Highly Efficient Spintronic Terahertz Emitter Enabled by Metal–Dielectric Photonic Crystal

Abstract

Spintronic terahertz (THz) emitter provides the advantages such as apparently broader spectrum, significantly lower cost, and more flexibility compared with the commercial THz emitters, and thus attracts great interest recently. In past few years, efforts have been made in optimizing the material composition and structure geometry, and the conversion efficiency has been improved close to that of ZnTe crystal. One of the drawbacks of the current designs is the rather limited laser absorption—more than 50% energy is wasted and the conversion efficiency is thus limited. Here, a novel device that fully utilizes the laser intensity and significantly improves the conversion efficiency is theoretically proposed and experimentally demonstrated. The device, which consists of a metal–dielectric photonic crystal structure, utilizes the interference between the multiple scattering waves to simultaneously suppress the reflection and transmission of the laser, and to reshape the laser field distributions. The experimentally detected laser absorption and THz generation show one‐to‐one correspondence with the theoretical calculations. The strongest THz pulse emission that presents a 1.7 times improvement compared to the currently designed spintronic emitter is achieved. This work opens a new pathway to improve the performance of spintronic THz emitter from the perspective of optics.

Terahertz Radiation Modulated by Confinement of Picosecond Current Based on Patterned Ferromagnetic Heterostructures

“…It has been demonstrated previously that the ultrafast spin‐dependent Seebeck effect generates an out‐of‐plane spin current j s ( t ) triggered by the femtosecond laser pulses . The inverse spin Hall effect (ISHE) converts the forward and backward transient j s ( t ) from the FM layer into the transverse (in‐plane) charge current pulse j c ( t ) in the NM layerswhere A is the absorbed fraction of the incident pump fluence ( F inc ), d is the entire metal thickness, d NM is the NM layer thickness, is the relaxation length of the ultrafast spin current, and is the spin Hall angle. As the opposite sign of for W and Pt, j c flows in the same direction within W and Pt layers ( j c = j c W + j c Pt ) and then radiates THz pulses in phase.…”

Ghost spintronic THz-emitter-array microscope

“…The STEA is capped with a 150-nm SiO 2 layer (n = 1.97) to protect it from being damaged by the fs laser. The output THz electric field E(t) is linearly polarized perpendicular to the applied magnetic field B, as described by E(t) ∝ (J c = J s × B) [21][22][23][24][25][26] , where J s represents the spin current induced by the fs laser and J c represents the charge current converted in the NM metals under B. To perform ghost imaging (see "Materials and methods" for details), the Walsh-Hadamard matrix 27 was used to code the STEA due to its unrivalled noise suppression performance among various measurement matrices [11][12][13][14][15][16][17] .…”

“…In this work, we utilize a spintronic THz emitter (STE) to illuminate an object in the near field. STEs are a novel type of THz emitter based on the spin-related effects 21,22 in ferromagnetic/nonmagnetic (FM/NM) heterostructures [23][24][25][26] , which are only a few nanometres thick but offer generation efficiency comparable to conventional milimetre-thick EO crystals. In principle, an STE can fully cover the 0.1-30 THz frequency range 24 without phonon absorption, which is superior to all the current solid emitters.…”

THz spintronic emitters: a review on achievements and future challenges

“…Temperature was able to switch the relative alignment of the Fe moments in the different GdFe layers and thus the strength of the THz emission. Another approach was presented by Feng et al [48] where the performance of the spintronic THz emitter was improved by utilizing optics. In particular, the work utilized metal (NM1/FM/NM2)-dielectric photonic crystal structure where the metal Pt (1.8 nm)/Fe(1.8 nm)/W (1.8 nm) served as the spintronic emitter and the dielectric interlayers was SiO 2 , while the maximum number of layer repeats in the photonic crystal was 3.…”