Nano Res. 2011, 4(8): 807– 815 807 Tailoring the Diameter of Single-Walled Carbon Nanotubes for Optical Applications Ying Tian 1 , Marina Y. Timmermans 1 , Samuli Kivistö 2 , Albert G. Nasibulin 1 ( , ), Zhen Zhu 1 , Hua Jiang 1 , Oleg G. Okhotnikov 2 , and Esko I. Kauppinen 1 ( , ) 1 NanoMaterials Group, Department of Applied Physics and Center for New Materials, Aalto University, Espoo, 00076, Finland 2 Optoelectronics Research Centre, Tampere University of Technology, P. O. Box 692, Tampere, 33101, Finland Received: 16 December 2010 / Revised: 20 March 2011 / Accepted: 11 April 2011 © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011 ABSTRACT Single-walled carbon nanotubes (SWCNTs) with specific diameters are required for various applications particularly in electronics and photonics, since the diameter is an essential characteristic determining their electronic and optical properties. In this work, the selective growth of SWCNTs with a certain mean diameter is achieved by the addition of appropriate amounts of CO 2 mixed with the carbon source (CO) into the aerosol (floating catalyst) chemical vapor deposition reactor. The noticeable shift of the peaks in the absorption spectra reveals that the mean diameters of the as-deposited SWCNTs are efficiently altered from 1.2 to 1.9 nm with increasing CO 2 concentration. It is believed that CO 2 acts as an etching agent and can selectively etch small diameter tubes due to their highly curved carbon surfaces. Polymer-free as-deposited SWCNT films with the desired diameters are used as saturable absorbers after stamping onto a highly reflecting Ag-mirror using a simple dry-transfer technique. Sub-picosecond mode-locked fiber laser operations at ~1.56 μm and ~2 μm are demonstrated, showing improvements in the performance after the optimization of the SWCNT properties. KEYWORDS Single-walled carbon nanotube, diameter-controlled synthesis, optical absorption spectroscopy, saturable absorption, mode-locked laser 1. Introduction Single-walled carbon nanotubes have been widely explored in attempts to take advantage of their out- standing electronic, mechanical and optical properties for many applications, and extensive efforts have been made to develop various fabrication techniques to optimize the growth of CNTs [1]. Among the different synthesis methods, chemical vapor deposition (CVD) processes have particularly attracted a great deal of attention due to their efficiency and flexibility in view of the potential applications of CNTs in nanoelectronics [2, 3]. The aerosol CVD approach, in particular, has potential for large scale SWCNT growth since it is a continuous process involving both catalyst particle formation and SWCNT growth. Recently, SWCNTs have been reported as excellent saturable absorbers for use in solid state and fiber lasers because of their fast recovery time, low saturation intensity, large flexibility and environmental robustness [4–6]. However, one bottleneck for their widespread applications is the limited control of diameter and Nano Res. 2011, 4(8): 807– 815 ISSN 1998-0124 DOI 10.1007/s12274-011-0137-6 CN 11-5974/O4 Research Article Address correspondence to Albert G. Nasibulin, albert.nasibulin@aalto.fi; Esko I. Kauppinen, esko.kauppinen@hut.fi