On-Chip Random Spectrometer

TL;DR

This paper presents a compact on-chip spectrometer utilizing multiple scattering in a random photonic structure to achieve high spectral resolution in a small footprint, suitable for lab-on-a-chip applications.

Contribution

The authors design and fabricate a high-resolution, silicon-based on-chip spectrometer leveraging engineered disorder and multiple scattering for enhanced spectral resolution.

Findings

Spectral resolution of 0.75 nm at 1500 nm wavelength.

Compact device size of 25 μm by 50 μm.

Reduced out-of-plane scattering loss through nanofabrication.

Abstract

Light scattering in disordered media has been studied extensively due to its prevalence in natural and artificial systems [1]. In the field of photonics most of the research has focused on understanding and mitigating the effects of scattering, 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 [2-5]. Here, we utilize 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 are able to efficiently channel the scattered light to the detectors, minimizing the reflection loss. We demonstrate spectral resolution of 0.75 nm at a wavelength of 1500 nm in a 25 {\mu}m by 50 {\mu}m random structure. Furthermore, the phenomenal control afforded by semiconductor nanofabrication technology enabled us to engineer 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 fabrication imperfections is well suited for lab-on-a-chip spectroscopy applications.