Building a FT-FIR Towards a THz version of the Flygare R. Braakman

1,*)

; M.J. Kelley

1)

, K. Cossel

1)

, G.A. Blake

2) 1)

Division of Chemistry & Chemical Engineering, California Institute of Technology

2)

Division of Geological & Planetary Sciences, California Institute of Technology

*)

contact: [email protected]

Introduction

THz radiation probes low energy states in a wide variety of systems, including rotational states in small molecules, phonons in solids, thermal emission from cold sources, and the lowest lying vibrational modes of large molecules. In our group the main push for the development of a THz spectrometer is the commissioning of the SOFIA/Herschel observatories over the next 2 to 3 years, which will allow high sensitivity access to interstellar THz spectra for the first time.

Due to technological difficulties, easily tunable intense THz radiation sources and sensitive detectors have been lacking. To circumvent this problem we will use a non-linear effect to generate THz radiation (explained below) from two 1.55

 m sources. The result is a stable yet easily tunable source. The THz power output is only on the order of ~0.1

 W, however, which is quite low for spectroscopic purposes.

We thus propose the development of a FT-FIR spectrometer based on the Flygare FTMW design (Ref. 1) to increase the sensitivity of the system.

THz generation

A photomixing scheme will be used to generate the THz radiation. In this scheme, two beams of light with roughly the same frequency are simultaneously directed onto the surface of a photomixer chip, whose characteristics allow for the generation of THz waves at the difference frequency of the two lasers.

A recently developed material consisting of ErAs/InGaAs that operates at 1.55

 m (Ref. 3) is

Figure 2. Photomixer setup. Adopted from Ref. 2

currently being fabricated into chips . This allows for a fully fiber based setup, in which a fixed frequency DFB laser and a tunable Agilent laser are coupled together in a 3 dB coupler and subsequently amplified simultaneously. After decoupling, the two beams are then directed onto the photomixer chip, where THz generation takes place.

“THz Flygare”

e.g. P,  meter THz photomixer 1.5  m Er: Amplifier 3 dB coupler Fixed tuned 1.55  m DFB laser Tunable 1.55  m Agilent laser 50:50 beam splitters mirror mirrors Wire grid polarizer (R~99.9%) Heterodyne HEB THz mixer Amplifier, filters Molecular nozzle Mirror (R>99.99%)

THz cavity

The power output of a photomixer device is several orders magnitude lower than the power used in a typical Flygare experiment (~0.1

 W vs. ~1 mW). However, using a Fabry-Perot resonator to build up a radiation field and subsequently recording the FID of the excited molecules in the time domain allows us to take advantage of several aspects of this system to compensate for this large difference.

• Different source stabilization method allows for longer radiation pulses in THz system which help build up higher field (~1 ms vs ~0.1

 s).

• Einstein A coefficients, and thus emitted power, tend to be significantly higher in the THz region. Comparing the fundamental transition (J = 0 – 1) for OCS at 12.163 GHz (= standard for Flygare system) and that for H 2 O at 1113.342 GHz, we find A = 1.8x10

-2 for H 2 O and A = 3.6x10

-9 s -1 for OCS, a difference of almost 7 orders of magnitude!

s -1 • The “loaded” Q-factor should be ~equivalent even at higher frequencies. A test cavity at 300 GHz (spectrum shown on right) is indeed found to have a Q-factor close to that of Flygare systems. The semi-confocal cavity employed allows for better coupling into the cavity than in a Flygare FTMW setup, resulting in less loss.

FWHM = 4 MHz, Q ~ 7.5 x 10 4 !!

Figure 3. Cavity absorption of prototype semi-confocal cavity at 300 GHz

THz detection

With a Q of ~10 4 the bandwidth of the cavity will be around 100 MHz. Therefore a detector that is both fast and sensitive is needed in this experiment.

New Hot Electron Bolometers (HEB) that have recently been developed in the Prober group at Yale meet both of these requirements. These Nb-based superconducting devices, which are currently being tested in a joint effort, have an electron-phonon cooling time of ~1 ns, which results in a possible detection bandwidth of at least 150 MHz.

HEBs are capable of attaining a system temperature (T sys ) of ~1000 K, which gives a NEP of 1.4x10

-20 W/Hz in heterodyne detection mode. This compares to a T sys K and a NEP of 1.4x10

-21 of ~100 W/Hz for a typical Schottky detector in the Flygare system. The increased bandwidth in the THz setup adds another factor of 10 2 to the noise level.

Figure 4.Nb based HEB detector (above) and their T-R response curve (below). Courtesy of Prober group.

Laser stabilization

Recording the FID of the molecules in the time domain followed by a Fourier transform to the frequency domain allows for very high resolution spectra, in principle. In the heterodyne detection method, however, the FID signal is mixed with a LO signal at the HEB mixer, and the resulting line width is thus dependent on the widest of the two incoming signals. It is therefore essential to stabilize both input lasers to ensure an LO signal with a narrow line width. In addition, long term drift due to thermal fluctuations can thus be countered. Prior to stabilization efforts, the line width of the beat note between DFB and Agilent lasers was ~10 MHz, with a long term drift of ~100 MHz. For the DFB laser a temperature feedback circuit will be used, which is shown below. A similar method based on an external cavity will be used for the Agilent.

T, I controller Lock-in amplifier HDO DFB EOM

Figure 5. Temperature feedback circuit for DFB laser. EOM sideband probes the HDO transition, which is detected as first derivative line shape by Lock-in amplifier. LIA then feeds positive or negative signal back to the temperature controller of the DFB

Summary

We propose the development of a FT-FIR instrument as a THz analog of the Flygare system. There are several disadvantages that must be overcome. The input power of the cavity from the photomixer is quite low compared with typical input powers in a Flygare system. In addition, the noise level of detectors is intrinsically higher at THz frequencies than at microwave frequencies and the higher detection bandwidth adds to this effect.

However, it is possible to take advantage of the Fabry Perot resonator to a greater extent than in the case of the Flygare by having better coupling and using longer radiation pulses to build up a higher field. Spectroscopic properties of molecules are also favorable at THz frequencies. Due to the significantly higher Einstein A coefficients, a much larger fraction of the absorbed radiation can be recovered in emission. In principle, these combined factors should sufficiently compensate for the disadvantages and allow higher sensitivity than at the wavelengths of a traditional Flygare system.

References

1. T.J. Balle & W. H. Flygare, 1981, Rev. Sci. Instr., 52 , 33-45 2. S. Matsuura et. al., 1999,

Appl. Phys. Lett.

,

74

, 2872-2874 3. M. Sukhotin et. al., 2003, Appl. Phys. Lett., 82 , 3116-3118