[Paper Review] Optically-Induced Symmetry Breaking via Nonlinear Phononics

Jared S. Ginsberg, Mohsen Jadidi+11 more|arXiv (Cornell University)|Jul 26, 2021

Advanced Fiber Laser Technologies23 references4 citations

TL;DR

This paper demonstrates that resonant excitation of transverse optical phonons in hexagonal boron nitride (hBN) induces strong optical nonlinearities, including enhanced four-wave mixing and third-harmonic generation. The authors observe sub-picosecond nonlinear signals and predict significant boosts in high-harmonic generation efficiency due to phonon-mediated nonlinear enhancement.

ABSTRACT

Optical nonlinearities in solids reveal information about both the in-plane rotational and out-of-plane inversion symmetries of a crystal. In the van der Waals material hexagonal boron nitride (hBN) both these symmetries and the linear vibrational properties have led to the rich physics of mid-infrared phonon-polaritons. However, the role of strong electron-phonon nonlinearities requires further study. In this work, we investigate both theoretically and experimentally the rich interplay of phonon anharmonicity and symmetry in phonon-polariton mediated nonlinear optics. We show that large enhancements (>30x) of third-harmonic generation occur for incident femtosecond pulses that are resonant with the hBN transverse optical phonons. In addition, we predict and observe large transient sub-picosecond duration second-harmonic signals during resonant excitation, which in equilibrium is forbidden by symmetry. This surprising result indicates that instantaneous crystal inversion symmetry breaking can be optically induced and controlled via phonon interactions by both the power and polarization of the pump laser.

Motivation & Objective

• To investigate how strong phonon resonances in hexagonal boron nitride (hBN) can enhance optical nonlinearities.
• To explore the role of nonlinear phononics in generating and amplifying ultrafast optical signals.
• To demonstrate time-resolved observation of lattice dynamics via four-wave mixing during resonant excitation.
• To predict and validate significant enhancements in high-harmonic generation efficiency through phonon-induced nonlinearities.

Proposed method

• Resonant excitation of hBN's transverse optical phonon mode using tailored laser pulses.
• Employing four-wave mixing (FWM) to generate and detect sub-picosecond nonlinear signals.
• Measuring third-harmonic generation (THG) under resonant pumping at the phonon frequency.
• Using theoretical modeling to predict phonon-enhanced nonlinearities and high-harmonic generation efficiencies.
• Analyzing time-resolved signals to extract dynamics of crystal motion and phonon excitation.
• Leveraging the strong anharmonicity of hBN's phonon modes to amplify nonlinear optical responses.

Experimental results

Research questions

• RQ1 How do resonant phonon excitations in hBN influence the strength and temporal profile of optical nonlinearities?
• RQ2 Can four-wave mixing be used to time-resolve lattice dynamics in hBN with sub-picosecond resolution?
• RQ3 To what extent does phonon resonance enhance third-harmonic generation in hBN?
• RQ4 What is the predicted enhancement in high-harmonic generation efficiency due to phonon-mediated nonlinearities?
• RQ5 How does the coupling between optical excitation and lattice vibrations lead to symmetry-breaking nonlinear responses?

Key findings

• The authors observe sub-picosecond duration four-wave mixing signals during resonant excitation of hBN's transverse optical phonons.
• Third-harmonic generation is significantly enhanced when pumping at the hBN transverse optical phonon frequency.
• Time-resolved FWM signals allow direct observation of crystal motion on the sub-picosecond timescale.
• Phonon-induced nonlinearities are predicted to yield large increases in high-harmonic generation efficiency.
• The strong phonon resonance in hBN acts as a nonlinear amplifier for optical signals, enabling enhanced nonlinear optical responses.
• Theoretical modeling confirms that the observed enhancements arise from the interplay between optical excitation and anharmonic phonon modes.