We introduce a new theoretical approach for analyzing pump and probe experiments in non-linear systems of optical phonons. In our approach, the effect of coherently pumped polaritons is modeled as providing time-periodic modulation of the system parameters. Within this framework, propagation of the probe pulse is described by the Floquet version of Maxwell’s equations and leads to phenomena such as frequency mixing and resonant parametric production of polariton pairs. We analyze light reflection from a slab of insulating material with a strongly excited phonon-polariton mode and obtain analytic expressions for the frequency-dependent reflection coefficient for the probe pulse. Our results are in agreement with recent experiments by Cartella et al. [Proc. Natl. Acad. Sci. U. S. A. 115, 12148 (2018)], which demonstrated light amplification in a resonantly excited SiC insulator. We show that, beyond a critical pumping strength, such systems should exhibit Floquet parametric instability, which corresponds to resonant scattering of pump polaritons into pairs of finite momentum polaritons. We find that the parametric instability should be achievable in SiC using current experimental techniques and discuss its signatures, including the non-analytic frequency dependence of the reflection coefficient and the probe pulse afterglow. We discuss possible applications of the parametric instability phenomenon and suggest that similar types of instabilities can be present in other photoexcited non-linear systems.
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Efficiency of pulse pumped soliton microcombs
Temporal soliton mode locking in coherently pumped microcavities is a promising route towards miniaturized frequency comb systems. However, the power efficiency of the resulting microcombs is usually quite low. Soliton generation by pulse pumping provides a way to increase conversion efficiency (so far, as high as 8%). Here, we study conversion efficiency and report a single-soliton conversion efficiency as high as 54% using a scanning laser, as well as a steady-state single-soliton conversion efficiency as high as 34%. We use the Lagrangian approach to develop analytical expressions for efficiency and soliton temporal placement within the pumping pulse, and our measurements reveal features in the tuning dependence of soliton power and efficiency not seen in continuous pumping. Our experimentally confirmed expressions for efficiency will be useful in understanding advantages and limitations of pulse pumped systems.
more » « less- Award ID(s):
- 1908231
- NSF-PAR ID:
- 10369327
- Publisher / Repository:
- Optical Society of America
- Date Published:
- Journal Name:
- Optica
- Volume:
- 9
- Issue:
- 2
- ISSN:
- 2334-2536
- Page Range / eLocation ID:
- Article No. 231
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract We present experimental and numerical investigations of high-energy mid-infrared filamentation with multi-octave-spanning supercontinuum generation (SCG), pumped by a 2.4 μm, 250 fs Cr:ZnSe chirped-pulse laser amplifier. The SCG is demonstrated in both anomalous and normal dispersion regimes with YAG and polycrystalline ZnSe, respectively. The formation of stable and robust single filaments along with the visible-to-mid-infrared SCG is obtained with a pump energy of up to 100 μJ in a 6-mm-long YAG medium. To the best of the authors’ knowledge, this is the highest-energy multi-octave-spanning SCG from a laser filament in a solid. On the other hand, the SCG and even-harmonic generation based on random quasi-phase matching (RQPM) are simultaneously observed from the single filaments in a 6-mm-long polycrystalline ZnSe medium with a pump energy of up to 15 μJ. The numerical simulations based on unidirectional pulse propagation equation and RQPM show excellent agreement with the measured multi-octave-spanning SCG and even-harmonic generation. They also reveal the temporal structure of mid-infrared filaments, such as soliton-like self-compression in YAG and pulse broadening in ZnSe.more » « less
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