An Experimental Investigation into the Enhancement of Second Harmonic Generation by the Addition of Indium Tin Oxide in Thin Films
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This experimental work was predominantly inspired by recent publications by Alam, et Al., and Koos, et Al. [1,2] Alam and Boyd proposed that the nonlinear optical response of Indium Tin Oxide (ITO) became enhanced by approximately 43 times when an incident wave was in the material-dependent Epsilon Near Zero (ENZ) range. While Alam & Boyd considered third order nonlinearity, n_(2(eff)), our work was aimed at investigating the wavelength dependent behavior of the second order nonlinearity of ITO in the ENZ range. This idea was then employed to expand on the work by Koos, as previously mentioned. They reported an experimental enhancement of second harmonic signal by A-B-C layer stacks of the centrosymmetric materials Alumina, Titanium Oxide (TiO2), and Hafnium Oxide (HfO2) with a total layer thickness of approximately 68nm, but did not offer any systematic approach to optimize the design. The significance of the A-B-C structure is that it may allow the creation of a non-centrosymmetric effective medium, which will yield second harmonic generation (SHG) due to an asymmetric optical potential, which will be discussed in a following section. Building upon these two works, we had two goals; first, to demonstrate that the addition of ITO into a thin film design may enhance second harmonic generation, and second, to collect enough data in order to allow the design of a systematic approach to tailoring the second harmonic response to a particular preference. In practice, this new understanding could be used to detect infrared wavelengths using visible light detectors, and exploring the effects of ITO in the ENZ region can further efforts for controlling the behavior of light on integrated circuit (IC) scales and ultra-confinement. As discussed in a paper by I. Liberal and N. Engheta, epsilon-near-zero properties may be used to enhance nonlinear optical response by alleviating the phase matching condition required for higher harmonic generation.  They claim that in ENZ materials, the need for this phase matching is eliminated, and coherent buildup of the propagating field is helped by an absence of phase progression. This principle was another source of inspiration for this investigation, and as the ellipsometry shows that the thin film samples did indeed have an epsilon near zero point in the near infrared range, if this claim is correct we should see an enhancement of SHG at such wavelengths.