A chip-scale second-harmonic source via self-injection-locked all-optical poling.

Second-harmonic generation allows for coherently bridging distant regions of the optical spectrum, with applications ranging from laser technology to self-referencing of frequency combs. However, accessing the nonlinear response of a medium typically requires high-power bulk sources, specific nonlinear crystals, and complex optical setups, hindering the path toward large-scale integration. Here we address all of these issues by engineering a chip-scale second-harmonic (SH) source based on the frequency doubling of a semiconductor laser self-injection-locked to a silicon nitride microresonator. The injection-locking mechanism, combined with a high-Q microresonator, results in an ultra-narrow intrinsic linewidth at the fundamental harmonic frequency as small as 41 Hz. Owing to the extreme resonant field enhancement, quasi-phase-matched second-order nonlinearity is photoinduced through the coherent photogalvanic effect and the high coherence is mapped on the generated SH field. We show how such optical poling technique can be engineered to provide efficient SH generation across the whole C and L telecom bands, in a reconfigurable fashion, overcoming the need for poling electrodes. Our device operates with milliwatt-level pumping and outputs SH power exceeding 2 mW, for an efficiency as high as 280%/W under electrical driving. Our findings suggest that standalone, highly-coherent, and efficient SH sources can be integrated in current silicon nitride photonics, unlocking the potential of χ(2) processes in the next generation of integrated photonic devices.

Integrated Backward Second-Harmonic Generation through Optically Induced Quasi-Phase-Matching.

Quasi-phase-matching for efficient backward second-harmonic generation requires sub-µm poling periods, a nontrivial fabrication feat. For the first time, we report integrated first-order quasiphase-matched backward second-harmonic generation enabled by seeded all-optical poling. The self-organized grating inscription circumvents all fabrication challenges. We compare backward and forward processes and explain how grating period influences the conversion efficiency. These results showcase unique properties of the coherent photogalvanic effect and how it can bring new nonlinear functionalities to integrated photonics.

Tunable photo-induced second-harmonic generation in a mode-engineered silicon nitride microresonator.

All-optical poling enables reconfigurable and efficient quasi-phase-matching for second-order parametric frequency conversion in silicon nitride integrated photonics. Here, we report broadly tunable milliwatt-level second-harmonic generation in a small free spectral range silicon nitride microresonator, where the pump and its second-harmonic are both always on the fundamental mode. By carefully engineering the light coupling region between the bus and microresonator, we simultaneously achieve critical coupling of the pump as well as efficient extraction of second-harmonic light from the cavity. Thermal tuning of second-harmonic generation is demonstrated with an integrated heater in a frequency grid of 47 GHz over a 10 nm band.

Bright and dark Talbot pulse trains on a chip.

Temporal Talbot effect, the intriguing phenomenon of the self-imaging of optical pulse trains, is extensively investigated using macroscopic components. However, the ability to manipulate pulse trains, either bright or dark, through the Talbot effect on integrated photonic chips to replace bulky instruments has rarely been reported. Here, we design and experimentally demonstrate a proof-of-principle integrated silicon nitride device capable of imprinting the Talbot phase relation onto in-phase optical combs and generating the two-fold self-images at the output. We show that the GHz-repetition-rate bright and dark pulse trains can be doubled without affecting their spectra as a key feature of the temporal Talbot effect. The designed chip can be electrically tuned to switch between pass-through and repetition-rate-multiplication outputs and is compatible with other related frequencies. The results of this work lay the foundations for the large-scale system-on-chip photonic integration of Talbot-based pulse multipliers, enabling the on-chip flexible up-scaling of pulse trains' repetition rate without altering their amplitude spectra.

Photo-induced cascaded harmonic and comb generation in silicon nitride microresonators.

