Multi-octave spectral comb from a gas-filled hollow fiber

Abstract

Attoscience is the production of light pulses lasting on the order of a millionth of a billionth of a second. These pulses make it possible to measure and control physical and chemical processes extremely rapidly. The pulses are conventionally produced from an initial optical excitation using high-harmonic generation (HHG) in a gas-phase (gaseous) medium. The result is an ultrabroad, comblike optical spectrum, with mutually coherent ‘teeth’ spanning spectral regions from the visible to XUV and soft x-ray. Up to now, HHG has demanded complex, cumbersome, pulsed lasers to drive the gas-phase medium. These pump lasers emit ultrashort pulses (<10fs) with extraordinarily high instantaneous power (≥1MW). The stringent requirements on the driving lasers limited their use to a few specialized laboratories. Recently, our team from the University of Bath collaborated with the Technical University of Denmark and the University of Oregon to demonstrate a much simpler system.1 We generated a coherent spectral comb, covering more than three octaves of frequency, using a single pump laser that emitted moderately powerful, quasi-continuous infrared pulses. The novel generation mediumwas a compact photonic structure consisting of an∼1m length of a newly designed, hollow-core photonic-crystal fiber (HC-PCF), shown in Figure 1. Although prior work had found unprecedented nonlinearity at low optical powers in HC-PCF, the spectral width was limited by the modest bandgap achievable in the fibers used. The results1 represent two advances: a new theory of mode guiding in these fibers, and harmonic generation with unprecedented low powers and long pulses. The first advance theoretically relates photonic guidance in HC-PCF to the ‘Von Neumann–Wigner’ bound and quasi-bound states within a continuum. In contrast to common intuition, the HC-PCF concurrently accommodates both discrete core-guided modes and Figure 1. The hollow-core photonic-crystal fiber (HC-PCF), as seen under an optical microscope.