Output power enhancement of a palmtop terahertz-wave parametric generator

Broadband terahertz (THz) waves were generated by optical parametric processes based on laser light scattering from the polariton mode of a nonlinear crystal. By using the parametric oscillation of a MgO-doped LiNbO3 crystal pumped by a nanosecond Q-switched Nd:YAG laser, we have realized a broadband, high-energy and compact THz-wave source. We report the development of a THz-wave parametric generator (TPG) using a small pump source with a short pulse width and a top-hat beam profile. These characteristics of the pump beam permit high-intensity pumping especially close to the output surface of the THz wave without thermal damage to the crystal surface. We also calculated the outcoupled THz wave for beams with two different intensity profiles: a top-hat beam (in this experiment) and a Gaussian beam (previously reported). The result shows the mechanism of the output energy and/or power enhancement.     

Terahertz astronomical telescopes and instrumentation

The terahertz (THz) region of the electromagnetic spectrum is of great importance for astrophysics. In fact, it is central to the whole field of experimental cosmology. This paper outlines the range of astrophysical problems that can be answered by observing at THz frequencies, describes some of the major THz astronomical telescopes that are being constructed, and gives a vision of the way in which the technological development of superconducting very-large-scale integration circuit technology and solid-state source technology will lead to major advances in the subject.     

Experimental comparison of performance degradation from terahertz and infrared wireless links in fog

We describe a lab setup for analyzing impairments of terahertz (THz) and infrared (IR) free space links caused by local refraction index changes in the signal's propagation paths that could be induced by turbulence, particles, humidity, etc. A THz signal comprising a 2.5 Gb/s data load modulated on a carrier at 625 GHz, is launched through a weather emulating chamber, detected, and its performance analyzed. An IR beam at 1.5 um wavelength carrying the same data load is superposed with the THz beam, propagating through the same weather conditions and also performance analyzed. We modulate the IR channel with a usual non-return-to-zero (NRZ) format but use duobinary coding for driving our THz source, which enables signaling at high data rate and higher output power. As both beams pass through the same channel perturbations and as their degradations are recorded simultaneously we can simultaneously compare the weather impact on both. We investigate scintillation and fog attenuation effects for the THz and IR signals by measuring bit error rates (BER), signal power, and phase front distortions.     

Arrayed silicon prism coupler for a terahertz-wave parametric oscillator

Using room-temperature parametric oscillation of a LiNbO(3) crystal pumped by a Q-switched Nd:YAG laser with a simple configuration, we have realized a widely tunable coherent terahertz- (THz-) wave source in the range between 1 and 3 THz. Inasmuch as the THz wave is affected by total internal reflection at the crystal edge, we used a Si prism coupler to couple out the THz wave. We introduce an arrayed Si-prism coupler that increases the efficiency and decreases the diffraction angle. By use of the arrayed-prism coupler, there is a sixfold increase in coupling efficiency and a 40% decrease in the far-field beam diameter, compared with the use of a single-prism coupler. We discuss the negative effect of the free carriers at the Si-prism surface that is excited by the scattered pump beam, and the positive effect of cavity rotation on the unidirectional radiation of the THz wave from a Si prism.     

RF Spectrum of a Signal after Frequency Multiplication; Measurement and Comparison with a Simple Calculation

A novel experimental technique is introduced and used to measure the effect of frequency multiplication on the RF spectrum of an oscillator. This technique makes it possible to produce the RF spectrum at X band?where measurements are relatively straightforward?that would have been produced by frequency multiplication of the 5-MHz source to any frequency from 9.2 GHz to 100 THz (1014 Hz). A simplified theory is developed and shown to reproduce the experimental results for the relative power in the carrier and noise pedestal, and the shape and the width of the carrier and noise pedestal, to within the measurement uncertainty of 2 or 3 dB, from 5 MHz to 10 THz. The calculations are easily made using analytical techniques from the measurement of the spectral density of phase fluctuations of the source, the effective input spectrum density and the bandwidth of the multiplier chain, and the frequency multiplication factor. It is shown that present 5-MHz-crystal-controlled oscillators are useful as a precision source to ~500 GHz. Suggestions for extending their range to ~100 THz are made.     

