Extending the principles of the Flygare Towards a

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 d. B coupler Mirror (R>99. 99%) mirror Amplifier, filters Fixed tuned Tunable 1. 55 m DFB laser Agilent laser

Cavity & Q-factor Beam splitter Polarizer In. Sb 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 • Er. As/In. Ga. As 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 < 1 k. Hz • 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 = k. Tsys ~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 12. 163 GHz - H 2 O 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