Organic Electro-optic Crystals Research Articles

Phonon-Suppressing Intermolecular Adhesives: Catechol-Based Broadband Organic THz Generators.

Solid‐state molecular phonons play a crucial role in the performance of diverse photonic and optoelectronic devices. In this work, new organic terahertz (THz) generators based on a catechol group that acts as a phonon suppressing intermolecular adhesive are developed. The catechol group is widely used in mussel‐inspired mechanical adhesive chemistry. Newly designed organic electro‐optic crystals consist of catechol‐based nonlinear optical 4‐(3,4‐dihydroxystyryl)‐1‐methylpyridinium (DHP) cations and 4‐(trifluoromethyl)benzenesulfonate anions (TFS), which both have multiple interionic interaction capability. Interestingly, compared to benchmark organic crystals for THz generators, DHP‐TFS crystals concomitantly achieve top level values of the lowest void volume and the highest crystal density, resulting in an exceptionally small amplitude of solid‐state molecular phonons. Simultaneously achieving small molecular phonon amplitude, large optical nonlinearity and good phase matching at infrared optical pump wavelengths, DHP‐TFS crystals are capable of generating broadband THz waves of up to 16 THz with high optical‐to‐THz conversion efficiency; one order of magnitude higher than commercial inorganic THz generators.

Terahertz Phonon Mode Engineering of Highly Efficient Organic Terahertz Generators

For terahertz (THz) wave generators based on organic electrooptic crystals, their intrinsic phonon modes are playing an essential role in THz generation characteristics. Here, this study proposes an effective design strategy for THz phonon mode engineering of organic electrooptic salt crystals for efficient optical‐to‐THz frequency conversion. To reduce phonon‐mode intensity, strongly electronegative trifluoromethyl group acting as strong hydrogen‐bond acceptor is incorporated into molecular anions. New 2‐(4‐hydroxy‐3‐methoxystyryl)‐1‐methylquinolinium 4‐(trifluoromethyl)benzenesulfonate (HMQ‐4TFS) crystals exhibit a relatively small absorption coefficient in the THz spectral range between 0.5 and 4 THz, which is attributed to suppressed molecular vibrations due to strong hydrogen bonds involving the 4TFS anion. In addition, HMQ‐4TFS crystals possess a very large macroscopic optical nonlinearity, comparable (or even higher) to benchmark stilbazolium crystals. Based on the low‐intensity THz phonon modes and the large optical nonlinearity, a 0.37 mm thick HMQ‐4TFS crystal pumped with 150 fs infrared laser pulses facilitates very efficient THz wave generation by optical rectification, delivering 23 times higher peak‐to‐peak THz electric field than the widely used standard inorganic ZnTe crystal (1.0 mm thick) and a broader spectral bandwidth. Therefore, strongly electronegative groups introduced into molecular salt electrooptic crystals provide a very promising design strategy of THz phonon mode engineering for developing intense broadband THz sources.

Terahertz spectrum study of organic electro-optic crystal 4-N, N-dimethylamino-4'-N'-methyl-stilbazolium tosylate

The ground-state structural optimization and the terahertz spectrum calculation of an organic electro-optical crystal of 4-N, N-dimethylamino-4'-N'-methyl-stilbazolium tosylate (DAST) are performed using dispersion-corrected density functional theory (DFT-D2). DAST consists of an organic pyridinium salt (cation), one of the most efficient non-linear optical active chromophores and a sulfonate (anion) for enhancing the stability of the noncentrosymmetric macroscopic crystal. Such an organic crystalline salt DAST exhibits highly electro-optical and nonlinear optical coefficients, and it is an efficient emitter of THz pulses. The steady ground-state structure of DAST is obtained by a step-by-step optimization method with gradually increasing the convergence accuracy. The calculated terahertz spectra in 0-4 THz are in good agreement with experimental measurements, implying the reasonability of DFT-D2 method. Moreover, the vibration displacement vector diagrams for DAST molecular structure are obtained using Cambridge sequential total energy package animation simulation function. The results indicate that the phonon modes of DAST crystal at 1.12 THz are attributed to the optical phonon modes of the anion and cation, and DAST cation (organic pyridinium salt) and anion (sulfonate) undergo translational vibrations in their respective (benzene ring) plane. In contrast the vibrations at 1.46 THz and 1.54 THz are mainly related to the vibration of the sulfonate, among which 1.46 THz vibration is caused by the rotation of the sulfonate along the a axis, while 1.54 THz is due to the motion of the whole sulfonate along the c axis. And the vibrations at 2.63 THz and 3.16 THz originate from the torsional vibrations of cations and the rotation of anions, respectively. The results presented in this work clearly illustrate the contributions of the anion and cation of DAST in the THz responses. The mode assignments provide important reference and guidance for further synthesis of new DAST derivatives with larger electro-optical coefficients. In particular, our results suggest that DFT method is a powerful theoretical tool for studying the THz photonics and it is helpful not only for better understanding the mechanisms of the THz responses of organic electro-optic crystals, but also for controlling their performances.

