Ultrafast DUV/VUV light

Spectrally bright deep (200-300 nm) and vacuum (100-200 nm) ultraviolet light has a wide range of potential applications, currently frustrated by a lack of easy-to-use table-top lasers in these spectral regions. Coherent broadband DUV/VUV sources could be used to produce sub-femtosecond pulses for precise temporal, spectral and spatial excitation of the electronic resonances of many atoms, molecules and bulk materials, and thus form the basis for advanced time-resolved pump-probe or photoemission spectroscopy.

The Division is working on the generation of broadband coherent light from the extreme UV to the mid-IR, using a variety of different techniques, and on high-harmonic generation with few-cycle pump pulses generated by self-compression in noble-gas-filled hollow core PCF. Much of the work deals with the interaction of solitons with photo-induced plasmas and ultrafast molecular dynamics. Increasingly these novel sources are being used in applications, often in collaboration with other groups.

The work has resulted in the formation of a start-up company, ultralumina.

Supercontinuum generation in the VUV

We have used soliton dynamics in neon- and helium-filled kagomé HC-PCFs to produce tunable, coherent, broadband VUV pulses from 120 nm to beyond 180 nm, with efficiencies exceeding 1% and VUV pulse energies in excess of 50 nJ [Ermolov (2015)]. In addition, in helium-filled fibres we were able to generate a flat and bright supercontinuum spanning from 115 to 1000 nm – over three octaves (2x2x2) wide (Fig. 1). 

In collaboration with a group at CFEL in Hamburg, pulses at 145 nm (8.6 eV) were generated in neon-filled kagomé-PCF and used for angle-resolved photoemission spectroscopy (ARPES) of the surface electron structure of the topological insulator Bi2Se3 [Bromberger (2015)].

The full supercontinuum generated in a He-filled kagomé-PCF. The blue curve was measured using a VUV spectrometer and the brown curve using a UV-NIR spectrometer. The solid black line is the simulated spectrum. The dashed vertical line marks the ZDW (N = normal, A = anomalous GVD).

High energy fs DUV pulses at 10 MHz

Using a gas-filled kagomé-PCF we demonstrated nonlinear compression of a 1030 nm, 300 fs (FWHM) fibre laser pulses to sub-30-fs, and high energy tunable ultraviolet emission at 9.7 MHz repetition rate through dispersive wave emission from self-compressed higher-order solitons (Fig. 2). The UV pulses are tunable between at least 270 and 320 nm, simply by changing the gas pressure in the fibre, and have up to 70 nJ of pulse energy and 0.7 W of average power [Köttig (2015)].

Fig. 2: (a) Experimental output spectrum when an ARR-PCF is filled with 10 bar of Ar, yielding a UV emission band at ~278 nm. (b) Average power scaling with repetition rate in the UV (~278 nm). (c) Average power in the UV for each pressure-tuned wavelength at 9.7 MHz repetition rate. The wavelength was tuned from ~278 to ~320 nm by increasing the Ar pressure from 10 to 15 bar.

Ultrafast 4-wave-mixing to the DUV

As an alternative to dispersive wave emission we generated DUV light pulses via four-wave mixing in a Ar–filled kagomé-PCF [Belli (2016)]. Ultrafast four-wave mixing permits independent control over both pulse bandwidth and central frequency, and potentially offers even broader bandwidth, higher conversion efficiencies, and reduced energy fluctuations (through gain saturation). We demonstrated up to 30% conversion efficiency from a pump wavelength of 400 nm to a signal at 266 nm, when also seeding at the idler wavelength of 800 nm. At the optimum phase-matching pressure, and optimal setting of the delay between the pump and idler, we achieved 490 nJ at the signal wavelength, with a 10.4 nm broad spectrum, which would support ~10 fs pulses. The energies required are easily within reach of high repetition-rate pump sources, offering average power scalability. 

(a) Experimental output energy for the signal, idler and pump as a function of input pump energy for 3.16 bar Ar and a fixed idler input energy of 0.26 µJ. (b) Experimental conversion efficiencies as a function of pressure at a fixed pump energy of 1.55 µJ and idler energy of 0.48 µJ (the inset shows the spectra achieved at two different pump/idler delays; (c) calculated gain for 3.25 bar Ar.

Mid IR supercontinuum in gas–filled single-ring HC-PCF

We made use of a single-ring ARR-PCF structure to generate an ultra-wide, high-energy supercontinuum extending into the mid-infrared [Cassataro (2016)]. The experiments were performed by pumping the single-ring HC-PCF with pulses generated by a commercial OPA system, wavelengths between 1.5 and 1.9 µm, duration 28 fs and energies between 1 and 20 µJ. The figure shows the supercontinuum generated in a 5-cm-long fibre filled with noble gases and pumped at 1.5-1.7 µm. In-fibre soliton dynamics (in particular dispersive-wave emission and subsequent soliton recoil) result in a supercontinuum spanning more than 3 octaves (at the –30 dB level), reaching 3 µm. Such broad spectra could be used to generate single-cycle pulses in the mid-IR. 

(a) Output spectrum generated in a 5-cm-long fibre with 51 µm diameter filled with 5 bar argon, pumped in the anomalous dispersion regime at 1.5 µm. (b) Output spectra generated in the same fibre filled with 18 bar krypton and pumped at 1.7 µm.

High harmonic generation

In recent work we explored the use of the plasma-driven soliton self-frequency blue-shift [Hoelzer (2011)] to frequency-tune high harmonic generation. A He-filled kagomé-style ARR-PCF was used to compress 25 fs, 800 nm pulses, with energies in the range 10-50 µJ, and to deliver them to a pulsed Ar-jet in a vacuum chamber so as to generate high harmonics in the extreme ultraviolet. The He pressure was selected so that the pulses propagated in the anomalous dispersion regime with relatively low soliton order (between 2 and 5, depending on the input energy). Under these conditions, the pulse gets shorter with increasing input energy, its peak intensity rises and the influence of ionization becomes more important, eventually causing the self-compressing pulse to blue-shift in frequency. As a result, the harmonics generated by blue-shifted solitons also shift to shorter wavelengths as the input energy is increased. In this way, the harmonic frequency could be continuously tuned over a range greater than the spacing between them [Tani (2016)].

High harmonic (left) and pump pulse (right) spectra for increasing launched pulse energy. The white curve on the right-hand side tracks 19 times the wavelength of the 19th harmonic.