Electro-optic modulators (EOMs) convert signals from the electrical to the optical domain, which are at the heart of high-capacity optical communication, broadband microwave photonics, sensing, and quantum technologies^[1-4]. To fulfill the requirements of booming information technologies, such as big data, internet of things (IoT), cloud-based services, and autonomous vehicles, as well as the artificial intelligence/machine learning (AI/ML), EOMs should be able to chip-scale integrate with electronics at high-density^[1,3]. However, EOMs have long suffered from the substantial size mismatch with electronic circuits together with issues of energy efficiency, bandwidth, optical loss, drive voltage, and signal distortion. Tremendous efforts have been made in the past few decades to optimize the performance of EOMs and improve the integration with electronics^[1,5]. Recent advances of silicon photonics provide dense integration of complex photonic functionalities, using mature technology and process toolsets from complementary metal–oxide–semiconductor (CMOS) electronics^[6–8]. Due to lack of intrinsic second-order non-linearity, silicon photonics rely on the plasma dispersion effect that derives from carrier injection by application of a voltage^[4,9]. This plasma perturbs the transmission of light and thus effects modulation. Unlike Pockels modulators, plasma dispersion modulators do not have a linear dependence of voltage and refractive index change Δn (thus, they show chirp). Most recently, various heterogeneous approaches that combine Pockels-type electro-optic (EO) materials with dielectric or plasmonic waveguide structures have been explored to introduce better solutions for silicon devices^[10–12].

Currently used materials for modulators include thermo-optic materials^[13], III–V semiconductor (electrical absorption materials, e.g., GaAs, InP)^[14], inorganic crystal materials (lithium niobite)^[15], liquid crystals (LC) and organic electro-optic (OEO) materials. The process of refractive index change for thermo-optic materials and LCs is relatively slow and they cannot be used in high-speed EO modulation. Crystalline inorganic materials, which generally have low optical loss and good stability, are widely used for a long time in fiber optic networks, especially the thin-film lithium niobite on insulator (LNOI) shows the good performance^[16,17]. However, due to the relatively low EO activity, the EO modulators based on lithium niobate have long propagation length and large footprints, which might be difficult to integrate with high-density. III–V semiconductors modulate light via changing the wavelength of an optical resonance caused by an applied electric field. During electro-absorption modulation, both index of refraction and absorbance change, which perhaps leads to the unwanted phenomenon of “chirp”, altering the signal waveform together with high optical loss near the half-maximum of absorption.

OEO materials are of great interest because they offer high EO coefficients, fast response speed, and extremely high bandwidth^[9,18–27]. The Pockels effect of OEO materials comes from the conjugated π-electron system of nonlinear optical (NLO) molecules, which has a few femtoseconds phase relaxation time and enables the theoretical bandwidth more than 15 THz. Significant improvements in device performance have been made by silicon-organic hybrid (SOH)^[28–37] and plasmonic-organic hybrid (POH)^[11,38–44] Pockels effect modulators, which have been proven to be effective photonic platforms for both analog and digital applications. OEO materials generally have larger EO coefficients (300–500 pm/V), and have a potential to achieve higher EO coefficients (>1000 pm/V) based on the theoretical predictions^[3,9,45]. The large EO coefficients are critical for low insertion loss, best energy efficiency, and the highest bandwidth operation. The EO coefficient r₃₃ of OEO materials is proportional to combination of chromophore hyperpolarizability (β), electric field poling-induced acentric order of the chromophores (<cos³θ>), and chromophore number density (ρ_N). The half-wave voltage-length parameterV_πL for an EO modulator is inversely proportional ton³r₃₃. The past two decades of research and development have demonstrated many effective molecular engineering strategies, such as side-chain engineering, self-assembly, multi-chromophore dendrimers, site-isolation, chromophore blending, and control of chromophore shape, to increaseρ_N<cos³θ> (increasingρ_N by eliminating the inactive polymer host, while maintaining good poling efficiency)^[46–59]. State-of-the-art POH modulators have already demonstrated ultra-high electro-optical bandwidths (> 500 GHz)^[41], small device footprints < 10μm², energy efficiencies ≤ 70 aJ/bit^[31,42], and voltage-length parameters (V_πL) of ≤ 50 V·μm with existing high-performance OEO materials^[12,60]. To continually shrink theV_πL values, POH and SOH modulators require novel device designs and engineering as well as improved EO activity.

