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Hands-on approach to adaptive optics: wavefront sensors, deformable mirrors and…

Precision spectroscopy of atomic hydrogen provides important tests of bound-state quantum electrodynamics (QED) and searches for new physics. As an independent approach to these tests heavier one-electron atoms can be investigates, as high-order QED terms scale with powers of Z. We aim to measure the 1S-2S transition in He+ with a combination of 790 nm and 32 nm photons using the Ramsey-comb spectroscopy method in combination with high-harmonic generation (HHG). As an important first step towards this goal, we present the first precision measurements using both these techniques on a transition in xenon at 110 nm. The relative accuracy of the measurement of 2.3x10-10 was not hampered by the HHG process and is the highest ever achieved with light generated through this process. A second important step was recently taken with the first observation of laser excitation of the 1S-2S transition in He+ using our unequal photon scheme. In this case, a single 150 fs pulse was sent through a neutral beam of He atoms to ionize them to He+, then excite the 1S-2S transition and ionize the excited ions further to He2+. This paves the way to a precision measurement in a single trapped He+ ion with Ramsey-comb spectroscopy.

We perform broadband cavity ring-down spectroscopy (CRDS) relying on the near-infrared frequency comb as the excitation source and a time-resolved mechanical Fourier transform spectrometer as detection device. The many decays corresponding to each spectral element are recorded simultaneously and sorted after Fourier transformation to yield the CRDS spectrum of CO in Ar contained in a 20’000-finesse cavity.

We present here an experimental demonstration of dual-comb spectroscopy performed around 2 μm, with the use of a new design of dispersion-controlled highly nonlinear fibre. The latter allows us to efficiently convert light via a four-wave mixing process from 1.55 μm to 2 μm, which is a spectral region suited for the detection of pollutants such as CO2 or N2O. Experimental measurements have been performed with these two molecules and show an excellent agreement with the HITRAN database.

Chip-scale Terahertz quantum cascade laser frequency combs: recent advanced and applications in near field-nanoscopy

In this presentation, we will discuss dissipative Kerr soliton generation on Lithium Niobate and Silicon Carbide microresonators. Additionally, I will introduce inverse-designed nonlinear Fabry-Perot resonators on Silicon Nitride and Silicon Carbide platforms for frequency comb generation

Repetition-rate locking in soliton microcombs via the injection of a weak second continuous-wave laser in the spectral wing of the soliton is studied experimentally, resulting in all-optical control and reduction of phase noise through optical division.

We present a tapered coupler design to achieve single-mode operation in multimode microring resonators and experimentally demonstrate Kerr comb generation in a high-Q, single-mode operated AlGaAs-on-insulator multimode microring resonator.

We report on the formation of temporal solitons in a Kerr fiber resonator that includes a phase modulator shorter than the fundamental limit imposed by the stimulated Raman scattering.

We report the observation of a stable and broadband optical frequency comb in a high-Q fiber Fabry Perot resonator. We evidence it arises from an unexpected mode-locking phenomena.

In this work we show Frequency Comb (FC) and short pulsed operation of mid-infrared Interband Cascade Lasers (ICLs) in a single long section. This is through the use of an adapted ultrafast Quantum Well Infrared Photodetectors (QWIPs), and correlating the microwave beatnotes with high resolution spectra of the ICL. In particular, we will show active mode-locking (ML) of single-section ICL that does not require RF optimisation of the ICL device and highlight its temporal characteristics using Shifted Wave Infrared Fourier Transform Spectroscopy (SWIFTS) analysis to reconstruct the intensity in the time domain.

We report on a multi-GHz repetition rate, femtosecond fiber laser operating in the burst mode, achieved by nonlinearly shaping and amplifying a phase-only modulated electro-optic comb at 1.03 μm. The system delivers an average power of 1.2 W with pulses compressible down to sub 100 fs.

Electro-optic frequency-comb is an interesting method for comb generation as it offers the possibility to electrically tune the generated frequency-comb by simply tuning the electrical signal applied on the modulator. Integrated modulators operating in a wide spectral range in the mid-IR have been demonstrated recently, relying on free carrier plasma dispersion effect in a Schottky diode embedded in a Ge-rich graded SiGe waveguide. Such integrated mid-infrared modulators have been used to generate electro-optic frequency-combs with more than 200 lines around the 8 µm wavelength optical carrier.

