Analytical Gas Sensing in the Terahertz Spectral Range.

Exploiting the terahertz (THz) part of the electromagnetic spectrum is attracting attention in various scientific and applied disciplines worldwide. THz technology has also revealed its potential as an effective tool for gas analysis in astronomy, biomedicine and chemical analysis. Recently, it has also become important in environmental applications for monitoring hazardous and toxic gases in the atmosphere.

Design of High-Performance Organic Nonlinear Optical and Terahertz Crystals by Controlling the van der Waals Volume.

In the development of new organic crystals for nonlinear optical and terahertz (THz) applications, it is very challenging to achieve the essentially required non-centrosymmetric molecular arrangement. Moreover, the resulting crystal structure is mostly unpredictable due to highly dipolar molecular components with complex functional substituents. In this work, new organic salt crystals with top-level macroscopic optical nonlinearity by controlling the van der Waals volume (V ), rather than by trial and error, are logically designed.

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.

Generation of strong-field spectrally tunable terahertz pulses.

The ideal laser source for nonlinear terahertz spectroscopy offers large versatility delivering both ultra-intense broadband single-cycle pulses and user-selectable multi-cycle pulses at narrow linewidths. Here we show a highly versatile terahertz laser platform providing single-cycle transients with tens of MV/cm peak field as well as spectrally narrow pulses, tunable in bandwidth and central frequency across 5 octaves at several MV/cm field strengths. The compact scheme is based on optical rectification in organic crystals of a temporally modulated laser beam.

Organic Broadband THz Generators Optimized for Efficient Near-Infrared Optical Pumping.

New organic THz generators are designed herein by molecular engineering of the refractive index, phonon mode, and spatial asymmetry. These benzothiazolium crystals simultaneously satisfy the crucial requirements for efficient THz wave generation, including having nonlinear optical chromophores with parallel alignment that provide large optical nonlinearity; good phase matching for enhancing the THz generation efficiency in the near-infrared region; strong intermolecular interactions that provide restraining THz self-absorption; high solubility that promotes good crystal growth ability; and a plate-like crystal morphology with excellent optical quality. Consequently, the as-grown benzothiazolium crystals exhibit excellent characteristics for THz wave generation, particularly at near-infrared pump wavelengths around 1100 nm, which is very promising given the availability of femtosecond laser sources at this wavelength, where current conventional THz generators deliver relatively low optical-to-THz conversion efficiencies.

MHz-repetition-rate, sub-mW, multi-octave THz wave generation in HMQ-TMS.

We demonstrate the first megahertz (MHz) repetition-rate, broadband terahertz (THz) source based on optical rectification in the organic crystal HMQ-TMS driven by a femtosecond Yb:fibre laser. Pumping at 1035 nm with 30 fs pulses, we achieve few-cycle THz emission with a smooth multi-octave spectrum that extends up to 6 THz at -30 dB, with conversion efficiencies reaching 10 and an average output power of up to 0.38 mW.

Supercontinuum generation in OHQ-N2S organic crystal driven by intense terahertz fields.

A laser supercontinuum is generated by cross-phase modulation (XPM) driven by an intense terahertz (THz) field in organic crystal OHQ-N2S. In this highly nonlinear medium, the THz electric field induces a time-varying optical phase modulation, which causes a spectacular spectral broadening and shifting of a co-propagating near-infrared laser pulse. The effect is enabled by the large electro-optic coefficient, the low absorption, and the good velocity matching between the laser and the THz pulse over the OHQ-N2S crystal thickness.

Generation of high-field terahertz pulses in an HMQ-TMS organic crystal pumped by an ytterbium laser at 1030 nm.

We present the generation of high-peak-electric-field terahertz pulses via collinear optical rectification in a 2-(4-hydroxy-3-methoxystyryl)-1-methilquinolinium-2,4,6-trimethylbenzenesulfonate (HMQ-TMS) organic crystal. The crystal is pumped by an amplified ytterbium laser system, emitting 170-fs-long pulses centered at 1030 nm. A terahertz peak electric field greater than 200 kV/cm is obtained for 420 µJ of optical pump energy, with an energy conversion efficiency of 0.

