Thin-film Modulator Research Articles

Folded thin-film lithium niobate modulator based on a poled Mach-Zehnder interferometer structure.

The thin-film lithium niobate structure has been used recently to construct compact and high-performance electro-optical modulators. Due to the moderate electro-optical coefficient of the lithium niobate material, the device length of such a modulator is still long, a few centimeters usually. Here, a folded Mach-Zehnder interferometer based modulator on x-cut thin-film lithium niobate is demonstrated. An effective poling procedure is developed to activate the device. The proposed modulator structure can shorten the device length without affecting its performance. The measured VπL product of a fabricated and completely poled folded modulator is about 2.74V⋅cm, and the 3 dB electro-optical bandwidth is about 55 GHz. They are close to those of a conventional Mach-Zehnder modulator with a straight modulation section.

Atomic Layer Modulation of Multicomponent Thin Films through Combination of Experimental and Theoretical Approaches

Atomic Layer Modulation of Multicomponent Thin Films through Combination of Experimental and Theoretical Approaches

Property Modulation of Two-Dimensional Lead-Free Perovskite Thin Films by Aromatic Polymer Additives for Performance Enhancement of Field-Effect Transistors.

The inevitable oxidation of Sn2+ and p-type self-doping has plagued the development of two-dimensional (2D) Sn-based perovskite field effect transistors. In this work, we demonstrate the modulation of the properties of phenethylammonium tin iodide ((PEA)2SnI4) perovskite thin films by introducing the aromatic polymer additives of poly(4-vinylphenol) (PVP) and poly(vinyl pyrrolidone) (PVPD) during the crystallization processes, keeping the 2D layered structure of (PEA)2SnI4 unchanged. The proposed formation mechanisms of the polymer-assisted (PEA)2SnI4:PVP and (PEA)2SnI4:PVPD films disclose that the interactions between the polymers and (PEA)2SnI4, such as hydrogen bonds, π-π interactions, and coordination bonds, lead to the improvement of the morphology and crystallization as well as the inhibition of Sn2+ oxidation of the films. However, the field-effect transistors based on the two polymer-assisted (PEA)2SnI4 thin films constructed on the dielectric of poly(vinyl alcohol) (PVA) modified by crosslinking PVP (CL-PVP) exhibit quite a different performance. Compared with the (PEA)2SnI4 transistor, without sacrificing the hole mobility, the on-off current ratio of the (PEA)2SnI4:PVP device increases by one order of magnitude, and the subthreshold slope declines slightly due to the reduced leakage current, which results from the reduction of p-type self-doping of the perovskite film and the improved quality of the perovskite/dielectric interface because of the strong π-π interactions between the benzene rings in CL-PVP and (PEA)2SnI4:PVP. In contrast, the (PEA)2SnI4:PVPD transistor exhibits relatively poor overall performance because of the N-vinylpyrrolidone of PVPD. More importantly, employing PVP and PVPD as additives can effectively enhance the chemical stability of (PEA)2SnI4 as well as the operational stabilities of the corresponding transistors. Our work provides an effective strategy for selecting chemical additives to improve 2D perovskite properties and suppress the oxidation of Sn-based perovskites, and paves a way toward the future applications of Sn-based perovskite optoelectronic devices with high performance and stability.

Integrated lithium niobate electro-optic modulators: when performance meets scalability

Electro-optic modulators (EOMs) convert signals from the electrical to the optical domain. They are at the heart of optical communication, microwave signal processing, sensing, and quantum technologies. Next-generation EOMs require high-density integration, low cost, and high performance simultaneously, which are difficult to achieve with established integrated photonics platforms. Thin-film lithium niobate (LN) has recently emerged as a strong contender owing to its high intrinsic electro-optic (EO) efficiency, industry-proven performance, robustness, and, importantly, the rapid development of scalable fabrication techniques. The thin-film LN platform inherits nearly all the material advantages from the legacy bulk LN devices and amplifies them with a smaller footprint, wider bandwidths, and lower power consumption. Since the first adoption of commercial thin-film LN wafers only a few years ago, the overall performance of thin-film LN modulators is already comparable with, if not exceeding, the performance of the best alternatives based on mature platforms such as silicon and indium phosphide, which have benefited from many decades of research and development. In this mini-review, we explain the principles and technical advances that have enabled state-of-the-art LN modulator demonstrations. We discuss several approaches, their advantages and challenges. We also outline the paths to follow if LN modulators are to improve further, and we provide a perspective on what we believe their performance could become in the future. Finally, as the integrated LN modulator is a key subcomponent of more complex photonic functionalities, we look forward to exciting opportunities for larger-scale LN EO circuits beyond single components.

