IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ

Terahertz focal plane arrays for astrophysics and remote sensing

Christopher Groppi Arizona State University

School of Earth and Space Exploration

IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ

Emission at 115 GHz from the CO molecule was first detected in 1970. Astronomers have been imaging THz light ever since.

IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ

This is what we can do today: a 120 square degree image of 13CO(1-0) at 110 GHz. Beamsize is ~1 arcminute in this image.

Every pixel is an integrated high resolution spectrum.

Full Moon

IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ

We can do the same trick looking at the thermal emission from small, solid particles (“dust”). Dust emits a blackbody spectrum,

peaking at THz frequencies at typical molecular cloud temperatures.

350 GHz continuum

SWIR image 1.2, 1.6, 2.1 microns

IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ

Blackbody emission from dust

Dust is 1% by mass of a molecular cloud, but is easy to detect and provides information about the cloud temperature and structure.

IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ

How do molecules emit light?

Blue / Higher Frequency Red / Lower Frequency

• Molecular clouds are 99% molecular gas, mostly H2.

• We can’t see H2 (no dipole moment) so we look at other, less abundant molecules instead (like carbon monoxide).

IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ

How do we make images? • Until recently, virtually all telescopes were

equipped with single pixel THz detectors.

• Images are made using the “On The Fly” Mapping technique. • Antenna is raster scanned across the source at a fixed

angular rate. • Receiver is read out rapidly (several Hz). • Lots of short integrations at closely spaced intervals are

convolved with a Gaussian kernel the size of the beam on a Nyquist sampled grid.

• Much faster than point by point mapping, since multiple spatial pixels share the same reference.

IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ

Tradeoff between spatial resolution and mapping speed. • Large antennas have smaller beams, resulting in better spatial

resolution in the final image.

• This also results in more pixels in an image of a given angular size. Bigger telescopes map a given area slower.

• If you want your cake and eat it too (wide areas AND high spatial resolution) you need multiple spatial pixels.

IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ

This is what we can do today: a 120 square degree image of 13CO(1-0) at 110 GHz. Beamsize is ~1 arcminute in this image.

Every pixel is an integrated high resolution spectrum.

Full Moon

IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ

Coherent array receivers • SEQUOIA was built for the 15m FCRAO antenna in

Massachusetts. • Operates from 85-115 GHz. • 32 cryogenic HEMT amplifier based pixels (16 in each linear

polarization).

IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ

PoleStar array for AST/RO

•810 GHz

•4 Pixel SIS superconducting mixers

•Solid state local oscillator source (~0.3 mW)

•L-band IF 1-2 GHz

•Trec~550-650 K

•4 channel array AOS backend spectrometer

IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ

2 8 channel downconverter modules

Omnisys Spectrometer 64x250 MHz complete system

Prototype 8 channel bias system (1 6U card with power supplies)

Spectrometer and bias control computer

LO System with 8 way power divider LO Optics LO Beamsplitter & dewar window CTI 350 cooler Sumitomo 4K cooler

Supercam System

IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ

Magnet DC connector

Bias DC connector Gilbert GPPO blind mate IF connectors

1x8 Mixer Module

Electromagnets

Horn Extension Block

IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ

Ground beamlead

SOI SIS Chip

Beamlead alignment tabs

IF Beamlead

Magnet probes Input matching network WBA13 MMIC chip

Bias circuitry

Output coax

A Closer Look

IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ

Low Noise Amplifiers

N. Wadefalk, J. Kooi, H. Mani & S. Weinreb,

Caltech

32 dB Gain, 5 K Noise at 8mW power dissipation

IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ

• 1x8 Downconverter Module (Caltech: G. Jones and J. Bardin)

• Total power metering

• 250 MHz and 500 MHz bandwidth modes (1 GHz with filter change)

• Digital attenuators

• Low cost surface mount components

Supercam IF Processing

IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ

• Real-Time FFT system • Virtex 4 SX55 FPGA • 4x 500 MHz or 2x 1 GHz per board • 1024 channels • power consumption 25W per board • Ethernet interface • SuperCam spectrometer initially uses 8

identical boards for 64 x 250 MHz operation

• Allan time (with IF processor) ~250 s

Supercam Spectrometer (Omnisys)

IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ

LO Multiplexing 64-Way Waveguide Power Divider

IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ

CNC Metal Micromachining

350 GHz TWT

650 GHz Sideband Separating Mixer

IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ

Technological Challenges and Proposed Solutions for Even Larger Coherent Arrays

1. Mechanical and Electrical Complexity Solution: Use 2D integration to simplify design

2. Detector yield and focal plane assembly Solution: Use simple, robust SIS device design with self aligning SOI chips

3. Heat load from LNAs becomes dominant with large pixel count Solution: Develop integrated ultra-low power dissipation LNAs

4. Economical and fast WG and feed fabrication Solution: use drillable feeds (e.g. Leech et al, this conference), CNC micromachining for WG.

5. RF and DC interconnects and wire count Solution: Develop multi-conductor cryo-ribbon interconnects

6. Magnetic field for SIS devices Solution: Use engineered permanent magnets to replace individually adjustable electromagnets

IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ

KAPPA FPU Concept

SiGe MMIC Bonding Pad

Waveguide

Via to surface mount G3PO

connector

6 mm

IF Microstrip

IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ

KAPPA SiGe LNA 500 um

800 um

SiGe MMIC Chip

• 0.5-4.5 GHz • 16 dB Gain • 7K noise temperature (predicted)

9K (measured) • 2 mW power dissipation

• |S11|<-11 dB • On-chip bias tee • Small chip size • 2 stage version also fabricated

(32 dB gain, 4-5 mW power dissipation)

IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ

LNA Integration

IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ

IF Flex Circuit • 16 channel stripline

flex circuit with microstrip terminations.

• 1m circuit has ~4 dB (simulated) loss at 4.5 GHz.

• Heat load with 9 micron thick copper conductors: ~5 mW (heat sunk at 15K, 150mm length.

• Heat load could be further reduced with cryogenically compatible metal replacing copper (i.e. phosphor-bronze).

10mm wide

Rogers Ultralam 3000 flexible LCP dielectric: 200um total thickness

18mm wide Stripline to microstrip transition