We present a planarized waveguide cavity with an integrated broadband output coupler that improves both the output power and far-field properties of THz quantum cascade laser frequency combs. The laser mirror reflectivity can be tuned by the shape of the end facet, which is obtained with an efficient inverse design algorithm, where the structure is iteratively updated to match the desired figure of merit. Surface emission is achieved with a broadband patch-array antenna, and all the components have been optimized for octave-spanning emission spectra (2-4 THz). Experimentally, we demonstrate a broadband surface-emitting THz quantum cascade laser frequency comb with optical bandwidths of up to 800 GHz, surface emission into a narrow beam with divergence below (20° x 20°) and a peak power of 13 mW.

This paper reports the development of a dual-comb spectroscopy system based on gain-switched single-mode laser diodes emitting at 1572 nm. The switching is performed by a train of short electrical pulses at a repetition rate of 100 MHz while the lasers are subjected to external optical injection. Under these conditions, broad and flat optical frequency combs (OFC) are generated, showing 100 tones within 10 dB and a line spacing fixed by the repetition rate of the switching pulses. The dual-comb generated by the interference of two such combs with slightly different line spacing, is used for measuring the absorption line of CO2 at 1572.02 nm. The transmission spectrum is analyzed and fitted to a Voigt profile. A deviation of 3% in the residuals of the fitting is obtained, denoting the high spectral performance of the spectroscopy system.

While the opportunity to perform fast and broadband spectroscopy in the Mid-IR portion of the electromagnetic spectrum is very appealing, it requires the use of compatible light-sources. Here, we strongly modulate a Mid-Infrared Quantum Cascade Laser at a frequency in the RF domain, which is low compared to the natural repetition frequency of the device. In this way, we demonstrate that it is possible to obtain an emission bandwidth of up to 250𝐜𝐦−𝟏. Finally, we employ it as a light source in an FTIR based on a rotational delay line, performing fast and broadband FTIR spectroscopy.

A key component of quantum communication is the distribution of entanglement through networks, however this comes with several challenges. One of the most significant is that direct transmission of quantum information between two nodes via a standard fibre link is unfeasible for transmission distances of more than a few hundred kilometres [1]. A solution is the ‘quantum repeater’ architecture, which distributes entanglement between two nodes via an intermediate repeater node [2]...

We designed a second harmonic generation (SHG) device utilizing the fundamental TE-TM modes to achieve phase matching, and experimentally investigated the relationship between the phase matching wavelength and the waveguide width on the AlGaAs-on-insulator platform. A SHG efficiency of 201% W-1 cm-2 is obtained with a broad bandwidth of 7.6nm in the 1-mm-long waveguide.

We demonstrate experimentally critical coupling for nonlinear conversion in grating-coupled Fabry-Pérot planar microcavities known as Cavity-Resonant Integrated Grating Filters (CRIGFs).

Novel asymmetric designs offer Q-factors from 1000 to 7000 and allow critical coupling with maximised SHG.

We developed an improved coupled-mode model for the linear and non-linear spectral response of CRIGFs which allows accurate insight on the intrinsic and coupling losses in these microcavities.

We designed an X-cut lithium niobate (LiNbO3) membrane dedicated to type I second harmonic generation (SHG) at telecom wavelength. A competitive conversion efficiency compared to a quasi-phase-matched configuration with the advantage of a broadband response of 100nm is shown.

Hybrid photonic devices represent a promising solution to the effective on-chip integration of all the components required for the generation, manipulation and detection of non-classical states of light encoding quantum information. We present an AlGaAs source of highly entangled photon pairs envisioned for the hybridization with silicon-on-insulator integrated platforms, in order to take benefit from the strong second order nonlinearity and the compliance with electrical pumping of the III-V platform and the maturity and CMOS compatibility of silicon photonic circuitry, enabling a wide variety of quantum information applications.

Highly anisotropic crystals have recently attracted considerable attention due to their ability to support polaritons with unique properties, such as hyperbolic dispersion, negative phase velocity, or extreme confinement. In particular, the biaxial van der Waals semiconductor α-phase molybdenum trioxide (α-MoO3) has received much attention [1] due to its ability to support in-plane hyperbolic phonon polaritons (PhPs) —infrared (IR) light coupled to lattice vibrations in polar materials— with ultra-low losses, offering an unprecedented platform for controlling the flow of energy at the nanoscale.

In this talk, we will show experimental demonstrations of the unique behavior of PhPs in these crystals, including the visualization of anomalous cases of the fundamental optical phenomena of refraction [2] and reflection [3], and the exotic phenomenon of canalization, in which PhPs propagate along a single direction with ultralow losses [4].

