Parametric Excitation of an Optically Silent Goldstone-Like Phonon Mode

Abstract

It has recently been indicated that the hexagonal manganites exhibit Higgs-and Goldstone-like phonon modes that modulate the amplitude and phase of their primary order parameter. Here, we describe a mechanism by which a silent Goldstone-like phonon mode can be coherently excited, which is based on nonlinear coupling to an infrared-active Higgs-like phonon mode. Using a combination of first-principles calculations and phenomenological modeling, we describe the coupled Higgs-Goldstone dynamics in response to the excitation with a terahertz pulse. Besides theoretically demonstrating coherent control of crystallographic Higgs and Goldstone excitations, we show that the previously inaccessible silent phonon modes can be excited coherently with this mechanism.Order parameters are physical observables that are used to quantify the different states of matter. Their amplitudes and phases can be excited by external stimuli, such as a laser pulse, leading to exotic states of matter that cannot be accessed in equilibrium [1]. Two particular excitations are Higgs and Goldstone modes, which correspond to the modulation of the amplitude and phase of an order parameter that breaks a continuous symmetry. Originally discussed in the context of particle physics [2] and superconductivity [3], Higgs and Goldstone excitations were found in cold atom systems, such as superfluids [4][5][6][7][8] or supersolids [9]. Higgs and Goldstone modes are well-studied in superconductors today [10][11][12][13][14][15][16][17][18][19][20][21], and similar manifestations have recently been reported in charge density wave (CDW) systems [22][23][24], antiferromagnets [25][26][27], and excitonic insulators [28]. It has been debated whether the optical and acoustic vibrational modes of solids can be considered Higgs and Goldstone excitations of the crystal lattice [29]. Several complex transition metal oxide compounds have shown signatures of optical Goldstone-like phonon modes that live in their symmetry-broken potential energy landscape [30][31][32][33][34].Coherent control over Raman-active phonons via impulsive stimulated Raman scattering using visible light pulses is well-established [35,36]. Recently, the excitation of infrared (IR)-active phonons with large amplitudes via IR absorption has become feasible through the development of high intensity terahertz and mid-IR sources. This progress has enabled selective control over the dynamics of the crystal lattice through nonlinear phonon interactions using ionic Raman scattering [37-39] and two-particle absorption mechanisms [40][41][42][43][44][45]. In contrast, phonon modes that do not respond to IR absorption or Raman scattering techniques, so called silent modes, can only be detected through hyper-Raman scattering. As hyper-Raman scattering is a third-order interaction of the electric field component of light with a * djuraschek@seas.harvard.edu † prineha@seas.harvard.edu phonon mode, it is inefficient and no coherent excitation has been achieved yet. Higgs and Goldstone modes, similar to silent p...

Light-control of materials via nonlinear phononics

“…Recent mid-IR pump experiments have observed long-lived metastable ferroelectricity in SrTiO 3 [27] and ferromagnetism in DyFeO 3 [41] and CoF 2 [42], but the precise mechanisms for these phenomena need to be clarified with further experimental studies. Other theoretical predictions based on the nonlinear phonon couplings that await experimental confirmations include indirect-to-direct band gap switching [43], phono-magnetic analogs of opto-magnetic effects [44], cavity control of nonlinear phonon interactions [45], and excitation of an optically silent mode in InMnO 3 [46].…”

Light-driven topological and magnetic phase transitions in thin-layer antiferromagnets

“…In turn, such an IR mode can couple nonlinearly with some Raman modes, provided their irreps satisfy [ΓIR × ΓIR] ⊗ ΓR ⊃ A1g [23]. This mechanism is referred as non-linear phononics [23,45,46], and has allowed experimental [19,[24][25][26][27][28][29][30][46][47][48] and theoretical manipulations of correlated states of matter [49][50][51][52][53][54][55][56][57][58][59][60][61].…”

Ferroelectric Polarization Reversal vs Pump Spot Shape

“…It was also shown that optical pumping with excitation of the electronic subsystem [18][19][20][21] leads only to a temporary decrease in polarization [22][23][24][25]. Following the rapid development and achievements of nonlinear phononics [26][27][28][29], it was proposed to pump the IR-active optical phonon mode, which is nonlinearly coupled with the ferroelectric soft mode [30]. However, in this way, only a transient switching of the polarization was achieved in a monodomain LiNbO 3 sample, followed by the polarization recovery to its initial value on a timescale of about 0.2 ps [31].…”