Ultrashort signals are integral for conducting high-resolution measurements. In the mid-infrared, the generation of ultrashort pulses is notoriously difficult to achieve and usually requires large optical setups. In our work we use direct sampling to demonstrate the spontaneous generation of stable ultrashort features in the time-domain signal of a mid-infrared quantum cascade laser frequency comb. The full-width at half-maximum of these features is measured to be ~500 fs, right below the Fourier-limit derived from the corresponding optical spectrum and RF-injection can be used for stabilization and manipulation. Using Maxwell-Bloch equation-based simulations, we can reproduce the generation of such features, including the position in relation to the instantaneous frequency and show their width can be lowered even further below the Fourier limit, thus opening new possibilities for high-resolution measurements based on quantum cascade laser frequency combs.
Because their phase can be compensated by a grating-based stretcher compressor and further controlled by RF injection, quantum cascade laser based optical frequency comb generation allowed the generation of pulse as short as 630fs after compression, value confirmed using an upconversion technique with a sub-picosecond time resolution. Another possibility is the direct generation of optical solitons using ring quantum cascade lasers in which, by using a very low lateral loss waveguide, the symmetric counter-propagating modes undergo a spontaneous symmetry breaking and generate solitons.
Quantum cascade laser combs enable the generation of high average power coherent optical frequency combs. Combination of RF injection in specially designed devices enable the recompression of the emission into high peak power pulses for non-linear optics applications. A combination of techniques are used to analyse the temporal profile of the emission.
We present strong radio-frequency current modulation close to their repetition frequency as a means to control the emitted state of quantum cascade laser frequency combs. In particular, more than doubling of the spectral bandwidth compared to free-running can be achieved throughout the dynamical range of the device. By changing the modulation frequency, the spectral bandwidth and center-frequency can be tuned and by fast switching between modulation frequencies we can multiplex spectral regions with negligible overlap from the same device. In the time-domain, we are able to transition from quasi-continuous to long-pulse output by injecting at high power.
In recent years, quantum cascade lasers have matured to become compact, powerful sources of coherent midinfrared light. Yet, the ultrafast carrier dynamics in these sources has so far restricted the formation of highintensity ultrashort pulses. In this work, we demonstrate the formation of ∼ 630 fs QCL pulses with a peak power of ∼ 4.3 W. We break the picosecond barrier in an approach similar to chirped pulse amplification, where we externally recompress the maximally chirped output of a quantum cascade laser frequency comb. Ultrashort pulse formation is confirmed with a novel asynchronous optical sampling technique. These results emphasise the potential of quantum cascade lasers also as sources for non-linear experiments in the mid-infrared.
Recently, on-chip quantum-cascade-laser-based frequency combs are gaining increasing attention both in the Mid-IR and in the THz spectral regions. THz devices offer the possibility of filling the gap of comb sources in a spectral region were no table-top comb is available. I will discuss direct THz comb generation from both homogeneous and heterogeneous quantum cascade lasers. Octave spanning emission spectra and comb operation on bandwidth larger than 1 THz are reported for heterogeneous cascades. I will also report on a series of new structures with homogeneous cascade design that feature a very low threshold current density (< 100 A/cm2), a bandwidth of roughly 1 THz centered a 3 THz and an extremely wide bandwidth (>1.8 THz) when driven in the NDR region. This extremely broadband emission in the NDR is studied as well with NEGF simulation and is based on an interplay between strong photon assisted transport due to the highly diagonal transition and domain formation.These structures are also showing RF injection locking with extremely reduced microwave powers. We will discuss locking experiments as well as a method to finely control the repetition rate of the laser based on controlled optical feedback.
Time resolved spectral measurements aimed to clarify the physics of field domains in the NDR will be also presented.
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