Compact and ultra-efficient broadband plasmonic terahertz field detector

3D Intelligent Cloaked Vehicle Equipped with Thousand-Level Reconfigurable Full-Polarization Metasurfaces

A crucial aspect in shielding a variety of advanced electronic devices from electromagnetic detection involves controlling the flow of electromagnetic waves, akin to invisibility cloaks. Decades ago, the exploration of transformation optics heralded the dawn of modern invisibility cloaks, which has stimulated immense interest across various physical scenarios. However, most prior research is simplified to low-dimensional and stationary hidden objects, limiting their practical applicability in a dynamically changing world. This study develops a 3D large-scale intelligent cloak capable of remaining undetectable even in non-stationary conditions. By employing thousand-level reconfigurable full-polarization metasurfaces, this work has achieved an exceptionally high degree of freedom in sculpting the scattering waves as desired. Serving as the core computational unit, a hybrid inverse design enables the cloaked vehicle to respond in real-time, with a rapid reaction time of just 70 ms. These experiments integrate the cloaked vehicle with a perception-decision-control-execution system and evaluate its performance under random static positions and dynamic travelling trajectories, achieving a background scattering matching degree of up to 93.3%. These findings establish a general paradigm for the next generation of intelligent meta-devices in real-world settings, potentially paving the way for an era of "Electromagnetic Internet of Things."

Fine-tuning of organic optical double-donor NLO chromophores with DA-supported functional groups

New strategic chromophores with updated fine-tuning of previously reported BLD1 and BLD3 chromophores were designed. BLD1 and BLD3 have silicon functional groups on the donor unit, and the bridge has a good chance of self-assembling, so in the present study we fine-tuned the isolating groups to the bulky cyclic alkene to improve their dipole moment and organic electro-optic (OEO) properties as well. To demonstrate the impact of cyclic alkenes on the electron-donating groups in sensible NLO chromophore designs, a thorough analysis and comparison of the chromophore synthesis, UV-Vis calculations, solvatochromic behavior of the chromophore, DFT quantum mechanical calculations, thermal stabilities, and much lower dipole moments was conducted.

All-optical free-space routing of upconverted light by metasurfaces via nonlinear interferometry

All-optical modulation yields the promise of high-speed information processing. In this field, metasurfaces are rapidly gaining traction as ultrathin multifunctional platforms for light management. Among the featured functionalities, they enable light-wavefront manipulation and more recently demonstrated the ability to perform light-by-light manipulation through nonlinear optical processes. Here, by employing a nonlinear periodic metasurface, we demonstrate the all-optical routing of telecom photons upconverted to the visible range. This is achieved via the interference between two frequency-degenerate upconversion processes, namely, third-harmonic and sum-frequency generation, stemming from the interaction of a pump pulse with its frequency-doubled replica. By tuning the relative phase and polarization between these two pump beams, we route the upconverted signal among the diffraction orders of the metasurface with a modulation efficiency of up to 90%. This can be achieved by concurrently engineering the nonlinear emission of the individual elements (meta-atoms) of the metasurface along with its pitch. Owing to the phase control and ultrafast dynamics of the underlying nonlinear processes, free-space all-optical routing could be potentially performed at rates close to the employed optical frequencies divided by the quality factor of the optical resonances at play. Our approach adds a further twist to optical interferometry, which is a key enabling technique employed in a wide range of applications, such as homodyne detection, radar interferometry, light detection and ranging technology, gravitational-wave detection and molecular photometry. In particular, the nonlinear character of light upconversion combined with phase sensitivity is extremely appealing for enhanced imaging and biosensing.

