Terahertz Wave Frequency Research Articles

Mapping of electromagnetic waves generated by free-running self-oscillating devices

Near-field mapping has proven to be a powerful technique for characterizing and diagnosing antennas in the microwave frequency range. However, conventional measurement methods based on a network analyzer cannot be applied to on-chip antenna devices extensively studied for future wireless communication in the millimeter wave (mm-wave) (30–300 GHz) and terahertz (THz) wave (0.1–10 THz) frequency regions. Here, we present a new asynchronous mapping technique to investigate the spatial distribution of not only the amplitude but also the phase of the electric field generated by free-running, self-oscillating generators including CMOS oscillators, Gunn oscillators, resonant tunneling diodes, and quantum cascaded lasers. Using a photonic-electronic hybrid measurement system, a wide frequency coverage, minimal invasiveness of the field to be measured, and phase distribution measurements with a theoretically-limited sensitivity are simultaneously achieved. As a proof-of-concept experiment, we demonstrate the mapping of a mm-wave (77 GHz) generated by a free-running Gunn oscillator and antenna characterization based on near-to-far field transformation.

Efficient generation and frequency modulation of quasi-monochromatic terahertz wave in Lithium Niobate subwavelength waveguide.

A kind of lateral excitation (LE) configuration is proposed for quasi-monochromatic terahertz generation via impulsive stimulated Raman scattering in a LiNbO3 (LN) slab waveguide by numerical simulation. In an individual waveguide, maximum efficiency frequency-selective excitation is achieved with linewidth narrower than 38 GHz when phase matching is fulfilled between the pump laser and the generated terahertz (THz) waves. As a result, the frequency and linewidth of narrowband THz waves can be tuned through changing the dispersion of THz waves, which is implemented by adjusting the thickness of host LN slab. Furthermore, Au-Air-LN-Air-Au multilayer LE structure is developed to realize a dramatic change of the dispersion to obtain quasi-monochromatic THz waves, of which the linewidth is achieved as narrow as 10 GHz. In addition, the frequency and linewidth of quasi-monochromatic THz waves are modulated dynamically by varying the distance between LN slab and Au mirrors flexibly. Consequently, the optimized LE structure is expected to boost the development of high-precision and real-time inspection and sensing.

Active modulation of the terahertz spectra radiated from two air plasmas.

A simple and energy-saving method has been proposed to actively modulate the spectra of terahertz (THz) waves radiated from two serial plasmas, which uses the background light to generate one plasma to make full use of the energy of the femtosecond laser. With this method, the modulation of the central frequency, spectral bandwidth, and spectral profile of the output THz waves have been observed. The shifting of the amplitude dip has been manipulated by changing the distance of the two serial plasmas. The manipulation results agree with the ones simulated by the transition-Cherenkov model. This proposed method provides a useful tool for getting the modulated THz spectra that can be used in the THz remote sensing.

Terahertz pulses in a periodic plasma channel via modulated electron beam

The proposed scheme offers a promising way to study the effect of modulated relativistic electron beam in a periodic plasma channel (wiggler). The fluid equations model is used to find out the numerical behaviour of the radiation wave. In this scheme, a modulated relativistic electron beam makes possible to generate and amplify the high power terahertz (THz) radiation pulses. Using this model, we have analyzed and calculated the nonlinear dispersion relation of the radiation wave. The radiation frequency of THz wave can be tuned by varying the wiggler wavelength and beam energy. The excitation of the radiation wave is being developed with the resonance between the modulated frequency of electron beam and radiation wave in the periodic plasma channel (wiggler field) via Cerenkov interaction along the axial direction of channel, which provides the maximum enhancement in the growth rate of the radiation wave. Additionally, the growth rate of the unstable mode of the radiation wave depends on beam energy and modulation index, and becomes larger as modulation index approaches unity.

Investigation of Terahertz Peak Frequencies From GaAs Photoconductive Antennas

To theoretically predict the peak frequencies of terahertz (THz) electromagnetic radiation generated by photoconductive antennas, we have calculated the peak frequency of THz waves from laser-induced plasma. The peak frequency is the frequency with the greatest generation probability from different volume elements in a laser pulse period. Using the methods of probability and statistics, the plasma density distribution function $F_{n}$ ( n ) is solved. Then, the probability density function ϕ $_{N}$ ( n ) is calculated. Finally, the maximum of probability density is calculated, which is the factor that is vital to calculating the peak frequency. The peak frequencies calculated theoretically are verified by experiment. The theoretical model serves as the basis for the design of a GaAs photoconductive antenna with a specific peak frequency.

