High modulation efficiency lithium niobate Michelson interferometer modulator

Reconfigurable AWGR based on LNOI with a tunable central wavelength and bandwidth used in elastic optical networking

Elastic optical networking introduces elasticity and adaptation into the optical domain, which highly depends on reconfigurable optical devices. In this paper, a tunable 4×4 arrayed waveguide grating router based on lithium niobate on insulator is designed. By using the electro-optic effect of lithium niobate, we design electrode regions with specific shapes in the array waveguide region to realize the tuning of the routing wavelength and bandwidth of the third output channel. The designed arrayed waveguide grating router (AWGR) has a dense channel spacing of 0.8 nm, and the minimum insertion loss is 2.3 dB. Experiments show that the tuning range of the central wavelength can reach 3.2 nm, and the 3 dB bandwidth can be expanded from 0.2 to 0.6 nm.

High-efficiency thin-film lithium niobate modulator with highly confined optical modes

We demonstrate a low-loss, high-efficiency lithium niobate electro-optic (EO) modulator with optical isolation trenches to achieve stronger field confinement and reduced light absorption loss. The proposed modulator realized considerable improvements, including a low half-wave voltage-length product of 1.2 V·cm, an excess loss of ∼2.4 dB, and a broad 3-dB EO bandwidth of over 40 GHz. We developed a lithium niobate modulator with, to the best of our knowledge, the highest reported modulation efficiency of any Mach-Zehnder interferometer (MZI) modulator.

Parametric amplification and instability in time-periodic dielectric slabs

We study the phenomenon of parametric amplification in the context of time-periodic dielectric slabs. These structures show particular promise inasmuch as they are capable of very large amplifications when illuminated by an electromagnetic wave of half the modulation frequency. Successive studies have corroborated this finding but none have yet been able to ascertain the nature of amplification in such devices. On top of that, some studies have raised speculations regarding the instability of a time-periodic slab which are off the mark. The problem lies in the poor understanding (or lack thereof) of the mathematical devices necessary to tackle such problems. We successfully carry out the tasks by tapping into the rich mathematical theory of Hill's equation. Specifically, we make use of the Folquet's theorem in its complete form which brings to light novel physical phenomena that the more prevalent simplified form fails to account for. Also, useful mathematical concepts such as coexistence are employed which to the best of our knowledge have not yet been applied in the field of time-varying optics. Our analytical method proves an effective means of assessing the amplifier's performance, e.g., estimating how long it takes for the device to reach steady state. We further delineate the link between amplification and instability and correct the misconceptions surrounding the subject by presenting a rigorous analysis of the instability problem in such structures.

Hybrid resonance metasurface for a lithium niobate electro-optical modulator

Electrically tunable metasurfaces can realize two-dimensional pixelated spatial light modulation and have a wide range of applications in optical switching, free-space communication, high-speed imaging, and so on, arousing the interest of researchers. Here, a gold nanodisk metasurface on a lithium-niobate-on-insulator (LNOI) substrate is fabricated and experimentally demonstrated as an electrically tunable optical metasurface for transmissive free-space light modulation. Using the hybrid resonance formed by the localized surface plasmon resonance (LSPR) of gold nanodisks and the Fabry-Perot (FP) resonance, the incident light is trapped in the gold nanodisk edges and a thin lithium niobate layer to realize field enhancement. In this way, an extinction ratio of 40% is achieved at the resonance wavelength. In addition, the proportion of hybrid resonance components can be adjusted by the size of the gold nanodisks. By applying a driving voltage of ± 2.8 V, a dynamic modulation of 135 MHz is achieved at resonant wavelength. The highest signal-to-noise ratio (SNR) is up to 48 dB at 75 MHz. This work paves the way for the realization of spatial light modulators based on CMOS-compatible LiNbO₃ planar optics, which can be used in lidar, tunable displays, and so on.

High-Quality Dry Etching of LiNbO3 Assisted by Proton Substitution through H2-Plasma Surface Treatment

The exceptional material properties of Lithium Niobate (LiNbO₃) make it an excellent material platform for a wide range of RF, MEMS, phononic and photonic applications; however, nano-micro scale device concepts require high fidelity processing of LN films. Here, we reported a highly optimized processing methodology that achieves a deep etch with nearly vertical and smooth sidewalls. We demonstrated that Ti/Al/Cr stack works perfectly as a hard mask material during long plasma dry etching, where periodically pausing the etching and chemical cleaning between cycles were leveraged to avoid thermal effects and byproduct redeposition. To improve mask quality on X- and Y-cut substrates, a H₂-plasma treatment was implemented to relieve surface tension by modifying the top surface atoms. Structures with etch depths as deep as 3.4 µm were obtained in our process across a range of crystallographic orientations with a smooth sidewall and perfect verticality on several crystallographic facets.