Thin-film lithium niobate electro-optic terahertz wave detector

The design, fabrication, and validation of a thin-film lithium niobate on insulator (LNOI) electro-optic (EO) time-domain terahertz (THz) wave detector is reported. LNOI offers unprecedented properties for the EO detection of freely propagating THz wave radiation pulses and transient electric fields because of the large EO coefficient of the material, engineering of the velocity matching of the THz wave and optical wave, and much reduced detector size. The proof-of-concept device is realized using thin-film lithium niobate optical waveguides forming a Mach-Zehnder interferometer with interferometer arms electrically poled in opposite directions. THz waves are coupled effectively to the fully dielectric device from free space without using antennas or plasmonics. The detection of THz waves with frequencies up to 800 GHz is successfully demonstrated. The detector allows for the detection of THz frequency electric fields up to 4.6 MV/m. The observed frequency response of the device agrees well with theoretical predictions.

Highly Efficient and Stable Binary Cross-Linkable/ Self-Assembled Organic Nonlinear Optical Molecular Glasses

The development of electro-optical materials with high chromophore loading levels that possess ultrahigh electro-optic coefficients and high long term alignment stability is a challenging topic. Anthracene-maleimide Diels-Alder (DA) reaction and π-π interaction of Anthracene-pentafluorobenzene and benzene-pentafluorobenzene are developed for making highly efficient binary cross-linkable/self-assembled dendritic chromophores FZL1-FZL4. A covalently or non-covalently cross-linked network is formed by DA reaction or π-π interaction after electric field poling orientation, which greatly improves the long-term alignment stability of the materials. An electro-optic coefficient up to 266 pm V-1 and glass transition temperature as high as 178 °C are achieved in cross-linked film FZL1/FZL2, and 272-308 pm V-1 is achieved for self-assembled films FZL1/FZL4 and FZL3/FZL4 due to high chromophore density (3.09-4.02 × 1020 molecules cm-3 ). Long-term alignment stability tests show that after heating at 85 °C for over 500 h, 99.73% of the initial r33 value is maintained for poled crosslinked electro-optic films 1:1 FZL1/FZL2. The poled self-assembled electro-optic films 1:1 FZL1/FZL4 and 1:1 FZL3/FZL4 can still maintain more than 97.11% and 98.23%, respectively, of the original electro-optic coefficient after being placed at room temperature for 500 h. The excellent electro-optic coefficient and stability of the material indicate the practical application prospects of organic electro-optic materials.

Highly Sensitive Bilirubin Biosensor Based on Photonic Crystal Fiber in Terahertz Region

An unstable bilirubin level in the human blood causes many dangerous health problems, such as jaundice, coronary artery disease, ulcerative colitis, and brain lesions. Therefore, the accurate and early detection of bilirubin concentrations in the blood is mandatory. In this work, a highly sensitive biosensor based on photonic crystal fiber (PCF) for monitoring bilirubin levels is proposed and analyzed. The sensor parameters, including relative sensitivity, effective mode area, confinement loss, and effective material loss, are calculated. The geometrical parameters are studied, and a modal analysis of the suggested sensor is carried out using the full-vectorial finite element method (FEM). The fabrication tolerance of the geometrical parameters is also studied to ensure the fabrication feasibility of the reported design. High sensitivities of 95% and 98% are obtained for the x-polarized and y-polarized modes, respectively. Furthermore, a small material loss of 0.00193 cm−1, a small confinement loss of 2.03 × 10−14 dB/cm, and a large effective mode area of 0.046 mm2 are achieved for the y-polarized mode. It is believed that the presented sensor will be helpful in health care and in the early detection of bilirubin levels in the blood.

Terahertz waveform synthesis in integrated thin-film lithium niobate platform

Bridging the "terahertz gap" relies upon synthesizing arbitrary waveforms in the terahertz domain enabling applications that require both narrow band sources for sensing and few-cycle drives for classical and quantum objects. However, realization of custom-tailored waveforms needed for these applications is currently hindered due to limited flexibility for optical rectification of femtosecond pulses in bulk crystals. Here, we experimentally demonstrate that thin-film lithium niobate circuits provide a versatile solution for such waveform synthesis by combining the merits of complex integrated architectures, low-loss distribution of pump pulses on-chip, and an efficient optical rectification. Our distributed pulse phase-matching scheme grants shaping the temporal, spectral, phase, amplitude, and farfield characteristics of the emitted terahertz field through designer on-chip components. This strictly circumvents prior limitations caused by the phase-delay mismatch in conventional systems and relaxes the requirement for cumbersome spectral pre-engineering of the pumping light. We propose a toolbox of basic blocks that produce broadband emission up to 680 GHz and far-field amplitudes of a few V m^-1 with adaptable phase and coherence properties by using near-infrared pump pulse energies below 100 pJ.

