Engineering the Berreman mode in mid-infrared polar materials

Area of Science:

• Materials Science
• Optics
• Condensed Matter Physics

Background:

• The Berreman mode is a leaky mode in the mid-infrared spectrum.
• Aluminum nitride (AlN) is an epsilon-near-zero material supporting the Berreman mode.
• Dielectric environments significantly influence optical mode properties.

Purpose of the Study:

• To demonstrate coupling to and control over the Berreman mode.
• To investigate the impact of different dielectric environments on the mode's broadening and dispersion.
• To engineer the dielectric environment using AlN/GaN heterostructures.

Main Methods:

• Fabrication of subwavelength AlN films on various dielectric substrates (weakly negative, strongly negative, and positive permittivity).
• Incorporation of ultra-thin AlN layers into GaN/AlN heterostructures.
• Angle-dependent reflection measurements near the longitudinal optical phonon energy to observe coupling.
• Numerical modeling to compare with experimental dispersion data.

Main Results:

• Successful coupling to the Berreman mode was observed in all fabricated samples.
• Measured Berreman mode dispersion showed good agreement with numerical simulations.
• Quantified differences in mode dispersion and broadening across various dielectric environments.
• Observed a 13 cm^-1 red-shift in the Berreman mode energy for the heterostructure sample.

Conclusions:

• The dielectric environment critically affects the broadening and dispersion of the Berreman mode.
• Engineering dielectric environments, including heterostructures, offers a method for controlling this mid-infrared leaky mode.
• The findings are relevant for applications utilizing epsilon-near-zero materials and mid-infrared optics.