Fully integrated and broadband Si-rich silicon nitride wavelength converter based on Bragg scattering intermodal four-wave mixing

Intermodal four-wave mixing(FWM)processes have recently attracted significant interest for all-optical signal processing applications thanks to the possibility to control the propagation properties of waves exciting distinct spatial modes of the same waveguide.This allows,in principle,to place signals in different spectral regions and satisfy the phase matching condition over considerably larger bandwidths compared to intramodal processes.However,the demonstrations reported so far have shown a limited bandwidth and suffered from the lack of on-chip components designed for broadband manipulation of different modes.We demonstrate here a silicon-rich silicon nitride wavelength converter based on Bragg scattering intermodal FWM,which integrates mode conversion,multiplexing and de-multiplexing functionalities on-chip.The system enables wavelength conversion between pump waves and a signal located in different telecommunication bands(separated by 60 nm)with a 3 dB bandwidth exceeding 70 nm,which represents,to our knowledge,the widest bandwidth ever achieved in an intermodal FWM-based system.

Multi-direction bending sensing based on spot pattern demodulation of dual-hole fiber

A multi-direction bending sensor based on spot pattern demodulation of a dual-hole fiber(DHF)is proposed.By using the interference and scattering in a DHF,the related multidirectional variations can be captured by the optical field.Furthermore,the multi-directional bending characteristics of the fiber are quantitatively described by the pattern of the output light spot,achieving multidirectional bending sensing.In addition,considering the subtle changes in the deformation patterns over time,a convolutional neural network(CNN)model based on deep learning is introduced for accurate recognition and prediction of the bending angle.The experimental results show that the sensor can perceive different bending angles in four directions.These outstanding results indicate that the multi-directional bending sensor based on dual-hole interference pattern decoding has potential applications in multi-directional quantitative sensing and artificial intelligence perception.

Spatiotemporally reconfigurable light in degenerate laser cavities

We show that a Ⅲ-V semiconductor vertical external-cavity surface-emitting laser(VECSEL) can be engineered to generate light with a customizable spatiotemporal structure. Temporal control is achieved through the emission of temporal localized structures(TLSs), a particular mode-locking regime that allows individual addressing of the pulses traveling back and forth in the cavity. The spatial profile control relies on a degenerate external cavity, and it is implemented due to an absorptive mask deposited onto the gain mirror that limits the positive net gain within two circular spots in the transverse section of the VECSEL. We show that each spot emits spatially uncorrelated TLSs. Hence, the spatiotemporal structure of the light emitted can be shaped by individually addressing the pulses emitted by each spot. Because the maximum number of pulses circulating in the cavity and the number of positive net-gain spots in the VECSEL can be increased straightforwardly, this result is a proof of concept of a laser platform capable of handling light states of scalable complexity. We discuss applications to three-dimensional alloptical buffers and to multiplexing of frequency combs that share the same laser cavity.

25 Gbps low-voltage hetero-structured silicongermanium waveguide pin photodetectors for monolithic on-chip nanophotonic architectures

Near-infrared germanium(Ge) photodetectors monolithically integrated on top of silicon-on-insulator substrates are universally regarded as key enablers towards chip-scale nanophotonics, with applications ranging from sensing and health monitoring to object recognition and optical communications. In this work, we report on the highdata-rate performance pin waveguide photodetectors made of a lateral hetero-structured silicon-Ge-silicon(Si-Ge-Si) junction operating under low reverse bias at 1.55 μm. The pin photodetector integration scheme considerably eases device manufacturing and is fully compatible with complementary metal-oxide-semiconductor technology. In particular, the hetero-structured Si-Ge-Si photodetectors show efficiency-bandwidth products of^9 GHz at-1 V and ~30 GHz at-3 V, with a leakage dark current as low as ~150 nA, allowing superior signal detection of high-speed data traffic. A bit-error rate of 10-9 is achieved for conventional 10 Gbps, 20 Gbps, and25 Gbps data rates, yielding optical power sensitivities of-13.85 dBm,-12.70 dBm, and-11.25 dBm, respectively. This demonstration opens up new horizons towards cost-effective Ge pin waveguide photodetectors that combine fast device operation at low voltages with standard semiconductor fabrication processes, as desired for reliable on-chip architectures in next-generation nanophotonics integrated circuits.

Ultra-compact on-chip metaline-based 1.3/1.6 μm wavelength demultiplexer

In this paper, we report an experimental demonstration of enabling technology exploiting resonant properties of plasmonic nanoparticles, for the realization of wavelength-sensitive ultra-minituarized(4 μm× 4 μm) optical metadevices. To this end, the example of a 1.3/1.6 μm wavelength demultiplexer is considered. Its technological implementation is based on the integration of gold cut-wire-based metalines on the top of a silicon-on-insulator waveguide. The plasmonic metalines modify locally the effective index of the Si waveguide and thus allow for the implementation of wavelength-dependent optical pathways. The 1.3/1.6 μm wavelength separation with extinction ratio between two demultiplexers' channels reaching up to 20 dB is experimentally demonstrated. The considered approach, which can be readily adapted to different types of material planar lightwave circuit platforms and nanoresonators, is suited for the implementation of a generic family of wavelength-sensitive guided-wave optical metadevices.

