Size reduction of a semiconductor nanowire laser using metal coating A. V. Maslov a and C. Z. Ning b a NASA Ames Research Center, Mail Stop 229-1, Moffett Field, CA 94035, USA; b Department of Electrical Engineering, Arizona State University, Tempe, AZ 95287, USA ABSTRACT We explore the possibility of coating semiconductor nanowires with metal (Ag) to reduce the size of nanowire lasers operating at photon energies around 0.8–2 eV. Our results show that the material gain of a typical III-V semiconductor in nanowire may be sufficient to compensate Joule losses of such metal as Ag. The most promising mode to achieve lasing seem to be TM 01 near its cutoff. To calculate the guiding properties of metal coated nanowires, we developed a finite-difference discretization approach, the details of which we also present. This approach allowed us to treat accurately the wire/metal interface and to include nonperturbatively the imaginary parts of dielectric constants of the semiconductor core and metal cladding. Keywords: nanowires, lasers, waveguide dispersion, plasmons, finite-difference discretization 1. INTRODUCTION Miniaturization in optoelectronics requires the development of devices with sizes comparable to or smaller than the operating wavelength in vacuum. The size reduction can be achieved using several approaches, from which we will mention just two. The first one is to use waveguides with high dielectric constant surrounded by air. For example, dielectric waveguides can support well-guided modes even for wire radii smaller than the wavelength [1]. This property is now used to make sub-wavelength waveguides using semiconductor or silica nanowires [2, 3]. The second one is to use surface plasmons polaritons guided by metal surfaces that become well localized close to the resonant condition. Semiconductor lasers and light emitting diodes (LEDs) – the most commonly used devices – can potentially benefit from these two approaches to miniaturization. Indeed, very recently lasing in semiconductor nanowires surrounded by air was demonstrated in ultraviolet (GaN, ZnO), visible (CdS) and infrared (GaSb) spectral ranges [4–7]. At the same time, there is a significant progress in using surface plasmon to achieve lasing. Although the possibility to obtain surface plasmon polariton amplification was suggested in the past [8], recently it attracted significant theoretical [9–11] and experimental interests [12]. The use of non-propagating surface plasmons to make lasers is also being explored [13,14]. Free standing nanowires have very poor mode confinement at long operating wavelengths, even despite the rather strong dielectric index contrast. The weak confinement also leads to small values of gain and small facets reflections [15, 16]. Thus, one can imagine increasing their confinement by surrounding a nanowire with metal and thus forming core-shell semiconductor-metal waveguide. In such a waveguide the lowest order modes differ from those in a dielectric waveguide or in a metal-wall waveguide operating in the millimeter and centimeter range: they are surface-plasmon polaritons which are localized at the semiconductor/metal interface. However, if the cross-section decreases they loose their evanescent character in the core region and become propagating. These modes are responsible for the operation of near-field scanning optical tips where they are excited by a HE 11 mode of the fiber, but with rather weak efficiency [17]. Here, we investigate the properties of metal-encased nanowires for the purpose of making ultrasmall lasers. This paper is organized as follows. In Sec. 2 we develop our numerical approach to study the guided properties of cylindrical multi-shell structures. Using this approach we obtain the propagation properties of guided modes and their localization (Sec. 3) as well as modal loss or gain (Sec. 4). In Sec. 4 we draw our conclusions regarding feasibility of using metal encapsulation to decrease the size of nanowire lasers. Further author information: A.V.M. is now at Canon Development Americas, Inc., Irvine, CA; e-mail: alexey.maslov@cda.canon.com.