Advances in diffractive nanophotonics enabled by ...

Invited Paper

Advances in diffractive nanophotonics enabled by commercial photoreduction lithography Christoph M. Greinera, Dmitri Iazikova, Thomas W. Mossberga a LightSmyth Technologies, Inc., 1720 Willow Creek Circle, Ste 520, Eugene, OR 97402 ABSTRACT We describe monolithic advanced-function diffraction grating arrays for instantaneous ultrawide spectral coverage and other uses that have inherent spectral and spatial self-calibration features. This new technology is made possible by recent advances in deep ultraviolet (DUV) reduction-lithographic fabrication. Keywords: Photoreduction lithography, wavelength calibration, monolithic grating arrays, binary gratings.

1. INTRODUCTION The technology for manufacturing diffraction gratings has remained essentially unchanged for over forty years. Diamond scribing with ruling engines and interference lithography (holographic fabrication) have been the dominant methods used to fabricate master gratings. Both approaches constrain the grating designer to relatively simple line patterns with the consequence that advanced function gratings depending on variable line spacing, general curvilinear line contour, and coherent arrays of dissimilar gratings have received limited attention. This situation is about to change. In the present article, we discuss some advanced features available through the use of monolithic coherent grating arrays produced using the tools of the semiconductor industry. Deep-ultraviolet (DUV) reduction photolithography, the workhorse tool of the semiconductor industry, provides nanopatterning capability with feature sizes below 100 nm and placement accuracy on the scale of nanometers (yielding high spatial coherence) throughout a field spanning nearly ten square centimeters [1-3]. For gratings, today’s typical DUV production optical stepper allows one to address and design more than one hundred billion pixels on an individual basis, enabling truly arbitrary patterning at the subwavelength level. Moreover, photolithographicallyfabricated gratings produced in volume can have very attractively low costs as masters and even lower costs through wafer level replication.

2. DIFFRACTION GRATINGS WITH INTEGRAL WAVELENGTH CALIBRATION FEATURES The ability to tailor the properties of individual diffractive lines arbitrarily via lithographic nanopatterning allows one to coherently integrate multiple gratings of different orientation on a single substrate with interferometric accuracy. Such a monolithic Silicon-substrate diffraction grating array is shown in Figure 1. The array consists of a single primary 1200 lines/mm grating and twelve auxiliary gratings (1 through 12) at the top of the substrate. The primary grating is used in traditional grating manner, but the 12 alignment/calibration gratings provide a family of wavelength calibration markers when illuminated by a monochromatic reference and viewed in the grating’s focal plane detection region. These markers allow highly precise and convenient wavelength calibration of the primary grating’s dispersed output. To illustrate this function consider Figure 2, where we schematically (top) depict the output signals of the primary grating and calibration array in the system’s detection (focal) plane. White light illumination is assumed. The solid black horizontal line is the primary’s dispersed output. The wavelength numbers above the horizontal line were obtained by the grating equation for the primary grating. The output marks of the calibration gratings are shown as red circles. These features are obtained by illuminating the calibration array with a HeHe laser co-propagating along the standard input direction. The marker spots

Advanced Fabrication Technologies for Micro/Nano Optics and Photonics II, edited by Thomas J. Suleski, Winston V. Schoenfeld, Jian Jim Wang, Proceedings of SPIE Vol. 7205, 72050L · © 2009 SPIE CCC code: 0277-786X/09/$18 · doi: 10.1117/12.811619 Proc. of SPIE Vol. 7205 72050L-1

Calibration Grating Array 1

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Fig. 1. Photograph (left) and schematic (right) of grating array consisting of primary 1200 lines/mm grating and auxiliary gratings (top) for integral wavelength calibration.

occur at precisely determined intervals on two horizontal lines parallel to the primary grating dispersion line. The wavelengths given below these horizontal lines indicate the wavelength that the primary grating diffracts to the location of the calibration marks. In combination with appropriate detection such as a 2D photo detector the calibration array thus allows one to fully calibrate the primary grating’s dispersion line for its entire spectral region with only a single source. Shown on the bottom of Figure 2 is a photograph of the output of a recently fabricated wavelength-calibrated diffraction grating for white light input and HeNe reference.

1200 (9)

900 nm 1100 (8)

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Figure 2. Top, detection plane output of primary grating (solid black line) and calibration array signals (red dots) for white light input and HeNe reference (incidence angle: 30°). Angles shown refer to the output direction signals travel relative to the substrate normal. Minus signs indicate that input and output are on the same side of the normal (-1st diffraction order). Numbers in parenthesis (bottom) indicate which calibration grating produces each signal.

Proc. of SPIE Vol. 7205 72050L-2

It is vital to note that, for a given reference source wavelength, the calibration marker wavelength values do not depend on the input angle to the grating. For different input angles, the detection plane pattern changes, but each marker point visible will always remain correlated with the same primary grating output wavelength. Other reference light sources may be employed as well, which enables one to change the calibration marker wavelength values. An immediately noticeable feature of the calibration gratings in the schematic of Figure 1 is that their grating lines exhibit a non-zero tilt with respect to the substrate vertical that is different for each grating. Tilting the grating lines provides critical function since the tilt rotates the calibration grating’s dispersion plane. Gratings of different tilt produce dispersed outputs that are angularly and thus vertically displaced from each other. The marker spots are positioned in the grating design via control of line tilt and line spacing.

3. WIDEBAND MONOLITHIC GRATING ARRAYS A different type of grating array is shown in Figure 3. Here, four primary gratings (1 - 4) designed to have slightly overlapping active spectral ranges occupy the single substrate. Auxiliary gratings (A through E) at the top and bottom of the substrate produce optical outputs used for wavelength calibration and system alignment.

