1 Introduction

Over the past few decades, terahertz technologies [1] have exhibited distinct superiority in ultrafast research [2, 3], non-destructive examination [4, 5], biomedical research [6] and security [7]. Terahertz wave owns good penetrability of the nonpolar materials, terahertz imaging [8] becomes one of the most valuable research direction in terahertz field. By employing sub-wavelength pinholes to obtain a high spatial resolution, the terahertz confocal imaging can be regarded as a potential diagnostic method in many applications where a high spatial resolution is demanded [9]. The terahertz transmission [10, 11] and reflection [12,13,14] confocal imaging systems have been implemented to acquire great lateral and axial resolutions.

With the development of terahertz photonic technologies, the terahertz quantum cascade lasers (QCL) [15] may make a further improvement of terahertz imaging technology. The superiority of the terahertz QCLs lies in the characteristics of high-efficiency, high output power, and easy-integration. And the terahertz QCL device is small in size and easy to operate. Compared with the common detectors (Golay-cell [16], bolometers [17], pyroelectrics [18], etc.), the terahertz quantum-well photodetector (QWP) [19] can provide a response time [21] at the picosecond level which is very useful for the fast terahertz imaging systems. As the fast Schottky diodes [15] operating at room temperature, the cooled terahertz QWPs can be adopted to fast detect the terahertz radiation emitted from the terahertz QCLs in the terahertz imaging systems [20].

The fast detection [21] of terahertz light is of great importance for many applications such as terahertz imaging, spectroscopy, and high-speed communications. Fast imaging systems are more practical than slow scanned systems in practical application. Some fast terahertz imaging systems [22] and terahertz devices such as diffractive gratings [23], quantum cascade laser with self-mixing [24] and metamaterial spatial light modulators [25] have been applied to shorten the time of the terahertz imaging. Some devices (micro-bolometer focal-plane array [26], superconducting tunnel junction detectors [27], Schottky diode mixer [15], etc.) and more effective imaging methods (compressed sensing [28], raster scan imaging [29], rapid-phase modulation [30], etc.) have been employed to raise the detection speed of the systems. For these terahertz imaging systems, the imaging time or the resolution should be improved further to meet the requirement of the practical application.

In this paper, a fast terahertz reflective confocal scanning imaging system is demonstrated based on a fast terahertz QWP and a fast rotating translational platform. The terahertz QWP is employed to detect the terahertz radiation generated from a terahertz QCL lasing at 4.3 THz. Lateral and axial resolutions better than 110 μm and 320 μm, respectively, are achieved. The terahertz two-dimensional images of some objects are obtained within 5 s.

2 Experimental setup

Figure 1 shows the experimental setup of the fast terahertz reflective confocal scanning imaging system. A dual-axis reflection mode is employed to reduce the energy loss of the imaging system, the angle of the reflection beam is 30° to the normal vertical direction, as shown in the front view of the platform. Two pinholes (PH1 and PH2) are placed in the two focuses of the off-axis parabolic mirrors to improve the spatial resolution of the terahertz imaging system. The scattered light generating from the laser will be filtered out by the first pinhole. The second pinhole at the last focus geometrically rejects the residual light scattered by the object and filters out the central region of the image [11]. Compared with the lenses, the off-axis parabolic mirrors can reduce the energy loss of the terahertz pulse and obtain a higher quality light spot. A pulsed electrically pumped terahertz beam at 4.3 THz is generated by the terahertz QCL, the terahertz QCL is placed in a cryostat at 11 K and driven by a 3.7 A current with a 3% duty cycle at 10 kHz. The power of the terahertz QCL is measured to be about 1 mW by a power meter of Ophir with a thermal sensor. After being reflected by two 90° off-axis parabolic mirrors (P1 and P2), with focus length of 127 mm, the terahertz beam is focused into a pinhole (PH1) with a size of 200 μm. Then the beam is focused in the position of the object by a pair of 90° off-axis parabolic mirrors (P3 and P4), with focus length of 127 mm. Before being reflected by the P4 mirror, the terahertz beam is transmitting in the XOY plane. The P4 mirror is rotated along the x-axis direction to change the transmission direction of the reflected terahertz beam to the YOZ plane. The angle between the optical axis of the reflected beam and the z-axis is 30°. The object is placed on a reflecting mirror which is installed on a three-dimensional fast rotating translational platform. The beam reflected by the object goes through a pair of 90° off-axis parabolic mirrors (P5 and P6), with a focal length of 127 mm and 154.2 mm, respectively. The P5 mirror is also rotated along the x-axis direction to collect the beam reflected by the object in the YOZ plane. After being reflected by the P5 mirror, the terahertz beam transmit back to the XOY plane. A pinhole (PH2), with a size of 200 μm, is placed in the position of the smallest waist of the converging beam. Finally, after focused by a pair of 90° off-axis parabolic mirrors (P7 and P8), with a focal length of 154.2 mm and 127 mm, respectively, the terahertz beam is detected by the terahertz QWP which is operating at 4.2 K with a bias voltage of 75 mV. In our experiment, All OAPs have a diameter of 76.2 mm except P2 and P3 (101.6 mm in diameter).

