IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 44, NO. 1, JANUARY 2008 67 Theoretical Investigation of Metal Cladding for Nanowire and Cylindrical Micropost Lasers Vivek Krishnamurthy, Student Member, IEEE, and Benjamin Klein, Member, IEEE Abstract—We investigate the transverse modal properties of cylindrical subwavelength metal-clad nanowire and micropost lasers via rigorous theoretical waveguide analysis, including the effects of finite thickness metal cladding and gain in the core. The results of this analysis show that air–metal surface guided and some hybrid surface guided modes suffer less loss but are less confined to the core, while core–metal surface guided modes are better confined to the core but suffer greater loss. An increase in the thickness of the metal cladding reduces the loss of the core–metal surface guided modes. The modal gain and confinement of the metal-clad cavity are compared to an unclad cavity. Index Terms—Air–metal surface guided mode, core-guided mode, core–metal surface guided mode, cylindrical waveguide, lasers, metal-clad, micropost, nanowire, surface plasmon. I. INTRODUCTION R ECENTLY there has been considerable interest in surface plasmon based lasers [1]–[3] for nanophotonic applica- tions. These devices attempt to achieve lasing with metal-di- electric surface plasmon modes, for which subwavelength-scale cavity confinement is possible. A surface plasmon based laser has been successfully demonstrated at long wavelengths [1]. Also, there has been an investigation of lossless transmission of surface plasmon modes when a metal surface is adjacent to a gain medium [4]. The coupling of stimulated emission into short range and long range surface plasmon modes was also calcu- lated in [3]. A proposal to exploit the stimulated emission in the active region to coherently strengthen the electric field of a sur- face plasmon mode has been made by [5] and [2]. However, all the above investigations have been restricted to planar waveg- uides. There has been an investigation of metallic cylindrical waveg- uides in the long wavelength range for low loss transmission of various modes in the core guided mode regime [8]–[10]. Surface plasmon modes and their loss properties in cylindrical waveg- uides [11], [12] were also calculated, but the analysis was re- stricted to purely TM modes in [11] and the cladding layer was assumed to be of infinite width in [12]. There has also been in- terest in various nanoscale vertically emitting sources, such as nanowire lasers [6] and micropost based cavities [7]. The objec- tive of the present work is to quantitatively investigate the effect of metal cladding with finite thickness on the confinement, loss, Manuscript received April 27, 2007; revised September 6, 2007. The authors are with the Department of Electrical and Computer Engi- neering, Georgia Institute of Technology, Savannah, GA 31407 USA (e-mail: vivek@ece.gatech.edu; ben.klein@gtsav.gatech.edu). Digital Object Identifier 10.1109/JQE.2007.910451 Fig. 1. Metal clad laser structure. and coupling of modes in cylindrical nanowire and micropost lasers. The paper is organized as follows. First, a description of the laser waveguide structure and the mode calculation technique is given. This is followed by results for various core guided modes and dielectic–metal surface guided modes (surface plasmon modes). Finally, conclusions are drawn based on the results. II. STRUCTURE The structure under consideration is shown in Fig. 1. It is a nanowire or micropost waveguide with metal cladding. The core semiconductor is taken to be Ga As [13], [14] with a refractive index of 3.6. The metal used in the calculation is gold with a refractive index of [15]. The entire waveguide is surrounded by air. The mirrors capping the laser cavity above and below are not considered for the purposes of this paper; the reflectivity of the endcaps can be made arbitrarily large using dielectric or metallic mirrors. Therefore, for our pur- poses the cylindrical laser waveguide is considered to be of in- finite length. All calculations are done for a fixed wavelength of 1 m. The gain in the active semiconductor cavity will depend on the detailed internal structure of the nanowire and the method of pumping. The majority of experimentally realized nanowire and micropost lasers are optically pumped [2], [3], [6], [7] in the visible and UV range. In many nanowire structures, elec- trical pumping is not possible because of the absence of pn junc- tion and controlled doping [16]. However, this difficulty can be overcome with advancements in materials science. For the pur- poses of this work, it is not necessary to restrict our analysis to 0018-9197/$25.00 © 2007 IEEE