Efficient Electro-optic Modulation Research Articles

Efficient wideband tunable radio frequency-optical conversion via triply resonant photonic molecules.

Electro-optic (EO) transduction of weak radio frequency (RF) and millimeter-wave signals, such as those received by an antenna, onto laser sidebands for processing in the optical domain requires efficient EO modulators. Microrings offer spatial density and efficiency advantages over Mach-Zehnder modulators (MZMs), but conventional single-ring modulators suffer a fundamental trade-off between resonantly enhanced conversion efficiency and the RF carrier frequency that it can accommodate. Dual-cavity "photonic molecule" modulators resolve this trade-off, allowing high efficiency independent of the RF carrier frequency by providing separate resonant supermodes to enhance the laser local oscillator (LO) and the narrowband RF-detuned sideband. However, the RF frequency is fixed at design time by geometry, with efficiency dropping quickly for RF carriers away from the design value. We propose a novel, to the best of our knowledge, triple-cavity configuration with an off-resonant middle ring acting as an effective tunable coupler between two active modulator cavities. This configuration provides wideband tunability of the target RF carrier while maintaining efficient sideband conversion. When the middle ring is passive (high Q), this configuration provides wide RF tunability with no efficiency penalty over the fixed dual-cavity case and could become an important building block for future RF/mm-wave photonic integrated circuits (PICs).

Lithium niobate thin film electro-optic modulator

Abstract The linear electro-optic effect offers a valuable means to control light properties via an external electric field. Lithium niobate (LN), with its high electro-optic coefficients and broad optical transparency ranges, stands out as a prominent material for efficient electro-optic modulators. The recent advent of lithium niobate-on-insulator (LNOI) wafers has sparked renewed interest in LN for compact photonic devices. In this study, we present an electro-optic modulator utilizing a thin LN film sandwiched between top and bottom gold (Au) film electrodes, forming a Fabry–Pérot (F–P) resonator. This resonator exhibits spectral resonance shifts under an applied electric field, enabling efficient modulation of reflected light strength. The modulator achieved a 2.3 % modulation amplitude under ±10 V alternating voltage. Our approach not only presents a simpler fabrication process but also offers larger modulation amplitudes compared to previously reported metasurface based LN electro-optic modulators. Our results open up new opportunities for compact electro-optic modulators with applications in beam steering devices, dynamic holograms, and spatial light modulators, and more.

Space-qualifying silicon photonic modulators and circuits.

Reducing the form factor while retaining the radiation hardness and performance matrix is the goal of avionics. While a compromise between a transistor's size and its radiation hardness has reached consensus in microelectronics, the size-performance balance for their optical counterparts has not been quested but eventually will limit the spaceborne photonic instruments' capacity to weight ratio. Here, we performed space experiments of photonic integrated circuits (PICs), revealing the critical roles of energetic charged particles. The year-long cosmic radiation exposure does not change carrier mobility but reduces free carrier lifetime, resulting in unchanged electro-optic modulation efficiency and well-expanded optoelectronic bandwidth. The diversity and statistics of the tested PIC modulator indicate the minimal requirement of shielding for PIC transmitters with small footprint modulators and complexed routing waveguides toward lightweight space terminals for terabits communications and intersatellite ranging.

Toroidal dipole Fano resonances driven by bound states in the continuum of lithium niobate metasurface for efficient electro-optic modulation

Free-space optical applications benefit greatly from actively reconfigurable metasurfaces. While static metasurfaces have demonstrated remarkable capabilities in manipulating optical wave characteristics, their fixed electromagnetic response faces a challenge for active device demands. This work presents an all-dielectric metasurface of lithium niobate with high Q-factor Fano resonances for enhanced electro-optic modulation. The resonance mechanism is investigated through multipole decomposition and near-field analysis, revealing that toroidal dipole contributions are dominant in all found three resonances. The strongest resonance arises from symmetry-protected bound states in the continuum (SP-BIC), and it is excited in an asymmetric metasurface by introducing a perturbation in the symmetric nanohole clusters, to enable polarization-independent operation. We numerically achieve a tuning efficiency of 0.65 nm/V and π-phase retardation driven by a bias of 0.53 V, due to the strong field confinement of quasi-BIC mode. Our work provides a scheme for compact, high-performance, and electrically tunable metasurfaces that can help in the development of spatial optical switches as well as tunable micro-lenses and displays.