Benzothiazolium Single Crystals: A New Class of Nonlinear Optical Crystals with Efficient THz Wave Generation.

Highly efficient nonlinear optical organic crystals are very attractive for various photonic applications including terahertz (THz) wave generation. Up to now, only two classes of ionic crystals based on either pyridinium or quinolinium with extremely large macroscopic optical nonlinearity have been developed. This study reports on a new class of organic nonlinear optical crystals introducing electron-accepting benzothiazolium, which exhibit higher electron-withdrawing strength than pyridinium and quinolinium in benchmark crystals.

Intense, carrier frequency and bandwidth tunable quasi single-cycle pulses from an organic emitter covering the Terahertz frequency gap.

In Terahertz (THz) science, one of the long-standing challenges has been the formation of spectrally dense, single-cycle pulses with tunable duration and spectrum across the frequency range of 0.1-15 THz (THz gap). This frequency band, lying between the electronically and optically accessible spectra hosts important molecular fingerprints and collective modes which cannot be fully controlled by present strong-field THz sources.

High efficiency THz generation in DSTMS, DAST and OH1 pumped by Cr:forsterite laser.

We investigated Terahertz generation in organic crystals DSTMS, DAST and OH1 directly pumped by a Cr:forsterite laser at central wavelength of 1.25 μm. This pump laser technology provides a laser-to-THz energy conversion efficiency higher than 3 percent.

High-energy terahertz pulses from organic crystals: DAST and DSTMS pumped at Ti:sapphire wavelength.

High-energy terahertz pulses are produced by optical rectification (OR) in organic crystals 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) and 4-N, N-dimethylamino-4'-N'-methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate (DSTMS) by a Ti:sapphire amplifier system with 0.8 μm central wavelength. The simple scheme provides broadband spectra between 1 and 5 THz, when pumped by a collimated 60 fs near-IR pump pulse, and it is scalable in energy.

High-power broadband organic THz generator.

The high-power broadband terahertz (THz) generator is an essential tool for a wide range of THz applications. Here, we present a novel highly efficient electro-optic quinolinium single crystal for THz wave generation. For obtaining intense and broadband THz waves by optical-to-THz frequency conversion, a quinolinium crystal was developed to fulfill all the requirements, which are in general extremely difficult to maintain simultaneously in a single medium, such as a large macroscopic electro-optic response and excellent crystal characteristics including a large crystal size with desired facets, good environmental stability, high optical quality, wide transparency range, and controllable crystal thickness.

Optical phase conjugation of picosecond pulses at 1.06 mum in Sn(2)P(2)S(6):Te for wavefront correction in high-power Nd-doped amplifier systems.

We report, for the first time to our knowledge, on picosecondpulse optical phase conjugation using photorefractive Sn(2)P(2)S(6) crystals. For 7.2-ps pulses at 1.

Influence of phenolic hydroxyl groups on second-order optical nonlinearity at an example of 2,4- and 3,4-dihydroxyl hydrazone isomorphic crystals.

We investigate the crystal structure and physical properties of 2,4- and 3,4-dihydroxybenzaldehyde-4-nitrophenylhydrazone (DHNPH) isomer crystals to understand the relation between molecular ordering with noncovalent interactions based on phenolic OH groups. The microscopic and macroscopic optical nonlinearities of 2,4- and 3,4-DHNPH crystals are investigated experimentally and theoretically by using density functional theory calculations. Although the two isomer crystals possess a very similar molecular orientation based on a similar supramolecular synthon, 2,4-DHNPH exhibits a 1.

Fast dynamic waveguides and waveguide arrays in photorefractive Sn(2)P(2)S(6) induced by visible light.

We report on dynamic waveguides and waveguide arrays induced beneath the surface of electro-optic Sn(2)P(2)S(6) crystals by visible light at 514 nm. The waveguide structures are generated by interband photoexcitation and drift or diffusion charge transport mechanism. These structures are probed nondestructively in the transverse direction with a beam at a longer wavelength.