Extreme multiexciton emission from deterministically assembled single-emitter subwavelength plasmonic patch antennas

Coupling nano-emitters to plasmonic antennas is a key milestone for the development of nanoscale quantum light sources.One challenge,however,is the precise nanoscale positioning of the emitter in the structure.Here,we present a laser etching protocol that deterministically positions a single colloidal CdSe/CdS core/shell quantum dot emitter inside a subwavelength plasmonic patch antenna with three-dimensional nanoscale control.By exploiting the properties of metal–insulator–metal structures at the nanoscale,the fabricated single-emitter antenna exhibits a very high-Purcell factor(>72)and a brightness enhancement of a factor of 70.Due to the unprecedented quenching of Auger processes and the strong acceleration of the multiexciton emission,more than 4 photons per pulse can be emitted by a single quantum dot,thus increasing the device yield.Our technology can be applied to a wide range of photonic nanostructures and emitters,paving the way for scalable and reliable fabrication of ultracompact light sources.

Stretching the spectra of Kerr frequency combs with self-adaptive boundary silicon waveguides

Dispersion engineering of optical waveguides is among the most important steps in enabling the realization of Kerr optical frequency combs.A recurring problem is the limited bandwidth in which the nonlinear phase matching condition is satisfied,due to the dispersion of the waveguide.This limitation is particularly stringent in high-index-contrast technologies such as silicon-on-insulator.We propose a general approach to stretch the bandwidth of Kerr frequency combs based on subwavelength engineering of single-mode waveguides with self-adaptive boundaries.The wideband flattened dispersion operation comes from the particular property of the waveguide optical mode that automatically self-adapts its spatial profile at different wavelengths to slightly different effective spatial spans determined by its effective index values.This flattened dispersion relies on the squeezing of small normal-dispersion regions between two anomalous spectral zones,which enables it to achieve two Cherenkov radiation points and substantially broaden the comb,achieving a bandwidth between 2.2 and 3.4μm wavelength.This strategy opens up a design space for trimming the spectra of Kerr frequency combs using high-index-contrast platforms and can provide benefits to various nonlinear applications in which the manipulation of energy spacing and phase matching are pivotal.

Ultrafast response of harmonic modelocked THz lasers

The use of fundamental modelocking to generate short terahertz(THz)pulses and THz frequency combs from semiconductor lasers has become a routine affair,using quantum cascade lasers(QCLs)as a gain medium.However,unlike classic laser diodes,no demonstrations of harmonic modelocking,active or passive,have been shown in THz QCLs,where multiple pulses per round trip are generated when the laser is modulated at the harmonics of the cavity’s fundamental round-trip frequency.Here,using time-resolved THz techniques,we show for the first time harmonic injection and mode-locking in which THz QCLs are modulated at the harmonics of the round-trip frequency.We demonstrate the generation of the harmonic electrical beatnote within a QCL,its injection locking to an active modulation and its direct translation to harmonic pulse generation using the unique ultrafast nature of our approach.Finally,we show indications of self-starting harmonic emission,i.e.,without external modulation,where the QCL operates exclusively on a harmonic(up to its 15th harmonic)of the round-trip frequency.This behaviour is supported by time-resolved simulations of induced gain and loss in the system and shows the importance of the electronic,as well as photonic,nature of QCLs.These results open up the prospect of passive harmonic modelocking and THz pulse generation,as well as the generation of low-noise microwave generation in the hundreds of GHz region.

Nonlinear optical properties of integrated GeSbS chalcogenide waveguides

In this paper, we report the experimental characterization of highly nonlinear Ge Sb S chalcogenide glass waveguides.We used a single-beam characterization protocol that accounts for the magnitude and sign of the real and imaginary parts of the third-order nonlinear susceptibility of integrated Ge23 Sb7 S70(GeSbS) chalcogenide glass waveguides in the near-infrared wavelength range at λ =1580 nm. We measured a waveguide nonlinear parameter of 7.0±0.7 W^(-1)· m(-1), which corresponds to a nonlinear refractive index of n_2=0.93±0.08 × 10^(-18) m^2∕W,comparable to that of silicon, but with an 80 times lower two-photon absorption coefficient βTPA=0.010± 0.003 cm∕GW, accompanied with linear propagation losses as low as 0.5 dB/cm. The outstanding linear and nonlinear properties of Ge Sb S, with a measured nonlinear figure of merit FOMTPA=6.0 ±1.4 at λ =1580 nm, ultimately make it one of the most promising integrated platforms for the realization of nonlinear functionalities.