Fabrication of Low Loss Lithium Niobate Rib Waveguides Through Photoresist Reflow

We present lithography and argon plasma etching of lithium niobate on insulator (LNOI) rib waveguides using reflowed photoresist etch masks and 405 nm photolithography. Melting the photoresist at temperatures greatly exceeding its glass transition temperature while minimizing feature distortion through photoresist adhesion control reduces sidewall surface roughness and allows the photoresist to be used both as the pattern mask and the hard etch mask. Waveguide sidewall surfaces exhibiting sub-nm root mean square roughness are fabricated. Dependence of sidewall roughness and angle on feature width, and propagation loss on thermal annealing of the fabricated devices is characterized. Measured quality factors on fabricated microresonators exceed one million. LNOI rib waveguides and resonators with low propagation loss increase nonlinear optical conversion efficiencies and are useful for efficient electro-optic modulation. Photolithography compatible fabrication of low loss LNOI photonic integrated circuits facilitates scalable commercialization.

Plasmonic Lithium Niobate Mach-Zehnder Modulators.

Lithium niobate Mach-Zehnder modulators (MZMs) are present in a wide range of technologies and though fulfilling the performance and reliability requirements of present applications, they are becoming progressively too bulky, power inefficient, and slow in switching to keep pace with future technological demands. Here, we utilize plasmonics to demonstrate the most efficient (VπL = 0.23 Vcm) lithium niobate MZM to date, consisting of gold nanostripes on lithium niobate that guide both plasmonic modes and electrical signals that control their relative optical phase delay, thereby enabling efficient electro-optic modulation. For high linearity (modulation depth of >2 dB), the proposed MZM inherently operates near its quadrature point by shifting the relative phase of the signal in the interferometric arms. The demonstrated lithium niobate MZM manifests the benefits of employing plasmonics for applications that demand compact (<1 mm2) and fast (>10 GHz) photonic components operating reliably at ambient temperatures.

Gigahertz free-space electro-optic modulators based on Mie resonances

Electro-optic modulators are essential for sensing, metrology and telecommunications. Most target fiber applications. Instead, metasurface-based architectures that modulate free-space light at gigahertz (GHz) speeds can boost flat optics technology by microwave electronics for active optics, diffractive computing or optoelectronic control. Current realizations are bulky or have low modulation efficiencies. Here, we demonstrate a hybrid silicon-organic metasurface platform that leverages Mie resonances for efficient electro-optic modulation at GHz speeds. We exploit quasi bound states in the continuum (BIC) that provide narrow linewidth (Q = 550 at {lambda }_{{{{{{{{rm{res}}}}}}}}}=1594 nm), light confinement to the non-linear material, tunability by design and voltage and GHz-speed electrodes. Key to the achieved modulation of frac{{{Delta }}T}{{T}_{max }}=67 % are molecules with r33 = 100 pm/V and optical field optimization for low-loss. We demonstrate DC tuning of the resonant frequency of quasi-BIC by {{Delta }}{lambda }_{{{{{{{{rm{res}}}}}}}}}= 11 nm, surpassing its linewidth, and modulation up to 5 GHz (fEO,−3dB = 3 GHz). Guided mode resonances tune by {{Delta }}{lambda }_{{{{{{{{rm{res}}}}}}}}}= 20 nm. Our hybrid platform may incorporate free-space nanostructures of any geometry or material, by application of the active layer post-fabrication.

Efficient silicon and side-cladding waveguide modulator with electro-optic polymer.

Efficient electro-optic (EO) modulation can be generated in the hybrid silicon modulator with EO polymer in the form of an in-plane coplanar waveguide and electrode structure. Strong confinement of the optical field in the hybrid structure is critical to performing efficient electric poling and modulation of the EO polymer. The waveguide consists of silica-based side claddings and an EO core for increasing the integral of the optical field and the overlap interaction between the optical field and the modulated electric field within the EO polymer. We discuss in detail the volume resistivity dependence of the efficiency of electric poling and modulation for various side-cladding materials. In a Mach-Zehnder interferometer modulator, the measured half-wave-voltage length product (VπL) is 1.9 V·cm at an optical communication wavelength of 1,550 nm under the TE optical mode operation. The high-speed signaling of the device is demonstrated by generating on-off-keying transmission at signal rates up to 52 Gbit/s with a Q factor of 6.1 at a drive voltage of 2.0 Vpp.

