Effects of material parameters on the nonlinear refraction of transparent conducting oxides in the epsilon-near-zero spectral region
This doctoral degree program was undertaken together with Instituto Tecnológico y de Estudios Superiores de Monterrey
Hosein Ghobadi is a PhD student in the department Optical Sciences. (Co)Supervisors are prof.dr.ir. H.L. Offerhaus, dr. J.A. Alvarez Chavez, prof. M. Morales Masis from the faculty of Science & Technology and prof.dr. I. de Leon Arizpe from Instituto Tecnológico y de Estudios Superiores de Monterrey.
The realization of high-speed computation and communication relies on all-optical modulation of the properties of light through nonlinear interaction with matter. This, in turn, requires materials capable of achieving large values of nonlinear change in refraction on a sub-picosecond time scale. Nonetheless, most materials are known to exhibit weak optical nonlinearities, which would significantly increase the power consumption and the device footprint. Therefore, researchers in the field of nonlinear optics have been pursuing alternative materials, which has recently led to the introduction of Transparent Conducting Oxides (TCOs) as highly-nonlinear materials. These materials exhibit large and ultrafast intensity-dependent refraction that originates from the enhanced light-matter interactions in the Epsilon-Near-Zero (ENZ) spectral region, where the electric permittivity of the material tends to zero. Although TCOs hold promise for high-speed data processing, there are still issues of power consumption and optical loss that need to be addressed before the practical implementation of devices based on these materials. The aim of this thesis has been to improve the mentioned drawbacks by tailoring the important material parameters of TCOs in the ENZ spectral region and at the same time investigating alternative TCO materials with an improved optical response. The obtained results can be summarized as follows:
Chapter 4 investigates the impact of the crystal quality of Indium-Tin Oxide (ITO) thin films on their material parameters and nonlinear refraction. The results reveal a considerable decrease in the optical loss of ITO due to the increased free-electron optical mobility. Furthermore, ITO films with higher crystal quality have larger high-frequency permittivity and smaller free-electron effective mass, which would improve the nonlinear refraction of these materials. This is confirmed by the calculations of figure-of-merit (FoM) for intensity-dependent refraction, revealing a significant increase in FoM as a function of the crystal quality. As a result, improving the crystal quality of TCO films would help to reduce the undesired attenuation of the signal subject to all-optical modulation and would reduce power consumption compared to films with poor crystal quality.
Chapter 5 presents the optical properties and nonlinear refraction of Zirconium-doped Indium Oxide (IZrO) thin films in the ENZ spectral region. IZrO films feature a lower optical loss compared to ITO, which would lead to smaller signal attenuation in the ENZ spectral region. An analysis of the effect of IZrO film's thickness on its optical properties reveals that the optical loss can be further reduced by growing thicker films. Moreover, the measurements of the nonlinear change in transmittance () over a range of wavelengths result in a broadband nonlinear response caused by a nonuniform permittivity distribution along the thickness of the film. The full-width-at-half-maximum bandwidth of response as a function of optical intensity reaches up to 260 nm covering the three consecutive C, L, and U telecommunication bands. According to these results, IZrO is a more promising candidate for all-optical signal processing.
Chapter 6 evaluates the impact of incidence angle, optical intensity, and film thickness on the nonlinear refraction of TCOs. Measurements of angle and intensity-dependent for multiple IZrO films show that the angle of incidence for maximum decreases with the thickness. In comparison, the effect of optical intensity is the opposite. The origin of the observed phenomenon is investigated through a series of calculations, linking it to the variation in absorption and field intensity enhancement as a function of material parameters and measurement conditions. Using these results an FoM is proposed that accounts for the effects of material and experimental parameters and estimates the trend of variation in for any TCO film, which helps to reduce the experimental efforts to a large extent. More importantly, this study shows that using the optimal measurement arrangements is as important as tailoring the material parameters of TCOs when attempting to overcome their drawbacks related to all-optical data processing.
Chapter 7 presents a pulsed laser deposition (PLD) process for fabricating ITO films with graded optical properties. These films have a significantly wide ENZ spectral region, which is expected to result in broadband nonlinear refraction, in turn allowing extraction of more functionalities from ENZ-TCOs. The PLD process developed in this thesis is based on tailoring the deposition atmosphere, which is straightforward and cost-effective compared to the processes based on the sputtering technique.
The research exploring the optical responses of TCOs has intensified in the past decade generating a wealth of knowledge critical for our understanding of the subject. Moving forward, efforts should be dedicated to optimizing the optical response of TCOs to overcome the material drawbacks hindering the practical implementation of all-optical devices. In this regard, the findings reported in this thesis would contribute to the development of ENZ-TCOs with improved performance to be employed in both fundamental studies and potential applications.