This PDF file contains the front matter associated with SPIE Proceedings Volume 11279, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

InGaAs and GaN HEMTs, InP and SiGe HBTs, and Si MOS demonstrated an efficient detection of terahertz (THz) radiation. The detection mechanism is the rectification of the decaying oscillations of plasma waves. These devices have been also used for homodyne and heterodyne detection, frequency mixing, and for the detection of THz pulses. A high detection speed, a wide dynamic range, and the sensitivity to the sign of the THz electric field make them very attractive for applications in the THz time resolved and time-domain spectroscopy. InP-based and SiGe HBTs have also demonstrated the THz operation. The missing link to revolutionizing the THz electronics is the lack of efficient and powerful THz electronic sources. The Dyakonov-Shur and plasmonic boom instabilities - the proposed mechanisms of generating THz radiation by FETs - require the resonant excitation of the plasma waves, i.e. very short device sizes and high materials and interface quality. The feature sizes of 7 nm and 5 nm of the current and emerging generations of Si CMOS are considerably smaller than the 15 to 30 nm mean free path in Si at room temperature. Ballistic transport in such nanoscale FETs should enable the resonant plasma wave regimes. GaN-based FETs, with extremely high sheet carrier densities and, as a consequence, with higher plasma frequencies, should demonstrate even better performance. The materials properties of p-diamond make it a promising candidate for plasmonic THz sources. New device designs - plasmonic crystals - using multiple resonant sections should improve coupling and increase power.

Terahertz (THz) range holds between infrared light and millimeter wave or microwave radiation. Moreover, THz waves is highly attenuated by the metal object or sensitive to an inter-molecular binding force. Therefore, imaging using THz range is attracted much attention for security, manufacturing, chemical imaging, and so on. In our research, the THz detector composed of Indium arsenide (InAs) high electron mobility transistor (HEMT) and one-sided directional slot antenna on a chip will be developed. In this paper, we focused on the antenna on a chip. The proposed antenna has three layers, namely, top antenna metal, dielectric substrate (BCB, benzocyclobutene) and bottom floating metal layer. There are a coplanar (CPW) feed lines and slots on the top antenna metal. By optimizing the size of the bottom floating metal layer, the radiation toward the back side is suppressed. The CPW feed line is connected the gate electrode on the InAs HEMT. In order to maximize the receiving THz signal form the antenna to InAs HEMT, antenna and gate input impedance is characterized by using the 3D electromagnetic simulator. It has been found that when the input impedance of the gate electrode changes from 10 ohms to 50 ohms, the voltage generated at the gate electrodes is tripled. The antenna was fabricated by the conventional photolithography process. The size of the radiation metal is 290 μm x 210 μm on the top metal with probe pads. The measured antenna gain is 5.57 dBi at 0.93 THz compared with the 5.96 dBi antenna gain at 1 THz from the simulation.

We report on the investigations of the fin-shaped GaN/AlGaN field effect transistors (FinFETs) with two lateral Schottky barrier gates exactly placed at the edges of the fin-shaped transistor channel. This kind of FinFET modification (called EdgeFET) allowed us to efficiently control the current flow in two-dimensional electron gas conduction channel. Moreover, due to depletion, regions of the channel at a certain range of reverse bias form a nanowire, which is beneficial for the tunable resonant THz detection. Our studies of current-voltage characteristics and response in the sub-terahertz frequency range confirm the validity of the approach.

In this work we demonstrate spectrometer based mid-infrared (MIR) optical coherence tomography (OCT) at 4 µm but with an increase of state-of-the-art imaging speed by at least 10 times. The improvement is based on exploiting a chirped periodically poled lithium niobate crystal. We show more than 3 kHz line rate OCT imaging. With this significant increase in imaging speed we hope to expand the efficacy of mid-infrared OCT.

In this paper, we report enhanced performance of quantum dot infrared photodetector (QDIP) by means of surface plasmonic structure. A 10-layer InGaAs quantum dot (QD) structure with AlGaAs barriers were grown on GaAs(001) substrate by molecular beam epitaxy. A periodic gold pillar array was fabricated on the surface of the wafer by i-line lithography and ICP dry etching. The wafer was processed to form a circular mesa of 300 μm diameter. The pillar pitch was varied from 2.0 to 2.5 μm and the pillar diameter was varied from 1.1 to 1.6 μm. The detectors were illuminated from substrate side to evaluate spectral responsivity and detectivity D* at temperature T = 78 K. The pillar array was found to enhance the detector performance at particular wavelength which depends on the pillar property. We found the optimal property of the pillar array whose enhancement peak matches the QDIP's response peak at 7.7 μm. At that wavelength, the responsivity and detectivity were almost doubled compared to the detector without the pillar array.

