Fibonacci or bifocal terahertz (THz) imaging based on silicon diffractive zone plate in a continuous wave mode at 0.6 THz is demonstrated. Silicon as a low absorbing material was used in the laser ablation process to fabricate the Fibonacci structures. Zone plates were designed by the three-dimensional finite-difference time-domain method. To illustrate the Fibonacci focusing, the performance was studied both theoretically and experimentally by determining spatial profiles, the distance between the foci and the focal depth at frequencies of 0.3 THz and 0.6 THz. Terahertz images of various packaged objects at the 0.6 THz frequency were simultaneously recorded with the spatial resolution of wavelength in two different planes separated by 7 mm distance. Imaging performance using the Fibonacci lens is compared with the operation of the conventional silicon phase zone plate.
Incoherent terahertz (THz) sources can be compact, stable, reliable and cheaper alternative to coherent emitters for compact THz systems. Low power of incoherent THz emitters can be compensated by benefit from extremely high sensitivity of THz micro-detectors. Incoherent THz torch device based on Ga(As,Bi)/AlGaAs parabolic quantum well (PQW) is modeled, fabricated and experimentally investigated paying special attention to the THz spectral range and optical properties. The structures for optical characterization were grown using MBE technique on GaAs substrates. The experimental study was carried out to measure the absorption by intersubband transition of electrons in PQW using vacuum Fourier transform spectroscopy, conventional farinfrared Fourier transform spectroscopy and THz time-domain spectroscopy. Transmittance spectra dependence on temperature and photo-excitation of undoped structures for THz torch device is studied exploring impact of the PQW on THz spectra.
In this work, a comparative research of biologically active organic molecules in its natural environment using the terahertz (THz) time domain spectroscopy (TDS) and Fourier transform spectroscopy (FTS) systems is carried out. Absorption coefficient and refractive index of Nicotiana tabacum L. leaves containing nicotine, Cannabis sativa L. leaves containing tetrahydrocannabinol, and Humulu lupulus L. leaves containing α-acids, active organic molecules that obtain in natural environment, were measured in broad frequency range from 0.1 to 13 THz at room temperature. In the spectra of absorption coefficient the features were found to be unique for N. tabacum, C. sativa and H. lupulus. Moreover, those features can be exploited for identification of C. sativa sex and N. tabacum origin. The refractive index can be also used to characterize different species.
Several pharmaceutical drugs, such as alprazolam, ibuprofen, acetaminophen, activated carbon and others, and caffeine-containing foods were tested using terahertz (THz) time domain spectroscopy in the range from 0.3 to 2 THz. The dry powder of pharmaceutical drugs was mixed with HDPE and pressed into the pellets using hydraulic press. The coffee grounds were also pressed into the pellets after ball-milling and mixing with HDPE. The caffeine containing liquid foods were dried out on the paper strips of various stacking. Experiments allow one to determine characteristic spectral signatures of the investigated substances within THz range caused by active pharmaceutical ingredients, like in the case of caffeine, as well as supporting pharmaceutical ingredients. Spectroscopic THz imaging approach is considered as a possible option to identify packaged pharmaceutical drugs. The caffeine spectral features in the tested caffeine containing foods are difficult to observed due to the low caffeine concentration and complex caffeine chemical surrounding.
Stimulated emission dynamics in InGaN-based multiple quantum wells (MQWs) is analyzed. The lasing threshold
measurements of the In0.09Ga0.91N/In0.02Ga0.98N MQWs revealed non-monotonous threshold dependence on the growth
temperature of the active MQW region. The optimal growth temperature range with the lowest stimulated emission
threshold (100 kW/cm2) in the active region was found to be 780 - 800°C. The influence of indium nano-clusters on
stimulated emission threshold is discussed. Optical gain in InGaN MQWs was measured using variable excitation stripe
length technique. The optical gain dependence on excitation stripe length and excitation power density was studied. The
onset of the gain saturation was observed on the high energy side of the stimulated emission peak. The onset exhibited
red-shift with increasing stripe length due to reduced electron-hole density caused by high optical transition rate.
Increase of excitation power density resulted in the strong blue-shift of the optical gain spectra. The maximal optical gain
coefficient values of 200 cm-1 and 300 cm-1 were obtained for the samples with the lowest and the highest stimulated
emission thresholds, respectively. The calculated optical confinement factor (3.4 %) for the samples yielded the net gain
coefficient of about 5900 cm-1 and 8800 cm-1, respectively