# A Low-Cost Terahertz Camera
## 2 Materials and Methods  
To perform our imaging comparison, a THz source based on the tilted-pulse-front pumping (TPFP) scheme in a lithium niobate crystal was employed, as illustrated in Figure 1a. The pumping source consists of an amplified Ti-Sapphire femtosecond laser providing 120 fs duration, 5 mJ pulses at a repetition rate of 1 kHz with a center wavelength of 800 nm. As shown in Figure 1b, the THz source delivered intense THz pulses centered at 1 THz with a peak electric field at the focus on the order of 500 kV/cm. According to its approximately 3 mJ in energy per pulse, the maximum average THz power available is approximately 3 mW. To perform our imaging demonstration, we used an uncooled microbolometer camera (IRXCAM-384 THz from INO [13]) as a reference sensor and compared its response with a low-cost thermal camera (Compact pro from Seek Thermal). Both cameras were tested at the focus of the THz source, as illustrated in Figure 1a. Notice that to record images or movies using the Compact Pro camera, either an Apple iPhone or Android device is required. Both cameras have a similar number of pixels and are based on the sensing technology of vanadium oxide material [13; 16]. The microbolometer array from INO is composed of \(384\times 288\) pixels with 35 um in pitch, whereas, the Compact Pro used an uncooled microbolometer array of \(380\times 240\) pixels with 12 um in pitch. Since vanadium oxide sensors are highly sensitive to ambient heat given their insulating to metal transition at \(\approx\)340 K, a mechanical chopper is included inside the Compact pro camera. This shutter is activated depending on the background information. On the other hand, the IRXCAM-384 THz is easily triggered via a BNC connector, thus enabling synchronous background subtraction when combined with a mechanical chopper. This option radically improved the imaging contrast of THz acquisitions and is obviously desirable for accurate measurements. In order to turn the Compact Pro thermal camera into a THz sensor, the small chalcogenide lens used to form images at the sensor position must be removed. Indeed, while highly suitable for the infrared wavelength range (i.e., from 1 um to 10 um), the absorption coefficient of such material is extremely high in the THz frequency range [17]. Figure 2a shows the difference in absorption coefficients between a high resistivity silicon wafer and a typical chalcogenide glass material in the THz frequency range. The inset in Figure 2a shows a zoomed view of the small 0.02 cm\({}^{-1}\) in power absorption of a high resistivity (\(>\) 20,000 ohm-cm) silicon wafer, in good agreement with [18]. By removing the 7-mm-thick chalcogenide lens, the vanadium oxide sensor can be directly illuminated with THz radiation without suffering from the high attenuation factor of chalcogenide glass (e.g., \(>\) 80 cm\({}^{-1}\) at 1 THz) [17]. We note that the use of the IRXCAM-384 THz camera for general pulse diagnostics involves illuminating the sensor without the accompanying optical lenses. Bare metallic reflective OAP mirrors are preferred to produce an image at the sensor position, especially when a broadband THz pulse source is used. Here, the focusing conditions were similar for both tests using OAPs. To attenuate the THz beam, several high resistivity silicon wafers were inserted in the collimated beam path, as illustrated in Figure 1a. Since the THz absorption in intrinsic silicon is negligible, the transmittance (T) and reflectance (R) of a THz beam depends solely on the Fresnel losses. The situation is illustrated in Figure 2b, with the ambient refractive index (n\({}_{1}\)), the silicon refractive index (n\({}_{2}\)) and an incidence angle (\(\theta\)). The transmittance T\({}_{12}\) denotes the transmittance through the interface, when THz light comes from medium 1 and passes into medium 2. The primarily transmitted light is again transmitted into the \"third medium\", which in our case is identical to the first one (i.e., air) with transmittance T\({}_{21}\).