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Guided mode resonances (GMR) can be made angle insensitive using conical mounting of the grating relative to the external illumination. Conical mounting has been previously utilized for linear optical applications, such as optical filtering, and sensing. Here we present Third Harmonic Generation (THG) enhancement from 10 nm amorphous silicon overlayer on silicon nitride-based one-dimensional sub-wavelength grating GMR structures under conical-mounting illumination. The designed structure comprising of 70 nm deep silicon dioxide gratings of 1054 nm pitch, 50% duty cycle over which 160 nm thick silicon nitride and 10 nm a-Si layers are deposited is resonant at 1580 nm for TE polarized excitation. With increased angular spread of the incident excitation, the GMR spectral width and contrast are known to degrade. The angular aperture of the GMR structures studied here, which is defined as the angular spread across which the resonance drops to 50% of its peak value is calculated as 0.46° and 5.2° for classical and full-conical illumination respectively, highlighting the angular insensitivity of the full-conical mounting condition. Rectangular aperture masks placed in the back focal plane of the objective lens are used to limit the illumination angle along the grating wave-vector direction when compared to the grating line direction, thus achieving conical mounting condition. Experimentally, we observe the THG enhancement, defined as the ratio of on- to off-grating THG, improves from 2860 to 4742 and 1.7x104 by utilizing 0.06 NA objective and illuminating in classical configuration (no aperture) with rectangular apertures of size 3x13 mm and 1.5x13 mm respectively.
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