Passively Q-switched femtosecond-laser-written thulium waveguide laser based on evanescent field interaction with carbon nanotubes ESROM KIFLE, 1 PAVEL LOIKO, 2 JAVIER RODRÍGUEZ VÁZQUEZ DE ALDANA, 3 CAROLINA ROMERO, 3 AIRÁN RÓDENAS, 1,4 SUN YUNG CHOI, 5 JI EUN BAE, 5 FABIAN ROTERMUND, 5 VIKTOR ZAKHAROV , 2 ANDREY VENIAMINOV , 2 MAGDALENA AGUILÓ, 1 FRANCESC DÍAZ, 1 UWE GRIEBNER, 6 VALENTIN PETROV , 6 AND XAVIER MATEOS 1, * 1 Universitat Rovira i Virgili, Departament Qu ı ´mica F ı ´sica i Inorgànica, F ı ´sica i Cristal·lografia de Materials i Nanomaterials (FiCMA-FiCNA)-EMaS, Campus Sescelades, E-43007 Tarragona, Spain 2 ITMO University, 49 Kronverkskiy pr., 197101 St. Petersburg, Russia 3 Aplicaciones del Láser y Fotónica, University of Salamanca, 37008 Salamanca, Spain 4 Istituto di Fotonica e Nanotecnologie, Consiglio Nazionale delle Ricerche (IFN-CNR), Piazza Leonardo da Vinci, 32, 20133 Milano, Italy 5 Department of Physics, KAIST, 291 Daehak-ro, Yuseong-gu, 34141 Daejeon, South Korea 6 Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, Max-Born-Str. 2a, D-12489 Berlin, Germany *Corresponding author: xavier.mateos@urv.cat Received 29 June 2018; revised 17 July 2018; accepted 17 July 2018; posted 17 July 2018 (Doc. ID 336536); published 26 September 2018 Surface channel waveguides (WGs) were fabricated in a monoclinic Tm 3 :KLuWO 4  2 crystal by femtosecond direct laser writing (fs-DLW). The WGs consisted of a half-ring cladding with diameters of 50 and 60 μm located just beneath the crystal surface. They were characterized by confocal laser microscopy and μ-Raman spectroscopy, indicating a reduced crystallinity and stress-induced birefringence of the WG cladding. In continuous-wave (CW) mode, under Ti:sapphire laser pumping at 802 nm, the maximum output power reached 171.1 mW at 1847.4 nm, corresponding to a slope efficiency η of 37.8% for the 60 μm diameter WG. The WG propagation loss was 0.7  0.3 dB∕cm. The top surface of the WGs was spin-coated by a polymethyl methacrylate film containing randomly oriented (spaghetti-like) arc-discharge single-walled carbon nanotubes serving as a saturable absorber based on evanescent field coupling. Stable passively Q -switched (PQS) operation was achieved. The PQS 60 μm diameter WG laser generated a record output power of 150 mW at 1846.8 nm with η  34.6%. The conversion efficiency with respect to the CW mode was 87.6%. The best pulse characteristics (energy/duration) were 105.6 nJ/98 ns at a repetition rate of 1.42 MHz. © 2018 Chinese Laser Press OCIS codes: (230.7380) Waveguides, channeled; (140.3540) Lasers, Q-switched; (140.3380) Laser materials. https://doi.org/10.1364/PRJ.6.000971 1. INTRODUCTION Waveguide (WG) lasers emitting in the spectral range of ∼2 μm are of interest for bio- and environmental sensing applications. This is because such a radiation matches spectrally with the ab- sorption of some relevant molecules, such as H 2 O or CO 2 . Typically, laser emission at ∼2 μm is achieved using thulium (Tm 3 ) or holmium (Ho 3 ) ions. In the former case, the emis- sion is due to the 3 F 4 → 3 H 6 transition [1]. Tm 3 ions are at- tractive because of a strong absorption at ∼0.8 μm ( 3 H 6 → 3 H 4 transition), efficient cross-relaxation (CR) enhancing the pump quantum efficiency up to 2 [2], low-threshold behavior, and typically large Stark splitting of the ground state leading to a broadband tuning feature. Highly efficient laser operation was achieved with continuous-wave (CW) Tm WG lasers. In Ref. [3], a “ mixed” Tm:KY, Lu, GdWO 4  2 crystalline channel WG laser gener- ated 1.6 W at 1.84 μm with a record slope efficiency η of 80%, promoted by an efficient CR at a high Tm 3 doping level. The active layer was fabricated by liquid phase epitaxy (LPE) [4]. The WG propagation loss δ was ∼0.1 dB∕cm [5]. The refractive index contrast with respect to the undoped KYWO 4  2 substrate Δn was ∼1 × 10 -3 . Note that the active material belongs to the crystal family of monoclinic double tungstates (DTs), which are well-known for Tm 3 ion doping due to their advantageous spectroscopic properties for polarized light [6]. Research Article Vol. 6, No. 10 / October 2018 / Photonics Research 971 2327-9125/18/100971-10 Journal © 2018 Chinese Laser Press