Epsilon-near-zero Enhancement of Linear and Nonlinear Thermo-optic Effects

Epsilon-near-zero (ENZ) materials, characterized by their near-zero permittivity, ease of fabrication, enhanced nonlinearity, and compatibility for nano-fabrication, have positioned themselves as alluring solutions for large-scale integrated systems-on-chips. However, systems operating within confined spaces inherently generate heat, presenting significant challenges for the functionality of ENZ materials, including CMOS-compatible transparent conductive oxides. Although the temperature sensitivity of their optical properties is well recognized, a systematic analysis of this critical dependence has been lacking, so far. This experimental study aims to clarify the linear and nonlinear thermo-optic ENZ effects within indium tin oxide (ITO). The investigation encompasses a comprehensive analysis of the temperature-dependent optical properties of ITO samples, spanning ENZ frequencies within the telecommunication O-band, C-band, and 2-µm-band. Notably, our findings reveal that ITO exhibits an unprecedented enhancement of thermo-optic effects within the ENZ region, ranging from 660% to 955%, vastly surpassing the conventional thermo-optic effect. This enhancement is achieved over a wide bandwidth of 70 to 93 THz. In addition to linear phenomena, we delve into thermo-optic nonlinearity, found to exhibit an exceptionally substantial enhancement, ranging from 1113% to 2866%, comparable to the reported enhancement of its Kerr nonlinearity. The outcomes of our investigation provide essential parameters and practical insights that are relevant for both optical and thermal engineering design considerations. This is especially pertinent for applications involving packaged photonic integrated circuits. Additionally, our findings suggest the potential of ITO as a novel platform for slow light applications and photonic emulation.