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 10-4 and an average output power of up to 0.38 mW. We assess the thermal damage limit of the crystal and conclude a maximum fluence of ∼1.8 mJ·cm-2 at 10 MHz with a 1/e2 pump beam diameter of 0.10 mm. We compare the performance of HMQ-TMS with the prototypical inorganic crystal gallium phosphide (GaP), yielding a tenfold electric field increase with a peak on-axis field strength of 7 kV·cm-1 and almost double the THz bandwidth. Our results further demonstrate the suitability of organic crystals in combination with fibre lasers for repetition-rate scaling of broadband, high-power THz sources for time-domain spectroscopic applications.

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. The XPM occurs when the THz field is aligned along the polar axis of the OHQ-N2S. The results display a promising pathway for ultrafast control of the spectral and temporal properties of laser pulses using THz stimuli.

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 0.26% - about two orders of magnitude higher than in common inorganic crystals collinearly pumped by amplified femtosecond lasers. An open-aperture Z-scan measurement performed on an n-doped InGaAs thin film using such terahertz source shows a nonlinear increase in the terahertz transmission of about 2.2 times. Our findings demonstrate the potential of this terahertz generation scheme, based on ytterbium laser technology, as a simple and efficient alternative to the existing intense table-top terahertz sources. In particular, we show that it can be readily used to explore nonlinear effects at terahertz frequencies.

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. This paper gives an overview of THz gas detection analytical methods for environmental and biomedical applications, starting with a brief introduction to THz technology and an explanation of the interaction of THz radiation with gaseous species and the atmosphere. The review focuses on several gaseous species and groups of air pollutants that have been or can be analysed by THz spectrometry. The review concludes that different but complementary THz detection methods allow unique detection, identification and quantification of gaseous and particulate air pollutants with high selectivity, specificity and sensitivity. THz detection methods also allow further technological improvements and open new application possibilities.

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 (VvdW ), rather than by trial and error, are logically designed. When the VvdW of molecular ionic components varies, the corresponding crystal symmetry shows an observable trend: change from centrosymmetric to non-centrosymmetric and back to centrosymmetric. All non-centrosymmetric crystals exhibit an isomorphic P1 crystal structure with an excellent macroscopic second-order nonlinear optical response. Apart from the top-level macroscopic optical nonlinearity, new organic crystals introducing highly electronegative fluorinated substituents with strong secondary bonding ability show excellent performance in efficient and broadband THz wave generation, high crystal density, high thermal stability, and good bulk crystal growth ability.

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.

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 1.06 mum, we have achieved phase-conjugate reflectivities of up to 45% with very fast build-up times, about 15 ms at an intensity of 23 W/cm(2) using Te-doped Sn(2)P(2)S(6). We furthermore demonstrate aberration-free 5 W optical output of 8-ps pulses at 1.06 mum from a side pumped Nd:YVO(4) amplifier using the Sn(2)P(2)S(6)-based phase-conjugate feedback.

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. Compared to a 1.0-mm-thick ZnTe crystal as an inorganic benchmark, the 0.28-mm-thick benzothiazolium crystal yields a 19 times higher peak-to-peak THz electric field with a broader spectral bandwidth (>6.5 THz) when pumped at 1140 nm. The present work provides a valuable approach toward realizing organic crystals that can be pumped by near-infrared sources for efficient THz wave generation.

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. We measured the fastest formation of light induced waveguides in the visible up to now. The recording times are below 200 mus for intensities above 0.1 W/cm2. By interfering two light beams, dynamic waveguide arrays are generated with waveguide spacings of 7 microm. If an electric field is applied to the crystal, these arrays can be spatially shifted by 1.5 mum for an applied field of E(0) = 1 kV/cm.

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 1.7 times larger powder second harmonic generation efficiency than 3,4-DHNPH, which is attributed to their different intermolecular interactions involving phenolic OH groups. We show that the microscopic nonlinearity of the DHNPH molecules is particularly sensitive to variations in phenolic OH characteristics such as the orientation and intermolecular interactions.

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. By adjusting the center wavelength of the master laser to the center wavelength of an emitter with an accuracy of less than 0.1 nm, locking of any emitter of the laser diode bar is demonstrated. We improve the spectral width of the emitter from 0.5 nm to below 2.5 x 10(-4) nm.

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. In the waveguide core region the nonlinear optical susceptibility is shown to be preserved to more than 90% of its bulk value. A model which relates the molecular changes to the measured macroscopic alteration of the refractive index and the nonlinear coefficient has been introduced to quantify the fraction of molecules modified by ion implantation. Furthermore, a first electro-optic modulation in ion implanted DAST waveguides has been demonstrated.

