High-resolution imaging in the terahertz (THz) spectral range remains fundamentally constrained by the limited numerical apertures of currently existing state-of-the-art imagers, which restricts its applicability across many fields, such as imaging in complex media or nondestructive testing. To address this challenge, we introduce a proof-of-concept implementation of THz Fourier Ptychographic imaging to enhance spatial resolution without requiring extensive hardware modifications. Our method employs a motorized kinematic mirror to generate a sequence of controlled, multi-angle plane-wave illuminations, with each resulting oblique-illumination intensity image encoding a limited portion of the spatial-frequency content of the target imaging sample. These measurements are combined in the Fourier domain using an aberration-corrected iterative phase-retrieval algorithm integrated with an efficient illumination calibration scheme, which enables the reconstruction of resolution-enhanced amplitude and phase images through the synthetic expansion of the effective numerical aperture. Our work establishes a robust framework for high-resolution THz imaging and paves the way for a wide array of applications in materials characterization, spectroscopy, and non-destructive evaluation.

THz generation from chirped and delayed laser pulses using periodically poled lithium niobate (PPLN) traditionally requires high-energy sources and bulky PPLN crystals. However, for compact, integrated, and miniaturized THz-based applications, such systems must be downsized. In this work, we numerically and experimentally demonstrate THz generation in a PPLN waveguide with a cross-section of 500 × 500 µm2, pumped with low pulse energies in the microjoule range at a wavelength of 1 µm. Simulations show that tightly focused optical pulses in this configuration achieve significantly higher THz generation efficiency compared to collimated pumping. For a propagation length of 1.6 cm, optimal parameters are identified as a pulse duration of 5 ps and a pump energy between 0.5 and 1 µJ. Experimental validation confirms these findings, yielding a narrowband THz spectrum centered around 488 GHz. This work opens new perspectives for the development of efficient, integrated THz sources for advanced applications in THz photonics, spectroscopy, and high-speed communications.

Phase imaging in the terahertz (THz) spectral range has gained significant traction as a potential technique for various industrial applications, including sensing and non-destructive inspection of non-polar material [1]. In this context, THz time-domain spectroscopy (TDS) has offered robust solutions by capturing images that encode hyperspectral amplitude and phase information. In such a direction, researchers have integrated TDS imaging systems with near-field electro-optic sampling [2], [3] or multi-angle illumination [4] approaches to achieve high-resolution imaging solutions. However, TDS-based techniques are frequently constrained by a lack of matrix detectors, leading to time-consuming raster scanning of imaging object. Moreover, TDS imaging systems rely on controllable lab environments and exhibit relatively lower stability.

Located between microwaves and infrared, the terahertz (THz) frequencies find various applications in recent technologies, notably in non-destructive imaging and telecommunications [1]. The method commonly used for their generation and detection is based on the conversion of infrared ultra-short pulses into THz waves by exciting photoconductive (PC) antennas. While being an efficient process offering a high dynamic range, the ultra-short optical pulse duration and high peak power bring some limitations, particularly for integrating devices toward a high-power level of optical excitation. Here, we present a novel method for photo-mixing generation in PCs using long chirped pulses to reduce peak power, thereby producing broadband, long-duration THz radiation. This approach offers new opportunities for temporal spectroscopy and the development of integrated devices.

Terahertz (THz) microscopy has attracted attention owing to distinctive characteristics of the THz frequency region, particularly non-ionizing photon energy, spectral fingerprint, and transparency to most nonpolar materials. Nevertheless, the well-known Rayleigh diffraction limit imposed on THz waves commonly constrains the resultant imaging resolution to values beyond the millimeter scale, consequently limiting the applicability in numerous emerging applications for chemical sensing and complex media imaging. In this theoretical and numerical work, we address this challenge by introducing, to our knowledge, a new imaging approach based on acquiring high-spatial frequencies by adapting the Fourier synthetic aperture approach to the THz spectral range, thus surpassing the diffraction-limited resolution. Our methodology combines multi-angle THz pulsed illumination with time-resolved field measurements, as enabled by the state-of-the-art time-domain spectroscopy technique. We demonstrate the potential of the approach for hyperspectral THz imaging of semi-transparent samples and show that the technique can reconstruct spatial and temporal features of complex inhomogeneous samples with subwavelength resolution.

Terahertz (THz) generation via photomixing on photoconductive antenna using twin delayed chirped pulses provides a long THz pulse with a narrow bandwidth. To generate a long pulse with broad bandwidth, we propose a new method that combines two long optical pulses with opposite chirps. The pulses exhibit temporal distributions of their instantaneous frequencies with opposite slopes. As a result, interaction between the beat frequency evolving over time and a photoconductor produces a broad THz spectrum with temporal variations. In our experimental setup, we generate a 12 ps-long pulse with a 1 THz bandwidth spectrum, featuring a frequency ramp of 90 GHz/ps, resembling a chirped THz pulse. This approach signifies a major advancement toward integrating photomixer technology, particularly in THz ranging applications.

