Enhanced and controlled light absorption, as well as field confinement in optically thin materials, are pivotal for energy‐efficient optoelectronics and nonlinear optical devices. Highly doped transparent conducting oxide (TCO) thin films can support the so‐called epsilon near zero (ENZ) modes in a frequency region of near‐zero permittivity, which can lead to the perfect light absorption and ultrastrong electric field intensity enhancement (FIE) within the films. To achieve full control over absorption and FIE, one must be able to tune the ENZ material properties as well as the film thickness. Here, engineered absorption and FIE are experimentally demonstrated in aluminum‐doped zinc oxide (AZO) thin films via control of their ENZ wavelengths, optical losses, and film thicknesses, tuned by adjusting the atomic layer deposition (ALD) parameters such as dopant ratio, deposition temperature, and the number of macrocycles. It is also demonstrated that under ENZ mode excitation, though the absorption and FIE are inherently related, the film thickness required for observing maximum absorption differs significantly from that for maximum FIE. This study on engineering ENZ material properties by optimizing the ALD process will be beneficial for the design and development of next‐generation tailorable photonic devices based on flat, zero‐index optics.
- Award ID(s):
- 1752295
- PAR ID:
- 10132727
- Date Published:
- Journal Name:
- Advanced materials interfaces
- Volume:
- Submitted
- ISSN:
- 2196-7350
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
Abstract -
Using basic considerations on the average power absorbed in ultra-thin conducting films, we derive a closed-form expression for the average electric- field intensity enhancement (FIE) due to epsilon-near-zero (ENZ) polariton modes. We show that FIE in ENZ media with realistic losses reaches a maximum value in the limit of ultra-small film thickness. The maximum value is reciprocal to the second power of ENZ losses. This is illustrated in an exemplary series of aluminum-doped zinc oxide nanolayers of varying thickness grown by atomic layer deposition technique. The limiting behavior of FIE is shown in exact cases of the perfect absorption, normal incidence, and in a case of ultra- thin lossless ENZ films. Only in the case of lossless ENZ films FIE is inversely proportional to the second power of film thickness as it was predicted by S. Campione, et al. [Phys. Rev. B 91, 121408(R) (2015)]. We also show that FIE could achieve values as high as 100,000 in ultra-thin polar semiconductor films, which have losses as small as 0.02 close to the longitudinal optic (LO) phonon frequency.more » « less
-
Abstract Using electrodynamical description of the average power absorbed by a conducting film, we present an expression for the electric-field intensity enhancement (FIE) due to epsilon-near-zero (ENZ) polariton modes. We show that FIE reaches a limit in ultrathin ENZ films inverse of second power of ENZ losses. This is illustrated in an exemplary series of aluminum-doped zinc oxide nanolayers grown by atomic layer deposition. Only in a case of unrealistic lossless ENZ films, FIE follows the inverse second power of film thickness predicted by S. Campione, et al. [ Phys. Rev. B , vol. 91, no. 12, art. 121408, 2015]. We also predict that FIE could reach values of 100,000 in ultrathin polar semiconductor films. This work is important for establishing the limits of plasmonic field enhancement and the development of near zero refractive index photonics, nonlinear optics, thermal, and quantum optics in the ENZ regime.more » « less
-
We use post-deposition vacuum annealing of epsilon-near-zero (ENZ) indium tin oxide (ITO) nanolayers in order to modify their structural properties and enhance the third-order optical nonlinear response around the ENZ wavelength. We find that room temperature magnetron sputtering deposition results in polycrystalline thin films with an intrinsic tensile strain and a ⟨110⟩ fiber axis preferentially oriented normal to the substrate. Moreover, we demonstrate that post-deposition vacuum annealing treatments produce a secondary anisotropic phase characterized by compressive strain that increases with the annealing temperature. Finally, we use the Z-scan optical technique to accurately measure the complex nonlinear susceptibility [Formula: see text] and the intensity-dependent refractive index change [Formula: see text] for samples with different structural properties despite featuring similar ENZ wavelengths. Our intensity-dependent analysis demonstrates that an enhancement of the optical nonlinearity can be achieved by tuning the structure of ENZ nanolayers with values as large as [Formula: see text]. This study unveils the importance of structural control and secondary phase formation in ITO nanolayers with ENZ optical dispersion properties for the engineering of integrated highly nonlinear devices and metamaterials that are compatible with the scalable silicon photonics platform.
-
Major technological breakthroughs are often driven by advancements in materials research, and optics is no different. Over the last few years, near-zero-index (NZI) materials have triggered significant interest owing to their exceptional tunability of optical properties and enhanced light-matter interaction, leading to several demonstrations of compact, energy-efficient, and dynamic nanophotonic devices. Many of these devices have relied on transparent conducting oxides (TCOs) as a dynamic layer, as these materials exhibit a near-zero-index at telecommunication wavelengths. Among a wide range of techniques employed for the deposition of TCOs, atomic layer deposition (ALD) offers advantages such as conformality, scalability, and low substrate temperature. However, the ALD process often results in films with poor optical quality, due to low doping efficiencies at high (>1020cm−3) doping levels. In this work, we demonstrate a modified ALD process to deposit TCOs, taking Al:ZnO as an example, which results in an increase in doping efficiency from 13% to 54%. Moving away from surface saturation for the dopant (aluminum) precursor, the modified ALD process results in a more uniform distribution of dopants (Al) throughout the film, yielding highly conductive (2.8×10−4Ω-cm) AZO films with crossover wavelengths as low as 1320nm and 1370nm on sapphire and silicon substrates, respectively.