Electro-optic Signal Research Articles

Surface Relief Diffraction Gratings for Modulation Enhancement in PLZT Thin Films

Azobenzene polymer surface relief diffraction gratings were inscribed on relaxor ferroelectric Lead Lanthanum Zirconate Titanate (PLZT) thin films having a (9/65/35) composition. Resonance structures were observed in the reflected intensity spectra of a laser light beam onto these films, indicating coupling of the incident light. An ac electric field was then applied along the thickness of the PLZT films and an electro-optic modulation signal was measured as a function of the incidence angle of the laser beam. It was found that the resonance structures increased the modulation signal of the PLZT thin films at constant field amplitude.

Non-invasive single bunch monitoring for ps pulse radiolysis

A single-shot electro-optic (EO) diagnostic has been installed on the ELYSE photocathode RF gun accelerator to monitor the electron bunch at the place and under the conditions of the ps pulse radiolysis experiments. The EO signal is due to the coulombic field of the electron bunch and to a contribution of a free-space THz radiation generated by the same electron pulse. This signal is recorded shot-to-shot at the repetition rate of the accelerator. The jitter of the arrival time of the electron bunch is characterized for the first time with a non-invasive method and is confirmed to be around 1 ps.

Numerical studies on the electro-optic detection of femtosecond electron bunches

The electro-optic (EO) effect is a powerful diagnostic tool for determining the time profile of ultrashort relativistic electron bunches. When a relativistic bunch passes within a few mm of an electro-optic crystal, its transient electric field is equivalent to a half-cycle THz pulse passing through the crystal. The induced birefringence can be detected with polarized femtosecond laser pulses. A simulation code has been written in order to understand the faithfulness and the limitations of electron bunch shape reconstruction by EO sampling. The THz pulse and the laser pulse are propagated as wave packets through the EO crystal. Alternatively, the response function method is applied. Using experimental data on the material properties of zinc telluride (ZnTe) and gallium phosphide (GaP), the effects of velocity mismatch, pulse shape distortion, and signal broadening are explicitly taken into account. The simulations show that the most severe limitation on the time resolution is given by the transverse-optical (TO) lattice oscillation in the EO crystal. The lowest TO frequency is 5.3 THz in ZnTe and 11 THz in GaP. Only the Fourier components below the TO resonance are usable for the bunch shape reconstruction. This implies that the shortest rms bunch length which can be resolved with moderate distortion amounts to $\ensuremath{\sigma}\ensuremath{\approx}90\text{ }\text{ }\mathrm{fs}$ in ZnTe and $\ensuremath{\sigma}\ensuremath{\approx}50\text{ }\text{ }\mathrm{fs}$ in GaP. The influence of the crystal thickness on the amplitude and width of the EO signal is studied. The optimum thickness is in the range from 100 to $300\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ for ZnTe and from 50 to $100\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ for GaP.

Low-Noise and High-Frequency Resolution Electrooptic Sensing of RF Near-Fields Using an External Optical Modulator

We developed an electrooptic (EO) sensing system using an external optical intensity modulator for radio frequency (RF) electric near-field measurements. This low-noise, high-frequency resolution system is based on the photonic heterodyning technique in which a RF near-field signal is mixed with modulated light by an EO crystal and downconverted to the difference frequency. Because the optical modulator has a bandwidth extending from DC to several gigahertz, the frequency resolution of this system is determined by that of the RF synthesizer it uses. The intensity noise of the modulated light is quite low, and the noise level of the EO signal can be reduced to almost the shot-noise level by using a balanced detection scheme.

Simple and highly sensitive optical pulse-characterization method based on electro-optic spectral signal differentiation

A very simple self-referenced, linear pulse-characterization technique based on spectral phase reconstruction by frequency-domain signal differentiation is introduced. This technique can be implemented using electro-optic intensity modulation of the pulse under test with a synchronized RF sinusoid. The pulse spectral phase profile can be accurately and unambiguously reconstructed from only two measured energy spectra, i.e., at the input and at the output of the modulator, using a direct analytic equation. The method is experimentally demonstrated by precisely characterizing microwatt-power picosecond pulses after linear dispersion through short sections (50-700 m) of conventional single-mode fiber.

