Cunlin Zhang

Chemical mapping of a single molecule by plasmon-enhanced Raman scattering

Visualizing individual molecules with chemical recognition is a longstanding target in catalysis, molecular nanotechnology and biotechnology. Molecular vibrations provide a valuable ‘fingerprint’ for such identification. Vibrational spectroscopy based on tip-enhanced Raman scattering allows us to access the spectral signals of molecular species very efficiently via the strong localized plasmonic fields produced at the tip apex. However, the best spatial resolution of the tip-enhanced Raman scattering imaging is still limited to 3−15 nanometres, which is not adequate for resolving a single molecule chemically. Here we demonstrate Raman spectral imaging with spatial resolution below one nanometre, resolving the inner structure and surface configuration of a single molecule. This is achieved by spectrally matching the resonance of the nanocavity plasmon to the molecular vibronic transitions, particularly the downward transition responsible for the emission of Raman photons. This matching is made possible by the extremely precise tuning capability provided by scanning tunnelling microscopy. Experimental evidence suggests that the highly confined and broadband nature of the nanocavity plasmon field in the tunnelling gap is essential for ultrahigh-resolution imaging through the generation of an efficient double-resonance enhancement for both Raman excitation and Raman emission. Our technique not only allows for chemical imaging at the single-molecule level, but also offers a new way to study the optical processes and photochemistry of a single molecule.

Plasmonic Airy Beam Generated by In-Plane Diffraction

Optical Airy beam, as a novel non-diffracting and self-accelerating wave packet, has generated great enthusiasm since its first realization in 2007, owing to its unique physics and exciting applications. Here, we report a new form of this intriguing wave packet - plasmonic Airy beam, which is experimentally realized on a silver surface for the first time. By particular diffraction processes in a carefully designed nano-array structure, a novel planar Airy beam of surface plasmon polariton (SPP) is directly generated and a structural dependent phase tuning method is proposed to modulate the beam properties. This SPP Airy beam is regarded as a two-dimensional (2D) subwavelength counterpart of the 3D optical Airy beam in free space, allowing for on-chip photonic manipulations. Moreover, it possibly suggests a breakthrough in recognition of this unique wave packet in a polariton regime after its previous evolution from free particle to pure optical wave.

Detection and identification of illicit drugs using terahertz imaging

We demonstrated an advanced terahertz imaging technique for detection and identification of illicit drugs by introducing the component spatial pattern analysis. As an explanation, the characteristic fingerprint spectra and refractive index of ketamine were first measured with terahertz time-domain spectroscopy both in the air and nitrogen. The results obtained in the ambient air indicated that some absorption peaks are not obvious or probably not dependable. It is necessary and important to present a more practical technique for the detection. The spatial distributions of several illicit drugs [3,4-methylenedioxymethamphetamine, methylenedioxyamphetamine, heroin, acetylcodeine, morphine, and ketamine], widely consumed in the world, were obtained from terahertz images using absorption spectra previously measured in the range from 0.2to2.6THz in the ambient air. The different kinds of pure illicit drugs hidden in mail envelopes were inspected and identified. It could be an effective method in the field of s...

Electrical and thermoelectric properties of single-wall carbon nanotube doped Bi2Te3

The effects of single-wall carbon nanotube (SWCNT) doping in n-type Bi2Te3 bulk samples on the electrical and thermal transport properties have been studied. Bi2Te3 samples doped with 0–5 wt. % SWCNTs were fabricated using solid state reaction and investigated using x-ray diffraction, transmission electron microscopy, and magneto transport measurements. Results show that the 0.5% doping results in the significant enhancement of the Seebeck coefficience to as high as −231.8 μV/K, giant magneto resistance of up to 110%, reduction of thermal conductivity, and change of sign of the Seebeck coefficient from n to p type depending on the doping level and temperature. The figure of merit, ZT, of the optimum SWCNT doped Bi2Te3 was increased by 25%–40% over a wide temperature range compared to the undoped sample.