Nonresonant detection of terahertz radiation in high-electron-mobility transistor structure using InAIAs/InGaAs/InP material systems at room temperature

In this paper, we report on nonresonant detection of terahertz radiation using the rectification mechanism of two-dimensional plasmons in high-electron-mobility transistors using InAIAs/InGaAs/InP material systems. The experiments were performed at room temperature using a Gunn diode operating at 0.30 THz as the THz source. The measured response was dependent on the polarization of the incident THz wave; The device exhibited higher response when the electric-field vector of the incident radiation was directed in the source-drain direction. The 2D spatial distribution image of the transistor responsivity extracted from the measured response shows a clear beam focus centered on the transistor position, which ensures the appropriate coupling of the terahertz radiation to the device. The device also demonstrated excellent sensitivity/noise performances of approximately 125 V/W and approximately 10(-11) W/Hz(0.5) under 0.30 THz radiation.     

Terahertz pulsed spectroscopy of human Basal cell carcinoma

Good contrast is seen between normal tissue and regions of tumor in terahertz pulsed imaging of basal cell carcinoma (BCC). To date, the source of contrast at terahertz frequencies is not well understood. In this paper we present results of a spectroscopy study comparing the terahertz properties (absorption coefficient and refractive index) of excised normal human skin and BCC. Both the absorption coefficient and refractive index were higher for skin that contained BCC. The difference was statistically significant over the range 0.2 to 2.0 THz (6.6 cm(-1) to 66.6 cm(-1)) for absorption coefficient and 0.25 to 0.90 THz (8.3 cm(-1) to 30 cm(-1)) for refractive index. The maximum difference for absorption was at 0.5 THz(16.7 cm(-1)). These changes are consistent with higher water content. These results account for the contrast seen in terahertz images of BCC and explain why parameters relating to the reflected terahertz pulse provide information about the lateral spread of the tumor. Knowing the properties of the tissue over the terahertz frequency range will enable the use of mathematical models to improve understanding of the terahertz response of normal and diseased tissue.     

频率漂移不敏感太赫兹波透射测量系统设计

当太赫兹波源频率发生漂移时,由于干涉效应会影响太赫兹波对样品透射测量结果的精度。设计了一种频率漂移不敏感的双光路透射测量系统。用MATLAB建立了F-P干涉效应仿真模型,对参数进行了优化,分析比较了太赫兹波源频率漂移对单路和双路透射测量系统测量精度的影响。结果表明,该测量系统有效地克服了太赫兹辐射源频率漂移对测量的影响,提高了测量精度。     

Differentiation of CO isotopic variants by frequency tuning a terahertz source

The transmission spectra of two CO isotopic variants, i.e., (12)CO and (13)CO, are measured with a recently developed widely tunable THz source. The pure rotational transition lines of J=6 --> 7, 10 --> 11, 11 --> 12, 12 --> 13, and 13 --> 14 have been identified in the spectra. The transition frequencies and the rotational constant of each CO isotopic variant are also presented. Experimental results indicate that the difference of the rotational constants between the two isotopic variants can be used to reliably differentiate (12)CO and (13)CO. Compared with the measurements made by using a Fourier transform infrared spectrometer, our tunable THz source has produced approximately the same accuracies for measuring transition frequencies or determining rotational constants.     

Terahertz Digital Holography

Abstract: Terahertz (THz) technology is combined with digital holography for THz imaging. The characteristics of the propagation behaviour of the THz pulse in free space are investigated by using numerical simulations. The algorithm is based on the angular spectrum theory. The spatiotemporal coupling of the THz pulse during propagation results in a significant time‐dependent beam diameter and wave front. The two‐dimensional dynamic evolution of the THz pulse passing through an aperture is obtained. The diffraction is time‐dependent as the pulse travels through the object, which can be clearly observed in simulations. The simulation algorithm and result have been used to reconstruct the original object, with the spatiotemporal amplitude recorded by using a charge‐coupled device (CCD). The implementation of THz digital holography is presented and the corresponding experimental result given.     

