Non-invasive near-field THz imaging using a single pixel detector

The terehertz radiation potentially has many interesting applications. From air port security, non-destructive evaluations of electronics and space shuttle panels, to non-ionizing photon energies with the potential to detect cancer growths and quality control of pharmaceutical tables, the list of potential applications is vast as shown in chapter 1. However, there is a lack of cheap, robust and efficient THz sources, detectors and modulators. Further, the long wavelengths render micron sized details unseeable with far-field imaging techniques. This has rendered most imaging applications unusable in the real world. This thesis is based around demonstrating an imaging technique that uses a near-field THz modulator to obtain sub-wavelength images. There are five distinct experimental demonstrations that show the full capacity of the imaging technique developed here. Chapter 2 gives an outline of the background physics knowledge needed to understand the entirety of the thesis. An outline of the mathematics used for modellingis given in the latter part of the chapter as well. Chapter 3 gives a background on the THz generation and detection techniques used in our THz-TDS system, optical rectification and electro-optic sampling in ZnTe. Further more, our system is capable of photoexciting a sample in conjunction to it being probed with a THz pulse. For the most part, we photoexcite a silicon wafer in order to use its photoconductive properties to modulate our THz pulse. Our photoexcitation pulse is spatially modulated, via a digital micromirror device, which in turn spatially modulates our THz pulse. This patterned THz pulse can then be used with a single-element detector to perform imaging. How to do this and the type of patterns needed is described in the latter part of chapter 3. Chapter 4 is the first demonstration that photo-induced conductivity in silicon can be used to manipulate evanescent THz fields for sub-wavelength imaging. For this, we imaged a 1D sub-wavelength slit and were able to obtain the slit profile with 65μm (λ/6 at 0.75T Hz) resolution. Chapter 5 demonstrates what limits the resolution in our imaging system. Namely, the distance which the patterned THz pulse propagates to the object from where itwas spatially modulated. We demonstrate 9μm (λ/45 at 0.75T Hz) resolution using an ultra-thin (6μm) silicon wafer. At such sub-wavelength scales polarization becomes an important factor. We show how one can use polarization in order to detect 8μm breaks in a circuit board hidden by 115μm of silicon. Chapter 6 concerns itself with showing how noise affects our images. Further more, our imaging system is compatible with compressed sensing where one can obtain an image using fewer measurements than the number of pixels. We investigate how different under-sampling techniques perform in our system. Note under-sampling at sub-wavelength resolutions, as is done here, is rather unusual and is of yet to be demonstrated for other part of the electro-magnetic spectrum. Chapter 7 shows that one does not need to photoexcite silicon. One can in principle illuminate any material, hence we photoexcite graphene with our spatially modulated optical pulses. This allows us to obtain the THz photoconductive response of our graphene sample with sub-wavelength resolution (75μm ≈ λ/5 at 0.75T Hz). We compare our results with Raman spectra maps. We find a clear correlation between THz photoconductivity and carrier concentration (extracted from Raman). Chapter 8 exploits the full capacity of our imaging system by performing hyper-spectral near-field THz imaging on a biological sample. For this, in our imaged field of view, we measured the full temporal trace of our THz pulse at a sub-wavelength spatial resolution. This has allowed us to extract the frequency dependent permittivity of our biological sample, articular cartilage, over our spectral range (0.2-2T Hz). We find the permittivity to change on a sub-wavelength scale in correlation with changes in the structure of our sample. However, the permittivity extraction procedures that have been developed make a far-field approximation. We mathematically show the presence of the THz near-fields to render the long wavelength spectral parts of our extracted permittivity to be wrong. Chapter 9 is where we conclude and point out the main problem that needs to be addressed in order to make the measurements presented here more accessible to others. Namely, the cost of the laser system powering the THz-TDS and how to further reduce the acquisition time.QinetiQ iCase award 1244057

Real-time terahertz imaging with a single-pixel detector

Terahertz (THz) radiation is poised to have an essential role in many imaging applications, from industrial inspections to medical diagnosis. However, commercialization is prevented by impractical and expensive THz instrumentation. Single-pixel cameras have emerged as alternatives to multi-pixel cameras due to reduced costs and superior durability. Here, by optimizing the modulation geometry and post-processing algorithms, we demonstrate the acquisition of a THz-video (32 × 32 pixels at 6 frames-per-second), shown in real-time, using a single-pixel fiber-coupled photoconductive THz detector. A laser diode with a digital micromirror device shining visible light onto silicon acts as the spatial THz modulator. We mathematically account for the temporal response of the system, reduce noise with a lock-in free carrier-wave modulation and realize quick, noise-robust image undersampling. Since our modifications do not impose intricate manufacturing, require long post-processing, nor sacrifice the time-resolving capabilities of THz-spectrometers, their greatest asset, this work has the potential to serve as a foundation for all future single-pixel THz imaging systems