Sizeable bandgap modulation in Y2Hf2O7 pyrochlore oxide thin films through B-site substitution

The high-quality (111)-oriented pyrochlore oxide thin films Y2(Hf1-xIrx)2O7 (YHIO) were prepared by pulsed laser deposition. The bandgap values of Ir-substituted YHIO thin films for x = 0–0.5 range from 4.6 to 1.9 eV. Taking account of spin orbital coupling and correlation effect (U), the first-principles calculations based on density functional theory were used to intercept this variation. The theoretical results illustrate that the variation is attributed to the lower Ir 5d orbital in energy and stronger orbital hybridization between the oxygen 2p orbital and the Ir 5d orbital near the Fermi surface. These results suggest that we can widely modulate the bandgap through B-site chemical substitution based on the Y2Hf2O7 thin film.

Thickness dependent electrical resistivity modulation in tensile-strained epitaxial thin films of calcium manganese oxide

We report our studies of the thickness dependence of electrical resistivity and lattice constants in strained epitaxial thin films of calcium manganese oxide. Our results indicate the potential of bi-axial lattice mismatch strain as a handle for modulating electrical resistivity. We observe thickness dependence of lattice constants consistent with what is expected for strain relaxation for films thicker than 400 Å. At lower thickness values, anomalies are observed suggestive of reduced oxygen stoichiometry. We observe a remarkable decrease in electrical resistivity with decreasing film thickness. The resistivity of our thinnest films (5–7 nm) is about three orders of magnitude lower than the resistivity of bulk CaMnO3. Resistivity increases as the film thickness increases, along with the progression of strain relaxation. It is noteworthy that the thickness dependence of resistivity we observe in CMO thin films is the opposite of what has been reported for their hole-doped rare earth manganite counterpart La0.67Ca0.33MnO3 (LCMO), where tensile lattice mismatch strain suppresses metallicity, leading to the increase in resistivity with film thickness. We believe that the enhanced conductivity in our thinnest films is related to the possible oxygen deficiency promoted by tensile strain. Recent x-ray absorption measurements have revealed reduced oxygen content and associated changes in Mn valence states in tensile-strained CMO thin films, as also predicted by density functional theory calculations. This report is the first observation of electrical transport behavior possibly indicative of strain–oxygen stoichiometry coupling.

Material Properties Modulation in Inorganic Perovskite Films via Solution-Free Solid-State Reactions

A simple solvent-free, versatile, and flexible route for material properties modulation in halide perovskite thin films has been developed using CsPbBr3 thin films. The method further enables compo...

Breaking voltage–bandwidth limits in integrated lithium niobate modulators using micro-structured electrodes

Electro-optic modulators with low voltages and large bandwidths are crucial for both analog and digital communication. Recently, thin-film lithium niobate modulators have emerged as a strong candidate for next generation electro-optic solutions. These modulators offer significantly improved voltage–bandwidth performances over the existing bulk lithium niobate modulators while preserving key material advantages such as linear response, high extinction ratio, high optical power handling ability, and low optical losses. However, reduced electrode gaps in miniaturized thin-film modulators lead to higher microwave losses, which limit electro-optic performances at high frequencies. Here we overcome this limitation to achieve a record combination of low RF half-wave voltage ( V π ) of 1.3 V while maintaining electro-optic response with 1.8 dB roll-off at 50 GHz using micro-structured electrodes. Our demonstration represents a significant improvement in voltage–bandwidth performance, one that is comparable to the performance gain in switching from legacy bulk to thin-film lithium niobate modulators. Such a micro-structured electrode design could enable sub-volt modulators with > 100 G H z bandwidth.