High-power picosecond terahertz-wave generation in photonic crystal fiber via four-wave mixing

We demonstrate picosecond terahertz (THz)-wave generation via four-wave mixing in an octagonal photonic crystal fiber (O-PCF). Perfect phase-matching is obtained at the pump wavelength of 1.55?μm and a generation scheme is proposed. Using this method, THz waves can be generated in the frequency range of 7.07-7.74?THz. Moreover, peak power of 2.55?W, average power of 1.53?mW, and peak conversion efficiency of more than -66.65?dB at 7.42?THz in a 6.25?cm long fiber are realized with a pump peak power of 2?kW.     

Identification and quantification of polymorphism in the pharmaceutical compound diclofenac acid by terahertz spectroscopy and solid-state density functional theory

Polymorph detection and quantification in crystalline materials is a principle interest of the pharmaceutical industry. Terahertz (THz) spectroscopy can be used for such analytical applications since this technique is sensitive to the intermolecular interactions of molecules in the solid state. Understanding the fundamental nature of the lattice vibrational motions leading to absorptions in THz spectra is challenging, but may be achieved through computational approaches. In this study, the THz spectra of two diclofenac acid polymorphs were obtained by THz spectroscopy, and the vibrational characters of the observed absorptions were analyzed using solid-state density functional theory (DFT). The results demonstrate the quantitative capacity of THz spectroscopy and the reliability and utility of solid-state DFT in the calculation of low-frequency vibrational motions.     

Development of a multi-Fourier-transform interferometer: imaging experiments in millimeter and submillimeter wave bands

We have developed a millimeter and submillimeter Michelson-type bolometric interferometer based on a Martin-Puplett-type Fourier-transform spectrometer named multi-Fourier-transform interferometer (MuFT). We have succeeded in proving that the MuFT is capable of performing broadband imaging observations as theoretically proposed by our previous paper (OHM) [Appl. Opt. 45, 2576 (2006)]. We succeeded in acquiring the mutual coherence signal for an extended source in broadband. By analyzing the obtained mutual coherence signal following the formula proposed in OHM, 2D source images for each wavenumber from 5 cm(-1) (150 GHz) to 35 cm(-1) (1.05 THz) with a wavenumber interval of 0.4 cm(-1) (12 GHz) were successfully extracted. The large dynamic range advantage of the MuFT proposed in OHM was confirmed experimentally.     

Three-dimensional terahertz wave imaging

Pulsed terahertz (THz) wave sensing and imaging is a coherent measurement technology. Like radar, based on the phase and amplitude of the THz pulse at each frequency, THz waves provide temporal and spectroscopic information that allows us to develop various three-dimensional (3D) terahertz tomographic imaging modalities. The 3D THz tomographic imaging methods we investigated include THz time-of-flight tomography, THz computed tomography (CT) and THz binary lens tomography. THz time-of-flight uses the THz pulses as a probe beam to temporally mark the target, and then constructs a 3D image of the target using the THz waves scattered by the target. THz CT is based on geometrical optics and inspired from X-ray CT. THz binary lens tomography uses the frequency-dependent focal-length property of binary lenses to obtain tomographic images of an object. Three-dimensional THz imaging has potential in such applications as non-destructive inspection. The interaction between a coherent THz pulse and an object provides rich information about the object under study; therefore, 3D THz imaging can be used to inspect or characterize dielectric and semiconductor objects. For example, 3D THz imaging has been used to detect and identify the defects inside a Space Shuttle insulation tile.     

A modified algebraic reconstruction technique taking refraction into account with an application in terahertz tomography

Terahertz (THz) tomography is a rather novel technique for non-destructive testing that is particularly suited for the testing of plastics and ceramics. Previous publications showed a large variety of conventional algorithms adapted from computed tomography or ultrasound tomography which were directly applied to THz tomography. Conventional algorithms neglect the specific nature of THz radiation, i.e. refraction at interfaces, reflection losses and the beam profile (Gaussian beam), which results in poor reconstructions. The aim is the efficient reconstruction of the complex refractive index, since it indicates inhomogeneities in the material. A hybrid algorithm has been developed based on the algebraic reconstruction technique (ART). ART is adapted by including refraction (Snell’s law) and reflection losses (Fresnel equations). Our method uses a priori information about the interface and layer geometry of the sample. This results in the ‘Modified ART for THz tomography’, which reconstructs simultaneously the complex refractive index from transmission coefficient and travel time measurements.     

