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Frequency comb and field-resolved broadband absorption spectroscopy are promising techniques for rapid, precise, and sensitive detection of short-lived atmospheric pollutants on-site. Enhancing detection sensitivity in absorption spectroscopy hinges on bright sources that cover molecular resonances and fast signal modulation techniques to implement lock-in detection schemes efficiently. Yb:YAG thin-disk lasers, combined with optical parametric oscillators (OPO), present a compelling solution to fulfill these requirements. In this work, we report on a bright OPO pumped by a Yb:YAG thin-disk Kerr-lens mode-locked oscillator delivering 2.8 W, 114 fs pulses at 2.06 {\mu}m with an averaged energy of 90 nJ. The OPO cavity operates at 30.9 MHz pulse repetition rates, the second harmonic of the pump cavity, allowing for broadband, efficient, and dispersion-free modulation of the OPO output pulses at 15.45 MHz rate. With 13% optical-to-optical conversion efficiency and a high-frequency intra-cavity modulation, this scalable scheme holds promise to advance the detection sensitivity and frontiers of field-resolved spectroscopic techniques.

Arrays of nonlinear resonators offer a fertile ground for a wide range of complex phenomena and opportunities for advanced photonic sensing and computing. Recently, significant attention has focused on studying coupled resonators in special-purpose configurations either on chips or in table-top experiments. However, a path to realizing a large-scale programmable network of nonlinear photonic resonators remains elusive because of the challenges associated with simultaneously achieving strong nonlinearity, independent operation of the resonators, and programmability of the couplings. In this work, we break these barriers by realizing large-scale, time-multiplexed optical parametric oscillators (OPOs) on a single lithium niobate nanophotonic chip. We show independent operation of 70 identical OPOs in an ultrafast nanophotonic circuit. The OPOs exhibit an ultra-low threshold of a few picojoules, substantially surpassing the strength of nonlinearity of other platforms. Using our ultrafast nanophotonic circuit, a network of N OPOs with programmable all-to-all couplings requires only a few additional components. The time-multiplexed nanophotonic OPOs can enable myriad applications, including ultrafast classical and quantum information processing., Comment: 19 pages, 5 figures

Photonics offers unique capabilities for quantum information processing (QIP) such as room-temperature operation, the scalability of nanophotonics, and access to ultrabroad bandwidths and consequently ultrafast operation. Ultrashort-pulse sources of quantum states in nanophotonics are an important building block for achieving scalable ultrafast QIP, however, their demonstrations so far have been sparse. Here, we demonstrate a femtosecond biphoton source in dispersion-engineered periodically poled lithium niobate nanophotonics. We measure 17 THz of bandwidth for the source centered at 2.09 \textmu m, corresponding to a few optical cycles, with a brightness of 8.8 GHz/mW. Our results open new paths towards realization of ultrafast nanophotonic QIP.

Traditional absorption spectroscopy has fundamental difficulty in resolving small absorbance from strong background due to the instability of laser sources. Existing background-free methods in broadband vibrational spectroscopy help to alleviate this problem but face challenges in realizing either low extinction ratios or time-resolved field measurements. Here, we introduce optical-parametric-amplification-enhanced background-free spectroscopy, in which the excitation background is first suppressed by an interferometer and then the free-induction decay that carries molecular signatures is selectively amplified. We show that this method can further improve the limit of detection in linear interferometry by order(s) of magnitude without requiring lower extinction ratios or time-resolved measurement, which can benefit sensing applications in detecting trace species., Comment: 11 pages, 5 figures

Over the past decade, cavity solitons have attracted substantial attention for their rich dynamics and their myriad potential applications. Recently, there has been growing interest in understanding cavity solitons in systems of coupled resonators, where both new physics and applications can emerge. While numerous works have theoretically studied the interplay between cavity solitons and lattice topology, experimental demonstrations of cavity solitons in topological lattices remain elusive. Here, we experimentally realize cavity solitons in a Su-Schrieffer-Heeger (SSH) lattice and illustrate that the synergy between topology and soliton formation dynamics can induce soliton formation at the boundaries of a topological SSH lattice. Our work illustrates the rich physics of cavity solitons in topological lattices and demonstrates a flexible approach to study solitons in large-scale coupled resonator arrays., Comment: 8 pages, 6 figures