Fig. 1
figure 1

Experimental setup of the fast terahertz reflective confocal scanning imaging system

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The fast response signal of the terahertz QWP is read and amplified by the source and preamplifier. The lock-in amplifier is employed to extract and amplify the peak response of the pulsed terahertz signal. The peak signal is gathered by a data acquisition (DAQ) pad which is controlled by a LabVIEW program in the computer. By utilizing a fast continuous-scanning mode instead of the traditional step-scanning mode, the imaging time of the system can be greatly shortened to 5 s by the three-dimensional fast rotating translational platform. The movement locus of the terahertz scanning is an Archimedes spiral. The image reconstruction is realized by collecting the location information of the spiral and corresponding amplitude of the terahertz pulse. The fast rotating translational platform is driven by a servo driver which is connected with the same LabVIEW program. The LabVIEW program is applied to collect the corresponding amplitude information and the location information to restore the object. At present, the imaging time of the scanning imaging system is limited by the fast rotating translational platform.

3 Performances of the device

Due to the special device structure of quantum-well, the terahertz QWP has a fast response time on the order of picosecond. Previous microwave measurements on similar QWPs showed response rates up to 6.2 GHz [21]. The similar terahertz QWP device [20] is applied to detect the terahertz light in our experiment. Using an oscilloscope, the drive signal of the 4.3 THz QCL and the response signal generated from the frequency-matched terahertz QWP are depicted in Fig. 2a. The results show that the fast-modulated terahertz light can be successfully detected by the fast terahertz QWP. The schematic diagram of the common←45° facet light-coupling structure and conduction band outline of an n-type terahertz QWP [31] are shown in Fig. 2b. The 45° facet light-coupled structure is adopted to enhance the coupling efficiency of the terahertz QWP.

Fig. 2
figure 2

Characterizations of the terahertz QWP. a The drive signal of the 4.3 THz terahertz QCL and the response signal generated from the frequency-matched terahertz QWP shown in an oscilloscope; b the schematic diagram and conduction band outline of a common n-type terahertz QWP with 45° facet light-coupling structure; c the emission spectra of the 4.3 THz terahertz QCL and the photoresponse spectra of the frequency-matched terahertz QWP; d the dark current of the terahertz QWP as a function of bias voltage; e the peak responsivity of the terahertz QWP as a function of bias voltage; f the experimental and fitting results of the noise equivalent power (NEP) of the terahertz QWP

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We characterize the optical and electrical performances of the terahertz QWP. Figure 2c gives the emission spectra of the 4.3 THz terahertz QCL and the photoresponse spectra of the frequency-matched terahertz QWP. Figure 2d plots the dark current of the terahertz QWP as a function of bias voltage at 5 K. To obtain the moderate background noise, the terahertz QWP works at the low dark current of 10−7 A below 150 mV, both in up and down sweeping mode. Figure 2e shows the peak responsivity of the terahertz QWP as a function of bias voltage. In the bias voltage range between 50 and 100 mV, the terahertz QWP obtains a peak responsivity of 0.5 A/W. The experimental and fitting results of the noise equivalent power (NEP) of the terahertz QWP are depicted in Fig. 2f. A NEP of 0.4 pW/Hz0.5 is acquired at the bias voltage of 75 mV. The performances of the device indicate that the terahertz QWP can be employed to fast and sensitive detect the terahertz radiation in the reflective confocal scanning imaging system.

