Second Harmonic Generation in Germanium Quantum Wells for Nonlinear Silicon Photonics
Journal article, 2021

Second-harmonic generation (SHG) is a direct measure of the strength of second-order nonlinear optical effects, which also include frequency mixing and parametric oscillations. Natural and artificial materials with broken center-of-inversion symmetry in their unit cell display high SHG efficiency, however, the silicon-foundry compatible group IV semiconductors (Si, Ge) are centrosymmetric, thereby preventing full integration of second-order nonlinearity in silicon photonics platforms. Here we demonstrate strong SHG in Ge-rich quantum wells grown on Si wafers. Unlike Si-rich epilayers, Ge-rich epilayers allow for waveguiding on a Si substrate. The symmetry breaking is artificially realized with a pair of asymmetric coupled quantum wells (ACQW), in which three of the quantum-confined states are equidistant in energy, resulting in a double resonance for SHG. Laser spectroscopy experiments demonstrate a giant second-order nonlinearity at mid-infrared pump wavelengths between 9 and 12 μm. Leveraging on the strong intersubband dipoles, the nonlinear susceptibility χ(2) almost reaches 105 pm/V, 4 orders of magnitude larger than bulk nonlinear materials for which, by the Miller's rule, the range of 10 pm/V is the norm.


nonlinear optics

silicon photonics


quantum wells


Jacopo Frigerio

Polytechnic University of Milan

Chiara Ciano

Universita degli studi - Roma Tre

Joel Kuttruff

University of Konstanz

Andrea Mancini

Sapienza University of Rome

Ludwig Maximilian University of Munich (LMU)

Andrea Ballabio

Polytechnic University of Milan

Daniel Chrastina

Polytechnic University of Milan

Virginia Falcone

Polytechnic University of Milan

Monica De Seta

Universita degli studi - Roma Tre

L. Baldassarre

Sapienza University of Rome

Jonas Allerbeck

University of Luxembourg

University of Konstanz

Daniele Brida

University of Luxembourg

Lunjie Zeng

Chalmers, Physics, Nano and Biophysics

Eva Olsson

Chalmers, Physics, Nano and Biophysics

Michele Virgilio

University of Pisa

M. Ortolani

Sapienza University of Rome

ACS Photonics

2330-4022 (eISSN)

Vol. 8 12 3573-3582

Areas of Advance

Nanoscience and Nanotechnology

Subject Categories

Atom and Molecular Physics and Optics

Other Physics Topics

Condensed Matter Physics


Chalmers Materials Analysis Laboratory



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