Noise Reduction in Terahertz Thin Film Measurements Using a Double Modulated Differential Technique

Abstract

Differential terahertz (THz) time-domain spectroscopy (TDS) is a technique for decreasing noise levels in THz thin film characterization experiments. Characterizing thin films in the GHz to THz range is critical for the development of fast integrated circuits and photonic systems, and is potentially applicable to biosensors and proteomics. This paper shows how the differential technique, combined with double modulation, enables the study of thin films with noise reduction over normal TDS that improves at the film gets thinner. Double modulated differential THz-TDS has enabled the characterization of films with less than 1-μm thickness.

The Detection of Agricultural Products and Food Using Terahertz Spectroscopy: A Review

“…Double-modulated differential (DTDS) THz-TDS has been presented as a way to overcome the difficulty of using the THz-TDS technique for thin-film sample measurement (33)(34)(35). Recently, terahertz biosensor systems based on DTDS have been successful in detection of trace amounts of biological and chemical materials with a very low detection limit (36,37).…”

“…A typical DTDS setup is shown in Figure 4 (33). The pump beam is first modulated at a higher frequency f 1 by the mechanical chopper and then modulated at a lower frequency f 2 by the shakable thin-film sample.…”

“…The second lock-in amplifier (LIA2), detecting the thin-film signal at f 2 and providing a complex output representing the DTDS waveform at a given pump-probe delay, is used to suppress the noise associated with the terahertz wave. Overall, the basic concept of DTDS is rapidly switching between a reference and a sample (33)(34)(35).…”

A Review on Thin-film Sensing with Terahertz Waves

“…In DTDS, the optical chopper typically used to modulate the laser pump beam is discarded in favor of the galvanometer, and only one lock-in amplifier is necessary. Even though DTDS can decrease the effects of laser fluctuations, the modulation rate of the galvanometer is too slow to avoid 1/ f noise from the laser [40,41]. To further improve the dynamic range and reduce the minimum sample thickness, an optical chopper can be added to the system with an expense of a second lock-in amplifier [42][43][44], as depicted in Fig.…”

Label-free bioaffinity detection using terahertz technology

“…The T-ray waveform measured by DTDS, y dif , is directly proportional to the difference between the reference and sample waveforms, y ref − y sam . Rapidly switching between the sample and the reference allows an increase in the signal-to-noise ratio (SNR) of the measurements by a factor of approximately S sam /S dif , where S is the Fourier spectrum of the time-domain waveform y (Mickan et al 2002b). Thus for very small differential signals S dif , there is a marked improvement in SNR.…”