High-Power Lensless THz Imaging of Hidden Objects

Thorsten Schultze,Clara J. Saraceno,Jan C. Balzer,Samira Mansourzadeh,Frank Wulf,Dilyan Damyanov,Bjorn Globisch,Robert B. Kohlhaas,Martin Hoffmann,Tim Vogel

doi:10.1109/access.2020.3048781

Abstract

The potential of pulsed THz radiation for time-of-flight imaging applications is well recognized. However, advances in this field are currently severely limited by the low average power of ultrafast THz sources. Typically, this results in impractically long acquisition times and a loss in resolution and contrast. These limitations make imaging of the objects in real-life scenarios impossible. Here, conclusively, the potential of state-of-the-art high-average power THz time-domain spectrometer (TDS), driven by a 100-W class, one-box ultrafast oscillator for imaging applications is shown by demonstrating lensless THz imaging in reflection mode of a dielectric sample with low reflectivity. Images obtained with our home-built 20-mW average power THz-TDS system show a significant contrast enhancement compared to a state-of-the-art commercial THz-TDS with less than $200~\mu \text{W}$ of average power. Our unique setup even allows us to obtain images of such an object with high-contrast hidden inside a medium-density fiberboard (MDF) box. This opens the door to THz time-of-flight imaging of concealed objects of unknown shape and orientation in various real-life scenarios which were so far impossible to realize.

Highlights

Terahertz (THz) radiation is well-known to have enormous potential for imaging in various fields such as security [1], non-destructive testing [2], biomedical research [3], as well as in fundamental research applications such as nanoscale imaging in a condensed matter [4]
In summary, we demonstrated the potential of state-of-theart high-average power THz-time-domain spectrometer (TDS) for THz imaging applications by performing lensless imaging of a 3D-printed mixed polymer object using a migration algorithm, which can reconstruct the image of the object without liming us to have a priori information about the shape and position of the object
Images obtained with our 20-mW average power THz time-domain spectroscopy (THz-TDS) shows a clear enhancement of dynamic range, which results in a significantly improved image contrast compared to the same measurements performed using a state-ofthe-art commercial TDS system with less than 200 μW of average power

Summary

Introduction

Terahertz (THz) radiation is well-known to have enormous potential for imaging in various fields such as security [1], non-destructive testing [2], biomedical research [3], as well as in fundamental research applications such as nanoscale imaging in a condensed matter [4] In applied fields such as security and testing, electromagnetic waves in the THz domain have the advantage that they are able to penetrate deep into materials which are usually opaque at visible and infrared wavelengths [5], [6] opening the door to imaging and characterizing material properties of concealed objects. Minuscule amounts of radiation reach the corresponding detectors, calling for high illumination power in combination with high detection sensitivity In this respect, THz time-domain spectroscopy (THz-TDS) offers the advantage of highly sensitive field-resolved detection [8], based on optimized photoconductive antennas [9]. THz-TDS suffers from a severe lack of high-average power, with typical sources in the

Methods

Results

Conclusion