A New Home for Optical Solitons

Developement of new enhancement cavities at the Laboratory for Attosecond Physics. (Source: T. Naeser)

Solitons are the most stable of all waves. Under condi­tions that result in the dispersion of all other waveforms, a soliton will continue undisturbed on its solitary way, without changing its shape or velocity in the slightest. The self-stabi­lizing properties of solitons explain their immense signi­ficance to the field of laser optics, in parti­cular for the generation of ultra­short light pulses. A team led by Ioachim Pupeza at the Laboratory of Atto­second Physics (LAP) in Munich, which is run jointly by the Max Planck Institute of Quantum Optics (MPQ) and the Ludwig-Maximilian Uni­versity (LMU), has now gene­rated optical solitons in passive free-space reso­nators for the first time.

The technique allows one to compress laser pulses while increasing their peak power, opening up new appli­cations for free-space enhance­ment cavities in the explora­tion of ultrafast dynamics and in precision spectro­scopy. The researchers coupled 350-femto­second infrared laser pulses with a wavelength of 1035 nano­meters and a repetition rate of 100 MHz, into a newly designed passive optical resonator made up of four mirrors and a thin sapphire plate. “The passage of the electro­magnetic field of the optical pulse causes a non-linear change in the refractive index of the crystal,” as Nikolai Lilien­fein explains.

“This results in a dynamic phase shift, which fully compensates for the disper­sion that occurs in the resonator, while at the same time broadening the spectrum of the pulse”, Lilien­fein adds. Since the power losses that inevitably occur in the resonator are simul­taneously compensated for by the interfero­metrically coupled laser source, a soliton can in principle circulate ad infinitum in the resonator. In addition, the researchers developed a highly efficient method for control­ling the energy input to the cavity soliton. In combi­nation, these measures allowed the team to compress the duration of input pulses by almost an order of magnitude to 37 femto­seconds while enhancing their peak power by a factor of 3200.

This enhance­ment-resonator tech­nology opens up new oppor­tunities for the gene­ration of trains of highly precise extreme ultra­violet (XUV) atto­second pulses. This in turn may enable researchers to charac­terize the dynamics of subatomic processes – and in particular to observe the motions of electrons – in even greater detail than was possible hitherto.

“Over the past several years, we have been able to make the unique advan­tages of enhance­ment resonators available for experiments in atto­second physics. This new technique opens a path towards further signi­ficant advances in the pulse power and stabi­lity attainable with such systems, while at the same time reducing the complexity of the experi­mental setup,” says Pupeza. These improve­ments would also be of benefit in the context of XUV frequency-comb spectro­scopy, which is central to the develo­pment of a new gene­ration of optical clocks based on quantum tran­sitions in atomic nuclei. (Source: MPQ)

Reference: N. Lilienfein et al.: Temporal solitons in free-space femtosecond enhancement cavities, Nat. Phot., online 21 January 2019; DOI: 10.1038/s41566-018-0341-y

Link: Attosecond Physics Division, Max-Planck-Institute for Quantum Optics, Garching, Germany