Polarized-Light-Induced Enhancement of the Electro-Optic Properties in a Photoreactive Maleimide Copolymer

A photocrosslinkable maleimide copolymer has been synthesized using a tricyanopyrrolidene (TCP)-based chromophore. The 2-(3-Cyano-4-(2-{4-[hexyl-(6-hydroxy-hexyl)-amino]-phenyl}vinyl)-5-oxo-1-{4-[4-(3-oxo-3-phenyl-propenyl)-phenoxy]-butyl}-1,5-dihydro-pyrrol-2-ylidene)-malononitrile chromophore (DHATCP-CH) exhibits nonlinear optical activity and contains a chalcone moiety that is highly sensitive to UV light for crosslink formation. The maleimide monomer was also functionalized with chalcone moieties. The electro-optic signal can be varied under irradiation with linearly polarized light. The enhancement of the electro-optic effect in the copolymer is attributed to an angular selective photocrosslink formation upon illumination with p-polarized UV light. A possible mechanism that can explain this phenomenon is presented.

Ultra small electro-optic field probe fabricated by aerosol deposition

We developed a microscopic electro-optic field probe by directly depositing a lead zirconate-titanate [PbZr0.3Ti0.7O3] film onto the optical fiber edge using aerosol deposition. Its fabrication process is low-cost and relatively easy because there are no complicated procedures such as fine lithography and etching. The lateral size of the film is 125µm, which is the same as the diameter of a typical single mode fiber, and the thickness is approximately 5µm. An RF electro-optic signal was successfully measured over a microstrip line. The capability for detecting GHz range fields is also shown. Because it is very tiny and thin, the developed electro-optic probe has great potential for detailed electrical characterization in the microscopic regions of high performance electronic products such as the interconnecting parts between LSI packages and printed circuit boards and spaces among different LSI chips in a package.

Macroscopic nonlinear optical properties of tricyanopyrrolidene chromophore containing amorphous polycarbonate: Effect of molecular lateral moiety in the conjugative structure

Tricyanopyrrolidene chromophores were prepared in order to compare their macroscopic nonlinear optical (NLO) properties with a conjugated structure through the long molecular axis. A thiophene or phenyl ring was tethered to an ethylenic bond; it may act as a lateral moiety to disrupt the planarity of a chromophore and lessen the electrostatic interaction. Thin film composites of these chromophores dissolved in amorphous polycarbonate (APC) were fabricated. Real time pole and probe method was employed to investigate the change of electro-optic (EO) signal during poling. The EO properties and their relaxation behaviors of the guest-host systems containing newly synthesized chromophores were investigated in detail.

A voltage calibration technique of electro-optic probing for characterization internal to IC’s chip

This paper simulates the electric potential distribution and the electro-optic (EO) signal amplitude in a longitudinal poled EO polymer external probe tip on the electric signal transmission lines under test. The influence of the circuit layout on the EO probing is discussed. A novel probing configuration with a poled polymer EO probe tip is built and demonstrated for the first time, by which the signal voltage level corresponding to the EO signal can be calibrated. Using the new probe tip, we examine the influence of the variations of the linewidth and the spacing between neighboring lines on the EO signal. The results indicate that when the vertical distance between the tested point on the lines and the reference electrode is not larger than that between the tested point and its neighboring conductor, the disturbance from circuit layout can be avoided so that the voltage calibration of EO signal can be carried out.

Electro-optic property of chromophore-terminated trifunctional dendrimer in a guest–host system

We prepared the new heterocyclic nonlinear optical (NLO) chromophores that show fairly high microscopic nonlinearity through intramolecular charge transfer. Phenothiazine unit played an important role to contribute high electron donability and connect the resonance pathway via conjugative effect in the cyclized ring beside the aromatic ring. Three NLO chromophores were tethered to the aromatic core to synthesize the 1st-generation trifunctional dendrimer. We compared the electro-optic (EO) properties of two samples with single chromophore or trifunctional dendritic chromophore. Real time pole and probe method was employed to investigate the change of the EO signal during poling. EO properties and their relaxation behaviors of the guest–host systems containing synthesized chromophores were thoroughly investigated.