In this work, we experimentally demonstrate the photo-inscription of complex waveguiding structures by a single diffracting Bessel beam (BB) propagating in a biased SBN crystal. Our optical platform enables all-optical control of the characteristics of such induced configurations by tailoring the parameters such as beam size, the electric field, and the input beam intensity. Our numerical results are in good agreement with our experimental work. In addition, we numerically study the interaction of two counterpropagating (CP) BBs under nonlinear conditions and the spatiotemporal dynamics of these photo-induced configurations. These results suggest more opportunities for fully controllable complex waveguiding structures and new all-optical solutions for active components in optical communication.

Spin-orbit interactions (SOI), describing the transfer of a spin degree of freedom to an orbital angular momentum (OAM), have been widely explored in recent opto-acoustic studies for applications mainly in spintronics and for topological insulators. We report the observation of SOI by Brillouin scattering in an optical nanofiber. Specifically, we describe the transfer of a spin degree of freedom from light incident to the nanofiber to an acoustic vortex with a topological charge of order 2 in the form of OAM. Coupled with the phase matching condition for the energy conservation during Brillouin scattering, it results in a backscattered wave with a spin opposite to the incident wave. This observation allows considering applications of opto-acoustic Brillouin memory based on polarization conversion through a SOI.

We report on the use of PMMA/DR1 coating to enhance the efficiency of photon pair generation in silica nanofibers. The coating improves the second-order nonlinear susceptibility of the nanofibers, leading to improved photon pair generation efficiency. We investigate the effect of varying the nonlinear optical properties of the composite material, and we characterize the photon pair generation efficiency of the coated silica nanofibers. Our modelling results show a significant enhancement in photon pair generation efficiency by a factor of 1000 compared to a bare silica nanofiber.

Optical clocks based on trapped laser-cooled atoms and single ions have demonstrated frequency stability and estimated systematic frequency uncertainty far surpassing the current generation of caesium microwave frequency standards, with the result that a future optical redefinition of the SI second is anticipated. However before this can happen several key challenges remain to be addressed.

Firstly, the uncertainty budgets of the optical clocks need to be validated through a programme of international comparisons between systems developed independently by different research groups around the world. Continuity with the current caesium-based definition must also be ensured, by performing absolute frequency measurements with as low an uncertainty as possible. And finally, we need to improve the robustness of optical clocks and automate their operation. This will enable them to be operated routinely as secondary representations of the second, regularly contributing to International Atomic Time (TAI) via reporting to the International Bureau of Weights and Measures, and being used to steer the local UTC(k) time scales maintained by national timing laboratories.

I will discuss recent progress towards addressing these challenges, in particular drawing on examples of work performed in the European collaborative project Robust Optical Clocks for International Timescales (ROCIT).

Using near-infrared light, we tightly-lock a mid-infrared quantum cascade laser frequency comb to another laser, achieving a residual integrated phase noise of 200 mrad. This high coherence is pertinent for highly-sensitive dual-comb spectroscopy and metrology.

We will report our recent progress toward H2+ spectroscopy by use of a SI-referenced Mid-IR source laser. H2+ molecular ions are very interesting candidates to improve the determination of fundamental constants, such as the proton to electron mass ratio mp/me and search for new physics beyond the standard model. At LKB, an erbium fibered frequency comb is phase locked to the LNE-SYRTE frequency standards thanks to the T-REFIMEVE network. By sum frequency generation in a AgGaSe2 crystal between a CO2 laser and an output of the comb at 1895 nm, a shifted frequency comb centered at 1560 nm is generated. The latter is then mixed with the original one to generate a beatnote used to stabilise the Mid-IR laser. As a first application, a narrow saturated absorption line in formic acid has been extensively studied. Pressure, power and modulation depth shifts and broadenings have been evaluated, leading to a determination of its central frequency at a sub ppt (10^-12) resolution, high enough for H2+ spectroscopy and fundamental constant determination.

Dual-comb systems have demonstrated their potential for metrology, e.g. spectroscopy vibrometry or ranging. However, the implementation of dual-combs is often complex and generally requires substantial optical and optoelectronic hardware. Here, we propose a simple and compact architecture based on a bidirectional frequency shifting loop, that provides more than 100 mutually coherent comb lines. The system makes use of a CW laser and a slow electronic detection chain (10 MSa/s). We have implemented two configurations enabling dynamic multi-heterodyne interferometry at 20kHz: the first one makes use of acousto-optic frequency shifters, and allows highly sensitive distributed acoustic sensing along a fiber. The second one involves electro-optic frequency shifters, and enables ranging with a sub-mm resolution.

