Terahertz Emission via Optical Rectification in a Metal-Free Perovskite Crystal

Abstract

We report on the emission of high-intensity pulsed terahertz radiation from the metal-free halide perovskite single crystal methyl-DABCO ammonium iodide (MDNI) under femtosecond illumination. The power and angular dependence of the THz output implicate optical rectification of the 800 nm pump as the mechanism of THz generation. Further characterization finds that, for certain crystal orientations, the angular dependence of THz emission is modulated by phonon resonances attributable to the motion of the methyl-DABCO moiety. At maximum, the THz emission spectrum of MDNI is free from significant phonon resonances, resulting in THz pulses with a temporal width of <900 fs and a peak-to-peak electric field strength of approximately 0.8 kV cm^-1-2 orders of magnitude higher than any other reported halide perovskite emitters. Our results point toward metal-free perovskites as a promising new class of THz emitters that brings to bear many of the advantages enjoyed by other halide perovskite materials. In particular, the broad tunability of optoelectronic properties and ease of fabrication of perovskite materials opens up the possibility of further optimizing the THz emission properties within this material class.

Figure 1

Structure of methyl-DABCO ammonium triiodide (MDNI). As with other halide perovskites, the B-site and halide species form an octahedral cage into which the A-site species are located.

Figure 2

Illustration of the THz emission setup employed in this study. The 800 nm pump and probe beams are displayed in pink, while the THz output is displayed in blue. Solid arrows denote the direction of travel, while dashed arrows display the direction of polarization. The rotation of the sample and the λ/2 plate are given in the top right and bottom insets, respectively. The relationship between the orientation of the MDNI crystal, the crystallographic axes, and the laboratory coordinate system is displayed in the top left inset.

Figure 3

Characterization of the optimized THz field. (a) Spectrum of the emitted THz radiation; the shaded regions represent the standard deviation of 20 individual spectra, while the dashed line represents the noise floor of the instrument; the dynamic range of our detection system is approximately 5 × 10⁴ Arb. U (47 dB); (b) time-varying electric field of the THz pulse whose spectrum is given in (a) (inset: the dependence of the peak-to-peak field on the fluence of the 800 nm pump beam).

Figure 4

Comparison of MDNI to other recently reported THz emitters, including the high-performance organic emitter DAST, as well as MAPbI₃ and FAPbI₃. The dashed line is an extrapolation of the linear fit given in Figure 2(a). References for the various literature values used in the figure are supplied in the SI.

Figure 5

Rotational characterization of the THz emission. (a) Dependence of the THz emission on crystal orientation. The solid points represent the experimentally obtained strengths of the THz emission peak, while the solid blue line represents the predicted angular dependence of the peak field as predicted via eq 2. (b) THz waveforms of the four extremal points marked (I–IV) in panel (a). (c) Power spectra of the THz waveforms given in panel (b).

Figure 6

Vibrational properties of MDNI. (a) Vibrational spectrum of MDNI across the IR and THz spectral regions comprising a THz-TDS spectrum (blue line) and an IR-ATR spectrum (orange line). (b, c) Angular dependence of the ∼0.8 THz absorption line of MDNI: (b) Relationship between the rotation of MDNI in the lab frame, the crystallographic orientation of MDNI, the dipoles (orange arrows) of the [111] aligned mDABCO molecules, and the electric field of the polarized THz pulse (blue arrow); (c) the peak absorption of the ∼1 THz resonance as a function of crystal angle, including a proposed relationship between the crystal orientation, the orientation of the mDABCO moieties, and the electric field of the THz pulse that gives rise to the angular dependencies.