4 Imaging results and discussion

Due to the Rayleigh diffraction limit, the resolution of terahertz imaging system is limited by the wavelength of the terahertz wave and the numerical aperture of the system. The traditional terahertz far-field imaging system can obtain a resolution of sub-millimeter. Two pinholes are employed to improve the spatial resolution of the fast reflective confocal imaging system. To represent the resolution of the system, a terahertz array was employed to obtain the beam profiles of the terahertz beam in the plane of the first pinhole (PH1) and the object, as shown in Fig. 3a, b, respectively. The beam profile in the object plane is acquired at 30° angle. The terahertz array is an uncooled micro-bolometer with a pixel points of 320 × 240 and a pixel dimension of 23.5 μm. The horizontal resolution Δx, vertical resolution Δy, and axial resolution Δz based on the full width at half-maximum (FWHM) of a dual-axis system can be, respectively, given by [14]:

Fig. 3
figure 3

The beam profiles of the terahertz beam in the plane of a the first pinhole (PH1) and b the object; the experimental and fitting results of the resolution in the c horizontal, d vertical and e axial dimensions

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$$\Delta x = \frac{0.37 \lambda }{{\sin \alpha_{i } \times \cos \theta }}, \Delta y = \frac{0.37 \lambda }{{\sin \alpha_{i } }}, \Delta z = \frac{0.37 \lambda }{{\sin \alpha_{i } \times \sin \theta }},$$
(1)

where λ is the wavelength of the terahertz wave, sinαi is the N.A. of the terahertz imaging system, and θ is the angle between the optical axis of the reflected beam and the z-axis. By theoretical calculation, the horizontal, vertical, and axial resolutions are 85, 74, and 148 μm, respectively.

The means of the scanning knife-edge technique [9, 32] was applied to test the resolution of the terahertz imaging system in the object plane. The experimental and fitting results of the horizontal and vertical directions are exhibited in Fig. 3c, d, respectively. According to the Rayleigh criterion, the resolutions are obtained from the fitting results to be about 110 μm and 80 μm in the horizontal and vertical dimensions, respectively. The FWHM of the echo, namely, the axial resolution of the system, is measured by moving the plane mirror installed on the object plane along the axial direction (Z) [14]. The experimental and Gaussian fitting results are plotted in Fig. 3e. The fitting result is basically consistent with the experiment. An axial resolution of 320 μm is obtained according to the fitting result.

The images of the terahertz system are a circular region with 5000 pixels and own a maximal diameter of 100 mm. The diameter of the terahertz image can be adjusted to fit the size of the object. And the signal noise radio (SNR) of the terahertz imaging system is about 21 dB. The terahertz image of the brush part of a plastic toothpick was obtained to explore the performance of the fast terahertz reflective confocal scanning imaging system. Figure 4a shows the photographic image of the brush part which has a length of about 10 mm. The terahertz image of the brush part fixed by the tape is exhibited in Fig. 4b. The enlarged terahertz image of the brush part is demonstrated in the inset of Fig. 4b for better observation. It is easy to distinguish the brush part in the terahertz image, even the region covered by the tape. To accurately characterize the quality of the terahertz image, the signal amplitude of the terahertz light along the line “m” in the inset of Fig. 4b is plotted in Fig. 4c. The contrast “C” of the terahertz image is evaluated as follows [33]:

Fig. 4
figure 4

THz imaging of the brush part of a plastic toothpick and the image analysis. a The photographic image and b THz image of the brush part fixed by the tape; c the signal amplitude along line “m” in the inset of b

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$$C = \frac{{I_{{\max} } - I_{{\min} } }}{{I_{{\max} } + I_{{\min} } }},$$
(2)

where \(I_{{\max} }\) and \(I_{{\min} }\) are the maximum and minimum signal amplitude. Finally, the contrast of the brush part is calculated to be about 0.82, the result shows that the brush part is distinguishable in the terahertz image. The real diameter of the branch of the brush part is about 200 μm which is comparable to the lateral resolution of the fast terahertz reflective confocal scanning imaging system.

