A broader spectrum without losing energy
A research team at Japan’s RIKEN institute has demonstrated a way to amplify an exceptionally brief mid-infrared laser pulse while retaining the broad range of wavelengths needed to make it shorter than one optical cycle. The system produced pulses with an energy of 50 millijoules, a duration of 6.7 femtoseconds and a spectrum spanning 1.4 octaves.
The work combines a newly developed waveform-synthesised seed generator with an amplification method known as HNC-DCOPA. The seed is the initial, relatively weak light that is strengthened by the laser system. Preserving its unusually broad spectrum during amplification matters because compressing a pulse into a fraction of a light cycle requires many wavelengths to remain coordinated.
The researchers reported that the output energy was more than 100 times higher than previously reported for sub-cycle laser pulses. The paper describes the system as a 100-millijoule-class laser, while its measured pulse energy was 50 millijoules.
Less than one cycle
The measured pulse lasted 6.7 femtoseconds. At the reported central wavelength of about 2.26 micrometres, one cycle of the light’s electric field takes roughly 7.5 femtoseconds. The pulse therefore contained about 0.88 cycles: it ended before the field completed one full oscillation.
That distinction is important for experiments that use intense laser fields to drive very rapid processes. A sub-cycle pulse can give researchers finer control over when the electric field rises and falls than a pulse containing several cycles. Producing one with substantial energy is challenging, however, because the pulse must retain a very broad spectrum as it is amplified.
To create the seed, the team generated different wavelength regions along separate optical paths and brought them together with their waveforms precisely aligned. The combined seed covered 1.4 octaves, spanning approximately 1.2 to 3.2 micrometres. The researchers then used HNC-DCOPA amplification, which combines different nonlinear crystals to amplify a wide range of wavelengths, while preserving that bandwidth.
Attosecond light demonstrated
The team also used the laser to drive high-harmonic generation in a gas, producing light in the attosecond range. They varied the carrier-envelope phase, which describes the position of the electric wave within the pulse’s overall envelope, and observed corresponding changes in the high-harmonic spectrum. The measured behaviour was compared with numerical simulations.
A duration of about 35 attoseconds was calculated as the Fourier-limited pulse width from the measured spectrum. This is a theoretical estimate of the shortest pulse that spectrum could support under ideal compression, rather than a direct measurement of an isolated attosecond pulse lasting 35 attoseconds.
The result builds on earlier work developing broadband amplification for ultrashort mid-infrared light. By extending the usable spectrum and raising the pulse energy, the new system offers a stronger driver for experiments seeking shorter and more energetic attosecond emissions.
Further steps remain prospective
The researchers say their amplification approach could be extended to a high-output half-cycle laser. That possibility is supported by numerical simulations, and further development is planned; the current experiment did not produce such a half-cycle output.
Likewise, the work does not report the creation of zeptosecond pulses. Such pulses, far shorter than attosecond pulses, are discussed as a possible longer-term application of advances in laser and high-harmonic technology. The present achievement is the measured sub-cycle laser and its demonstrated use in attosecond-generation experiments.
The research was led by Kaito Nishimiya, Rambabu Rajpoot and Eiji J. Takahashi at RIKEN’s Center for Advanced Photonics. The study appeared online on 22 October.
