The vital precursor to attosecond pulses
Is it humanly possible to understand a period of time just 10-18 seconds, or a billion billionth of a second in duration? It’s a true testament to our technology that it can generate such attosecond (as) pulses, which are to a second what a second is to the age of the Universe. On this unimaginably short timescale it’s possible to track closely how electrons surrounding atoms behave – something scientists already do. That knowledge will surely prove beneficial, but will attosecond laser pulses themselves become widely used?
The push for attosecond pulses builds on existing ultrafast capabilities that produce longer femtosecond pulses, already proven as beneficial in micromachining and surgery. Their fleeting pulses make cuts before any heating can damage surrounding materials, or any induced plasma shielding can reduce the effectiveness and reproducibility of cutting. Pulses use miniscule amounts of energy and remove only tiny amounts of material, allowing high-precision cutting. This is especially important for medical surgery, particularly laser eye surgery. Yet it’s questionable whether shrinking pulse durations by orders of magnitude further will improve such commercial uses. And any broader benefits that may emerge also remain some way off – though some are more optimistic about realising them than others.
Central to producing ultrafast pulses is shepherding a broad range of light wavelengths. In the ‘mode-locking’ approach a pulse is created at the point when these different wavelengths are in phase. Titanium-doped sapphire (Ti:sapphire) crystals that emit near-infrared (NIR) 680-1,080nm light when pumped by a laser with an approximately 530nm wavelength have been vital in enabling this. Today Ti:sapphire systems emit few-femtosecond pulses, near the fundamental limit for this material, spanning just a few cycles of the light’s electric field.
Producing attosecond pulses requires shorter wavelength, higher energy laser light, with extreme ultraviolet (EUV) necessary for creating 100as pulses. Attosecond EUV pulses can be generated from NIR femtosecond pulses through processes such as high-order harmonic generation (HHG). ‘A 6fs pulse with a centre wavelength of 1µm contains 1.8 optical cycles,’ explained Károly Osvay, research technology director of the Extreme Light Infrastructure Attosecond Light Pulse Source (ELI-ALPS) being established in Szeged, Hungary. ‘HHG “transfers” the wavelength from 1µm down to something like 40nm and much lower, keeping the number of optical cycles more or less untouched. A 1.8 optical cycle pulse at 40nm therefore lasts 240 attoseconds.’
But before this conversion can be achieved, the NIR pulses need to be amplified in energy from nanojoules to millijoules. This can be done in Ti:sapphire, but steps must be taken to avoid the higher energies damaging the crystal. The best-established technique is chirped-pulse amplification (CPA), where pulses are stretched to reduce their peak intensity and recompressed following amplification.
Much can happen to disturb the different wavelengths involved, noted Nigel Gallagher, senior product manager for research laser systems at Santa Clara, California, headquartered ultrafast laser manufacturer Coherent. ‘Fluctuations of pump power and non-linear effects in Ti:sapphire crystals influence the relative optical paths of the various colours,’ he said. That noise can result in the phase of the optical carrier wave slipping relative to the pulse envelope. Because very short optical pulses contain only a few optical cycles, such phase slip and noise have a significant impact.
Coherent therefore markets its Vitara Ti:sapphire oscillator with a carrier envelope phase (CEP) stabilisation option that controls phase slip. ‘It’s locking the oscillating electric field of the light pulse under the overall pulse envelope,’ Gallagher said. ‘We’re actively stabilising the carrier offset frequency to reduce CEP noise. We detect the carrier offset frequency and generate a signal that goes to the Vitara oscillator.’
Pulses from the Vitara can then be amplified from nanojoule pulse energies up to several millijoules in Coherent’s Legend Elite Ti:sapphire regenerative amplifier. Today the shortest pulses commercial CPA Ti:sapphire amplifiers can produce are around 20fs, said Marco Arrigoni, the director of marketing of Coherent’s scientific business unit. However, such pulses are not short enough to be converted to attosecond durations, he added, so an extra intermediate hollow-fibre compressor amplifier is first needed to shrink them to 5-10fs.
