Waveguide-based electro-absorption modulator performance: comparative analysis

Investigation on the monolithically integrated chaotic optical transmitting chip based on parallel EAMs

Chaotic optical communication ensures information security at the physical layer. However, the monolithic integration of lasers and lithium niobate Mach-Zehnder modulators remains a challenge, limiting the progress of integrated chaotic optical communication systems based on an electro-optic feedback. Here, we propose the monolithically integrated chaotic optical transmitting chip based on the parallel EAMs and validate its performance from the perspectives of phase portraits, fast Fourier transform (FFT), probability density function (PDF), largest Lyapunov exponents, and bifurcation. The results demonstrate the feasibility of the chip, which is beneficial for the miniaturization and integration of the system.

On-chip low-loss all-optical MoSe2 modulator

Monolayer transition metal dichalcogenides (TMDCs), like MoS₂, MoSe₂, WS₂, and WSe₂, feature direct bandgaps, strong spin-orbit coupling, and exciton-polariton interactions at the atomic scale, which could be harnessed for efficient light emission, valleytronics, and polaritonic lasing, respectively. Nevertheless, to build next-generation photonic devices that make use of these features, it is first essential to model the all-optical control mechanisms in TMDCs. Herein, a simple model is proposed to quantify the performance of a 35-μm-long Si₃N₄ waveguide-integrated all-optical MoSe₂ modulator. Using this model, a switching energy of 14.6 pJ is obtained for a transverse-magnetic (TM) and transverse-electric (TE) polarized pump signals at λ = 480 nm. Moreover, maximal extinction ratios of 20.6 dB and 20.1 dB are achieved for a TM and TE polarized probe signal, respectively, at λ = 500 nm with an ultra-low insertion loss of <0.3 dB. Moreover, the device operates with an ultrafast recovery time of 50 ps, while maintaining a high extinction ratio for practical applications. These findings facilitate modeling and designing novel TMDC-based photonic devices.

Optimizing epsilon-near-zero based plasmon assisted modulators through surface-to-volume ratio

Plasmonic-based integrated nanophotonic modulators, despite their promising features, have one key limiting factor of large insertion loss (IL), which limits their practical potential. To combat this, we utilize a plasmon-assisted approach through the lens of surface-to-volume ratio to realize a 4-slot based EAM with an extinction ratio (ER) of 2.62 dB/µm and insertion loss (IL) of 0.3 dB/µm operating at ∼1 GHz and a single slot design with ER of 1.4 dB/µm and IL of 0.25 dB/µm operating at ∼20 GHz, achieved by replacing the traditional metal contact with heavily doped indium tin oxide (ITO). Furthermore, our analysis imposes realistic fabrication constraints, and material properties, and illustrates trade-offs in the performance that must be carefully optimized for a given scenario.

Optical switch with ultra high extinction ratio using electrically controlled metal diffusion

An optical switch with ultra high extinction ratio is proposed. Optical switching is realized using the resistive switching effect through the lateral coupling between the input nanophotonic waveguide and output waveguide at a wavelength of 1550 nm. The coupled waveguide system is engineered to increase the number of mode beats in a unit length of the device. An increase in the number of mode beats and controlled diffusion of metal ions through a thin dielectric layer with an applied electric field is responsible for a high optical extinction ratio of 27 dB for a 20 µm long device. Compared to electrical control by plasma dispersion in silicon, the resistive switching effect enables a reduction in the coupling length and an increase in the waveguide absorption, leading to an almost 100 times higher extinction ratio. The proposed compact on-chip silicon-based nanophotonic resistive device is a potential candidate for a large-scale integrated photonic circuit for applications in optical switching, modulation, memory, and computation.

Heterogeneously integrated ITO plasmonic Mach-Zehnder interferometric modulator on SOI

Densely integrated active photonics is key for next generation on-chip networks for addressing both footprint and energy budget concerns. However, the weak light-matter interaction in traditional active Silicon optoelectronics mandates rather sizable device lengths. The ideal active material choice should avail high index modulation while being easily integrated into Silicon photonics platforms. Indium tin oxide (ITO) offers such functionalities and has shown promising modulation capacity recently. Interestingly, the nanometer-thin unity-strong index modulation of ITO synergistically combines the high group-index in hybrid plasmonic with nanoscale optical modes. Following this design paradigm, here, we demonstrate a spectrally broadband, GHz-fast Mach–Zehnder interferometric modulator, exhibiting a high efficiency signified by a miniscule V_πL of 95 V μm, deploying a one-micrometer compact electrostatically tunable plasmonic phase-shifter, based on heterogeneously integrated ITO thin films into silicon photonics. Furthermore we show, that this device paradigm enables spectrally broadband operation across the entire telecommunication near infrared C-band. Such sub-wavelength short efficient and fast modulators monolithically integrated into Silicon platform open up new possibilities for high-density photonic circuitry, which is critical for high interconnect density of photonic neural networks or applications in GHz-fast optical phased-arrays, for example.