An advanced infrared emitter, consisting of a non-periodic silicium-microstructure and a platinium-nano-composition, which enables extraordinary highly emission intensities is presented. A spectral broadband emission coefficient ε of nearly 1 is achieved. The foundation of the emitter is a MEMS hot plate design containing a high temperature stable molybdenum silicide resistance heater layer embedded in a multilayer membrane consisting of silicon nitride and silicon oxide. The temperature resistance of the silicon-platinum micro-nanostructure up to 800 °C is secured by a SiO₂ protection layer. The long-term stability of the spectral behavior at 750 °C has been demonstrated over 10,000 h by FTIR measurements. The low thermal mass of the multilayer MEMS membrane leads to a time constant of 28 ms which enables high chopper frequencies. A precondition for long term stability under rough conditions is a real hermetic housing. High temperature stable packaging technologies for infrared MEMS components were developed.

Since the observation of pyroelectric properties in oxygen depleted semiconducting Y-Ba-Cu-O, the interest of its amorphous phase (a-YBCO) obtained at low deposition temperature (150 °C) has been demonstrated for near-infrared (NIR) detection. At the core of the uncooled thermal detector development, there is the material choice for sensing the incoming radiation. Apart from its manufacturing compatibility with silicon technologies for further integration with readout electronics, a low noise level and a high value of the pyroelectric coefficient are highly desirable material properties. In the first part of this paper, we investigate room temperature noise performances of planar and trilayer detectors fabricated on silicon substrates. The best noise equivalent power (NEP) and detectivity D^*, which are at the state of the art, were observed at 10 kHz modulation frequency: NEP = 2.0 pW/Hz^1/2 and D^* = 6.6×10⁹ cm·Hz^1/2/W for planar structures; NEP = 2.6 pW/Hz^1/2 and D^* = 5.7×10⁹ cm·Hz^1/2/W for trilayers. These detectors also exhibit a very fast response (time constant τ = 1.9 μs for planar, and τ = 0.12 μs for trilayer devices) as compared to commercially available pyroelectric sensors. In the second part, we examine the pyroelectric response of a-YBCO to extract the pyroelectric coefficient p. A first estimate of p gave a value as high as 600 μC·m⁻²·K⁻¹ at 300 K. The pyroelectric figure of merit F_d which takes into account dielectric properties of the material (dielectric constant and dielectric losses) is also discussed with respect to results published for pyroelectric oxide thin films sputtered on silicon substrates.

Terahertz Spectral Characterization of NIR Nanomaterials Jillian P. Martin, M.E. Gkikas, C.S. Joseph and R.H. Giles Biomedical Terahertz Technology Center, University of Massachusetts Lowell, MA, USA Near-infrared responsive (NIR) nanomaterials are currently being developed for use in photothermal and photodynamic cancer treatment therapies. The NIR nanomaterials under investigation for such therapies include gold nanorods, gold nanoshells, lanthanide-doped nanomaterials, and graphene oxide. To date, researchers have shown that terahertz (THz) time-domain spectroscopy (TDS) can be used to track the concentration of gold nanorods. In this work, a THz-TDS system is utilized to investigate the terahertz spectral characteristics of various solutions of NIR nanomaterials.

Terahertz pulse time-domain holography (THz PTDH) is an ultimate technique both for the measurement of object properties in the THz range and broadband wavefront sensing. In this proceeding, we reveal the key principles of the technique, including the layout solutions for recording a collimated THz wavefront in the form of spatio-temporal profiles. The possibilities to investigate ultrashort THz field propagation dynamics based on the data measured in one transverse plane is discussed. The evolution for both transverse and longitudinal components of the electromagnetic field thus can be estimated. We illustrate these possibilities on the example of Bessel-Gaussian pulsed THz beam propagation formed by an on axicon lens.

Computational sampling methods have been implemented to spatially characterize terahertz (THz) fields. Previous methods usually rely on either specialized THz devices such as THz spatial light modulators, or complicated systems requiring assistance from photon-excited free-carriers with high-speed synchronization among multiple optical beams. Here, by spatially encoding an 800 nm near-infrared (NIR) probe beam through the use of an optical SLM, we demonstrate a simple sampling approach that can probe THz fields with a single-pixel camera. This design does not require any dedicated THz devices, semiconductors or nanofilms to modulate THz fields. By using computational algorithms, we successfully measure 128×128 field distributions with a 62 μm transverse spatial resolution, more than 15 times smaller than the central wavelength of the THz signal (940 μm. Benefiting from the non-invasive nature of THz radiation and sub-wavelength resolution of our system, this simple approach can be used in applications such as biomedical sensing, inspection of flaws in industrial products, and so on.

