THz signal can be simultaneously obtained. In addition, the transverse and longitudinal
components of the THz field can be separately measured by using detection crystals with
different crystalline orientations. In previous reports, the field distribution of the longitudinal
THz component has been observed on the surface of a plasmonic device [18] and in the free
space [19, 20]. However, the experimental time was consumed and the sampling rate was
limited in these works because the THz images were built by the raster scanning method.
In recent years, a THz digital holographic imaging system with high resolution, enough
signal-to-noise ratio, and polarization detection ability has been developed. This system has
been applied in various areas, including the measurement of THz waveguide modes [21],
performance demonstration of the THz metasurface elements [22], and observation of the
diffraction process of the THz field [23]. In this paper, this imaging system is adopted to
coherently probe the vector fields of converging THz beams with linear, circular, and
cylindrical vortex polarization, respectively. The transverse and longitudinal components of
the THz field around the focal point are achieved by employing a <110> or a <100> ZnTe
crystal, respectively. Besides, the vector diffraction theory is adopted to simulate the
propagation of vector THz beams and a good agreement between the experimental results and
theoretical expectation are found.
2. Experimental Setup
Fig. 1. (a) THz digital holographic imaging system. (b) A quartz THz quarter wave plate
(TQWP) is used to convert the linear polarization to the circular polarization. (c) A TQWP and
a THz wire radial polarizer (TWRP) are used to convert the linear polarization into the
cylindrical vortex polarization.
In this work, a THz digital holographic imaging system shown in Fig. 1(a) is utilized to
characterize the diffraction features of the converging THz beams with different polarization.
The used light source is a Spectra-Physics laser amplifier system with 800 nm central
wavelength, 50 fs pulse duration, 1 kHz repetition ratio, and 900 mW average power. The
femtosecond laser is divided into the pump beam with 890 mW to generate the THz radiation
and the probe beam with 10 mW to detect the THz radiation, respectively. A <110> ZnTe
crystal with 3 mm thickness [is not shown in Fig. 1(a)] is used to radiate the THz wave via the
optical rectification effect. The diameter of the THz beam is about 21 mm. The incident THz
beam with the linear polarization (x direction) is focused by a silicon (Si) lens with a focal
length of 25 mm. In the path of the probe beam, a half wave plate (HWP) and a polarizer (P)
are used to adjust the probe polarization for measuring different THz polarization components
[24]. The probe beam is reflected into a detection crystal by a 50/50 non-polarization beam
splitter (BS). In the crystal, the two dimensional THz field information is loaded on the
polarization change of the probe beam. The modulated probe beam is reflected into the
imaging module of the system, which consists of a quarter wave plate (QWP), a Wollaston
prism (PBS), two lenses (L1 and L2) and a CY-DB1300A CCD camera (Chong Qing Chuang
Yu Optoelectronics Technology Company). The CCD camera is synchronously controlled
Received 6 Aug 2014; revised 22 Sep 2014; accepted 24 Sep 2014; published 1 Oct 2014
6 October 2014 | Vol. 22, No. 20 | DOI:10.1364/OE.22.024622 | OPTICS EXPRESS 24624