Second harmonic generation spectroscopy in the Reststrahl band of SiC using an infrared free-electron laser

Abstract

The Reststrahl spectral region of Silicon Carbide has recently attracted much attention owing to its potential for mid-infrared nanophotonic applications based on surface phonon polaritons (SPhPs). Studies of optical phonon resonances responsible for surface polariton formation, however, have so far been limited to linear optics. In this Letter, we report the first nonlinear optical investigation of the Reststrahl region of SiC, employing an infrared free-electron laser to perform second harmonic generation (SHG) spectroscopy. We observe two distinct resonance features in the SHG spectra, one attributed to resonant enhancement of the nonlinear susceptibility χ (2) and the other due to a resonance in the Fresnel transmission. Our work clearly demonstrates high sensitivity of mid-infrared SHG to phonon-driven phenomena, and opens a route to studying nonlinear effects in nanophotonic structures based on SPhPs.PACS numbers: Second Harmonic, Phonons, Silicon Carbide In recent years, there has been considerable interest in the mid-infrared Reststrahl 1 spectral region of Silicon Carbide (SiC), 2-6 since it holds much promise for a novel approach to low-loss, mid-infrared (IR) nanophotonic applications based on surface phonon polaritons (SPhPs). 4,5 Similarly to surface plasmon polaritons in metals, these surface phonon waves in the Reststrahl band of polar dielectrics can be tailored using resonant optical nano-antennas 4,5 as the fundamental building block of future nanophotonic devices. 7,8 Most importantly, nanophotonics based on SPhPs could solve the intrinsic optical loss-problem of plasmonics, 9 making use of the much smaller damping rates of phonons as compared to plasmons. 6 The peculiar linear optical properties in the Reststrahl region responsible for SPhP formation are dominated by dielectric response of the optical phonons in polar dielectrics. 10 The polar bonding character leads to splitting of the transversal optical (TO) and longitudinal optical (LO) phonon branches at the Brillouin zone-center and IR activity of the zone-center TO phonon. As a result, the TO phonon appears as a sharp resonance in the dielectric function ε(ω) = ε 1 + iε 2 , accompanied with a zero-crossing of ε 1 at the zone-center TO frequency ω T O , whereas the macroscopic polarization associated with LO modes produces the second zero-crossing of ε 1 at ω LO . 10 Therefore, ε 1 is negative between TO and LO phonon frequencies constituting the bulk phonon polariton gap where only SPhPs can exist, defining the Reststrahl band of near-perfect reflectivity. 1,10 In order to improve the engineering of future nanophotonic devices based on SPhPs, exact knowledge of the dielectric response in the Reststrahl region is desired. Howa) Electronic mail: alexander.paarmann@fhi-berlin.mpg.de ever, linear optical techniques like reflectivity only provide indirect information, and require differentiation to extract the resonances. 11,12 As one possible solution, nonlinear optical spectroscopy could provide improved sensitivity to critical featur...

Second harmonic generation from strongly coupled localized and propagating phonon-polariton modes

“…The characteristic SHG spectrum shown in Fig. 1a, consistent with previous results of SiC SHG spectroscopy 35,36 , indicates the higher importance of the electric field enhancement factor, giving rise to the steadily increasing SHG background at larger frequencies in the spectral range of 830 − 950 cm −1 .…”

“…In what follows, we shall focus on those features and do not discuss the SHG peaks at the frequencies of the transverse (≈ 790 cm −1 ) and longitudinal (≈ 980 cm −1 ) optical phonons in SiC which are the fingerprints of the SiC crystalline symmetry. These features have been analyzed previously 35,36 and are insensitive to the pitch d of the nanopillar array.…”

Femtosecond single-shot timing and direct observation of subpulse formation in an infrared free-electron laser

“…Overall, cavity detuning provides the possibility to control the duration and bandwidth of the emitted FEL radiation. On the one hand, short pulses for time-resolved experiments are available; on the other hand, narrowband radiation can be used for high-resolution nonlinear spectroscopy [35,36] or selective excitation of quantum states in matter.…”

Coherent Field Transients below 15 THz from Phase-Matched Difference Frequency Generation in 4H-SiC

“…2(a)], the other extending from 18 THz (16 μm) down to the sub-terahertz range [ Fig. 2(b)] owing to the high phonon frequencies of the Si-C dimers [23]. Among the most prominent features, we can also list a high second-order nonlinearity [19,24,25], excellent mechanical robustness (Mohs hardness 9 [26]), and outstanding thermal properties.…”