High-dispersion mirrors shrink femtosecond laser

Ti:sapphire femtosecond lasers currently use optical systems based on prisms or diffraction gratings to stretch and recompress pulses before and after amplification. These optical systems are complex, rather lossy and alignment sensitive. Researchers from Ferenc Krausz's group at the Ludwig-Maximilians University and Max-Planck Institute of Quantum Optics, both in Garching, Germany, believe high-dispersion mirrors (HDMs) are the solution (Optics Express 16 10220).

Vladimir Pervak and his colleagues believe low-loss, HDMs can take over the role of prisms and possibly gratings in conventional chirped-pulse amplifier (CPA) systems with the added benefit of providing high-order dispersion control.

The group has demonstrated the usability of HDMs in high-energy femtosecond oscillators, such as a chirped pulse Ti:Sapphire oscillator and an Yb:YAG disk oscillator. In both cases a group delay dispersion (GDD) of the order of 2 × 104 fs2 was introduced, accompanied with an overall transmission loss as low as ∼ 2%.

The penetration depth of spectral components into the HDM structure. The electrical field components at 830 nm penatrate much deeper into the multilayer structure than the components at 770 nm. This means that the 830 nm components become delayed relatively to the 770 nm components.

The group had to make mirrors with very high dispersion in order to replace prisms and gratings. To make the mirrors, the researchers used magnetron sputtering to deposit alternate layers of tantalum pentoxide (Ta2O5) and silicon dioxide (SiO2). These materials have high (2.12 @ 800 nm) and low (1.47 @ 800 nm) refractive index, respectively. The resultant HDMs have layer thicknesses ranging between 25 nm and 400 nm, and a total physical thickness of approximately 10 µm. The total group delay (GD) in the HDM structure is a result of two combined effects: penetration effect (used in a conventional dispersive mirror); and an interferometer effect. "For our HDMs, the maximal GD that can be obtained by the pure penetration effect is 100 fs," said Pervak. "But our HDM provides a total GD of 150 fs. Therefore, 50 fs of the delay can be attributed to the interferometer effect."

He admits that making HDMs is challenging and that this has been the limiting factor to their use in this application. "But the advantages they offer means that it is worth the effort," he said. "When compared with using prisms, HDMs offer a much higher output efficiency; have no wavelength bandwidth limit; enable a more compact system; and give a clean pulse with no satellite pulses."

However, extraordinary sensitivity of the HDM design to manufacturing errors suggests that it may be difficult to manufacture a HDM with well-established technologies, such as electron-beam (ion-assisted) evaporation and ion-beam sputtering.

Ultrafast technology shifts to wafer scale

Could compact femtosecond laser be used to clock the multicore computer processors of the future? Optics & Laser Europe magazine recently reports ultrafast pioneer Ursula Keller of ETH Zurich, Switzerland, to find out about her latest idea.

Ultrafast VECSELs(vertical external-cavity surface-emitting lasers) have three main cavity elements: the gain structure, an output coupler and a semiconductor saturable absorber mirror. The essence of Keller's idea is to integrate the gain and the saturable absorber into a single structure. With the only other cavity element being an output coupler, Keller thinks that MIXSELs(modelocked integrated external-cavity surface-emitting laser) could become a true wafer-scale technology.

The full file link: http://images.iop.org/objects/optics/analysis/13/7/2/pdf.pdf

Ultrashort pulses create ultrabroad source

By sending laser pulses with a duration of just 5 femtoseconds through a helium cell held at high pressure, researchers have created a coherent supercontinuum with near-uniform spectral intensity spanning the range 270 to 1000 nm. The result relies on a process known as self-channeling and gives the team a new tool with which to explore electron motion inside atoms (Optics Letters 33 1407).

"Our ultimate goal is to generate coherent continuum light that spans several optical octaves," researcher Eleftherios Goulielmakis from the Max-Planck Institute for Quantum Optics in Garching, Germany, told optics.org. "In attosecond physics, we aim to steer the electron motion on atomic scales of space and time. To do this, we require fields that can be precisely controlled and shaped with sub-cycle (attosecond) accuracy and that are intense enough to enable nonlinear interactions with matter."

High and near-uniform efficiency are the prerequisites for generating light fields on a sub-cycle scale. While supercontinuum generation has been at the forefront of ultrafast research for several years, with groups using nonlinear propagation in photonic crystal fibres and solids, this prerequisite combination has remained elusive.

