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1. Giant optical second- and third-order nonlinearities at a telecom wavelength

   https://doi.org/10.1364/OL.528340  

   Chen, Ching-Fu ; Tian, Fanglin ; Zhou, Junxiao ; Wagner, Jackson_C ; Xiong, Wei ; Liu, Zhaowei ( June 2024 , Optics Letters)

   A material platform that excels in both optical second- and third-order nonlinearities at a telecom wavelength is theoretically and experimentally demonstrated. In this TiN-based coupled metallic quantum well structure, electronic subbands are engineered to support doubly resonant inter-subband transitions for an exceptionally high second-order nonlinearity and provide single-photon transitions for a remarkable third-order nonlinearity within the 1400–1600 nm bandwidth. The second-order susceptibilityχ⁽²⁾reaches 2840 pm/V at 1440 nm, while the Kerr coefficientn₂arrives at 2.8 × 10⁻¹⁰cm²/W at 1460 nm. The achievement of simultaneous strong second- and third-order nonlinearities in one material at a telecom wavelength creates opportunities for multi-functional advanced applications in the field of nonlinear optics.

    

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2. Photocarrier‐Induced Active Control of Second‐Order Optical Nonlinearity in Monolayer MoS 2

   https://doi.org/10.1002/smll.201906347  

   Taghinejad, Mohammad ; Xu, Zihao ; Wang, Hua ; Taghinejad, Hossein ; Lee, Kyu‐Tae ; Rodrigues, Sean P. ; Adibi, Ali ; Qian, Xiaofeng ; Lian, Tianquan ; Cai, Wenshan ( January 2020 , Small)

   Atomically thin transition metal dichalcogenides (TMDs) in their excited states can serve as exceptionally small building blocks for active optical platforms. In this scheme, optical excitation provides a practical approach to control light‐TMD interactions via the photocarrier generation, in an ultrafast manner. Here, it is demonstrated that via a controlled generation of photocarriers the second‐harmonic generation (SHG) from a monolayer MoS₂crystal can be substantially modulated up to ≈55% within a timeframe of ≈250 fs, a set of performance characteristics that showcases the promise of low‐dimensional materials for all‐optical nonlinear data processing. The combined experimental and theoretical study suggests that the large SHG modulation stems from the correlation between the second‐order dielectric susceptibility χ⁽²⁾and the density of photoexcited carriers in MoS₂. Indeed, the depopulation of the conduction band electrons, at the vicinity of the high‐symmetryK/K′points of MoS₂, suppresses the contribution of interband electronic transitions in the effective χ⁽²⁾of the monolayer crystal, enabling the all‐optical modulation of the SHG signal. The strong dependence of the second‐order optical response on the density of photocarriers reveals the promise of time‐resolved nonlinear characterization as an alternative route to monitoring carrier dynamics in excited states of TMDs.

    

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3. 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)

   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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4. Electro-optic spatial light modulator from an engineered organic layer

   https://doi.org/10.1038/s41467-021-26035-y  

   Benea-Chelmus, Ileana-Cristina ; Meretska, Maryna L. ; Elder, Delwin L. ; Tamagnone, Michele ; Dalton, Larry R. ; Capasso, Federico ( October 2021 , Nature Communications)

   Tailored nanostructures provide at-will control over the properties of light, with applications in imaging and spectroscopy. Active photonics can further open new avenues in remote monitoring, virtual or augmented reality and time-resolved sensing. Nanomaterials withχ⁽²⁾nonlinearities achieve highest switching speeds. Current demonstrations typically require a trade-off: they either rely on traditionalχ⁽²⁾materials, which have low non-linearities, or on application-specific quantum well heterostructures that exhibit a highχ⁽²⁾in a narrow band. Here, we show that a thin film of organic electro-optic molecules JRD1 in polymethylmethacrylate combines desired merits for active free-space optics: broadband record-high nonlinearity (10-100 times higher than traditional materials at wavelengths 1100-1600 nm), a custom-tailored nonlinear tensor at the nanoscale, and engineered optical and electronic responses. We demonstrate a tuning of optical resonances by Δλ = 11 nm at DC voltages and a modulation of the transmitted intensity up to 40%, at speeds up to 50 MHz. We realize 2 × 2 single- and 1 × 5 multi-color spatial light modulators. We demonstrate their potential for imaging and remote sensing. The compatibility with compact laser diodes, the achieved millimeter size and the low power consumption are further key features for laser ranging or reconfigurable optics.

    

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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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