An official website of the United States government
Abstract 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.
more »
« less
This content will become publicly available on February 25, 2026
Berreman-assisted optical characterization of sub-percolation threshold, ultrathin near-zero-index films
Abstract Due to its transparent and conductive nature, indium tin oxide (ITO) offers substantial benefits in several industries, such as thin film transistors, displays, and nanophotonics. Previous studies on ultrathin ITO have so far focused on its electrical properties but have neglected the technologically important epsilon-near-zero (ENZ) optical features due to the difficulty of extracting the refractive index and the thickness-dependent degradation of the optical properties. Here, we demonstrate a complementary metal-oxide-semiconductor (CMOS)-compatible deposition procedure for sub-percolation thickness (below 4 nm) ITO using a dry-etch assisted radiofrequency magnetron sputtering technique that yields continuous films in a precisely controlled manner. Through interface engineering and post-deposition annealing optimization, we show that these ITO films can retain high carrier mobility (43 cm2V−1s−1) while achieving a tunable near-zero-index (NZI) regime throughout the telecommunications band using a Berreman-assisted optical characterization technique. Our result opens the possibility of efficiently designing ENZ/NZI materials at the nanoscale using a robust fabrication approach for applications in nanophotonics.
more »
« less
- Award ID(s):
- 2011924
- PAR ID:
- 10589194
- Publisher / Repository:
- IOP Publishing
- Date Published:
- Journal Name:
- Journal of Physics: Photonics
- Volume:
- 7
- Issue:
- 2
- ISSN:
- 2515-7647
- Page Range / eLocation ID:
- 025009
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Enhanced and controlled light absorption as well as field confinement in an optically thin material are pivotal for energy-efficient optoelectronics and nonlinear optical devices. Highly doped transparent conducting oxide (TCO) thin films with near-zero permittivity can support ENZ modes in the so-called epsilon near zero (ENZ) frequency region, which can lead to perfect light absorption and ultra-strong 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, we experimentally demonstrate engineered absorption and FIE 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 number of macro-cycles. We also demonstrate 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 tunable photonic devices based on flat, zero-index optics.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
-
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 The epsilon‐near‐zero (ENZ) frequency regime of transparent conducting oxide materials is known to yield large enhancements in their optical nonlinearity and electro‐optic response. Here, Faraday rotation is investigated in Gd and In‐doped CdO films and it is found that the Verdet constant peaks at values >3 105 deg T−1 m−1near the ENZ frequency, which is tunable in the wavelength range 2 < λ< 10 µm by varying the doping concentration. These results are among the highest reported to date in the mid‐infrared spectral range and are in good agreement with the Drude model, which confirms that the magneto‐optic response of doped CdO derives from its free carriers. The combination of a tunable Verdet constant, low optical loss compared to other plasmonic materials, and the ability to deposit CdO on Si with no loss in performance make this material a promising platform for integrated magneto‐optic and magnetoplasmonic devices that operate across the mid‐infrared.more » « less