Rapid advancements in deep learning over the past decade have fueled an insatiable demand for efficient and scalable hardware. Photonics offers a promising solution by leveraging the unique properties of light. However, conventional neural network architectures, which typically require dense programmable connections, pose several practical challenges for photonic realizations. To overcome these limitations, we propose and experimentally demonstrate Photonic Neural Cellular Automata (PNCA) for photonic deep learning with sparse connectivity. PNCA harnesses the speed and interconnectivity of photonics, as well as the self-organizing nature of cellular automata through local interactions to achieve robust, reliable, and efficient processing. We utilize linear light interference and parametric nonlinear optics for all-optical computations in a time-multiplexed photonic network to experimentally perform self-organized image classification. We demonstrate binary classification of images in the fashion-MNIST dataset using as few as 3 programmable photonic parameters, achieving an experimental accuracy of 98.0% with the ability to also recognize out-of-distribution data. The proposed PNCA approach can be adapted to a wide range of existing photonic hardware and provides a compelling alternative to conventional photonic neural networks by maximizing the advantages of light-based computing whilst mitigating their practical challenges. Our results showcase the potential of PNCA in advancing photonic deep learning and highlights a path for next-generation photonic computers.

Ultrabroadband frequency combs coherently unite distant portions of the electromagnetic spectrum. They underpin discoveries in ultrafast science and serve as the building blocks of modern photonic technologies. Despite tremendous progress in integrated sources of frequency combs, achieving multi-octave operation on chip has remained elusive mainly because of the energy demand of typical spectral broadening processes. Here we break this barrier and demonstrate multi-octave frequency comb generation using an optical parametric oscillator (OPO) in nanophotonic lithium niobate with only femtojoules of pump energy. The energy-efficient and robust coherent spectral broadening occurs far above the oscillation threshold of the OPO and detuned from its linear synchrony with the pump. We show that the OPO can undergo a temporal self-cleaning mechanism by transitioning from an incoherent operation regime, which is typical for operation far above threshold, to an ultrabroad coherent regime, corresponding to the nonlinear phase compensating the OPO cavity detuning. Such a temporal self-cleaning mechanism and the subsequent multi-octave coherent spectrum has not been explored in previous OPO designs and features a relaxed requirement for the quality factor and relatively narrow spectral coverage of the cavity. We achieve orders of magnitude reduction in the energy requirement compared to the other techniques, confirm the coherence of the comb, and present a path towards more efficient and wider spectral broadening. Our results pave the way for ultrashort-pulse and ultrabroadband on-chip nonlinear photonic systems for numerous applications., Comment: 8 pages, 4 figures

Mode-locked lasers (MLLs) have enabled ultrafast sciences and technologies by generating ultrashort pulses with peak powers substantially exceeding their average powers. Recently, tremendous efforts have been focused on realizing integrated MLLs not only to address the challenges associated with their size and power demand, but also to enable transforming the ultrafast technologies into nanophotonic chips, and ultimately to unlock their potential for a plethora of applications. However, till now the prospect of integrated MLLs driving ultrafast nanophotonic circuits has remained elusive because of their typically low peak powers, lack of controllability, and challenges with integration with appropriate nanophotonic platforms. Here, we overcome these limitations by demonstrating an electrically-pumped actively MLL in nanophotonic lithium niobate based on its hybrid integration with a III-V semiconductor optical amplifier. Our MLL generates $\sim$4.8 ps optical pulses around 1065 nm at a repetition rate of $\sim$10 GHz, with pulse energy exceeding 2.6 pJ and a high peak power beyond 0.5 W. We show that both the repetition rate and the carrier-envelope-offset of the resulting frequency comb can be flexibly controlled in a wide range using the RF driving frequency and the pump current, paving the way for fully-stabilized on-chip frequency combs in nanophotonics. Our work marks an important step toward fully-integrated nonlinear and ultrafast photonic systems in nanophotonic lithium niobate.

