Blanca Ruiz

Generation of terahertz pulses through optical rectification in organic DAST crystals: theory and experiment

We present a combined theoretical and experimental investigation of the generation of few-cycle terahertz (THz) pulses via the nonlinear effect of optical rectification and of their coherent detection via electro-optic sampling. The effects of dispersive velocity matching, absorption of the optical and the THz waves, crystal thickness, pulse diameter, pump pulse duration, and two-photon absorption are discussed. The theoretical calculations are compared with the measured spectra of THz pulses that have been generated and detected in crystals of the highly nonlinear organic salt 4-N,N-dimethylamino-4′-N′-methyl stilbazolium tosylate (DAST). The results are found to be in agreement with the theory. By the selection of the optical pump wavelength between 700 and 1600 nm, we achieved several maxima of the overall generation and detection efficiency in the spectral range between 0.4 and 6.7 THz, with an optimum at 2 THz generated with 1500 nm laser pulses.

Synthesis and crystal structure of a new stilbazolium salt with large second-order optical nonlinearity

The novel stilbazolium salt 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium 2-naphthalenesulfonate (DSNS), has been synthesized and small single crystals were grown. The crystal structure of the resulting crystals was determined via X-ray crystallography and it was found to be non-centrosymmetric with the chromophores aligned perfectly parallel. The Kurtz powder test of the crystals at λ = 1.9 µm reveals that they have around 50 percent higher second-order nonlinearity than the presently best organic nonlinear optical crystal 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate (DAST).

Polymorphism, crystal growth and characterization of an organic nonlinear optical material: DAPSH

We have investigated the crystal growth, structural and physical properties of the organic nonlinear optical salt trans-4′-(dimethylamino)-N-phenyl-4-stilbazolium hexafluorophosphate (DAPSH). The chromophore in this recently created material shows a first molecular hyperpolarizability considerably larger than that of the related cation in the well-studied salt DAST (trans-4′-(dimethylamino)-N-methyl-4-stilbazolium tosylate). Through recrystallization of DAPSH using various solvents and conditions we have obtained three different crystalline phases. Phase a of DAPSH has a large second-order nonlinear optical susceptibility χ2. It crystallizes from acetone solutions and grows as purple needles, which are non-centrosymmetric with the space group Cc being suitable for electro-optic applications. Phase b with the triclinic non-polar space group P appears as reddish-brown thin plates when grown from DMSO solutions. It shows large third-order nonlinear optical susceptibility χ3 and can be easily grown as thin films, making it a good candidate for all-optical switching applications. Phase c with the non-polar space group P21/m grows as bright orange plates when crystallized from (50 : 50 vol%) ethanol–acetone solutions. This material shows no significant bulk nonlinear optical activity.

Organic electro-optic single crystalline films for integrated optics

We have fabricated organic electro-optic single crystalline thin films on various inorganic substrates. A high refractive index contrast of up to Δn = +0.6 at 1.55 μm with respect to glass substrates and up to Δn = -1.9 at 1.55 μm with respect to silicon substrates has been achieved. The single crystalline films can be grown quasi-epitaxially without lattice matching and also on amorphous substrates providing appropriate interface interactions and solid-liquid phase equilibrium conditions. The thickness of the single-crystalline films can vary between less than 30 nm and above 5000 nm; they are therefore appropriate for optical waveguiding structures, as well as nano-size electro-optic structures needed for future nanophotonics. Several organic electro-optic crystalline materials have been employed using solution or melt-based processing. The techniques are suitable for the fabrication of conventional wire electro-optic waveguides, silicon-organic hybrid electro-optic waveguides, as well as more complex organic-inorganic structures such as single-crystalline electro-optic microring resonators.

Terahertz emission in organic crystals pumped by conventional laser wavelength

Intense Terahertz radiation in organic crystals is typically generated by optical rectification of short wavelength infrared femtosecond lasers between 1.3 and 1.5 μm. In this wavelength range high energy ultrashort pump sources are hardly available. Here we present results on powerful THz generation by using DAST and DSTMS pumped directly by the widely used and well-established Ti:sapphire laser technology, emitting at 0.8 μm. This approach enables straightforward THz generation by optical rectification. We present systematic studies on nIR-to-THz conversion efficiency, damage threshold, and on the emitted THz spectrum and field strength.

New Organic Salts for Electro-optics and THz Generation

Organic materials with high second order optical nonlinearities have attracted a lot of attention because of their potential for optical frequency conversion, THz generation, electro-optical and integrated optics applications. However, the theoretical limits for the macroscopic second-order nonlinearities in organic crystals are still far from being achieved. In addition, the growth of high quality nonlinear optical bulk or thin film organic crystals such as DAST remains a big challenge.