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Graphene takes light to a higher level
High harmonic generation is a useful nonlinear effect in which the light-matter interaction within a material results in the conversion of one wavelength to a shorter one. Typically performed in atomic gases, there is now interest in extending such a process to the solid state. Yoshikawa et al. pumped single-layer graphene with intense polarized pulses of infrared light to generate ultraviolet light, up to the ninth harmonic. Theoretical analysis of the process suggests that the effect could be transferred to other solid-state systems, providing a possible route to develop coherent light sources across the spectrum.
Science, this issue p. 736
Abstract
The electronic properties of graphene can give rise to a range of nonlinear optical responses. One of the most desirable nonlinear optical processes is high-harmonic generation (HHG) originating from coherent electron motion induced by an intense light field. Here, we report on the observation of up to ninth-order harmonics in graphene excited by mid-infrared laser pulses at room temperature. The HHG in graphene is enhanced by an elliptically polarized laser excitation, and the resultant harmonic radiation has a particular polarization. The observed ellipticity dependence is reproduced by a fully quantum mechanical treatment of HHG in solids. The zero-gap nature causes the unique properties of HHG in graphene, and our findings open up the possibility of investigating strong-field and ultrafast dynamics and nonlinear behavior of massless Dirac fermions.
