Zhengwei Shi

Terahertz All-Optical Modulation in a Silicon-Polymer Hybrid System

Although gigahertz-scale free-carrier modulators have been demonstrated in silicon, intensity modulators operating at terahertz speeds have not been reported because of silicon’s weak ultrafast nonlinearity. We have demonstrated intensity modulation of light with light in a silicon–polymer waveguide device, based on the all-optical Kerr effect—the ultrafast effect used in four-wave mixing. Direct measurements of time-domain intensity modulation are made at speeds of 10 GHz. We showed experimentally that the mechanism of this modulation is ultrafast through spectral measurements, and that intensity modulation at frequencies in excess of 1 THz can be obtained. By integrating optical polymers through evanescent coupling to silicon waveguides, we greatly increase the effective nonlinearity of the waveguide, allowing operation at continuous-wave power levels compatible with telecommunication systems. These devices are a first step in the development of large-scale integrated ultrafast optical logic in silicon, and are two orders of magnitude faster than previously reported silicon devices.

Highly efficient electro-optic polymers through improved poling using a thin TiO2-modified transparent electrode

A sol-gel derived thin titanium dioxide (TiO2) layer was spin-coated onto indium-tin-oxide substrate to improve poling efficiency of recently developed electro-optic (E-O) polymers. The thin TiO2 layer significantly blocks excessive charge injection and reduces the leakage current during high field poling. Ultralarge E-O coefficients, up to 160–350 pm/V at 1310 nm, have been achieved. These results show higher poling efficiency (enhancement of 26%–40%) compared to the results of poled films without the TiO2 layer. This enhancement can be explained by field distribution flattening effect at high injection barrier with the insertion of TiO2 barrier layer.

A Highly Selective Mitochondria-Targeting Fluorescent K+ Sensor

Regulation of intracellular potassium (K(+) ) concentration plays a key role in metabolic processes. So far, only a few intracellular K(+) sensors have been developed. The highly selective fluorescent K(+) sensor KS6 for monitoring K(+) ion dynamics in mitochondria was produced by coupling triphenylphosphonium, borondipyrromethene (BODIPY), and triazacryptand (TAC). KS6 shows a good response to K(+) in the range 30-500 mM, a large dynamic range (Fmax /F0 ≈130), high brightness (ϕf =14.4 % at 150 mM of K(+) ), and insensitivity to both pH in the range 5.5-9.0 and other metal ions under physiological conditions. Colocalization tests of KS6 with MitoTracker Green confirmed its predominant localization in the mitochondria of HeLa and U87MG cells. K(+) efflux/influx in the mitochondria was observed upon stimulation with ionophores, nigericin, or ionomycin. KS6 is thus a highly selective semiquantitative K(+) sensor suitable for the study of mitochondrial potassium flux in live cells.

Facile structure and property tuning through alteration of ring structures in conformationally locked phenyltetraene nonlinear optical chromophores

A series of phenyltetraene-based nonlinear optical (NLO) chromophores 1a–c with the same donor and acceptor groups, but different tetraene bridges that are partly connected by various sizes of aliphatic rings, have been synthesized and systematically investigated. The interposed conjugated tetraene segments in three chromophores studied are based on isophorone, (1S)-(−)-verbenone, and 3,4,4-trimethyl-2-cyclopentenone, respectively. This kind of structural alteration has significant effect on the intrinsic electronic structures and physical properties of these highly polarizable chromophores as revealed by a variety of characterization techniques. The introduction of the verbenone- and trimethylcyclopentenone-based tetraene bridges could significantly improve the glass-forming ability of chromophores 1b and 1c in comparison with the highly crystalline characteristics of isophorone-based chromophores 1a. More importantly, chromophores 1a–c exhibited distinct optical features in absorption band shape, solvatochromic behavior, as well as energy band gap from the UV-vis-NIR absorption measurements. Quantum mechanical calculations using density functional theory (DFT) were also used to evaluate second-order NLO properties of these chromophores. The electro-optic (EO) coefficients of 1a–c in poled polymers with the 10 wt% chromophore content showed an apparent decrease from 78 pm V−1 for 1a to 42 pm V−1 for 1c. This decrease is attributed to the gradual decrease of the molecular hyperpolarizability (β) of the chromophores which is associated with the progressive cyanine-like electronic structure from the isophorone-based 1a to the cyclopentenone-based 1cchromophore.

