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1. Strongly enhanced second-order optical nonlinearity in CMOS-compatible Al1− x Sc x N thin films

   https://doi.org/10.1063/5.0061787  

   Yoshioka, Valerie; Lu, Jian; Tang, Zichen; Jin, Jicheng; Olsson, III, Roy_H; Zhen, Bo (October 2021, APL Materials)

   Silicon photonics has enabled large-scale production of integrated optical devices for a vast array of applications. However, extending its use to nonlinear devices is difficult since silicon does not exhibit an intrinsic second-order nonlinearity. While heterogeneous integration of strongly nonlinear materials is possible, it often requires additional procedures since these materials cannot be directly grown on silicon. On the other hand, CMOS-compatible materials often suffer from weaker nonlinearities, compromising efficiency. A promising alternative to current material platforms is scandium-doped aluminum nitride (Al1−xScxN), which maintains the CMOS compatibility of aluminum nitride (AlN) and has been used in electrical devices for its enhanced piezoelectricity. Here, we observe enhancement in optical second-order susceptibility (χ(2)) in CMOS-compatible Al1−xScxN thin films with varying Sc concentrations. For Al0.64Sc0.36N, the χ(2) component d33 is enhanced to 62.3 ± 5.6 pm/V, which is 12 times stronger than intrinsic AlN and twice as strong as lithium niobate. Increasing the Sc concentration enhances both χ(2) components, but loss increases with a higher Sc concentration as well, with Al0.64Sc0.36N exhibiting 17.2 dB/cm propagation loss at 1550 nm and Al0.80Sc0.20N exhibiting 8.2 dB/cm at 1550 nm. Since other material properties of this alloy are also affected by Sc, tuning the Sc concentration can balance strong nonlinearity, loss, and other factors depending on the needs of specific applications. As such, Al1−xScxN could facilitate low cost development of nonlinear integrated photonic devices. 

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2. Large optical nonlinearity enabled by coupled metallic quantum wells

   https://doi.org/10.1038/s41377-019-0123-4  

   Qian, Haoliang; Li, Shilong; Chen, Ching-Fu; Hsu, Su-Wen; Bopp, Steven Edward; Ma, Qian; Tao, Andrea R.; Liu, Zhaowei (January 2019, Light: Science & Applications)

   Abstract New materials that exhibit strong second-order optical nonlinearities at a desired operational frequency are of paramount importance for nonlinear optics. Giant second-order susceptibilityχ⁽²⁾has been obtained in semiconductor quantum wells (QWs). Unfortunately, the limited confining potential in semiconductor QWs causes formidable challenges in scaling such a scheme to the visible/near-infrared (NIR) frequencies for more vital nonlinear-optic applications. Here, we introduce a metal/dielectric heterostructured platform, i.e., TiN/Al₂O₃epitaxial multilayers, to overcome that limitation. This platform has an extremely highχ⁽²⁾of approximately 1500 pm/V at NIR frequencies. By combining the aforementioned heterostructure with the large electric field enhancement afforded by a nanostructured metasurface, the power efficiency of second harmonic generation (SHG) achieved 10⁻⁴at an incident pulse intensity of 10 GW/cm², which is an improvement of several orders of magnitude compared to that of previous demonstrations from nonlinear surfaces at similar frequencies. The proposed quantum-engineered heterostructures enable efficient wave mixing at visible/NIR frequencies into ultracompact nonlinear optical devices. 

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3. Demonstration of the DC-Kerr effect in silicon-rich nitride

   https://doi.org/10.1364/OL.432359  

   Friedman, Alex; Nejadriahi, Hani; Sharma, Rajat; Fainman, Yeshaiahu (August 2021, Optics Letters)

   We demonstrate the DC-Kerr effect in plasma enhanced chemical vapor deposition (PECVD) silicon-rich nitride (SRN) and use it to demonstrate a third order nonlinear susceptibility, ${\mathrm{\chi <\#comment/>}}^{\left(3\right)}$, as high as $(6\pm <\#comment/>0.58)\times <\#comment/>{10}^{-<\#comment/>19}{\mathrm{m}}^2/{\mathrm{V}}^2$. We employ spectral shift versus applied voltage measurements in a racetrack resonator as a tool to characterize the nonlinear susceptibilities of these films. In doing so, we demonstrate a ${\mathrm{\chi <\#comment/>}}^{\left(3\right)}$larger than that of silicon and argue that PECVD SRN can provide a versatile platform for employing optical phase shifters while maintaining a low thermal budget using a deposition technique readily available in CMOS process flows. 

