Researchers in Switzerland have designed and tested a new nonlinear material that is both easy to grow and has a large phase-matchable nonlinear optical coefficient of about 29 pm/V—its figure of merit is 16 times that of potassium niobate (KNbO3). As well as being highly nonlinear, crystals of 5-(methylthio)-thiophenecarboxaldehyde-4 nitrophenyl-hydrazone (MTTNPH) exhibit good thermal stability and high optical quality. Researchers say they can be used as efficient optical parametric oscillators (OPOs) at infrared wavelengths when pumped with, for example, Ti:sapphire lasers.
The new material was developed by the organic materials group of the Nonlinear Optics Laboratory at the Swiss Federal Institute of Technology Institute of Quantum Electronics in Zurich. Among other projects, the group has been involved in growing the largest-ever crystals of DASTTM, a nonlinear material now being commercialized by Molecular OptoElectronics Corp. (Watervliet, NY) and Rainbow Photonics (Zurich).
Optimizing properties
When designing nonlinear materials, researchers must optimize two properties—the nonlinearity of the individual dipoles in the material and the nonlinearity of the crystal as a whole. This involves balancing the electronic response of the molecule, its chemical structure, and the properties of the crystal that it forms.
In particular, three characteristics have been designed into MTTNPH to make it suitable for nonlinear applications.1 First, each molecule consists of two chromophoric parts (C6H6NS2 and C6H5N2O2) that are connected head to tail. These two sections react very differently to an applied electric field, resulting in a push-pull effect that provides one source of nonlinearity. Second, charge transferred during an electronic interaction travels down the entire length of the molecule from donor to acceptor. The fact that this process is extended and that the molecule is so long increase the nonlinear reaction of the molecule to an applied electric field. The molecule has many electrons that are actively involved in the interaction, and this adds to the complexity of the system.
Both of these properties add to the nonlinearity of the individual dipoles within the MTTNPH crystal, but the shape of the molecule also affects the macroscopic behavior of the material. Unusually, the MTTNPH molecule is bent, which has two useful byproducts. The first is that the two chromophores—because they are at different angles to the incoming light—have different effects on its polarization. Second, the bend angle is variable, which allows the molecule to adopt a crystalline structure in the first place: more-rigid molecules are unable to conform to the crystal lattice. Even better, the bent shape also means that the resulting crystal is not centrosymmetric, thus adding another source of nonlinearity.
Growing these crystals involves a two-step process. First, the MTTNPH is dissolved in acetonitrile solvent, which is then allowed to slowly evaporate. This produces a number of small plate-shaped crystals, the best of which are prepared for use as seeds. These can then be grown further in a 40°C saturated solution of MTTNPH that is allowed to cool slowly. Polarized microscopy shows that, when this process is carefully controlled, the crystals have good optical quality.
The researchers have calculated the conditions that would allow the crystals to operate as phase-matched OPOs, basing the calculations on a pump laser with a wavelength of approximately 800 nm—compatible with Ti:sapphire lasers or low-power laser diodes. The results suggest that, by changing the angle of the incoming pump beam, the OPO output should be tunable from 1000 to 2400 nm.
REFERENCE
1. F. Pan et al., Appl. Phys. Lett. 71(15), (13 Oct. 1997).
Sunny Bains | Contributing Editor
Sunny Bains is a contributing editor for Laser Focus World and a technical journalist based in London, England.