Fig. 2: Experimental characterization of visible-frequency SHG enabled by cMQWs.

a Schematic diagram of the SHG measurement. b SHG emission spectrum under a 920-nm light pulse (100-fs pulse width, 80-MHz repetition rate, 5-μm spot radius) excitation with an average power of 3 mW (peak intensity of 0.48 GW/cm²). Solid black circles are the experimental result, which is fitted by a Gaussian lineshape function (black line). Left inset shows the experimental configuration where the pump light (thick red arrow) with the electric field E (black arrow), polarized at angle φ to the out-of-plane (with respect to the incident plane) direction, is obliquely incident on the sample surface with the incident angle θ. Right inset shows the total (transmitted and reflected) SHG emission at various incident powers. The experimental results fall on the quadratic theoretical curve. c CCD images at the back-aperture plane of the objective (equivalent to the Fourier plane) showing the evolution of emissions at 460 nm as the polarization angle φ and thus E_z increases. The dashed yellow circle highlights the collection angle of θ = 30°, while the solid white circle indicates the maximum collection angle of the objective. d Wavelength dependence of χ⁽²⁾ for a single unit of the cMQWs. The error bars represent the variation in the mean value measured over ten different locations on three identical samples. The presence of a resonant peak for the wavelength-dependent χ⁽²⁾ is evidence of the double resonant transition in the single cMQW unit