SIPM Makes Progress in The Study Of Strong Terahertz-induced Nonlinear Behavior Of Platinum Selenide

Recently, a research team from the State Key Laboratory of Strong-Field Laser Physics, Shanghai Institute of Optics and Precision Machinery (SIPM), Chinese Academy of Sciences (CAS), has made progress in the study of the nonlinear behavior and mechanism of platinum selenide in the terahertz band. The team systematically investigated the spectral and intensity characteristics of platinum diselenide under strong terahertz pulse excitation, revealing two nonlinear processes dominated by the real and imaginary parts of the nonlinear polarizability. The related results are presented as "Terahertz-triggered ultrafast nonlinear optical activities in two-dimensional centrosymmetric PtSe2" in Optics Letters.
Terahertz is a region of the electromagnetic spectrum between millimeter-wave and infrared optics, and exploring potential materials for applications in the terahertz band is critical to the development of terahertz technology. The two-dimensional topological semimetallic platinum selenide has shown excellent performance in terahertz generation and modulation thanks to its properties of broadband optical and optoelectronic response. However, there is still a lack of systematic studies on the fundamental nonlinear optical properties of Pt selenide under strong terahertz effects. Therefore, it is of great significance to explore the nonlinear phenomena and intrinsic mechanisms of platinum selenide generated in the terahertz domain.
In this study, the research team carried out a study of the interaction of terahertz pulses with platinum selenide thin films using ultrafast terahertz pumping-infrared photoprobe technology. The strong terahertz pulse breaks the inversion center of symmetry of platinum selenide by nonlinear polarization and radiates a strong second harmonic signal using the real part of its nonlinear polarization rate. The time scale of this second harmonic signal is comparable to that of the terahertz pulse and has high signal-to-noise and switching ratios, confirming that this property can be applied to terahertz modulation and logic gates. On the other hand, the conductivity of platinum selenide is subjected to strong terahertz modulation and exhibits enhanced nonlinear absorption due to the nonlinear polarizability imaginary part. This work reveals the nonlinear nature of platinum selenide in the terahertz region, realizes the transient reversible inversion symmetry modulation of platinum selenide, and broadens the potential of platinum selenide-based two-dimensional materials for future applications in optoelectronic devices and logic circuits.
The related work is supported by the National Natural Science Foundation of China.

Fig. 1 (a) Schematic diagram of terahertz-pumped-infrared light detection system. (b) Waveform of terahertz pump source. (c) Reflectance spectra with and without terahertz pumping.

Fig. 2 (a) Second-harmonic spectrum of platinum selenide obtained under terahertz pumping-IR optical detection system. (b) Comparison of the ultrafast dynamic process extracted at 725 nm with the square of the terahertz waveform. (c) Second harmonic signal intensity versus terahertz field strength. (d) Polarization nature of the second harmonic signal strength.

Fig. 3 (a) Transmittance of platinum selenide film versus terahertz field strength. (b) Platinum selenide conductivity versus terahertz field strength.