Poled Silica/DR1 Films with Thermally Stability and LargeElectro-Optic Coefficient Applying in External Probe Tip

Sol-gel-processed silica films doped with Disperse Red 1(DR1) wereprepared at 80°C aging temperature and 120°C bakingtemperature with corona poling to obtain stable and large electro-optic(EO) coefficient and film strength. A large EO coefficient ofγ33 = 56 pm/V was measured for the film of 0.5-μm thicknessat the wavelength of 1300 nm, and the value was unvaried at roomtemperature. Moreover, an external EO probe tip using the film wasfabricated for the first time, and a signal voltage level correspondingto the EO signal was calibrated successfully.

Polyamic alkyl ester system promising for side-chain electro-optic polymer

A polyamic alkyl ester was examined for the first time as a new candidate for an electro-optic (EO) polymer. Disperse Red 1 was easily introduced into a polyamic acid synthesized from 2,2-bis(4-aminophenyl)hexafluoropropane and oxydiphthalic anhydride by the Mitsunobu reaction. The EO polyamic alkyl ester film is amorphous. The dipole orientation in the film is slightly aligned without a poling process, showing ∼5 pm/V. The best poling temperature is ca. 145 °C with regard to a rearrangement of the EO polyamic alkyl ester into the corresponding polyimide. The EO coefficient of the poled film is around 25 pm/V at a wavelength of 1.55 μm when the film is poled with an applied voltage of 150 V/μm. The EO signal is thermostable.

Bilateral switching of the modulated electrooptic contrast in PLZTN ceramics

Spatial distribution of ferroelectric polarization in Pb(Zr,Ti)O 3 ceramics doped with lanthanum and niobium has been investigated using scanning electrooptic confocal microscopy. Measurement of the modulated electrooptic signal in presence of a sinusoidal electric field reveals ferroelectric domains at microscopic scale not accessible to conventional polarized optical imaging. Both electrooptic images and local electrooptic loops have been observed after applying a bias electric field to the ceramic samples, being initially non-ferroelectric macroscopically. Bilateral reversal of the electrooptic contrast during the poling cycle has been detected and explained taking into account non-180° switching processes in ferroelectric grains of arbitrary orientation.

Modulation depth enhancement for highly sensitive electro-optic RF near-field measurement

A modulation depth enhancement technique for highly sensitive electro-optic (EO) sensing of RF near-fields is presented. This technique increases EO signal intensity without increasing total power received by a photodiode (PD). Therefore, sensitivity limitation owing to saturation of the PD can be avoided. Sensitivity improvement of 20 dB compared to the conventional configuration is successfully achieved.

POLYAMIC ALKYL ESTER SYSTEM AS AN ELECTRO-OPTIC POLYMER

A polyamic alkyl ester (PAE) was examined for the first time as a new candidate electro-optic (EO) polymer. An EO PAE was easily prepared from Disperse Red 1 (DR1) and a polyamic acid (PAA) synthesized from 2,2-bis(4-aminophenyl)hexafluoropropane and oxydiphthalic anhydride. The EO coefficient (r33) of the poled EO PAE film is around 25 pm/V at a wavelength of 1.55 μm when the film is poled with an applied voltage of 150 V/μm. The EO signal is thermostable. However, the chromophore is slightly released over 150°C.

A stable host–guest electro-optic polymer system with polyisoimide as a host

A stable host–guest electro-optic (EO) polymer system has been prepared for improving not only the EO effect but also the thermal stability of the effect using a polyisoimide as a host. Disperse Red 1 (DR1) and a polyisoimide (PII) synthesized from 2,2-bis(4-aminophenyl)hexafluoropropane and oxydiphthalic anhydride were used as the guest and the host, respectively. Analyses of a model compound reaction and the host–guest film after annealing at various temperatures show that the reaction between the isoimide groups in the PII and the hydroxyl groups in DR1 occurs at around 140 °C. The EO coefficient obtained at a wavelength of 1.55 μm was 13.7 pm/V from an EO polymer film with 30 wt% of DR1 as added guest. The EO signal was thermostable at high temperature due to the chemical reaction between the host and guest during poling.