Silicon nitride (Si3N4) is an ever-maturing integrated platform for nonlinear optics but mostly considered for third-order [χ(3)] nonlinear interactions. Recently, second-order [χ(2)] nonlinearity was introduced into Si3N4 via the photogalvanic effect, resulting in the inscription of quasi-phase-matched χ(2) gratings. However, the full potential of the photogalvanic effect in microresonators remains largely unexplored for cascaded effects. Here, we report combined χ(2) and χ(3) nonlinear effects in a normal dispersion Si3N4 microresonator. We demonstrate that the photo-induced χ(2) grating also provides phase-matching for the sum-frequency generation process, enabling the initiation and successive switching of primary combs. In addition, the doubly resonant pump and second-harmonic fields allow for effective third-harmonic generation, where a secondary optically written χ(2) grating is identified. Last, we reach a broadband microcomb state evolved from the sum-frequency-coupled primary comb. These results expand the scope of cascaded effects in microresonators.

Linear Electro-optic Effect in Silicon Nitride Waveguides Enabled by Electric-Field Poling.

Stoichiometric silicon nitride (Si3N4) is one of the most mature integrated photonic platforms for linear and nonlinear optical applications on-chip. However, because it is a centrosymmetric material, second-order nonlinear processes are inherently not available in Si3N4, limiting its use for multiple classical and quantum applications. In this work, we implement thermally assisted electric-field poling, which allows charge carrier separation in the waveguide core, leading to a depletion zone formation and the inscription of a strong electric field reaching 20 V/µm. The latter results in an effective second-order susceptibility (χ(2)) inside the Si3N4 waveguide, making linear electro-optic modulation accessible on the platform for the first time. We develop a numerical model for simulating the poling process inside the waveguide and use it to calculate the diffusion coefficient and the concentration of the charge carriers responsible for the field formation. The charge carrier concentration, as well as the waveguide core size, is found to play a significant role in determining the achievable effective nonlinearity experienced by the optical mode inside the waveguide. Current findings establish a strong groundwork for further advancement of χ(2)-based devices on Si3N4.

Near perfect two-photon interference out of a down-converter on a silicon photonic chip.

Integrated entangled photon-pair sources are key elements for enabling large-scale quantum photonic solutions and address the challenges of both scaling-up and stability. Here we report the first demonstration of an energy-time entangled photon-pair source based on spontaneous parametric down-conversion in silicon-based platform-stoichiometric silicon nitride (Si3N4)-through an optically induced second-order (χ(2)) nonlinearity, ensuring type-0 quasi-phase-matching of fundamental harmonic and its second-harmonic inside the waveguide. The developed source shows a coincidence-to-accidental ratio of 1635 for 8 µW pump power. We report two-photon interference with remarkable near-perfect visibility of 99.36±1.94%, showing high-quality photonic entanglement without excess background noise. This opens a new horizon for quantum technologies requiring the integration of a large variety of building functionalities on a single chip.

Difference-frequency generation in optically poled silicon nitride waveguides.

Difference-frequency generation (DFG) is elemental for nonlinear parametric processes such as optical parametric oscillation and is instrumental for generating coherent light at long wavelengths, especially in the middle infrared. Second-order nonlinear frequency conversion processes like DFG require a second-order susceptibility χ (2), which is absent in centrosymmetric materials, e.g. silicon-based platforms. All-optical poling is a versatile method for inducing an effective χ (2) in centrosymmetric materials through periodic self-organization of charges. Such all-optically inscribed grating can compensate for the absence of the inherent second-order nonlinearity in integrated photonics platforms. Relying on this induced effective χ (2) in stoichiometric silicon nitride (Si3N4) waveguides, second-order nonlinear frequency conversion processes, such as second-harmonic generation, were previously demonstrated. However up to now, DFG remained out of reach. Here, we report both near- and non-degenerate DFG in all-optically poled Si3N4 waveguides. Exploiting dispersion engineering, particularly rethinking how dispersion can be leveraged to satisfy multiple processes simultaneously, we unlock nonlinear frequency conversion near 2 µm relying on all-optical poling at telecommunication wavelengths. The experimental results are in excellent agreement with theoretically predicted behaviours, validating our approach and opening the way for the design of new types of integrated sources in silicon photonics.