Systematic Study on Nonlinear Optical Chromophores with Improved Electro-Optic Activity by Introducing 3,5-Bis(trifluoromethyl)benzene Derivative Isolation Groups into the Bridge

A series of novel chromophores A, B, C, and D, based on the julolidinyl donor and the tricyanofuran (TCF) and CF₃-tricyanofuran (CF₃-Ph-TCF) acceptors, have been synthesized and systematically investigated. The 3,5-bis(trifluoromethyl)benzene derivative isolation group was introduced into the bridge in the chromophores C and D. These nonlinear optical chromophores showed good thermal stability, and their decomposition temperatures were all above 220 °C. Density functional theory (DFT) was used to calculate the energy gaps and first-order hyperpolarizability (β). The macroscopic electro-optic (EO) activity was measured using a simple reflection method. The highest EO coefficient of poled films containing 35 wt% of chromophore D doped in amorphous polycarbonate afforded values of 54 pm/V at 1310 nm. The results indicate that the 3,5-bis(trifluoromethyl)benzene isolation group can suppress the dipole-dipole interaction of chromophores. The moderate r33 value, good thermal stability, and good yield of chromophores suggest their potential use in the nonlinear optical area.

Electro-optic crosslinkable chromophores with ultrahigh electro-optic coefficients and long-term stability

The development of organic electro-optic materials with ultrahigh electro-optic coefficients and high long-term alignment stability is the most challenging topic in this field. Next-generation crosslinkable nonlinear optical chromophore molecular glasses were developed to address this problem. A highly stable EO system including crosslinkable binary chromophores QLD1 and QLD2 or crosslinkable single chromophore QLD3 and multichromophore QLD4 with large hyperpolarizability was synthesized using tetrahydroquinoline as the donor. When the temperature continues to rise after poling, the chromophores modified with anthracene and acrylate can undergo Diels-Alder crosslinking reaction to fix the oriented chromophores through chemical bonds. After crosslinking, the QLD1/QLD2 and QLD2/QLD4 films achieved very high maximum r ₃₃ values of 327 and 373 pm V^-1, respectively, which are the highest values reported for crosslinkable chromophore systems. After Diels-Alder cycloaddition, the glass transition temperature of the EO film increased by ∼90 °C to 185 °C, which is higher than for any other pure chromophore films. After being annealed at 85 °C, 99.63% of the initial r ₃₃ value could be maintained for over 500 h. The ultrahigh electro-optic activity and high long-term alignment stability of these materials showed new breakthroughs in organic EO materials for practical device explorations.

Monolithic Metamaterial-Integrated Graphene Terahertz Photodetector with Wavelength and Polarization Selectivity

The frequency spectra and polarization states of terahertz waves can convey significant information about physical interactions and material properties. Compact and miniaturized on-chip platforms for effective capturing of these quantities are being extensively investigated because of their promising potential for paramount applications of terahertz technology such as in situ sensing and characterization. Here, we present a metamaterial-graphene hybrid device that integrates the functions of photodetection, wavelength, and polarization selectivity into a monolithic architecture. Leveraging the ultrahigh design freedom of metamaterial optical properties and the electronically controllable hot-carrier-assisted photothermoelectric effect in graphene, our detector shows resonantly enhanced photoresponse at two specific target wavelengths with orthogonal polarizations. We demonstrate its versatile capabilities for spectrally selective and polarization resolved imaging on a single-chip platform that is free from advanced optical components. Our strategy is beneficial to the future development of multifunctional, compact, and low-cost terahertz sensors.