A 100 Gbaud On-off-Keying Silicon-Polymer Hybrid Modulator Operating at up to 110°C

We demonstrate 100-Gbaud on-off-keying (OOK) transmission driven at a low voltage using an electro-optic (EO) polymer modulator combined with a silicon Mach-Zehnder interferometer waveguide. Various types of organic- and polymer-based modulators have been reported to perform efficient EO modulation and high-speed data transmission at over 100 Gbaud. However, there are critical concerns regarding the practical application of polymer devices in terms of environmental stability. In particular, long-term thermal storage and stability during operation require improvements before EO polymer modulators can be applied in practical systems. We have developed an EO polymer with enhanced thermophysical stability and used it to fabricate an efficient EO polymer modulator. In this study we extend our earlier work on high-speed EO polymer modulators by performing fiber-link 100-Gbaud OOK transmission at various operating temperatures. A thermal stability test revealed that the EO polymer modulator can survive high-temperature exposure up to 110°C. Error-free signal transmissions over a distance of 2.0 km was successfully demonstrated with a driving voltage of 1.9 V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pp</sub> and a bit error rate below the 7% overhead forward error correction threshold. The driving voltage, bandwidth, bit error rate, and fiber-link performance of the device are presented.

Demonstration of high-speed thin-film lithium-niobate-on-insulator optical modulators at the 2-µm wavelength.

Optical communication wavelength is being extended from the near-infrared band of 1.31/1.55 µm to the mid-infrared band of 2 µm or beyond for satisfying the increasing demands for high-capacity long-distance data transmissions. An efficient electro-optic (EO) modulator working at 2 µm is highly desired as one of the indispensable elements for optical systems. Lithium niobate (LiNbO3) with a large second-order nonlinear coefficient is widely used in various EO modulators. Here, we experimentally demonstrate the first Mach-Zehnder EO modulator working at 2 µm based on the emerging thin-film LiNbO3 platform. The demonstrated device exhibits a voltage-length product of 3.67 V·cm and a 3-dB-bandwidth of >22 GHz which is limited by the 18 GHz response bandwidth of the photodetector available in the lab. Open eye-diagrams of the 25 Gb/s on-off keying (OOK) signals modulated by the fabricated Mach-Zehnder EO modulator is also measured experimentally with a SNR of about 14 dB.

Wavefront shaping and modulation with resonant electro-optic phase gradient metasurfaces

Phase gradient metasurfaces have revolutionized modern optical components by significantly reducing the path length of bulk optics while maintaining high performance. However, their geometric design makes dynamic modulation challenging, with devices facing a trade-off between the modulation range and efficiency. Here, we introduce Silicon-on-Lithium Niobate (LNO) high-Quality-factor (high-Q) metasurfaces for efficient electro-optic wavefront shaping and modulation. Periodic perturbations within Si metasurface elements allow incident light to be weakly coupled into guided modes, generating high-Q resonances that manifest in the far-field diffraction spectrum. The near field of each Si element spatially overlaps with the LNO substrate, enabling electrically tunable modulation of the resonant frequency. Using full-field simulations, we demonstrate near-infrared, dynamically tunable beamsteering, and beamsplitting metasurfaces. First, we demonstrate beamsteering metasurfaces whose +1st order diffracted intensity can be modulated from 70% to 7% absolute efficiency near the resonant frequency with applied electric fields of order V/μm. Next, we design a tunable beam splitter, switching between direct, 0th order transmission and beamsplitting with the application of 30 V across the metasurface. Our high-Q electro-optic metasurfaces provide a foundation for efficient, time-dependent transfer functions in subwavelength footprints.

Device architectures for low voltage and ultrafast graphene integrated phase modulators.