Hybrid silicon slotted photonic crystal waveguides:how does third order nonlinear performance scale with slow light?

We investigate in this paper the influence of slow light on the balance between the Kerr and two-photon absorption(TPA) processes in silicon slotted hybrid nonlinear waveguides. Three typical silicon photonic waveguide geometries are studied to estimate the influence of the light slow-down factor on the mode field overlap with the silicon region, as well as on the complex effective nonlinear susceptibility. It is found that slotted photonic crystal modes tend to focalize in their hollow core with increasing group index(n_G) values. Considering a hybrid integration of nonlinear polymers in such slotted waveguides, a relative decrease of the TPA process by more factor of 2 is predicted from n_G=10 to n_G=50. As a whole, this work shows that the relative influence of TPA decreases for slotted waveguides operating in the slow light regime, making them a suitable platform for third-order nonlinear optics.

Mode selection and dispersion engineering in Bragg-like slot photonic crystal waveguides for hybrid light-matter interactions

We introduce a family of slot photonic crystal waveguides(SPh CWs) for the hybrid integration of low-index active materials in silicon photonics with energy-confinement factors of ~30% in low-index regions. The proposed approach, which is based on a periodic indentation of the etched slot in the middle of the SPh CW, makes it possible to reconcile a simultaneously narrow and wide slot for exploiting the two modes of even symmetry of a SPh CW. The resulting mode-selection mechanism allows a flexible choice of the modes to be used. Furthermore,the proposed structure offers tremendous flexibility for adjusting the dispersive properties of the slot-confined modes, in particular of their slow-light effects. Flat band slow light in a bandwidth of about 60 nm with a group velocity dispersion factor jβ_2 j below 1 ps^2∕mm is numerically demonstrated by this approach, corresponding to a normalized delay bandwidth product of around 0.4. These results, obtained from hollow-core periodic waveguides that are directly designed in view of hybrid integration of active materials in mechanically robust structures(not based on free-standing membranes) could pave the way for the realization of on-chip slow-light bio-sensing,active hybrid-silicon optoelectronic devices, or all-optical hybrid-silicon nonlinear functionalities.

Direct observation of zero modes in a non-Hermitian optical nanocavity array

Zero modes are symmetry protected ones whose energy eigenvalues have zero real parts. In Hermitian arrays, they arise as a consequence of the sublattice symmetry, implying that they are dark modes. In non-Hermitian systems that naturally emerge in gain/loss optical cavities, particle-hole symmetry prevails instead;the resulting zero modes are no longer dark but feature π∕2 phase jumps between adjacent cavities. Here, we report on the direct observation of zero modes in a non-Hermitian three coupled photonic crystal nanocavities array containing quantum wells. Unlike the Hermitian counterparts, the observation of non-Hermitian zero modes upon single pump spot illumination requires vanishing sublattice detuning, and they can be identified through far-field imaging and spectral filtering of the photoluminescence at selected pump locations. We explain the zero-mode coalescence as a parity-time phase transition for small coupling. These zero modes are robust against coupling disorder and can be used for laser mode engineering and photonic computing.

Nonlinear gallium phosphide nanoscale photonics[Invited]

We introduce a nanoscale photonic platform based on gallium phosphide. Owing to the favorable material properties, peak power intensity levels of 50 GW∕cm^2 are safely reached in a suspended membrane. Consequently,the field enhancement is exploited to a far greater extent to achieve efficient and strong light–matter interaction.As an example, parametric interactions are shown to reach a deeply nonlinear regime, revealing cascaded fourwave mixing leading to comb generation and high-order soliton dynamics.

Versatile metasurface platform for electromagnetic wave tailoring

The emergence of metasurfaces provides a novel strategy to tailor the electromagnetic response of electromagnetic waves in a controlled manner by judicious design of the constitutive meta-atom.However,passive metasurfaces tend to perform a specific or limited number of functionalities and suffer from narrow-frequency-band operation.Reported reconfigurable metasurfaces can generally be controlled only in a 1D configuration or use p-i-n diodes to show binary phase states.Here,a 2D reconfigurable reflective metasurface with individually addressable meta-atoms enabling a continuous phase control is proposed in the microwave regime.The response of the meta-atom is flexibly controlled by changing the bias voltage applied to the embedded varactor diode through an elaborated power supply system.By assigning appropriate phase profiles to the metasurface through voltage modulation,complex beam generation,including Bessel beams,vortex beams,and Airy beams,is fulfilled to demonstrate the accurate phase-control capability of the reconfigurable metasurface.Both simulations and measurements are performed as a proof of concept and show good agreement.The proposed design paves the way toward the achievement of real-time and programmable multifunctional meta-devices,with enormous potential for microwave applications such as wireless communication,electromagnetic imaging,and smart antennas.