This work addresses simulations of terahertz waves for the determination of layer thicknesses, in particular to analyze the top layers of multilayer coatings. For such analyses, a relatively short measuring time window is possible, which leads to time savings. However, not every simulation method takes time limitations of measuring windows into account. Therefore, we compare different simulation methods and adapt one of them to our application.

We analyze Terahertz (THz) echoes by reflection on a sunflower leaf in order to evaluate the internal leaf structure (geometry, complex indices and thicknesses). The analysis is based on the thin film multilayer formalism in time and frequency domains. A high agreement is emphasized between experiment and theory, and we evaluate how realistic the multilayer solution can be in regard to our knowledge related to the sunflower leaf. A test campaign is performed in Charles Coulomb laboratory, which is equipped with the THz spectrometer.

The demand for high speed Terahertz measurement devices for industrial and research applications is growing rapidly. From plant biology to nondestructive testing in the automotive industry, there is a long list of applications demanding rapid scanning of femtosecond pulses. We present a novel approach to speed up the repetition rate control by a factor of 10 compared to conventional approaches using electro-optical means . Hence, we have demonstrated repetition rate changes of 100 kHz in less than 1 ms. This increase in speed enables novel applications in Terahertz spectroscopy which will be presented.

We employ thickness gauging with a fast terahertz time-domain spectroscopy (TDS) system based on electronically controlled optical sampling (ECOPS) and compare the results with those of a benchmark conventional terahertz TDS system and a mechanical micrometer gauge. The results of all technologies are in good agreement. We show that the ECOPS system is suitable for fast inline thickness measurements, owing to high measurement rate of 1600 traces per second. Moreover, we characterize the system with respect to signal quality. The time-domain dynamic range is ~60 dB for a single-shot measurement, and ~90 dB with 1000 trace averages, which are completed within less than a second (i.e., 0.625 seconds). The time-domain signal-to-noise ratio amounts to ~50 dB and ~80 dB for 1 and 1000 averages, respectively.

Stable THz waves are obtained from the multimode-laser diode excited photoconductive antennas using a laser chaos. Because the many longitudinal modes oscillate simultaneously. This THz wave is suitable for the spectroscopy. Stable THz waves are obtained from the multimode-laser diode excited photoconductive antennas using a laser chaos. Because the many longitudinal modes oscillate simultaneously. This THz wave is suitable for the spectroscopy.

We present a novel system architecture for coherent cw THz spectrometers. The system features 2.5 THz bandwidth with an acquisition rate of 58 Hz and provides full phase information without active phase modulation. We achieve passive amplitude and phase modulation at a fixed intermediate frequency by using a fast sweeping laser in combination with a static optical fiber delay: By heterodyning the incoming THz signal with the frequency-shifted optical beatnote on a photomixing receiver, we can extract amplitude and phase of the signal with a lock-in detector. To the best of our knowledge, this is the fastest coherent cw THz system demonstrated so far.

We demonstrate real-time and ultrabroadband THz time-domain spectroscopy in a compact optimized system. The system is based on a femtosecond fiber laser at 1560 nm as pump source and optical rectification and detection in organic electro-optic DSTMS crystals. Terahertz time-domain spectroscopy with a spectrum extending beyond 20 THz with a maximum bandwidth 70 dB has been reached with this system. We present spectroscopic measurements of various biomedical and pharmaceutical substances in transmission and reflection geometry at frequencies up to 20 THz.

We demonstrate a high dynamic range, broadband THz-TDS system that is compatible with 1 μm femtosecond lasers. In order to improve the dynamic range and bandwidth, we designed and fabricated photoconductive terahertz sources and detectors equipped with arrays of plasmonic nano-antennas fabricated on an epitaxially-grown In0.24Ga0.76As substrate. Plasmonic nano-antennas concentrate the photo-generated carriers close to the antenna-photoconductor interface. This ensures a superior performance both in terahertz generation and detection by increasing the induced ultrafast current at the antenna terminals. We demonstrate a THz-TDS system with more than a 100 dB dynamic range and a 4 THz bandwidth.

Terahertz (THz) spectroscopy is expected as a new technique for nondestructive inspection of pharmaceutical tablets, because THz waves pass through a wide variety of pharmaceutical materials. We have been developing a frequency domain THz spectrometer with an injection-seeded THz parametric generation (is-TPG) technique. In this study, the frequency range of the spectrometer was successfully expanded to 0.9-4.2 THz by generating widely tunable THz waves by shifting a nonlinear crystal to balance gain and absorption of the THz waves in the crystal, and detecting the THz waves with an angle-compensation method that utilized the dispersion of the crystal. Absorption spectra of the crystal polymorphs of carbamazepine forms I and III were obtained with the THz spectrometer, and these polymorphs were identified by specific absorption peaks related to their solid state.

