We present a free-space plasmonic modulator based on a single heavily-doped semiconductor layer. We investigate its ability to modulate both the linear and nonlinear response at mid-infrared frequencies slightly below the plasma frequency of the semiconductor. We demonstrate electric control of the linear transmittance and reflectance, and of the efficiency of third-harmonic generation with a field-effect gate structure. We discuss further performance optimization of the device in terms of modulation speed and depth towards a fast modulator with very simple active material requirements. Our results establish a viable route toward practical plasmonic modulators and mixers operating in the mid-infrared atmospheric window available for free-space communications at wavelengths between 8 and 12 um.

Laser frequency combs (LFCs) are a promising technology for wavelength calibration of astronomical high-resolution spectrographs requiring utmost accuracy and stability, since they directly translate the fundamental SI time standard from the radio frequency regime to optical frequencies. However, they have so far seen limited use in practice, due to their complexity, incomplete wavelength coverage, but also the challenges in the data analysis they imply. Here, we present a detailed test of a 34 GHz electro-optic modulation comb with the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO) spectrograph. Using thin-film lithum-niobate waveguides for broadening and harmonic generation, the setup provides partial coverage of the infrared, visible, and near-ultraviolet spectral ranges. We focus on assessing the quality of the delivered spectra and their capability to facilitate accurate and stable wavelength calibration. We present a detailed analysis of the spectrally diffuse background, the line width, and characterize the line-spread function over a broader width than possible with the ESPRESSO facility LFC. Comparing both combs, we find strong local discrepancies in the wavelength calibration accuracy up to 15 m s(-1), which correlate with the echellogram structure. These do not originate from the lasers, but from misalignments in the ESPRESSO calibration unit, highlighting the strong need to make instrument fibre feeds more robust to light-injection effects. Nevertheless, we demonstrate excellent stability of the wavelength calibration, with a scatter of only 17 cm s(-1). This, however, can only be achieved when accurately modelling the non-Gaussian line-spread function, showcasing the need for advanced data analysis techniques when dealing with LFC spectra.

Periodically poled thin film lithium niobate waveguides provide simultaneous access to efficient second- and third-order nonlinear processes, enabling broadband generation of coherent laser light. Here, we demonstrate the generation of a broadband mid-infrared continuum in a nanophotonic lithium niobate waveguide pumped by a telecom-wavelength femtosecond source. Specifically, our dual-stage design includes third-order nonlinear spectral broadening followed by a dedicated periodically poled waveguide section performing efficient broadband intrapulse difference-frequency generation. Driven by sub-100 fs pulses with an estimated on-chip pulse energy of around 200 pJ, the generated mid-infrared light covers wavelengths from 3200 to 4800 nm. Cascaded harmonic generation also extends the spectrum into the visible and ultraviolet domains, resulting in an overall spectral bandwidth ranging from 350 to 4800 nm.

Astronomical precision spectroscopy supports searches for exoplanets and may enable the direct observation of the Universe’s expansion. A key challenge in these efforts is the precise and long-term calibration of astronomical spectrographs. White light-illuminated Fabry–Pérot cavities, cross-calibrated with hollow-cathode lamps, are reliable calibrators. However, they exhibit temporal variability and nontrivial dispersion of the resonance frequencies, and the calibration lamps provide only sparse spectra with complex line shapes, which are also prone to aging. In contrast, laser frequency combs (“astrocombs”) can offer SI-referenced accuracy and precision, but the requirement of resolvable comb lines implies pulse repetition rates >10 GHz, which is challenging to implement. Here, we explore an alternative technique where lower-repetition rate dual-frequency comb lasers (comb lines unresolvable by the spectrograph) measure the modes of a 12 GHz Fabry–Pérot cavity across a spectral span of 30 THz (175–205 THz). This effectively transfers the comb’s accuracy to the cavity and enables precise near real-time tracking of the cavity modes. These measurements are of immediate relevance to currently used calibration techniques, and with further advances, this approach could potentially support spectrograph calibration at the level needed to search for Earth-like exoplanets.

Lightwave-driven scanning tunnelling microscopy (STM) at near-IR frequencies promises an unprecedented combination of atomic spatial resolution and temporal resolution approaching the attosecond range. To achieve this goal, high-sensitivity optical control and detection of sub-cycle tunnelling currents must be achieved at the STM junction. Here, we demonstrate the generation and detection of coherent ultrafast currents across the junction of an STM illuminated by near-infrared single-cycle pulses. We introduce a modulation scheme that avoids time-dependent thermal loading while selectively isolating carrier-envelope phase (CEP)-dependent photocurrents. All artifacts arising from periodic modulation of laser power and thermal coupling are efficiently suppressed, enabling a clean readout of the coherent portion of the ultrafast tunneling current.