Electro-optic crystals grown in confined geometry with optimal crystal characteristics for THz photonic applications

We report a new crystal engineering technique for the thickness-controlled growth of large-area organic electro-optic crystals in confined geometry optimal for THz wave photonics. We investigate the relationship between the material properties and THz generation characteristics to determine the optimal crystal characteristics for THz applications. Two different state-of-the-art nonlinear optical organic crystals are chosen as model crystals, ionic quinolinium-based and non-ionic polyene-based crystals. Both kinds of electro-optic crystals grown in confined geometry exhibit good optical quality, large area and sub-millimeter-scale thickness, which are optimal crystal characteristics for efficient THz wave generation. In order to investigate the relationship between crystal characteristics and optical-to-THz conversion characteristics, THz experiments with quinolinium crystals are performed and compared with theoretical calculations. The characteristics of the generated THz waves from as-grown quinolinium-based single crystals in confined geometry match well with those from very flat quinolinium crystals prepared from bulk crystals by a cutting method and also with theoretical calculations.

Quinolinium-based organic electro-optic crystals: Crystal characteristics in solvent mixtures and optical properties in the terahertz range

We investigate crystal characteristics of ionic organic π-conjugated crystals based on 4-hydroxy-3-methoxystyryl quinolinium (HMQ), which are recently developed acentric crystals showing a large nonlinear optical response and a high THz generation efficiency. Among a few of HMQ derivatives, HMQ-T crystal consisting of the HMQ cation and the 4-methylbenzenesulfonate anion, is chosen for the investigation of solubility, morphology and crystal growth characteristics including optical properties in various single solvents and solvent mixtures. Compared to the solvent mixture of non-aromatic methanol and acetonitrile, HMQ-T crystals in a solvent mixture containing an aromatic solvent (methanol/toluene mixture) exhibits a similar solubility variation with the solvent composition ratio, but based on distinguishable intermolecular interactions with solvent molecules. In addition, solvent mixtures containing an aromatic solvent show a significantly different equilibrium between benzenoid and quinoid forms of the HMQ cation. In contrast to the strong influence of solvent characteristics on solubility behavior, the investigated solvent systems only slightly influence the crystal morphology. Large HMQ-T single crystals with size up to 37 × 6 × 2 mm3 are grown by slow cooling method. We also determine the refractive index and absorption of as-grown single HMQ-T crystals in the broad THz frequency range of 0.3–10 THz.

Phonon Modes of Organic Electro-Optic Molecular Crystals for Terahertz Photonics

We investigate theoretically and experimentally the phonon modes of organic electro-optic crystals for THz-wave photonics. THz phonon modes significantly influence the characteristics of THz-wave generation and detection through the THz absorption and refractive-index dispersion. For a model as THz generation material, electro-optic PNP (2-(N-prolinol)-5-nitropyridine) molecular crystal containing a relatively small numbers of atoms in the unit cell is chosen for phonon-mode calculation. THz phonon modes with their frequencies and intensities are calculated by periodic density functional theory, and the resulting spectrum exhibits a good agreement with the experimental absorption spectrum measured by THz time-domain spectroscopy. THz-wave generation experiment is performed with PNP as THz generator and the organic electro-optic OH1 (2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene)malononitrile) crystal as THz detector. Considering the phase matching condition and THz absorption, which are both influenced by the THz phonon modes of THz generation and detection materials, the calculated THz generation spectrum also matches well with the experimental THz spectrum obtained by optical rectification at the pump wavelength of 1300 nm. Therefore, the phonon mode identification is very important for optimizing and designing new THz source and detection materials as well as for THz spectroscopy.

