Most of the research in the field of photonics has focused on understanding and mitigating the effects of disorder which are often detrimental. For certain applications, however, intentionally introducing disorder can actually improve the device performance, e.g., in photovoltaics optical scattering improves the efficiency of light harvesting. We have utilized multiple scattering in a random photonic structure to build a compact on-chip spectrometer. The probe signal diffuses through a scattering medium generating wavelength-dependent speckle patterns which can be used to recover the input spectrum after calibration. Multiple scattering increases the optical pathlength by folding the paths in a confined geometry, enhancing the spectral decorrelation of speckle patterns and thus increasing the spectral resolution. By designing and fabricating the spectrometer on a silicon wafer, we efficiently channel the scattered light to the detectors, minimizing the reflection loss. We demonstrate the wavelength resolution of 0.75 nm at the center wavelength of 1500 nm in a 25 μm by 50 μm random structure. Furthermore, the phenomenal control afforded by semiconductor nanofabrication technology enables engineering of the disorder to reduce the out-of-plane scattering loss. Such a compact, high-resolution spectrometer that is integrated on a silicon chip and robust against the fabrication imperfections is well suited for lab-on-a-chip spectroscopy applications.
Birck Technology Center, Room 2001, Purdue University, West Lafayette, IN