Spatial pattern separation of chemicals and frequency-independent components by terahertz spectroscopic imaging

We separated the component spatial patterns of frequency-dependent absorption in chemicals and frequency-independent components such as plastic, paper, and measurement noise in terahertz (THz) spectroscopic images, using known spectral curves. Our measurement system, which uses a widely tunable coherent THz-wave parametric oscillator source, can image at a specific frequency in the range 1-2 THz. The component patterns of chemicals can easily be extracted by use of the frequency-independent components. This method could be successfully used for nondestructive inspection for the detection of illegal drugs and devices of bioterrorism concealed, e.g., inside mail and packages.     

High field broadband THz generation in organic materials

Organic salt crystals, e.g. DAST, OH1, and DSTMS, pumped by ultra-short infrared laser are efficient THz emitters. We review our latest results on the generation in organic crystals of THz single-cycle transients with field strength of 1.5?MV/cm. The energy conversion reaches 2% with photon conversion efficiency up to 200%. THz radiation produced in such crystals offers excellent beam propagation properties and can be focused down to diffraction limited spot size in order to realize the highest field. This source covers the full spectral range between 0.1 and 10?THz. Further, we discuss the possibility to control the absolute phase and the polarity of the THz field.     

High efficiency terahertz-wave photonic crystal fiber optical parametric oscillator

We theoretically propose phase matched terahertz (THz)-wave generation via degenerate four-wave mixing (FWM) in a fiber optical parametric oscillator (FOPO) with our newly designed photonic crystal fiber (PCF). Perfect phase matching is realized when we locate the pump wavelength in the normal group-velocity dispersion (GVD) regime. The generated THz-wave can be tuned from 4.7578 to 5.9015?THz by varying the pump wavelength. Moreover, peak power of 27.38?W at 5.9015?THz with conversion efficiency of 1.37% is realized when the pump peak power of 2000?W is at 4.675?μm in our FOPO.     

Terahertz time-domain spectroscopy and the quantitative monitoring of mechanochemical cocrystal formation

Terahertz (THz) radiation probes intermolecular interactions through crystal lattice vibrations, allowing the characterization of solid materials. Thus, THz spectroscopy is a promising alternative to mainstream solid-state analytical tools such as X-ray diffraction or thermal analysis. The method provides the benefits of online measurement, remote sampling and three-dimensional imaging, all of which are attractive for quality control and security applications. In the context of pharmaceutical solids, THz spectroscopy can differentiate and quantify different forms of active pharmaceutical ingredients. Here, we apply this technique to monitor a dynamic process involving two molecular crystals. In particular, we follow the mechanochemical construction of a two-component cocrystal by grinding together phenazine (phen) and mesaconic acid (mes). To rationalize the observed changes in the spectra, we conduct lattice dynamics calculations that lead to the tentative assignment of at least one feature in the cocrystal THz spectrum.     

Toward 6G Communication Networks: Terahertz Frequency Challenges and Open Research Issues

Future networks communication scenarios by the 2030s will include notable applications are three-dimensional (3D) calls, haptics communications, unmanned mobility, tele-operated driving, bio-internet of things, and the Nano-internet of things. Unlike the current scenario in which megahertz bandwidth are sufficient to drive the audio and video components of user applications, the future networks of the 2030s will require bandwidths in several gigahertzes (GHz) (from tens of gigahertz to 1 terahertz [THz]) to perform optimally. Based on the current radio frequency allocation chart, it is not possible to obtain such a wide contiguous radio spectrum below 90 GHz (0.09 THz). Interestingly, these contiguous blocks of radio spectrum are readily available in the higher electromagnetic spectrum, specifically in the Terahertz (THz) frequency band. The major contribution of this study is discussing the substantial issues and key features of THz waves, which include (i) key features and significance of THz frequency; (ii) recent regulatory; (iii) the most promising applications; and (iv) possible open research issues. These research topics were deeply investigated with the aim of providing a specific, synopsis, and encompassing conclusion. Thus, this article will be as a catalyst towards exploring new frontiers for future networks of the 2030s.