Fig. 2.

Blue-shifted harmonics. a Schematic of the experimental configuration, showing the mid-IR field (linearly polarized, 70-fs pulses at 3.1 μm wavelength, i.e., E₀ = 0.4 eV photon energy) before and after propagation through 5 monolayers of graphene that are supported on a 0.4-mm thick CaF₂ substrate. Pulses of different durations and degrees of elliptical polarization are also investigated. b Measured fundamental spectrum, along with the emission at the third and fifth harmonics (blue-shifted). The dashed curves represent the nominal position of the third and fifth harmonic. c The resulting third harmonic is blue-shifted by 1.8% from 3E₀ and the blue-shift is independent of driving field intensity. d The blue-shift depends inversely on pulse duration for both the third and the fifth harmonic. Note that, while in (b) we plot the absolute spectral shift, in (d) we plot the relative shift δω_THG/3ω and δω_FHG/5ω. The measurement errors are derived according to the error propagation law from the instrument measurement uncertainties, i.e., from the spectrometer, power meter, beam profiler, and frequency resolved optical gating