Optical frequency combs and their spectra of evenly spaced discrete laser lines are essential to modern time and frequency metrology. Recent advances in integrated photonic waveguides enable efficient nonlinear broadening of an initially narrowband frequency comb to multi-octave bandwidth. Here, we study the nonlinear dynamics in the generation of such ultrabroadband spectra where different harmonics of the comb can overlap. We show that a set of interleaved combs with different offset frequencies extending across the entire spectrum can emerge, which can be arranged into a practically evenly spaced ultrabroadband frequency comb when the initial comb is offset-free.

Gallium nitride (GaN) is a promising material for nonlinear optics. It offers a second-order nonlinearity up to 20 pm/V and a third-order nonlinearity that is one order of magnitude higher than in silicon nitride at 1550 nm [1], [2]. Owing to its large bandgap of 3.4 eV GaN has a wide transparency range and is free of two-photon and three-photon absorption in the technologically important erbium-gain window. Recent advances have explored integrated GaN waveguide as a new platform for integrated photonics, demonstrating second-harmonic generation, four-wave mixing, soliton generation [2]–[4], supercontinuum generation and chip-based f-2f interferometry [5].

Continuous-wave (CW) laser-driven integrated Kerr microresonators enable broadband optical frequency combs with high repetition rates and low threshold power, in a compact footprint. A drawback of such microcombs is the low conversion efficiency from the pump laser to the comb lines, which is often in the few percent range or below. In recent works, improved conversion efficiency has been demonstrated by leveraging soliton crystal states [1], interferometric back-coupling to suppress the residual pump light [2], or tuning the resonance frequency of the pumped mode [3]. While these approaches achieve improved conversion efficiency, they add complexity to the design and operation of the system, relying on non-deterministic states, dynamic control, and/or a potentially reduced number of comb lines.

Nonlinear optics is the precursor for many of the modern-day applications of photonics, including femtosecond pulse synthesis, precision spectroscopy, and metrology. In the last decade, nanophotonic waveguides have not only boosted the efficiencies of nonlinear effects but also unlocked new degrees of freedom in the design process and enabled the monolithic integration of multiple nonlinear devices. Now, the advent of thin-film variants of platforms with a strong second-order nonlinearity such as lithium niobate-on-insulator (LNOI) enables entirely new applications while further improving efficiency for the existing ones. However, suitable fabrication processes are needed to exploit the full potential of these new platforms. Here, we introduce a process for fabricating high-confinement lithium niobate waveguides with periodic poling. Our waveguide designs enable both third-order nonlinear χ(3) broadening and sum frequency generation (SFG) up to the fourth harmonic through a quasi-phase-matched χ(2) section. In supercontinuum (SC) experiments, our devices produce multi-octave SC spectra when pumped with an 80 fs mode-locked laser at 1560 nm.

Supercontinua are broadband spectra that are essential to optical spectroscopy, sensing, imaging, and metrology. They are generated from ultrashort laser pulses through nonlinear frequency conversion in fibers, bulk media, and chip-integrated waveguides. For any generating platform, balancing the competing criteria of strong nonlinearity, transparency, and absence of multiphoton absorption is a key challenge. Here, we explore supercontinuum generation in integrated gallium nitride (GaN) waveguides, which combine a high Kerr nonlinearity, mid-infrared transparency, and a large bandgap that prevents two- and three-photon absorption in the technologically important telecom C-band, where compact erbium-based pump lasers exist. Using this type of laser, we demonstrate tunable dispersive waves and gap-free spectra extending to almost 4 mu m mu m in wavelength, which is relevant to functional group chemical sensing. Additionally, leveraging the material's second-order nonlinearity, we implement on-chip f-to-2f interferometry to detect the pump laser's carrier-envelope offset frequency, which enables precision metrology. These results demonstrate the versatility of GaN-on-sapphire as a platform for broadband nonlinear photonics.

Femtosecond laser pulses enable the synthesis of light across the electromagnetic spectrum and provide access to ultrafast phenomena in physics, biology, and chemistry. Chip-integration of femtosecond technology could revolutionize applications such as point-of-care diagnostics, biomedical imaging, portable chemical sensing, or autonomous navigation. However, current sources lack the required power, and the on-chip amplification of femtosecond pulses is an unresolved challenge. Here, addressing this challenge, we report >50-fold amplification of 1 GHz-repetition-rate chirped femtosecond pulses in a CMOS-compatible photonic chip to 800 W peak power with 116 fs pulse duration. Nonlinear effects, usually a hallmark of integrated photonics but prohibitive to pulse amplification are mitigated through all-normal dispersion, large mode-area rare-earth-doped gain waveguides. These results offer a pathway to chip-integrated femtosecond technology with power-levels characteristic of table-top sources.