Optical solitons have long been of interest both from a fundamental perspective and because of their application potential. Both cubic (Kerr) and quadratic nonlinearities can lead to soliton formation, but quadratic solitons can practically benefit from stronger nonlinearity and achieve substantial wavelength conversion. However, despite their rich physics, quadratic cavity solitons have been used only for broadband frequency comb generation, especially in the mid-IR. Here, we show that the formation dynamics of mid-IR quadratic cavity solitons can be effectively leveraged to enhance molecular sensing. We demonstrate significant sensitivity enhancement while circumventing constraints of traditional cavity enhancement mechanisms. We perform experiments sensing CO$_2$ using quadratic cavity solitons around 4 $\mu$m and achieve an equivalent path length enhancement of 6000. Additionally, we demonstrate large sensitivity at high concentrations of CO$_2$, beyond what can be achieved using an equivalent high-finesse linear cavity by orders of magnitude. Our results highlight a path for utilizing quadratic cavity nonlinear dynamics and solitons for molecular sensing beyond what can be achieved using linear methods., Comment: 22 pages, 4 figures

Powerful and efficient optical frequency combs in the mid-infrared (MIR) spectral region are highly desirable for a broad range of applications. Despite extensive efforts utilizing various techniques, MIR frequency comb sources are still lacking power, efficiency, or bandwidth for many applications. Here, we report the generation of an intrinsically locked frequency comb source centered at 4.18 {\mu}m from an optical parametric oscillator (OPO) operating in the simulton regime, in which formation of purely quadratic solitons lead to enhanced performance. We show advantages of operation in the simulton regime in direct experimental comparisons to the conventional regime, which are also supported by simulation and theory. We achieve 565 mW of average power, 900 nm of instantaneous 3-dB bandwidth, 350% of slope efficiency, and 44% of conversion efficiency; a performance that is superior to previous OPO demonstrations and other sources in this wavelength range. This work opens a new avenue toward MIR frequency comb generation with high power and efficiency and suggests the great potential of soliton generation based on quadratic nonlinearity in the MIR spectral region., Comment: 12 pages, 6 figures (main text); 3 pages, 0 figure (supplementary materials)

One of the most fundamental quantum states of light is squeezed vacuum, in which noise in one of the quadratures is less than the standard quantum noise limit. Significant progress has been made in the generation of optical squeezed vacuum and its utilization for numerous applications. However, it remains challenging to generate, manipulate, and measure such quantum states in nanophotonics with performances required for a wide range of scalable quantum information systems. Here, we overcome this challenge in lithium niobate nanophotonics by utilizing ultrashort-pulse phase-sensitive amplifiers for both generation and all-optical measurement of squeezed states on the same chip. We generate a squeezed state spanning over more than 25 THz of bandwidth supporting only a few optical cycles, and measure a maximum of 4.9 dB of squeezing ($\sim$11 dB inferred). This level of squeezing surpasses the requirements for a wide range of quantum information systems. Our results on generation and measurement of few-optical-cycle squeezed states in nanophotonics enable a practical path towards scalable quantum information systems with THz clock rates and open opportunities for studying non-classical nature of light in the sub-cycle regime., Comment: 8 pages, 4 figures

In recent years, the computational demands of deep learning applications have necessitated the introduction of energy-efficient hardware accelerators. Optical neural networks are a promising option; however, thus far they have been largely limited by the lack of energy-efficient nonlinear optical functions. Here, we experimentally demonstrate an all-optical Rectified Linear Unit (ReLU), which is the most widely used nonlinear activation function for deep learning, using a periodically-poled thin-film lithium niobate nanophotonic waveguide and achieve ultra-low energies in the regime of femtojoules per activation with near-instantaneous operation. Our results provide a clear and practical path towards truly all-optical, energy-efficient nanophotonic deep learning.