Spatial terahertz-light modulators for single-pixel cameras

Terahertz imaging looks set to become an integral part of future applications from semiconductor quality control to medical diagnosis. This will only become a reality when the technology is sufficiently cheap and capabilities adequate to compete with others. Single-pixel cameras use a spatial light modulator and a detector with no spatial-resolution in their imaging process. The spatial-modulator is key as it imparts a series of encoding masks on the beam and the detector measures the dot product of each mask and the object, thereby allowing computers to recover an image via post-processing. They are inherently slower than parallel-pixel imaging arrays although they are more robust and cheaper, hence are highly applicable to the terahertz regime. This chapter dedicates itself to terahertz single-pixel cameras; their current implementations, future directions and how they compare to other terahertz imaging techniques. We start by outlining the competing imaging techniques, then we discuss the theory behind single-pixel imaging; the main section shows the methods of spatially modulating a terahertz beam; and finally there is a discussion about the future limits of such cameras and the concluding remarks express the authors’ vision for the future of single-pixel THz cameras

Spatial Terahertz-Light Modulators for Single-Pixel Cameras

Terahertz imaging looks set to become an integral part of future applications from semiconductor quality control to medical diagnosis. This will only become a reality when the technology is sufficiently cheap and capabilities adequate to compete with others. Single-pixel cameras use a spatial light modulator and a detector with no spatial-resolution in their imaging process. The spatial-modulator is key as it imparts a series of encoding masks on the beam and the detector measures the dot product of each mask and the object, thereby allowing computers to recover an image via post-processing. They are inherently slower than parallel-pixel imaging arrays although they are more robust and cheaper, hence are highly applicable to the terahertz regime. This chapter dedicates itself to terahertz single-pixel cameras; their current implementations, future directions and how they compare to other terahertz imaging techniques. We start by outlining the competing imaging techniques, then we discuss the theory behind single-pixel imaging; the main section shows the methods of spatially modulating a terahertz beam; and finally there is a discussion about the future limits of such cameras and the concluding remarks express the authors’ vision for the future of single-pixel THz cameras

Super sub-nyquist single-pixel imaging by total variation ascending ordering of the Hadamard absis

Single pixel imaging (SPI) captures images without array detectors or raster scanning. When combined with compressive sensing techniques it enables novel solutions for high-speed optical imaging and spectroscopy. However, when it comes to the real-time capture and analysis of a fast event, the challenge is the inherent trade-off between frame rate and image resolution. Due to the lack of sufficient sparsity and the intrinsic iterative process, conventional compressed sensing techniques have limited improvement in capturing natural scenes and displaying the images in real time. In this work, we demonstrate a novel alternative compressive imaging approach employing an efficient and easy-implementation sampling scheme based on reordering the deterministic Hadamard basis through their total variation. By this means, the number of measurements and acquisition are reduced significantly without needing complex minimization algorithms. We can recover a 128 × 128 image with a sampling ratio of 5% at the signal peak signal-to-noise ratio (PSNR) of 23.8 dB, achieving super sub-Nyquist sampling SPI. Compared to other widely used sampling e.g. standard Hadamard protocols and Gaussian matrix methods, this approach results in a significant improvement both in the compression ratio and image reconstruction quality, enabling SPI for high frame rate imaging or video applications

Convolutional Neural Network based denoising method for rapid THz Imaging

We propose our own convolutional neural network (CNN) structure as a post-processing method for the rapid THz Imaging in Terahertz Time Domain Spectroscopy(THz-TDS). The experiment results show that our approach can greatly reduce the noise and artifacts in the collected under-sampled THz images, which in turn benefits the rapid THz imaging technique by allowing higher acquisition rates. In our setup, 5 times higher acquisition rates can be realized. Moreover, this approach needs no extra hardware cost, and since the training has been done offline, the processing time in practice can be ignored. To the best of our knowledge, this is the first time applying deep learning(DL) methods into rapid THz imaging technique, which would inspire researchers to explore more DL based applications and technologies for THz technique development

Rapid imaging of pulsed terahertz radiation with spatial light modulators and neural networks

We report the first use of a convolutional neural network for terahertz (THz) imaging combined with a single-pixel camera to achieve high quality hyperspectral THz imaging 10× faster than the current commercial systems, with faster potential only limited by noise. Imaging with a spatial light modulator (SLM) relies on projecting a set of spatial patterns onto an object and recording the transmission with a single-pixel detector, and an optical delay unit (ODU) is used to obtain the terahertz time-of-flight information. A key breakthrough in this work is to synchronize the equipment to not need any time to instruct the ODU to move while projecting the patterns