Entering a new timescale in ultrafast lasers
The repetition rate – the number of pulses in a period of time – that such a CPA system can achieve is limited by several factors. They include fluorescence lifetime, available pump power and heating in the Ti:sapphire crystal. An alternative approach is parametric amplification using an amplifying medium that doesn’t absorb light, eliminating the heating problem. Stockport, UK’s Laser Quantum uses such non-absorbing non-linear crystals in its optical parametric chirped-pulse amplifier (OPCPA). The OPCPA transfers energy from 500fs ytterbium-based fibre pump pulses to a broadband signal from the Venteon Dual Ti:sapphire oscillator, explained Thomas Binhammer, managing director of Venteon Laser Technologies, designer of the OPCPA and a Laser Quantum subsidiary.
The Venteon Dual offers a broader wavelength range than typical Ti:sapphire oscillators, added Binhammer, which can be compressed to less than two optical cycles. ‘The Laser Quantum product generates a sub-5fs pulse, with microjoule level energy, and high repetition rate,’ he said. ‘OPCPA technology can directly amplify the broad spectrum associated with these pulses, so that we maintain the pulse duration without complex broadening mechanisms. Laser Quantum’s combination of expertise in ultrashort laser pulses and in-house developed fibre-laser technology allowed us to be the first company to provide a full commercial OPCPA system.’
OPCPAs provide a unique light source for attosecond science, asserted Binhammer, combining short pulses, high intensities and high repetition rates. ‘Attosecond pulses allow us to look at nature at a timescale that was inaccessible to mankind in the past,’ Binhammer underlined. As well as directly producing ultrashort pulses required to generate single attosecond pulses, the Laser Quantum instrument’s high intensities provide a strong electric field that overcomes atomic binding forces. And the benefits of high repetition rates include shortening measurement times by providing data points faster.
Whereas the Venteon OPCPA offers microjoule-energy femtosecond pulses over repetition rates ranging from 200-1,000kHz, Ti:sapphire CEP systems are geared towards the low kilohertz and millijoule level, said Arrigoni. He noted that Coherent also produces a ytterbium pump laser, called Monaco, that could work well in an OPCPA. And he further added that while OPCPAs and Ti:sapphire systems are both suited to CEP stabilisation, it’s not always necessary.
‘CEP is specifically needed if you want isolated attosecond pulses, where you want to know each pulse is a single attosecond pulse,’ Arrigoni said. Sending an unstabilised femtosecond pulse into HHG will instead result in a 5-10fs EUV pulse containing a ‘bunch’ of attosecond pulses. ‘People realise that the complexity of what they have to do after the laser with single isolated pulses is such that if they can do experiments using bunches of attosecond pulses, they would rather do so,’ Arrigoni explained. ‘More than half the people who buy CEP stabilisation systems use it to do bunches. They like the idea of being able to do isolated pulses eventually or that the system is strongly stabilised.’
At Lund University in Sweden, Anne L’Huillier’s team uses both trains of attosecond pulses originating from CEP-stabilised femtosecond beams, and trains that originate from unstabilised beams. Trains are ‘sometimes easier to use and provide natural interferometric measurement, which is useful in some experiments’, L’Huillier explained.
Taking attosecond sources from academia to industry
The Swedish scientists do HHG in a low-pressure gas cell, driven by a variety of femtosecond laser inputs. ‘Attosecond sources are very versatile,’ L’Huillier said. Higher energy, lower repetition rate femtosecond sources are more useful generating pulses for studies such as attosecond pump and attosecond probe experiments, L’Huillier explained. An example would be a pump pulse putting a molecule into an excited state, and probe pulses that follow almost immediately afterwards, collecting information on that excited state.
