Terahertz digital holography using angular spectrum and dual wavelength reconstruction methods

Real-time millimeter wave holography with an arrayed detector

Millimeter and terahertz wave imaging has emerged as a powerful tool for applications such as security screening, biomedical imaging, and material analysis. However, intensity images alone are often insufficient for detecting variations in the dielectric constant of a sample, and extraction of material properties without additional phase information requires extensive prior knowledge of the sample. Digital holography provides a means for intensity-only detectors to reconstruct both amplitude and phase images. Here we utilize a commercially available source and detector array, both operating at room temperature, to perform digital holography in real-time for the first time in the mm-wave band (at 290 GHz). We compare the off-axis and phase-shifting approaches to digital holography and discuss their trade-offs and practical challenges in this regime. Owing to the low pixel count, we find phase-shifting holography to be the most practical and high fidelity approach for such commercial mm-wave cameras even under real-time operational requirements.

High-throughput terahertz imaging: progress and challenges

Many exciting terahertz imaging applications, such as non-destructive evaluation, biomedical diagnosis, and security screening, have been historically limited in practical usage due to the raster-scanning requirement of imaging systems, which impose very low imaging speeds. However, recent advancements in terahertz imaging systems have greatly increased the imaging throughput and brought the promising potential of terahertz radiation from research laboratories closer to real-world applications. Here, we review the development of terahertz imaging technologies from both hardware and computational imaging perspectives. We introduce and compare different types of hardware enabling frequency-domain and time-domain imaging using various thermal, photon, and field image sensor arrays. We discuss how different imaging hardware and computational imaging algorithms provide opportunities for capturing time-of-flight, spectroscopic, phase, and intensity image data at high throughputs. Furthermore, the new prospects and challenges for the development of future high-throughput terahertz imaging systems are briefly introduced.

Single-pixel three-dimensional imaging of the terahertz-wave by complex-field synthesis

We propose a novel three-dimensional (3D) imaging technique by terahertz (THz) waves. Specifically, we modulate the THz wave using diffusers to produce three different speckle-like illumination patterns. The object is raster scanned by the three illumination patterns to generate three raw images via the single-pixel detection method. Subsequently, we synthesize a complex field using the three raw images. Finally, the retrieved image is calculated using the phase correlation of the complex point spread function. The proposed imaging system is simple and highly cost-effective. Therefore, it is a promising technique that can be adopted for industrial inspection and security screening.

Resolution and contrast in terahertz pulse time-domain holographic reconstruction

Here, we present a comprehensive study of the reconstruction quality in terahertz (THz) pulse time-domain holography. We look into single wavelength reconstructions, as well as broadband recovery enabled by the ultrabroadband nature of radiation and coherent detection enabled by electro-optic or photoconductive sensing. We demonstrate the transverse resolution dependence for amplitude and phase objects on the solid angle of the inline recorded time-domain THz hologram, and then turn to the contrast of reconstructed binary amplitude objects, and further to longitudinal resolution of phase objects. We show that transverse resolution can reach values comparable to the wavelength of the radiation used, and longitudinally, phase objects can be resolved with even greater precision. We compare the obtained resolution with theoretical estimates and show that THz pulse time-domain holography is a powerful non-contact imaging tool.

THz coherent lensless imaging

Imaging with THz radiation has proved an important tool for both fundamental science and industrial use. Here we review a class of THz imaging implementations, named coherent lensless imaging, that reconstruct the coherent response of arbitrary samples with a minimized experimental setup based only on a coherent source and a camera. After discussing the appropriate sources and detectors to perform them, we detail the fundamental principles and implementations of THz digital holography and phase retrieval. These techniques owe a lot to imaging with different wavelengths, yet innovative concepts are also being developed in the THz range and are ready to be applied in other spectral ranges. This makes our review useful for both the THz and imaging communities, and we hope it will foster their interaction.