This paper reviews the recent advances in OEO materials and organic-based hybrid EO modulators. To get groundbreaking performance, it is necessary to carry out synergistic innovation from rational design of OEO materials to device engineering and advancements in communication systems. Theory-guided design has been proven to be a very effective and highly efficient strategy to get high-performance chromophores. The combination of experimental and theoretical studies affords better understanding of structure-property relationships, as well as the prospects for implementation in nanoscale photonic devices. The theory-guided advances in OEO materials greatly promote the development of SOH and POH devices. In summary, the design paradigms for both materials and devices have changed dramatically over the past several years, and this review summarizes and discusses the importance of these changes.

Organic-EO materials are generally composed of two parts: the first part is the polymer that provides the substrate, which determines the refractive index and thermal properties of the material, the second part is the active chromophore that performs the EO conversion function^[5].

The influencing factors of the EO performance is as shown in Eq. (1):

r33=2β(ω,ε)ρN〈cos3θ〉g(ω,n),

whereg(ω, n) is the modified Lorentz−Onsager factor taking into account the dielectric properties of the environment surrounding the chromophores.n is the refractive index of the material at the operational wavelength.

To activate the macroscopic EO effect, the organic chromophores need to be poled in the polymer network by heating them close to their glass-transition temperaturesT_g to induce the alignment while applying an external electric field. An active push-pull chromophore contains a p-conjugated bridge which is end-capped with a donor and an acceptor (D-π-A). To optimize the molecular first hyperpolarizabilityβ of the EO chromophores, the electron density distribution needs to be fine-tuned through chemical modification of molecular constituents. The approaches for improving molecularβ generally include using stronger donor/acceptor combination, extending conjugating system, or low-aromatic π-bridge, and so on. With the enhancement of molecularβ, however, the molecular dipole moments rapidly increase, induce the strong anti-parallel aggregation between chromophores and make it very difficult to get acentric order under electric field poling. The chromophores with high-β need to be well-engineered with enough steric hindrance to inhibit unwanted aggregations. There are many structure-engineering strategies that have been proved effective to isolate the chromophores with each other, such as side-chain engineering, dendritic structure, and so on. Our group designed a series of high-performance chromophores with stronger electron-donors that also serve as the bulky groups to isolate with each other, as shown inFig. 1.

Furthermore, 1,1,7,7-tetramethyl-julolidine was proved to provide not only the strong electron-donating ability but also the excellent steric hindrance in our group inFig. 1 (Chromophore representative: WJ1, A and A2)^{[21,22,24,51,52,59]}. WJ1 was synthesized using a facile route to get a high yield (36%)^[52]. The conjugating structure of the chromophore was protected by the tetramethyl groups, thiophene ring and chlorohexyloxy group, of which the steric effect and electron-donating ability enhance the molecular nonlinearity. Due to the greatly weakened dipole-dipole interactions, WJ1 can be doped in amorphous polycarbonate (APC) at a higher concentration (40 wt%) with no terrible dipole-dipole aggregations compared with most of the reported guest-host EO polymers. The bestr₃₃ value of electric-poled WJ1/APC films is 337 pm/V, which is comparable with some outstanding materials. CLD- and FTC-types chromophores were also synthesized using 1,1,7,7-tetramethyl-julolidine as the donor. FTC-type chromophore A2 showed a relatively highr₃₃ value of 223 pm/V in APC films by 25 wt%^[21].

