Shape-from-focus for real-time terahertz 3D imaging.

Thanks to significant advances in real-time terahertz imaging in terms of resolution and image quality, adapting and extending optical methods for 3D imaging at the millimeter scale is now promising. The shape-from-focus algorithm is a post-processing tool used in optical microscopy to reconstruct the external shape surface of a convex surface object. Images acquired at different distances from the object-side focal plane are implemented in this algorithm. We localize the best focus position in the stack of images for each pixel and then reconstruct the object in 3D due to the short depth of field. In this Letter, we propose an application of this algorithm in active and real-time terahertz imaging. We achieve the experimental reconstruction in 3D with a terahertz waves imaging system composed of a powerful source and a real-time terahertz camera.

Liquid index matching for 2D and 3D terahertz imaging.

Two-dimensional (2D) terahertz imaging and 3D visualization suffer from severe artifacts since an important part of the terahertz beam is reflected, diffracted, and refracted at each interface. These phenomena are due to refractive index mismatch and reflection in the case of non-orthogonal incidence. This paper proposes an experimental procedure that reduces these deleterious optical refraction effects for a cylinder and a prism made with polyethylene material. We inserted these samples in a low absorption liquid medium to match the sample index. We then replaced the surrounding air with a liquid with an optimized refractive index, with respect to the samples being studied. Using this approach we could more accurately recover the original sample shape by time-of-flight tomography.

Bulk magnetic terahertz metamaterials based on dielectric microspheres.

Rigid metamaterials were prepared by embedding TiO2 microspheres into polyethylene. These structures exhibit a series of Mie resonances where the lowest-frequency one is associated with a strong dispersion in the effective magnetic permeability. Using time-domain terahertz spectroscopy, we experimentally demonstrated the magnetic nature of the observed resonance. The presented approach shows a way for low-cost massive fabrication of mechanically stable terahertz metamaterials based on dielectric microresonators.

Terahertz imaging and tomography as efficient instruments for testing polymer additive manufacturing objects.

Additive manufacturing (AM) technology is not only used to make 3D objects but also for rapid prototyping. In industry and laboratories, quality controls for these objects are necessary though difficult to implement compared to classical methods of fabrication because the layer-by-layer printing allows for very complex object manufacturing that is unachievable with standard tools. Furthermore, AM can induce unknown or unexpected defects. Consequently, we demonstrate terahertz (THz) imaging as an innovative method for 2D inspection of polymer materials. Moreover, THz tomography may be considered as an alternative to x-ray tomography and cheaper 3D imaging for routine control. This paper proposes an experimental study of 3D polymer objects obtained by additive manufacturing techniques. This approach allows us to characterize defects and to control dimensions by volumetric measurements on 3D data reconstructed by tomography.

Low-frequency noise effect on terahertz tomography using thermal detectors.

In this paper, the impact of low-frequency noise on terahertz-computed tomography (THz-CT) is analyzed for several measurement configurations and pyroelectric detectors. We acquire real noise data from a continuous millimeter-wave tomographic scanner in order to figure out its impact on reconstructed images. Second, noise characteristics are quantified according to two distinct acquisition methods by (i) extrapolating from experimental acquisitions a sinogram for different noise backgrounds and (ii) reconstructing the corresponding spatial distributions in a slice using a CT reconstruction algorithm. Then we describe the low-frequency noise fingerprint and its influence on reconstructed images. Thanks to the observations, we demonstrate that some experimental choices can dramatically affect the 3D rendering of reconstructions. Thus, we propose some experimental methodologies optimizing the resulting quality and accuracy of the 3D reconstructions, with respect to the low-frequency noise characteristics observed during acquisitions.

Ordered subsets convex algorithm for 3D terahertz transmission tomography.

We investigate in this paper a new reconstruction method in order to perform 3D Terahertz (THz) tomography using a continuous wave acquisition setup in transmission mode. This method is based on the Maximum Likelihood for TRansmission tomography (ML-TR) first developed for X-ray imaging. We optimize the Ordered Subsets Convex (OSC) implementation of the ML-TR by including the Gaussian propagation model of THz waves and take into account the intensity distributions of both blank calibration scan and dark-field measured on THz detectors. THz ML-TR reconstruction quality and accuracy are discussed and compared to other tomographic reconstructions.

Propagation beam consideration for 3D THz computed tomography.

In this paper, a model of the beam propagation is developed according to the physical properties of THz waves used in THz computed tomography (CT) scan imaging. This model is first included in an acquisition simulator to observe and estimate the impact of the Gaussian beam intensity profile on the projection sets. Second, the model is introduced in several inversion methods as a convolution filter to perform efficient tomographic reconstructions of simulated and real acquired objects. Results obtained with three reconstruction methods (BFP, SART and OSEM) are compared to the techniques proposed in this paper. We will demonstrate an increase of the overall quality and accuracy of the 3D reconstructions.

