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1. Ultrafast second-order nonlinear photonics—from classical physics to non-Gaussian quantum dynamics: a tutorial

   https://doi.org/10.1364/AOP.495768  

   Jankowski, Marc; Yanagimoto, Ryotatsu; Ng, Edwin; Hamerly, Ryan; McKenna, Timothy P; Mabuchi, Hideo; Fejer, M M (June 2024, Advances in Optics and Photonics)

   Photonic integrated circuits with second-order (χ⁽²⁾) nonlinearities are rapidly scaling to remarkably low powers. At this time, state-of-the-art devices achieve saturated nonlinear interactions with thousands of photons when driven by continuous-wave lasers, and further reductions in these energy requirements enabled by the use of ultrafast pulses may soon push nonlinear optics into the realm of single-photon nonlinearities. This tutorial reviews these recent developments in ultrafast nonlinear photonics, discusses design strategies for realizing few-photon nonlinear interactions, and presents a unified treatment of ultrafast quantum nonlinear optics using a framework that smoothly interpolates from classical behaviors to the few-photon scale. These emerging platforms for quantum optics fundamentally differ from typical realizations in cavity quantum electrodynamics due to the large number of coupled optical modes. Classically, multimode behaviors have been well studied in nonlinear optics, with famous examples including soliton formation and supercontinuum generation. In contrast, multimode quantum systems exhibit a far greater variety of behaviors, and yet closed-form solutions are even sparser than their classical counterparts. In developing a framework for ultrafast quantum optics, we identify what behaviors carry over from classical to quantum devices, what intuition must be abandoned, and what new opportunities exist at the intersection of ultrafast and quantum nonlinear optics. Although this article focuses on establishing connections between the classical and quantum behaviors of devices withχ⁽²⁾nonlinearities, the frameworks developed here are general and are readily extended to the description of dynamical processes based on third-orderχ⁽³⁾nonlinearities. 

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2. Quantum critical electro-optic and piezo-electric nonlinearities

   https://doi.org/10.1126/science.adx8657  

   Anderson, Christopher P; Scuri, Giovanni; Chan, Aaron; Eun, Sungjun; White, Alexander D; Ahn, Geun Ho; Jilly, Christine; Safavi-Naeini, Amir; Van_Gasse, Kasper; Li, Lu; et al (October 2025, Science)

   Although electro-optic (EO) nonlinearities are essential for many quantum and classical photonics applications, a major challenge is inefficient modulation in cryogenic environments. Guided by the connection between phase transitions and nonlinearity, we identify the quantum paraelectric perovskite SrTiO₃as a strong cryogenic EO [>500 picometers per volt (pm/V)] and piezo-electric material (>90 picocoulombs per newton) atT= 5 K, at frequencies to at least 1 megahertz. Furthermore, by tuning SrTiO₃toward quantum criticality, we more than double the EO and piezo-electric effects, demonstrating a linear Pockels coefficient above 1000 pm/V. Our results probe the link between quantum phase transitions, dielectric susceptibility, and nonlinearity, unlocking opportunities in cryogenic optical and mechanical systems and providing a framework for discovering new nonlinear materials. 

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3. Highly sensitive detection of broadband terahertz waves using aqueous salt solutions

   https://doi.org/10.1364/OE.472753  

   Zhang, Ming-Hao; Xiao, Wen; Wang, Wei-Min; Zhang, Rui; Zhang, Cun-Lin; Zhang, Xi-Cheng; Zhang, Liang-Liang (January 2022, Optics Express)

   Water-based coherent detection of broadband terahertz (THz) wave has been recently proposed with superior performances, which can alleviate the limited detection bandwidth and high probe laser energy requirement in the solid- and air-based detection schemes, respectively. Here, we demonstrate that the water-based detection method can be extended to the aqueous salt solutions and the sensitivity can be significantly enhanced. The THz coherent detection signal intensity scales linearly with the third-order nonlinear susceptibilityχ⁽³⁾or quadratically with the linear refractive indexη₀of the aqueous salt solutions, while the incoherent detection signal intensity scales quadratically withχ⁽³⁾or quartically withη₀, proving the underlying mechanism is the four-wave mixing. Both the coherent and incoherent detection signal intensities appear positive correlation with the solution concentration. These results imply that the liquid-based THz detection scheme could provide a new technique to measureχ⁽³⁾and further investigate the physicochemical properties in the THz band for various liquids. 

