ИСТИНА

Terahertz time-domain spectroscopy (THz-TDS) provides non-destructive capabilities to investigate substances. This region is very promising for various organic molecules having absorption lines due to rotation, phonon and collective vibration modes. Sometimes the reflection mode of THz spectroscopy is the only method for quantity characterization of media. This method is very useful for distant measurements, for instance for industry control and security. Other virtue of reflection THz-TDS is the ability to get spectral information about shielded object. A reliable method for estimating the T-ray characteristics of dielectrics uses a model based on simple Fresnel equations and an iterative fit algorithm. A complex refractive index of the medium can be obtained from real and imaginary parts of reflection spectra. Probing THz emission was generated by femtosecond laser. Depending on parameters of sample (dimension and surface roughness) the THz probing beam spot diameter on the surface can be changed by variation of optical system parameters. State of surface roughness can be led to change of ratio between of specular and scattered parts of reflected signal. Registered THz signal is a dependence of electric part of electromagnetic field E(t), where t is a delay-time. THz signal spectra is a result of Fourier transform of E(t) data. Our experimental results demonstrate possibility of resonant substances detection even in ambient conditions and hidden under dielectric shield. This simple technique enables distant characterization in the THz range. THz-TDS data is processed by numerical algorithm to extract optical properties of substance.