Iterative phase-retrieval-assisted off-axis terahertz digital holography

In terahertz digital holography, the off-axis configuration is the appropriate choice when the investigated object is non-sparse and complex. The limitation of recording distance in the off-axis configuration restricts the imaging quality. Either low-resolution or spectra overlap can potentially occur. We propose an iterative phase-retrieval approach to improve the quality of reconstruction results obtained from an off-axis hologram. One additional capture of object wave intensity is recorded to perform iterative phase retrieval with off-axis reconstruction as the initial guess. Apodization operation can be applied to the object wave intensity capture to suppress undesired border diffraction effects. The image quality using the proposed method has been improved both from simulation and experimental verification.

Video-rate terahertz digital holographic imaging system

Terahertz (THz) imaging has been demonstrated in numerous applications from medical to non-destructive evaluation (NDE), but current systems require expensive components, provide slow frame-rates and low resolutions. THz holography offers a potentially low-cost, high-performance alternative. Here we demonstrate the first full video-rate THz digital holography system at 2.52 THz (118.8 µm) using low-cost optical components. 2D digital reconstructions of samples are performed at frame-rates of 50 Hz - an order of magnitude higher than previous systems, whilst imaging of samples concealed in common packaging types demonstrates suitability for NDE applications. A lateral resolution of 250 µm was determined using a 1951 USAF target.

Topography of hidden objects using THz digital holography with multi-beam interferences

We present a method for the separation of the signal scattered from an object hidden behind a THz-transparent sample in the framework of THz digital holography in reflection. It combines three images of different interference patterns to retrieve the amplitude and phase distribution of the object beam. Comparison of simulated with experimental images obtained from a metallic resolution target behind a Teflon plate demonstrates that the interference patterns can be described in the simple form of three-beam interference. Holographic reconstructions after the application of the method show a considerable improvement compared to standard reconstructions exclusively based on Fourier transform phase retrieval.

[Three-dimensional morphology of C6/36 cells infected by dengue virus: a study based on digital holographic microscopy]

OBJECTIVE

To monitor the 3-dimensional (3D) morphological changes of C6/36 cells during dengue virus (DENV) infection using a live-cell imaging technique based on digital holographic microscopy and provide clues for better understanding the mechanisms of DENV infection.

METHODS

C6/36 cells were seeded in 6-well plates to determine the optimal imaging density under a holographic cell imager, and the morphological changes of the cells were recorded in response to a culture temperature change from 28 degrees celsius; to 37 degrees celsius; C6/36 cells were infected with 4 DENV strains with different serotypes at 28 degrees celsius; and incubated at 37 degrees celsius; for 24 h, and the 3D holograms and relevant morphological parameters were recorded at different time points using HoloMonitor M4 holographic cell imaging and analysis system.

RESULTS

The holograms of C6/36 cells inoculated at the optimal density for imaging (4×10⁵ per well) showed unified 3D morphologies of the single cells with minimal dispersions in the cell area, thickness and volume (P<0.05), which did not undergo obvious changes when the cells were incubated at 37 degrees celsius; for 24 h (P>0.05). The cell area and volume of the cells infected with the 4 DENV strains all increased and the cell thickness was reduced during incubation. Among the 4 strains, DENV-1 and DENV-2 caused reduced cell thickness while DENV-3 and DENV-4 increased the cell thickness, and the pattern and degree of such changes differ among the 4 strains.

CONCLUSIONS

Digital holographic microscopy allows monitoring of the complex morphological changes of cells during DENV infection. The 4 DENV strains with different serotypes causes characteristic cell damages during infection.

Resolution and quality enhancement in terahertz in-line holography by sub-pixel sampling with double-distance reconstruction

We demonstrate the enhancement of resolution and image quality in terahertz (THz) lens-free in-line digital holography by sub-pixel sampling with double-distance reconstruction. Multiple sub-pixel shifted low-resolution (LR) holograms recorded by a pyroelectric array detector (100 μm × 100 μm pixel pitch, 124 × 124 pixels) are aligned precisely to synthesize a high-resolution (HR) hologram. By this method, the lateral resolution is no more limited by the pixel pitch, and lateral resolution of 150 μm is obtained, which corresponds to 1.26λ with respect to the illuminating wavelength of 118.8 μm (2.52 THz). Compared with other published works, to date, this is the highest resolution in THz digital holography when considering the illuminating wavelength. In addition, to suppress the twin-image and zero-order artifacts, the complex amplitude distributions of both object and illuminaing background wave fields are reconstructed simultaneously. This is achieved by iterative phase retrieval between the double HR holograms and background images at two recording planes, which does not require any constraints on object plane or a priori knowledge of the sample.