"Using few-cycle pulses dramatically improves the situation," explained Goulielmakis. "Once the duration of the pulse approaches the oscillating period (around 2.5 fs) of the light wave, phenomena like ionization-induced blue shift and shockwave effects result in a dramatic enhancement of the generation of light in the blue wing of the spectrum. We have been able to generate light that extends into the UV part of the spectrum at nearly uniform intensity."

The team focused 5 fs pulses with a central wavelength of 750 nm into a gas cell filled with helium. Self-channeling sets in at a pressure of around 25 bar, which results in a 5 cm long channel and a substantial reduction of the beam divergence in the far field. A intensity-calibrated fibre spectrometer monitored the emerging supercontinuum.

With this impressive result under its belt, the team now has several new experiments in the pipeline. "We plan to extend the supercontinuum source into the VUV by means of quasi-monocyle (~ 1.5 cycles of the field) laser pulses recently realized in out laboratories," said Goulielmakis.

A second follow-on experiment will see Goulielmakis and colleagues split the supercontinuum into narrower bands. The plan is to control properties such as the duration, phase and amplitude of these narrower bands separately before recombining them to synthesize intense light waveforms with a desired shape. "We plan to use these waveforms to control the generation of intense attosecond soft x-ray pulses from atoms," said Goulielmakis.

Other partners in the team come from the Technical University of Vienna, Austria; the Lomonosov Moscow State University, Russia; and the Ludwig-Maximilians University, also in Garching, Germany.

Source: optics.org

FROG reaches the attosecond scale

Researchers in Germany have developed a new FROG retrieval method for characterizing the electric field of attosecond pulses. The technique works by analysing attosecond streaking measurements and could be used to probe numerous physical phenomena that occur on the attosecond timescale (Applied Physics B 92 25).

"Conventional FROG (frequency-resolved optical gating) algorithms lose their accuracy when we try to characterize shorter pulses," Justin Gagnon of the Max Planck Institute for Quantum Optics in Garching told optics.org. "But ours retains its accuracy and robustness no matter how short (or broadband) the XUV pulse."

No electronic device is fast enough to record the electric field of ultrashort attosecond (10-18 s) pulses. Researchers therefore rely on less direct techniques, such as attosecond streaking spectroscopy, to characterize these fields.

Attosecond streaking spectroscopy measures streaked electron kinetic energy spectra by photoionizing an atom with an attosecond XUV pulse in the presence of an infrared laser field. By varying the delay between the XUV and IR pulses, a sequence of streaked spectra, known as a streaking spectrogram, is obtained.

This spectrogram contains complete phase and amplitude information about the XUV and IR field that can be extracted using FROG retrieval. "Extracting phase information from the spectrogram is an example of a 2D phase retrieval problem, known to possess a solution," explained Gagnon. "By applying alternating constraints between the frequency and time domains, the FROG retrieval algorithm can identify the XUV and IR pulses that reproduce the measured spectrum."

The y-axis is electron energy, and the x-axis is the delay between the XUV and IR fields. This spectrogram shows how the electron spectrum is modified by the laser field, as a function of the delay. (See the right picture)

The new technique is able to characterize pulses that last just 80 as, compared with the 130 as achieved by previous researchers. Gagnon and co-workers have also optimized FROG retrieval and have established the range of experimental parameters for which this technique can be used.

Attosecond pulses are essential tools that will allow us to probe physical phenomena occurring on the attosecond timescale, said Gagnon. These include electron dynamics in atoms, molecules and metal surfaces, Auger decay and autoionization. Attosecond streaking spectroscopy can be used to glean information about electron wave packets in such processes because a streaking spectrogram contains information about the time-related behaviour of these wave packets – and therefore time-related information about the attosecond process itself.

Chirped fibre laser drills faster

A chirped-pulse amplification (CPA) fibre laser is helping researchers in Germany drill holes in metal faster than conventional laser sources. The high-energy femtosecond pulses emitted by the source are allowing the team to investigate laser-metal interactions at repetition rates approaching 1 MHz for the first time (Optics Express 16 8958).

"Both heat accumulation and particle shielding are investigated for the first time at high repetition rates," Antonio Ancona, a visiting scientist at the Friedrich-Schiller University in Jena, told optics.org. "We have found that our fibre CPA system decreases the process time, but still produces a high-quality hole, which is important for industrial applications."

The precision, quality and reliability of holes produced using laser systems that deliver pulses of around 1 ns or longer is limited. This is because the ablation of metals is often accompanied by the formation of large heat-affected zones and a throw-out of the molten material.

