We demonstrate the first room temperature continuous wave THz sources based on intracavity difference frequency generation from mid-infrared quantum cascade lasers. This accomplishment was enabled by integration of several key technologies, resulting in a new high efficiency waveguide design and improved thermal dissipation. Room temperature single mode emissions at 3.6 THz with an emitting power of 3 μW and a mid-IR-to-THz conversion efficiency of 0.44 mW/W² are obtained in continuous wave mode. THz peak power up to 1.4 mW in pulsed mode operation with a mid-IRto- THz conversion efficiency of 0.8 mW/W2 at 3.5 THz is also demonstrated.

We report our research on processing of AlGaAs/GaAs structures for THz quantum-cascade lasers (QCLs). We focus on the processes of fabrication of Ti/Au claddings for metal-metal waveguides and the wafer bonding with indium solder. We place special emphasis on the optimum technological conditions of these processes, leading to working devices. The wide range of technological conditions was studied, by use of test structures and analyses of their electrical, optical, chemical and mechanical properties, performed by electron microscopic techniques, energy dispersive X-ray spectrometry, secondary ion mass spectroscopy, atomic force microscopy, fourier-transform infra-red spectroscopy and circular transmission line method. On the basis of research a set of technological conditions was selected, and devices lasing at the maximum temperature 130K were fabricated from AlGaAs/GaAs structures grown by molecular beam epitaxy (MBE) technique. Their threshold-current densities were about 1.5kA/cm². Additionally we report our initial stage research on fabrication of Cu-based claddings, that theoretically are more promising than the Au-based ones for fabrication of low-lossy waveguides for THz QCLs.

A photomixer-based THz-wave emitter having linear polarization characteristics was developed by integrating a unitraveling- carrier photodiode (UTC-PD) and a self-complementary reflection-symmetry antenna. The fabricated module was operated at frequencies from 120 GHz to 2.6 THz, and the typical level of output power was 2.8 μW at 1 THz for a photocurrent of 9 mA at a bias voltage of -0.4 V. It also exhibited stable principal polarization axis angles within ±1° at frequencies from 200 GHz to 2 THz, and the generated signals were linearly polarized with extinction ratios below 5% at frequencies from 200 to 800 GHz. The fabricated module was successfully applied to sub-THz-wave ellipsometry without using an external polarizer.

The terahertz (THz) imaging is an advanced technique on the basis of the unique characteristics of terahertz radiation. Due to its noncontact, non-invasive and high-resolution capabilities, it has already shown great application prospects in biomedical observation, sample measurement, and quality control. The continuous-wave terahertz in-line digital holography is a combination of terahertz technology and in-line digital holography of which the source is a continuous-wave terahertz laser. Over the past decade, many researchers used different terahertz sources and detectors to undertake experiments. In this paper, the pre-process of the hologram is accomplished after the holograms’ recording process because of the negative pixels in the pyroelectric detector and the air vibration caused by the chopper inside the camera. To improve the quality of images, the phase retrieval algorithm is applied to eliminate the twin images. In the experiment, the pin which terahertz wave can’t penetrate and the TPX slice carved letters “THz” are chosen for the samples. The amplitude and phase images of samples are obtained and the twin image and noise in the reconstructed images are suppressed. The results validate the feasibility of the terahertz in-line digital holographic imaging technique. This work also shows the terahertz in-line digital holography technique’s prospects in materials science and biological samples’ detection.

Terahertz imaging systems have received substantial attention from the scientific community for their use in astronomy, spectroscopy, plasma diagnostics and security. One approach to designing such systems is to use focal plane arrays. Although the principle of these systems is straightforward, realizing practical architectures has proven deceptively difficult. A different approach to imaging consists of spatially encoding the incoming flux of electromagnetic energy prior to detection using a reconfigurable mask. This technique is referred to as coded aperture"or Hadamard"imaging. This paper details the design, fabrication and testing of a prototype coded aperture mask operating at WR-1.5 (500-750 GHz) that uses the switching properties of vanadium dioxide(VO₂). The reconfigurable mask consists of bowtie antennas with vanadium dioxide VO₂ elements at the feed points. From the symmetry, a unit cell of the array can be represented by an equivalent waveguide whose dimensions limit the maximum operating frequency. In this design, the cutoff frequency of the unit cell is 640 GHz. The VO₂ devices are grown using reactive-biased target ion beam deposition. A reflection coefficient (S₁₁) measurement of the mask in the WR-1.5 (500-750 GHz) band is conducted. The results are compared with circuit models and found to be in good agreement. A simulation of the transmission response of the mask is conducted and shows a transmission modulation of up to 28 dB. This project is a first step towards the development of a full coded aperture imaging system operating at WR-1.5 with VO₂ as the mask switching element.

