Engineered optical materials combined with new linear (waveguides, gratings, couplers, etc.) and nonlinear (nested waveguides, 1-D and 2-D periodic poling, parametric oscillators, etc.) elements have begun to dramatically broaden the optical functions available to both the researcher and the photonic circuit designer.

All-optical wavelength interchange

Much like their linear counterparts, photonic crystals, patterned periodic and aperiodic nonlinear materials give rise to a new class of optical functions. Our nonlinear photonic crystal, patterned in LiNbO₃, provides one of the first practical demonstrations of this capability. This lattice has a unique spatial-spectral characteristic that uses a two-dimensional phase matching to produce simultaneous wavelength interchange of data between two optical carriers at different wavelengths.

Guided Wave Entangled Photon Source

A new generation of optical communication – which truly deserves the name photonics – is beginning to emerge. One manifestation is the entanglement of photons, the non-classical correlation between separate quantum mechanical systems. A number of intriguing functions which blur the line between communications and computation are beginning to emerge, including quantum teleportation (transferring the quantum state of one photon to another), entanglement swapping (teleportation of an entangled state), quantum dense coding, and quantum cryptography (in which two remote parties generate a common secret key which is immune to eavesdropping).

Entangled photon pairs can be produced by the process of spontaneous parametric down conversion (ϖ₃ → ϖ₁ + ϖ₂) in 2nd order nonlinear materials such as LiNbO₃. We are exploring the use of new waveguide geometries and 2-D nonlinear photonic lattices to phase match and spatially separate entangled daughter photons. In addition, guided wave optics provides a platform on which to integrate the linear optical operations needed for entanglement applications, including coherent coupling (beam splitting), spectral filtering, and multi-GHz modulation of amplitude, phase, and polarization control.

Guided Wave Structures for THz Generation

While the Far-IR (THz) is nominally defined from ~ 0.3 – 10 THz (1 mm > λ > 30 μm), the frequency range above 1 THz provides a unique spectroscopic probe of a molecular environment by way of inter-molecular interactions. Here, changes in the rotational, breathing, and vibrational modes of molecular subunits of larger molecular moieties provide an identifying signature of the larger molecular host. Spectroscopic access can be obtained by way of conventional absorption measurements and by nonlinear optical resonances. A combination of the two, sometimes referred to as coherence spectroscopy, probes the interaction of molecular subunits. For example, the interaction between the carboxyl and amide groups (C–O ⇔ N–H) is a key element in protein's secondary structure (e.g., α-helix), and can be thought of as a nonlinear (χ⁽³⁾ ) process, resonant in the far infrared (THz frequencies), and therefore accessible by nonlinear spectroscopy such as four wave mixing with a coherent, tunable, THz source as one pump component. THz spectroscopy would provide a new window to hydrogen-bonding resonance dynamics of molecules with liquid water, as well as polypeptide secondary and tertiary structures in general. The output from such nonlinear spectroscopy (intensities, or better, amplitudes and phases) can be displayed in n-dimensions (multi-dimensional coherence spectroscopy), unfolding closely related molecular phases and the relative signs of their transition dipole moments.

Such a spectroscopic system requires that the far-infrared (THz) source be tunable and have sub-MHz linewidth. Unfortunately, there is usually a trade-off between available optical power and linewidth in THz sources, making most impractical for high-resolution spectroscopy. By making use of a co-axial guided wave structure fabricated in LiNbO₃, we have been able to produce THz light with ~ 2kHz linewidth at a conversion efficiency some 25 times larger than the largest previously reported value. More recent calculations show that an additional 80-fold increase in conversion efficiency can be achieved in a an AlGaAs structure. Unlike the quantum cascade laser, this source operates at room temperature and is broadly tunable.