Thin-film lithium niobate modulators for non-invasive sensing of high-frequency electric fields

We propose a non-invasive, large-bandwidth electro-optic (EO) detection scheme for high-frequency electric fields using thin-film lithium niobate Mach–Zehnder interferometers. Our proof-of-concept device is capable of detecting high-strength electric fields, such as those present in x-ray free electron lasers and linear accelerators. The proposed detection scheme utilizes an off-the-shelf C-band fiber optic continuous-wave laser and optical spectrum analyzer, which has a lower system cost and footprint compared to bulk-crystal-based schemes. Towards this objective, fabricated devices are characterized in the 1–40 GHz frequency range to estimate the detection threshold, detection bandwidth, and EO modulation strength. The characterized device exhibits a 0.13 V ⋅ m − 1 ⋅ H z − 1 / 2 normalized electric field sensitivity. By extrapolating the measured frequency response, it is expected that the characterized device will be able to detect free-space electric fields up to 150 GHz.

High-performance thin-film electro-optical modulator based on heterogeneous silicon and lithium niobate platform (Invited)

High-performance thin-film electro-optical modulator based on heterogeneous silicon and lithium niobate platform (Invited)

Modulating extraordinary terahertz transmissions in multilayer plasmonic metasurfaces

Manipulations of light–matter interaction via sub-wavelength plasmonic structures have opened up many new research opportunities in photonics from microwave to the visible spectrum, including the relatively underdeveloped but technologically important terahertz regime. Here, we have studied manipulation of terahertz transmission through a bilayer metasurface consisting of a metallic hole array and a complimentary patch array separated by an ultra-thin dielectric spacer. The terahertz transmission spectra through our studied metasurfaces exhibit characteristic resonances due to the surface plasmon induced extraordinary transmission peak. Our study reveals a counterintuitive blue shift of the transmission peak with increasing spacer thickness, which is explained by reduced Coulomb interaction between two plasmonic layers. The measured quality factor exhibits a strong dependence on the spacer thickness, and the maximum quality factor is observed for a spacer thickness of around λ/30, indicating strong electric-field confinement inside the dielectric spacer. These bilayer plasmonic structures will aid in realizing next-generation terahertz plasmonic devices such as ultrasensitive thin-film sensors, modulators, narrow-band filters, and other nonlinear components.

The effect of MoS2 modulated doping with molybdenum-oxide on the photovoltaic performance for MoS2/n-Si heterojunction solar cells

Achieving large Fermi energy splitting under illumination is critical for improving the performance of photovoltaic devices. In this work, we conducted the energy band structure modulation of MoS2 thin films via MoOx doping scheme to realize its photovoltaic operation on n-type c-Si by magnetron sputtering. It is found that, by capping a MoOx overlayer, the MoS2 electrons density decreased and the Fermi level shifted ~0.4 eV towards valence band, consequently the MoS2 conductive type changing. With this doped MoS2 layer, the built-in electric field of fabricated MoS2/n-Si heterojunction devices greatly improved and exceeded 400 mV owing to the larger interface energy-level differences. Meanwhile, the defects recombination losses reduced benefiting from the holes-selective behavior of O-deficiencies in MoOx, so that the photo-generated carriers extraction process were promoted. As a result, the doped MoS2/n-Si devices showed excellent rectification behavior with rectifying ratio to 105 and ideality of 1.12. The TRPL spectrums corroborated the enhanced carrier transport mechanism at MoS2/n-Si interface by MoOx modulation doping effect. Correspondingly, the significant improvement of doped MoS2/n-Si solar cells performance (Voc of 289 mV, FF of 60.72% and Jsc of 31.25 mA/cm2) was achieved, exhibiting an optimized conversion efficiency of 5.47%. The UPS analysis revealed that the doping mechanism of MoOx/MoS2 stacks lies in energy band overlap and electrons contact transfer between these two materials. This observation of MoOx modulation doping effect on large-area sputtering MoS2 could help in building a better interface carrier transport in photovoltaic and optoelectronic applications of 2D materials.