I//IIIIIII//III/III//Iiiiii/

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Fig. 3. Photograph (left) and schematic (right) of monolithic grating array. 1-4, primary array gratings; A-E, auxiliary gratings provide output wavelength calibration and alignment assistance.

To illustrate the wideband operation afforded by this type of array consider the Czerny-Turner-type spectrometer of Figure 4 where the stationary array replaces the typically used single rotating grating. A twodimensional detector array is employed to record the grating array output. The inset to Figure 2 shows a schematic of the grating array output as seen on the 2D detector array when illuminated by white light and a co-propagating HeNe laser. Each near horizontal line comprises output of one of the four primary gratings and corresponds to the spectral range indicated. Monolithic grating arrays are substantially advantageous over traditional diffraction gratings in that they uniquely provide instantaneous and contiguous access to optical bandwidths multiple times that of a single grating but with the same spectral resolution. This feature makes grating arrays particularly suitable for spectroscopic applications where substantial optical bandwidths have to be acquired in a single shot or on time scales inconsistent with slow mechanical grating rotation. While the setup of Figure 2 simplifies the Czerny-Turner design by eliminating the rotating grating turret, further reduction of system component count can be achieved by designing the collimation and focusing function of the two spherical mirrors into the array gratings itself. Such array gratings that focus incident signal onto the 2D detector while spectrally dispersing it can be conveniently designed via the methods of computer-generated holography. The grating pattern is simply derived from the interference pattern computed between desired input and output beams and then realized via photolithography and etch [4]. Reduction in system components is desirable both in terms of cost as well as system footprint/compactness, ruggedness and insensitivity to perturbations in the instrument’s environment such as temperature and vibration.


Grating 1 Output 380 nm - 520 nm Grating 2 Output 510 nm - 670 nm Calibration marks

marks don’t overlap

Calibration marks

center marks indicate tilt

Grating 3 Output 680 nm - 935 nm Grating 4 Output 930 nm - 1270 nm

unaligned 2D Detector

marks overlap

LightSmyth Monolithic Grating Array

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aligned Fig. 4. Schematic illustrating operation of monolithic grating array in Czerny-Turner spectrometer with 2D detector and blow-up of array output signal on detector. The array provides instantaneous aggregate spectral coverage in excess of 1.5 octaves (380 - 1270 nm). Right side, illustration of spatial alignment via optical alignment markers.

4. ARRAY ALIGNMENT AND OUTPUT CALIBRATION FEATURES Six small gratings at the top and bottom of the array (see Figure 3) provide marker signals that, when mixing a reference source into the system input, allow one to spectrally calibrate the primaries’ output and also aid in system alignment. When the detector surface properly coincides with the focal plane of the post-array focusing mirror, two pairs of the six alignment marks are designed (via identical line tilt and line density) to produce overlapping output spots on the detector (see right side of Figure 4). Two center marks originating from the remaining two gratings (without line tilt) enable correct horizontal alignment of array and detector surface. At the same time, left and right reference marks indicate the beginning and endpoints of the spectral coverage provided by the primary gratings and allow the user to calibrate wavelength versus position along the primaries’ dispersion lines. As in Figure 2, the calibrated wavelength range is independent on the input angle. When combined with an appropriate feedback routine the present spectral calibration and alignment features comprise inherently intelligent references that allow for fully automatic grating-detector alignment, correction and spectral range calibration making gratings with reference/calibration structures ideally suitable for implementation in hostile or difficult-to-access environments where essentially autonomous system function is highly desirable.

5. INNER WORKINGS OF THE ARRAY GRATINGS Similar to the calibration gratings of Figure 1 the primary gratings of Figure 3 have a non-zero line tilt in order to displace their respective dispersion planes from each other as shown in the far field schematic of Figure 5 (solid black lines). The solid dot in the schematic is the extrapolated intersection point of all the dispersion planes and indicates the specular reflection (zeroth order) output common to all gratings. The primaries line densities are different and chosen so that the four gratings produce contiguous spectral coverage over approximately a factor of three in wavelength in the four separate output stripes within a predetermined output angular range. Shown at the bottom of Figure 5 is a photograph of the far field grating array output for white light input and a HeNe reference source. The region


photographed is bracketed by the bright red HeNe calibration marks shown both in the photograph and in the output schematic. Alternative reference wavelengths can be used to change the wavelength spans bracketed by the calibration output markers. λβ

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Fig. 5. Schematic (top) and photograph (bottom) of far-field output from monolithic grating array for white light input, HeNe reference and incidence angle of 20°. Solid black lines, primary gratings’ dispersion planes. Dashed lines, dispersed output from calibration gratings. Solid red dots, calibration reference marks indicating beginning and end of design spectral coverage of primaries. Angle conventions are the same as for Figure 2.

REFERENCES [1] [2] [3]

[4]

Greiner, C., Iazikov, D. and Mossberg, T. W., “Lithographically-fabricated planar holographic Bragg reflectors,” J. Lightwave Technol. 22, 136-145 (2004). Greiner, C., Iazikov, D. and Mossberg, T. W., “Integrated-holographic coarse-wavelength-division multiplexers patterned by DUV photolithography,” J. Lightwave Technol. 25, 146-150 (2007). Huang, Y.-K., Glesk, I., Greiner, C. M., Iazikov, D., Mossberg, T. W., Wang, T. and Prucnal, P. R., “Single integrated device for optical CDMA code processing in dual-code environment,” Opt. Express 15, 7327-7334 (2007). Greiner, C. M., Iazikov, D. and Mossberg, T. W., “Diffraction-limited performance of flat-substrate reflective imaging gratings patterned by DUV photolithography,” Opt. Express 14, 11952 – 11957 (2006).