The chromophores using 4-(diarylamino)phenyl as donor have demonstrated significantly enhanced thermo- and photostability compared to their 4-(dialkylamino)phenyl analogues^[61]. Due to the weak electron-donating ability and strongπ–π stacking interactions, however, these chromophores have poor performance in achieving larger₃₃ in poled films. Ther₃₃ values derived from these chromophores are lower than lithium niobate. Dalton’s group and Jen’s group have designed many chromophores with stronger 4-(diarylamino)phenyl donors by introducing alkoxyl groups into donor, or replacing the phenyl-ring by heteroaromatic rings that could increase donor strength and reduce aromaticity and ionization potential^[61,62]. We designed a new electron-donor bis-(4-diarylamino)phenyl group based on bis-(4-alkoxylphenyl)phenyl group by replacing alkoxyl groups with alkylamino groups. The bis-(4-alkylamino)phenyl-amino donor exhibits stronger electron-donating ability than bis-(4-alkoxylphenyl)phenyl-amino donors. InFig. 1, the chromophore T3 showed the bestr₃₃ value of 95 pm/V which was 1.6-fold of T2 performance^[18].

A series of Y-type chromophores using Michlers ethylketone as the donor was first reported by our group, which had strong electron-donating ability and large steric hindrance, as shown inFig. 1^{[54,58,63–65]}. The molecular conformation, including the hindrance of the donor and π-bridge, effectively isolates the chromophores and weakens the dipole–dipole interactions. Density functional theory (DFT) was used to calculate the HOMO–LUMO energy gaps and first-order hyperpolarizability of these chromophores. These chromophores showed better thermal stability with their decomposition temperatures all above 220 °C. Most importantly, the high molecular hyperpolarizability of these chromophores can be effectively translated into large EO coefficients in poled polymers due to the bulky donor structure. The poled films of FTC-yh1 and FTC-yh2 displayedr₃₃ values of 149 and 143 pm/V at the concentration of 25 wt% in amorphous polycarbonate (APC)^[63]. To improve EO activity based on FTC-yh1, two long alkoxy chains were introduced into thiophene-bride to effectively isolate with other chromophores, and using stronger electron acceptor CF₃-Ph-tricyanofuran (CF₃-Ph-TCF) rather than TCF can significantly improve theβ value. The poled films of 25 wt% and 35 wt% chromophore F in APC exhibited 305 and 358 pm/V respectively which were comparable with high-performance CLD-type chromophores^[54].

In recent years, the research focus has been shifted from electrically poled polymer composite materials to high-number-density neat (pure) chromophore materials that yieldr₃₃ larger than 500 pm/V in thin films^[66]. Moreover, theory has led the way to utilization of OEO materials with controlled lattice dimensionality (e.g., two-dimensional or Bessel lattice materials). The dipole-dipole aggregations can be harnessed by well-engineered molecular configurations. Dalton’s group tried to do the poling for the neat JRD1 and YLD124 films that were prepared without EO inactive polymers, as shown inFig. 2^[56,66]. Due to the greatly reduced electric resistance of neat chromophore films in poling, large leakage current was observed that will cause huge poling field loss. To overcome this problem, many kinds of dielectric materials have been used as charge blocking layers (CBLs) by inserting between EO layer and electrode^[66,67]. By using CBLs, the poled JRD1 film exhibited greatly enhancedr₃₃ value of 556 pm/V^[66].

Theoretical predictions suggest that the performance of OEO materials can be considerably greater than 1000 pm/V. Recently, significant progress has been made in theory-guided design for OEO chromophores that permits screening of many structures by evaluating their calculated hyperpolarizabilities, band gaps, and dipole moments, and finding materials optimized for multiple design criteria^[68,69]. Dalton’s group designed and synthesized several new chromophores with hugely improved molecularβ andr₃₃ values. As shown inFig. 2, BAY1 and BAH13 exhibited the maximumr₃₃ values close to 1100 pm/V. Titanium oxide and hafnium oxide were used as the CBLs. The EO performance of BAH13 was also evaluated by POH devices in which BAH13 could greatly reduce theV_πL.

