Resonant Optical Nonlinearities in Quantum Cascade Lasers

The determining factor in the efficiency of any nonlinear process is the nonlinear susceptibility of the material to be used for frequency conversion. In the case of a second order process (second harmonic generation or sum/difference generation), the conversion efficiency of converting the fundamental frequency to a generated frequency is proportional to the square of the second order susceptibility, χ(2). Typically these values are small in most bulk materials ( 100pm/V in GaAs) and to attain reasonable conversion efficiencies phase matching is imperative to allow a long interaction length.

The nonlinearities, however, can be enhanced by orders of magnitude if any or all of the interacting beams are resonant with a ‘real’ transition of the material in question. In this work, the “ultra-fast THz spectroscopy” group at LPA (ENS) has demonstrated that efficient frequency mixing can be attained between an optical photon (energy eV) and a terahertz (THz) photon ( meV) within the cavity of a compact and practical THz laser – a quantum cascade laser (QCL). Here the optical transition is resonant with the bandgap, enhancing the susceptibility to 104pm/V, permitting frequency conversion without the considerations of phase matching.

As well as of fundamental interest, this work opens up applications in the domains of all-optical telecommunication where the QCL is used as an optical wavelength shifter or THz detection where the QCL up-converts the THz emission to the optical domain.


Reference :
J. Madéo, et al. “All-optical wavelength shifting in a semiconductor laser using resonant nonlinearities”, Nature Photonics 6, 519-524 (2012).

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