Terahertz Nonlinear Ghost Imaging via Plane Decomposition: Toward Near-Field Micro-Volumetry

Terahertz microscopy through complex media

Manipulating broadband fields in scattering media is a modern challenge across photonics and other wave domains. Recent studies have shown that complex propagation in scattering media can be harnessed to manipulate broadband light wave packets in space-time for focusing, imaging, and computing applications. Interestingly, while many proposed methodologies operate on intensity-based assessment of scattered fields, often in the spectral domain, from a pure transmission-function perspective, scattering operates as a linear field-level combinatory process, i.e., the superposition of transformation of unit excitations. As a result, we recently demonstrated that gaining experimental access to instantaneous scattered fields, as available through time-domain spectroscopy in the terahertz (THz) spectral range, in conjunction with sparse light excitation typical of ghost imaging, provides a key advantage in enabling the functionalisation of scattering, exposing a novel modelling paradigm. In this paper, we provide experimental proof of reconstructing 1-dimensional object features through a scattering medium using a fully broadband THz time-domain approach.

Flyscan terahertz multi-plane lensless imaging with suppressed coherent noise

Coherent lensless imaging usually suffers from coherent noise and twin-image artifacts. In the terahertz (THz) range, where wavelengths are 2 to 4 orders of magnitude longer than those in the visible spectrum, the coherent noise manifests primarily as parasitic interference fringes and edge diffraction, rather than speckle noise. In this work, to suppress the Fabry-Pérot (F-P) interference fringes, we propose a novel method, which involves the averaging over multiple diffraction patterns that are acquired at equal intervals within a sample's half-wavelength axial shift. To address edge diffraction, as well as non-uniform illumination, a normalization operation is applied. As the twin-image disturbances when dealing with a single diffraction pattern, multi-plane configuration is employed. With all these strategies combined, we propose a flyscan THz multi-plane lensless imaging technique that enables subwavelength resolution, and high-quality, full-field, and rapid complex-valued THz imaging. Furthermore, we refine two algorithms for image reconstruction: one based on the regular multi-plane alternating projection and the other based on an optimization model with total variation regularization. We experimentally verify the proposed methods, achieving a lateral resolution of 88 µm (0.74λ) at 2.52 THz, and showcase its potential for biomedical applications by imaging a section of mouse brain tissue.

Increasing the Resolution of Transmission Electron Microscopy by Computational Ghost Imaging

By means of numerical simulations, we demonstrate the innovative use of computational ghost imaging in transmission electron microscopy to retrieve images with a resolution that overcomes the limitations imposed by coherent aberrations. The method requires measuring the intensity on a single pixel detector with a series of structured illuminations. The success of the technique is improved if the probes are made to resemble the sample and the patterns cover the area of interest evenly. By using a simple 8 electrode device as a specific example, a twofold increase in resolution beyond the aberration limit is demonstrated to be possible under realistic experimental conditions.

Terahertz optical pattern recognition with rotation and scaling enhanced by a 3D-printed diffractive deep neural network

Optical pattern recognition (OPR) has the potential to be a valuable tool in the field of terahertz (THz) imaging, with the advantage of being capable of image recognition with single-point detection, which reduces the overall system costs. However, this application is limited in the traditional OPR that rotation and scaling of the input image will bring about an offset of the recognition spot. Here we demonstrate a full-diffractive method to maintain the recognition spot at a fixed position, even when the input image is rotated or scaled, by using an all-optical diffractive deep neural network. The network is composed of two layers of diffractive optical elements (DOEs) without a 4f-system, and 3D-printed all-in-one. Experimental results show that our device can achieve a stable recognition of the input image regardless of its rotation (from 0° to 360°) or scaling (with a ratio from 1 to 1/1.9). This work is expected to provide enhanced functionality for compact THz systems in imaging and security applications.

Spatial and spectral beam characteristics in a terahertz broadband sub-wavelength imaging system using a solid immersion lens

This study focuses on the spatial and spectral beam characteristics in a terahertz (THz) broadband sub-wavelength imaging system using a solid immersion lens (SIL). Previously, we demonstrated a broadband sub-wavelength THz imaging system by integrating a SIL with a THz time-domain spectrometer (TDS). Key parameters that influence beam characteristics and, consequently, imaging performance, such as SIL misalignment tolerances and beam propagation from the SIL, constitute the primary focus of this investigation. Numerical simulations demonstrate that the system can tolerate millimeter-level transverse and longitudinal SIL position displacements, underscoring its robustness for sub-wavelength imaging in a wide frequency range. Additionally, numerical simulations of beam propagation characteristics reveal that the system achieves sub-wavelength imaging resolution up to 1 mm from the SIL at 0.5 THz, highlighting its potential for non-destructive testing of subsurface structures. These findings gain experimental validation through imaging stacked utility knife blades with sub-wavelength structures ranging from 0.2 to 2 THz.

