Full-Field Terahertz Imaging at Kilohertz Frame Rates Using Atomic Vapor

There is much interest in employing terahertz (THz) radiation across a range of imaging applications, but so far, technologies have struggled to achieve the necessary frame rates. Here, we demonstrate a THz imaging system based upon efficient THz-to-optical conversion in atomic vapor, where full-field images can be collected at ultrahigh speeds using conventional optical camera technology. For a 0.55-THz field, we show an effective $1\text{−}{\mathrm{cm}}^2$ sensor with near diffraction-limited spatial resolution and a minimum detectable power of $(190\pm 30)\mathrm{fW}{\mathrm{s}}^{-1/2}$ per $(40\times 40)\mu {\mathrm{m}}^2$ pixel capable of video capture at 3000 frames per second. This combination of speed and sensitivity represents a step change in the state of the art of THz imaging and will likely lead to its uptake in wider industrial settings.

Popular Summary

The terahertz (THz) frequency band lies on the electromagnetic spectrum between the infrared and microwave regions. Because THz radiation is not harmful and passes easily through materials such as paper, cloth, and plastics, it has many potential applications, including security screening, biomedical imaging, and production-line monitoring. For these and other applications, it is desirable to be able to acquire images at high frame rates, something that current THz technologies have struggled to achieve. Here, we demonstrate a THz imaging system based on THz-to-optical conversion in a laser-excited vapor, allowing us to capture images at high speeds.

We show how room-temperature cesium atoms that have been excited using infrared lasers emit green light when illuminated with THz radiation. This green light can be captured on any optical camera, making this a highly versatile system. We use this process to develop an imaging system with a 1-mm spatial resolution capable of capturing images at 3000 frames per second.

While this scheme is still in the development phase, it presents an entirely new technique for imaging THz radiation, one which enables imaging at previously unattainable speeds. This advance may allow for the development of commercial THz-based imaging systems or the further exploration of high-speed phenomena in the THz regime.

Figure 1

THz imaging using atomic vapor. (a) The internal energy structure of caesium, highlighting the excitation and principal decay pathways used in this work. The red arrows correspond to laser excitations, the purple arrow to the excitation driven by the THz field, and the orange and green arrows to optical fluorescence emitted by decay from the $14{\mathrm{P}}_{3/2}$ and $13{\mathrm{D}}_{5/2}$ states, respectively. The large green arrow represents the principal decay pathway. (b) The spectral characteristics of the fluorescence from the vapor, both with and without the THz field (green and orange lines, respectively). The shaded region highlights the optical filter used to isolate the spectral region of interest. Inset: The dependence of the fluorescence signal on THz detuning, from which we extract a linewidth of $(14.0\pm 0.4)\mathrm{MHz}$. (c) A diagram of the imaging setup described in this work. Infrared pump lasers form a sheet of excited caesium atoms contained within a quartz cell. The THz field passes through an object that is imaged onto the atoms using PTFE lenses. Interaction of the THz field with the excited atoms leads to the emission of green fluorescence, which is captured using an optical camera.

Figure 2

Demonstration of spatial resolution. (a) A metal mask (center) placed in the object plane of the system. To the left and right are true-color images taken with camera A (above) and false-color images taken with camera B (below) for a 0.50-mm-diameter pinhole (left) and a “$\mathrm{\Psi }$”-shaped aperture (right). (b) Radially averaged intensity profiles of the measured PSF (solid blue line) and the simulated ideal PSF (dashed orange line). Both PSFs were scaled such that the total power in each image was equal. The ratio of peak heights gives a Strehl ratio of 0.57. (c) Real (top panel) and simulated (bottom panel) images of two 0.50-mm-diameter pinholes separated by 1.00 mm.

Figure 3

Ultrahigh-speed THz video. (a) Subsequent frames from a THz video of an optical chopper wheel rotating at 700 rpm, imaged at a frame rate of 3 kHz. The white arrow is added to highlight the movement of one spoke of the wheel between frames. (b) A significantly slower frame rate of 500 Hz, is sufficient to capture the dynamics of a water droplet in free fall shortly after being released from a buret, every fourth frame of which is shown here. Full video files are available online [44].

Figure 4

Minimum detectable power. (a) The fluorescence signal ${\mathrm{THz}}_{\text{on}}-{\mathrm{THz}}_{\text{off}}$ (open blue circles) as a function of incident THz power for a total integration time of 1 second. The linear trendline is extrapolated from that in panel (b) to highlight the saturation point at THz powers above 20 pW per $(40\times 40)\mu {\mathrm{m}}^2$ pixel. The vertical dashed lines highlight the range of the data used in panel (b). (b) Plotting the measured fluorescence signal (open blue circles), we can map the linear response of the system (solid green line) plus its associated uncertainty (green shaded region) to the point at which the fluorescence signal is no longer reliably detectable (dashed orange line), at $(190\pm 30)\mathrm{fW}$ per $(40\times 40)\mu {\mathrm{m}}^2$ pixel for a 1-s integration time.