To address the challenge of long-term and high-temperature stability of OEO materials, a crosslinkable EO system consisting of two chromophores, HLD1/HLD2, that can be electric field poled and then thermally crosslinked in situ has been used to form a stable system, as shown inFig. 3(a). A maximum ofr₃₃ of 290 ± 30 pm/V was obtained in a cross-linked film and greater than 99% of the initialr₃₃ value was maintained after heating to 85 °C for 500 h inFig. 3(b)^[27].

Though achieving very larger₃₃ values of about 1000 pm/V in bulk thin film, the EO performance of OEO materials in POH or SOH devices is about 200 or 400 pm/V through the calculation of device’s half-wave voltage. In the future, the research will focus on how to improve the EO performance in SOH, POH devices or other type nano-photonics.

Photonic crystals are a type of artificial functional material that was first proposed in the late 1980s, which are formed by periodic arrangement of dielectric materials with different dielectric constants in space. If a line defect is introduced into a photonic crystal, then a photonic crystal optical waveguide (PCW) can be generated.

In 2008, Brosiet al. proposed a slot-photonic crystal slow-light phase modulator with a length of only 80μm, a bandwidth of 78 GHz and a transmission speed of 100 Gbit/s^[70]. As shown inFig. 4(a), the slow-light phase modulator consists of slotted photonic crystal waveguides (150 nm) filled with nonlinear OEO materials which form the interaction region. Due to the slow-light effect of the slot-photonic crystal waveguide, the velocity of the light field group in the modulation region decreased, the interaction time between the light field and the microwave electric field is significantly increased, and then the length of the modulator is reduced. In 2010, Chen’s group demonstrated a PCOH modulator integrating silicon photonic crystal (PC) slot waveguide (75 nm) and AJ-CKL1/APC electro-optic polymer. An ultra-efficient EO modulation based on a half-wave voltage length productV_πL = 0.56 V·mm and an effective electro-optic coefficientr₃₃ = 132 pm/V is obtained by using slow-light effect and strong field constraints in a slot waveguide^[71]. A new EO polymer-filled silicon slot PC waveguide modulator was reported and the structural diagram is shown inFig. 4(b). The half-wave voltageV_π of the PCOH modulator is 0.97 V, and the half-wave voltage length productV_πL is 0.291 V·mm and then the 3dB bandwidth is raised to 15 GHz in 2016^[72,73].

Silicon photonics offers great potential for its CMOS compatibility with electronics and low-cost optoelectronic integration technologies. In recent years, various silicon-based active and passive optical waveguide devices have developed rapidly, and silicon photonics is a promising platform for highly integrated photonic chips due to its high-index-contrast and the compatibility with the mature complementary. However, silicon has nearly no electro-optic effect, so the traditional silicon modulators mainly rely on free carrier dispersion effect to achieve electro-optic modulation. Therefore, it is difficult for silicon modulators to achieve 100 GHz bandwidth and eliminate the nonlinear response^[17].

Integrating the excellent electro-optic materials into a mature silicon photonics platform is an effective means to overcome bandwidth and loss limits that stem from the physics of the plasma dispersion effect. SOH modulator combines with silicon-on-insulator (SOI) waveguides and the functional organic nonlinear materials with larger₃₃ to realize the large bandwidth and low half-wave voltage ultra-compact modulator. Around 2011, a SOH phase modulator was first demonstrated by the Christian Koos and Juerg Leuthold's group at a data rate of 42.7 Gbit/s with a widely open eye diagrams^[74]. In the SOH device, the silicon slot waveguide by increasing the doping concentration of the silicon strips can realize low resistivity, and the EO bandwidth is increased according to the corresponding RC time constant. As shown inFig. 5(a), the electro-optic polymer M1 consists of chromophores dispersed in APC with the electro-optic coefficient of 70 pm/V, and the slot width of 120 nm and the 1.7 mm long phase shifting section is adopted in the device with aV_π of 5.3 V. The SOH phase modulator was reported by the same group with the frequency response of more than 100 GHz by using a device as short as 500μm, a highly conductive electron accumulation layer and an improved gate insulator inFig. 5(b)^[10]. The results indicate that the RC time constant is not a speed limitation of the device at 100 GHz.