Terahertz (THz) wave generation via four-wave mixing (FWM) in silicon membrane waveguides is investigated with mid-infrared pump. The silicon membrane waveguides with width of 12 μm and heights varied from 14 μm to 17 μm, which can confine the THzwave ranging from 7.5 THz to 10 THz due to the large refractive index contrast of the waveguide core and cladding, are designed to realize the collinear phase matching for THz-wave generation via FWM. Compared with the conventional parametric amplification or wavelength conversion based on FWM in silicon waveguides, which needs a pump wavelength located in the anomalous groupvelocity dispersion (GVD) regime to realize broad phase matching, the pump wavelength located in the normal GVD regime is required to realize phase matching because of the large signal-pump frequency detuning. Phase matching for a tunable THz-wave ranging from 8.57 THz to 10 THz can be realized by tuning the pump wavelength from 4.2 μm to 4.4 μm in the silicon waveguide with rib height of 15 μm. Whilst, the phase matching bandwidth of THz-wave ranging from 7.7 THz to 10 THz can be achieved by tailoring the waveguide height from 14 μm to 17 μm when the pump wavelength is 4.3μm. Moreover, the conversion efficiency of the THz-wave generation is studied with different pump wavelengths and waveguide heights, the maximum conversion efficiency of 1.25 % at 9.2 THz can be obtained in a 6-mm long silicon waveguide when the pump wavelength is 4.3 μm and the waveguide height is 15 μm.

Nowadays, the detection and identification of dangerous substances at long distance (several metres, for example) by using of THz pulse reflected from the object is an important problem. The main problem with this technique is the absorption of THz energy by water vapor. However, using THz pulsed radiation is possible at distance of some metres as it is well-known. Below we demonstrate possibility of THz signal measuring reflected from a flat metallic mirror placed about 4 metres from the parabolic mirror. Investigated object is placed before this mirror. Therefore, at present time our measurements contain features of both transmission and reflection modes. The reflecting mirror is used because of weak averaged power of femtosecond laser. This power is about 1 W. Moreover, the laser beam splits many times. Therefore, the averaged power falling on the THz emitter decreases at least 8 times. The pulse duration generated by the femtosecond laser is equal to 68 fs. Measurements were provided at room temperature and humidity about 70%. The aim of investigation was the detection of a substance in real conditions. We discuss new features of the detection of a substance covered under various ordinary materials and possible way for their influence deleting on the detection using reflected THz pulse. We discuss also details of action of THz pulse with a few circles on media. The main feature of such interaction is its dependence from absolute phase of the THz pulse. We demonstrate results of computer simulations as well as physical experiment results for propagation of such laser pulses.

This paper reviews recent advances in graphene plasmonic heterostructures for new types of terahertz lasers. We theoretically discovered and experimentally manifested that the excitation of surface plasmons in population-inverted graphene by the terahertz photons results in propagating surface plasmon polaritons with a giant gain in a wide terahertz range. Furthermore, double graphene layer heterostructures consisting of a tunnel barrier insulator sandwiched with a pair of gated graphene monolayers are introduced. Photoemission-assisted quantum-mechanical resonant tunneling can be electrically tuned to meet a desired photon energy for lasing, resulting in enormous enhancement of the terahertz gain. Current injection structures are also addressed.

THz response of a number of samples based on CdTe/CdMgTe quantum wells grown by a molecular beam epitaxy was investigated at low temperatures and high magnetic fields. The experiments involved magnetotransport, photocurrent, and transmission measurements carried out with a monochromatic THz sources or a Fourier spectrometer. Samples of different geometry, with and without a gate metallization were used. We observed excitations of a two-dimensional plasma in the form of optically-induced Shubnikov-de Haas oscillations, cyclotron resonance transitions and magnetoplasmon resonances. A polaron effect was observed at magnetic fields higher than 10 T. A point contact device processed with an electron beam lithography was investigated as a detector of THz radiation. It was shown that the main mechanism responsible for a THz performance of the point contact was excitation of magnetoplasmons with a wave vector defined by geometrical constrictions of the device mesa.

The coupling of THz and intersubband excitations leads to THz polaritons and antipolaritons and has great potential for device applications. In this paper, based on the dielectric function formalism, we investigate the relevance of controlling cavity resonance and dephasing in the THz polariton and antipolariton dispersions. The input optical dielectric constant stems from numerically exact nonequilibrium many body solutions which are adjusted to a simplified nonlinear dielectric susceptibility. The resulting expression is inserted in the wave equation to describe the coupling of TE- polarized THz radiation and intervalence band transitions in GaAs/Al_0.3Ga_0.7As multiple quantum well structures with various resonance conditions.