Enhanced room-temperature terahertz detection and imaging derived from anti-reflection 2D perovskite layer on MAPbI3 single crystals

Terahertz (THz) detection technology is getting increasing attention from scientists and industries alike due to its superiority in imaging, communication, and defense. Unfortunately, the detection of THz electromagnetic waves under room temperature requires a complicated device architecture design or additional cryogenic cooling units, which increase the cost and complexity of devices, subsequently imposing an impediment in its universal application. In this work, THz detectors operated under room temperature are designed based on the thermoelectric effect with MAPbI₃ single crystals (SCs) as active layers. With solution-processed molecular growth engineering, the anti-reflection 2D perovskite layers were constructed on SCs' surfaces to suppress THz reflection loss. Simultaneously, by finely regulating the main carrier types and the direction of the applied bias across the inclined energy level, the thermoelectric effect is further promoted. As a result, THz-induced ΔT in MAPbI₃ SCs reaches 4.6 °C, while the enhancement in the bolometric and photothermoelectric effects reach ∼4.8 times and ∼16.9 times, respectively. Finally, the devices achieve responsivity of 88.8 μA W^-1 at 0.1 THz under 60 V cm^-1, noise equivalent power (NEP) less than 2.16 × 10^-9 W Hz^-1/2, and specific detectivity (D*) of 1.5 × 10⁸ Jones, which even surpasses the performance of state-of-the-art graphene-based room-temperature THz thermoelectric devices. More importantly, proof-of-concept imaging gives direct evidence of perovskite-based THz sensing in practical applications.

Synthesis of Bis(N,N-diethyl)aniline-Based, Nonlinear, Optical Chromophores with Increased Electro-Optic Activity by Optimizing the Thiolated Isophorone Bridge

Six nonlinear, optical chromophores, Z1–Z6, based on the bis(N,N-diethyl)aniline-derived donor and thiolated isophorone bridge, were designed and synthesized. The bis(N,N-diethyl)aniline-derived donor was applied in a chromophore with thiolated isophorone as an electron bridge for the first time. In particular, the bridge parts of chromophores Z2–Z6 were modified with different functional groups, including tert-butyltrimethylsilane and tert-butyl(methyl)diphenylsilane derivative: 1,3-bis(trifluoromethyl)benzene and alkylaniline cyanoacetate, respectively. Density functional theory calculations suggested this series of chromophores show much greater hyperpolarizability than traditional, nonlinear, optical chromophores due to strong electron donor ability. These chromophores, Z1–Z6, showed very high poling efficiencies due to the large steric hindrance and hyperpolarizability of the chromophores. A large poling efficiency (2.04 ± 0.08 nm2/V2) and r33 value (193 pm/V) were achieved for polymeric thin films doped with 25 wt% chromophore Z6 at 1310 nm.

Reconfigurable terahertz metamaterials: From fundamental principles to advanced 6G applications

Terahertz (THz) electromagnetic spectrum ranging from 0.1THz to 10THz has become critical for sixth generation (6G) applications, such as high-speed communication, fingerprint chemical sensing, non-destructive biosensing, and bioimaging. However, the limited response of naturally existing materials THz waves has induced a gap in the electromagnetic spectrum, where a lack of THz functional devices using natural materials has occurred in this gap. Metamaterials, artificially composed structures that can engineer the electromagnetic properties to manipulate the waves, have enabled the development of many THz devices, known as "metadevices". Besides, the tunability of THz metadevices can be achieved by tunable structures using microelectromechanical system (MEMS) technologies, as well as tunable materials including phase change materials (PCMs), electro-optical materials (EOMs), and thermo-optical materials (TOMs). Leveraging various tuning mechanisms together with metamaterials, tremendous research works have demonstrated reconfigurable functional THz devices, playing an important role to fill the THz gap toward the 6G applications. This review introduces reconfigurable metadevices from fundamental principles of metamaterial resonant system to the design mechanisms of functional THz metamaterial devices and their related applications. Moreover, we provide perspectives on the future development of THz photonic devices for state-of-the-art applications.

