two-dimensional materials, such as three-dimensional topological insulators or Weyl semimetals?

Yes, the principles underlying the proposed method of enhancing the Berry Curvature Dipole (BCD) using Floquet engineering, namely breaking inversion symmetry and driving the system near a topological transition, can be generalized to other systems beyond two-dimensional materials. Here's how it could apply to three-dimensional topological insulators and Weyl semimetals:

Three-Dimensional Topological Insulators: These materials possess insulating bulk but conducting surface states protected by time-reversal symmetry. While the bulk BCD is typically zero, it can be induced by breaking time-reversal symmetry, for instance, by applying a magnetic field or interfacing with a ferromagnet. Floquet engineering with circularly polarized light could provide an alternative route to break time-reversal symmetry and induce a BCD in the surface states. The challenge lies in ensuring that the optical driving frequency is lower than the bulk bandgap to avoid exciting bulk carriers, which would complicate the response.

Weyl Semimetals: These materials host Weyl nodes, points in momentum space where two bands touch linearly. Weyl nodes act as sources and sinks of Berry curvature and are associated with intriguing transport phenomena, including the chiral anomaly. Applying a static electric field to a Weyl semimetal generates a nonlinear Hall current due to the Berry curvature dipole associated with the Weyl nodes. Floquet engineering could further enhance this BCD response by driving the system with light, potentially leading to larger nonlinear Hall currents. Additionally, by tuning the driving frequency and polarization, one could selectively manipulate the separation and energy of the Weyl nodes, leading to even richer nonlinear optical and transport properties.
The key takeaway is that the concepts of inversion symmetry breaking, topological transitions, and Berry curvature manipulation are not limited to two-dimensional systems. Floquet engineering provides a versatile tool for exploring and controlling these properties in a wide range of materials, including three-dimensional topological insulators and Weyl semimetals, opening up exciting possibilities for novel device applications.

If the manipulation of the BCD allows for the control of nonlinear optical properties, what potential applications in optical computing or information processing could be envisioned?

The ability to manipulate the Berry Curvature Dipole (BCD) and consequently control nonlinear optical properties holds immense potential for applications in optical computing and information processing. Here are some possibilities:

All-Optical Switching: The strong dependence of the nonlinear Hall current on the incident light intensity, as shown in Figure 6(c), suggests a mechanism for all-optical switching. By tuning the light intensity near the critical value (A0c), one could switch the nonlinear Hall current on or off, enabling the creation of ultrafast optical transistors and logic gates.

Frequency Conversion and Harmonic Generation: The nonlinear optical response associated with the BCD can lead to efficient frequency conversion processes, such as second-harmonic generation (SHG) and sum-frequency generation (SFG). By engineering the BCD through Floquet driving, one could enhance the efficiency of these processes and potentially achieve tunable frequency conversion, enabling applications in optical communications and spectroscopy.

Optical Modulation and Signal Processing: The sensitivity of the BCD to external stimuli, such as light intensity or electric fields, makes it suitable for optical modulation and signal processing. By modulating the driving light or applying an external electric field, one could control the nonlinear optical response and encode information onto an optical signal. This could lead to the development of high-speed optical modulators and signal processors for applications in optical communications and data transmission.

Nonreciprocal Light Propagation: The BCD breaks time-reversal symmetry, which is a prerequisite for achieving nonreciprocal light propagation. This means that light traveling in opposite directions experiences different refractive indices, leading to the possibility of creating optical isolators and circulators. These devices are essential for protecting sensitive optical components from back reflections and for routing optical signals in photonic circuits.

Topological Photonics: The concepts of Berry curvature and topology are not limited to electronic systems but also extend to photonic systems. By engineering the BCD in photonic crystals or metamaterials, one could create topological photonic devices with robust optical properties immune to disorder and imperfections. This could lead to the development of novel optical waveguides, lasers, and other photonic components with enhanced performance.
In conclusion, the ability to manipulate the BCD through Floquet engineering opens up a plethora of opportunities for controlling nonlinear optical properties. This control paves the way for developing novel devices and technologies for optical computing, information processing, and beyond.