Extending the principles of the Flygare: Towards a FT-THz spectrometer Rogier Braakman Chemistry & Chemical Engineering California Institute of Technology Geoffrey A. Blake Geological and Planetary Science California Institute of Technology Matthew J. Kelley Kevin Cossel 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 Gas, sample Signal MW pulse Confocal cavity setup problematic at THz: factor ~100 smaller  Antenna / Nozzle disrupt cavity Much lower power, want maximal coupling

“THz Flygare” THz photomixer 3 dB coupler Tunable 1.55  m Agilent laser Fixed tuned 1.55  m DFB laser e.g. P, meter 1.5  m Er doped fiber Amplifier mirrors Wire grid polarizer (R~99%) Molecular nozzle 50:50 beam splitters mirror Heterodyne HEB THz mixer Amplifier, filters Mirror (R>99.99%)

Cavity & Q-factor InSb Bolometer Frequency multiplier chain Beam splitterPolarizer Mirror E and  both depend on Q! Flygare: Q=10 4 at 10 GHz Possible to scale in and maintain high Q? Test cavity at 300 GHz (setup above) Q L = /  E ~ P·Q

Cavity spectrum  = 4 MHz  Q L ~ 7.5 x 10 4 Possible to maintain high Q with semi-confocal cavity at THz frequencies!

THz photomixers Advantage: Tunability Disadvantage: Low power Difference frequency generation ErAs/InGaAs material (UCSB), bandgap at 1.55  m THz output: ~0.1  W  much lower than Flygare source!

Stabilizing lasers T, I controller HDO Lock-in amplifier DFB EOM DFB:Agilent: Lock to external cavity ‘Hop scanning’ between cavity modes gives tunability +/- LIA signal feedback to T controller Very close to working Possible to follow up with lamb dip locking

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 T c : max sensitivity In principle active throughout THz, range limited by antenna

Results and implications Response time ~1 ns  bandwidth: 150 MHz Able to cover cavity width Potentially no switching needed! From M. Reese, Prober group at Yale NEP (heterodyne): P = kT sys  T sys = h /  T sys ~1000 K (Flygare: 100 K)

Molecular properties Much stronger absorption and emission: Intrinsically: A ~ v 3 Extra increase for torsional transitions Example: OCS 1-0 at GHz - H 2 O at GHz A = 3.6 x s -1 A = 1.8 x s -1

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 Summary Characterize frequency response of detectors Remeasure beatnote after stabilization Work on THz generation w/ new photomixers (hopefully) Plenty more….

Acknowledgements Caltech Zmuidzinas group: Dave Miller Tasos Vayonakis Frank Rice Chip Sumner Blake group Yale Prober group: Matt Reese Daniel Santavicca Funding NASA NSF