Polariton scattering from the lowest A1-symmetry TO vibrational mode in lithium niobate presents a promising approach for generating THz radiations. Nevertheless, a notable challenge hindering this technique is the generation of frequencies exceeding 2 THz, primarily attributed to the gain spectrum of Lithium Niobate centered at 2 THz and the substantial absorption coefficient at higher frequencies. Although THz output beyond this frequency has been intermittently observed in THz generators utilizing stimulated polariton scattering, the generation of higher frequency outputs remains largely unexplored. In this study, we report a direct measurement of a broadband THz field with a bandwidth of approximately 4 THz centered at 3.1 THz. This electric field is generated from ultra-short 1 mu m wavelength pump pulses focused in a waveguide made of LiNbO3. The theoretical phonon-polariton dispersion curve, calculated parametric gain, and the measured signal spectrum generated in the near-infrared confirm the THz field emission obtained from an electro-optic detection. Published by Optica Publishing Group under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

This study investigates the application of terahertz frequency-modulated continuous-wave (FMCW) imaging for the non-destructive inspection of a historical enamel plate, using both reflection and transmission modes. A 300 GHz FMCW radar system was employed to capture high-resolution images of the plate’s internal and surface structures. Through optimized data acquisition and processing, the system successfully revealed subsurface features such as fractures, as well as surface-level textural variations linked to the decorative glazes. Although pigment differentiation remains a challenge, contrast variations observed in THz images suggest correlations with material composition. The results highlight the potential of FMCW terahertz imaging as a compact, rapid, and non-contact diagnostic tool for cultural heritage analysis. Its practicality and adaptability make it particularly suitable for in situ inspections in museums or restoration contexts.

Terahertz (THz) microscopy has garnered significant attention due to the unique properties of THz radiation, making it appealing for chemical sensing, biomedical diagnostics, and imaging of complex media applications. However, conventional THz imaging systems encounter fundamental resolution limitations due to the Rayleigh diffraction limit. To overcome this challenge, we propose an innovative theoretical and numerical framework by adopting Fourier synthetic aperture principles in the THz spectral range. Our approach synthesizes high spatial frequency components by illuminating the sample with broadband THz pulses from multiple angles and capturing the time-resolved scattered fields through state-of-the-art time-domain spectroscopy (TDS). Our method offers a field-sensitive and experimentally viable solution for surpassing the diffraction limit in THz imaging. It paves the way for high-resolution, label-free material characterization and noninvasive imaging across various domains, including materials science, biomedical imaging, and security screening.

Terahertz (THz) imaging systems have become indispensable tools for analysing the chemical and material composition of samples, driving the evolution of THz science as a distinct and expanding discipline [1]. Despite their transformative potential, THz imaging systems face critical technological limitations, particularly in achieving a wide field-of-view (FOV) and high spatial resolution imaging solutions [2]. To address these challenges, synthetic aperture imaging techniques have emerged as a promising route. The synthetic aperture approach involves capturing multiple diffraction patterns, each containing a limited spectrum of spatial frequency components, and combining them coherently to extend the system's spatial frequency bandwidth [3]. Following this, we theoretically explore the unique potential by devising a spatiotemporal, coherent synthetic aperture framework in the THz domain [4]. Our methodology, which outperforms the resolution limit in conventional THz imaging, involves synthesising an expanded spatial frequency bandpass filter scanned across the Fourier space by illuminating the sample with a broadband THz pulse at various angles. In our imaging analysis, we then incorporate a propagation of the temporal traces obtained through time-domain spectroscopy (TDS) detection, facilitating the reconstruction of the full-field response of the sample, including phase and temporal delay contributions. This approach permits us to acquire extensive spatiotemporal information on the light-matter interaction between the THz beam and the sample.

Phase imaging in the terahertz (THz) regime poses both fundamental and practical challenges, necessitating innovative computational imaging strategies. In such direction, interferometric reconstruction and digital holography have been widely implemented; however, these techniques are inherently limited by their reliance on phase-stable reference beams, making them susceptible to noise and system instabilities. In this work, we address such limitations and present theoretical and experimental demonstrations of a reference-free phase retrieval approach based solely on direct intensity measurements. We investigate two experimental configurations for phase retrieval. The first employs a lensless imaging approach based on recording axial diffraction patterns at multiple planes. The second configuration captures defocused intensity patterns around the focal plane of a 4f imaging system. In both cases, we implement a nonlinear convex optimization framework to recover the complex phase from the stack of intensity measurements. Our proposed approach offers a robust and computationally efficient solution to the phase retrieval problem, effectively overcoming the limitations of slow convergence and stagnation commonly associated with traditional multi-plane methods. Such results pave the way for the broader adoption of computational THz imaging algorithms in diverse applications, including material characterization, biomedical diagnostics, and security screening.