Optical nonlinear functions are crucial for various applications in integrated photonics, such as all-optical information processing, photonic neural networks and on-chip ultrafast light sources. Due to the weak nonlinearities in most integrated photonic platforms, realizing optical nonlinear functions typically requires large driving energies in the picojoules level or beyond, thus imposing a barrier for most applications. Here, we tackle this challenge and demonstrate an integrated nonlinear splitter device in lithium niobate nano-waveguides by simultaneous engineering of the dispersion and quasi-phase matching. We achieve non-resonant all-optical switching with ultra-low energies down to tens of femtojoules, a near instantaneous switching time of 18 fs, and a large extinction ratio of more than 5 dB. Our nonlinear splitter simultaneously realizes switch-on and -off operations and features a state-of-the-art switching energy-time product as low as $1.4 \times10^{-27}$ J$\cdot$s. We also show a path toward attojoule level all-optical switching by further optimizing the device geometry. Our results can enable on-chip ultrafast and energy-efficient all-optical information processing, computing systems, and light sources.

Dual-comb spectroscopy has been proven a powerful tool in molecular characterization, which remains challenging to implement in the mid-infrared region due to difficulties in the realization of two mutually locked comb sources and efficient photodetection. Moreover, the detection capability of dual-comb spectroscopy is fundamentally limited by the strong excitation background and detector saturation. Here we introduce a variant of dual-comb spectroscopy called cross-comb spectroscopy, in which a mid-infrared comb is upconverted via sum-frequency generation with a near-infrared comb of a shifted repetition rate and then interfered with a spectral extension of the near-infrared comb. We show that cross-comb spectroscopy can have superior signal-to-noise ratio, sensitivity, dynamic range, and detection efficiency compared to other dual-comb-based methods and avoid the limits of the background excitation and detector saturation. We experimentally demonstrate a proof-of-concept measurement of atmospheric CO2 around 4.25 $\mu$m, with a 233-$cm^{-1}$ instantaneous bandwidth, 28000 comb lines, a single-shot SNR of 167, and a figure of merit of $2.4\times10^{6} Hz^{1/2}$. Cross-comb spectroscopy can be realized using up- or down-conversion and offers an adaptable and powerful spectroscopic method outside the well-developed near-IR region. This approach opens new avenues to high-performance molecular sensing with wavelength flexibility, which can impact a wide swath of applications., Comment: 12 pages, 5 figures (main text); 34 pages, 14 figures (supplementary materials)

Necessary conditions for asymptotically optimal sliding-block or stationary codes for source coding and rate-constrained simulation of memoryless sources are presented and used to motivate a design technique for trellis-encoded source coding and rate-constrained simulation. The code structure has intuitive similarities to classic random coding arguments as well as to ``fake process'' methods and alphabet-constrained methods. Experimental evidence shows that the approach provides comparable or superior performance in comparison with previously published methods on common examples, sometimes by significant margins., Comment: To appear in the IEEE Transactions on Information Theory, 13 pages

Source coding theorems and Shannon rate-distortion functions were studied for the discrete-time Wiener process by Berger and generalized to nonstationary Gaussian autoregressive processes by Gray and by Hashimoto and Arimoto. Hashimoto and Arimoto provided an example apparently contradicting the methods used in Gray, implied that Gray's rate-distortion evaluation was not correct in the nonstationary case, and derived a new formula that agreed with previous results for the stationary case and held in the nonstationary case. In this correspondence it is shown that the rate-distortion formulas of Gray and Hashimoto and Arimoto are in fact consistent and that the example of of Hashimoto and Arimoto does not form a counter example to the methods or results of the earlier paper. Their results do provide an alternative, but equivalent, formula for the rate-distortion function in the nonstationary case and they provide a concrete example that the classic Kolmogorov formula differs from the autoregressive formula when the autoregressive source is not stationary. Some observations are offered on the different versions of the Toeplitz asymptotic eigenvalue distribution theorem used in the two papers to emphasize how a slight modification of the classic theorem avoids the problems with certain singularities., Comment: Submitted to IEEE Transactions on Information Theory