Hybrid architectures for terahertz molecular polaritonics.

Atoms and their different arrangements into molecules are nature's building blocks. In a regime of strong coupling, matter hybridizes with light to modify physical and chemical properties, hence creating new building blocks that can be used for avant-garde technologies. However, this regime relies on the strong confinement of the optical field, which is technically challenging to achieve, especially at terahertz frequencies in the far-infrared region. Here we demonstrate several schemes of electromagnetic field confinement aimed at facilitating the collective coupling of a localized terahertz photonic mode to molecular vibrations. We observe an enhanced vacuum Rabi splitting of 200 GHz from a hybrid cavity architecture consisting of a plasmonic metasurface, coupled to glucose, and interfaced with a planar mirror. This enhanced light-matter interaction is found to emerge from the modified intracavity field of the cavity, leading to an enhanced zero-point electric field amplitude. Our study provides key insight into the design of polaritonic platforms with organic molecules to harvest the unique properties of hybrid light-matter states.

AlGaAs Nonlinear Integrated Photonics.

Practical applications implementing integrated photonic circuits can benefit from nonlinear optical functionalities such as wavelength conversion, all-optical signal processing, and frequency-comb generation, among others. Numerous nonlinear waveguide platforms have been explored for these roles; the group of materials capable of combining both passive and active functionalities monolithically on the same chip is III-V semiconductors. AlGaAs is the most studied III-V nonlinear waveguide platform to date; it exhibits both second- and third-order optical nonlinearity and can be used for a wide range of integrated nonlinear photonic devices. In this review, we conduct an extensive overview of various AlGaAs nonlinear waveguide platforms and geometries, their nonlinear optical performances, as well as the measured values and wavelength dependencies of their effective nonlinear coefficients. Furthermore, we highlight the state-of-the-art achievements in the field, among which are efficient tunable wavelength converters, on-chip frequency-comb generation, and ultra-broadband on-chip supercontinuum generation. Moreover, we overview the applications in development where AlGaAs nonlinear functional devices aspire to be the game-changers. Among such applications, there is all-optical signal processing in optical communication networks and integrated quantum photonic circuits.

Relaxed Phase-Matching Constraints in Zero-Index Waveguides.

The utility of all parametric nonlinear optical processes is hampered by phase-matching requirements. Quasi-phase-matching, birefringent phase matching, and higher-order-mode phase matching have all been developed to address this constraint, but the methods demonstrated to date suffer from the inconvenience of only being phase matched for a single, specific arrangement of beams, typically copropagating, resulting in cumbersome experimental configurations and large footprints for integrated devices. Here, we experimentally demonstrate that these phase-matching requirements may be satisfied in a parametric nonlinear optical process for multiple, if not all, configurations of input and output beams when using low-index media. Our measurement constitutes the first experimental observation of direction-independent phase matching for a medium sufficiently long for phase matching to be relevant. We demonstrate four-wave mixing from spectrally distinct co- and counterpropagating pump and probe beams, the backward generation of a nonlinear signal, and excitation by an out-of-plane probe beam. These results explicitly show that the unique properties of low-index media relax traditional phase-matching constraints, which can be exploited to facilitate nonlinear interactions and miniaturize nonlinear devices, thus adding to the established exceptional properties of low-index materials.

Cross-polarized surface lattice resonances in a rectangular lattice plasmonic metasurface.

Multiresonant metasurfaces could enable many applications in filtering, sensing, and nonlinear optics. However, developing a metasurface with more than one high-quality-factor or high-Q resonance at designated resonant wavelengths is challenging. Here, we experimentally demonstrate a plasmonic metasurface exhibiting different, narrow surface lattice resonances by exploiting the polarization degree of freedom where different lattice modes propagate along different dimensions of the lattice. The surface consists of aluminum nanostructures in a rectangular periodic lattice. The resulting surface lattice resonances were measured around 640 nm and 1160 nm with Q factors of ∼50 and ∼800, respectively. The latter is a record-high plasmonic Q factor within the near-infrared type-II window. Such metasurfaces could benefit such applications as frequency conversion and all-optical switching.

Fourier-Engineered Plasmonic Lattice Resonances.

