A wide broadband terahertz spectrometer was used to illustrate unique applications of terahertz spectroscopy as an analytical
tool. Following a brief discussion on spectrometer calibration, illustrative examples are outlined for a chemical compound
from the Department of Homeland Security, nanoparticles with ligand, and contamination in gasoline. It was found that the
technique is suitable for interrogating molecular motions that are not easily visible by other methods such as UV, infrared,
or Raman spectroscopy.
Spectroscopy is the tool of choice for investigations of molecular structures. Probing molecular properties such as resonances
is the key for a number of real-world applications including sensing, detection, identification, and diagnostics. Recently,
scientists have been increasingly confronted with the term "terahertz spectrometry," which is not commonly found in the standard
spectroscopy textbooks, nor does it appear in the classical molecular spectroscopy bible by Gerhard Herzberg (1). There are
terms such as "far-infrared spectroscopy" or submillimeter waves that have been used in the literature for portions of the
electromagnetic spectrum implying a part of the terahertz region.
Although the term "spectrophotometer" has traditionally been used based on the fact that most spectrometer's functionality
depends on the measurements of light of one kind or another, the terahertz spectrometer deploys a different mode of measurements;
that is, the time-domain measurements based on electro-optic sampling. Therefore, most authors in this area use the term "terahertz
time-domain spectroscopy" (THz-TDS). Increasingly, the practitioners of molecular spectroscopy are expecting the branch of
terahertz science and technology to be the mainstream of spectroscopy because of some unique capabilities offered by this
newer tool. Until recently, this portion of the electromagnetic spectrum used to be referred to as the "terahertz gap." However,
with the discovery of compact and high efficiency terahertz sources, it is no longer a gap; instead terahertz spectroscopy
is emerging as the main mode of spectroscopic investigation.
Table I: Comparison of measured polyethylene peaks with those reported in the literature
Most electronic-based technologies are built around the magic material group known as "semiconductors." As such, photonics
technologies have also been trying to follow the time-honored tradition of riding on the proven semiconductor technology;
perhaps this is the main reason that photonics-based devices are still not in the forefront of modern day applications. Even
though lasers are generally considered a photonics device, it is exclusively a special application of semiconductors except
for a few organic materials-based efforts. Nevertheless, lasers qualify as "photonics" because the word photonics implies
control of photons (or light). The biggest handicap for the flourishing of photonics technologies comes from the inability
of forming an "integrated circuit" with photons, because inherently photons lack a charge state of being either negative or
positive (unlike the electrons). Consequently, to define the binary 1's and 0's by the photons is still not possible. Another
factor is that for photonics integration there is no magic material like silicon or semiconductor in general. Therefore, fabrication
of most functional photonic devices requires marrying of the different materials for instilling required functionalities;
this is the so-called hybrid integration, which is often prone to failure by thermal degradation.
Figure 1: (a) Molecular structure of a third-generation dendrimer. The terminal groups each have two sites available where
dopant molecules may be attached. Thus, a distribution of dipoles per molecule is possible via chromophore doping. (b) Schematic
of energy level diagram in dendrimer molecule resulting from chromophore doping and poling. A distribution of dipole moments
will create multiple bands via which a broadband emission dendrimer dipole excitation (DDE) is energetically possible.
In this article, some unique capabilities of terahertz technology are outlined. In particular, some background of terahertz
generation from electro-optic dendrimer is outlined and some practical experimental systems are used to introduce this emerging
branch of spectroscopy.
Figure 2: Time-domain signal of the empty spectrometer exhibits a very high signal-to-noise ratio.