Circular polarization is no longer a barrier for High Harmonic Generation
Contrary to long-standing scientific belief, ICFO researchers have demonstrated that circularly polarized light can, in fact, generate high harmonics –provided that the light contains sufficiently strong fluctuations.
High harmonic generation (HHG) is a process where an intense laser interacts with a material to produce new light at much higher frequencies –the harmonics of the incoming laser’s frequency. This light only lasts a few attoseconds (10-18 seconds), making it an invaluable tool for observing electronic and nuclear motions within atoms and molecules, which typically occur on timescales too fast for longer light pulses to capture.
Despite extensive research aimed at understanding HHG, some open questions remain. For instance, it has been observed that when the incoming laser field is a coherent, classical light source, its polarization plays a crucial role in the outcome. In particular, when the light is circularly polarized—meaning its electric field rotates as it propagates— this typically suppresses the generation of high harmonics in atoms.
ICFO researchers Dr. Javier Rivera-Dean, Philipp Stammer, led by ICREA Prof. Maciej Lewenstein, in collaboration with Prof. Dr. Marcelo Ciappina from Guangdong Technion – Israel Institute of Technology, have overcome this limitation, overturning the long-standing belief that circularly polarized light cannot produce high harmonics. After working for months, they have theoretically shown that high harmonics can indeed re-emerge by introducing strong enough fluctuations into the laser light. The results were published today in Physical Review Letters.
Specifically, the team considered light with engineered quantum fluctuations that cannot be described by classical physics alone, caused by a quantum phenomenon known as squeezing. Not only did they show the emission of high harmonics, but they also found that their frequency and intensity depend sensitively on the quantum properties of light, particularly on the type of squeezing applied. Finally, they linked these observations to how the underlying behavior of electrons changes during the process.
The researchers remark that non-classical light is not strictly required to enable HHG with circular polarization; instead, strong fluctuations, regardless of their origin, are the key. However, according to Dr. Javier Rivera-Dean, lead author of the paper: “Our work opens the exciting possibility of using structured quantum fluctuations in the light to modify the HHG process, and therefore ask questions about both its nature and the consequences it could have in more involved attosecond science applications.” Therefore, this could provide powerful new methods for controlling and studying ultrafast electron dynamics in complex systems, and further advance the emerging field of attosecond quantum optics.
Reference:
J. Rivera-Dean, P. Stammer, M. F. Ciappina, and M. Lewenstein. Structured squeezed light allows for high-harmonic generation in classical forbidden geometries. Phys. Rev. Lett. (2025)
DOI: https://doi.org/10.1103/4hdl-bdwj
Acknowledgements:
ICFO-QOT group acknowledges support from: European Research Council AdG NOQIA; MCIN/AEI (PGC2018-0910.13039/501100011033, CEX2019-000910-S/10.13039/501100011033, Plan National FIDEUA PID2019-106901GB-I00, Plan National STAMEENA PID2022-139099NB, I00, project funded by MCIN/AEI/10.13039/501100011033 and by the “European Union NextGenerationEU/PRTR" (PRTRC17.I1), FPI); QUANTERA DYNAMITE PCI2022-132919, QuantERA II Programme co-funded by European Union’s Horizon 2020 program under Grant Agreement No 101017733; Ministry for Digital Transformation and of Civil Service of the Spanish Government through the QUANTUM ENIA project call – Quantum Spain project, and by the European Union through the Recovery, Transformation and Resilience Plan - NextGenerationEU within the framework of the Digital Spain 2026 Agenda; Fundació Cellex; Fundació Mir-Puig; Generalitat de Catalunya (European Social Fund FEDER and CERCA program; Barcelona Supercomputing Center MareNostrum (FI-2023-3-0024); Funded by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union, European Commission, European Climate, Infrastructure and Environment Executive Agency (CINEA), or any other granting authority. Neither the European Union nor any granting authority can be held responsible for them (HORIZON-CL4-2022-QUANTUM-02-SGA PASQuanS2.1, 101113690, EU Horizon 2020 FET-OPEN OPTOlogic, Grant No 899794, QU-ATTO, 101168628), EU Horizon Europe Program (This project has received funding from the European Union’s Horizon Europe research and innovation program under grant agreement No 101080086 NeQSTGrant Agreement 101080086 — NeQST); ICFO Internal “QuantumGaudi” project; P. Stammer acknowledges support from: The European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 847517. M. F. C. acknowledges support from: the National Key Research and Development Program of China (Grant No. 2023YFA1407100), the Guangdong Province Science and Technology Major Project (Future functional materials under extreme conditions - 2021B0301030005) and the Guangdong Natural Science Foundation (General Program project No. 2023A1515010871).