Synthetic aperture in terahertz in-line digital holography for resolution enhancement

Terahertz digital holography is a combination of terahertz technology and digital holography. In digital holography, the imaging resolution is the key parameter in determining the detailed quality of a reconstructed wavefront. In this paper, the synthetic aperture method is used in terahertz digital holography and the in-line arrangement is built to perform the detection. The resolved capability of previous terahertz digital holographic systems restricts this technique to meet the requirement of practical detection. In contrast, the experimental resolved power of the present method can reach 125 μm, which is the best resolution of terahertz digital holography to date. Furthermore, the basic detection of a biological specimen is conducted to show the practical application. In all, the results of the proposed method demonstrate the enhancement of experimental imaging resolution and that the amplitude and phase distributions of the fine structure of samples can be reconstructed by using terahertz digital holography.

Real-time terahertz digital holography with a quantum cascade laser

Coherent imaging in the THz range promises to exploit the peculiar capabilities of these wavelengths to penetrate common materials like plastics, ceramics, paper or clothes with potential breakthroughs in non-destructive inspection and quality control, homeland security and biomedical applications. Up to now, however, THz coherent imaging has been limited by time-consuming raster scanning, point-like detection schemes and by the lack of adequate coherent sources. Here, we demonstrate real-time digital holography (DH) at THz frequencies exploiting the high spectral purity and the mW output power of a quantum cascade laser combined with the high sensitivity and resolution of a microbolometric array. We show that, in a one-shot exposure, phase and amplitude information of whole samples, either in reflection or in transmission, can be recorded. Furthermore, a 200 times reduced sensitivity to mechanical vibrations and a significantly enlarged field of view are observed, as compared to DH in the visible range. These properties of THz DH enable unprecedented holographic recording of real world dynamic scenes.

Markov chain Monte Carlo sampling based terahertz holography image denoising

Terahertz digital holography has attracted much attention in recent years. This technology combines the strong transmittance of terahertz and the unique features of digital holography. Nonetheless, the low clearness of the images captured has hampered the popularization of this imaging technique. In this paper, we perform a digital image denoising technique on our multiframe superposed images. The noise suppression model is concluded as Bayesian least squares estimation and is solved with Markov chain Monte Carlo (MCMC) sampling. In this algorithm, a weighted mean filter with a Gaussian kernel is first applied to the noisy image, and then by nonlinear contrast transform, the contrast of the image is restored to the former level. By randomly walking on the preprocessed image, the MCMC-based filter keeps collecting samples, assigning them weights by similarity assessment, and constructs multiple sample sequences. Finally, these sequences are used to estimate the value of each pixel. Our algorithm shares some good qualities with nonlocal means filtering and the algorithm based on conditional sampling proposed by Wong et al. [Opt. Express18, 8338 (2010)10.1364/OE.18.008338OPEXFF1094-4087], such as good uniformity, and, moreover, reveals better performance in structure preservation, as shown in numerical comparison using the structural similarity index measurement and the peak signal-to-noise ratio.

THz holography in reflection using a high resolution microbolometer array

We demonstrate a digital holographic setup for Terahertz imaging of surfaces in reflection. The set-up is based on a high-power continuous wave (CW) THz laser and a high-resolution (640 × 480 pixel) bolometer detector array. Wave propagation to non-parallel planes is used to reconstruct the object surface that is rotated relative to the detector plane. In addition we implement synthetic aperture methods for resolution enhancement and compare Fourier transform phase retrieval to phase stepping methods. A lateral resolution of 200 μm and a relative phase sensitivity of about 0.4 rad corresponding to a depth resolution of 6 μm are estimated from reconstructed images of two specially prepared test targets, respectively. We highlight the use of digital THz holography for surface profilometry as well as its potential for video-rate imaging.