We show that our developed free-running, bidirectional ring Ti:Sa laser cavity meets the requirements for Dual Comb Spectroscopy in the UV range (UV-DCS). Two counter-propagative frequency combs with slightly different repetition rate are generated in such a cavity and we show quantitatively that this repetition rate difference can be explained by the self-steepening effect. Molecular absorption lines of the O2 A-band centered around 760~nm are measured with a 1,5 GHz spectral resolution, demonstrating that the mutual coherence of the two combs allows GHz-resolution DCS measurements. Moreover, we demonstrate that the generated output peak power allows for efficient second harmonic generation (SHG), in the scope of developing laboratory and open-path UV-DCS experiments.

ITO is a transparent conductive material commonly used in everyday life and for its potential in material research. In nonlinear optics for instance, ITO has shown great capabilities for harmonic generation in its epsilon-near-zero configuration. In this article, we demonstrate a completely new nonlinear behaviour from ITO thin layers. After a Ga-Focused Ion Beam (FIB) milling of the thin film, we observe nonlinear photoluminescence produced for tightly focused femtosecond pulses. The signal shares strong similarities with that commonly detected from noble metals. We show that the arising of this nonlinear photoluminescence originates from the radiative decay of metal-like hot electron distribution in the material and is associated to a modification of the optical properties by the Ga ions.

In this talk we introduce two new European projects DYNAMOS and ADOPTION, which will develop advanced integrated photonic technologies for WDM hyperscale data centre architectures including a novel stamped metallic micro-mirror array for advanced PIC-to-fibre coupling with the potential to dramatically reduce PIC design and assembly costs. We also discuss international standardization efforts for these corresponding optical interconnect technologies for future hyperscale data centre, HPC and 6G and Quantum environments.

We review our recent progress on advanced silicon photonic devices and photonic circuits, including advanced grating couplers, modulators, mode and polarization division multiplexing and integrated optical signal processors for use in high capacity data center interconnects. The use of shifted polysilicon overlay gratings on waveguide grating couplers to improve coupling efficiency and polarization independence will be described. We also present our recent results on silicon microring and silicon-germanium electroabsorption modulators for 100Gbaud data transmission and their use polarization and mode division multiplexed optical fiber interconnects. We present novel integrated optical signal processors which can unscramble the mixing of polarization and mode data lanes that will occur after fiber transmission and demonstrate 400Gb/s per wavelength intensity modulation direct-detection silicon photonic transceivers.

We discuss recent results related to the realization of integrated waveguide and fibre-based phase filtering devices for high-speed linear passive logic applications. A record frequency resolution of 1 GHz (a 10-fold improvement over standard optical waveshaper technology) is achieved using fibre Bragg grating-based phase filters. We have also succeeded in realizing Bragg grating devices with sub-mm2 device footprint based on spiral waveguides, paving the way for on-chip implementation. Using (all-pass) phase-only filtering devices, NOT and XNOR logic operations are demonstrated at ultrafast operation speeds with a few-fJ/bit energy consumption.

At Zero Point Motion we create low noise chipscale optical inertial sensors by combining photonic integrated circuit (PIC) structures with micro-electro-mechanical systems (MEMS) mechanical structures. Our technology is derived from cavity optomechanics which uses the coupling between optical resonances and mechanical motion to detect picometer to femtometre displacements, producing lower noise readout compared with capacitive devices. Our platform comprises ring resonators with whispering gallery mode resonances, that are evanescently coupled to the motion of MEMS accelerometer and vibratory gyroscope test-mass. We report on progress in realising an integrated device, with both MEMS and PIC structures die-to-die bonded and then packaged with light sources and detectors on-chip.

Here, we report on a CMOS-compatible thulium-doped high power continuous wave (CW) optical amplifier, leveraging large mode-area gain waveguides. The amplifier structure combines a silicon nitride waveguide

above which a sputtered 1250 nm-thick thulium-doped alumina gain layer is deposited. We demonstrate > 220 mW output signal power at center wavelength of 1850 nm inside a 9-cm-long amplifier. Small signal gain of > 15 dB is achieved.

We propose a novel polarization rotator concept using two-dimensional (2D) anisotropic materials, such as molybdenum trioxide, on top of 3 µm silicon waveguides. The in-plane anisotropic behavior of 500 nm thick MoO3 flake is analyzed and confirmed with Raman spectroscopy.