New Acentric Core Structure for Organic Electrooptic Crystals Optimal for Efficient Optical‐to‐THz Conversion

Organic electrooptic crystals based on a new acentric core structure 2-(4-hydro­xystyryl)-1-methylquinolinium provide optimal molecular packing for maximizing the electrooptic response. They allow intense broadband terahertz generation with over 30 times higher terahertz generation efficiency compared to the widely used inorganic ZnTe. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Dielectric, optical and mechanical studies of phenolic polyene OH1 organic electrooptic crystal

Abstract 2-{3-[2-(4-Hydroxyphenyl) vinyl]-5, 5-dimethylcyclo-hex-2-en-1-ylidene}malononitrile (OH1) phenolic locked polyene organic material has been synthesized by the Knoevenagel condensation method. OH1 single crystals were grown in methanol by a slow evaporation method. In order to avoid the multinucleation and reduce the metastable zone width, phosphoric acid is added in different concentrations. The linear optical property of OH1 crystal has been studied using UV–vis–NIR spectroscopy in the wavelength range 190–1100 nm and optical constants are calculated theoretically. The magnitude of nonlinear refractive index (10 −12 m 2 /W), nonlinear absorption (10 −6 m/W) and third order nonlinear susceptibility (10 −6 esu) has been studied using a Z -scan technique. Dielectric property of OH1 crystal has been studied in frequency range 50 Hz–5 MHz. Photoluminescence spectrum was recorded using a xenon lamp in the range of 450–700 nm. Laser optical damage threshold of OH1 crystal was obtained (0.62 GW/cm 2 ) using a pulsed Nd-YAG laser (1064 nm) of repetition rate 10 ns.

Electro-optic single-crystalline organic waveguides and nanowires grown from the melt

Organic nonlinear optical materials have proven to possess high and extremely fast nonlinearities compared to conventional inorganic crystals, allowing for sub-1-V driving voltages and modulation bandwidths of over 100 GHz. Compared to more widely studied poled electro-optic polymers, organic electro-optic crystals exhibit orders of magnitude better thermal and photochemical stability. The lack of available structuring techniques for organic crystals has been the major drawback for exploring their potential for photonic structures. Here we present a new approach to fabricate high-quality electro-optic single crystal waveguides and nanowires of configurationally locked polyene DAT2 (2-(3-(2-(4-dimethylaminophenyl)vinyl)-5,5-dimethylcyclohex-2-enylidene)malononitrile). The high-index-contrast waveguides (delta(n) = 0.54 +/- 0.04) are grown from the melt between two anodically bonded borosilicate glass wafers, which are structured and equipped with electrodes prior to bonding. Electro-optic phase modulation is demonstrated for the first time in the non-centrosymmetric DAT2 single crystalline channel waveguides at a wavelength of 1.55 microm. We also show that this technique in combination with DAT2 material allows for the fabrication of single-crystalline nanostructures inside large-area devices with crystal thicknesses below 30 nm and lengths of above 7 mm.

Thin film electro-optic modulator based on single crystal of N-(4-nitrophenyl)-(L)-prolinol (NPP) grown from melt by the modified Bridgman method

We discuss the plate-guiding method of growing micrometer- thick single crystal films of electro-optic organic material N-(4- nitrophenyl)-(L)-prolinol. This approach has the advantages of full control over the thickness of the film and selection of a proper seed for initial crystal growth. Characterization of the electro-optic properties of the films is performed using the intensity modulation technique with a low (of the order of 10 V) alternating driving voltage. Both the fundamental and double-frequency responses are used for the characterization. The electro-optic effect is pseudolongitudinal since the configuration of the experiment is that of a longitudinal intensity modulator. The main contri- bution to the effect originated from the transverse component of the ex- ternal electric field. By purely electro-optic means we determine the ori- entation of the dielectric axes and measure the half-wave voltage, the figure of merit, and electro-optic coefficientsr 12 and r 22 to be 3.24 kV and 99.2, 461, and 154 pm/V, respectively. We also find that electro-optic coefficientsr 61 and r 63 are two orders of magnitude less than r 12 and r 22 . © 2003 Society of Photo-Optical Instrumentation Engineers. (DOI: 10.1117/1.1542595) Subject terms: electro-optic modulator; plate-guiding method; single crystal film; electro-optic coefficients; organic electro-optic crystals; single-crystal films.

