Amirhossein Nateghi1 Jieun Myung1 Tae Yoon Jeon1 Axel Scherer1

1, California Institute of Technology, Pasadena, California, United States

Mid-IR radiation covers light in the spectral region of 4000–400 cm−1 or 2-20 um which can be used to provide information on molecular structure as well as to gain insight into a molecule’s local environment. Here, we present design and experimental demonstration of a sample collection and interrogation geometry that enables the spectroscopic measurement of materials in the mid-IR region. In this spectral region, silicon is relatively transparent, and a cuvette consisting of two silicon wafers bonded together can be made of this material. Such cuvettes however suffer from progressively higher reflectivity, resulting in low transmitted light. Although different anti-reflection methods can be used over small wavelength ranges, only few approaches work throughout the broad spectral regions needed for spectroscopy. Here we show a promising technique by micro-fabricating a geometric anti-reflection structure analogous to the moth-eye on silicon. This anti-reflection geometry can drastically reduce the reflectance from a silicon surface from ~30% to ~1% . We have built and characterized antireflection microstructures and have shown how our geometry can be used for sample collection on a surface of patterned silicon. Low reflectivity is measured by incorporating two moth-eye structures on both sides of a silicon wafer. This new method can be easily implemented to produce high transmission efficiencies through high-reflectivity materials over a large wavelength range and still obtain large surface areas both of which are necessary for spectroscopy.
The surface of the microfabricated moth-eye structure can collect sample within the crevices between the microfabricated pillars through surface tension. We can also geometrically filter materials on this surface by excluding particles, cells or other materials with sizes larger than the crevices formed between adjacent pillars, and thus the microfabricated anti-reflection geometry can function as a size-filter. If further coated with antibodies, aptamers or hybridization binding chemistries, this structure will be able to collect specific analytes from a complex solution for quantification and can be used not only to filter, but also to pre-concentrate materials for analysis. This is desirable when small concentrations of analytes are to be quantified from a complex solution, such as, for example, insulin in blood, urine, and sputum.