In recent years, THz modulators have been improved by 2D materials, yet facing a conflict between the demand for high modulation depth and the limitation of low pump power. Previously, by pumping continuous-wave laser on a highresistivity silicon wafer, we achieved modulation depth >95% in 0.3-1.5 THz, demanding continuous-wave pump power of 11.8 W. In this work, we added a mono-layer WSe2 on the high-resistivity silicon wafer by mechanical exfoliation, raising the modulation depth of THz pulse from 20% to 58% under 0.05 W femtosecond laser pump. The modulation depth can be further enhanced by raising pump power. The modulation behavior is most significant from 1.3 THz to >1.5 THz. This modulation enhancement is due to the interface state between WSe2 and silicon, as well as the direct bandgap of mono-layer WSe2. If exposed to the air, WSe2 starts oxidization at a low exciting power of <1 mW. Experiments shows that even if WSe2 is partly oxidized to WO3, the modulation depth is only slightly weakened, yet still better than the bare silicon substrate. Also, even if WSe2 does not fully cover the range of THz wave, or if the pump laser beam does not fully cover the range of THz wave, the modulation enhancement is also partly effective. Our work suggests a lowcost way to improve the efficiency of all-optical THz modulators.
Femtosecond control of electron spin not only promises the capability of satisfying the ever-increasing demand of storage information and ultrafast manipulation of magnetization in mediums, but also delivering controllable, highlyefficient, cost-effective and compact terahertz sources. Femtosecond spin dynamics have been extensively investigated these years with the methods of ultrafast magnetic-optical Kerr effect, inverse Faraday effect, inverse spin Hall effect and so on. Recently emerged coherent terahertz emission spectroscopy can also be employed to study this ultrafast spin dynamics with its unique advantages. For example, terahertz emission spectroscopy is a coherent, time-resolved, contactless Ampere-meter, which can be used to deduce the spin-charge conversion. However, femtosecond laser interaction with magnetic mediums is a rather complex process, there are still lots of physical mechanisms waiting to be unveiled. Here, we systematically investigate the femtosecond spin dynamics in ferromagnetic materials via polarization-resolved terahertz emission spectroscopy. We obtain detectable electromagnetic field radiation with its polarization parallel to the external magnetic field direction, which was not observed in the same materials in previous work. Inverse spin-orbit torque tilting is responsible for the observed phenomenon. Based on this mechanism, the efficiency and polarization of the generated terahertz waves can be coherently controlled and manipulated not only by the external magnetic fields, but also by the sample structures and the pumping femtosecond laser pulses. Our work not only helps further deepen understanding of the physical mechanism of all-optical magnetization reversal, boosting future spin recording technology, but also offers a very promising way for developing novel and efficient terahertz functional sources and devices.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.