Low energy consumption is a problem that must be considered. In 2015, the power consumption of a 1-mm-long SOH modulator was reduced to 0.7 fJ/bit, and the device was able to be operated at data rates of up to 40 Gbit/s^[31]. In Ref. [34], a SOH modulator with very low half-wave voltage (0.21 V) in a 1.5-mm-long device was reported (Fig. 5(c)). Organic chromophore JRD1 as a cladding, and the maximumn³r₃₃ amounts to 2300 pm/V withr₃₃ of 390 pm/V was achieved in the SOH device. To overcome the RC limitation of SOH modulator, BaTiO₃ with high-κ can avoid the resistive slab and form a large coupling capacitor between the optical slot waveguides. A bandwidth of 76 GHz and aV_π of 1.3 V are achieved in the 1-mm-long SOH capacitively coupled modulator (Fig. 5(d))^[75].

The reported results showed that the poled polymer film can still maintain good electro-optic activity in the cryogenic region of 7 K^[76]. However, the thermal stability and long-term reliability of OEO materials have always been concerned because thermal relaxation of chromophores will happen when its working and storage temperature are higher thanT_g. It illustrates that the thermal stability of the OEO materials seems to have been solved in the following work. As shown in theFig. 5(e), the SOH modulator combined an ultra-highT_gof 172 °C EO polymer with silicon rib waveguides. The modulator showed 200 Gbit/s PAM4 modulations, and feature excellent reliability at ambient temperatures up to 110 °C^[77].

Silicon slot waveguides filled with OEO polymers (r₃₃ = 150 pm/V) in a SOH Mach Zehnder modulator (MZM) were reported in internal research group. According to the simulation results,V_πL of 0.45 V·mm and bandwidth of 70 GHz could be achieved^[78]. The overall structure and the cross-section structure of the modulator are shown inFig. 5(f). The strip-to-slot mode converter has a cone-shaped coupling efficiency of 99.5%, which greatly reduces the loss caused by mode conversion.

Highly compact nanometer optoelectronic integrated chips require smaller device sizes. However, the mode size of traditional dielectric waveguide is more than half of its wavelength length due to the diffraction limit. Surface plasmon photonics are a means of providing ultra-compact and high-speed components. Surface plasmon polariton (SPP) devices have developed rapidly, such as SPPs waveguide, SPPs modulator, SPPs sensor and so on. Plasmonic−organic hybrid modulators have attracted much attention in recent years which combine many advantages of the large RF bandwidth of a travelling wave modulator, the compactness of a photonic-crystal modulator, and the energy efficiency of a ring modulator.

The size variation of the various organic-based hybrid modulators (all-organic, SOH, and POH) is shown inFig. 6(a)^[1]. All-organic thin film OEO devices consist of upper and lower cladding layer and core OEO layer which are all made of polymer. The three layers are sandwiched between top and bottom electrode. To achieve low half-wave voltage, devices of 1–2 cm in length are typically required, because the electrode separations of greater than 7μm (thickness of three-layer films)^[79]. Although all-organic devices achieved significant drive voltage and bandwidth, they still suffered from a large footprint for integration of electronics and photonics. As shown inFig. 6(b), in the SOH modulator, the electrical field drops only across the silicon slot waveguide filled with the OEO materials, and this allows strong overlap of optical and RF electric fields, reducing device length to a few hundred micrometers^[79]. POH modulator uses metal-slot waveguides rather than silicon slot waveguides and exploits stronger confinement of light for surface plasmon polaritons, thereby reducing device lengths to a few tens of micrometers and electrode spacing to less than 50 nm. For the 2 V CMOS drive, the phase shifter lengths for lithium niobate and Si PN junction are 1 cm and 2 mm, while phase shifter lengths for SOH and POH are only 200 and 25μm, respectively. Therefore, the SOH and POH showed the small footprint for the CMOS process-compatible chip-scale integration.V_πL values from 40 to 50 V·μm have been reported (symbols) in JRD1 and DLD164 for POH devices inFig. 6(c), and even lower values based on demonstrated improvements in new materials^[69].

