Flying Particle Microlaser and Temperature Sensor in Hollow-Core Photonic Crystal Fiber

Whispering-gallery mode (WGM) resonators combine small optical mode volumes with narrow resonance linewidths, making them exciting platforms for a variety of applications. Here we report a flying WGM microlaser, realized by optically trapping a dye-doped microparticle within a liquid-filled hollow-core photonic crystal fiber (HC-PCF) using a CW laser and then pumping it with a pulsed excitation laser whose wavelength matches the absorption band of the dye. The laser emits into core- guided modes that can be detected at the endfaces of the HC-PCF. Using radiation forces, the microlaser can be freely propelled along the HC-PCF over multi-cm distances—orders of magnitude further than in previous experiments where tweezers and fiber traps were used. The system can be used to measure temperature with high spatial resolution, by exploiting the temperature-dependent frequency shift of the lasing modes, and also for precise delivery of light to remote locations.

1. INTRODUCTION
   Whispering-gallery mode (WGM) resonators are powerful tools for a variety of applications including quantum optics, nonlinear optics [1] and sensing [2]. By exploiting the sensitivity of the cavity resonances to changes in the resonator geometry and environment, sensing of physical quantities [3,4] as well as chemical and biological samples even down to the single molecule level [5], has been demonstrated. In most experiments passive resonators are used, relying on evanescent excitation of WGMs using tapered waveguides or prism coupling [6]. These techniques require close proximity between the resonator and the coupler. In contrast, active WGM resonators containing a gain medium permit remote excitation and collection of the emitted spectrum, even if the resonator is placed in inaccessible environments such as biological samples [7]. Moreover, when operated above threshold, linewidth narrowing of the resonances and increased oscillator strength improves the sensing performance compared to passive resonators [8].
   Optical tweezers and related techniques [9,10] allow precise control of the position of microparticles, potentially permitting position-dependent WGM-based sensing [11]. It was shown that active WGM resonators can lase when captured in an optical tweezer or fiber trap [12,13]. Although considerable effort has been made to enhance the manipulation range of optical tweezers and fiber traps, it is limited to a few mm or less by the Rayleigh range or the dimensions of the optical system, even when complex experimental configurations involving spatial light modulators [14] or non-diffracting Bessel beams are used [15]. Hollow-core photonic crystal fiber (HC-PCF) allows optical trapping and propulsion of individual microparticles within the fiber core over distances limited only by the fiber loss. The particle can be freely moved along the fiber axis and is protected from unwanted external perturbations [16]. Recently, a “flying particle sensor” using trapped particles inside HC-PCF was reported [17,18]
   Here we report a “flying” WGM microlaser consisting of a dye-doped particle optically trapped and propelled within the core of a liquid-filled HC-PCF. The microparticle laser is pumped by sub-ns pulses at 532 nm, which are launched into the fundamental core mode along with a CW trapping beam at 1064 nm. Laser emission at ~590 nm is collected at the fiber endfaces. Using the thermally-induced frequency-shift of the lasing modes, we show that this flying WGM microlaser can be used to remotely measure the temperature distribution along a HC-PCF with a spatial resolution of ~5 mm.
2. EXPERIMENTAL SETUP The experimental setup is sketched in Fig. 1. One end of a kagomé-style HC-PCF with 30 µm core diameter (SEM shown in the inset) was mounted inside a liquid cell connected to a syringe pump to allow filling with D2O. The typical fiber length in the experiment was ~30 cm. The loose end was placed inside a polydimethylsiloxane (PDMS) chamber containing a solution of commercial dye-doped polystyrene microparticles (λex = 532 nm, λem = 570 nm from microParticles GmbH) with diameter of ~ 15.5 µm. Light was launched into the HC-PCF via glass windows on the PDMS chamber and the liquid cell, and the system was optimized to ensure that mainly the LP01-like core mode was excited.   An external optical tweezer system (operating at 1064 nm wavelength, not shown in Fig. 1) was installed below the PDMS chamber and used to trap a single particle and position it in front of the fiber core. Once the particle was trapped, the tweezing power was reduced and the 1064 nm propulsive power incre

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