All-Dielectric Electrooptic Sensor Based on a Polymer Microresonator Coupled Side-Polished Optical Fiber

A novel electrooptic (EO) electric field (E-field) sensor based on side-polished fiber coupled with an EO polymer microring resonator is proposed and demonstrated. An EO polymer waveguide with a ring shape is fabricated on the polished flat of an optical fiber. Light in the fiber evanescently couples into the resonator and forms resonant modes for certain wavelengths and produces notches in the output intensity of the fiber. External electric fields change the index of refraction of the ring waveguide and therefore dither its resonant wavelengths. For light of wavelength on the slope of a resonance notch, a change in the output intensity can be detected. The sensor is all dielectric without metal layers to distort the measured E-field. The resonant structure allows the sensor to potentially have much higher sensitivity than other electrooptic sensors based on Mach-Zehnder or polarization modulation. Since electrooptic polymers have higher electrooptic coefficients, lower dielectric constants and faster electrooptic responses than inorganic crystals, higher sensitivity, lower invasiveness, and higher bandwidth of E-field sensing can be expected. This sensor eliminates unreliable fiber-to-waveguide butt coupling as well as the high propagation loss encountered in the long straight EO polymer waveguides of sensors based on Mach-Zehnder structures. By using the fiber itself as the supporting substrate of the ring waveguide, the sensor can have small size and low disturbance to the measured electric field. The concept is demonstrated using AJLS103 EO polymer. A sensitivity of 100 mV/m has been achieved at frequencies up to 550 MHz (limited by the measurement system)

Achieving excellent electro-optic activity and thermal stability in poled polymers through an expeditious crosslinking process

A series of highly efficient and thermally stable electro-optic (EO) polymers have been developed by poling and crosslinking in situ the blend of high glass-transition temperature (Tg) anthracene-containing polymers and acrylate-functionalized dendritic nonlinear optical (NLO) chromophores. By molecular engineering of the shape, nonlinearity, Tg, and crosslinking moieties of the chromophores and polymers, the resultant materials showed significantly enhanced EO activities (r33 values as high as 126 pm V−1 at 1310 nm) and alignment stability (up to 200 °C). Poling efficiency of these EO polymers could be improved by 35–50% by using simplified lattice hardening and poling protocols. The combined good processability, large EO activities, and high temperature stability endow these materials as promising candidates for device exploration in the CMOS-based photonics.

Molecular Mobility in Self-Assembled Dendritic Chromophore Glasses

Increasing complexity in bottom-up molecular designs of amorphous structures with multiple relaxation modes demands an integrated and cognitive design approach, where chemical synthesis is guided by both analytical tools and theoretical simulations. In particular, this is apparent for novel organic second-order nonlinear optical materials of self-assembling molecular glasses involving dendritic arene stabilization moieties (phenyl, naphthyl, and anthryl) with electro-optical activities above 300 pm/V. In this study, nanoscale thermo-mechanical analyses yield direct insight into the molecular enthalpic and entropic relaxation modes. Arene-perfluoroarene interactions for coarse self-assembly are found to impose three phase relaxation regimes, with intermediate regimes of 8-15 degrees C in width and apparent activation energies between 40 and 60 kcal/mol to be the most effective for poling. Energetic analyses based on intrinsic friction microscopy (IFA) identify increasing temporal stability with increasing arene size for the low-temperature regime. Electric field poling efficiency is found to be inversely proportional to entropic cooperative contributions that can make up 80% of the overall apparent relaxation energy for the high-temperature regime. The origin for the activation energies below the incipient glass transition temperature, based on complementary molecular dynamic simulations, is tied primarily to noncovalent interactions between chromophore (dipole), dendritic (quadrupole) moieties, and combinations thereof.