In 2014, J. Leuthold's group first reported a new POH phase modulator, and the structure diagram is shown inFig. 7(a)^[11]. Light enters the plasmon waveguide from the silicon waveguide and propagates in the surface plasmon (SPP) mode. The gap is filled with OEO materials, and the phase modulation is realized by applying voltage to the metal. The device is only 29μm long, operates up to 40 Gbit/s, and has a frequency response of at least 65 GHz. Next, a POH intensity MZM was demonstrated with a bandwidth of 70 GHz in 2015, and the scanning electron microscope image of the MZM component is shown inFig. 7(b). The device has aV_πL of 0.06 V·mm and an energy consumption of 25 fJ/bit, which is attributed to the filling of the slot with a polymer called DLD-164^[12].

In contrast to the previous two works, the all-plasmonic MZM without any silicon waveguide is reported inFig. 7(c)^[38]. All the elements (grating couplers, splitters, polarization rotators and active section) are included in a single metal layer in this POH MZM. The length of phase shifter is 16μm, and the half-wave voltage is 10 V. The frequency response of the device exhibits no speed limitation up to 70 GHz and 116 Gbit/s PAMs modulation formats.

The electron motion generates heat through ohmic losses for plasmonics, which has led to a broad view that plasmonics is too lossy for optical transmission. To reduce the intertion loss of POH modulator, a plasmonic electro-optic ring modulator was reported in theFig. 7(d). The ohmic losses can be bypassed by using “resonant switching”. The intertion loss of the device is only 2.5 dB which can be reduced by more than 6 dB relative to non-resonant MZM device. The bandwidth of the plasmonic resonator is beyond 110 GHz^[40].

Plasmonics is indeed a viable path to an ultracompact, highest-speed, and low-cost technology modulator that can be used in a wide range of fields. The applications of POH modultors have been carried out. Terahertz wireless applications demands modulators with frequency responsed to the sub-terahertz range, high power handing, and very low nonlinear distortions. Beyond 500 GHz, V_π of 3 V, and high linearity 25μm-long POH modulator was experimentally demonstrated, which can achieve a 222 GBd on-off-keying transmitter in theFigs. 8(a) and8(b)^[41,80]. The POH modulator shows good performance, not only in the digital signals but also in the sub-terahertz radio-over-fiber analog optical link. The POH modulator has high linearity with a third-order input intercept point (IIP3) of 18.9 dBm. Meanwhile, an analog RoF link over 100 GHz bandwidth (220–325 GHz) was used to prove the capability of direct terahertz-to-optical-to-terahertz conversion of the POH modulator, and the experiment represents the link has a flat response over the complete 220–325 GHz frequency range. Based on these results, POH modulators have potential applications in the general field of microwave photonics, 5G wireless communications, antenna remoting, internet of things, sensing and more analogy links for the sub-terahertz range.

Heniet al. demonstrated an in-phase/quadrature (IQ) POH modulators, and the active region was 4 × 25µm × 3µm with sub-1-V driving electronics. As shown inFig. 8(c), the devices exhibit low electrical energy consumptions of 0.07 fJ/bit at 50 Gbit/s, 0.3 fJ/bit at 200 Gbit/s, and 2 fJ/bit at 400 Gbit/s with organic chromophore HD-BB-OH/YLD124 filling the slot^[42]. The IQ POH modulator can be used in long-haul and short-haul communications.

