Spintronic Terahertz Emission Spectroscopy System Driven by Direct Diode Pumped Ti:Sapphire Ultrafast Oscillator (Invited)

Objective The spintronic terahertz (THz) emitters developed in recent years have shown numerous advantages such as ultrabroadband, low cost, easy integration, and tunable polarization. As a result, they are increasingly demonstrating significant practical value in THz technology applications. However, advancing the development of THz time-domain spectroscopy (THz-TDS) systems crucially depends on combining high-efficiency, broadband spintronic THz emitters with low-cost, miniaturized titanium sapphire (Ti:sapphire) laser oscillators. In our study, we use a miniaturized direct diode-pumped Ti:sapphire ultrafast oscillator to drive the THz spectroscopy system, achieving efficient THz emission from antiferromagnetic | ferromagnetic | heavy metal (IrMn3|Co20Fe60B20|W) heterostructures under no external magnetic field conditions. We not only verify the spintronic THz radiation mechanism but also find that the heterostructure has a stronger radiation signal before the focusing lens focus. These experiments demonstrate that miniaturized, low-cost direct diode pumped Ti:sapphire ultrafast oscillators are the preferred choice for direct application in spintronic THz emission spectroscopy systems, which is a powerful tool for analyzing the interactions between femtosecond lasers and different materials. This research lays the foundation for further promoting the application of miniaturized ultrabroadband THz time-domain spectral imaging technology driven by femtosecond lasers. Methods We utilize a direct diode pumped Ti:sapphire ultrafast oscillator to drive the THz time-domain spectroscopy system. This laser adopts a new direct diode pumping scheme, combining the advantages of traditional Ti:sapphire lasers and fiber femtosecond lasers, and features high performances with small size and low cost. It can provide laser pulses with a center wavelength of 800 nm, pulse duration of 47 fs, repetition rate of 80 MHz, and output power of 625 mW. The laser output beam is divided into three paths: the first excites a photoconductive antenna to generate THz pulses; the second excites another photoconductive antenna to detect THz time-domain waveforms; the third passes through a focusing lens with a focal length of 20 mm and a chopper with a frequency of 1600 Hz. The laser power on the THz emitter is about 130 mW, which excites THz waves on the studied materials. We characterize the performances of the THz time-domain spectroscopy system by testing the absorption peaks of nano metasurface samples at different positions under TE and TM wave incidences. Results and Discussions In this work, we use THz emission spectroscopy to investigate the THz radiation of IrMn3|Co20Fe60B20|W in the atmospheric environment. The results and discussions are summarized as follows: first, the sample is placed at the focal point of the focusing lens and pumped by a femtosecond laser without an external magnetic field. We observe an emitted THz signal up to 1.5 THz. The amplitude of THz signals changes periodically along with the rotation of the sample's azimuth angle (Fig. 4). These experimental results are consistent with the spintronic THz emission mechanism. The exchange bias effect between antiferromagnetic and ferromagnetic materials causes Co20Fe60B20 to saturate magnetization, and the spin current generated inside it undergoes spin-to-charge conversion (SCC) at the interface due to the inverse spin Hall effect (ISHE), radiating a THz wave. Consequently, we can flexibly modulate the polarization of the THz waves by changing the sample azimuth angle. Second, we change external conditions to investigate the dependence of THz radiation on the laser pump power, in-plane magnetic field, and incident plane. We exclude other mechanisms by conducting left-right flipping and up-down flipping experiments (Fig. 5). It is confirmed that THz emission is induced by SCC caused by ISHE because both flipping methods result in THz pulses with opposite phases. In addition, the materials have not been magnetized due to the incomplete exchange bias effect of the sample. When the in-plane magnetic field is applied to IrMn3|Co20Fe60B20|W, the THz radiation increases by approximately 66.7%. An opposite polarity THz pulse is obtained by reversing the in-plane magnetic field (Fig. 6). Furthermore, THz emission intensity is enhanced by increasing the pump power, showing a linear dependence relationship (Fig. 6). Finally, we move the position of the sample before and after the focusing lens focus and measure the THz radiation output. It is found that THz emission amplitude is maximum when the sample is positioned 8 mm in front of the focus point instead of being located exactly at the focus point (Fig. 8). We have ruled out the possibility of the laser pump fluence reaching the sample saturation threshold. The reason might be that the coupling between the THz signal and the photoconductive antenna is better when the laser is converging, resulting in a larger detected THz amplitude. Conclusions Our research takes the lead in using a compact direct diode-pumped Ti:sapphire laser oscillator as the pumping source to drive the THz spectroscopy system and studies the THz radiation mechanism and performance of the IrMn3|Co20Fe60B20|W heterostructure at room temperature without external magnetic field conditions. We not only observe the phenomenon of THz emission enhanced by applying an in-plane magnetic field, increasing the pump power, and placing the sample before the focus but also realize the control of THz linear polarization. This research promotes the development of THz spectroscopy technology based on direct diode pumped Ti:sapphire laser oscillators with ultrashort pulses (less than 15 fs) to achieve broader band, smaller volume, and more flexible THz manipulation.