Terahertz frequency domain

Terahertz imaging approaches have typically used either short-pulsed laser or continuous wave (CW) THz generation and detection. The short-pulsed method usually involves the generation and detection of sub-picosecond THz pulses using either photoconductive antenna structures or optical rectification in a non-linear crystal. Pulsed sources seem to be more favorable (in particular for close proximity applications) because they can be used for acquiring depth information. Spectral information is retrieved by a Fourier transform of the time-domain data to the frequency domain. 

Fig. 4 Comparison of Zero-Field techniques to determine the Zero-Field Splitting in Mni2Ac. (a) Frequency Domain Magnetic Resonance Spectroscopy [105]. (b) Frequency Domain Fourier-Transform Terahertz Spectroscopy [88] (Schnegg, Personal communication), (c) Terahertz Time-Domain Spectroscopy, adapted from [102]. Used with permission. 2001 American Physical Society, (d) Inelastic Neutron Scattering, adapted from [106]. Used with permission. 1999 American Physical Society. Readers may view, browse, and/or download material for temporary copying purposes only, provided these uses are for noncommercial personal purposes. Except as provided by law, this material may not be further reproduced, distributed, transmitted, modified, adapted, performed, displayed, published, or sold in whole or part, without prior written permission from the American Physical Society...

By comparison with the phrase far-infrared spectroscopy, the term terahertz spectroscopy or terahertz time-domain spectroscopy is a relatively recent one, which probably began to be used in the early 1990s. As was already described in Section 1.2.1, the wavenumber region of 400 to 10cm corresponds to the frequency region of (12-0.3) x 10 Hz... 

In practice, an electromagnetic pulse with an infinitely short width does not exist, but ultrashort laser pulses are now used for various spectroscopic measurements. Terahertz spectrometry described in Chapter 19 is based on femtosecond laser pulses. In Chapter 20, time-resolved infrared spectroscopic methods using picosecond to femtosecond laser pulses are described. Such ultrashort laser pulses have large spectral widths in the frequency domain. Let us discuss the relation between the pulse width in the time domain and its spectral width expressed in either frequency or wavenumber. 

However, the frequency area involved in the design of an atomic clock was changed recently in a radical way. Equation 11.1 shows that the performances of atomic clocks can be improved when the nominal frequency is increased toward the optical domain. Forbidden optical transitions (with a very weak excitation probability) have narrow natural line widths (ca 1 Hz), resulting in a high Q-factor (cfl 10, at frequencies of several hundreds of terahertz). During the past ten or so years, many projects for optical atomic clocks have been initiated. Laser cooling... 

Whereas CM in bulk materials is usually determined in photocurrent device measurements, that is, by collecting the carriers, CM in QDs is studied by (optical) spectroscopic measurements, in which the orbital occupation of the QDs is probed on ultrafast (picosecond) timescales. Hence, the commonly used experimental procedures to determine CM in QDs (ultrafast spectroscopy) and in bulk (device measurements) are rather different. While time-resolved optical and IR spectroscopies are ideally suited to probe carrier populations in colloidal QDs, " light of terahertz (THz) frequencies interacts strongly with free carriers in the bulk material and allows the direct characterization of carrier density and mobility. From THz-time domain spectroscopy (TDS) experiments, one can quantitatively assess the number of photogenerated carriers in bulk semiconductors picoseconds after the light is absorbed. Additionally, as a result of the contact-free nature of the THz probe, it is possible to determine the CM factor in isolated samples of bulk semiconductors without the need to apply contacts, which is necessary in the device measurements. For these reasons, THz-TDS experiments have been employed to quantify CM in bulk PbSe and PbS on ultrafast timescales " in order to make a bulk-QD comparison in the context of the CM controversy. The CM factor in bulk PbS and PbSe was determined for excitation with various photon energies from the UV to the IR. 

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