Enhanced temporal stability of a highly efficient guest–host electro-optic polymer through a barrier layer assisted poling process

Significantly enhanced temporal stability of poled electro-optic (E-O) polymers could be achieved by inserting a thin sol–gel derived titanium dioxide (TiO2) barrier layer in the high electric field poling process. The resulting poled film can retain >90% of its original r33 value (169 pm V−1 at 1310 nm) after being annealed at 85 °C for 500 h. This is significantly higher (∼30%) compared to that obtained without the TiO2 layer. This barrier approach is also applicable to a variety of dielectric polymers although the degree of enhancement varies. The enhanced temporal stability of E-O polymers is attributed to reduced charge injection/accumulation that improves the stability of screening charges for poled films.

Spontaneous thermal crosslinking of a sydnone-containing side-chain polymer with maleimides through a convergent [3 + 2] dual cycloaddition/cycloreversion process for electro-optics

A convergent [3 + 2] dual cycloaddition/cycloreversion process between sydnone and multi-functional maleimides is reported as an efficient protocol for thermal crosslinking of polymers. To alleviate the commonly observed pre-crosslinking problem, the reactivity of 1,3-dipolar sydnone moieties is tuned by increasing its steric effect through a sydnone-containing side-chain polymer (SCP) that is synthesized using a Mitsunobu post-functionalization method. Thin films and amorphous solid composites derived from SCP and a passive bis-maleimide cross-linker (BMI-1) are used to conduct a model study of the crosslinking process. The results from spectroscopic and thermal analyses show that the SCP/BMI-1 composites react efficiently at a modest curing temperature of ∼100 °C and can be converted into a highly cross-linked polymer network with excellent thermal stability. The spontaneity and high conversion efficiency of thermal cross-linking for SCP/BMI-1 suggest that an efficient three-step cascade reaction with minimal side reactions can be achieved in the solid-state. This lattice hardening process was implemented in a new cross-linkable electro-optic (EO) polymer SCP/BMI-2/TMI possessing a dipolar polyene chromophore crosslinker BMI-2. Through poling and in situ cross-linking, the poled films of SCP/BMI-2/TMI exhibit both large EO coefficient (r33 of 117 pm V−1 at 1.31 μm) and excellent long-term alignment stability at 85 °C.

Broadband electric field sensor with electro-optic polymer micro-ring resonator on side-polished optical fiber

A novel broadband E-field sensor based on electro-optic polymer micro-ring resonator directly coupled to the core of optical fiber is proposed and demonstrated. A flat is made on the side of the optical fiber by polishing and an electro-optic polymer waveguide in the shape of a ring is placed on the polished flat. One side of the ring is directly above the core of the fiber and light is evanescently coupled between the fiber and the micro-ring. External electric fields change the index of refraction of the ring resonator and therefore its resonant wavelengths. The sensor is all dielectric without metal layers to distort the measured E-field. The resonance structure allows the sensor to potentially have much higher sensitivity than other electro-optic sensors based on interferometry or polarization modulation. Since electro-optic polymers have higher electro-optic coefficients, lower dielectric constants and faster electro-optic responses than inorganic crystals, higher sensitivity, lower invasiveness and higher bandwidth of E-field sensing can be expected. The sensor with EO polymer micro-ring directly coupled to side-polished fiber eliminates unreliable and possibly lossy fiber to waveguide butt coupling as well as the high propagation loss which comes from the long straight EO polymer waveguides. Unlike devices based on waveguide technology, a supporting substrate is not necessary in this device. This leads to sensors of small size and low disturbance to the measured electric field. In the proof-of-concept experiment, a sensitivity of 100 mV/m has been achieved at frequencies up to 550 MHz (limited by the measurement system) using AJLS103 electro-optic polymer.