Epitaxial Aluminum-on-Sapphire Films as a Plasmonic Material Platform for Ultraviolet and Full Visible Spectral Regions

Abstract

In comparison to noble metals (gold and silver), aluminum is a sustainable and widely applicable plasmonic material owing to its abundance in the Earth’s crust and compatibility with the complementary metal–oxide–semiconductor (CMOS) technology for integrated devices. Aluminum (Al) has a superior performance in the ultraviolet (UV) regime with the lowest material loss and good performance in the full visible regime. Furthermore, aluminum films can remain very stable in ambient environment due to the formation of surface native oxide (alumina) acting as a passivation layer. In this work, we develop an epitaxial growth technique for forming atomically smooth aluminum films on transparent c-plane (0001) sapphire (Al-on-Sapphire, ALOSA) by molecular-beam epitaxy (MBE). The MBE-grown ALOSA films have small plasmonic losses and enable us to fabricate and utilize high-quality plasmonic nanostructures in a variety of optical configurations (reflection, transmission, and scattering). Here, the surface roughness and crystal orientation of ALOSA films are characterized by atomic force microscopy (AFM) and X-ray diffraction (XRD). Moreover, the formation of smooth native oxide layer and abrupt heterointerfaces are investigated by transmission electron microscopy (TEM). We have also measured the optical dielectric function of epitaxial aluminum films by using spectroscopic ellipsometry (SE). These results show that the structural and optical properties of epitaxial aluminum films grown by MBE are excellent compared to polycrystalline aluminum films grown by other deposition methods. To illustrate the capability of device applications for the full visible spectrum, we demonstrate clear surface plasmon polarition (SPP) interference patterns using a series of double-groove surface interferometer structures with varied groove–groove separations under white-light illumination. Finally, we show the device performance of zinc oxide (ZnO) nanowire (UV) and indium gallium nitride (InGaN) nanorod (blue and green) plasmonic lasers prepared by using the epitaxial Al films. The measured lasing thresholds are comparable with the best available data obtained on the Ag films. According to these result, we suggest that epitaxial ALOSA films are a versatile plasmonic material platform in the UV and full visible spectral regions.

Plasmonic Nanolasers: Pursuing Extreme Lasing Conditions on Nanoscale

“…Owing to the ultralow surface roughness and excellent uniformity, these films exhibited the lowest loss reported ever. For example, the ε′′ of a 45 nm epitaxial with 2 nm Al 2 O 3 /1.5 nm MgO capped Ag film is 2 times smaller at VIS region than that previously reported, and from NIR to ultraviolet (UV), the ε′′ of an epitaxial Al film on Al 2 O 3 substrate is also smaller than previous data …”

“…Metal films grown by thermal evaporation or electron‐beam evaporation usually present granular or polycrystalline structures with issues of surface roughness and grain boundaries, which may lead to additional damping of plasmonic propagation . Recently, epitaxial Ag/Al films with atomically smooth surface have been grown on some different dielectric platforms, such as Ag on Si(111) and Al on Al 2 O 3 (0001). Owing to the ultralow surface roughness and excellent uniformity, these films exhibited the lowest loss reported ever.…”

“…Benefited from the diverse semiconductor materials, the lasing spectrum has been extended to UV and NIR that covering the whole VIS region . For example, in 2014, Lu et al demonstrated an all‐color plasmonic nanolasers with ultralow thresholds using In x Ga 1− x N/GaN core–shell nanorods supported on an Al 2 O 3 ‐capped epitaxial Ag film (Figure c) .…”

“…To reduce the lasing threshold, a number of efforts, including improving surface quality of the plasmonic materials and using higher gain materials have been successfully made. Especially, based on the axial InGaN/GaN quantum wells and Ag substrate (Figure e,f), Tao et al realized a room‐temperature continuous‐wave‐pumped plasmonic nanolaser at 490 nm wavelength with threshold down to 300 W cm −2 …”

Low threshold room-temperature UV surface plasmon polariton lasers with ZnO nanowires on single-crystal aluminum films with Al2O3 interlayers

“…22 Through MBE, epitaxial growth of atomically smooth aluminum on silicon has been achieved. 23,24 The purpose of inserting a dielectric interlayer is to alleviate optical loss caused by metallic regions in addition to matching the boundary conditions for electromagnetic wave at the interface. Plasmonic lasers with different thicknesses of Al 2 O 3 layer were prepared to nd ultra-strong connement of optical eld in the subwavelength regime and obtain ultralow threshold laser operation, which is considerably lower than ZnO/Al and Zno/Al 2 O 3 /Al plasmonic lasers reported previously.…”