Transcript THz_Flygare_OSU.ppt

Extending the principles of the Flygare:
Towards a FT-THz spectrometer
Rogier Braakman
Chemistry & Chemical Engineering
California Institute of Technology
Geoffrey A. Blake
Matthew J. Kelley
Kevin Cossel
Geological and Planetary Science
California Institute of Technology
Chemistry & Chemical Engineering
California Institute of Technology
Outline
• Motivation
• Overview proposed spectrometer
• Scaling from MW to THz
– Sensitivity: crucial part
• Different aspects of proposed setup
THz spectroscopy & astronomy
• Domain of “soft” interactions: rotations of small
molecules, torsions of large molecules, hydrogen
tunneling, etc.
• THz Astronomy: The next step in search for
complex molecules. Should decrease detection
limits and move away from Boltzman peak and
congested spectra of microwave region.
• Urgent need for spectral characterization of new
target molecules and known interstellar species
before commissioning of Herschel !!
Basics of Flygare
Signal
Gas, sample
MW pulse
Confocal cavity setup problematic at THz:
•  factor ~100 smaller  Antenna / Nozzle disrupt cavity
• Much lower power, want maximal coupling
“THz Flygare”
Molecular nozzle
mirrors
Wire grid polarizer (R~99%)
THz photomixer
e.g. P,  meter
1.5 m Er doped
fiber Amplifier
50:50 beam splitters
Heterodyne HEB
THz mixer
3 dB coupler
Mirror (R>99.99%)
mirror
Amplifier, filters
Fixed tuned
Tunable 1.55 m
1.55 m DFB laser Agilent laser
Cavity & Q-factor
Beam splitter
Polarizer
InSb
Bolometer
Frequency multiplier chain
Mirror
QL= /
E ~ P·Q
• E and  both depend on Q!
• Flygare: Q=104 at 10 GHz
• Possible to scale in  and maintain high Q?
• Test cavity at 300 GHz (setup above)
Cavity spectrum
 = 4 MHz  QL ~ 7.5 x 104
Possible to maintain high Q with semi-confocal
cavity at THz frequencies!
THz photomixers
• Difference frequency generation
• ErAs/InGaAs material (UCSB),
bandgap at 1.55 m
• THz output: ~0.1 W  much
lower than Flygare source!
Advantage: Tunability
Disadvantage: Low power
Stabilizing lasers
DFB:
Agilent:
T, I controller
Lock-in amplifier
HDO
DFB
EOM
• +/- LIA signal feedback to T controller
• Very close to working
• Possible to follow up with lamb dip locking
• Lock to external cavity
• ‘Hop scanning’ between
cavity modes gives
tunability
Radiation sources & pulse length
Flygare:
• Phase-locked oscillator  linewidth < 1kHz
• Short pulses (~s) used to Fourier broaden
signal to match to cavity mode
THz ‘Flygare’:
• Signal broadened by FM modulating source
• Longer pulses (~ms) possible!
Detection: Yale Nb HEBs
• Superconducting device
• Electron-phonon cooled
• Bias (V) at Tc: max sensitivity
• In principle active throughout
THz, range limited by antenna
Results and implications
From M. Reese,
Prober group at Yale
• Response time ~1 ns 
bandwidth: 150 MHz
NEP (heterodyne):
• Able to cover cavity width
Tsys = h / 
• Potentially no switching
needed!
P = kTsys 
Tsys ~1000 K (Flygare: 100 K)
Molecular properties
Much stronger absorption and emission:
• Intrinsically: A ~ v3
• Extra increase for torsional transitions
Example:
OCS 1-0 at 12.163 GHz - H2O at 1113.342 GHz
A = 3.6 x 10-9 s-1
A = 1.8 x 10-2 s-1
Summary
• THz analog of Flygare proposed
• Possible to maintain high Q using semi-confocal cavity,
and to compensate low source power with longer pulses
as well as advantageous molecular properties in THz
• DFB laser stabilization routine and new HEB detectors
nearly functional
Next steps
• Characterize frequency response of detectors
• Remeasure beatnote after stabilization
• Work on THz generation w/ new photomixers (hopefully)
• Plenty more….
Acknowledgements
Caltech
Yale
Zmuidzinas group:
Dave Miller
Tasos Vayonakis
Frank Rice
Chip Sumner
Prober group:
Matt Reese
Daniel Santavicca
Blake group
Funding
NASA
NSF