Resonances in optical systems are useful for many applications, such as frequency comb generation, optical filtering, and biosensing. However, many of these applications are difficult to implement in optical metasurfaces because traditional approaches for designing multiresonant nanostructures require significant computational and fabrication efforts. To address this challenge, we introduce the concept of Fourier lattice resonances (FLRs) in which multiple desired resonances can be chosen a priori and used to dictate the metasurface design. Because each resonance is supported by a distinct surface lattice mode, each can have a high quality factor. Here, we experimentally demonstrate several metasurfaces with flexibly placed resonances (e.g., at 1310 and 1550 nm) and Q-factors as high as 800 in a plasmonic platform. This flexible procedure requires only the computation of a single Fourier transform for its design, and is based on standard lithographic fabrication methods, allowing one to design and fabricate a metasurface to fit any specific, optical-cavity-based application. This work represents a step toward the complete control over the transmission spectrum of a metasurface.

Ultra-high-Q resonances in plasmonic metasurfaces.

Plasmonic nanostructures hold promise for the realization of ultra-thin sub-wavelength devices, reducing power operating thresholds and enabling nonlinear optical functionality in metasurfaces. However, this promise is substantially undercut by absorption introduced by resistive losses, causing the metasurface community to turn away from plasmonics in favour of alternative material platforms (e.g., dielectrics) that provide weaker field enhancement, but more tolerable losses. Here, we report a plasmonic metasurface with a quality-factor (Q-factor) of 2340 in the telecommunication C band by exploiting surface lattice resonances (SLRs), exceeding the record by an order of magnitude. Additionally, we show that SLRs retain many of the same benefits as localized plasmonic resonances, such as field enhancement and strong confinement of light along the metal surface. Our results demonstrate that SLRs provide an exciting and unexplored method to tailor incident light fields, and could pave the way to flexible wavelength-scale devices for any optical resonating application.

Hyperpolarizability of Plasmonic Meta-Atoms in Metasurfaces.

Plasmonic metasurfaces are promising as enablers of nanoscale nonlinear optics and flat nonlinear optical components. Nonlinear optical responses of such metasurfaces are determined by the nonlinear optical properties of individual plasmonic meta-atoms. Unfortunately, no simple methods exist to determine the nonlinear optical properties (hyperpolarizabilities) of the meta-atoms hindering the design of nonlinear metasurfaces. Here, we develop the equivalent RLC circuit (resistor, inductor, capacitor) model of such meta-atoms to estimate their second-order nonlinear optical properties, that is, the first-order hyperpolarizability in the optical spectral range. In parallel, we extract from second-harmonic generation experiments the first-order hyperpolarizabilities of individual meta-atoms consisting of asymmetrically shaped (elongated) plasmonic nanoprisms, verified with detailed calculations using both nonlinear hydrodynamic-FDTD and nonlinear scattering theory. All three approaches, analytical, experimental, and computational, yield results that agree very well. Our empirical RLC model can thus be used as a simple tool to enable an efficient design of nonlinear plasmonic metasurfaces.

Ultrafast modulation of the spectral filtering properties of a THz metasurface.

We demonstrate ultrafast tuning of a plasmonic spectral filter at terahertz (THz) frequencies. The device is made of periodically spaced gold crosses deposited on the surface of an undoped silicon wafer in which transient free carriers can be optically injected with a femtosecond resonant pulse. We demonstrate the concept by measuring the transmission spectrum of a notch filter using time-domain THz spectroscopy. Proper synchronization of the THz probe and visible excitation pulses leads to an enhanced transmission at the resonance by more than two orders of magnitude. Finite-difference time-domain simulations, which are in agreement with the experimental results, show that the underlying mechanisms responsible for the resonance blueshift and linewidth broadening can be attributed to the photoinduced change in dielectric properties of the substrate. This is supported by the numerically simulated field distribution and reflection/transmission coefficients. The device can be used in future pulse shaping and ultrafast switching experiments.

Propagation of broadband THz pulses: effects of dispersion, diffraction and time-varying nonlinear refraction.

