Spintronic terahertz emitters (STEs), based on optical excitation of nanometer thick ferromagnetic/heavy metal (FM/HM) heterojunctions, have become important sources for the generation of terahertz (THz) pulses. However, the efficiency of the optical-to-THz conversion remains limited. Although optical techniques have been developed to enhance the optical absorption, no investigations have studied the application of THz cavities. Here, to enhance the THz efficiency of STEs in a selected THz spectral range, FM/HM structures are realized on ultra-thin sapphire layers with metallic mirrors to create λ/4 THz resonant cavities. THz emission time domain spectroscopy of these STE/sapphire/mirror heterostructures, with sapphire thicknesses ranging from 110 µm to 25 µm, shows enhancement of the emitted THz field that fits the λ/4 cavity resonance with up to a doubling of the field in the spectrum, and in agreement with temporal simulations of the emitted THz pulse. By taking advantage of birefringent materials, we further show the potential of control of the polarization state of the emitted THz pulse. This work shows the potential of enhancing and engineering THz emission from STEs using THz cavities that can be controlled over a broad spectral range, which can be easily combined with optical cavities.

2D materials, such as transition metal dichalcogenides, are ideal platforms for spin‐to‐charge conversion (SCC) as they possess strong spin–orbit coupling (SOC), reduced dimensionality and crystal symmetries as well as tuneable band structure, compared to metallic structures. Moreover, SCC can be tuned with the number of layers, electric field, or strain. Here, SCC in epitaxially grown 2D PtSe 2 by THz spintronic emission is studied since its 1T crystal symmetry and strong SOC favor SCC. High quality of as‐grown PtSe 2 layers is demonstrated, followed by in situ ferromagnet deposition by sputtering that leaves the PtSe 2 unaffected, resulting in well‐defined clean interfaces as evidenced with extensive characterization. Through this atomic growth control and using THz spintronic emission, the unique thickness‐dependent electronic structure of PtSe 2 allows the control of SCC. Indeed, the transition from the inverse Rashba–Edelstein effect (IREE) in 1–3 monolayers (ML) to the inverse spin Hall effect (ISHE) in multilayers (>3 ML) of PtSe 2 enabling the extraction of the perpendicular spin diffusion length and relative strength of IREE and ISHE is demonstrated. This band structure flexibility makes PtSe 2 an ideal candidate to explore the underlying mechanisms and engineering of the SCC as well as for the development of tuneable THz spintronic emitters.

2D materials, such as transition metal dichalcogenides, are ideal platforms for spin-to-charge conversion (SCC) as they possess strong spin-orbit coupling (SOC), reduced dimensionality and crystal symmetries as well as tuneable band structure, compared to metallic structures. Moreover, SCC can be tuned with the number of layers, electric field, or strain. Here, SCC in epitaxially grown 2D PtSe2 by THz spintronic emission is studied since its 1T crystal symmetry and strong SOC favor SCC. High quality of as-grown PtSe2 layers is demonstrated, followed by in situ ferromagnet deposition by sputtering that leaves the PtSe2 unaffected, resulting in well-defined clean interfaces as evidenced with extensive characterization. Through this atomic growth control and using THz spintronic emission, the unique thickness-dependent electronic structure of PtSe2 allows the control of SCC. Indeed, the transition from the inverse Rashba-Edelstein effect (IREE) in 1-3 monolayers (ML) to the inverse spin Hall effect (ISHE) in multilayers (>3 ML) of PtSe2 enabling the extraction of the perpendicular spin diffusion length and relative strength of IREE and ISHE is demonstrated. This band structure flexibility makes PtSe2 an ideal candidate to explore the underlying mechanisms and engineering of the SCC as well as for the development of tuneable THz spintronic emitters.

As a two-dimensional (2D) material for terahertz (THz) applications, platinum diselenide (PtSe 2 ) can be uniquely tuned from a semiconductor in the near infrared to a semimetal with the number of atomic layers, in contrast to other transition metal dichalcogenides (TMDs). Consequently, the material has unique photonic properties at THz frequencies that can be enhanced by atomic layer engineering. Here, we demonstrate that a controlled THz nonlinearity - tuned from monolayer to bulk PtSe 2 - can be realized in wafer size PtSe 2 through the generation of ultrafast photocurrents and the engineering of the bandstructure valleys. Further, we show layer dependent circular dichroism, where the sign of the ultrafast currents and hence the phase of the emitted THz pulse can be controlled through the excitation of different bandstructure valleys. In particular, we show that a semimetal has a strong dichroism that is absent in the monolayer and few layer semiconducting limit. The microscopic origins of this TMD bandstructure engineering is highlighted through detailed DFT simulations, and shows the circular dichroism can be controlled when PtSe 2 becomes a semimetal and when the K-valleys can be excited. As well as showing that PtSe 2 is a promising material for THz generation through layer controlled optical nonlinearities, this work opens up new class of circular dichroism materials beyond the monolayer limit that has been the case of traditional TMDs, and impacting a range of domains from THz valleytronics to harmonic generation.

Platinum diselenide (PtSe2) is a promising two-dimensional (2D) material for the terahertz (THz) range as, unlike other transition metal dichalcogenides (TMDs), its bandgap can be uniquely tuned from a semiconductor in the near infrared to a semimetal with the number of atomic layers. This gives the material unique THz photonic properties that can be layer-engineered. Here, we demonstrate that a controlled THz nonlinearity-tuned from monolayer to bulk PtSe2-can be realized in wafer size polycrystalline PtSe2 through the generation of ultrafast photocurrents and the engineering of the bandstructure valleys. This is combined with the PtSe2 layer interaction with the substrate for a broken material centrosymmetry, permitting a second order nonlinearity. Further, we show layer dependent circular dichroism, where the sign of the ultrafast currents and hence the phase of the emitted THz pulse can be controlled through the excitation of different bandstructure valleys. In particular, we show that a semimetal has a strong dichroism that is absent in the mono layer and few layer semiconducting limit. The microscopic origins of this TMDb and structure engineering are highlighted through detailed DFT simulations, and shows the circular dichroism can be controlled when PtSe2 becomes a semimetal and when the K-valleys can be excited. As well as showing that PtSe2 is a promising material for THz generation through layer controlled optical nonlinearities, this work opens up a new class of circular dichroism materials beyond the monolayer limit that has been the case of traditional TMDs, and impacting a range of domains from THz valleytronics, THz spintronics to harmonic generation.