Structural phase control and thermochromic modulation of VO2 thin films by post thermal annealing

Monoclinic M1 to rutile structural phase transition (SPT) in thermochromic VO2 is accompanied by the insulator to metal transition and transition from an infrared (IR) transparent to IR opaque phase. Fine control over the phase stabilization and the functional properties without additional doping can offer the ability to tailor VO2 thin films for their respective applications. In this work, post deposition thermal annealing was used to control the phase stabilization and the modulation in thermochromic performance of polycrystalline VO2 thin films. Monoclinic M1 to rutile SPT in VO2 thin films was tracked by in-situ temperature dependent synchrotron X-ray diffraction measurements and an increase in the SPT temperature was observed with increasing annealing temperature. Intermediate monoclinic M2 phase emerges near the SPT temperature at higher annealing temperature. Temperature dependent X-ray absorption spectroscopy (XAS) measurements revealed the higher electron correlation effect among the V 3d|| states across the phase transition in VO2 thin film having the intermediate phase. Spectral weight of V 3d|| states was found to increase with the annealing temperature. Lower annealing temperature drives the IR switching temperature towards room temperature while the higher annealing temperature improves the IR switching performance of VO2 thin films.

Influence of precursor dose and residence time on the growth rate and uniformity of vanadium dioxide thin films by atomic layer deposition

The growth of vanadium dioxide (VO2) thin films using tetrakis (ethyl-methyl) amino vanadium (TEMAV) and H2O by atomic layer deposition (ALD) has been investigated as a function of the exposure dose and residence time. A novel multiple pulse mode has been employed to mitigate the small deposition rate brought about by the low vapor pressure of TEMAV. Compared to the conventional ALD cycle with a single pulse of precursor, the use of multiple pulsing with very short pulse time allows lower consumption of precursor, but larger exposure dose and longer residence time on the growth surface, resulting in a higher growth rate for a low volatility precursor, while maintaining a good film uniformity across 4-in. wafers. The Raman analysis and the electrical resistivity modulation of the VO2 thin films show that the films synthesized by the multiple pulse mode is comparable to the films synthesized by the conventional single pulse mode.

Interlayer effect on photoluminescence enhancement and band gap modulation in Ga-doped ZnO thin films

Nano-dimensional ZnO have drawn much attention due to its wonderful transmittance , high UV emission, better environmental and thermal stability, abundant resources and low price for use in many advanced applications. In this context, here we report photoluminescence and structural properties of Ga-doped ZnO thin films , synthesized using pulsed laser deposition . The profound effect of Au and Ag as interlayer on Ga-doped ZnO thin films has been presented systematically. The crystalline texture of the grown films was properly oriented along (002) lattice planes with hexagonal wurtzite structure as determined by X-ray diffraction analysis. Surface study was carried out by means of field emission scanning electron microscope revealing low roughness on surface of the films. These films reveal an enhancement of twofold photoluminescence behavior in the near band edge emission region with the insertion of metallic interlayer. The band gap energies of the films obtained were 3.29–3.52 eV using IR spectrophotometer . Reflectance spectra reveal less reflectance (<16%) and more absorption in the UV-NIR region which provide a novel way to replace other UV and IR photodetectors with Ga-doped ZnO. • Synthesis of pulsed laser deposited highly crystalline Ga-doped ZnO thin films. • Utility of Au and Ag as buffer layers between Ga-doped ZnO thin films. • Smooth surface and grain morphology were obtained. • Dominance of quantum confinement in enhancing near band edge luminescence spectra.

Magnetic property modulation of Ni thin films transferred onto flexible substrates

The integration of highly crystallized thin films on flexible plastic substrates has so far been limited by the instability of the plastic substrates at high growth temperatures. In this paper, we report a robust and facile method to transfer magnetic Ni thin films onto flexible substrates, which enables the imposition of strain on the substrates to take advantage of the magnetoelastic properties of the films. Ni thin films were grown on water-soluble NaCl substrates by sputtering at high temperature and, thereafter, successfully transferred onto flexible polyethylene terephthalate substrates. Magnetic hysteresis loops with strong magnetic anisotropy were observed for the films grown at high temperatures on NaCl substrates; the anisotropy increased after the films were transferred onto the plastic substrate. The magnetic anisotropy of the transferred films was modulated under bending; this phenomenon of magnetic modulation is more apparent at a small radius of curvature. The modulation of magnetic anisotropy in magnetic thin films on flexible substrates using our innovative thin film transfer method presents significant potential for application in future flexible multifunctional devices.