We have presented graphene-based metasurface tri-layer highly efficient broadband solar absorber. Below metasurface and above dielectric layer a monolayer graphene sheet is inserted to achieve maximum average absorption in visible terahertz (430 THz to 770 THz) spectrum. We demonstrated a broadband solar absorber with 85% absorptance in the visible terahertz band. The single C shaped unit cell of metasurface solar absorber made up of tungsten separated by the tungsten ground plane by a silicon dioxide dielectric layer. The proposed absorber also investigated to manipulated absorbtance of absorber by varying different parameters of structure. The graphene metasurface solar absorber has a potential application for the development of terahertz lasers and sensors.

We employ tilted-pulse-front techniques to control the propagation direction of the THz pulses emitted from semiconductor photo-switches. In a first step, we successfully demonstrate the manipulation of the THz emission angle from an electrically biased, large-area photoconductive switch on semi-insulating GaAs. With a double-metal waveguide with an optically transparent thin metal window we then show that, at a proper tilt angle of the optical pulse front, the generated THz pulse propagates along the waveguide to radiate off at its end facet. The technique is suitable for optical pumping of THz gain media with short excited-state lifetime, and for THz pulse control and beam steering using any type of coherent THz emitters.

A new generation of photoconductive antennas (PCAs) compatible with 1550 nm excitation for terahertz time-domain spectroscopy is presented. Iron (Fe) doped InGaAs grown by molecular beam epitaxy is used as the underlying photoconductor. Due to the advantageous combination of ultrashort carrier lifetime and excellent electronic properties, InGaAs:Fe based PCAs increase the dynamic range for frequencies from 1 THz – 6 THz by more than 10 dB compared to the state-of-the-art.

The linear and nonlinear optical properties are investigated in a <110>-cut, 485 μm-thick AgGaSe₂ crystal. The linear optical properties are studied by performing terahertz time-domain spectroscopy measurements, which allow the refractive indices and the extinction coefficients to be calculated for the uniaxial AgGaSe₂ crystal. Optical rectification measurements are conducted to investigate the crystal’s nonlinear optical properties, which produce broadband terahertz radiation pulses having spectral components between 0.02 and 4 THz. Due to the large optical rectification coherence length exhibited by the AgGaSe₂ crystal, in-phase terahertz radiation is generated across the entire length of this 485 μm-thick crystal. Therefore, this crystal can be utilized as a source of broadband terahertz radiation in areas such as medicine and security.

Terahertz (THz) waves are electromagnetic waves with frequencies between 0.1 THz and 10THz. With the rapid development of wireless communication, the existing spectrum resources have become increasingly scarce. Developing the new frequency band of wireless communication has gradually become a consensus to solve this contradiction. There are a lot of unexploited resources in THz frequency range, making terahertz play a decisive role in the future development of wireless communication. Three-dimensional (3D) graphene with connection carbon nanomaterials is expected to possess better optical and electrical properties than single-layer graphene. In this paper, we studied a room temperature ultra-broadband photodetector based on 3D graphene and investigated the different photoresponse at 0.22, 2.52, 30 THz. Obvious photocurrents and ultra-broadband absorption from infrared spectrum to terahertz (THz) region can be measure in the three 3D graphene. A high photoresponsivity of 15.3 mA W^-1 and a fast time response of 20 ms have been achieved at 2.52 THz. The results reveal 3D graphene a good candidate for room-temperature broadband Terahertz detector.

We demonstrate terahertz imaging using a terahertz nonlinear quantum cascade laser source (THz NL-QCL). THz NL-QCLs are ultrabroad terahertz source which can be operated at room temperature. The maximum operating temperature of conventional THz-QCLs has been limited to 210.5 K so far. Therefore, THz NL-QCL sources are the only electrically pumped monolithic terahertz semiconductor sources operable at room temperature. Currently, various room temperature compact THz sources have been reported. However, the operation frequencies of these sources were basically below 1 THz. Although several devices demonstrate THz emission above 1THz, output powers are still quite low; thus, it is very difficult to apply to practical THz applications. THz NL-QCL sources are able to operate above 1 THz, and the average THz output powers have exceeded 10 μW (duty cycle >5%) at room temperature, which can potentially be applied to THz applications. Also, in edge-emitting metal-metal THz QCLs, ring-like fringe patterns in their far-field beams are frequently observed due to far-field interference of coherent radiation in deep sub -wavelength apertures. Otherwise, the beam profile of THz NLQCL is Gaussian-like far-field pattern. The beam quality of nonlinear quantum cascade laser is better than that of conventional terahertz quantum cascade laser. Therefore, THz NL-QCL sources are suitable for terahertz imaging. We demonstrated terahertz imaging with the THz NL-QCL sources.

