Fanzhen Meng

THz Active Imaging Systems With Real-Time Capabilities

This paper presents a survey of the status of five active THz imaging modalities which we have developed and investigated during the last few years with the goal to explore their potential for real-time imaging. We start out by introducing a novel waveguide-based all-electronic imaging system which operates at 812 GHz. Its salient feature is a 32-pixel linear detector array heterodyne-operated at the eighth subharmonic. This array in combination with a telescope optics for object distances of 2-6 m reaches a data acquisition speed suited for real-time imaging. The second system described then is again an all-electronic scanner (now for around 300 GHz ), designed for object distances of ≥ 8 m , which combines mechanical scanning in vertical direction, synthetic-aperture image generation in horizontal direction, and frequency-modulated continuous-wave sweeping for the depth information. The third and fourth systems follow an optoelectronic approach by relying on several- to multi-pixel parallel electrooptic detection. One imager is based on a pulsed THz-OPO and homodyne detection with a CCD camera, the other on either continuous-wave electronic or femtosecond optoelectronic THz sources and a photonic-mixing device (PMD) camera. The article concludes with a description of the state of the art of imaging with focal-plane arrays based on CMOS field-effect transistors.

Coherent electro-optical detection of terahertz radiation from an optical parametric oscillator

We report the realization of coherent electro-optical detection of nanosecond terahertz (THz) pulses from an optical parametric oscillator, which is pumped by a Q-switched nanosecond Nd:YVO4 laser at 1064 nm and emits at approximately 1.5 THz. The beam profile and wavefront of the THz beam at focus are electro-optically characterized toward the realization of a real-time THz camera. A peak dynamic range of approximately 37 dB/radical Hz is achieved with single-pixel detection.

Characterizing large-area electro-optic crystals toward two-dimensional real-time terahertz imaging

We have characterized the homogeneity of large-area (>10 mm x 10 mm) CdTe(110) and ZnTe(110) crystals using a raster electro-optic scanning method to assess their usability in two-dimensional electro-optic terahertz (THz) imaging with parallel read out. The spatial variation in the detected THz signal (at 0.2 and 0.645 THz, respectively) is due to nonuniform residual birefringence and scattering. For CdTe, this depends critically on the growth method, and has an important contribution from slip planes in the crystals, as is evident in the scanned images. For the highest-quality CdTe(110) crystals investigated, the rms signal variations are less than 15%, comparable to those for ZnTe(110). For electro-optic scanning, we introduce a hybrid measurement system based on a fs Nd:glass laser and a continuous-wave electronic THz source.

Terahertz propagation properties of free-standing woven-steel-mesh metamaterials: Pass-bands and signatures of abnormal group velocities

We explore steel-mesh structures, which are commercially available as insets of chemical particle filters, in terms of their metamaterial properties below 1 THz, including both single- and multi-layer structures. Their pass-band characteristics are very similar to those reported for hole arrays in metal films, exhibiting power transmission as high as 88% for the lowest transmission band of single-layer structures. The transmission minima are explained in terms of the Rayleigh-Wood anomaly. The phase properties of the transmitted THz pulses reveal negative group delays of several picoseconds, i.e., abnormal group velocities, in spectral regions between pass-bands.

Pump/probe THz spectroscopy of the conductivity of TTF-TCNQ films

We perform optical-pump/THz-probe measurements on TTF-TCNQ films in the metallic state to study the picosecond conductivity dynamics. The data suggest charge-density-wave formation on a time scale of 10 ps following an ultrafast thermalization process.

Wire-cloth-mesh metamaterials for GHz and THz frequency regime

Wire-cloth-mesh structures are introduced as metamaterials. They are experimentally investigated in terms of their metamaterial properties. It is shown that they exhibit pronounced pass-band characteristics in GHz and THz bands. Transmission minima are explained in terms of Rayleigh-Wood anomaly. While slow-light propagation occurs in pass-band region, fast-light outside the mentioned region. This allows us to investigate abnormal propagation effects in these materials. These meshes are commercially available as precisely fabricated chemical particle filters and come in many sizes which facilitate their application as metamaterials in optical devices, antennas, multilayer structures, and so on.

CCD-camera-based electro-optical detection of nanosecond THz pulses from an optical parametric oscillator

We report realization of a THz camera based on electro-optical detection of THz radiation from an optical parametric oscillator, using a silicon-based CCD camera as the optical detector. The dynamic range of 16.5 dB for a 6000-frame average (2-minute data acquisition time) is limited by laser noise.

THz pulse propagation through woven-steel-mesh metamaterials

We experimentally study single- and multi-layer woven metallic meshes and investigate their pass-band and dispersion characteristics. The structures are commercially available as precisely fabricated chemical particle filters, which facilitates their application as metamaterials.

Coherent electro-optical detection of nanosecond THz pulses from a parametric oscillator

We successfully realized electro-optical detection of nanosecond THz pulses based on a THz optical parametric oscillator. A maximum dynamic range of ~30 dB/ Hz is achieved in the electro-optical measurements.

Development of a hybrid THz camera using synchronized two-color laser radiation

We demonstrate a hybrid system for THz raster scan imaging using frequency-stabilized DFB lasers for electro-optic detection. The system combines a 0.65-THz micro-electronic narrow-band emitter with two synchronized diode lasers, providing two-color laser radiation. The functional principle promises three-dimensional real-time imaging capabilities.