![[object Object],[object Object],[object Object],[object Object],[object Object],New Integrated Materials Approaches to Photonic Crystal-based THz Sources Thomas Kuech and Leon McCaughan, Univ. Wisconsin - Madison, DMR-0505775 Structures being developed by undergraduate researchers: Guided-wave/guided-pump tunable THZ source. Unlike conventional photonic crystal structures, the THz light is allowed to propagate through the crystal, taking advantage of its strong dispersive nature to provide tunable phase matching. Thinned LiNbO 3 Patterned Si photonic crystal](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
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New Integrated Materials for THz Sources
- 1. New Integrated Materials Approaches to Photonic Crystal-based THz Sources Thomas Kuech and Leon McCaughan, Univ. Wisconsin - Madison, DMR 0505775 Terahertz (THz) radiation is of great current interest for imaging science and technology. Applications include time-domain spectroscopy (e.g., the dielectric response of molecules in the far infrared), medical imaging in a new radiation band, detection and imaging for homeland security and defense applications, as well as communications. Calculations predict that a long channel waveguide buried in a LiNbO 3 /Al 2 O 3 ( or SiO 2 ) heterostructure would efficiently produce THz radiation at 1.5 THz. These structures are fabricated through application of materials integration techniques of wafer bonding and materials growth, yielding a new class of structures. The design and development of new processes to fabricate heterostructures in nonlinear optical materials opens the range of microstructures for optical communications and radiation sources. U.S. Patent Application, Bright, Tunable, CW Coherent THz Source , filed 12/28/06.
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- Aim of the project : The aim of this project is to develop and exploit entirely new classes of synthesis techniques and subsequently structures in optical materials based on lithium niobate. This is the workhorse of the optical communication community but has not benefited from new approaches in materials growth and processing. We are developing these processes and growth techniques. A key target is new heterostructures which can be used to emit radiation efficiently in the THz range. Research Results 1. Bulk LiNbO 3 possesses a very large optical nonlinearlity for producing coherent THz radiation by way of difference frequency generation (DFG) – but only if cooled to cryogenic temperatures to reduce re-absorption of the THz product. Our calculations demonstrate that a dielectric heterostructure consisting of a LiNbO 3 film sandwiched between low loss, low index cladding layer (Si, Al 2 O 3 , crystalline SiO 2 ) will produce a confined THz wave at room temperature. 2. Unlike conventional DFG, the THz product in our structure grows exponentially (vs. quadratic) with pump-THz interaction length. 3. A rotatable 2-D photonic crystal cladding of (100) Si permits a continuously tunable phase matching from ~0.5 to 1.5 THz. We believe this to be the first use of the strong dispersive nature of photonic crystals to provide tunable phase matching . 4. We recently fabricated a doubly-confining (pumps and THz product) SiO 2 /LiNbO 3 /SiO 2 heterostructure with Ti-diffused channel waveguides (for 1550 nm pumps) in an 11 mm LiNbO 3 film. THz characterization is underway. Significance of the results : Terahertz (THz) radiation is of great current interest for imaging science and technology. Interest is especially intense in the regions 0.3 to 10 THz, where few (if any) viable sources exist. The two major contenders under development are the quantum cascade laser (QCL) and frequency mixing via nonlinear optics. Temperature is a stumbling block for both. While the QCL has recently generated ~ 13 mW peak power at 3.5 THz in pulsed operation (0.03% duty cycle) at 5°K, emission is quenched above 100°K. Ferroelectrics offer large nonlinear optical coefficients for producing THz radiation by way of difference frequency generation (DFG) and optical rectification (OR). Because of their large absorption coefficient in the THz, the crystals must be cooled to cryogenic temperatures to reduce the temperature-broadened TO phonon absorption band and the optical pumps must be pulsed (to achieve sufficient peak power). Second order nonlinear materials without THz absorption bands (e.g., ZnGeP 2 ) require pulsed pumps to overcome their small nonlinear coefficients. All of these sources have no or limited tunability. Our goal was to design a LiNbO 3 film heterostructure for generating narrow band tunable room temperature THz. Future plans : Future work will continue in several major directions: We will further test these structures for their ability and efficacy in THz generation. We will use the integration of Si wafer-scale photonic crystals to provide tunability in these structures. We will continue to use new CVD source materials to provide alternative means to fabricate these materials and heterostructures broadening the range of oxides available for structure formation.