Second-order optical nonlinearities in materials are of paramount importance for optical wavelength conversion techniques, which are the basis of new high-resolution spectroscopic tools. Semiconductor technology now makes it possible to design and fabricate artificially asymmetric quantum structures in which optical nonlinearities can be calculated and optimized from first principles. Extremely large second-order susceptibilities can be obtained in these asymmetric quantum wells. Moreover, properties such as double resonance enhancement or electric field control will open the way to new devices, such as fully solid-state optical parametric oscillators.

https://science.sciencemag.org/content/sci/271/5246/168.full.pdf

Quantum Engineering of Optical Nonlinearities

Continuously graded buffers for InGaAs/GaAs structures grown on GaAs

Second and third harmonic generation associated to infrared transitions in a Morse quantum well under applied electric and magnetic fields

The contributions of homogeneous and inhomogeneous broadening mechanisms to the linewidth of an intersubband transition in an ${\mathrm{In}}_{0.45}$${\mathrm{Ga}}_{0.55}$As/${\mathrm{Al}}_{0.45}$${\mathrm{Ga}}_{0.55}$As multiple quantum well are determined by line-shape fitting and saturation intensity spectroscopy. An intersubband transition at 327 meV is found with a total linewidth of 36 meV and an underlying homogeneous linewidth of 14 meV. The sizes of various sources of inhomogeneous broadening are estimated and the implications for intersubband nonlinear optics are discussed.

Inhomogeneous broadening of intersubband transitions in In 0.45 Ga 0.55 As/ Al 0.45 Ga 0.55 As multiple quantum wells

We observed room-temperature intersubband transitions at 1.26 μm in n-doped type-II In0.3Ga0.7As/AlAs strained quantum wells. An improved tight-binding model was used to optimize the structure parameters in order to obtain the shortest wavelength intersubband transition ever achieved in a semiconductor system. The corresponding transitions occur between the first confined electronic levels of the well following mid-infrared optical pumping of electrons from the barrier X- valley into the well ground state.

1.26 μm intersubband transitions in In0.3Ga0.7As/AlAs quantum wells

We demonstrate intersubband absorption and second harmonic generation (SHG) in asymmetric coupled In0.6Ga0.4As/AlAs n‐type quantum wells (QWs) grown on a GaAs substrate. Intersubband absorption at 4.1 and 2.1 μm wavelengths, corresponding to the 1 to 2 and 1 to 3 transitions, respectively, are observed. SHG of 2.0 μm light is demonstrated in this doubly resonant QW. This is the shortest wavelength SHG to date in any n‐type QW system. The second order nonlinear susceptibility χ(2) is measured using a free electron laser by interference of the second harmonic fields from the QW and substrate. At a pump wavelength of 4.0 μm, a large asymmetry in the SHG power with rotation angle of the sample arising from SHG from the QW is observed, and a χ(2) of magnitude 20±8 nm/V, approximately 100 times that of bulk GaAs, and phase 63°±34° relative to the GaAs substrate is measured. Comparison of both the linear and nonlinear properties to a simple model is discussed.

Doubly resonant second harmonic generation of 2.0 μm light in coupled InGaAs/AlAs quantum wells

We report short wavelength second‐harmonic generation (SHG) spectroscopy of asymmetric coupled In0.6Ga0.4As/AlAs quantum wells (QWs). The QW is designed to show maximum second‐order nonlinear susceptibility χ(2) for SHG of 4 and 2 μm wavelengths by single and double resonance effects, respectively. SHG spectroscopy across the midinfrared is measured using both a CO2 and a free electron laser as pumps. The χ(2) of the QW is extracted from interference of the second‐harmonic fields from the QW and GaAs substrate, determined by the azimuthal dependence of the SHG power. We measure χ(2) of the QW for harmonic wavelengths between 5.36 and 1.85 μm. This is the shortest wavelength SHG to date by any QW intersubband interaction. Good agreement of experiment with theory for the dispersion of χ(2) for both singly and doubly resonant conversion is observed throughout the midinfrared.

Short wavelength (5.36–1.85 μm) nonlinear spectroscopy of coupled InGaAs/AlAs intersubband quantum wells

Proton beam-dump experiments are a high-intensity source of secondary muons and provide an opportunity to probe muon-speciﬁc dark sectors. We adopt a simpliﬁed-models framework for an exotic light scalar particle coupling predominantly or exclusively to muons. Equipped with state-of-the-art muon simulations, we compute the sensitivity reach in the parameter space ( m S , g µ ) of the dark mediator, examining in detail the examples of the experiment NA62 in beam-dump mode and the proposed experiment SHiP. We ﬁnd a signiﬁcant yield of such exotics in the sub-GeV mass range. Our projections are competitive with those of primary muon-beam experiments and complementary to current constraints, spanning uncharted parameter space and accessing new physics potentially responsible for the ( g − 2) µ anomaly.

pdf/searching-for-muonphilic-dark-sectors-with-proton-beams-87sy000e.pdf

Searching for muonphilic dark sectors with proton beams

We report on some applications of high indium content quantum wells for mid-infrared (2–7μm) applications. Studies have been done to explore intra-valence band absorption in the mid-infrared between valence band hole states. Second harmonic generation spectroscopy is demonstrated in the conduction band near 5.5 μm.

Applications of High Indium Content InGaAs/AlGaAs Quantum Wells in the 2–7 μm Regime

The quantization of the mirror curve to a toric Calabi-Yau threefold gives rise to quantum-mechanical operators, whose fermionic spectral traces produce factorially divergent power series in the Planck constant. These asymptotic expansions can be promoted to resurgent trans-series. They show infinite towers of periodic singularities in their Borel plane and infinitely-many rational Stokes constants, which are encoded in generating functions expressed in closed form in terms of $q$-series. We provide an exact solution to the resurgent structure of the first fermionic spectral trace of the local $\mathbb{P}^2$ geometry in the semiclassical limit of the spectral theory, corresponding to the strongly-coupled regime of topological string theory on the same background in the conjectural Topological Strings/Spectral Theory correspondence. Our approach straightforwardly applies to the dual weakly-coupled limit of the topological string. We present and prove closed formulae for the Stokes constants as explicit arithmetic functions and for the perturbative coefficients as special values of known $L$-functions, while the duality between the two scaling regimes of strong and weak string coupling constant appears in number-theoretic form. A preliminary numerical investigation of the local $\mathbb{F}_0$ geometry unveils a more complicated resurgent structure with logarithmic sub-leading asymptotics. Finally, we obtain a new analytic prediction on the asymptotic behavior of the fermionic spectral traces in an appropriate WKB double-scaling regime, which is captured by the refined topological string in the Nekrasov-Shatashvili limit.

Resurgence, Stokes constants, and arithmetic functions in topological string theory

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