Lower energy, high repetition rate femtosecond sources are useful for exploring physics at surfaces, for example using photoelectron emission spectroscopy. ‘If you have too many photons on your sample you may ionise several electrons at once; they repel each other and you lose all resolution and that destroys your measurement,’ L’Huillier said. ‘So you don’t want too much energy per pulse, but on the other hand you’re very happy if you have many events per second to obtain both spatial and temporal resolution.’ While OPCPA technology enables the Lund University scientists to do such studies, ‘it needs to be more stable and reliable to enable such experiments’, L’Huillier said.
Perhaps the highest-profile attosecond laser project is ELI-ALPS, which Osvay explained will include four major high peak power ultrafast laser systems. Their few-cycle pulses will produce attosecond pulses through HHG in gases, and also a less commonly-used ‘relativistic oscillating mirror effect’ on solid surfaces. Likewise, the two laser systems especially involved in attosecond generation will employ one more familiar approach to producing femtosecond precursor pulses, and one more novel.
‘The single-cycle laser, called SYLOS, was designed and is being manufactured by two Vilnius, Lithuania-based companies, Light Conversion and EKSPLA,’ Osvay said. ‘It produces sub-6fs pulses at 1kHz via amplification of CEP stabilised broadband pulses in an OPCPA, reaching compressed pulse energy of 100mJ. The other, high repetition system, or HR laser, was developed by a consortium based in Jena, Germany, led by Active Fiber Systems. It generates sub-6fs pulses at 100kHz in a way that has not yet been used for any big facility laser systems. A few 100fs laser pulses are generated and amplified in several fibre amplifiers in parallel. These amplified millijoule-level pulses are coherently combined to get a single pulse with energy of tens of millijoules. In the last step, these pulses are sent through two hollow-core fibres subsequently where their bandwidth is extended, hence their duration is compressed down to the sub-6fs level.’
Single-attosecond pulses should appear at ELI-ALPS’ user stations at the end of 2016, said Osvay, after the driving SYLOS and HR laser systems are delivered in May and July 2016. ‘There is a strong intention to make the facility attractive to industry both with regards to research technology and scientific expertise,’ he stressed. ‘The industrial R&D may involve the femtosecond lasers, and/or the generated attosecond pulses. If there is any industrial interest in launching projects for laser or other light source development, we will be happy to meet those requests.’
For now, commercial exploitation of attosecond pulses will not proceed much further than this, according to Arrigoni. ‘At Coherent we believe that there’s no general commercial application at this point that requires pulses shorter than 100fs,’ he said. ‘Anything below that is highly speculative. Not because it can’t be done, but because the cost-benefit is really limited.’
Gallaher emphasised that Coherent’s CEP systems value can be best seen in labs where scientists are researching electron motion and chemical bonding. ‘There’s no doubt that it’s going to add understanding to fundamental science, but using attoseconds directly in industrial applications seems very far away.’ ‘I think it will be after we retire,’ admitted Arrigoni.
Binhammer pointed out that conversion from NIR femtosecond to EUV attosecond pulses via HHG is still ineffective. However he believes that the work he is doing to create ‘a turnkey OPCPA system with high photon flux’ is an important step towards industrial applications.
‘We are seeing more scientific applications every year,’ he noted. ‘Normally with such new technologies there’s a spin-off that leads to applications that find their way into industry. I don’t see it in the next five years, but I am quite optimistic that there will be downstreaming of this technology into industrial applications.’ The first target would be commercial systems that generate the EUV radiation associated with attosecond pulses, which would also be useful in applications like lithography, Binhammer added.
L’Huillier highlighted one specific example as justification for her own similar hopes. KMLabs, a startup founded by University of Colorado academics Henry Kapteyn and Margaret Murnane, received $5.5 million funding from investors including Intel Capital in November 2015. The funding is intended to accelerate product development efforts for ultrashort-pulse and extreme ultraviolet laser sources. ‘There are more and more applications in research, and there is some interest from industry,’ L’Huillier asserted.
Andy Extance is a freelance science writer based in Exeter, UK.