Recently, a symbolic rate over 100 GBd single-chip electro-optical transmitter that combines advanced bipolar CMOS with silicon plasmonic was reported. This is a great progress, because it addresses key challenges in single-chip integration through the co-design of electronics and plasmonic layers in theFig. 8(d)^[44]. The HLD1/HLD2 organic material in the plasmonic modulator shows very excellent electro-optic and thermal-stability performance. The POH modulator based on the above organic materials can operate stable data modulation at temperatures above 112 °C.

The calculated capacitance of POH modulator is approximately several fF, so the RC bandwidth can break through the limitation of RC time constant and be much greater than 1 THz. Meanwhile, the Pockels effect is based on the reorganization of π-electrons within a nanometer-long molecule of OEO materials and is estimated to be on the timescale of femtoseconds or attoseconds. The Pockels bandwidth will be much greater than 1 THz. Therefore, the POH modulator has a great space for the ultra-large bandwidth.

The recent developments of OEO materials and organic-based hybrid modulators were reviewed. The outstanding characteristic of OEO materials is their design flexibility which makes OEO materials have high electro-optic activity, fast response speed, high processability, good thermal stability and low cost. Meanwhile, the OEO material is flexible and compatible with many processes (CMOS, silicon photonics) without bonding. The defect for both datacom and telecom system is the lack of performance, specifically as it relates to very high speeds and very low power consumption. Heterogeneous integration based on the organic materials is a very promising solution for high-speed optical modulators and communication system.

Table 1 presents various performance for the high-speed modulators with operating principle of Pockels effect in SOH, POH, PCOH, LNOI, and LN/Si (LN film bonded to SOI platform). The linear Pockels effect can offer intrinsically good linearity. SOH has the lowest half-wave voltage (0.21 V) because of the large EO coefficient (r₃₃ = 390 pm/V) of OEO polymer in the device. The bandwidth of SOH is intermediate between POH and thin film lithium niobite based modulators. POH modulators have the highest bandwidth and the smallest footprint, which make them promising materials for ultra-high speed and low energy consumption integrated system on a chip (SOC). Compared with the LNOI, POH modulators show the large propagation losses which can be solved through waveguide structure and manufacturing process^[40]. PCOH modulators are less studied, perhaps because they need to fabricate complex photonic crystals. The LNOI modulator exhibits ultra-low propagation losses, but may be difficult to achieve low driving voltage for CMOS-level and small size simultaneously owing to the fixedr₃₃.

It is reasonable to find suitable applications for different materials because discrete device applications will likely be based on low optical loss materials such as lithium niobite and silicon. For the application of chip-scale integration of electronics and photonics, the requirement of small size close toμm and CMOS-level driving voltage less than 1 V will likely favor OEO materials. OEO materials are suitable for the satellite and space telecommunications because they may be used to develop small size, lightweight, excellent efficiency and radiation resistance devices^[79]. The range of applications for electro-optics is broad, besides the high-capacity telecommunications and ultra-bandwidth microwave photonics, electro-optics can be used in Lidar, sensor, computing, THz and other areas.

In the future, new OEO materials can be designed and synthesized to optimize the EO performance, refractive index, optical loss and EO thermal-stability, and offer many potential possibilities for high-performance EO modulators. SOH modulator shows lowV_πL and energy consumption, and the aim is to improve the bandwidth and maturity of processing technology. A POH device is a highly efficient modulator to achieve unprecedented performance (very high speed, low power consumption) and compactness, and designing the new waveguide structure and optimizing preparation process should be attracted attention to reduce optical loss. More and more research institutions, universities and companies (e.g., Lightwave Logic) have already studied the organic EO materials devices, and high-performance optical circuits and system could be expected.

We are grateful to the National Natural Science Foundation of China (No. 62175267), the Beijing Municipal Natural Science Foundation (No. 4192061), the Fundamental Research Funds for the Central Universities (2020MDJC13) and the Beijing Talents Foundation (2018000021223ZK45) for the financial support.