Terahertz in-line digital holography of human hepatocellular carcinoma tissue

Terahertz waves provide a better contrast in imaging soft biomedical tissues than X-rays, and unlike X-rays, they cause no ionisation damage, making them a good option for biomedical imaging. Terahertz absorption imaging has conventionally been used for cancer diagnosis. However, the absorption properties of a cancerous sample are influenced by two opposing factors: an increase in absorption due to a higher degree of hydration and a decrease in absorption due to structural changes. It is therefore difficult to diagnose cancer from an absorption image. Phase imaging can thus be critical for diagnostics. We demonstrate imaging of the absorption and phase-shift distributions of 3.2 mm × 2.3 mm × 30-μm-thick human hepatocellular carcinoma tissue by continuous-wave terahertz digital in-line holography. The acquisition time of a few seconds for a single in-line hologram is much shorter than that of other terahertz diagnostic techniques, and future detectors will allow acquisition of meaningful holograms without sample dehydration. The resolution of the reconstructions was enhanced by sub-pixel shifting and extrapolation. Another advantage of this technique is its relaxed minimal sample size limitation. The fibrosis indicated in the phase distribution demonstrates the potential of terahertz holographic imaging to obtain a more objective, early diagnosis of cancer.

Observation of hydrofluoric acid burns on osseous tissues by means of terahertz spectroscopic imaging

Terahertz technologies have gained great amount of attention for biomedical imaging and tissue analysis. In this study, we utilize terahertz imaging to study the effects of hydrofluoric acid on both compact bone tissue and cartilage. We compare the differences observed in the exposure for formalin fixed and raw, dried, tissue as well as those resulting from a change in hydrofluoric (HF) concentration. Measurements are performed with THz-TDS, and a variety of spectroscopic-based image reconstruction techniques are utilized to develop contrast in the features of interest.

Terahertz in-line digital holography of dragonfly hindwing: amplitude and phase reconstruction at enhanced resolution by extrapolation

We report here on terahertz (THz) digital holography on a biological specimen. A continuous-wave (CW) THz in-line holographic setup was built based on a 2.52 THz CO(2) pumped THz laser and a pyroelectric array detector. We introduced novel statistical method of obtaining true intensity values for the pyroelectric array detector's pixels. Absorption and phase-shifting images of a dragonfly's hindwing were reconstructed simultaneously from single in-line hologram. Furthermore, we applied phase retrieval routines to eliminate twin image and enhanced the resolution of the reconstructions by hologram extrapolation beyond the detector area. The finest observed features are 35 μm width cross veins.

Terahertz holography for imaging amplitude and phase objects

A non-monochromatic THz Quantum Cascade Laser and an uncooled micro-bolometer array detector with VGA resolution are used in a beam-splitter free holographic set-up to measure amplitude and phase objects in transmission. Phase maps of the diffraction pattern are retrieved using the Fourier transform carrier fringe method; while a Fresnel-Kirchhoff back propagation algorithm is used to reconstruct the complex object image. A lateral resolution of 280 µm and a relative phase sensitivity of about 0.5 rad are estimated from reconstructed images of a metallic Siemens star and a polypropylene test structure, respectively. Simulations corroborate the experimental results.

Imaging through scattering microfluidic channels by digital holography for information recovery in lab on chip

We tackle the problem of information recovery and imaging through scattering microfluidic chips by means of digital holography (DH). In many cases the chip can become opalescent due to residual deposits settling down the inner channel faces, biofilm formation, scattering particle uptake by the channel cladding or its damaging by corrosive substances, or even by condensing effect on the exterior channels walls. In these cases white-light imaging is severely degraded and no information is obtainable at all about the flowing samples. Here we investigate the problem of counting and estimating velocity of cells flowing inside a scattering chip. Moreover we propose and test a method based on the recording of multiple digital holograms to retrieve improved phase-contrast images despite the strong scattering effect. This method helps, thanks to DH, to recover information which, otherwise, would be completely lost.

Study of image reconstruction for terahertz indirect holography with quasi-optics receiver

In this paper, an indirect holographic image reconstruction algorithm was studied for terahertz imaging with a quasi-optics receiver. Based on the combination of the reciprocity principle and modified quasi-optics theory, analytical expressions of the received spatial power distribution and its spectrum are obtained for the interference pattern of target wave and reference wave. These results clearly give the quantitative relationship between imaging quality and the parameters of a Gaussian beam, which provides a good criterion for terahertz quasi-optics transceivers design in terahertz off-axis holographic imagers. To validate the effectiveness of the proposed analysis method, some imaging results with a 0.3 THz prototype system are shown based on electromagnetic simulation.