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.

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. Numerical simulations of the refractive index evolution and beam propagation show a good agreement with the observations.

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). Due to the very favorable growth characteristics, large-area (> 150 mm(2)) single crystalline thin films with very good optical quality and thickness between 0.05-10 microm have been grown on amorphous glass substrates. We have developed and optimized optical lithography and reactive ion etching processes for the fabrication of wire optical waveguides with dimensions of w x h = 3.4 x 3.5 microm(2) and above. The technique is capable of producing low loss integrated optical waveguides having propagation losses of 2 dB/cm with a high refractive index contrast between core-cladding and core-substrate of delta n = 1.23 and 0.72, respectively at 980 nm. Electro-optic phase modulation in these waveguides has been demonstrated at 632.8 nm and 852 nm. Calculations show that with an optimized electrode configuration the half-wave voltage x length product V(pi) x L can be reduced from 8.4 Vcm, as obtained in our device, to 0.3 Vcm in the optimized case. This allows for the fabrication of sub-1 V half-wave voltage, organic electro-optic modulators with highly stable chromophore orientation.

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. The low-loss modulators (down to 0.6 dB per round trip), with a radius of 50 microm, were produced by micro-embossing and exhibit either highly asymmetric and steep Fano resonances with large 43-GHz modulation bandwidth or strong resonances with 11-dB extinction ratio. We show that Charon microresonators can lead to 1-V half wave voltage all-polymer micrometer-scale devices with larger tolerances to coupler fabrication limitations and wider modulation bandwidths than classical ring resonators.

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 2.5 THz for a pump laser wavelength range from 1200 to 1460 nm. OH1 crystals show a higher figure of merit for THz generation and detection in the optimized range compared to the benchmark inorganic semiconductor crystals ZnTe and GaAs and the organic ionic salt crystal 4-N,N-dimethylamino-4'-N'-methyl stilbazolium tosylate (DAST). The material shows a low THz absorption coefficient alpha3 in the range between 0.3 and 2.5 THz, reaching values lower than 0.2mm(-1) between 0.7 and 1.0 THz. This is similar as in ZnTe and GaAs, but much lower than in DAST in the respective optimum frequency range. A peak THz electric field of 100 kV/cm and a photon conversion efficiency of 11 percent have been achieved at a pump pulse energy of 45 microJ.

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 = -0.3 at a probing wave-length of 633 nm has been measured for an electron fluence of 2.6mC/cm(2). Furthermore, a new concept of direct channel waveguide patterning in bulk crystals is presented and waveguiding has been demonstrated in the produced structures by end-fire coupling. Mach-Zehnder modulators have been successfully realized and a first electro-optic modulation at a wavelength of lambda= 1.55mum has been demonstrated therein.

Ion implanted optical waveguides in nonlinear optical organic crystal.

We report for the first time to our knowledge optical waveguiding in an organic crystalline waveguide produced by ion implantation. Using H+ ions a refractive index barrier suitable for waveguiding has been realized in the highly nonlinear optical organic crystal 4-N, N-dimethylamino-4'-N'-methyl-stilbazolium tosylate (DAST). The refractive index changes in the waveguiding region as a function of the distance from the surface have been measured. Maximal refractive index changes of up to -0.2 and -0.1 at wavelengths of 633nm and 810nm have been realized, respectively. The waveguide refractive index profiles as a function of the ion fluence have been determined. Planar waveguiding has been demonstrated by polishing sharp edges and using conventional end-fire coupling. The measured losses are approximately 7 dB/cm at 1.57mum.

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. The benzothiazolium crystals consisting of new acentric core HMB (2-(4-hydroxy-3-methoxystyryl)-3-methylbenzo[d]thiazol-3-ium) exhibit extremely large macroscopic optical nonlinearity with optimal molecular ordering for maximizing the diagonal second-order nonlinearity. HMB-based single crystals prepared by simple cleaving method satisfy all required crystal characteristics for intense THz wave generation such as large crystal size with parallel surfaces, moderate thickness and high optical quality with large optical transparency range (580-1620 nm). Optical rectification of 35 fs pulses at the technologically very important wavelength of 800 nm in 0.26 mm thick HMB crystal leads to one order of magnitude higher THz wave generation efficiency with remarkably broader bandwidth compared to standard inorganic 0.5 mm thick ZnTe crystal. Therefore, newly developed HMB crystals introducing benzothiazolium with extremely large macroscopic optical nonlinearity are very promising materials for intense broadband THz wave generation and other nonlinear optical applications.