The atomic layer thin geometry and semi-metallic band diagram of graphene can be utilized for significantly improving the performance matrix of integrated photonic devices. Its semiconductor-like behavior of Fermi-level tunability allows graphene to serve as an active layer for electro-optic modulation. As a low loss metal layer, graphene can be placed much closer to active layer for low voltage operation. In this work, we investigate hybrid device architectures utilizing semiconductor and metallic properties of the graphene for ultrafast and energy efficient electro-optic phase modulators on semiconductor and dielectric platforms. (1) Directly contacted graphene-silicon heterojunctions. Without oxide layer, the carrier density of graphene can be modulated by the directly contact to silicon layer, while silicon intrinsic region stays mostly depleted. With doped silicon as electrodes, carrier can be quickly injected and depleted from the active region in graphene. The ultrafast carrier transit time and small RC constant promise ultrafast modulation speed (3dB bandwidth of 67 GHz) with an estimated Vπ·L of 1.19 V·mm. (2) Graphene integrated lithium niobite modulator. As a transparent electrode, graphene can be placed close to integrated lithium niobate waveguide for improving coupling coefficient between optical mode profile and electric field with minimal additional loss (4.6 dB/cm). Numerical simulation indicates 2.5× improvement of electro-optic field overlap coefficient, with estimated V π of 0.2 V.

Strong electro-optic absorption spanning nearly two octaves in an all-fiber graphene device

Abstract An efficient electro-optic transition control is reported in all-fiber graphene devices over a broad spectral range from visible to near-infrared. The ion liquid–based gating device fabricated onto a side-polished fiber with high numerical aperture significantly enhances the light-matter interaction with graphene, resulting in strong and nonresonant electro-optic absorption of up to 25.5 dB in the wavelength ranging from 532 to 1950 nm. A comprehensive analysis of the optical and electrical properties of the device fabricated with monolayer and bilayer graphene revealed that the number of graphene layers significantly impacts on the performance of the device, including modulation depth and driving voltage. Wavelength-dependent optical response is also measured, which clearly characterizes the electronic bandgap dispersion of graphene. The device exhibited more efficient electro-optic modulation in the longer wavelength region, where the maximum light modulation efficiency of 286.3%/V is achieved at a wavelength of 1950 nm.

Optical switches and modulators in deep freeze.

The integration of silicon-based waveguides with barium titanate thin films enables the realization of efficient electro-optic switches and modulators operating at cryogenic temperatures, offering promising opportunities for quantum technologies.

Plasmonic Bragg microcavity as an efficient electro-optic modulator.

Plasmonic electro-optic modulators might play a pivotal role in the development of compact efficient communication devices. Here, we introduce a novel electro-optic modulator based on a plasmonic Bragg microcavity and a pockels active material. We investigate detailed design and optimization protocols of the proposed structure. With 2D scanning of geometrical parameters, an extinction ratio of 19.8 dB, insertion loss of 2.8 dB and modulation depth of 0.99 with a driving voltage of ±5 V are obtained.

Sensitivity and noise in THz electro-optic upconversion radiometers

This paper presents a study of noise in room-temperature THz radiometers that use THz-to-optical upconversion followed by optical detection of thermal radiation. Despite some undesired upconverted thermal noise, no noise is intrinsically introduced by efficient electro-optic modulation via a sum-frequency-generation process in high quality factor (Q) whispering-gallery mode (WGM) resonators. However, coherent and incoherent optical detection results in fundamentally different noise characteristics. The analysis shows that the upconversion receiver is quantum limited like conventional amplifiers and mixers, only when optical homodyne or heterodyne detection is performed. However, this type of receiver shows advantages as a THz photon counter, where counting is in the optical domain. Theoretical predictions show that upconversion-based room-temperature receivers can outperform state-of-the-art cooled and room-temperature THz receivers based on low-noise amplifiers and mixers, provided that a photon conversion efficiency greater than 1% is realized. Although the detection bandwidth is naturally narrow due to the highly resonant electro-optic modulator, it is not fundamentally limited and can be broadened by engineering selective optical coupling mechanisms to the resonator.

