  Integrated mid-infrared micro-spectrometers have a great potential for
applications in environmental monitoring and space exploration.
Silicon-on-insulator (SOI) is a promising platform to tackle this integration
challenge, due to its unique capability for large volume and low-cost
production of ultra-compact photonic circuits. However, the use of SOI in the
mid-infrared is restricted by the strong absorption of the buried oxide layer
for wavelengths beyond 4 {\mu}m. Here, we overcome this limitation by utilizing
metamaterial-cladded suspended silicon waveguides to implement a spatial
heterodyne Fourier-transform (SHFT) spectrometer operating near 5.5{\mu}m
wavelength. The metamaterial-cladded geometry allows removal of the buried
oxide layer, yielding measured propagation loss below 2 dB/cm between 5.3{\mu}m
and 5.7{\mu}m wavelengths. The SHFT spectrometer comprises 19 Mach-Zehnder
interferometers with a maximum arm length imbalance of 200 {\mu}m, achieving a
measured spectral resolution of 13cm-1 and a free-spectral range of 100 cm-1
near 5.5{\mu}m wavelength.