We theoretically investigate the propagation of broadband single-cycle terahertz (THz) pulses through a medium with a nonlinear optical response. Our model takes into account non-paraxial effects, self-focusing and diffraction, as well as dispersion, in both the linear and nonlinear optical regimes. We investigate the contribution of non-instantaneous Kerr-type nonlinearity to the overall instantaneous and delayed Kerr effect at the THz frequencies. We show how increasing the nonlinear relaxation time and its dispersion modifies the THz pulse after the propagation through a transparent medium. We also discuss the effect of linear dispersion on self-action during the pulse propagation.

Multiresonant High-Q Plasmonic Metasurfaces.

Resonant metasurfaces are devices composed of nanostructured subwavelength scatterers that generate narrow optical resonances, enabling applications in filtering, nonlinear optics, and molecular fingerprinting. It is highly desirable for these applications to incorporate such devices with multiple high-quality-factor resonances; however, it can be challenging to obtain more than a pair of narrow resonances in a single plasmonic surface. Here, we demonstrate a multiresonant metasurface that operates by extending the functionality of surface lattice resonances, which are the collective responses of arrays of metallic nanoparticles. This device features a series of resonances with high-quality factors (Q ∼ 40), an order of magnitude larger than what is typically achievable with plasmonic nanoparticles, as well as a narrow free spectral range. This design methodology can be used to better tailor the transmission spectrum of resonant metasurfaces and represents an important step toward the miniaturization of optical devices.

Design optimization and tolerance analysis of a spot-size converter for the taper-assisted vertical integration platform in InP.

We report on the design optimization and tolerance analysis of a multistep lateral-taper spot-size converter based on indium phosphide (InP), performed using the Monte Carlo method. Being a natural fit to (and a key building block of) the regrowth-free taper-assisted vertical integration platform, such a spot-size converter enables efficient and displacement-tolerant fiber coupling to InP-based photonic integrated circuits at a wavelength of 1.31 µm. An exemplary four-step lateral-taper design featuring 0.35 dB coupling loss at optimal alignment of a standard single-mode fiber; ≥7 µm 1 dB displacement tolerance in any direction in a facet plane; and great stability against manufacturing variances is demonstrated.

Nonlinear photonics on-a-chip in III-V semiconductors: quest for promising material candidates.

We propose several designs of nonlinear optical waveguides based on quaternary III-V semiconductors AlGaAsSb and InGaAsP. These semiconductor materials have been widely used for laser sources. Their nonlinear optical properties, however, yet remain unexplored, while the materials definitely hold promise for nonlinear photonics on-a-chip. The latter argument is based on the fact that III-V compounds tend to exhibit high values of the nonlinear optical susceptibilities, while the nonlinear absorption in these materials can be minimized in the wavelength range of interest through a proper selection of the material composition. We present the modal analysis for the designed waveguide structures and show that the effective mode area much less than 1 µm2 can be achieved through a design optimization in each of the two compounds. We also present specific waveguide designs that demonstrate zero dispersion at the wavelengths of interest. The designed AlGaAsSb and InGaAsP waveguides are thus expected to demonstrate high values of the nonlinear coefficient and efficient nonlinear optical interactions.

Ultra-strong polarization dependence of surface lattice resonances with out-of-plane plasmon oscillations.

The interplay between localized surface plasmon (LSP) resonances and their collective responses, known as surface lattice resonances (SLRs), in metal nanoparticle arrays can lead to resonances with high Q-factors (∼100). These responses have in the past usually been studied for LSP resonances in the plane of the array of the nanoparticles (assumed to be nonmagnetic), thus restricting efficient coupling to particles separated along a specific direction. In the present study, we demonstrate that LSPs oscillating perpendicular to the plane of the surface can lead to stronger inter-particle coupling, which enhances the SLRs. This stronger coupling occurs because the out-of-plane oscillations can couple in all directions within the plane of the array. We study the resulting SLRs for square and hexagonal lattices using the discrete-dipole approximation, and we predict much larger Q-factors in the wavelength range near 650 nm. This prediction suggests that SLRs could be very useful in enhancing various optical processes, and in many applications such as sensing and nonlinear optical wave mixing.

Enhanced spectral sensitivity of a chip-scale photonic-crystal slow-light interferometer.

