1. Introduction

THz imaging systems are interesting both for research and commercial applications. For instance the unique transitions of molecules in THz range, due to rotational and vibration levels, can be exploited to study specific functions of the samples or as fingerprint for security reasons [1–4]. Moreover, thanks to the low photon energies, the use of THz radiation in biomedical applications could prevent the screening exposure of biological tissues to harmful radiations such as x-rays [5,6]. The good transparency to the THz radiation of non-metallic materials, such as plastics, ceramics and polymers and the high penetration depth, makes this radiation valuable for applications as non-destructive material quality control and homeland security [3,7–9].

The main requirements for THz systems in most of the applications are high resolution images and fast detection. In addition, in order to be able to bring the THz imaging out of the laboratory environment, compactness and portability of the measurements apparatus are key elements. In this work we will show that it is possible to combine these properties, presenting a stand-alone system of less than 15 kg of weight, which can achieve a resolution of 200 µm (2.2 times the wavelength) for real time acquisition.

Among the different realizations of THz imaging systems two basic principles can be distinguished: either ultra-short pulses are used to generate a broadband THz radiation or sharp continuous-wave sources are employed [10].

In the first case coherent imaging is achieved by raster scanning a sample trough the focus of the THz beam and full spectroscopic information is recorded for each pixel. However this approach is far to be compact and it is inherently slow: few minutes are needed to acquire an image of 400x400 pixels in the fastest systems [10]. On the other side, thanks to optimized optical schemes based on the confocal principle, spatial resolution of ~2λ has been demonstrated for these systems [11]; values close to λ have been reported with quasi-near-field techniques [12]. To overcome the speed limitation of the measurement apparatus based on ultra-short pulses, promising results have been reported using a compressing sensing scheme [13], where random acquisition and special reconstruction routines can reduce the acquisition time of the 70%; however this is still far from a real time measurement.

To achieve real time operation and frequency-sensitive measurements, imaging systems based on continuous wave sources and an incoherent system of detection, as micro-bolometer arrays, have been developed. The possible sources of radiation available are solid-state frequency multipliers at sub-millimeter-wave frequencies, far-infrared gas laser, two colors external-cavity lasers [14] or quantum cascade lasers (QCL). Because of their compact size, QCLs are one of the most attractive sources. However, these lasers are generally limited in performance in terms of combination of low power consumption, beam quality, spectral control and output optical power, limiting their use in compact systems for THz imaging. Good results for real time imaging using QCLs have been reported in references [15,16] and in reference [17] over long distance (> 25 m), but the highest resolution reported is 7.2 λ [15] and the systems were not optimized for portability. Promising results based on raster scanning acquisition and QCLs have been demonstrated for compact apparatus in reference [18] and for dual-frequency imaging in reference [19].

In this work we present the successful implementation of a stand-alone set-up for real-time THz imaging using as source a THz QCL emitting at 3.4 THz [20], based on a third order distributed-feedback cavity [21]. Thanks to this laser geometry, improved performance of the laser are obtained; we demonstrate high resolution imaging using a single lens with a high numerical aperture and a single frame acquisition. Images and movies of imaging in the THz of more extended sample but with lower resolution are also presented.

2. Experimental set up and laser source

Experimental set-up

The THz imaging system presented in this work includes as source a THz QCL operating in a thermo-mechanical cryo-cooler (Sunpower, CryoteTel CT) at a temperature of 50K and a duty cycle of 50%. The detector is a commercial available un-cooled micro-bolometer camera (INO, IRXCAM) with 160 x 120 elements (pixel pitch is 52 µm), optimized for operation at 3 THz. The Noise equivalent power of the system is evaluated by the manufacturer in Ref [22] to a value ~80 pW using an high-resistive Si objective with f/0.95. The single frame used in the measurements is 0.033 seconds. The camera is connected to a computer laptop for the acquisition of the images; the laptop controls as well the cryo-cooler. The optical elements are diffractive and made in high-density polyethylene.

The electrical power dissipation of the entire system is around 265 W: 206 W for the cryo-cooler, 150 mW for the laser, 8 W for the camera and an average of 50 W for the computer. The total weight of all the elements is less than 15 kg: 5 kg for the cryo-cooler, 4.5 kg for the laser power supply, 1 kg for the cryo-cooler power supply, 0.5 kg for the camera and 0.05 kg for the optical elements. Moreover the physical dimensions of the elements are such that the entire system could be compacted in a parallelepiped of 70 cm x 60 cm x 60cm (See Fig. 1a ). In Fig. 1b is shown a packed version of the system, where the cryo-cooler and the power supplies are placed together.

 

Fig. 1 Photos of the stand-alone system for THz imaging. a) Detailed vision of all the elements. b) Packed version operating in transmission mode

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The limited power consumption, the small weight load and the compactness of the THz imaging system presented in this work demonstrate its portability and its validity as stand-alone system.

Laser source

The laser source is a THz QCL emitting at 3.4 THz. The active region is based on a four well design with an extraction mechanism based on the emission of LO phonons [18]. The sample has been processed following standard double metal procedure and a third-order distributed feedback cavity has been engineered for it [21].

As shown in reference [23], thanks to this cavity design, it is possible to improve the out-coupling efficiency of the laser and to achieve good beam quality, high output optical power and spectral control of the laser emission. Moreover the device presented in this work has been processed in very narrow waveguides (15 µm wide, 800 µm long) in order to reduce the power consumption in the laser.