Highly tunable second-harmonic generation in all-optically poled silicon nitride waveguides.

The availability of nonlinear parametric processes, such as frequency conversion in photonic integrated circuits is essential. In this contribution, we demonstrate a highly tunable second-harmonic generation in a fully complementary metal-oxide-semiconductor (CMOS)-fabrication-compatible silicon nitride integrated photonic platform. We induce the second-order nonlinearity using an all-optical poling technique with the second-harmonic light generated in the fundamental mode, and a narrow quasi-phase matching (QPM) spectrum by avoiding higher-order mode mixing. We are then able to broadly tune the phase-matched pump wavelength over the entire C-band (1540 nm to 1560 nm) by varying the poling conditions. Fine-tuning of QPM is enabled by thermo-optic effect with the tuning slope Δλ/ΔT in our device being 113.8 pm/°C. In addition, we exploit the measurable variation of the 3 dB QPM bandwidth to confirm how the length of the all-optically inscribed grating varies with exposure time.

Formation Rules and Dynamics of Photoinduced χ(2) Gratings in Silicon Nitride Waveguides.

Silicon nitride has emerged as a prominent platform for building photonics integrated circuits. While its nonlinear properties based on third-order effects have been successfully exploited, an efficient second harmonic generation in standard stoichiometric silicon nitride (Si3N4) waveguides can also be achieved after all-optical poling, as was recently shown. The root of such a phenomenon has been attributed to the inscription of a self-organized periodic space-charge grating along the waveguide, allowing an effective χ(2) and automatic quasi-phase-matching of pump and second harmonic. However, the different parameters and their role in increasing the efficiency of the process are still not fully comprehended. In this work, we use optical means to identify the general conditions of mode matching occurring during all-optical poling. The overlap integral between pump and second harmonic optical modes is shown to be the governing parameter in determining the features of the χ(2) gratings. Two-photon microscopy measurements of the χ(2) gratings reveal the presence of a secondary periodicity in some of the waveguides used in the study. According to overlap integral simulations, such an effect can occur due to mode mixing in the waveguide bends. From a study of poling dynamics, we observe that poling efficiency and rate increase as a function of optical pump power and waveguide length. However, in order to initiate poling, a critical pump intensity, which is lower for longer waveguides, must be coupled into a waveguide. Temporal and thermal stability tests reveal the nature of charge traps responsible for grating inscription. After applying thermally activated hopping as a conductivity mechanism in our samples, we show that only shallow traps seem to be activated during the all-optical poling process.

All-organic electro-optic waveguide modulator comprising SU-8 and nonlinear optical polymer.

In this paper we describe the principles of operation as well as the fabrication and testing steps of an all-organic waveguide modulator. The modulator comprises an SU-8 core and an electro-optic host-guest polymer cladding. The polymer properties are tuned in order to achieve single mode operation. We used direct-write laser lithography in two steps for the preparation of the devices. The electro-optic coefficient of the polymer is estimated from observing the modulation of the device operated in push-pull mode.

Poling induced mass transport in thin polymer films.

In this study we report investigation of the polymer film morphology modifications during their corona poling for fabrication of nonlinear optically (NLO) active materials. We demonstrate that at certain poling conditions surface and spatial inhomogeneities in the poled area of the sample appear. Densities of the inhomogeinities depend on the strength of the poling field, the sample temperature during the poling, and the prepoling conditions. Optimization of the poling conditions directed toward avoiding surface modifications enables us to increase the overall observable effective nonlinearity of the sample up to 10 times. To investigate, understand, and eventually explain the formation of the spatial and surface structure inhomogeinites in the poled material we have used optical, second harmonic, and scanning electron microscope measurements, as well as the conductivity measurements of the thin films. We present results of poled polymer host-guest films where (dimethylamino)benzylidene-1,3-indandione and low dipole moment 2,2',2″-(4,4',4″-nitrilotribenzylidene)triindan-1,3-dione were used as guests in poly(methyl methacrylate), polystyrol, and polysulfone matrixes doped at 10 wt %.