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4. HfO ₂ -based platform for high-index-contrast visible and UV integrated photonics

   https://doi.org/10.1364/OL.553552  

   Jaramillo, Oscar; Natarajan, Vighnesh; Mahmud_Rivy, Hamim; Tensuan, Joshua; Massai, Leonardo; Mehta, Karan K (January 2025, Optics Letters)

   Ultraviolet and visible integrated photonics enable applications in quantum information, sensing, and spectroscopy, among others. Few materials support low-loss photonics into the UV, and the relatively low refractive index of known depositable materials limits the achievable functionality. Here, we present a high-index integrated photonics platform based on HfO₂and Al₂O₃composites deposited via atomic layer deposition (ALD) with low loss in the visible and near UV. We show that Al₂O₃incorporation dramatically decreases bulk loss compared to pure HfO₂, consistent with inhibited crystallization due to the admixture of Al₂O₃. Composites exhibit refractive indexnfollowing the average of that of HfO₂and Al₂O₃, weighted by the HfO₂fractional compositionx. Atλ = 375 nm, composites withx = 0.67 exhibitn = 2.01, preserving most of HfO₂’s significantly higher index, and 3.8(7) dB/cm material loss. We further present fully etched and cladded waveguides, grating couplers, and ring resonators, realizing a single-mode waveguide loss of 0.25(2) dB/cm inferred from resonators of 2.6 million intrinsic quality factor atλ = 729 nm, 2.6(2) dB/cm atλ = 405 nm, and 7.7(6) dB/cm atλ = 375 nm. We measure the composite’s thermo-optic coefficient (TOC) to be 2.44(3) × 10⁻⁵RIU/°C nearλ = 397 nm. This work establishes (HfO₂)_x(Al₂O₃)_1−xcomposites as a platform amenable to integration for low-loss, high-index photonics spanning the UV to NIR. 

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5. 20 GHz fiber-integrated femtosecond pulse and supercontinuum generation with a resonant electro-optic frequency comb

   https://doi.org/10.1063/5.0165681  

   Sekhar, Pooja; Fredrick, Connor; Carlson, David R; Newman, Zachary L; Diddams, Scott A (November 2023, APL Photonics)

   Frequency combs with mode spacing of 10–20 GHz are critical for increasingly important applications such as astronomical spectrograph calibration, high-speed dual-comb spectroscopy, and low-noise microwave generation. While electro-optic modulators and microresonators can provide narrowband comb sources at this repetition rate, a significant remaining challenge is a means to produce pulses with sufficient peak power to initiate nonlinear supercontinuum generation spanning hundreds of terahertz (THz) as required for self-referencing. Here, we provide a simple, robust, and universal solution to this problem using off-the-shelf polarization-maintaining amplification and nonlinear fiber components. This fiber-integrated approach for nonlinear temporal compression and supercontinuum generation is demonstrated with a resonant electro-optic frequency comb at 1550 nm. We show how to readily achieve pulses shorter than 60 fs at a repetition rate of 20 GHz. The same technique can be applied to picosecond pulses at 10 GHz to demonstrate temporal compression by 9× and achieve 50 fs pulses with a peak power of 5.5 kW. These compressed pulses enable flat supercontinuum generation spanning more than 600 nm after propagation through multi-segment dispersion-tailored anomalous-dispersion highly nonlinear fibers or tantala waveguides. The same 10 GHz source can readily achieve an octave-spanning spectrum for self-referencing in dispersion-engineered silicon nitride waveguides. This simple all-fiber approach to nonlinear spectral broadening fills a critical gap for transforming any narrowband 10–20 GHz frequency comb into a broadband spectrum for a wide range of applications that benefit from the high pulse rate and require access to the individual comb modes. 

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