English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Tools annual

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools monthly

Global: Zendy Plus monthly

Presents the access of premium content as premium feature

Premium Content

Global: Zendy Plus annual

Global: Zendy Free Plan

The existence of large nonlinear optical coefficients is one of thepreconditions for using nonlinear optical materials in nonlinear opticaldevices. For a crystal, such large coefficients can be achieved by matchingphoton energies with resonant energies between different bands, and so thedetails of the crystal band structure play an important role. Here wedemonstrate that large third-order nonlinearities can also be generallyobtained by a different strategy: As any of the incident frequencies or the sumof any two or three frequencies approaches zero, the doped or excitedpopulations of electronic states lead to divergent contributions in the inducedcurrent density. We refer to these as intraband divergences, by analogy withthe behavior of Drude conductivity in linear response. Physically, suchresonant processes can be associated with a combination of inraband andinterband optical transitions. Current-induced second order nonlinearity,coherent current injection, and jerk currents are all related to suchdivergences, and we find similar divergences in degenerate four wave mixing andcross-phase modulation under certain conditions. These divergences are limitedby intraband relaxation parameters, and lead to a large optical response from ahigh quality sample; we find they are very robust with respect to variations inthe details of the band structure. To clearly track all of these effects, weanalyze gapped graphene, describing the electrons as massive Dirac fermions;under the relaxation time approximation, we derive analytic expressions for thethird order conductivities, and identify the divergences that arise indescribing the associated nonlinear phenomena.

Intraband divergences in third order optical response of 2D systems