Terahertz Generation for Spectroscopy Use

Figure 3: Fourier transform magnitude spectrum card of the polyethylene card obtained from the time-domain signal of Figure 2. The absorbance peaks (downward peaks) are compared in Table I.

Here, a nanomaterial called dendrimer plays an important role by enabling monolithic photonic integration (2). Terahertz generation from dendrimer exploits its one special property, that is, its multifunctional molecular architecture that is capable of being doped by chromophores and thereby creates a high second-order susceptibility (χ⁽²⁾) material, termed an electro-optic dendrimer. The process is described elsewhere (3), and suffice it to mention here that more than 5 mW continuous wave (CW) terahertz power has been obtained with very high stability. Such a source is suitable for spectroscopy applications. Furthermore, a wide broadband terahertz range of up to >30 THz has also been possible (4), which is important for probing closely spaced molecular phenomena. The key property of dendrimer is the ability to create a dipole moment distribution via the distribution of charge centers in a doped dendrimer molecule (see Figure 1): μ(r) = Ql(r), where μ is the dipole moment and Q is the charge. Note here that l, the separation between the charge centers, is not fixed but is a function of the coordinate, l(r), of the charge centers themselves. Consequently, the energy level diagram in Figure 1b has multiple quasi states within the bandgap. When excited by a suitable pump laser combination, this dipole moment distribution generates a wide-range terahertz radiation via broadband emission. This is termed the dendrimer dipole excitation (DDE) process. A spectrometer was designed based on this principle and has been described elsewhere (4).

Spectrometer Calibration

Figure 4: The front-end for spectral data acquisition.

Unlike the predecessor spectrometers, a terahertz spectrometer carries out its measurements in time-domain. As illustrated in the literature (5), when a stationary beam is scanned by an interrogating beam, an interferogram is generated because of the interference of the two beams. The intensity distribution is then captured by a pair of detectors (5). Figure 2 exhibits the time-domain pulses (or interferogram) generated when no sample is present and when a polyethylene card is placed in the spectrometer. A self-calibrating algorithm is implemented to minimize the effect of atmospheric moisture. A sample must be placed in the spectrometer for it to be measured; otherwise, the spectrometer will produce the "empty" interferogram. A Fourier transform magnitude spectrum corresponding to the time-domain signal of the polyethylene card (Figure 2) is shown in Figure 3 and reveals that the terahertz spectrum spans up to ~40 THz. In practical measurements, the polyethylene card spectrum will serve as the background but when the sample is placed on another substrate (for example, a glass slide), then the blank substrate spectrum will serve as the background.

Figure 5: Terahertz demonstration sample (diameter: 2 in.; thickness: ~0.35 in.) from Dr. Wilkinson's laboratory. Left: smooth side; right: rough side.

Calibration of a spectrometer involves reproducing known absorption peaks of standard materials to ensure measurement accuracies in a given spectral region. As previously mentioned, a terahertz spectrometer deploys time-domain measurement where the sample response resulting from the molecular interaction is recorded; the frequency spectrum is then obtained by means of Fourier analysis. Here, the absorbance is expected to exhibit established peaks of known standards. While the known peaks of polyethylene have been reproduced, however, the spectrometer yields additional peaks because of its high sensitivity. We hypothesize that these additional peaks become visible by the interaction of terahertz radiation with the polyethylene matrix. Terahertz radiation is sensitive to resonances because of translational, rotational, vibrational, and torsional motions. Additional peaks are thus expected, and therefore, justify the emergence of a new spectrometer where indeed additional information is generated that is not available from its predecessors.

Figure 6: Time-domain signal (interferogram or terahertz pulse) of the DHS sample shown in Figure 5. The pulses are generated by an arrangement where the detection system was very close to the reflecting surface. The frequency domain spectra are shown in Figure 7.

The Fourier transform process has a number of different manifestations to suit the versatility of experimental measurements. In addition, short-time Fourier transform and wavelet transform allow examining data simultaneously in time- and frequency-domain via 3-D plots. Furthermore, techniques such as Prony frequency spectrum, auto regressive spectrum, and eigen analysis spectrum allow one to learn details about the specimen's molecular properties (8).