Octave Spanning Supercontinuum Generation in Ta₂O₅ Waveguides Driven by Femtosecond $2.1-\mu \mathrm{m}$ Laser

Optical frequency combs beyond 1.5 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu\mathrm{m}$</tex> are increasingly attractive for applications in attosecond science and spectroscopy due to their ability to provide precise and stable frequency references. A fully stabilized frequency comb requires measurement and stabilization of the carrier-envelope offset frequency <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(f_{\text{CEO}}$</tex>), typically achieved using an f-to-2f interferometer and a coherent octave-spanning supercontinuum, calling for short-pulse driving lasers. For example, a multi-stage Tm-fiber-based system has been developed to achieve a pulse duration of 29 fs at 600-nJ pulse energy, where 4% of the pulses are coupled into a highly nonlinear fiber for characterizing and subsequently stabilizing <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_{\text{CEO}}$</tex> [1]. Meanwhile, integrated waveguides have emerged as an attractive approach for supercontinuum generation and self-referencing of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_{\text{CEO}}$</tex> at very low driving pulse energies, due to their high nonlinearity and dispersion engineering capabilities [2]. Recently, researchers have demonstrated 3-octave-spanning supercontinuum generation in periodically poled thin-film lithium niobate using a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2-\mu \mathrm{m}$</tex> optical parametric oscillator [3]. An <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_{\text{CEO}}$</tex> beat was detected with only 22-pJ input pulse energy. However, to the best of our knowledge, direct <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_{\text{CEO}}$</tex> detection of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2-\mu\mathrm{m}$</tex> oscillators using integrated waveguides has not been reported yet. Here we demonstrate octave-spanning supercontinuum generation in chip-based Ta<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>O<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf> waveguides pumped by a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2.1-\mu\mathrm{m}$</tex> femtosecond mode-locked laser, showing the potential for self-referencing <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text {of}f_{\text{CEO}}$</tex> in the mid-infrared.