Terahertz (THz) imaging is an attractive alternate to ultrasonic based Non-destructive Evaluation (NDE) especially for Fiber Reinforced Polymers (FRPs) such as Glass FRP (GFRP) composites as the latter demands proximity and additional coupling medium for the best performance. Typically, THz imaging system uses a single emitter-detector configuration employing raster scan method for image acquisition. The image acquisition speed is greatly limited by the speed of the mechanical stages and hence its usage in real-time industrial NDT applications such as in-line quality control has been limited. Alternatively, having an array of detectors will significantly increase the system cost. As an optimal compromise for speed and cost, line scanners are highly desirable. In this work, rapid imaging performance of a THz line scanner has been studied by imaging closely spaced defects in GFRP composites using a 100 GHz source. The total acquisition time for imaging the GFRP sample of dimensions 55× 35 mm² is 10 s, which is >100 times faster compared to a conventional raster scanning technique. In addition, image deconvolution techniques such as Lucy Richardson and Weiner deconvolution have been adopted to improve the quality of the acquired THz images. The results show that the THz line scanners can successfully be employed for rapid defect detection in GFRP composites.

Traditional imaging systems including microscopes depend on the generation of real images to be recorded by the sensor element, which is only sensitive to the intensity and not the phase. The object distance, which is crucial for the spatial resolution of the system, therefore is restricted to be larger than the focal length of the objective lens. This leads to a limitation of the achievable lateral, diffraction-limited spatial resolution. In order to reach a resolution enhancement with the same system components, we explore – in the sub-THz frequency regime – heterodyne detection of the scene’s complex-valued spatial Fourier spectrum in the image-sided focal plane of the optical system. The existence of the Fourier spectrum is independent of the object distance. The measured complex-valued field distribution enables a numerical back-propagation to any place in the object-sided free space from which radiation has reached the detector. Heterodyne Fourier imaging hence enables 3D imaging and – the relevant theme here – it lifts the restriction of the imaging distance. This enables object distances smaller than the focal length of the objective lens and, with it, an enhanced diffraction-limited spatial resolution. In the experiments presented here, the heterodyne data acquisition of the 0.3-THz continuous wave radiation employs Si CMOS TeraFET detectors, i.e., THz sensors based on antenna-coupled field-effect transistors which have been developed in our laboratory. First imaging results show an enhancement of the maximal resolution by a factor of 1.4 for the specific measurement conditions of our experiments, with considerable room for improvement.

Much effort has been focused on the development of frequency modulation continuous waves (FMCW) light sources which are used in Light Detection And Ranging (LiDAR) systems. However there has not appeared a method or instrument which can measure the instantaneous frequency and dynamic linewidth of the FMCW light source over the whole frequency sweep range of about several hundreds of GHz with an accuracy down to kHz. In this paper a new method is proposed for measuring the dynamic frequency properties of FMCW light sources in real-time and several experimental setups are built for the feasibility study.

Brillouin spectrum engineering for the enhancement of slope-assisted Brillouin dynamic sensing is investigated by simulation. By superimposing the Brillouin gain with Brillouin losses, the tradeoff between the dynamic range and the frequency-to-amplitude sensitivity becomes more flexible. Compared to a conventional dynamic Brillouin sensor, a simultaneous enhancement of the dynamic range by 60.27% and the sensitivity by 51.21% has been achieved with the proper parameters. Due to these enhancements, the proposed sensor is 33.87% more robust to system noise and provides a more than 2.2 times of accuracy improvement for strain signal recovery.

Freedom Photonics and the University of Virginia have developed high power, wide-bandwidth balanced photodetectors based on vertically-illuminated modified uni-traveling carrier (MUTC) photodiode technology. These balanced pairs are based on single photodiodes which achieve 3-dB bandwidths of 25 GHz, coupled with output powers above 23 dBm, as well as 35 GHz photodiodes with output powers greater than 19 dBm. A balanced configuration of these devices offers advantages in common-mode noise reduction, increasing the signal-to-noise ratio. In a photonic link, high-power, balanced photodiodes support high link gain and large bandwidths, while the high linearity of these devices maximizes spurious-free dynamic range (SFDR).

We present a newly developed 90-GHz high O/E conversion efficient, high power photoreceiver for radio over fiber transmission, which is driven by photonic power supplies. The photoreceiver consisted of a hybrid integration with a 100 GHz photodetector and a 22-dB high gain RF amplifier. To save the fiber resources in the PoF system, bias-free operational design was employed for the photodetector. In the RF measurement result, we successfully achieved the high output power of +15 -dBm at 90-GHz, high linearity in RF output power against the input photo-current, and high 3dB bandwidth of 14 GHz (85-99 GHz).

