Flare coupled metal parallel-plate waveguides for high resolution terahertz
time-domain spectroscopy
Michael Theuer,
1,2
S. Sree Harsha,
1
and D. Grischkowsky
1,a͒
1
School of Electrical and Computer Engineering, Oklahoma State University, Stillwater,
Oklahoma 74078, USA
2
Department of Terahertz Measurement and Systems, Fraunhofer Institute for Physical Measurement
Techniques, 67663 Kaiserslautern, Rhineland-Palatinate, Germany
͑Received 12 July 2010; accepted 14 October 2010; published online 3 December 2010͒
We report on a new coupling scheme for high resolution terahertz spectroscopy of microcrystalline
films using parallel-plate waveguides. Metal flares are used to couple the terahertz radiation into and
out of the waveguide. Very good coupling ratios as high as 35% at 1 THz from a collimated
free-space beam into a subwavelength gap are obtained. This microwave approach is compared in
terms of coupling ratio and spectral characteristics to the established technique of quasioptic
coupling to parallel-plate waveguides using silicon lenses. Various samples at room and cryogenic
temperatures are measured to show the capabilities of flare coupling for high resolution terahertz
spectroscopy. © 2010 American Institute of Physics. ͓doi:10.1063/1.3516307͔
I. INTRODUCTION
Terahertz ͑THz͒͑far-infrared͒ absorption spectroscopy is
an important technique for the characterization of molecules
and crystals. The probed transitions are either short range
͑rotational
1
or vibrational
2
͒, or long distance interactions
such as lattice vibrations,
3
bending modes or van-der-Waals
interactions.
4
Depending on the aggregate state and the par-
ticular sample, the characteristic frequencies of the transi-
tions can be in the THz frequency range. This band of the
electromagnetic spectrum covers frequencies between 100
GHz and 10 THz, corresponding to vacuum wavelengths be-
tween 3 mm and 30
m. To address a large fraction of the
THz band, THz time-domain spectroscopy ͑THz-TDS͒͑Ref.
5͒ has proven to be a useful tool if broadband radiation be-
tween 0.1 and 4 THz is needed.
To obtain an unambiguous spectral fingerprint of mo-
lecular solids under study at THz frequencies, special care
needs to be given to grow single crystals which is often a
complicated process.
3
Alternatively, for the pellet technique,
which involves measuring a diluted compressed powder of
the substance,
6
the spectral resolution is limited by many
broadening mechanisms. These are due to the heterogeneous
distribution of microcrystals within the pellet and their ran-
dom orientation with respect to the incident THz radiation,
resulting in broadened lineshapes of molecular absorption.
Melinger et al.
7
have recently demonstrated a THz spec-
troscopic technique based on metal parallel-plate waveguides
͑PPWGs͒.
8
A thin polycrystalline film of a sample is dropcast
on one of the metal plates comprising the PPWG and then
probed using the transverse electromagnetic ͑TEM͒ mode of
the metal PPWG. This technique only requires microgram
sample quantities due to the spatial confinement, and it also
facilitates the measurement of high resolution THz absorp-
tion spectra of the samples. Significant reduction in inhomo-
geneous broadening is obtained, due to the formation of high
quality microcrystals which are highly oriented to the metal
surface. Further reduction in homogeneous ͑thermal͒ broad-
ening is obtained by cooling the sample to cryogenic tem-
peratures. A detailed description of sample preparation is
presented in Ref. 3.
In this article, we will discuss the possibilities of flare
coupling to PPWGs as a simplification of the developed qua-
sioptical technique using silicon lenses. This easy-to-use and
low-cost alternative can match the performance ͑coupling ra-
tio, spectral resolution, required sample quantity͒ of the
PPWG with lens coupling. We will first describe WG THz-
TDS with quasioptical coupling, then introduce flared cou-
pling, and finally compare the two approaches.
II. THz WG SPECTROSCOPY
The experimental setup consists of three main compo-
nents: The THz-TDS system, the WG, and the sample. The
THz-TDS system is based on the coherent sampling of the
THz transients using photoconductive switches gated by
femtosecond laser pulses
5
͑for setup see Fig. 1͒. The THz
pulses are generated by a photoconductive antenna ͑Tx͒, col-
lected via a silicon lens, collimated by a pair of parabolic
a͒
Electronic mail: daniel.grischkowsky@okstate.edu.
FIG. 1. ͑Color online͒ Beam path of the THz-TDS system. In the waist of
the collimated beam, the PPWGs are measured between the two parabolic
mirrors. Here a PPWG is shown with silicon lens coupling.
JOURNAL OF APPLIED PHYSICS 108, 113105 ͑2010͒
0021-8979/2010/108͑11͒/113105/6/$30.00 © 2010 American Institute of Physics108, 113105-1