Microstructure morphology and concentration modulation of nanocomposite thin-films during simulated physical vapor deposition

Metallic, nanocomposite, thin-films with intertwined morphologies can be synthesized via physical vapor deposition of two immiscible metals. Understanding the phase-ordering kinetics controlling the way the microstructure develops is crucial to obtaining reliable and enhanced functionalities in these nanostructured thin-films. Here, we study the complex relationship between the vapor-deposition conditions and the resulting self-assembled nanoscale morphologies by using phase-field simulations. Our phase-field model accounts for the deposition of the incident vapor phase of a binary alloy onto a substrate, surface interdiffusion, and the subsequent spinodal decomposition in the resulting elastically inhomogeneous thin-film. We systematically investigated the effects of deposition rate, dissimilar bulk and surface kinetics, phase fraction, and dissimilar elastic response on the resulting microstructure. Four distinct classes of achievable self-assembled microstructure morphologies are observed throughout: lateral, vertical, random, and nanoprecipitate concentration modulations. Through our systematic investigation of competing mechanisms, we provide insights on the complex relationships between alloy species and deposition conditions to obtain specific nanostructured morphologies of binary, nanocomposite, thin-films.

A heterogeneously integrated silicon photonic/lithium niobate travelling wave electro-optic modulator.

Silicon photonics is a platform that enables densely integrated photonic components and systems and integration with electronic circuits. Depletion mode modulators designed on this platform suffer from a fundamental frequency response limit due to the mobility of carriers in silicon. Lithium niobate-based modulators have demonstrated high performance, but the material is difficult to process and cannot be easily integrated with other photonic components and electronics. In this manuscript, we simultaneously take advantage of the benefits of silicon photonics and the Pockels effect in lithium niobate by heterogeneously integrating silicon photonic-integrated circuits with thin-film lithium niobate samples. We demonstrate the most CMOS-compatible thin-film lithium niobate modulator to date, which has electro-optic 3 dB bandwidths of 30.6 GHz and half-wave voltages of 6.7 V×cm. These modulators are fabricated entirely in CMOS facilities, with the exception of the bonding of a thin-film lithium niobate sample post fabrication, and require no etching of lithium niobate.

Halide Perovskite High-k Field Effect Transistors with Dynamically Reconfigurable Ambipolarity

Despite the remarkable optoelectronic properties of halide perovskites, achieving reproducible field effect transistor (FET) action in polycrystalline films at room temperature has been challenging and represents a fundamental bottleneck for understanding electronic charge transport in these materials. In this work, we report halide perovskite-based FET operation at room temperature with negligible hysteresis. Extensive measurements and device modeling reveal that incorporating high-k dielectrics enables modulation of the channel conductance. Furthermore, continuous bias cycling or resting allows dynamical reconfiguration of the FETs between p-type behavior and ambipolar FET with balanced electron and hole transport and an ON/OFF ratio up to 104 and negligible degradation in transport characteristics over 100 cycles. These results elucidate the path for achieving gate modulation in perovskite thin films and provide a platform to understand the interplay between the perovskite structure and external stimuli such as photons, fields, and functional substrates, which will lead to novel and emergent properties.

Ferroelectric [formula omitted] thin film based broadband waveguide coupled surface plasmon electro-optic modulator

Present article demonstrates the enhanced dynamically tuneable EO modulation characteristics in ferroelectric Strontium Barium Niobate SBN,Sr0.6Ba0.4Nb2O6 thin film modulator based on waveguide coupled surface plasmon resonance. The alternating modulating signals having broad frequency range (1 kHz to 10 MHz) with varying amplitude (40kV/cm to 400kV/cm) was applied to estimate the resonance frequency (~1 MHz) using differential modulated intensity interrogation signal. A high value of EO coefficient r=310pm/V leading to Modulation index of 56% was achieved with sensitivity of about 0.33 at resonance possessing a characteristics length of 54.64 μm. These waveguide modes coupled with the plasmon modes paves a way for development of EO modulators utilizing materials with high losses. Therefore, a thin film based WCSPR-EO modulator with compact footprint, low operation voltage, broad operation spectrum and faster speed of modulation was fabricated which paves the way for on-chip optical communications.