Photonic-crystal waveguides have led to a new generation of integrated high-Q Kerr-nonlinear microresonators, in which linear and nonlinear dynamics can be strongly influenced and tailored. We discuss ultrashort pulse and frequency comb formation in such resonators, novel self- and sideband injection locking dynamics, as well as, methods for full phase-stabilization as needed for chip-based optical precision metrology.

Continuous-wave (CW) laser-driven integrated Kerr microresonators enable broadband optical frequency combs with high repetition rates and low threshold power, in a compact footprint. A drawback of such microcombs is the low conversion efficiency from the pump laser to the comb lines, which is often in the few percent range or below. Here, complementing previously demonstrated approaches to increase conversion efficiency, we demonstrate a novel approach that leverages a chip-based rare-earth (Tm3+)-doped optical gain medium to boost the pump-to-comb conversion efficiency by more than one order of magnitude. Importantly, the gain medium does not require an additional pump laser, but recycles residual pump light from the Kerr-comb: the CW pump of the Kerr-comb (1610 nm) coincides with the pump wavelength of the on-chip gain medium, allowing unconverted pump power to be absorbed and transferred to the comb lines within gain window (1700 - 1900 nm). This enables a new class of highly efficient Kerr-combs for applications e.g. in data centers and optical computing.

In this work, we introduce a technical approach to harness liquids for highly stable and efficient Supercontinuum Generation (SCG) at up to few hundreds of kHz. Using a differential pressure scheme, the velocity at which the liquid interacting with the laser is exchanged, is optimized to achieve pump source limited stability. This approach is validated by generating a SC in water pumped at the Fundamental Wavelength (FW) and the Second Harmonic (SH) of a Yb:KGW laser amplifier at 50 kHz and 100 kHz. In addition to its high stability, the resulting SC signal is more broadband and has a higher spectral intensity compared to the signals obtained with the established crystals Yttrium Aluminum Garnat (YAG) and sapphire.

We demonstrate for the first-time f emtosecond p ulse a mplification in a CMOS-compatible photonic chip. We report > 50-fold amplification of 1 GHz-repetitionrate chirped femtosecond pulses to 800 W of on-chip peak power with 116 fs pulse duration.

The investigation of optical phenomena in the strong-field regime requires few-cycle laser pulses at field strengths exceeding gigavolts per meter (GV/m). Surprisingly, such conditions can be reached by tightly focusing pJ-level pulses with nearly octave spanning optical bandwidth onto plasmonic nanostructures, exploiting the field-enhancement effect. In this situation, the Gouy phase of the focused beam can deviate significantly from the monochromatic scenario. Here, we study the effect of the Gouy phase of a pulse exploited to drive coherent strong-field photocurrents within a plasmonic gap nanoantenna. While the influence of the specific Gouy phase profile in the experiment approaches the monochromatic case closely, this scheme may be utilized to identify more intricate phase profiles at sub-diffraction scale. Our results pave the way for Gouy phase engineering at picojoule (pJ) pulse energy levels, enabling the optimization of strong-field optical phenomena.

Astronomical precision spectroscopy underpins searches for life beyond Earth, direct observation of the expanding Universe and constraining the potential variability of physical constants across cosmological scales. Laser frequency combs can provide the critically required accurate and precise calibration to the astronomical spectrographs. For cosmological studies, extending the calibration with such astrocombs to the ultraviolet spectral range is highly desirable, however, strong material dispersion and large spectral separation from the established infrared laser oscillators have made this exceedingly challenging. Here, we demonstrate for the first time astronomical spectrograph calibrations with an astrocomb in the ultraviolet spectral range below 400 nm. This is accomplished via chip-integrated highly nonlinear photonics in periodically-poled, nano-fabricated lithium niobate waveguides in conjunction with a robust infrared electro-optic comb generator, as well as a chip-integrated microresonator comb. These results demonstrate a viable route towards astronomical precision spectroscopy in the ultraviolet and may contribute to unlocking the full potential of next generation ground- and future space-based astronomical instruments.

Precision astronomical spectroscopy is vital for seeking life beyond Earth and often relies on detecting very small wavelength shifts over years. Precision of these instruments are ensured by regular wavelength calibration and laser frequency combs stabilized with frequency standards have recently emerged as suitable sources. In this work, we demonstrate wavelength calibration of an astronomical spectrograph in ultraviolet spectrum below 400 nm. This is achieved using second- and third- order nonlinear effects in thin-film, periodically poled lithium niobate waveguides with an infrared electro-optic comb generator at 18 GHz.