In Fig. 2a is reported the measured emitted optical power of the laser in continuous wave mode versus the dissipated power, for different temperatures. It is clear that this kind of devices have less stringent requirements for the cooling power needed for operation and compact cryo-cooler can be employed. It is also important to notice that there is almost no difference in the laser performance at 10 K and 50 K. The output optical power at high temperature, despite of the limited power consumption, is comparable to devices dissipating more power [24].

 

Fig. 2 a) Measured optical power versus electrical power dissipation of the THz quantum cascade laser at different temperatures. Inset: measured frequency emission. b) Measured beam pattern of the laser. A FWHM of 22° X 30° for the laser emission is demonstrated

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In Fig. 2b is reported the measured emission beam of the laser output any intermediate optical element. Despite of the subwavelength lateral dimensions of the device, thanks to the third order grating, a narrow beam of full-width-half-maximum of 22° x 30° is demonstrated.

3. Experimental results

The materials studied in this work are different kinds of plastics such as polyethylene, polypropylene and polystyrene. Similar results have been obtained for all these materials so in the following we show mainly the results for plastics opaque in the visible. These materials are extremely interesting due to their large use in very different applications. Polyethylene and high density polyethylene are used, for instance, for milk container, shampoo and detergent bottles, food container and dishware, water pipes; polypropylene is used for auto parts, soft drinks bottles, food container and dishware; polystyrene is used for disposable cups and plates, packing materials and CD cases.

We studied either the contrast given by the difference in thickness in the plastic itself or the contrast due to objects hidden in the plastic. All the images presented in the following have been obtained by single frame acquisition and real time movie are reported in the multimedia files Media 1, Media 2, Media 3, Media 4, and Media 5. Compared to the single frame images, real time acquisition takes advantage of the integration ability of the eye and the pattern recognition function of the brain, reaching a higher quality of the images.

We used two different schemes of the optical elements to perform THz imaging. The first one is for high resolution images: the sample is illuminated directly from the laser light and the image is collected from a single lens with short focal length (See Fig. 3 ). The lens parameters are: focal length f = 30 mm and the diameter is d = 40 mm; the distance a between the object and the lens is 6 cm and the distance b between the lens and the camera is 6 cm. The numerical aperture (NA) of the system is 0.32 ($\text{sin(arctan(d/2/a))}$). The highest resolution achievable is then 220 µm, according to Abbe’s theory (0.82λ/NA). The experimental results are shown in Fig. 3, where the THz images of some shapes imprinted in different plastics are reported. Objects separated by distance of ~220 µm can be clearly resolved, attending the expected resolution. The illuminated area in this configuration is 3 mm x 6 mm. In this configuration the single frame signal-to-noise ratio of the system is 5 and is obtained by measuring the average value of the voltage readout of the individual pixel when the laser source is switched on and off.

 

Fig. 3 Single-frame THz imaging of some shapes imprinted in different kinds of plastics (details presented in the first column of the figure). The real time imaging of the second row is reported in the file Media 1. The last row presents a characterization of the system resolution with 10 slits of varying widths cut out on a metal piece (widths are, from left to right: 1mm, 500 −370-280-210-160-120-90-70-50 µm). The three THz images are relative to the 1 mm and 0.5 mm, the 370-280-210-160 µm and the 160-120-90-70-50 µm. On the top: schematic of the experimental set-up

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A further characterization of the resolution achievable from the system has been performed by imaging a set of 10 slits of different widths (1mm, 500 −370-280-210-160-120-90-70-50 µm) cut out by laser machining on a metallic foil. As visible from Fig. 3, bottom row, we are able to see transmitted THz through all the slits and to resolve the size change down to to the 5th slit from the left, which has a width of 210 µm. This confirms our resolution claim for the setup in this configuration.

In the second configuration we used two lenses of focal length f = 30 mm and f1 = 95 cm (See Fig. 4 ). The object is placed in between the two lenses where a nearly parallel beam is expected. The image is then focused on the camera by the second lens. The illuminated area in this case is 1.1 cm x 1.4 cm. The experimental results are presented in Fig. 4. Good contrast is demonstrated for metallic object hidden in plastic but also for non-conducting small element as a pencil lead. Liquids in a bottle can also clearly be measured, presenting the potentiality of this system for the detection of different substances in container opaque in the visible. A writing cut out from standard paper has been concealed in a DVD box: transmitted signal through the two box surfaces allows to clearly distinguish the hidden writing “ETH”. Imaging of larger features imprinted in the plastic, compared to what shown in Fig. 3, is also demonstrated. These data is also presented as movies in the multimedia material section.

 

Fig. 4 First column: Images in the visible. Second and third columns: single-frame THz transmission images. First row: metallic blade in polystyrene. Second row: Lead pencil in polystyrene. Third row: Water droplets in high- density polyethylene bottle. Fourth row: writing “ETH” inside a DVD box. The real time imaging is in the multimedia: Media 2, Media 3, Media 4, and Media 5, respectively. On the top: schematic of the experimental set-up

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4. Conclusion

In conclusion we reported the successful demonstration of a stand-alone system for high resolution real-time THz imaging, with low power consumption and reduced physical dimension. A similar system could be adapted in the future for spectroscopic measurements and heterodyne receivers, opening the route toward a more diffuse use of THz quantum cascade laser as valid THz sources. We gratefully acknowledge financial support from NCCR “Quantum Photonics” and SNF through the project “Terascope” and the technical support from Alpes Lasers.