Transition metal dichalcogenides integrated waveguide modulator and attenuator in silicon nitride platform

Embedding transition metal dichalcogenides (TMDs) into optical devices enhance the light-matter interaction, which holds a great promise for designing compact integrated photonic components. The chemical composition and thickness of TMDs affect their electronic and optical properties. The optical properties demonstrate stable and strong gate tunable optical response near the excitonic transitions. These materials are, therefore, promising candidates for designing electro-optic modulators and attenuators. Here, an electro-absorption modulator is investigated based on integrating different TMD monolayers on silicon nitride waveguides near the excitonic binding energy. A comparison of absorption changes due to electrostatically induced charges in MoS₂, MoSe₂, WS₂, WSe_2, and graphene has been presented for modulator design. The results show that with the confinement factor of about 0.10% in the monolayer TMDs, the modulation strength is 10x higher in WS₂ as compared to the graphene-based modulator design. The WS₂ based modulator shows the highest modulation strength with an improvement by a factor of 5 as compared to Mo based designs. Further, the change in the spectral response of these materials with thickness and chemical composition has been exploited for the design of attenuator. A micro-opto-mechanical system technology with TMD integrated supersubstrate above a Si₃N₄ waveguide affecting the optical response is investigated. By replacing the TMD in the supersubstrate with Se atom instead of S in the MX₂ and WX₂ compound, the attenuation is shifted from visible to near-infrared range allowing tuning from 620 to 750 nm. The tuning of the attenuation wavelength will help the designer choose the best material for visible light photonic applications.

Graphene-based dual-mode modulators

Mode-division multiplexing (MDM) has attracted broad attention as it could effectively boost up transmission capability by utilizing optical modes as a spatial dimension in optical interconnects. In such a technique, different data channels are usually modulated to the respective carriers over different spatial modes by using individual parallel electro-optic modulators. Each modulated channel is then multiplexed to a multi-mode waveguide. However, the method inevitably suffers from a high cost, large device footprint and high insertion loss. Here, we design intensity and phase dual-mode modulators, enabling simultaneous modulations over two channels via a graphene-on-silicon waveguide. Our method is based on the exploration of co-planar interactions between structured graphene nanoribbons (GNs) and spatial modes in a silicon waveguide. Specifically, the zeroth-order transverse electric (TE₀) and first-order transverse electric (TE₁) modes are modulated separately and simultaneously by applying independent driving electrodes to different GNs in an identical modulator. Our study is expected to open an avenue to develop high-density MDM photonics integrated circuits for tera-scale optical interconnects.

Towards Deep Integration of Electronics and Photonics

A combination of computational power provided by modern MOSFET-based devices with light assisted wideband communication at the nanoscale can bring electronic technologies to the next level. Obvious obstacles include a size mismatch between electronic and photonic components as well as a weak light–matter interaction typical for existing devices. Polariton modes can be used to overcome these difficulties at the fundamental level. Here, we review applications of such modes, related to the design and fabrication of electro–optical circuits. The emphasis is made on surface plasmon-polaritons which have already demonstrated their value in many fields of technology. Other possible quasiparticles as well as their hybridization with plasmons are discussed. A quasiparticle-based paradigm in electronics, developed at the microscopic level, can be used in future molecular electronics and quantum computing.

Neuromorphic photonics with electro-absorption modulators

Photonic neural networks benefit from both the high-channel capacity and the wave nature of light acting as an effective weighting mechanism through linear optics. Incorporating a nonlinear activation function by using active integrated photonic components allows neural networks with multiple layers to be built monolithically, eliminating the need for energy and latency costs due to external conversion. Interferometer-based modulators, while popular in communications, have been shown to require more area than absorption-based modulators, resulting in a reduced neural network density. Here, we develop a model for absorption modulators in an electro-optic fully connected neural network, including noise, and compare the network's performance with the activation functions produced intrinsically by five types of absorption modulators. Our results show the quantum well absorption modulator-based electro-optic neuron has the best performance allowing for 96% prediction accuracy with 1.7×10^-12 J/MAC excluding laser power when performing MNIST classification in a 2 hidden layer feed-forward photonic neural network.