Progress in the Study of Nonlinear Behavior of Platinum Selenide Induced by Strong Terahertz at Shanghai Optics and Machinery Institute

Recently, the research team of the State Key Laboratory of Intense Field Laser Physics of the Chinese Academy of Sciences Shanghai Institute of Optics and Fine Mechanics has made progress in the research on the nonlinear behavior and mechanism of platinum selenide in terahertz band. The research team systematically studied the spectral and optical intensity characteristics of platinum selenide under strong terahertz pulse excitation, revealing two nonlinear processes dominated by the real and imaginary parts of nonlinear polarization. The related achievements were published in Optics Letters under the title "Terahertz triggered ultra fast non-linear optical activities in two dimensional centrosymmetric PtSe2".

Terahertz is an electromagnetic spectrum region between millimeter waves and infrared optics, and exploring potential materials for application in the terahertz band is crucial for the development of terahertz technology. The two-dimensional topological semi metallic platinum selenide exhibits excellent performance in terahertz generation and modulation due to its broadband photoresponse and photoelectric response characteristics. 
However, there is still a lack of systematic research on the basic nonlinear optical properties of platinum selenide under strong terahertz interaction. Therefore, exploring the nonlinear phenomena and underlying mechanisms of platinum selenide in the terahertz domain is of great significance.

In this study, the research team utilized ultrafast terahertz pumping infrared detection technology to investigate the interaction between terahertz pulses and platinum selenide thin films. The strong terahertz pulse breaks the inversion symmetry center of platinum selenide through nonlinear polarization and radiates a strong second harmonic signal using its nonlinear polarization real part effect. The time scale of the second harmonic signal is comparable to that of terahertz pulses, and it has a high signal-to-noise ratio and switching ratio, confirming that this property can be applied to terahertz modulation and logic gates. On the other hand, due to the effect of the imaginary part of nonlinear polarization, the conductivity of platinum selenide is modulated by strong terahertz, exhibiting a phenomenon of enhanced nonlinear absorption. This work reveals the nonlinear properties of platinum selenide in the terahertz region, achieving transient reversible inversion symmetry control of platinum selenide, and expanding the application potential of platinum selenide based two-dimensional materials in future optoelectronic devices and logic circuits.

The related work has received support from the National Natural Science Foundation of China and other organizations.

Figure 1 (a) Schematic diagram of terahertz pump infrared light detection system. (b) Waveform diagram of terahertz pump source. (c) Reflection spectra with and without terahertz pumping.

Figure 2 (a) Second harmonic spectrum of platinum selenide obtained under terahertz pumping infrared light detection system. (b) Comparison of the square of the terahertz waveform with the ultrafast dynamic process extracted at 725 nm. (c) The relationship between second harmonic signal strength and terahertz field strength. (d) Polarization properties of second harmonic signal intensity.

Figure 3 (a) The relationship between the transmittance of platinum selenide thin films and terahertz field strength. (b) The relationship between the conductivity of platinum selenide and terahertz field strength.

Source: Shanghai Institute of Optics and Precision Machinery