The images of three different metallic objects are demonstrated in Fig. 5. These metallic objects are placed on a piece of paper and can easily reflect the terahertz radiation. Figure 5a gives the photographic image of a razor blade which owns a size of about 40 mm. It is easy to discern the outline of the razor blade according to the terahertz image, as shown in Fig. 5b. The uneven brightness part of the terahertz image is caused by the non-planar surface of the razor blade. The terahertz image of a square plate (Fig. 5c) with some holes is shown in Fig. 5d. We can clearly make out the square plate and the holes without uneven brightness from the terahertz image. The terahertz image of a coin is exhibited in Fig. 5f with a diameter of 30 mm. The raised characters and numbers of the coin can be identified from the terahertz image with some fine details. The face value “5” of the coin stands out from the background surface on account of the height difference and the terahertz scattering. The little deformation in the center of the terahertz image is mainly caused by the inaccurate position of the rotating translational platform. The great collimation of the razor edge and undistorted outline of the holes in the square plate indicate the small image distortion of the terahertz imaging system. The well axial selectivity of the system is exhibited by the large strength difference between the raised part and the surface of the coin.

Fig. 5
figure 5

The images of three different metallic objects; photographic images and THz images of a razor blade (a, b), a square plate (c, d), and a coin (e, f)

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Finally, we pay attention to the axial selectivity of the terahertz reflective confocal scanning imaging system. We performed imaging of a combination which is composed of the coin and the square plate in Fig. 5. The coin, with back side up, was placed on the top of the square plate. The planform and the front view of the combination are demonstrated in Fig. 6a, b. The “Z” direction expressed on the right of Fig. 6b represents the axial direction of this terahertz imaging. The rectangular region in Fig. 6b is zoomed into view, as shown in Fig. 6c. The terahertz three-dimensional sections of the combination in different axial positions are exhibited in Fig. 6d. The position of the first section of Fig. 6d is stipulated to be the null point of the “Z” direction. With increasing of the axial position “Z”, the clearest images of the coin and the square plate are obtained in the position of 0.5 mm and 2.16 mm, respectively. The two positions indicate that the terahertz beam is focused on the back of the coin and the upper surface of the square plate. By calculating the difference between the two positions, the thickness of the coin can be estimated to be 1.66 mm which is very close to the true value of 1.68 mm.

Fig. 6
figure 6

The planform view (a), the front view (b), and the zoomed view (c) of a combination which is composed of the coin and the square plate in Fig. 5; d the terahertz three-dimensional sections of the combination in different axial positions

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5 Conclusion

We report on implementation of a fast terahertz reflective confocal scanning imaging system with a fast rotating translational platform and a fast terahertz quantum-well photodetector (QWP). The imaging time of the terahertz imaging system can be shortened to be 5 s. The performances of the terahertz devices indicate that the 4.3 THz radiation generated from the pulsed electrically pumped terahertz quantum cascade laser (QCL) can be detected by the terahertz QWP. Two 200 μm pinholes are employed to improve the resolution of the imaging system; we achieve a lateral resolution better than 110 μm and an axial resolution of about 320 μm. The terahertz two-dimensional images of some objects are obtained within 5 s with a high contrast. The imaging time of the scanning imaging system is limited by the fast rotating translational platform at present. The terahertz three-dimensional sections of a combination in different axial positions exhibit the great axial selectivity of the terahertz imaging system. In the future, more pixels can be added to improve the quality of the terahertz images based on the fast terahertz QWP, and faster rotating translational platform can be employed to increase the imaging speed of the system. This terahertz reflective confocal scanning imaging system is expected to be applied in the non-destructive inspection, biomedicine and the research of the fast physical or chemistry process.