Light deflection and modulation through dynamic evolution of photoinduced waveguides.

Light induced waveguides produced by lateral illumination of a hotorefractive crystal show a complex dynamic evolution upon removal of the sustaining applied electric field. Using this effect, deflection and modulation of the guided light is realized by taking advantage of the screening and counter-screening of the space charge distribution. The spot separation upon deflection can exceed 10 times the original waveguide width.

A hydrogen-bonded organic nonlinear optical crystal for high-efficiency terahertz generation and detection.

Broadband THz pulses have been generated in 2-[3-(4- hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene]malononitrile (OH1) by optical rectification of sub-picosecond laser pulses. We show that OH1 crystals allow velocity-matched generation and detection of THz frequencies in the whole range between 0.3 and 2.

Fabrication and phase modulation in organic single-crystalline configurationally locked, phenolic polyene OH1 waveguides.

A novel and promising technique for the fabrication of electro-optically active single crystalline organic waveguides from 2-{3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene}malononitrile (OH1) is presented. OH1 is an interesting material for photonic applications due to the large electro-optic coefficients (r333 = 109+/-4 pm/V at 632.8 nm) combined with a relatively high crystal symmetry (orthorhombic with point group mm2).

Double phase conjugate mirror using Sn2P2S6 for injection locking of a laser diode bar.

We demonstrate double phase-conjugation in pure and Te-doped Sn(2)P(2)S(6), a semiconducting ferroelectric material, at the wavelength of 685 nm. We observe a phase conjugate reflectivity of more than 800% at an intensity ratio of the pump beams of 44 for Te-doped Sn(2)P(2)S(6). Using a laser diode bar emitting at 685 nm, we demonstrate double phase conjugation of three independent emitters of the laser diode bar with a single mode master laser.

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.

Electro-optic and nonlinear optical properties of ion implanted waveguides in organic crystals.

We report on the electro-optic and nonlinear optical properties of waveguides produced by low fluence (Phi = 1.25x10(14) ions/cm(2)) H+ ion implantation in the organic nonlinear optic crystal 4-N,N-dimethylamino-4'-N'-methyl-stilbazolium tosylate (DAST). The profile of the nonlinear optical susceptibility has been determined by measuring the reflected second-harmonic generation efficiency from a wedged-polished sample at a fundamental wavelength of lambda(omega) = 1176nm.

Electro-optic Charon polymeric microring modulators.

We propose and demonstrate a new type of electro-optic polymeric microring resonators, where the shape of the transmission spectrum is controlled by losses and phase shifts induced at the asymmetric directional coupler between the cavity and the bus waveguide. The theoretical analysis of such Charon microresonators shows, depending on the coupler design, three different transmission characteristics: normal Lorentzian dips, asymmetric Fano resonances, and Lorentzian peaks. The combination of the active azo-stilbene based polyimide SANDM2 surrounded by the hybrid polymer Ormocomp allowed the first experimental demonstration of electro-optic modulation in Charon microresonators.

Photochemical stability of nonlinear optical chromophores in polymeric and crystalline materials.

We compare the photochemical stability of the nonlinear optical chromophore configurationally locked polyene 2-{3-[2-(4-dimethylaminophenyl)vinyl]-5,5-dimethylcyclohex-2-enylidene} malononitrile (DAT2) embedded in a polymeric matrix and in a single-crystalline configuration. The results show that, under resonant light excitations, the polymeric compound degrades through an indirect process, while the DAT2 crystal follows a slow direct process. We show that chromophores in a crystalline environment exhibit three orders of magnitude better photostability as compared to guest-host polymer composites.

Direct electron beam writing of channel waveguides in nonlinear optical organic crystals.

We report on optical channel waveguiding in an organic crystalline waveguide produced by direct electron beam patterning. The refractive index profile as a function of the applied electron fluence has been determined by a reflection scan method in the nonlinear optical organic crystal 4-N, N-dimethylamino-4'-N'-methyl-stilbazolium tosylate (DAST). A maximal refractive index reduction of Deltan1 = -0.