Polarization sensitive terahertz time-domain spectroscopy for birefringent materials

We present a polarization sensitive terahertz detection method which is able to measure both orthogonal components of the terahertz electric field. It allows for the study of polarization dependent properties of materials. Azimuthal angle dependent transmittance in 0.2–2.6 THz frequency region for crystalline quartz is measured. Polarized terahertz transmission spectroscopy shows that birefringence can result in transmission minima. This work suggests that polarization sensitive detection is effective for removing fake absorption features caused by material birefringence.

Aperiodic optical superlattices engineered for optical frequency conversion

An aperiodic optical superlattice is designed. The designing method is universal and can be applied to all frequency conversion processes by using the coupling of quasiphase matching, without any limitations to special materials and to given fundamental wavelengths.

Terahertz wave reference-free phase imaging for identification of explosives

We present terahertz reference-free phase imaging for identification of three explosive materials (HMX, RDX, and DNT. We propose a feature extraction technique to locate the spectral position of an unknown material’s absorption lines without using the reference signal. The samples are identified by their absorption peaks extracted from the negative first-order derivative of the sample signal phase divided by the frequency at each pixel. This technique will greatly benefit the future development of standoff distance, large size focal-plane terahertz imaging system.

Specified value based defect depth prediction using pulsed thermography

Several methods have been reported in the literature using pulsed thermography for quantitative measurement of defect depth or sample thickness. In this paper, based on the analysis of a theoretical one-dimensional solution of pulsed thermography, a new method was proposed to first multiply the original temperature decay with square root of the corresponding time to obtain a monotonically increasing function f. A specific time was obtained by setting f equals to a predefined value, the theoretical model shows that the obtained specific time has linear relation with square of defect depth, which was verified with the experimental results of one aluminum and one glass fiber reinforced polymer specimen machined with six flat-bottom wedges as simulated defects. This linearity can be used for defect depth prediction in pulsed thermography.

Evaluating PM2.5 at a Construction Site Using Terahertz Radiation

Economic and industrial development has led to increasing problems with particulate pollution in many countries. Particulate matter with the diameter of less than 2.5 μm (PM2.5) is of concern in many cities. In this study, a total of 70 samples of PM2.5 were collected from dusty environments and analyzed using terahertz (THz) radiation. The transmission spectrum of PM2.5 had two distinct absorption bands between 2.5 and 7.5 THz. Their center frequencies were 3.36 and 6.91 THz, respectively. Based on the THz absorbance spectra, the elemental compositions were studied by monitoring PM2.5 masses in conjunction with two-dimensional correlation spectroscopy. Correlations between absorption bands and cross peaks in the synchronous and asynchronous plots indicated that the metallic oxides showed absorption features in the range from 2.5 to 7.5 THz. The vibration modes of anions and cations were located at relatively low and high frequencies, respectively. Statistical methods, including partial least squares and back propagation artificial neural network, were used to quantitatively characterize the PM2.5 content with the input of absorbance over the whole frequency range. This research indicates that THz spectral analysis of PM2.5 is a promising tool for investigating the composition and mass of pollutants.

Possible way for increasing the quality of imaging from THz passive device

Using the passive THz imaging system developed by the CNU-THz laboratory, we capture the passive THz image of human body with forbidden objects hidden under opaque clothes. We demonstrate the possibility of significant improving the quality of the image. Our approach bases on the application of spatial filters, developed by us for computer treatment of passive THz imaging. The THz imaging system is constructed with accordance to well known passive THz imaging principles and to the THz quasi-optical theory. It contains a scanning mechanism, which has a detector approximately with 1200μm central wavelength, a data acquisition card and a microcomputer. To get a clear imaging of object we apply a sequence of the spatial filters to the image and spectral transforms of the image. The treatment of imaging from the passive THz device is made by computer code. The performance time of treatment of the image, containing about 5000 pixels, is less than 0.1 second. To illustrate the efficiency of developed approach we detect the liquid explosive, knife, pistol and metal plate hidden under opaque clothes. The results obtained demonstrate the high efficiency of our approach for the detection and recognition of the hidden objects and are very promising for the real security application.