We report an approach to build a tunable spectral imaging system operating in the long-wave infrared (λ=8-12 μm). In addition to broadband images, the imaging systems allows for tunable selection of particular passbands. The tunable passband selection allows the imaging system to focus on the infrared signatures (including both thermal emission signature related to object’s temperature and characteristic absorption signature) of object of interest. This is achieved by incorporating a fast tunable spectral filter comprising dispersive magneto-optic (MO) materials in the imaging system. The MO materials in the spectral filter are sandwiched between polarizers, and dispersion in the Faraday Effect rotates the polarization of different wavelengths of light by different amounts, which results in a wavelength-dependent attenuation by the subsequent polarizer. Multiple stages with different thicknesses of the MO material can be stacked (Lyot configuration) to further narrow down the transmission peak. The central wavelength of the filter can be tuned either by tuning the magnetic field B along the direction of the optical axis, or by rotating the polarizers while applying a fixed magnetic field. In the long-wave infrared, n-doped InSb proves to be a prominent MO material. We present detailed measurements of Verdet constant, absorption coefficient and calculated figure of merit (FOM) for a range of carrier concentrations near 1017 cm-3 in the long-wave infrared. A prototype tunable bandpass filter is constructed and demonstrated.
On-chip optical isolators constitute an essential building block for photonic integrated circuits. Monolithic magnetooptical isolators on silicon, while featuring unique benefits such as scalable integration and processing, fully passive operation, large dynamic range, and simple device architecture, had been limited by their far inferior performances compared to their bulk counterparts. Here we discuss our recent work combining garnet material development and isolator device design innovation, which leads to a monolithic optical isolator with an unprecedented low insertion loss of 3 dB and an isolation ratio up to 40 dB. To further overcome the bandwidth and polarization limitations, we demonstrated broadband optical isolators capable of operating for both TM and TE modes. These results open up exciting opportunities for scalable integration of nonreciprocal optical devices with chip-scale photonic circuits.