Ultra-small low-threshold mid-infrared plasmonic nanowire lasers based on n-doped GaN

Abstract

An ultra-small mid-infrared plasmonic nanowire laser based on n-doped GaN metallic material is proposed and studied by the finite-difference time-domain method. In comparison with the noble metals, nGaN is found to possess superior permittivity characteristics in the mid-infrared range, beneficial for generating low-loss surface plasmon polaritons and achieving strong subwavelength optical confinement. The results show that at a wavelength of 4.2 µm, the penetration depth into the dielectric is substantially decreased from 1384 to 163 nm by replacing Au with nGaN, and the cutoff diameter of nGaN-based laser is as small as 265 nm, only 65% that of the Au-based one. To suppress the relatively large propagation loss induced by nGaN, an nGaN/Au-based laser structure is designed, whose threshold gain has been reduced by nearly half. This work may pave the way for the development of miniaturized low-consumption mid-infrared lasers.

Keywords: Laser; Mid-infrared; Nanowire; Plasmonic; n-doped GaN.

Fig. 1

Wavelength-dependent real and imaginary parts of the permittivity of Au a and nGaN ($n=3.7\times {10}^{20}{\text{cm}}^{-3}$) b

Fig. 2

Schematic structure of plasmonic NW laser a and mode effective index of plasmonic NW lasers b

Fig. 3

Wavelength-dependent penetration depth into dielectric a and metallic b layer of SPPs on Au and nGaN substrates

Fig. 4

The transverse modal profiles of plasmonic NW laser on Au a and nGaN substrate b (where the NW diameter = 500 nm). The inset on the left shows the normalized electric field distribution along Y-axis

Fig. 5

Diameter-dependent mode confinement factor (CF) a, end facet reflectivity b, propagation loss c, and threshold gain d for Au-based (gold lines) and nGaN-based (red lines) NW lasers. The inset in d provided a physical intuition for the cutoff diameter’s difference

Fig. 6

a Schematic structure of the nGaN/Au-based NW laser. b Electric field magnitude distribution of nGaN and nGaN/Au-based NW lasers along the direction perpendicular to the substrate surface. The transverse electric field distributions of these plasmonic NW lasers are c and d, respectively

Fig. 7

Propagation loss a and threshold gain b versus NW diameter for the plasmonic laser on nGaN (red lines) and nGaN/Au (green lines) substrate