Large spin Hall effect and tunneling magnetoresistance in iridium-based magnetic tunnel junctions

Magnetic tunnel junctions(MTJs)switched by spin-orbit torque(SOT)have attracted substantial interest owing to their advantages of ultrahigh speed and prolonged endurance.Both field-free magnetization switching and high tunneling magnetoresistance(TMR)are critical for the practical application of SOT magnetic random access memory(MRAM).In this work,we propose an MTJ structure based on an iridium(Ir)bottom layer.Ir metal is a desirable candidate for field-free SOT switching owing to its strong intrinsic spin Hall conductivity(SHC),which can be enhanced via doping.Herein,we study TMR in Ir-based MTJs with symmetric and asymmetric structures.Ir-based MTJs exhibit large TMR,which can be further enhanced by heavy metal symmetry owing to the resonant tunneling effect.Our comprehensive investigations illustrate that Ir-based MTJs are promising candidates for realizing SOT switching and high TMR.

On-chip generation of hybrid polarization-frequency entangled biphoton states

We demonstrate a chip-integrated semiconductor source that combines polarization and frequency entanglement,allowing the generation of entangled biphoton states in a hybrid degree of freedom without post-manipulation.Our Al Ga As device is based on type-Ⅱ spontaneous parametric downconversion in a counterpropagating phasematching scheme in which the modal birefringence lifts the degeneracy between the two possible nonlinear interactions. This allows the direct generation of polarization–frequency entangled photons at room temperature and telecom wavelength, and in two distinct spatial modes, offering enhanced flexibility for quantum information protocols. The state entanglement is quantified by a combined measurement of the joint spectrum and Hong–Ou–Mandel interference(raw visibility 70.1% ± 1.1%) of the biphotons, allowing to reconstruct a restricted density matrix in the hybrid polarization–frequency space.

Ge-on-Si modulators operating at mid-infrared wavelengths up to 8 μm

We report mid-infrared Ge-on-Si waveguide-based PIN diode modulators operating at wavelengths of 3.8 and8 μm. Fabricated 1-mm-long electro-absorption devices exhibit a modulation depth of >35 dB with a 7 V forward bias at 3.8 μm, and a similar 1-mm-long Mach–Zehnder modulator has a Vπ· L of 0.47 V · cm. Driven by a 2.5 Vpp RF signal, 60 MHz on-off keying modulation was demonstrated. Electro-absorption modulation at 8 μm was demonstrated preliminarily, with the device performance limited by large contact separation and high contact resistance.

Supercontinuum generation in silicon photonics platforms

Nonlinear optics has not stopped evolving,offering opportunities to develop novel functionalities in photonics.Supercontinuum generation,a nonlinear optical phenomenon responsible for extreme spectral broadening,attracts the interest of researchers due to its high potential in many applications,including sensing,imaging,or optical communications.In particular,with the emergence of silicon photonics,integrated supercontinuum sources in silicon platforms have seen tremendous progress during the past decades.This article aims at giving an overview of supercontinuum generation in three main silicon-compatible photonics platforms,namely,silicon,silicon germanium,and silicon nitride,as well as the essential theoretical elements to understand this fascinating phenomenon.

Direct observation of photonic Landau levels and helical edge states in strained honeycomb lattices

We report the realization of a synthetic magnetic field for photons and polaritons in a honeycomb lattice of coupled semiconductor micropillars.A strong synthetic field is induced in both the s and p orbital bands by engineering a uniaxial hopping gradient in the lattice,giving rise to the formation of Landau levels at the Dirac points.We provide direct evidence of the sublattice symmetry breaking of the lowest-order Landau level wavefunction,a distinctive feature of synthetic magnetic fields.Our realization implements helical edge states in the gap between n=0 and n=±1 Landau levels,experimentally demonstrating a novel way of engineering propagating edge states in photonic lattices.In light of recent advances in the enhancement of polariton–polariton nonlinearities,the Landau levels reported here are promising for the study of the interplay between pseudomagnetism and interactions in a photonic system.

Cavity-based photoconductive sources for real-time terahertz imaging

Optically driven photoconductive switches are one of the predominant sources currently used in terahertz im-aging systems.However,owing to their low average powers,only raster-based images can be taken,resulting in slow acquisition.In this work,we show that by placing a photoconductive switch within a cavity,we are able to generate absolute average THz powers of 181μW with the frequency of the THz emission centered at 1.5 THz-specifications ideally adapted to applications such as non-destructive imaging.The cavity is based on a metal-insulator-metal structure that permits an en hancement of the average power by almost 1 order of magnitude compared to a standard structure,while conserving a broadband spectral response.We demonstrate proof-of-principle real-time imaging using this source,with the broadband spectrum permitting to eliminate strong diffraction artifacts.