Design and fabrication of a tunable fishnet metamaterial for potential THz beam steering device is presented. Split ring resonators have been the most popular unit-cell geometry for THz metamaterials. However, we have adopted the fishnet geometry for the application and utilizing polymer disperse liquid crystal (PDLC) as the tunable substrate. One of the advantages of using PDLC over conventional liquid crystal is the elimination of a cell to confine and encapsulate the liquid crystals. The trade-off of PDLC versus conventional liquid crystal performance is that a higher voltage is required to turn the PDLC on (approximately 2-10V/micron) as opposed to the conventional liquid crystal (1-5V/micron). In this report, we will present our latest development in the PDLC and its application in a tunable gradient fishnet metamaterial (TGFMM) array. The objective is to achieve a high Q and broader tunability of resonant frequency while maintaining a low operating voltage. We will present the latest simulation and experiment results of a polarization independent rotated square fishnet metamaterial design.

Thin film lithium niobate optical modulators allow modulation of optical signals up to several THz due to perfect phase matching between RF signal and optical signal that can be achieved using thin film devices. The platform uses a ridge waveguide fabricated by direct etching of lithium niobate thin film fabricated on silicon substrates. The lithium niobate thin film has been developed and optimized in our facility. Transmission spectrum of fabricated micro-ring resonators on this platform shows a linewidth of approximately 7 pm corresponding to a Q value of 2.2×10⁵ and an optical waveguide loss of 2 dB/cm. A coupling loss of -5 dB per coupler is obtained using grating couplers. Measured fiber to fiber insertion loss of the device is -10 dB. The measured 3-dB optical bandwidth of the fiber to fiber optical coupler is 45 nm. A Mach-Zehnder modulator consisting of two MMIs and 6 mm long arms were designed and fabricated on X-cut thin film of lithium niobate. Measured V_π of the device is 7.5 V at low frequencies (i.e. 10KHz) for a device with 7 μm gap between the electrodes. The measured half-wave voltage-length product, V_π.L, is equal to ~4.5 V.cm. High speed measurement results of the device response are presented. A THz electric field of 10kV/mHz^0.5 is detected with a low dynamic range OSA and it is estimated that a THz electric field with a strength as low as ~100V/mHz^0.5 is detectable by modulating the optical signal using these modulators.

Nanosilica incorporation in cement has been of great interest for its accelerating effect on the hydration process as well as providing higher compressive strength and durability. During hydration, cement constituents, such as tricalcium silicate (C3S) and dicalcium silicate (β-C2S) react with water to form key hydration products, such as calcium silicate hydrate (C-S-H) and calcium hydroxide (Ca(OH)₂, CH). In this work, Mid-infrared and Terahertz spectroscopy has been employed to study the effect of nanosilica incorporation in cement hydration. The acceleration due to the presence of nanosilica has been demonstrated by the reduction in peak intensity of the resonances related to Si-O stretching (925 cm^-1) and Si-O bending modes (520 cm^-1) which confirms faster consumption of the cement constituents. Furthermore, the formation of the hydration products C-S-H and CH is vital since C-S-H contributes to the early stage strength development in concrete and CH is an undesirable hydration product. CH content in the cement matrix can be minimized by nanosilica incorporation resulting in pozzolanic reactions as CH reacts with nanosilica to produce more C-S-H. Formation of C-S-H has been demonstrated by the prominence of the resonances related to deformations of SiO₄ chains around 455 cm^-1 and 1100 cm^-1. The type of C-S-H can also be predicted by tracking the shift of resonances to higher/lower wavenumbers, denoting polymerization which is more prominent for the nanosilica incorporated sample. Formation of the other key hydration product is observed as the resonance related to CH around 314 cm^-1 is seen to get sharper with hydration. This study has also been able to show a reduced carbonation effect in the nanosilica incorporated sample as evident from the less prominent carbonate peaks around 1425 cm^-1 after 28 days of hydration.

We present that the linearity of silicon ring modulators in microwave photonics links can be improved by manipulating the quality factor in the cavity. By reducing Q factors of silicon ring modulators from 11000 to 5880 and tuning the operation wavelength for modulation, the measured Spurious-Free Dynamic Ranges of the third-order intermodulation distortion are improved from 98.5 dB·Hz^2/3 to 104.3 dB·Hz^2/3 and from 90.6 dB·Hz^2/3 to 94.7 dB·Hz^2/3 at 1 GHz and 10 GHz, respectively.

