Hong-Liang Cui

Determination of Critical Micelle Concentrations of Surfactants by Terahertz Time-Domain Spectroscopy

A novel detection method is presented to determine the critical micelle concentrations (CMCs) of surfactants by using a simple and convenient terahertz time-domain spectroscopy (THz-TDS) technology which has been successfully used to study the picosecond dynamics of water in reverse micelles. Absorption coefficients or refractive indices of aqueous solutions of an ionic surfactant sodium dodecyl sulfate and a nonionic surfactant nonaethylene glycol monododecyl ether in 0.2-1.8 THz frequency range at different concentrations are measured through the THz-TDS. A clear intersection of THz absorption coefficient or refractive index caused by the micelle formation is observed. This phenomenon is strongly associated with the change of component ratio between surfactant, bulk water and hydration water during the formation of micelles. The corresponding concentration is determined as the CMC of the surfactant which is in good agreement with the most existing results using traditional methods. For comparison, absorption coefficients of the two surfactants in hydrochloric acid solutions are also measured and differences in contrast to those in aqueous solutions are observed and explained. Our findings indicate that THz spectroscopy technology can be applied to determine the CMCs of surfactants in a sensitive, accurate, and label-free way.

Signal detection techniques for scattering-type scanning near-field optical microscopy

ABSTRACT Scattering-type scanning near-field optical microscopy (s-SNOM) has been playing more and more important roles in investigating electromagnetic properties of various materials and structures on the nanoscale. In this technique, a sharp tip is employed as the near-field antenna to measure the samples properties with a high spatial resolution. As the scattered near-field signal from the tip is extremely weak and contaminated by strong background noise, the effective detection, and subsequent extraction of the near-field information from the detected signals is the key issue for s-SNOM. In this review, we give a systematic explanation of the underlying mechanisms of s-SNOM, and summarize and interpret major signal detection techniques involved, including experimental setups, theories for signal analysis and processing, and exposition of advantages and disadvantages of such techniques. By this, we hope to provide a practical guide and a go-to source of detailed information for those interested in and/or working on s-SNOM.

Hilbert-Transform-Based Accurate Determination of Ultrashort-Time Delays in Terahertz Time-Domain Spectroscopy

Accurate determination of time delays is crucial for distinguishing a samples different interfaces in terahertz time-domain spectroscopy (THz-TDS), especially for ultrathin samples or interfacial gaps. In this paper, Hilbert transform (HT) is invoked, along with posttransform signal spectral estimation of several varieties, including the conventional, that based on multiple-signal classification spectrum estimation (HT-MUSIC), and that based on autoregressive spectrum estimation using Yule–Walker law (HT-AR), to determine the samples different interfaces. The results are compared with the traditional method of obtaining time delays in THz-TDS, along with analysis of the resolution and accuracy, as well as advantages and deficiencies of the various approaches. It is demonstrated that HT is an effective method for determining different interfaces of an ultrathin sample by simulation and experiments. The simulation results show that MUSIC-HT has the best resolution, at about 0.35 ps and AR-HT has the best accuracy, whose error is less than 0.075 ps. Experimental results for ultrathin air gaps and polymer films confirmed that MUSIC-HT can distinguish time delays as short as 0.44 ps from the THz time-domain spectra.

Solid-state nanopores fabricated by pulse-controlled dielectric breakdown

This article describes a promising technique to fabricate individual solid-state nanopores directly on dielectric membranes by using adjustable pulses controlled dielectric breakdown in electrolyte. The pulse parameters are adjustable with the running of fabrication process, which is suitable for various thickness and material of the membrane and control precision with sub-nanometer. Moreover, double-stranded DNA translocations through the reliable nanopores fabricated in this manner were successfully demonstrated, exhibiting excellent electrical signals and long DNA translocation times with high signal-to-noise ratio. This economical and timesaving method shows enormous promise for the manufacturing of future nanopore-based technologies and lays the foundation for future research in the field of single molecule detection.

A microfluidic chip for terahertz spectral detection

Fluidic chips as sample storage devices have been widely used to control and analyze traces of solutions by terahertz time domain spectroscopy (THz-TDS). This paper presents a fluidic chip fabricated with ultra-thick micro-fluidic structures utilizing SU-8 photoresist for THz spectral detection. The chip consists of 2 pieces of quartz windows and a photoresist structure serves as liquid cell between the windows. In order to reduce the attenuation of THz energy by the windows and guarantee measurement stability, an infrared quartz with thickness of 1mm is adopted as the windows. For the purpose of improving the signal-to-noise ratio (SNR) in the frequency band between 1.0 THz-1.5 THz, and making the liquid film thickness match the THz optical path, the structure depth is designed to be 50μm. Water absorption coefficient at 0.1THz-1.5THz was detected with this chip. The result indicates that the chip has a good stability for terahertz spectral detection.

Near-field Terahertz Wave Measurement Based on Intensity and Electric-field Detections

In a self-homodyne detection scheme for the near-field terahertz wave under ambient conditions, it has been demonstrated that the intensity detection technique was superior to the electric field detection technique.

Label-free protein detection using terahertz time-domain spectroscopy

In the proposed method the membrane made of nitrocellulose after being moved on proteins is directly scanned by using terahertz time domain spectroscopy system (THz-TDS) to qualitatively and quantitatively protein species and quantity.

A Novel IF Signal Processing Scheme for CW THz Nondestructive Testing

To improve the low resolution of differential frequency signal processing associated with CW THz nondestructive testing, we proposed a novel IF signal processing scheme, with a significantly improved resolution, without sacrifice in dynamic range.

Investigation of W-band dielectric constant of coals by free space method

The dielectric constant of Shandong anthracite coals of China was studied in the frequency range of 75~110 GHz (W-band), using the free space method for the first time. The measurement system is based on the Vector Network Analyzer of Agilent Technology and a VDI extension module with frequency range from 75 GHz to 110GHz. The dielectric constants of coals were calculated from the scattering parameters by implementing an algorithm. Correctness of the test results is verified by measuring the dielectric constant of air and timber. The dielectric constant of each selected coal with different moisture contents is investigated. It is found that both the real and imaginary parts of selected coals exhibit an apparent increasing trend with increasing water content of coals. The real part of coals with different water content varies from 2.61 to 4.97, and the imaginary part from 0.06 to 0.41 at the frequency of 110 GHz. We also obtained the diversification of the dielectric constant by increasing the frequency at the W-band. The real part of coals with different frequency varies from 3.85 to 3.91, and the imaginary part from 0.32 to 0.37 at W-band.