Cross-waveband optical computing imaging

A novel, to the best of our knowledge, cross-spectral optical computing imaging experiment has been achieved through a single exposure of a charge-coupled device. The experimental setup integrates single-pixel imaging (SPI) with ghost imaging (GI) through a photoelectric conversion circuit and a synchronous modulation system. The experimental process involves modulation in one wavelength band (in SPI) and demodulation using the GI algorithm in another. Significantly, our approach utilizes optical computing demodulation, a departure from the conventional electronic demodulation in GI (SPI), which involves the convolution between the bucket optical signals and the modulated patterns on the digital micromirror device. A proof-of-concept cross-band imaging experiment from near-infrared to visible light has been carried out. The results highlight the system's ability to capture images at up to 20 frames per second using near-infrared illumination, which are then reconstructed in the visible light spectrum. This success not only validates the feasibility of our approach but also expands the potential applications in the SPI or GI fields, particularly in scenarios where two-dimensional detector arrays are either unavailable or prohibitively expensive in certain electromagnetic spectra such as x-ray and terahertz.

High-quality coherent ghost imaging of a transmission target

When the test detector of ghost imaging (GI) is a point-like detector and the detector's transverse size is smaller than the transverse coherence length of the light field at the detection plane, this case is corresponding to coherent GI (CGI) and the imaging result recovered by traditional GI (TGI) reconstruction algorithm is usually bad for a transmission target. Here a CGI scheme of a transmission target is proposed and a corresponding CGI reconstruction algorithm is developed to stably recover the target's image. The validity of the proposed method is verified by both simulation and experiments. Both the simulation and experimental results demonstrate that the target's transmission function can be perfectly reconstructed by CGI. We also show that the imaging quality of CGI with a point-like detector is better than that of TGI with a bucket detector if detection noise exists in the sampling process. Performance comparisons between CGI reconstruction and TGI reconstruction are also discussed.

Terahertz Spatiotemporal Wave Synthesis in Random Systems

Complex media have emerged as a powerful and robust framework to control light-matter interactions designed for task-specific optical functionalities. Studies on wavefront shaping through disordered systems have demonstrated optical wave manipulation capabilities beyond conventional optics, including aberration-free and subwavelength focusing. However, achieving arbitrary and simultaneous control over the spatial and temporal features of light remains challenging. In particular, no practical solution exists for field-level arbitrary spatiotemporal control of wave packets. A new paradigm shift has emerged in the terahertz frequency domain, offering methods for absolute time-domain measurements of the scattered electric field, enabling direct field-based wave synthesis. In this work, we report the experimental demonstration of field-level control of single-cycle terahertz pulses on arbitrary spatial points through complex disordered media.

Advances in Ghost Imaging of Moving Targets: A Review

Ghost imaging is a novel imaging technique that utilizes the intensity correlation property of an optical field to retrieve information of the scene being measured. Due to the advantages of simple structure, high detection efficiency, etc., ghost imaging exhibits broad application prospects in the fields of space remote sensing, optical encryption transmission, medical imaging, and so on. At present, ghost imaging is gradually developing toward practicality, in which ghost imaging of moving targets is becoming a much-needed breakthrough link. At this stage, we can improve the imaging speed and improve the imaging quality to seek a more optimized ghost imaging scheme for moving targets. Based on the principle of moving target ghost imaging, this review summarizes and compares the existing methods for ghost imaging of moving targets. It also discusses the research direction and the technical challenges at the current stage to provide references for further promotion of the instantiation of ghost imaging applications.

High-fidelity and high-robustness free-space ghost transmission in complex media with coherent light source using physics-driven untrained neural network

It is well recognized that it is challenging to realize high-fidelity and high-robustness ghost transmission through complex media in free space using coherent light source. In this paper, we report a new method to realize high-fidelity and high-robustness ghost transmission through complex media by generating random amplitude-only patterns as 2D information carriers using physics-driven untrained neural network (UNN). The random patterns are generated to encode analog signals (i.e., ghost) without any training datasets and labeled data, and are used as information carriers in a free-space optical channel. Coherent light source modulated by the random patterns propagates through complex media, and a single-pixel detector is utilized to collect light intensities at the receiving end. A series of optical experiments have been conducted to verify the proposed approach. Experimental results demonstrate that the proposed method can realize high-fidelity and high-robustness analog-signal (ghost) transmission in complex environments, e.g., around a corner, or dynamic and turbid water. The proposed approach using the designed physics-driven UNN could open an avenue for high-fidelity free-space ghost transmission through complex media.

Elimination of signal amplitude disturbance in ghost imaging using an auxiliary laser channel

Ghost imaging can be used to detect objects in a nonstationary environment or in the presence of variable ambient light, making it attractive when conventional imaging methods are ineffective. However, the conventional ghost imaging algorithm is susceptible to temporal fluctuations in the detected signal. In this work, we propose a polarization-multiplexed auxiliary laser channel propagating along the same optical path with the main one. The signal in the auxiliary channel is used as a reference and allows the elimination of signal disturbance. A quantitative analysis and comparison of the proposed method's performance to the high-pass filtering method are demonstrated. For an illumination pattern refresh rate of 10 Hz, effective suppression of bucket signal fluctuations has been experimentally demonstrated. For a disturbance frequency from 1 Hz to 10 Hz, the auxiliary channel method demonstrated a ghost image Pearson correlation coefficient (PCC) of not less than 0.70, while the high-pass filtering method showed a PCC sharp drop from 0.65 to 0.02.