Dielectric dispersion and piezoelectric resonance in an organic electro-optic crystal: (−)−2−α-methylbenzylamino-5-nitropyridine

The frequency response of the dielectric constant (εr), dielectric loss tangent (tan δ) and impedance (Z) of an organic electro-optic crystal, (−)−2−α- methylbenzylamino-5-nitropyridine (MBA-NP) monitored along the polar axis, is characterized by the presence of strong peaks in the kHz region. The observed resonance behavior, which is strongly dependent on the geometric size and shape of the sample, is attributed to its piezoelectric nature. The resonance peaks were found to shift steadily towards lower frequencies with increasing temperature and towards higher frequencies with increasing pressure. The temperature dependence of the stiffness coefficient (C) of the crystal has been defined using the resonance data. The temperature dependence of the damping of these piezoelectric vibrations has been monitored by the resonance–anti-resonance method. The influence of the application of pressure and the variation of temperature on εr, monitored along the polar axis of the crystal, has been studied.

A Novel and Perfectly Aligned Highly Electro−Optic Organic Cocrystal of a Merocyanine Dye and 2,4-Dihydroxybenzaldehyde

A Novel and Perfectly Aligned Highly Electro−Optic Organic Cocrystal of a Merocyanine Dye and 2,4-Dihydroxybenzaldehyde

A new organic electrooptic crystal: 2,6-dibromo-N-methyl-4-nitroaniline (DBNMNA)

A new organic electrooptic crystal, 2,6-dibromo-N-methyl-4-nitroaniline, is reported. The crystal structure was determined by X-ray diffraction (orthorhombic, space group Fdd2, point group mm2, Z=16, a=11.745 AA, b=29.640 AA, c=10.807 AA). The refractive indexes were measured at several wavelengths between 500 and 1100 nm (at 632.8 nm, n/sub a/=1.90, n/sub b/=1.62, and n/sub c/=1.48) and fit the data to a single oscillator Sellmeier equation. A report is presented on measurements of the linear electrooptic effect, yielding the combined coefficients n/sub a//sup 3/r/sub 13/-n/sub c//sup 3/r/sub 33/, as well as the first known measurements of the quadratic electrooptic effect in an organic crystal, yielding the coefficients r/sub 42/ and r/sub 51/ at 514.5, 632.8, and 810 nm. Good agreement is found between the observed dispersion in the electrooptic coefficients and that predicted by the simple two-level dispersion model.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A novel organic electro-optic crystal: 3-(1,1-dicyanoethenyl)-1-phenyl- 4,5-dihydro-1H-pyrazole

Details are given of a new organic electro-optic crystal, 3-(1,1-dicyanoethenyl)-1-phenyl-4,5-dihydro-1H-pyrazole, having a reduced half-wave voltage of 370 V (approximately 8 times lower than lithium niobate). The molecular design, synthesis, and molecular characterization of this compound are outlined and a description is given of growth of 1-cm dimension crystals. These crystals have been characterized in terms of their structure, linear optical properties (refractive index, orientation of optical axes, absorption spectra), and also of their linear electro-optic (Pockels) effect. The compound crystallizes in the noncentrosymmetric space group Cc, with the molecules showing a high degree of parallelism. This structure is discussed in terms of the applicability of the crystals to different nonlinear optical effects. Calculated molecular hyperpolarizabilities are used, in conjunction with the determined crystal structure, to estimate the coefficients of the crystalline χ(2) tensor, which is dominated by the single coefficient χ333. The related dominant electro-optic coefficient, r333, is measured to be 8.7×10−11 m/V.