Abstract
The determination of the second-order susceptibility (χ(2)) of thin film samples can be a delicate matter since well-established χ(2) measurement methodologies such as the Maker fringe technique are best suited for nonlinear materials with large thicknesses typically ranging from tens of microns to several millimeters. Here we compare two different second-harmonic generation setups and the corresponding measurement methodologies that are especially advantageous for thin film χ(2) characterization. This exercise allows for cross-checking the χ(2) obtained for identical samples and identifying the main sources of error for the respective techniques. The development of photonic integrated circuits makes nonlinear thin films of particular interest, since they can be processed into long waveguides to create efficient nonlinear devices. The investigated samples are ABC-type nanolaminates, which were reported recently by two different research groups. However, the subsequent analysis can be useful for all researchers active in the field of thin film χ(2) characterization.
Highlights
Materials are both active fields of research
The technique is based on Maker’s finding that the pattern of the transmitted second-harmonic (SH) power generated in a bulk nonlinear crystal reveals the so-called Maker fringes when the crystal is rotated[26]
These fringes occur since the second-harmonic generation (SHG) process is not phase matched and the SH waves generated at different locations in the crystal can interfere constructively or destructively depending on the effective length of the crystal that is changed by the rotation
Summary
Materials are both active fields of research. This makes the existence of easy and reliable methods for the characterization of the second-order nonlinearity in thin films undoubtedly an asset. The technique is based on Maker’s finding that the pattern of the transmitted second-harmonic (SH) power generated in a bulk nonlinear crystal reveals the so-called Maker fringes when the crystal is rotated[26] These fringes occur since the SHG process is not phase matched and the SH waves generated at different locations in the crystal can interfere constructively or destructively depending on the effective length of the crystal that is changed by the rotation. In a typical experiment the strength of the transmitted or reflected SHG is measured as a function of the angle of incidence and/or the polarization state[38] This allows determination of the surface χ(2) tensor elements, which on its turn provides information about the atomic structure of the interface, the concentration and orientation of adsorbed molecules, etc. As in surface SHG the χ(2) layer is only a few atoms thick, no Maker fringes are observed
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