Imaging live humans through smoke and flames using far-infrared digital holography

The ability to see behind flames is a key challenge for the industrial field and particularly for the safety field. Development of new technologies to detect live people through smoke and flames in fire scenes is an extremely desirable goal since it can save human lives. The latest technologies, including equipment adopted by fire departments, use infrared bolometers for infrared digital cameras that allow users to see through smoke. However, such detectors are blinded by flame-emitted radiation. Here we show a completely different approach that makes use of lensless digital holography technology in the infrared range for successful imaging through smoke and flames. Notably, we demonstrate that digital holography with a cw laser allows the recording of dynamic human-size targets. In this work, easy detection of live, moving people is achieved through both smoke and flames, thus demonstrating the capability of digital holography at 10.6 μm.

Digital holographic interferometry with CO2 lasers and diffuse illumination applied to large space reflector metrology [Invited]

Digital holographic interferometry in the long-wave infrared domain has been developed by combining a CO(2) laser and a microbolometer array. The long wavelength allows large deformation measurements, which are of interest in the case of large space reflectors undergoing thermal changes when in orbit. We review holography at such wavelengths and present some specific aspects related to this spectral range on our measurements. For the design of our digital holographic interferometer, we studied the possibility of illuminating specular objects by a reflective diffuser. We discuss the development of the interferometer and the results obtained on a representative space reflector, first in the laboratory and then during vacuum cryogenic test.

Experimental research on terahertz Gabor inline digital holography of concealed objects

Terahertz (THz) radiation has the characteristics of penetrating nonmetallic and nonpolar materials that are opaque to visible light, which makes THz digital holography have an application potential of imaging concealed objects with certain barriers. A CO(2) pumped continuous THz Gabor inline digital holographic imaging system was utilized to conduct experimental researches on imaging concealed objects. Paper, Teflon, a plastic express envelope, and silicon wafers were used as barriers. High-quality reconstructed images were obtained. Compared with the reconstruction results without any barriers, the results verify the feasibility of THz Gabor inline digital holography in imaging concealed objects.

Continuous-wave terahertz in-line digital holography

A terahertz (THz) in-line digital holography project has been proposed based on a CO2 pumped 2.52 THz continuous-wave laser and a pyroelectric-array camera used as the detector. The THz Gabor in-line digital holograms have been obtained and then the high resolution reconstruction of THz in-line digital holography was realized. The resolution of an in-line digital holography system has been tested by the use of a series of objects. High-quality and high-resolution reconstructed images have been obtained, and the real lateral resolution is higher than 0.2 mm. It can be inferred from the results that the THz Gabor in-line digital holography system has the abilities of real-time and high-resolution imaging.

Terahertz interferometric synthetic aperture tomography for confocal imaging systems

Terahertz (THz) interferometric synthetic aperture tomography (TISAT) for confocal imaging within extended objects is demonstrated by combining attributes of synthetic aperture radar and optical coherence tomography. Algorithms recently devised for interferometric synthetic aperture microscopy are adapted to account for the diffraction-and defocusing-induced spatially varying THz beam width characteristic of narrow depth of focus, high-resolution confocal imaging. A frequency-swept two-dimensional TISAT confocal imaging instrument rapidly achieves in-focus, diffraction-limited resolution over a depth 12 times larger than the instrument's depth of focus in a manner that may be easily extended to three dimensions and greater depths.

Coaxial waveguide mode reconstruction and analysis with THz digital holography

Terahertz (THz) digital holography is employed to investigate the properties of waveguides. By using a THz digital holographic imaging system, the propagation modes of a metallic coaxial waveguide are measured and the mode patterns are restored with the inverse Fresnel diffraction algorithm. The experimental results show that the THz propagation mode inside the waveguide is a combination of four modes TE₁₁, TE₁₂, TM₁₁, and TM₁₂, which are in good agreement with the simulation results. In this work, THz digital holography presents its strong potential as a platform for waveguide mode charactering. The experimental findings provide a valuable reference for the design of THz waveguides.