Ultra-Low Index-Contrast Polymeric Photonic Crystal Nanobeam Electro-Optic Modulator

A novel nanocavity-based polymer-on-insulator (POI) electro-optic modulator (EOM) is proposed. It consists of a polymeric photonic-crystal nanobeam cavity (PCNC) with ultra-low index-contrast ( n $_{\text{cavity}}$ /n $_{bg}$ = 1.17). Based on three-dimensional (3D) finite-difference time-domain (FDTD) method, the PCNC design and optimization are investigated theoretically. A high quality-factor ( Q ) of 3.4 $\,\times\, 10^4$ and small mode-volume of 22.8 ${({\lambda }/n_{SEO}})^{3}$ can be achieved. In order to judge the efficiency of the cavity design for electro-optic (EO) modulation, the optical modes and the electric field distribution are computed using an electromagnetic finite-element solver. Benefiting from the fast and strong EO effect in polymers, the modulator shows an EO modulation efficiency of up to 16 pm/V, which is over an order of magnitude higher than that in lithium niobate (LN) PCNCs. Moreover, the device is only 80 $\mu$ m in length, leading to a voltage-length product $V_{\pi }$ × L = 0.05 V $\cdot$ cm, which is much smaller than those of Mach-Zehnder modulators. To the best of our knowledge, this is the first on-chip POI-based EOM that features both ultra-compact size and high modulation efficiency. Hence, it is potentially an ideal platform for applications in optical communications, electric-field sensing, and tunable photonic circuits.

Plasmonic monolithic lithium niobate directional coupler switches

Lithium niobate (LN) has been the material of choice for electro-optic modulators owing to its excellent physical properties. While conventional LN electro-optic modulators continue to be the workhorse of the modern optoelectronics, they are becoming progressively too bulky, expensive, and power-hungry to fully serve the needs of this industry. Here, we demonstrate plasmonic electro-optic directional coupler switches consisting of two closely spaced nm-thin gold nanostripes on LN substrates that guide both coupled electromagnetic modes and electrical signals that control their coupling, thereby enabling ultra-compact switching and modulation functionalities. Extreme confinement and good spatial overlap of both slow-plasmon modes and electrostatic fields created by the nanostripes allow us to achieve a 90% modulation depth with 20-μm-long switches characterized by a broadband electro-optic modulation efficiency of 0.3 V cm. Our monolithic LN plasmonic platform enables a wide range of cost-effective optical communication applications that demand μm-scale footprints, ultrafast operation and high environmental stability.

Surface Plasmon Resonance assisted optical analysis of Strontium Barium Niobate thin films

Preferred c-axis (0 0 1) oriented SrxBa1-xNb2O6 (SBN60) thin films were grown exploiting Pulsed Laser deposition (PLD) technique. For development of an efficient Electro-Optic (EO) modulator based on SBN60 thin film, observation of minutiae change in refractive index is essential. Highly sensitive Surface Plasmon Resonance (SPR) tool is used to examine the optical property of SBN60 film. Significant increase from 1.94 to 2.50 in refractive index values with increasing deposition pressure from 8mTorr to 12mTorr is observed. Refractive index dispersion with the incident radiation wavelength has been also analysed for SBN60 thin films using SPR technique and theoretically fitted with Sellmeier equation.

Graphene-Incorporated Soft Capacitors for Mechanically Adjustable Electro-Optic Modulators.

In addition to ultrahigh capacity and speed in data management, future communication networks require enhanced performance via system reconfigurability under limited resources. Extremely high-speed operation renders optical data managing devices as excellent candidates to hybridize with current electronic devices; however, they still need tunability for system reconfiguration in an integrated scheme. We demonstrate an efficient electro-optic (EO) modulator that is mechanically tunable on a multiple optical waveguide system that functioned with a soft capacitor structure incorporating graphene and poly(methyl methacrylate) (PMMA). The flexible capacitor that generates optical signals by temporal light absorption depending on electrical signals can be mechanically detached and reattached from and onto a rigid surface of the waveguide. It provides either the on or off state of the modulating operation, and enables switching of the working waveguides, following the reconfigured data routes. Quality-controlled graphene mainly provides the EO operation, and PMMA plays an important role as both the flexible dielectric layer in the capacitor and the passivation layer for graphene protection. The modulation effects of the manually prepared graphene-PMMA capacitor mechanically adjusted onto a side-polished optical fiber (D-shaped fiber) are investigated in terms of the extinction ratio (ER) of the transmitting light and the operational bandwidth. We successfully display an ER of the modulator up to 19.8 dB with a voltage control ranging from -50 to 50 V. Its stable operation is verified with a modulation speed up to 2.5 MHz.