MXenes are a new family of two-dimensional transition metal carbides, nitride and carbonitrides with high conductivity and versatile chemical structures. Here we have used THz spectroscopy to study microscopic conductivity and photoinduced carrier dynamics in two Mo-based MXenes, Mo₂Ti₂C₃T_z and Mo₂TiC₂T_z. Both exhibit high intrinsic carrier densities (~ 10²⁰ cm^-3 in Mo₂Ti₂C₃T_z, and ~ 10¹⁹ cm^-3 in Mo₂TiC₂T_z), mobilities, and high conductivities within individual nanosheets. We also observe that optical excitation increases their conductivity, unlike Ti₃C₂T_z, in which photoexcitation suppresses conductivity for nanoseconds. Vacuum annealing improves the long-range transport of photoinduced carriers and further increases their lifetime, as it results in de-intercalation of water and other species from van der Waals gaps between the nanosheets in the films. High and long-lived photoinduced conductivity suggests Mo-based MXenes a promising candidate for optoelectronic, sensing and photoelectrochemical applications.

Nowadays, interactions between pulsed electromagnetic field and biological cells and tissues are particularly investigated to prevent any noxious effects and/or to develop therapeutic processes able to improve cancer treatment. Electrochemotherapy is an example which allows drugs delivery improvement thanks to local pulsed electric field application. Consequently, high voltage electrical nanosecond pulse generation has received great interest from the researchers working in the bioelectromagnetic domain. In that framework, we demonstrated that picosecond kilovolt generator activated by a nanosecond and femtosecond laser sources can be coupled with a Multiplex Coherent Anti- Stokes Raman Scattering system (M-CARS) to study the impact of electric pulses on biological cells. We generated kilovolt picosecond electric pulses by using a frozen-wave generator which integrated silicon PhotoConductive Semiconductors Switches (PCSSs). Because of the linear switching regime no temporal jitter is observed during the optical switching thus, coherent combining of short electric pulses can be obtained. Two semiconductors are activated with adjustable time delay, generating unipolar pulses with less than 60 ps rise time (pulse duration 100 ps). Balanced and unbalanced bipolar pulses have been also obtained with a peak to peak voltage of 1.4 kV and a total duration of 244 ps. The initial optical pulse may be used to produce a supercontinnum extended from the visible and up to infrared domain (2.4 μm). Thus synchronized M-CARS diagnosis can be realized when nanosecond and picosecond electric pulse excitation is applied to biological samples.

In this paper, stimulated Brillouin scattering induced noise in Brillouin optical time domain analyzers is experimentally and theoretically investigated. The noise mainly comes from the beating between the probe wave and the spontaneous Brillouin scattering component and from phase-to-intensity conversion. The noise in the time and frequency domain has been measured along the fiber. The results reveal that, compared to gain based sensors, the loss based ones show a lower Brillouin induced noise level. Furthermore, the Brillouin noise is characterized in dependence on the spatial resolution. This investigation provides a deep insight to the frequency dependence of the noise distribution, which might contribute to signal-to-noise ratio enhancement in Brillouin-based distributed sensing.

H-terminated diamond MESFETs are emerging devices for RF and power electronics applications, but few studies reported in the literature analyze their charge-trapping phenomena and long-term stability.
We analyze the effects of stress in off-state condition at increasing drain bias. The main variations are an increase in on resistance (R_on) and in threshold voltage (V_th) and a decrease in the peak transconductance value, whereas the gate diode remains stable during the test. The R_on and V_th variations are correlated, suggesting a common degradation mechanism for both effects.
By means of sampled filling and recovery measurements, it is possible to highlight a trapping process occurring in off-state condition and dependent on the drain filling bias. This process is related to deep levels located both in the access regions and in the region under the gate, since it results in both R_on and V_th shifts. By means of temperature-dependent recovery measurements, a dominant thermal activation energy of 0.30 eV was found. The good fit quality according to the “stretched exponential” model indicates that the deep levels are extended defects or energy mini-bands.
The amplitude of the recovery transient collected after the same filling condition increases after stress at higher drain voltage, confirming that the deep levels causing the detected dynamic variation are increasing in concentration as a consequence of the stress. The good correlation between the amplitude and the R_on variation suggests that the detected variation in concentration is the root cause for the degradation of the device.

Single-mode fiber plays an important role in the applications of THz technology. Serving as one of the optimal choices of THz fiber, hollow-core anti-resonant fiber (ARF) with single-layer anti-resonant elements has advantages of low transmission loss, high damage threshold, low dispersion but disadvantage of single mode which can by realized by the high-order-mode (HOM) suppression effect. In this paper, the key factor affecting the HOM suppression effect in THz ARF is investigated and confirmed to be the cross-sectional area ratio of cladding tubes inner side with fiber core, besides, the ratio will slightly change with the variation of the anti-resonant period of THz ARF where the cladding tubes have different shell thickness. By optimizing the anti-resonant elements of THz ARF from round tubes to semielliptical tubes, a high-performance single-mode THz fiber is proposed and the confinement loss of high-order-mode is confirmed larger than 24 dB/m at the same time that of fundamental-mode is controlled less than 0.4 dB/m.