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4. Search for direct production of electroweakinos in final states with one lepton, missing transverse momentum and a Higgs boson decaying into two b-jets in $$pp$$ collisions at $$\sqrt{s}=13$$ TeV with the ATLAS detector

   https://doi.org/10.1140/epjc/s10052-020-8050-3  

   Aad, G.; Abbott, B.; Abbott, D. C.; Abud, A. Abed; Abeling, K.; Abhayasinghe, D. K.; Abidi, S. H.; AbouZeid, O. S.; Abraham, N. L.; Abramowicz, H.; et al (August 2020, The European Physical Journal C)

   null (Ed.)

   Abstract The results of a search for electroweakino pair production $$pp \rightarrow \tilde{\chi }^\pm _1 \tilde{\chi }^0_2$$ p p → χ ~ 1 ± χ ~ 2 0 in which the chargino ( $$\tilde{\chi }^\pm _1$$ χ ~ 1 ± ) decays into a W boson and the lightest neutralino ( $$\tilde{\chi }^0_1$$ χ ~ 1 0 ), while the heavier neutralino ( $$\tilde{\chi }^0_2$$ χ ~ 2 0 ) decays into the Standard Model 125 GeV Higgs boson and a second $$\tilde{\chi }^0_1$$ χ ~ 1 0 are presented. The signal selection requires a pair of b -tagged jets consistent with those from a Higgs boson decay, and either an electron or a muon from the W boson decay, together with missing transverse momentum from the corresponding neutrino and the stable neutralinos. The analysis is based on data corresponding to 139  $$\mathrm {fb}^{-1}$$ fb - 1 of $$\sqrt{s}=13$$ s = 13 TeV pp collisions provided by the Large Hadron Collider and recorded by the ATLAS detector. No statistically significant evidence of an excess of events above the Standard Model expectation is found. Limits are set on the direct production of the electroweakinos in simplified models, assuming pure wino cross-sections. Masses of $$\tilde{\chi }^{\pm }_{1}/\tilde{\chi }^{0}_{2}$$ χ ~ 1 ± / χ ~ 2 0 up to 740 GeV are excluded at 95% confidence level for a massless $$\tilde{\chi }^{0}_{1}$$ χ ~ 1 0 . 

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5. Degenerate optical parametric amplification in CMOS silicon

   https://doi.org/10.1364/OPTICA.478702  

   Heydari, David; Cătuneanu, Mircea; Ng, Edwin; Gray, Dodd_J; Hamerly, Ryan; Mishra, Jatadhari; Jankowski, Marc; Fejer, M_M; Jamshidi, Kambiz; Mabuchi, Hideo (March 2023, Optica)

   Silicon is a common material for photonics due to its favorable optical properties in the telecom and mid-wave IR bands, as well as compatibility with a wide range of complementary metal–oxide semiconductor (CMOS) foundry processes. Crystalline inversion symmetry precludes silicon from natively exhibiting second-order nonlinear optical processes. In this work, we build on recent works in silicon photonics that break this material symmetry using large bias fields, thereby enablingχ⁽²⁾interactions. Using this approach, we demonstrate both second-harmonic generation (with a normalized efficiency of 0.20%W⁻¹cm⁻²) and, to our knowledge, the first degenerateχ⁽²⁾optical parametric amplifier (with an estimated normalized gain of 0.6dBW^−1/2cm⁻¹) using silicon-on-insulator waveguides fabricated in a CMOS-compatible commercial foundry. We expect this technology to enable the integration of novel nonlinear optical devices such as optical parametric amplifiers, oscillators, and frequency converters into large-scale, hybrid photonic–electronic systems by leveraging the extensive ecosystem of CMOS fabrication. 

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