Investigation on reconstruction methods applied to 3D terahertz computed tomography.

3D terahertz computed tomography has been performed using a monochromatic millimeter wave imaging system coupled with an infrared temperature sensor. Three different reconstruction methods (standard back-projection algorithm and two iterative analysis) have been compared in order to reconstruct large size 3D objects. The quality (intensity, contrast and geometric preservation) of reconstructed cross-sectional images has been discussed together with the optimization of the number of projections. Final demonstration to real-life 3D objects has been processed to illustrate the potential of the reconstruction methods for applied terahertz tomography.

Broadband dielectric terahertz metamaterials with negative permeability.

We present a design of dielectric metamaterials exhibiting a broad range of negative effective permeability in the terahertz spectral region. The investigated structures consist of an array of high-permittivity rods that exhibit a series of Mie resonances giving rise to the effective magnetic response. The spectral positions of resonances depend on the geometrical parameters of the rods and on their permittivity, which define the resonant confinement of the electromagnetic field within the rods. The electromagnetic coupling between the adjacent rods is negligible. With a suitable aspect ratio of the rods, a broadband magnetic response can be obtained.

Emission characteristics of ion-irradiated In(0.53)Ga(0.47)As based photoconductive antennas excited at 1.55 microm.

We present a detailed study of the effect of the carrier lifetime on the terahertz signal characteristics emitted by Br(+)-irradiated In(0.53)Ga(0.47)As photoconductive antennas excited by 1550 nm wavelength femtosecond optical pulses. The temporal waveforms and the average radiated powers for various carrier lifetimes are experimentally analyzed and compared to predictions of analytical models of charge transport. Improvements in bandwidth and in average power of the emitted terahertz radiation are observed with the decrease of the carrier lifetime on the emitter. The power radiated by ion-irradiated In(0.53)Ga(0.47)As photoconductive antennas excited by 1550 nm wavelength optical pulses is measured to be 0.8 muW. This value is comparable with or greater than that emitted by similar low temperature grown GaAs photoconductive antennas excited by 780 nm wavelength optical pulses.

Active optical control of the terahertz reflectivity of high-resistivity semiconductors.

We study theoretically and demonstrate experimentally light-controllable terahertz reflectivity of high-resistivity semiconductor wafers. Photocarriers created by interband light absorption form a thin conducting layer at the semiconductor surface, which allows the terahertz reflectivity of the element to be tuned between antireflective (R <3%) and highly reflective (R >85%) limits by means of the intensity and wavelength of the optical illumination.

On the validity of the independent hot-spot model.

The results of the independent hot-spot (IHS) model are compared to those of the underlying stochastic amplifier in the regime where the coupling of the amplifier is close to its critical value. The considered case is that of a 1D linear amplifier with at most one hot spot per interaction length. It is shown that the validity of the critical coupling given by the IHS model depends on the correlation function of the pump field and should be discussed in each particular case. The discrepancy between the IHS model and the underlying amplifier is shown to be due to the random fluctuations of the hot-spot field around its dominant, deterministic, component.

Effects of spatial and temporal smoothing on stimulated brillouin scattering in the independent-hot-spot model limit

The influence of laser beam smoothing on stimulated Brillouin backscattering (SBBS) is studied analytically in the limit of the independent hot spot model. It is shown that the temporal beam smoothing can reduce the SBBS reflectivity significantly even though the laser bandwidth is smaller than the growth rate for the average intensity. The explanation of this reduction effect is given in terms of SBBS growth in the statistically significant hot spots. The minimum laser bandwidth corresponding to an important reduction of the reflectivity is thus determined properly. The dependence of this reduction effect on the acoustic damping for a given laser bandwidth is discussed.

Experimental evidence of the effect of heat flux on thomson scattering off ion acoustic waves

Thomson self-scattering measurements are performed in a preionized helium gas jet plasma at different locations along the laser propagation direction. A systematic and important variation of the intensity ratio between the blue and the red ion spectral components is observed, depending on whether the location of the probed region is in front of or behind the focal plane. A simple theoretical calculation of Thomson scattering shows that this behavior can be qualitatively understood in terms of a deformation of the electron distribution function due to the return current correlated with the classical thermal heat flux.

Gas filter correlation instrument for air monitoring at submillimeter wavelengths.

A gas filter correlation (GFC) instrument for air monitoring at submillimeter wavelengths has been developed. We used a high-power terahertz radiation source in combination with a specific gas filter cell to obtain a highly selective instrument to differentiate species present in an unknown mixture. This approach provides a new method for survey measurements in the spectrum from 100 to 1000 GHz, in which many molecular rotational lines appear. Basic operational considerations and preliminary experiments with hydrogen sulfide are described.