Resonantly Increased Optical Frequency Conversion in Atomically Thin Black Phosphorus

Abstract

Optical frequency conversion via the nonlinear effect of third harmonic generation is shown to be resonantly enhanced in few-layer black phosphorus. This feature is believed to be a consequence of exciton-related resonance, as the enhancement is strongly correlated with the observation of exciton-recombination photoluminescence. Few-layer thicknesses are obtained both via mechanical exfoliation and laser thinning.

Optical third harmonic generation in black phosphorus

“…Using a nonorthogonal TB model to compute the energy dispersion and eigenstates of bP, we evaluate the low-energy THG conductivity. Results for bulk bP agree, at least qualitatively, with the experimental reports of THG in bulk or many-layer samples bP [31][32][33]. Furthermore, the results show the importance of the mixed inter-intraband processes for the nonlinear response at energy scales comparable with the optical gap, particularly for systems whose electronic properties are accurately captured by two bands at this energy scale.…”

“…The anisotropy of the system manifests itself clearly for both the bulk and monolayer bP, with the patterns dominated by the contribution of I x /I 0 |σ 11 | 2 cos 6 θ . To the best of our knowledge, experimental data on THG in bP is limited to bulk or several layer [31][32][33] and results for the intensity dependence on the polarization angle appear to be inconsistent, e.g., pattern of total intensity presented by Ref. [32] exhibits maxima along the crystal x direction, whereas Refs.…”

“…[32] exhibits maxima along the crystal x direction, whereas Refs. [31,33] shows maxima align with directions other than the primitive lattice directions, namely θ ∼ {±π/6, ± 5π/6}. Additionally, the pattern for I y in Ref.…”

“…It is worth noting that recent results of photoluminescence in high-quality samples [17] have shown that the linear response along the y direction is vanishingly small, indicating that the apparently higher response along the y direction can be attributed to mechanisms other than the intrinsic response of the system, such as disorder. In addition, the estimate of the magnitude of χ (3) eff and its ratio with regards to graphene's χ (3) eff remains an open question, as experimental reports indicate different results that span several orders of magnitude [31][32][33]. Our results indicate that both bulk and monolayer THG conductivities athω = 0.793 eV (λ ∼ 1560 nm) have magnitudes ∼20σ 3 , which corresponds to 035431-7 a nonlinear susceptibility χ (3) eff ∼ 10 × 10 −19 nm 2 /V 2 , similar to recent reported results for bulk bP [32,33].…”

“…Recent reports have demonstrated that the electronic and transport properties of * fh@nano.aau.dk † tgp@nano.aau.dk bP can be used for several applications, including field-effect transistors [16,[26][27][28]. The electronic properties of bP provide fertile ground for opto-electronics devices, such as photodetectors [28,29], dichroic absorption [30] and nonlinear optics, including THG [31][32][33] and high harmonic generation [19]. In addition, theoretical studies indicate that the anisotropic characteristics of bP can be harnessed and tuned by strain [34][35][36], opening a door for strain-sensitive or strain-enhanced optoelectronic devices based in bP.…”