Advancements in terahertz (THz) technology have substantially propelled the capabilities of imaging systems; however, retrieving the complex phase distribution of arbitrary test objects within the THz spectral range continues to face fundamental and practical challenges. In this context, interferometry-based imaging schemes have been widely adopted for THz phase imaging. However, the efficiency of such techniques is fundamentally constrained by their reliance on phase-stable reference beams, rendering them particularly susceptible to noise and inherent imaging system instabilities. In this work, we address such limitations and experimentally investigate two non-interferometric imaging configurations integrated with a computational phase retrieval framework, enabling accurate recovery of complex-valued wavefronts from intensity-only data. The first configuration employs a lens-less imaging approach based on recording axial diffraction patterns at multiple planes. The second configuration captures defocused intensities near the focal plane of a 4f imaging system. Our proposed computational framework formulates the phase retrieval task as a nonlinear least-squares error optimization problem. Specifically, we employ an accelerated second-order optimization strategy by iteratively computing both the gradient and the Hessian of the cost function, which outperforms previously demonstrated multi-plane phase reconstruction approaches. Our imaging framework marks a significant step forward in computational THz imaging, with promising implications for diverse applications, such as non-destructive testing for materials characterization, biomedical diagnostics, and security screening. (c) 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

This publisher's note contains a correction to Opt. Lett.48, 5463 (2023)10.1364/OL.499478.

The presentation will focus on polariton parametric scattering in a rectangular waveguide made of Lithium Niobate to generate THz radiation. We highlight an emission of ultra-short THz pulse with a spectrum centered around 3 THz with a large bandwidth of 4 THz.

The acquisition speed of millimeter wave and terahertz raster scan imaging system is limited by the inertia of the optical parts moving back and forth in relation to the object to be imaged. This limitation is a major obstacle to the use of this radiation for industrial non-destructive testing (NDT). In this paper, we demonstrate the ability to drastically increase the frame rate thanks to a patent pending beam steering device. This imaging device is compatible with conveyor and allows an acquisition speed of 5000 pixels per second distributed over 25 lines of 250 pixels at millimeter scale resolution.

The presentation will focus on THz generation by photo-mixing of chirped optical pulses on a GaBiAs dipole photo-conductive antenna. We demonstrate the emission of long THz pulses with either a narrow or a broad band depending on the relative sign of the chirp.

In this paper, we present the potential of Terahertz Time-Domain Imaging (THz-TDI) as a tool to perform non-invasive 3D analysis of an ancient enamel plate manufactured by Longwy Company in France. The THz data collected in the reflection mode were processed using noise filtering procedures and an advanced imaging approach. The results validate the capability to identify glaze layers and the thickness of ceramic materials. To characterize the nature of the pigments, we also use with X-ray images, visible near-infrared hyperspectral imaging spectroscopy, and p-XRF (portable X-ray fluorescence) to qualitatively and quantitively identify the materials used. The obtained information enables a better understanding of the decoration chromogens nature and, thus, to determine the color palette of the artists who produced such decorative object. We also establish the efficiency of a focus, Z-tracker, which enables to perform THz imaging on non-flat samples and to attenuate artifacts obtained with a short focus lens. Then, 3D images are extracted and generated, providing a real vision. We also report the evaluation of the internal damage state through the detection of fractures.

In this investigation, we explore the phenomenon of spectral broadening in a gas-filled hollow-core capillary induced by ultra-short pulses, with a focus on nonlinear mode coupling at a power lower than the critical power. As the pulse propagates through the capillary, the spectrum primarily broadens due to self-phase modulation, and concurrently, high-order modes are generated through interactions between different modes. Both experimental and numerical results consistently demonstrate that the spectral region near the center of the spectrum remains within the fundamental mode, while the spectral edges propagate in higher-order modes, predominantly in the LP _12 mode generated through nonlinear mode coupling within the gas-filled hollow-core capillary. This observation is relatively weak but is of paramount importance for the design of post-compression systems in gas-filled hollow-core capillaries with a high-quality beam profile when the power starts to reach the critical level. Understanding and then accommodating the dynamics of nonlinear mode coupling are crucial for achieving the desired characteristics and beam quality while optimising the performance of such systems.