Design and synthesis of chromophores with enhanced electro-optic activities in both bulk and plasmonic-organic hybrid devices

This study demonstrates enhancement of in-device electro-optic activity via a series of theory-inspired organic electro-optic (OEO) chromophores based on strong (diarylamino)phenyl electron donating moieties. These chromophores are tuned to minimize trade-offs between molecular hyperpolarizability and optical loss. Hyper-Rayleigh scattering (HRS) measurements demonstrate that these chromophores, herein described as BAH, show >2-fold improvement in β versus standard chromophores such as JRD1, and approach that of the recent BTP and BAY chromophore families. Electric field poled bulk devices of neat and binary BAH chromophores exhibited significantly enhanced EO coefficients (r₃₃) and poling efficiencies (r₃₃/E_p) compared with state-of-the-art chromophores such as JRD1. The neat BAH13 devices with charge blocking layers produced very large poling efficiencies of 11.6 ± 0.7 nm² V^-2 and maximum r₃₃ value of 1100 ± 100 pm V^-1 at 1310 nm on hafnium dioxide (HfO₂). These results were comparable to that of our recently reported BAY1 but with much lower loss (extinction coefficient, k), and greatly exceeding that of other previously reported OEO compounds. 3 : 1 BAH-FD : BAH13 blends showed a poling efficiency of 6.7 ± 0.3 nm² V^-2 and an even greater reduction in k. 1 : 1 BAH-BB : BAH13 showed a higher poling efficiency of 8.4 ± 0.3 nm² V^-2, which is approximately a 2.5-fold enhancement in poling efficiency vs. JRD1. Neat BAH13 was evaluated in plasmonic-organic hybrid (POH) Mach-Zehnder modulators with a phase shifter length of 10 μm and slot widths of 80 and 105 nm. In-device BAH13 achieved a maximum r₃₃ of 208 pm V^-1 at 1550 nm, which is ∼1.7 times higher than JRD1 under equivalent conditions.

Continuous-wave frequency upconversion with a molecular optomechanical nanocavity

[Figure: see text].

Electro-optic spatial light modulator from an engineered organic layer

Tailored nanostructures provide at-will control over the properties of light, with applications in imaging and spectroscopy. Active photonics can further open new avenues in remote monitoring, virtual or augmented reality and time-resolved sensing. Nanomaterials with χ⁽²⁾ nonlinearities achieve highest switching speeds. Current demonstrations typically require a trade-off: they either rely on traditional χ⁽²⁾ materials, which have low non-linearities, or on application-specific quantum well heterostructures that exhibit a high χ⁽²⁾ in a narrow band. Here, we show that a thin film of organic electro-optic molecules JRD1 in polymethylmethacrylate combines desired merits for active free-space optics: broadband record-high nonlinearity (10-100 times higher than traditional materials at wavelengths 1100-1600 nm), a custom-tailored nonlinear tensor at the nanoscale, and engineered optical and electronic responses. We demonstrate a tuning of optical resonances by Δλ = 11 nm at DC voltages and a modulation of the transmitted intensity up to 40%, at speeds up to 50 MHz. We realize 2 × 2 single- and 1 × 5 multi-color spatial light modulators. We demonstrate their potential for imaging and remote sensing. The compatibility with compact laser diodes, the achieved millimeter size and the low power consumption are further key features for laser ranging or reconfigurable optics.