Although femtosecond lasers offer improved performance, one of the main drawbacks is a low processing speed, which results from the lower repetition rate of these sources. On the other hand, at high repetition rates, heat accumulation effects might lead to melting and increased heat-affected zones. A further restriction on the highest useful repetition rate could be the interaction of the generated plasma or ablated particles with subsequent laser pulses as this will distort or shield the laser radiation.

The group's approach was to develop a CPA fibre laser system and investigate the effects of particle shielding and heat accumulation on the process time and hole quality for repetition rates ranging from 50–975 kHz. The 1030 nm source emit 800 fs pulses with energies ranging from 10–70 µJ and a corresponding average power of up to 68 W. The laser unit consists of a passively modelocked Yb:KGW oscillator, a dielectric grating stretcher-compressor unit, an acousto-optic modulator for pulse selection and two ytterbium-doped photonic crystal fibres, both in single-pass configuration, as amplification stages.

Two drilling techniques were studied: percussion – in which consecutive pulses are superimposed at the same focal spot, and trepanning – which involves moving the laser beam on a circular path relative to the target. The researchers found that the percussion approach offers faster drilling, but laser trepanning produces higher quality holes. "However, we found that working at such high repetition rates and pulse energies allows us to considerably reduce the processing time even when using the laser trepanning technique," commented Ancona.

The next steps for the Jena group are to investigate the influence of pulse duration from femtoseconds to tens of picoseconds in laser ablation processes at high average power and high repetition rates. "We are developing new processing strategies using the CPA laser system to further improve the processing speed and produce the highest quality holes," concluded Ancona. "We will also be scaling the output laser parameters to higher powers."

Antonio Ancona holds a permanent position at the CNR-INFM Regional Laboratory "LIT3" of Bari, Italy. The CNR-INFM of Bari is supported by the Italian Ministry of Research and University (MIUR) under the project 297 "FIBLAS". The University of Jena was supported by the German Federal Ministry of Education and Research (BMBF) under the PROMPTUS project.

Light Pulse Speed Record Set

Researchers have set a new record in ultrafast metrology, producing the first light pulses lasting only 80 attoseconds (a billionth of a billionth of a second).

The 80-attosecond achievement marks the first time scientists have achieved light pulse speeds below 100 attoseconds and was accomplished by a team of physicists led by professor Ferenc Krausz at the Max Planck Institute for Quantum Optics (MPQ) in Garching and professor Ulf Kleineberg at Ludwig Maximilians University Munich, working in cooperation with colleagues at the Advanced Light Source at Berkeley Lab in California.

To generate attosecond pulses, the Garching physicists use the strong electric field of flashes in the near-infrared spectrum. In the hypershort laser flashes this field performs hardly more than a single strong oscillation with a period of about 2.5 femtoseconds (a femtosecond is 1000 attoseconds). That is: the light wave now comprises just two high wave peaks and a deep wave valley between them. The force exerted by the electric light field on the electrons is strongest at the summits and the lowest point of the valley; strong enough to liberate electrons which are ejected from rare-gas atoms in the experiment at Garching. This leaves ion rumps.

The vacuum chamber for attosecond metrology: Attosecond pulses of extreme ultraviolet light (depicted as a blue beam) are focused by a mirror (right) on a jet of neon atoms effusing from a thin valve. At the same time an infrared beam is striking the atoms. Both beams in combination allow real-time observation of the motion of electrons in the neon atoms and measurement of the duration of the attosecond pulse.

With the oscillation of the light field the force changes direction and very soon hurls the electrons back to the ion rumps. The recolliding free electrons induce extremely fast electron oscillations which last just attoseconds and emit light flashes of the same duration. These flashes are then in the region of extreme ultraviolet light (XUV, a wavelength of approximately 10 to 20 nm).

Controlled production of this single strong light oscillation within a hypershort flash has now allowed the Garching research team for the first time to release electrons exactly three times during a single laser pulse. On returning to the ion they then emit exactly three attosecond pulses. Each femtosecond laser flash generates three attosecond pulses. One of these pulses has a particularly high intensity, providing more than 100 million photons in a period of just 80 attoseconds.

This pulse is filtered out with special x-ray mirrors from Kleineberg, resulting in a single isolated x-ray pulse lasting 80 attoseconds.

A paper on their work, "Single-cycle Nonlinear Optics," appeared in the June 20 edition of Science.

Source: Photonics.com