Dan Jaffe Silicon Diffractive Optics for Infrared Spectroscopy

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: m =d(sin  +sin  ) The grating equation gives allowable directions. The blaze tells you how much power goes to which.

Key Science for High Resolution Near-IR Spectroscopy: Probe the kinematics, densities, and chemical properties of unresolved disks. Molecular lines at 3-5  m Star Formation

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 18 O/ 16 O

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.

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. An immersion grating gains you a factor of n 2 -n 3 in spectrograph volume.

Assuming random errors in groove spacing, allowable wave front RMS error is  rms =25nm How precise does your grating have to be?

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 6.16 o 63.4 o 54.7 o We produce immersion gratings by a process of photolithography and chemical micromachining.

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 Silicon substrate Passivation layer Photoresist layer

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: Si 3 N 4 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 - + 2H 2 O  SiO 2 (OH) H 2

Production The exposed (111) crystal planes are smooth on an atomic scale.

Production Step 11. Remove the remaining passivation layer:Remaining Si 3 N 4 is etched in concentrated phosphoric acid at ~150 o C.

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 Flat entrance face antireflection coated Device completed by aluminizing the grooves along the hypotenuse Note: Only the bottom of the coating matters elliptical C.A. corresponds to a 24 mm circular beam 36 mm

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 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  m by 5  m area of G2. RMS roughness is 1.6 nm. Current grooves are 80  m 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  m.

Actual Flight Hardware for SOFIA

Planned Instrument for NASA IRTF

GMTNIRS

Planned instrument for the Giant Magellan Telescope