Research in the area of terahertz (THz) waveguides has seen a rapid progress recently and has led to demonstration of THz waveguides with transmission losses comparable to losses in air. We will discuss dielectric-lined hollow metallic waveguides, in which THz waves propagate with attenuation as low as 1 dB/m and dispersion of 6 ps/(THz·m), and compare them to other low-loss THz waveguides. As a key technique for THz waveguide research, we will discuss the application of THz near-field microscopy in combination with THz time-domain spectroscopy for waveguide characterization. This technique allows us to map spatial profiles of normal modes and to measure transmission loss and dispersion spectra for individual modes within the THz range.

Magnetic-field tunable semiconductor detectors are used in THz spectroscopy due to their sensitivity and possibility to respond to photons in a broad frequency range. We compare THz detectors processed on high electron mobility GaAs/GaAlAs and CdTe/CdMgTe quantum wells. Transmission, photocurrent and photovoltage measurements were carried out as a function of the magnetic field at a constant energy of incident THz photons from a THz laser. The samples investigated were grid-gated and grid-free. The spectra show features resulting from excitation of the cyclotron resonance and magnetoplasmons. Theoretical models allow to analyze quantitatively the frequency of observed excitations and determine plasmon dispersion relations. This study allows to point at advantages and disadvantages of THz cyclotron-resonance and plasmonic detectors fabricated on GaAs- and CdTe-based quantum wells as well as to compare these two types of devices.

Resonant THz antenna-coupled micro-bolometers are considered as a potential candidates for room temperature THz imaging, as well as spectroscopic applications. Micromachining technology is found to be well-suitable to fabricate a micro-meter bolometer sensor suitable for MEMS implementation. The sensitivity of the sensor is determined to be up to 1000V/W and the noise equivalent power (NEP) – is down to 5pW /√Hz. The sensor parameters are designed to be easily implemented with a low cost standard preamplifier array which increases the pixel sensitivity to 10⁶V/W without compromising the noise equivalent power.

Continuing advances in scaling of conventional semiconductor devices are enabling mainstream electronics to operate in the millimeter-wave through THz regime. At the same time, however, novel devices and device concepts are also emerging to address the key challenges for systems in this frequency range, and may offer performance and functional advantages for future systems. In addition to new devices, advances in integration technology and novel system concepts also promise to provide substantial system-level performance and functionality enhancements. Several emerging devices and device concepts, as well as circuit-level concepts to take advantage of them, are discussed. Based on unconventional semiconductor device structures and operational principles, these devices offer the potential for significantly improved system sensitivity and frequency coverage. When combined in arrays, features such as polarimetric detection and frequency tunability for imaging can be achieved. As examples of emerging devices for millimeter-wave through THz sensing and imaging, heterostructure backward diodes in the InAs/AlSb/GaSb material system and GaN-based plasma-wave high electron mobility transistors (HEMTs) will be discussed. Based on interband tunneling, heterostructure backward diodes offer significantly increased sensitivity and extremely low noise for direct detection applications, and have been demonstrated with cutoff frequencies exceeding 8 THz. The plasma-wave HEMT is an emerging device concept that, by leveraging plasma-wave resonances in the two-dimensional electron gas within the channel of the HEMT, offers the prospect for both tunable narrowband detection as well as low-noise amplification at frequencies well into the THz. These emerging devices are both amenable to direct integration within compact planar radiating structures such as annular slot antennas for realization of polarimetric detection and frequency tuning for spectroscopy and imaging.

In this paper, we introduce the self-mixing phenomenon in terahertz quantum cascade lasers (THz QCLs) and present recent advancements in the development of coherent THz imaging and sensing systems that exploit the self-mixing effect. We describe an imaging method which utilises the interferometric nature of optical feedback in a THz QCL to employ it as a homodyning transceiver. This results in a highly sensitive and compact scheme. Due to the inherently low penetration depth of THz radiation in hydrated biological tissue, imaging of superficial skin is an ideal application for this technique. We present results for imaging of excised skin tissue, showing high-contrast between different tissue types and pathologies.

Terahertz (THz) tomography is a recently developed imaging technique allowing 3D inspection of opaque objects. In this paper, we develop an ordered subsets convex algorithm for THz transmission tomography (THz-OSC). Since the reconstruction quality is highly depending on the THz beam energy, we investigate afterwards a multienergy version of the algorithm in order to provide a more accurate reconstruction of the acquired sample. This multi-energy approach is validated by reconstructing data from tomographic acquisitions measured with a 84/287 GHz transmission scanner. Then we discuss how this dual-energy approach could be able to extract physical properties of acquired samples in addition to improving 3D reconstruction.