Terahertz polariton propagation in patterned materials.

Generation and control of pulsed terahertz-frequency radiation have received extensive attention, with applications in terahertz spectroscopy, imaging and ultrahigh-bandwidth electro-optic signal processing. Terahertz 'polaritonics', in which terahertz lattice waves called phonon-polaritons are generated, manipulated and visualized with femtosecond optical pulses, offers prospects for an integrated solid-state platform for terahertz signal generation and guidance. Here, we extend terahertz polaritonics methods to patterned structures. We demonstrate femtosecond laser fabrication of polaritonic waveguide structures in lithium tantalate and lithium niobate crystals, and illustrate polariton focusing into, and propagation within, the fabricated waveguide structures. We also demonstrate a 90 degrees turn within a structure consisting of two waveguides and a reflecting face, as well as a structure consisting of splitting and recombining elements that can be used as a terahertz Mach-Zehnder interferometer. The structures permit integrated terahertz signal generation, propagation through waveguide-based devices, and readout within a single solid-state platform.

Space charge effects in polymer-based light-emitting diodes studied by means of a polarization sensitive electroreflectance technique

We used a single wavelength electro-optical reflection technique to study the creation of space charge distributions in a polymer light emitting diode, by monitoring the real third order nonlinear optical response of the electroluminescent layer. The diode is based on an aluminum/polymer/indium tin oxide stack, where the polymer is a derivative of poly(p-phenylene-vinylene) in which oxidiazole groups were grafted as side chains. The measured signal is strongly influenced by a space charge distribution and by screening in the organic layer. The space charge distribution is the superposition of a long lived contribution, close to indium tin oxide, due to charges trapped in deep levels, and of that of relatively mobile charges injected in the polymer film. In the frame of a model which takes into account Debye–Hückel screening, with screening length equal to 100 nm we estimated the carrier density of the long lived charge distribution to be 4.8×1023 m−3, for a uniform distribution in a 110-nm-thick layer. The influence of the injection of mobile charges on the electro-optic signal is an order of magnitude smaller than that of trapped charges. The real third order susceptibility of the polymer is estimated to be χ(3)=4×10−21 m2/V2.

Temporal stability of the electro-optic effect in poled polymethacrylate and polynaphthalate bearing methylsulfonyl benzothiazolyl azo dyes below the glass transition temperature

We synthesized second-order nonlinear optical (NLO) polymers for studying the structural effect on the electro-optic (E/O) properties. A heterocyclic azo chromophore was synthesized to be tethered to the methacrylate and naphthalate polymer backbones. Particularly, 2-amino-6-methylsulfonylbenzothiazole was employed to prepare the NLO azo chromophore. The real time pole and probe method was employed to investigate the change of the poled order during poling. The increasing rates of the E/O effect were considered in two polymer systems during poling. The decaying behaviors of the electro-optic signal were observed at different temperatures below the glass transition temperature. The decaying rates of the E/O effect were compared and the effect of structural difference was investigated.

Stability of second-order nonlinear optical properties in sol–gel matrix bearing silylated chalcone and disperse red 1

We report the syntheses of two triethoxysilanes for thin film fabrication. One silane (SGDR1) contains disperse red 1 (DR1) which is a second-order nonlinear optical (NLO) active chromophore. The other silane (SGCHC) bears a chalcone derivative. In order to improve the poor temporal stability of second-order NLO effects in the resulting poled sol–gel film, photo-crosslink was induced between the double bonds in chalcone units after mixing SGDR1 and SGCHC. Under UV exposure, the photocrosslink was studied with UV–visible absorption and IR vibrational spectroscopy. The decaying behavior of the molecular order parameter ( A 2) after corona poling was investigated using polarized absorption spectroscopy (PAS). Second-order non-linear optical properties of the sample were studied using second-harmonic generation (SHG) and linear electro-optic (E/O) signal measurement. Poled and UV-cured film of SGDR1/SGCHC exhibited much better stability than thermally poled sample with respect to the molecular order parameter and linear electro-optic effect.