Terahertz pulse time-domain holography (THz PTDH) is an ultimate technique both for the measurement of object optical properties and broadband wavefront sensing. However, THz PTDH has valuable restriction connected with low signal-to-noise ratio which becomes a serious issue in coherent measurements. This noise problem could be solved by filtering with use of modern block-matching algorithms based on nonlocal similarity of small patches of images existing in investigated objects. Here we present the study on the use of denoising algorithms applied for hyperspectral THz data in the spatio-temporal and spatial-spectral domain. We provide a numerical simulation of denoising in case of broadband uniform topologically charged (BUTCH) beam of pulsed THz radiation.

The frequency modulation continuous wave (FMCW) light detection and ranging (LIDAR) has drawn enormous attention and intense effort. In this paper a nonlinear tuning technique for a DFB laser which is driven by a home-made LD driver is proposed and developed experimentally for generation of linearly chirped light which may be applied in FMCW LIDAR systems. A frequency predistortion procedure based on nonlinear tuning technique can be applied to the LD driver which could be controlled by a field-programmable gate array (FPGA) module. And the frequency predistortion procedure can be used iteratively until the desired linearity of the chirped optical waves is achieved before the PLL is introduced in. This technique can be seen as an effective way to increase the acquisition bandwidth of the PLL significantly so that the trade-off between large acquisition frequency and narrow bandwidth of the PLL is no longer a problem.

We present a step-swept light source with high linearity and tunable frequency step based on the lightwave synthesized frequency sweeper (LSFS). Different from the traditional internal modulation, this light source can effectively avoid complex frequency-calibration and deduce the spectrum stitching error by using the external modulation and a time delayed spectrum stitching technique. Experimentally, a k-space swept with > -0.99997 swept linearity is achieved. The swept range are 5.135 nm with 10 GHz swept step and 3.902 nm with 7.5 GHz swept step. The spectrum stitching-error is less than 2.92% and 0.65% respectively.

Here, we present 3D solutions of Maxwell’s equations using the finite element method (FEM) for sub-wavelength plasmonic THz antennas in array configurations. We show that mutual coupling between neighboring antennas causes the arrays spectral response to change with different array densities and arrangements. The simulation results of the spectral response of the antennas are in very good agreement to the experiments. Finally, in order to investigate the sensing performance of the antennas, simulations with different feed gap materials were carried out.

The challenge nowadays is to build high-resolution 2D images with purpose shows the internal structures and properties of objects or study materials hence the importance of implementing a system of high precision sampling to optimize the work of obtaining images using Terahertz (THz) technology. The exactitude translation stage was built to move 3D objects by making two-dimensional paths of 1mm±1% with periods of programmable waiting time. The objective is to place the sample in reading position of each pixel, facing to propagation of the THz wave. For the operation was made to several programs capable of controlling the positioning, time and speed required in sampling.

Semiconductor materials are widely used in integrated circuit and are important material platform in the fabrication and development of different technologies. Their material properties, e.g. complex permittivity and permeability, must be accurately known when designing high frequency microwave and millimeter wave devices. The applications of terahertz (THz) waves is receiving considerable attention because of the many potential applications. In this paper we provide results of an initial study to characterize low resistance silicon. First a free-space measurement setup is used to measure the characteristics of four commercial silicon wafers. These differently doped silicon wafers have different resistivity, i.e. 10, 20, 30 & 40 Ω cm. The complex relative dielectric constant and the loss tangent of the four types of silicon are measured. Second, the absorption properties of the same samples are measured using a compact terahertz time domain spectrometer (THz-TDS). In both measurements the results show that the sample doped using Phosphorus has a higher loss (absorption) and lower permittivity in the THz range compared the three wafers doped with Boron. In the case of the three Boron doped samples the loss tangent was found to depend on the resistivity of the material.

THz imaging is being increasingly applied to produce images of the interiors of objects and to identify variations. THz detectors are steadily improving, but lag behind visible light systems in terms of their resolution, i.e. due in part to the limited pixel number and the size of the array detector area. Synthetic aperture methods seek to increase the numerical aperture (and so image resolution). For example, the position of the detector is altered, and multiple images are captured in order to obtain the entirety of the image plane information. High precision registration and image fusion algorithms are then required to stitch together the individual images captured. Here, we propose a novel subpixel estimation method for THz imaging, which enables real-time operation with high temporal and spatial resolution. A THz imaging system is implemented using a continuous-wave THz source emitting at 300 GHz and a THz camera with 16×16 pixels. The subpixel estimation method is applied to the resulting THz images. We demonstrate that this method can be used to calculate and calibrate image position in a conventional THz imaging system, with significantly reduced computational expense.