High-speed compact folded Michelson interferometer modulator

We propose and experimentally demonstrate a novel compact folded Michelson interferometer (FMI) modulator with high modulation efficiency. By folding the 0.5 mm-long phase shift arms, the length of the modulation area of the FMI modulator is only 0.25 mm. Meanwhile, the traveling wave electrode (TWE) is also shorter, which decreases the propagation loss of the RF signal and contributes to a small footprint. The V_π-L of the present device is as low as 0.87 V·cm at -8 V bias voltage. The minimum optical insertion loss is 3.7 dB, and the static extinction ratio (ER) is over 25 dB. The measured 3-dB electro-optical (EO) bandwidth is 17.3 GHz at a -6 V bias. The OOK eye diagram up to 40 Gb/s is demonstrated under 2 V driver voltage.

40 GHz high-efficiency Michelson interferometer modulator on a silicon-rich nitride and thin-film lithium niobate hybrid platform

We propose and demonstrate a Michelson interferometer modulator with integrated Bragg reflectors on a silicon-rich nitride-thin-film lithium niobate hybrid platform. High-reflectivity Bragg reflectors are placed at the ends of both arms, which double the electro-optic (E-O) interaction length and reduce the velocity mismatch between the microwave and optical wave. The presented Michelson interferometer modulator achieves a measured half-wave voltage length product as low as 1.06 V cm and high-speed modulation up to 70 Gbps. A 3-dB E-O bandwidth beyond 40 GHz is also achieved, which is, to the best of our knowledge, the highest modulation bandwidth of Michelson interferometer modulators.

Lithium niobate photonic-crystal electro-optic modulator

Modern advanced photonic integrated circuits require dense integration of high-speed electro-optic functional elements on a compact chip that consumes only moderate power. Energy efficiency, operation speed, and device dimension are thus crucial metrics underlying almost all current developments of photonic signal processing units. Recently, thin-film lithium niobate (LN) emerges as a promising platform for photonic integrated circuits. Here, we make an important step towards miniaturizing functional components on this platform, reporting high-speed LN electro-optic modulators, based upon photonic crystal nanobeam resonators. The devices exhibit a significant tuning efficiency up to 1.98 GHz V⁻¹, a broad modulation bandwidth of 17.5 GHz, while with a tiny electro-optic modal volume of only 0.58 μm³. The modulators enable efficient electro-optic driving of high-Q photonic cavity modes in both adiabatic and non-adiabatic regimes, and allow us to achieve electro-optic switching at 11 Gb s⁻¹ with a bit-switching energy as low as 22 fJ. The demonstration of energy efficient and high-speed electro-optic modulation at the wavelength scale paves a crucial foundation for realizing large-scale LN photonic integrated circuits that are of immense importance for broad applications in data communication, microwave photonics, and quantum photonics.

Lithium niobate (LN) devices are promising for future photonic integrated circuits. Here, the authors demonstrate an electro-optic LN modulator with a very small modal volume based on photonic crystal resonator architecture.

Visible-frequency meta-gratings for light steering, beam splitting and absorption tunable functionality

Diffractive grating and plasmonic metasurface have always been developing as two parallel optical domains, which have not met for studying their hybridization to discover new applications and potentials. Here, we proposed a novel meta-grating design, which hybridizes the metasurface interfacial gradient with the blazed grating profile. The unique architecture takes advantage of both grating effect and plasmonic resonances with minimum cross-coupling, thus leading to the polarization-selective behaviors to steer different polarized light to drastically inverse directions (> 90°). Furthermore, the hybridized surface also exhibits angle-dependent broadband absorptive tunability (∼ 5% - 86%) by migrating the strong blazed order and plasmonic order at the far field. We believe that the integrated meta-grating device would suggest various potential applications including polarization beam splitters, high signal-to-noise ratio (SNR) optical spectrometer, high-efficiency plasmonic couplers and filter, etc.