Ultrafast Sub‐100 fs All‐Optical Modulation and Efficient Third‐Harmonic Generation in Weyl Semimetal Niobium Phosphide Thin Films

Abstract

Since their experimental discovery in 2015, Weyl semimetals have generated a large amount of attention due their intriguing physical properties that arise from their linear electron dispersion relation and topological surface states. In particular, in the field of nonlinear (NL) optics and light harvesting, Weyl semimetals have shown outstanding performances and achieved record NL conversion coefficients. In this context, the first steps toward Weyl semimetal nanophotonics are performed here by thoroughly characterizing the linear and NL optical behavior of epitaxially grown niobium phosphide (NbP) thin films, covering the visible to the near‐infrared regime of the electromagnetic spectrum. Despite the measured high linear absorption, third‐harmonic generation studies demonstrate high conversion efficiencies up to 10−4% that can be attributed to the topological electron states at the surface of the material. Furthermore, nondegenerate pump–probe measurements with sub‐10 fs pulses reveal a maximum modulation depth of ≈1%, completely decaying within 100 fs and therefore suggesting the possibility of developing all‐optical switching devices based on NbP. Altogether, this work reveals the promising NL optical properties of Weyl semimetal thin films, which outperform bulk crystals of the same material, laying the grounds for nanoscale applications, enabled by top‐down nanostructuring, such as light‐harvesting, on‐chip frequency conversion, and all‐optical processing.

Ultrafast All-Optical Modulation and Efficient Third-Harmonic Generation in Two-Dimensional Perovskite Heterostructures

“…This outperforms the single n -phase films that have been reported, which are subject to the effects of electronic disorder, low-dimensional dielectric confinement, and lattice softening that accelerates the cooling of the carriers in the heterostructures of 2D HOIPs. Note that the maximum absorption change of −4.85% for the heterostructure under 570 nm probe wavelength is comparable to the ultrafast all-optical processing materials reported previously. , This implies that devices based on HOIP heterostructures have an all-optical modulation bandwidth of up to 5.3 terahertz, which is comparable to the reported performance of optical modulation materials in the epsilon-near-zero system , and the Weyl semimetal system, demonstrating that HOIP heterostructures are highly promising candidates for high-performance all-optical signal processing applications.…”

Optically induced changes in the band structure of the Weyl charge-density-wave compound (TaSe4)2I

“…The performances of (TaSe 4 ) 2 I are found to be comparable or even better than other materials used as IR photodetectors. Weyl semimetals thus represent promising candidates for several applications in optoelectronics operating at different time scales, ranging from the µs dynamics of (TaSe 4 ) 2 I to the sub-100 fs modulation of the optical properties of NbP thin films [56].…”

Symmetry transformation of nonlinear optical current of tilted Weyl nodes and application to ferromagnetic MnBi2Te4

“…The gapless excitations make Weyl semimetals an ideal platform to perform low-energy photon detection, particularly in the infrared and terahertz regimes. [1] Various nonlinear optical responses, such as harmonic generations, [2][3][4][5] bulk photovoltaic effect [6][7][8][9][10][11] and four-wave mixing, [12,13] have been widely studied in Weyl semimetals. Furthermore, it has recently been recognized that the geometry and topology of electronic band structures can have nontrivial impacts on optical conductivities.…”