We demonstrate new opportunities for the detection of concealed objects and of clothes components due to using of computer processing of images captured by passive THz cameras, manufactured by various companies. Computer processing of images results in a temperature resolution enhancing of cameras. We achieve good quality of the image due to applying various spatial filters to show independence of processed images on math operations. This result demonstrates a validity of observable objects. We discuss also a possibility of temperature trace observing on human skin if there is a difference in temperature inside the body. We consider images produced by THz passive cameras manufactured by Microsemi Corp., and ThruVision Corp., and Capital Normal University (Beijing, China).

Terahertz (THz) imaging is a relatively new non-destructive analytical technique that is transitioning from established application research areas such as defense and biomedicine to studies of cultural heritage artifacts. Our research adopts a THz medical imaging system, originally designed for in vivo tissue hydration sensing, to acquire high contrast imagery of painted plaster samples in order to assess the ability of the system to image the Byzantine wall paintings at the Enkleistra of St. Neophytos in Paphos, Cyprus. The original 12th century paintings show evidence of later painting phases overlapping earlier iconography. A thin layer of lead white (2PbCO₃·Pb(OH)₂) underlies, in parts, later wall paintings, concealing the original painting scheme beneath. Traditional imaging modalities have been unable to image the underlying iconography due to a combination of absorption and scattering. We aim to use THz imaging and novel optical design to probe beyond the visible surface and perform in situ analysis of iconography beneath the lead white layer. Imaging results of painted plaster mock-ups covered with a thin layer of lead white and/or chalk, as well as of a painted wooden panel with obscured writing, are presented, and from these images sufficient contrast for feature identification is demonstrated. Preliminary results from the analysis of these mock-ups confirmed the utility of this technique and its potential to image concealed original paintings in the Enkleistra of St. Neophytos. The results encourage analysis of THz scattering within paint and plaster materials to further improve spatial resolution and penetration depth in THz imaging systems.

THz imaging system design will play an important role making possible imaging of targets with arbitrary properties and geometries. This study discusses design consideration and imaging performance optimization techniques in THz quasioptical imaging system optics. Analysis of field and polarization distortion by off-axis parabolic (OAP) mirrors in THz imaging optics shows how distortions are carried in a series of mirrors while guiding the THz beam. While distortions of the beam profile by individual mirrors are not significant, these effects are compounded by a series of mirrors in antisymmetric orientation. It is shown that symmetric orientation of the OAP mirror effectively cancels this distortion to recover the original beam profile. Additionally, symmetric orientation can correct for some geometrical off-focusing due to misalignment. We also demonstrate an alternative method to test for overall system optics alignment by investigating the imaging performance of the tilted target plane. Asymmetric signal profile as a function of the target plane’s tilt angle indicates when one or more imaging components are misaligned, giving a preferred tilt direction. Such analysis can offer additional insight into often elusive source device misalignment at an integrated system. Imaging plane tilting characteristics are representative of a 3-D modulation transfer function of the imaging system. A symmetric tilted plane is preferred to optimize imaging performance.

Terahertz (THz) detection has been proposed and applied to a variety of medical imaging applications in view of its unrivaled hydration profiling capabilities. Variations in tissue dielectric function have been demonstrated at THz frequencies to generate high contrast imagery of tissue, however, the source of image contrast remains to be verified using a modality with a comparable sensing scheme. To investigate the primary contrast mechanism, a pilot comparison study was performed in a burn wound rat model, widely known to create detectable gradients in tissue hydration through both injured and surrounding tissue. Parallel T2 weighted multi slice multi echo (T2w MSME) 7T Magnetic Resonance (MR) scans and THz surface reflectance maps were acquired of a full thickness skin burn in a rat model over a 5 hour time period. A comparison of uninjured and injured regions in the full thickness burn demonstrates a 3-fold increase in average T2 relaxation times and a 15% increase in average THz reflectivity, respectively. These results support the sensitivity and specificity of MRI for measuring in vivo burn tissue water content and the use of this modality to verify and understand the hydration sensing capabilities of THz imaging for acute assessments of the onset and evolution of diseases that affect the skin. A starting point for more sophisticated in vivo studies, this preliminary analysis may be used in the future to explore how and to what extent the release of unbound water affects imaging contrast in THz burn sensing.

Terahertz detectors based on GaAs/AlGaAs heterostructure were investigated at low temperatures and high magnetic fields. A response of detectors showed a line caused by a cyclotron resonance transition which was accompanied by several peaks originated form excitation of magnetopasmons. Illumination with a visible light caused an increase of the plasma concentration and resulted in a change of the magnetoplasmon spectrum. An analysis of spectra allowed to determine changes in the plasmon dispersion relation with a visible light which gives a tool to tune a THz response of a plasmonic detector.

In this paper, analytical expressions for optical nonlinearities are applied in the simulations of the absorption and photoluminescence of mid infrared materials. The fast and efficient approximations reproduce recent experimental data for dilute nitride semiconductors with very good agreement.