CHOU ET AL . VOL. 9 ’ NO. 4 ’ 3978–3983 ’ 2015 www.acsnano.org 3978 April 08, 2015 C 2015 American Chemical Society Ultrastrong Mode Confinement in ZnO Surface Plasmon Nanolasers Yu-Hsun Chou, †,‡ Bo-Tsun Chou, § Chih-Kai Chiang, ) Ying-Yu Lai, ‡ Chun-Ting Yang, ‡ Heng Li, ‡ Tzy-Rong Lin, ) ,^ Chien-Chung Lin, † Hao-Chung Kuo, ‡ Shing-Chung Wang, ‡ and Tien-Chang Lu * ,‡ † Institute of Lighting and Energy Photonics, National Chiao Tung University, Tainan, Taiwan, ‡ Department of Photonics and § Department of Electronics Engineering, National Chiao Tung University, Hsinchu, Taiwan, and ) Institute of Optoelectronic Sciences and ^ Department of Mechanical and Mechatronic Engineering, National Taiwan Ocean University, Keelung, Taiwan T he quest for small coherent light sources has never been stopped, as these micro- to nanoscale light sources not only are essential for small-footprint, low power consumption, high-density, and parallel signal processing applications 1À11 but also provide an insightful way to inves- tigate the interaction between light and matter. 5À7,9,10,12À15 Several designs have been developed to scale down the optical cavity volume to contain only a few photon modes, such as photonic crystal defect type lasers, 2,16 microdisk lasers, 17 and nanowire lasers. 1,11 These lasers, however, require a cavity size on the order of a few (λ/n) 3 to sustain a proper mode profile with a reason- able cavity Q value. Recently, optical cavities surrounded by metal claddings have been developed to reduce the cavity volume because the optical field penetrating into the metal claddings decays so rapidly that the optical mode can be further shrunk at the cost of a lower cavity Q value because of the strong absorption of the metal. 3,8,18 However, the metal can provide a method for drastically diminishing the cavity mode beyond the diffraction limit by forming a surface plasmon at the interface of the metal and dielectric layers. 19 The ultrasmall electromagnetic field distribution of the surface plasmon mode can substantially facilitate the interaction between light and matter by enhancing the Purcell factor. 20À22 Furthermore, a novel type of surface plasmon- based amplification of stimulated emission of radiation (spaser) has been proposed 15 and demonstrated. 5 A group of Au nanospheres 5 and Au nanorods 23 coated with a gain medi- um and a two-dimensional array of protruding Au bowtie nanostructures 14 sitting on the active medium facilitates the spasing action based particularly on the localized surface plasmon mode. Additionally, an unambiguous demonstration of a single nanolaser has been performed using the nanowire gain medium lying on the metal separated by a thin di- electric layer to form a FabryÀPerot-type sur- face plasmon (or surface plasmon polariton) cavity. 6,10 Because the emitted photons have one-to-one corresponding characteristics to the cavity surface plasmons, these photons also show a coherent signature. A higher cavity Q value can be realized using a nano- square to form a whispering gallery surface plasmon mode facilitated by the total inter- nal reflection. 7 With the improvement of a cavity Q value, nanolaser operation can be performed at room temperature. * Address correspondence to timtclu@mail.nctu.edu.tw. Received for review December 28, 2014 and accepted April 8, 2015. Published online 10.1021/acsnano.5b01643 ABSTRACT Nanolasers with an ultracompact footprint can provide high-intensity coherent light, which can be potentially applied to high-capacity signal processing, biosensing, and subwavelength imaging. Among various nanolasers, those with cavities surrounded by metals have been shown to have superior light emission properties because of the surface plasmon effect that provides enhanced field confinement capability and enables exotic lightÀmatter interaction. In this study, we demonstrated a robust ultraviolet ZnO nanolaser that can operate at room temperature by using silver to dramatically shrink the mode volume. The nanolaser shows several distinct features including an extremely small mode volume, a large Purcell factor, and a slow group velocity, which ensures strong interaction with the exciton in the nanowire. KEYWORDS: nanolaser . surface plasmon . ZnO . nanowire . silver ARTICLE Downloaded by NATL CHIAO TUNG UNIV on September 15, 2015 | http://pubs.acs.org Publication Date (Web): April 14, 2015 | doi: 10.1021/acsnano.5b01643