Highly efficient terahertz photoconductive metasurface detectors operating at microwatt-level gate powers

Despite their wide use in terahertz (THz) research and technology, the application spectra of photoconductive antenna (PCA) THz detectors are severely limited due to the relatively high optical gating power requirement. This originates from poor conversion efficiency of optical gate beam photons to photocurrent in materials with sub-picosecond carrier lifetimes. Here we show that using an ultra-thin (160 nm), perfectly absorbing low-temperature grown GaAs metasurface as the photoconductive channel drastically improves the efficiency of THz PCA detectors. This is achieved through perfect absorption of the gate beam in a significantly reduced photoconductive volume, enabled by the metasurface. This Letter demonstrates that sensitive THz PCA detection is possible using optical gate powers as low as 5 µW-three orders of magnitude lower than gating powers used for conventional PCA detectors. We show that significantly higher optical gate powers are not necessary for optimal operation, as they do not improve the sensitivity to the THz field. This class of efficient PCA THz detectors opens doors for THz applications with low gate power requirements.

A Review on Terahertz Technologies Accelerated by Silicon Photonics

In the last couple of decades, terahertz (THz) technologies, which lie in the frequency gap between the infrared and microwaves, have been greatly enhanced and investigated due to possible opportunities in a plethora of THz applications, such as imaging, security, and wireless communications. Photonics has led the way to the generation, modulation, and detection of THz waves such as the photomixing technique. In tandem with these investigations, researchers have been exploring ways to use silicon photonics technologies for THz applications to leverage the cost-effective large-scale fabrication and integration opportunities that it would enable. Although silicon photonics has enabled the implementation of a large number of optical components for practical use, for THz integrated systems, we still face several challenges associated with high-quality hybrid silicon lasers, conversion efficiency, device integration, and fabrication. This paper provides an overview of recent progress in THz technologies based on silicon photonics or hybrid silicon photonics, including THz generation, detection, phase modulation, intensity modulation, and passive components. As silicon-based electronic and photonic circuits are further approaching THz frequencies, one single chip with electronics, photonics, and THz functions seems inevitable, resulting in the ultimate dream of a THz electronic-photonic integrated circuit.

Theoretical Analysis of Terahertz Dielectric-Loaded Graphene Waveguide

The waveguiding of terahertz surface plasmons by a GaAs strip-loaded graphene waveguide is investigated based on the effective-index method and the finite element method. Modal properties of the effective mode index, modal loss, and cut-off characteristics of higher order modes are investigated. By modulating the Fermi level, the modal properties of the fundamental mode could be adjusted. The accuracy of the effective-index method is verified by a comparison between the analytical results and numerical simulations. Besides the modal properties, the crosstalk between the adjacent waveguides, which determines the device integration density, is studied. The findings show that the effective-index method is highly valid for analyzing dielectric-loaded graphene plasmon waveguides in the terahertz region and may have potential applications in subwavelength tunable integrated photonic devices.

Helicity-Sensitive Plasmonic Terahertz Interferometer

Plasmonic interferometry is a rapidly growing area of research with a huge potential for applications in the terahertz frequency range. In this Letter, we explore a plasmonic interferometer based on graphene field effect transistor connected to specially designed antennas. As a key result, we observe helicity- and phase-sensitive conversion of circularly polarized radiation into dc photovoltage caused by the plasmon-interference mechanism: two plasma waves, excited at the source and drain part of the transistor, interfere inside the channel. The helicity-sensitive phase shift between these waves is achieved by using an asymmetric antenna configuration. The dc signal changes sign with inversion of the helicity. A suggested plasmonic interferometer is capable of measuring the phase difference between two arbitrary phase-shifted optical signals. The observed effect opens a wide avenue for phase-sensitive probing of plasma wave excitations in two-dimensional materials.

Plasmonics for Telecommunications Applications

Plasmonic materials, when properly illuminated with visible or near-infrared wavelengths, exhibit unique and interesting features that can be exploited for tailoring and tuning the light radiation and propagation properties at nanoscale dimensions. A variety of plasmonic heterostructures have been demonstrated for optical-signal filtering, transmission, detection, transportation, and modulation. In this review, state-of-the-art plasmonic structures used for telecommunications applications are summarized. In doing so, we discuss their distinctive roles on multiple approaches including beam steering, guiding, filtering, modulation, switching, and detection, which are all of prime importance for the development of the sixth generation (6G) cellular networks.