Silicon waveguides are a promising candidate for integrated nonlinear optics applications. Nonlinear coefficients of Silicon on Insulator (SOI) waveguides have been previously measured using techniques such as Z-scan, D-scan, Four Wave Mixing (FWM) and Self-phase modulation. However, they have several drawbacks such as they operate at high power or are cumbersome to setup and require multiple measurements to determine all the coefficients. In this work, we develop a direct and single measurement technique to characterize the nonlinear processes in SOI waveguides. This is achieved by employing a heterodyne interferometric technique to accurately measure minute nonlinear response. The measured nonlinear amplitude and phase shifts at 1550 nm wavelength are fit to extract third-order nonlinear coefficients of Two-photon absorption, Kerr nonlinear index, Free carrier absorption and Free carrier dispersion. The obtained coefficients for SOI waveguides are close to that found in literature measured using the above mentioned techniques. The advantages of this method include easy interpretation of the output signal and relatively low power of operation. It is especially advantageous for studying materials such as Phase Change Materials (PCM) in which phase changes occur dynamically. This aspect is quite promising for characterizing emerging materials for integrated photonics applications.

Ray-wave correspondence has proven a powerful tool in mesoscopic optics, in particular in the description of deformed microdisc cavities with a versatile application potential ranging from microlasers to sensors. New material classes such as graphene-based systems have enriched the field by adding Dirac Fermion optics as well as anisotropic material properties as further system parameters. The trigonally warped dispersion relation in bilayer-graphene billiards generalizes the concept of birefringence and opens unconventional ways of trajectory control in the interplay of dispersion relation and the cavity geometry as we illustrate in this contribution.

The longwave infrared and terahertz bands present unique obstacles for the generation of compact broadband frequency combs. I will discuss some of our work that aims to address this grand challenge, focusing on two types of combs. First, I will discuss our work on quantum cascade laser-based frequency combs, light sources that are able to directly generate broadband combs through an active cavity nonlinearity. In particular, I will discuss how our work on terahertz combs led to the discovery of frequency-modulated combs, a fundamental comb state that can manifest in any laser at any wavelength. This mode of operation is well-suited for efficient and broadband comb generation in semiconductor lasers. Following this, I will discuss some of our recent work on low-loss passive photonic platforms in the longwave infrared based on hybrid photonic integration. This approach allowed us to create optical microresonators in the longwave infrared with quality factors two orders of magnitude better than the state-of-the-art, offering a promising direction for the production of broadband microresonator-based solitons.

Inverse design is used to spectrally shape Kerr microcombs via dispersion

optimization. We experimentally demonstrate flexible ‘meta’ dispersion control using

selective multimode hybridization in photonic crystal ring resonators, and present initial

comb shaping results.

Frequency combs (FC) generated by quantum cascade lasers (QCLs) are a promising tool for precision spectroscopy and gas sensing. Recently, ring QCLs have emerged as a new platform for generating FC with unique advantages over Fabry-Perot geometry. While the bandwidth of such Fabry-Perot devices is determined by the device geometry and dispersion, radio-frequency injected devices with circular geometry enable the exploitation of the full gain bandwidth in a controlled manner. Together with this platform, a predictive analytical model that shows excellent agreement with the experimental data was developed. Our results pave the way for a new approach for frequency comb generation based on fast-gain saturation.

Locking multiple modes into a frequency comb is key for multiple metrological applications, and a great effort has been therefore invested in this challenge over the last decade. The most common techniques are based on either nonlinearities or modulation of the cavity, while the latter is considered the more controllable method to produce frequency combs. The modulation couples cavity modes and creates a lattice in a synthetic dimension with coherent walk dynamics, but typically these dynamics are overthrown by the dissipative processes, leading to a spectrum that is narrow relatively to the full frequency ladder potential. Here we propose and demonstrate that by using fast-gain we preserve the full potential of the coherent walk and lock the frequency comb at its maximum possible bandwidth. Moreover, we find in our system a unique regime of dissipative fast-gain Bloch oscillations. We demonstrate these dynamics in RF-modulated quantum cascade laser ring devices.

We present a dual-comb spectrometer based on quantum cascade lasers operating at 7.7 µm with a stabilization scheme that enables coherent averaging. We show that by illuminating a low cost near-infrared light source of the front facet of the quantum cascade laser, we can tightly lock one comb line of the dual-comb spectrum, resulting in narrow linewidth with sub-radian integrated phase noise for all RF comb lines.