For the last forty years, manufacturing technology for diffraction gratings has not changed significantly. Mechanical ruling and interferometric (holographic) exposure have been the two predominant approaches used to fabricate gratings. Both approaches provide limited freedom in terms of the complexity of grating lines (spacings and curvatures) that can be written.
On the other hand, it is well known that gratings can be designed to provide more advanced functionality such as imaging/focusing etc. in addition to simple spectral dispersion, yet such functionality requires complex line shapes that are difficult or even impossible to realize by traditional fabrication means. Deep-ultraviolet (DUV) reduction photolithography, the workhorse fabrication tool of the semiconductor industry, provides nanopatterning capability with feature sizes below 100 nm and control of feature placement on the scale of nanometers (yielding high spatial coherence) throughout a field spanning nearly ten square centimeters. For gratings, today’s typical DUV production optical stepper allows one to address and design more than 1011 pixels on an individual basis, enabling truly arbitrary patterning at the subwavelength-level. The ability to tailor grating lines arbitrarily and monolithically integrate multiple gratings on a single substrate using the DUV approach makes it possible to integrate new functions into diffraction gratings in an unprecedented manner as is described here.
The left side of Figure 2 is a photograph of a monolithic, single-substrate, silicon grating array fabricated via DUV photoreduction lithography that provides instantaneous high-resolution access to optical bandwidths that substantially exceed that of a single grating. First, the new grating eliminates the need for moving parts. As detailed below, monolithic grating arrays are consistent with single shot data acquisition for many broadband applications (e.g. laser-induced breakdown spectroscopy) and can help reduce system component numbers dramatically.
Each array consists of multiple primary (probing) gratings on a single sub- strate that provide a broad aggregate and instantaneous bandwidth. A schematic of the array is shown on the right side of Figure 2. The array consists of four primary gratings (1 through 4), occupying most of the substrate, and six additional smaller reference gratings (A through E) that are located at top and bottom of the substrate. A noticeable feature of the primary gratings is that their grating lines exhibit a non-zero tilt with respect to the substrate vertical, which is different for each grating. Tilting the grating lines provides critical function since the tilt rotates the grating’s dispersion plane so that gratings of different tilt produce dispersed outputs that are angularly, and thus vertically, displaced from each other (Note: Line tilts are not obvious in the photograph.)
To illustrate the operation of the monolithic grating array as compared to a regular diffraction grating consider Figure 3, where a stationary grating array replaces an ordinary rotating diffraction grating in a standard spectrometer. A two-dimensional detector array is being used for array output detection. Figure 4 shows a schematic of the grating array output, as would be detected for input light illuminating the entire array bandwidth and a HeNe calibration source. From top to bottom the diffracted outputs of the four primary gratings are shown, with typical spectral ranges indicated next to the detector. Primary grating outputs appear as spatially separate lines on the detector surface. Also shown as red dots are the reference marks generated by the reference gratings that allow calibration of the output field, generated in this case by the use of a HeNe laser.
The six small gratings at the top and bottom of the array (see Figure 2) provide calibration markers in the output field as well as assistance in system alignment. Their corresponding six calibration marks for HeNe illumination can be seen in Figure 5 schematically. The calibration marks provide two principal functions: For one, they indicate the beginning and endpoints of the spectral coverage provided by the larger gratings and allow the user to calibrate the wavelength as a function of position along each of the primary gratings’ dispersion lines (Note: The wavelength range denoted by the calibration spots is independent of the grating input angle, making the LightSmyth grating array versatile with respect to possible device layouts.) Second, the auxiliary gratings aid in system alignment. When the detector surface is properly positioned in the focal plane of the post-array focusing mirror, the two pairs of alignment marks are designed to coincide indicating proper far-field operation. The two center marks enable correct horizontal alignment of array and detector surface.