Abstract

Fourth and higher order even harmonics have previously been generated at solid surfaces only by irradiation far above the damage threshold,¹ where the high harmonic properties are determined by high intensity optical interaction with an azimuthally isotropic solid density plasma. In this work we demonstrate, for the first time to our knowledge, azimuthally anisotropic fourth harmonic generation (FHG) at a GaAs(001) interface by 5 μJ, 200-fs pulses at fluences more than 10 times below the damage threshold, where the fourth harmonic properties are determined by the crystalline symmetry of the surface region and by the 43m crystalline symmetry of the underlying noncentrosymmetric bulk. Our results reveal a strong four-fold anisotropic surface polarization, not present in second harmonic generation (SHG),2 that uniquely distinguishes surface from bulk FHG, and uniquely determines the symmetry of the contributing surface region as 4 mm, a determination that is ambiguous in SHG because of the alternation of four-fold and two-fold symmetric atomic layers.3 Although the current results were obtained with a fixed fundamental wavelength of 804 nm, they demonstrate the viability of FHG as a solid state interface spectroscopy.

© 1997 Optical Society of America