Transcript Slide 1
Silicon Diffractive Optics for Infrared Spectroscopy Dan Jaffe Why Infrared? Why Spectroscopy? Tools of the trade How to make a better mousetrap out of silicon Infrared lets you see through dust into star forming regions. It also gives you access to physical processes you cannot study in the visible. Diffraction gratings bend light like prisms, but through a greater angle for a given wavelength separation. This larger dispersion allows you to resolve more closely spaced spectral features. Grating Equation: ml=d(sina+sinb) The grating equation gives allowable directions. The blaze tells you how much power goes to which. Key Science for High Resolution Near-IR Spectroscopy: Star Formation Probe the kinematics, densities, and chemical properties of unresolved disks. Molecular lines at 3-5 mm Pre-Main Sequence Stellar Astrophysics •Rotational Properties (vsini) •Magnetic Field Strength (Zeeman splitting) •Accretion Rates (Brackett Line Profiles) •Cluster Kinematics (radial velocities) •Fundamental Parameters (Teff, log-g, abundances) •Courtesy K. Covey (U. Washington/CfA) Enable planet searches around low mass stars and brown dwarfs. Sensitivity for cooler, low mass stars is much better than in the visible. Put color argument here Determine the chemical evolution of the Galaxy. Study isotopic ratios such as 18O/16O The problem is that everything radiates in the infrared and so your entire spectrograph must be cooled to almost absolute zero. Immersion Gratings are the Key to the New Spectrographs An immersion grating is a grating in which grooves are immersed in a medium with an index of refraction n. ml nG (sin a sin b ) Rmax 2nG L sin l d b 2nG tan b dl l An immersion grating gains you a factor of n2-n3 in spectrograph volume. Idea dates back to Fraunhofer (1822). Reinvented in 1954 by Hulthén and Neuhaus Patented in 1984 by Sica. Never made to work because of production difficulties. How precise does your grating have to be? Assuming random errors in groove spacing, allowable wave front RMS error is 2 2 n exp 2 RMS 0 l RMS RMS / sin rms=25nm We produce immersion gratings by a process of photolithography and chemical micromachining. Step 1. Purchase a boule of high purity material Step 2. Cut the boule into disks Material needs to be oriented to achieve various blaze angles 6.16o 54.7o 63.4o Production Step 3. Polish disks using chemical – mechanical polish (CMP). Step 4. Deposit passivation layer. The passivation layer will be patterned into an etching mask which defines the grating period. Production Step 5. Deposit photoresist. •Spin-on photoresist at 3500 rpm Photoresist layer Passivation layer Silicon substrate Production Step 6. UV exposure through contact photolithography mask (contact is a critical issue) Step 7. Develop exposed photoresist We have the image of the mask in photoresist: Production Step 8. Etch the passivation layer: Si3N4 is etched via reactive ion etching (RIE). + + + + + Substrate Production Step 9. Photoresist removal Positive image of grating mask pattern in the passivation layer. Production Step 10. Anisotropic silicon etch in a KOH solution Si + 2OH- + 2H2O SiO2(OH)2-- + 2H2 Production The exposed (111) crystal planes are smooth on an atomic scale. Production Step 11. Remove the remaining passivation layer: Remaining Si3N4 is etched in concentrated phosphoric acid at ~150oC. Production Step 12. Cut the disk into a prism Production Step 13: anti-reflective coating on the entrance face and reflective coating on the groove surfaces Si Immersion Grating Production Grating etched into silicon puck Puck cut into prism and then polished 36 mm elliptical C.A. corresponds to a 24 mm circular beam Flat entrance face antireflection coated Device completed by aluminizing the grooves along the hypotenuse Note: Only the bottom of the coating matters Evaluation Combination of tests: • efficiency measurements • interferometric measurements • point-spread function (PSF) measurements • analysis of grating defects and aberrations (ghosts, scattered light) Tests give us consistent results on grating performance and help us analyze the sources of errors. Evaluation Sample 1D monochromatic spectra Evaluation Interferometric tests done at 632.8 nm. Evaluation Point spread function measurements also test diffraction limited performance. • Compare to optical performance of a flat mirror • Analyze errors (ghosts) and compare to periodic errors observed with interferometric tests • Determine resolving power from FWHM Demonstrated resolving power of up to 75,000 with G1 Diffuse scattered light is caused by surface microroughness and various macro defects. Atomic force microscopy of a 5 mm by 5 mm area of G2. RMS roughness is 1.6 nm. Current grooves are 80 mm wide and 40 mm long. If this were a 1m wide sidewalk, it would be 0.5 km long. The bump in the picture would be 60 mm. Actual Flight Hardware for SOFIA Planned Instrument for NASA IRTF GMTNIRS Planned instrument for the Giant Magellan Telescope