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Broadband mid-infrared waveform generation - Nature Photonics

Coherent multi-octave mid-infrared waveforms are created and manipulated by cascaded intrapulse difference-frequency generation, demonstrating absolute phase control, and adding to the growing arsenal of techniques for arbitrary light-wave control.
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Ultrashort, tailored optical waveforms with oscillating electromagnetic fields of less than a single optical cycle in duration are of great interest for the control of electronic coherence in atoms and molecules. Motivated by a plethora of applications and aided by advances in ultrafast lasers, control of sub-optical-cycle pulses is now possible in the mid-infrared region.
Now reporting in Nature Photonics , Steinleitner et al. produce coherent emission over a broad range of the infrared spectrum, 0.9–12 μm, in the form of a 0.7-cycle optical pulse. The approach brings the intrinsic ability to manipulate the emerging waveform by controlling the phase of different spectral components 1 .
Achieving control over such a wide bandwidth is not trivial, yet the researchers do so by means of a relatively simple approach. Single-cycle, carrier-envelope phase (CEP)-stabilized pulses at 2.2 μm are produced by a Cr:ZnS laser oscillator and a post-compression stage. The pulses are focused on a nonlinear ZnGeP 2 crystal, where they undergo frequency down-conversion via cascaded intrapulse difference-frequency generation (IPDFG). In this process, the pulses drive multiple orders of difference frequency mixing in the ZnGeP 2 crystal, creating overlapping spectral bands that generate an infrared supercontinuum radiation spanning 3.7 octaves (Fig. 1 ). Importantly, interference between different orders is affected by the CEP of the driving pulses. The resulting sub-cycle waveform can therefore be manipulated through the CEP locking electronics. For instance, the temporal profile of the waveform can be transformed from a clean cosine-pulse-like behaviour to a sine-like profile, while maintaining sub-cycle duration. This level of control has the potential to advance applications in which light fields are used to mould the flow of electrons with attosecond precision, for example, in low-gap semiconductors.
Fig. 1: Broadband waveform synthesis via cascaded intrapulse difference-frequency generation. Two narrowband laser pulses (dashed black curves) act as driving pulses. Down-converted radiation of different orders (zero–third) is produced in the nonlinear crystal. For a single broadband driving pulse, the progressive spectral broadening of the different orders will overlap to form a continuum (grey and light red curves). When the driving laser has a flat spectral phase, the CEP of the odd orders (shaded in blue) follows the CEP of the driving laser whereas the CEP of even orders (shaded in orange) does not. Thus, by varying the CEP of the driving laser (between Δ φ = 0 and ±π, for instance, where Δ φ = 0 is referenced to the red curve) the continuum waveform can be modified, as seen by the difference between the grey and red curve. The waveform can vary by up to an order of magnitude in spectral intensity when there is strong interference as observed in the vicinity of 60 THz (shaded in red). Credit: reproduced from ref. 1 , Springer Nature Ltd.
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A crucial prerequisite to obtain these results is the high stability of the CEP of the Cr:ZnS laser oscillator pulses used to drive the cascaded IPDFG process. The CEP noise achieved by Steinleitner et al. exhibits a root-mean-square jitter of only 11 mrad, equivalent to only 14 attoseconds of timing jitter between the carrier field and pulse envelope. This impressive capability is the culmination of two-decade long efforts aimed at achieving active CEP control. Owing to the unavoidable linear dispersion and nonlinear processes, the output from a mode-locked laser oscillator generally exhibits a pulse-to-pulse CEP slip that results in a carrier-envelope offset frequency, f ceo . An elegant self-referencing nonlinear interferometry technique to measure the f ceo was demonstrated in 2000, in which the f ceo can then be stabilized through a phase-locked loop that feeds back to the laser oscillator 2 , marking the beginning of a new era of optical phase control. Now, if the f ceo is locked to the N th subharmonic of the repetition rate, f rep , constant and stable CEP can be achieved by pulse picking every N th pulse in the pulse train 3 . Steinleitner et al. use another newly developed CEP stabilization concept that ingeniously locks f ceo to 0 with a phase-locked loop. The two beat notes around f rep , f rep – f ceo and f rep + f ceo , are extracted and used to generate the error signal in the phase-locked loop for locking f ceo to 0 (ref. 4 ). Pulse picking is thus not required, and the method does not suffer from the slow CEP drift observed in other methods 5 . Optimizing this new procedure allowed the team to demonstrate an unprecedented stability of the CEP, which is key for the reproducible generation and control of the broad mid-infrared waveforms.
These results increase the growing arsenal of methods available to shape pulses precisely by controlling the phase degrees of freedom of optical supercontinua. As bandwidth is widened to the levels needed for producing single- and sub-cycle transients, precise phase control is of utmost importance. This is true in terms of both the carrier-envelope and spectral phases. Although the underlying field oscillation of a light pulse must be stable from pulse to pulse to ensure a steady waveform, short pulses are themselves created by the constructive interference of a wide bandwidth of electric field oscillations. Constructive interference can be measured by the flatness of the group delay dispersion, a measure of spectral phase variation at quadratic order and higher. If group delay varies across the bandwidth by no more than roughly the Fourier-limited duration of a spectrum, the pulse will be fully compressed. Slight deviations beyond this maximum group delay variation cause dramatic changes to the pulse shape. For a 10-fs pulse, this means the optical path length difference for all frequencies within a pulse must be no more than a few micrometres to maintain a compressed pulse or any other pulse shape desired for an application.
Steinleitner et al. take a simple yet successful approach that relies on an octave-spanning wave that already has flat spectral phase, and perform the nonlinear interaction to widen the bandwidth in a very short medium that imparts no significant group delay dispersion. The incident pulse is already compressed and of high peak power, thus enabling the nonlinear frequency broadening interaction, while the short medium length allows it to remain compressed during the nonlinear interaction. Finally, the flat spectral phase ensures strong interference between overlapping orders of the cascaded IPDFG.
The path forward to realization of truly arbitrary control of light waves needed for the next generation of applications are likely to involve the use of several complementary approaches. Exquisite CEP control together with highly nonlinear techniques such as cascaded IPDFG allow controllable sub-cycle waveforms to be generated, and down to very low frequencies. Other approaches provide additional pulse-tailoring capabilities that widen the sphere of control. Adiabatic frequency conversion provides a way to robustly translate ultra-broadband pulses with precisely shaped spectral phases to other frequencies 6 , 7 and to intrinsically engineer their spectral phases 8 . Coherent pulse synthesis allows the coherent combination of any of these phase-controlled pulses into a waveform with even broader bandwidth 9 , and can be scaled to high pulse energies 10 . Combined, these tools allow multi-octave optical continua to be tuned precisely in phase to all orders, making for a truly powerful technology for manipulating and probing materials with light.
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Applied & Engineering Physics, Cornell University, Ithaca, NY, USA
Jeffrey Moses
Electrical, Computer, and Energy Engineering, University of Colorado Boulder, Boulder, CO, USA
Shu-Wei Huang
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Moses, J., Huang, SW. Broadband mid-infrared waveform generation. Nat. Photon. 16, 481–482 (2022). https://doi.org/10.1038/s41566-022-01027-6
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Published : 01 July 2022
Issue Date : July 2022
DOI : https://doi.org/10.1038/s41566-022-01027-6
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