Fresnel-Reflection-Free Self-Aligning Nanospike Interface between a Step-Index Fiber and a Hollow-Core Photonic-Crystal-Fiber Gas Cell Riccardo Pennetta, * Shangran Xie, Frances Lenahan, Manoj Mridha, David Novoa, and Philip St.J. Russell Max Planck Institute for the Science of Light, Staudtstrasse 2, 91058 Erlangen, Germany (Received 25 April 2017; revised manuscript received 29 May 2017; published 14 July 2017) We report a fully integrated interface delivering efficient, reflection-free, single-mode, and self-aligned coupling between a step-index fiber and a gas-filled hollow-core photonic crystal fiber. The device offers a universal solution for interfacing solid and hollow cores and can be sealed to allow operation either evacuated or at high pressure. Stimulated Raman scattering and molecular modulation of light are demonstrated in a H 2 -filled hollow-core photonic crystal fiber using the device. DOI: 10.1103/PhysRevApplied.8.014014 I. INTRODUCTION The development over the past 20 years of low-loss hollow-core photonic crystal fibers (HC PCFs), that guide by antiresonant reflection or photonic band-gap effects, has fostered advances in applied fields such as high power laser delivery, telecommunications and fiber sensing [1]. By eliminating beam diffraction, HC PCFs make possible intense light-gas interactions over distances thousands of times longer than the Rayleigh length, leading to funda- mental advances in nonlinear and quantum optics [2,3], atomic physics [4–6], and gas spectroscopy [7]. In this connection, macroscopic gas cells or vacuum chambers are typically employed and the laser light is launched into the system through windows using standard optical compo- nents. While this approach is very effective, the dimensions of the system are limited by the size of the gas cell and scaling down is difficult. The sensitivity of the system alignment to external perturbations also becomes a crucial issue, especially at high power levels, since even tiny misalignments can increase the overlap between the incident beam and the fiber microstructure. An alternative solution involves the direct fusion splicing of a solid-core single-mode fiber (SMF) to the HC PCF [8]. While the resulting gas cell is much more compact and stable compared to free-space arrangements, an efficient coupling (1–2-dB insertion loss) can only be achieved if the mode field diameters (MFDs) match [9]. This strongly limits the versatility; for example, the MFD for a standard SMF is about 10 μm, so that low-loss single-mode coupling to HC PCF is only possible for hollow-core diameters of about 14.5 μm. The glass-air interface also creates unavoid- able back reflections, which can be highly detrimental, especially at high power [10]. In addition, broadband- guiding HC PCFs with ultrathin walls (e.g., kagome-style or single-ring HC PCFs designed for guidance in the ultraviolet [11]) are unsuitable for fusion splicing, since the microstructure is easily damaged even at modest splicing temperatures, resulting in dramatic optical loss. Recently, a new technique for launching light from a SMF to a HC PCF was reported [12]. It consists of a silica nanospike, fabricated by thermally tapering a SMF to a tip FIG. 1. (a) Photograph of the gas cell beside a one-Eurocent coin. (b) Optical micrograph of the central portion of the gas cell containing a nanospike inserted into the HC PCF. (c)–(f) The fabrication procedure (see text). The black arrows indicate the insertion direction of the capillaries, and the red arrows show the positions where the vacuum epoxy is applied. * riccardo.pennetta@mpl.mpg.de PHYSICAL REVIEW APPLIED 8, 014014 (2017) 2331-7019=17=8(1)=014014(5) 014014-1 © 2017 American Physical Society