We experimentally demonstrate that the spectral sensitivity of a Mach-Zehnder (MZ) interferometer can be enhanced through structural slow light. We observe a 20-fold resolution enhancement by placing a dispersion-engineered, slow-light, photonic-crystal waveguide in one arm of a fiber-based MZ interferometer. The spectral sensitivity of the interferometer increases roughly linearly with the group index, and we have quantified the resolution in terms of the spectral density of interference fringes. These results show promise for the use of slow-light methods for developing novel tools for optical metrology and, specifically, for compact high-resolution spectrometers.

Tuneable four-wave mixing in AlGaAs nanowires.

We have experimentally demonstrated broadband tuneable four-wave mixing in AlGaAs nanowires with the widths ranging between 400 and 650 nm and lengths from 0 to 2 mm. We performed a detailed experimental study of the parameters influencing the FWM performance in these devices (experimental conditions and nanowire dimensions). The maximum signal-to-idler conversion range was 100 nm, limited by the tuning range of the pump source. The maximum conversion efficiency, defined as the ratio of the output idler power to the output signal power, was -38 dB. In support of our explanation of the experimentally observed trends, we present modal analysis and group velocity dispersion numerical analysis. This study is what we believe to be a step forward towards realization of all-optical signal processing devices.

Post-process wavelength tuning of silicon photonic crystal slow-light waveguides.

Silicon photonic crystal waveguides have enabled a range of technologies, yet their fabrication continues to present challenges. Here, we report on a post-processing method that allows us to tune the operational wavelength of slow-light photonic crystal waveguides in concert with optical characterization, offsetting the effects of hole-radii and slab thickness variations. Our method consist of wet chemical surface oxidation, followed by oxide stripping. Theoretical modelling shows that the changes in optical behavior were predictable, and hence controlled tuning can be achieved by changing the number of processing cycles, where each cycle removes approximately 0.25 nm from all exposed surfaces, producing a blueshift of 1.6±0.1 nm in operating wavelength.

Integrated optical temporal Fourier transformer based on a chirped Bragg grating waveguide.

We experimentally demonstrate the first integrated temporal Fourier transformer based on a linearly chirped Bragg grating waveguide written in silica glass with a femtosecond laser. The operation is based on mapping the energy spectrum of the input optical signal to the output temporal waveform by making use of first-order chromatic dispersion. The device operates in reflection, has a bandwidth of 10 nm, and can be used for incident temporal waveforms as long as 20 ps. Experimental results, obtained through both temporal oscilloscope traces and Fourier transform spectral interferometry, display a successful Fourier transformation of in-phase and out-of-phase pairs of input optical pulses, and demonstrate the correct functionality of the device for both amplitude and phase of the temporal output.

Compact highly-nonlinear AlGaAs waveguides for efficient wavelength conversion.

We report on the efficient nonlinear optical interactions in AlGaAs strip-loaded waveguides with a wafer composition specifically designed to increase the nonlinear coefficient. We demonstrate a broad-band self-phase modulation with a nonlinear phase shift up to 6π, and four-wave mixing with a 20-nm tuning range and signal-to-idler conversion efficiency up to 10 dB. Our samples are several times shorter than similar devices used for wavelength conversion by XPM and FWM in previous reports, but the efficiency of the observed effects is similar. Our experimental studies demonstrate the high potential of AlGaAs for all-optical networks.

Broadband self-phase modulation, cross-phase modulation, and four-wave mixing in 9-mm-long AlGaAs waveguides.

We demonstrate efficient self-phase modulation with a nonlinear phase shift of up to 3π, broadband cross-phase modulation, and four-wave mixing with a 14 nm tunability range in AlGaAs waveguides with a specially designed composition. The length of our sample is only 9 mm, but the efficiency of the observed effects is high.

Observation of a microscopic cascaded contribution to the fifth-order nonlinear susceptibility.

Typically, low-order nonlinearities are much stronger than high-order nonlinearities. In this Letter, we demonstrate a procedure by which strong high-order nonlinearities can be synthesized out of low-order nonlinearities. Our procedure involves the use of the previously largely overlooked process of microscopic cascading, which results from local-field effects. We have performed an experiment that allows us to distinguish the influence of microscopic cascading from the more-well-known process of macroscopic cascading, and we find conditions under which microscopic cascading can be the dominant effect. The ability to create a large high-order nonlinear response could prove useful for applications in quantum-information science that require the detection of the simultaneous presence of N entangled photons.