Thin-wall tubes for coupling terahertz waves to metal wires.

Bare metal wires are among the most promising waveguides for guiding terahertz (THz) surface plasmon polaritons. In this study, a thin-wall tube is proposed for coupling THz waves to a metal wire with ultrahigh efficiency, which results from three high mode matchings for the two waveguides: field distributions, polarization directions, and wave vectors. According to the mode-overlap calculation, the coupling efficiency can be always between 84% and 94% when the frequency of THz waves is in the range of 0.2-3 THz and the metal wire radius is 0.5 mm. The maximum efficiency is as high as 94% at 0.5 THz, which is much higher than that obtained by the previous methods. We further conclude that the optimal coupling efficiency can be obtained when the outer tube radius is equal to the wire radius and simultaneously the real propagation constants of modes in the two waveguides are the same.

Nonlinear radiation response of n-doped indium antimonide and indium arsenide in intense terahertz field**Project supported by the National Natural Science Foundation of China (Grant Nos. 11574105 and 61177095), the Natural Science Foundation of Hubei Province, China (Grant Nos. 2012FFA074 and 2013BAA002), the Wuhan Municipal Applied Basic Research Project, China (Grant No. 20140101010009), and the Fundamental Research Funds for the Central

The nonlinear radiation responses of two different n-doped bulk semiconductors: indium antimonide (InSb) and indium arsenide (InAs) in an intense terahertz (THz) field are studied by using the method of ensemble Monte Carlo (EMC) at room temperature. The results show that the radiations of two materials generate about 2-THz periodic regular spectrum distributions under a high field of 100 kV/cm at 1-THz center frequency. The center frequencies are enhanced to about 7 THz in InSb, and only 5 THz in InAs, respectively. The electron valley occupancy and the percentage of new electrons excited by impact ionization are also calculated. We find that the band nonparabolicity and impact ionization promote the generation of nonlinear high frequency radiation, while intervalley scattering has the opposite effect. Moreover, the impact ionization dominates in InSb, while impact ionization and intervalley scattering work together in InAs. These characteristics have potential applications in up-convension of THz wave and THz nonlinear frequency multiplication field.

Stereolithographic Additive Manufacturing of Diamond Photonic Crystal Composed of Titania and Alumina Micro Lattices

Three dimensional micro photonic crystals with a diamond structure made of a dense titania and alumina were fabricated successfully by using stereolithographic additive manufacturing. Photonic band gap properties were investigated in gigahertz and terahertz wave frequency ranges. Acrylic diamond lattice structures with nanometer sized particles of titania and alumina dispersion at 40 vol. % were fabricated by the stereolithography. The forming accuracy was 10 μm. After dewaxing and sintering process, the titania and alumina diamond lattice structures were obtained. The relative density reached above 98 %. Electromagnetic wave transmittances were measured by using a W-band millimeter waveguide connected with a network analyzer and a terahertz wave time domain spectroscopy. In the transmission spectra for the Γ-X <100> direction, a forbidden band was observed at 90 – 110 GHz and 0.4 – 0.6 THz. The band gap frequencies well agreed with calculated results by plane wave expansion (PWE) method. Additionally, simulated results by transmission line modeling (TLM) method indicated that a localized mode can be obtained by introducing a plane defect between twinned diamond lattice structures.

Compact and Tunable-Multichannel Terahertz Wave Filter

A compact terahertz tunable multichannel filter based on two cascaded photonic crystal waveguide directional couplers, formed by two closely spaced line defect waveguides in a two-dimensional photonic crystal of silicon rods in the air, is numerically studied using plane wave expansion and finite difference time domain methods. The filtering properties of the present terahertz wave filter, including channel number and frequency, can be tuned by adjusting excitation of external applied magnetic field. The dynamical shift of channels with external applied magnetic field intensity is studied in detail. The transmittances of the present filter are higher than 95% for each terahertz wave frequency with a compact size of 3.8 mm $\times$ 10 mm. The designed device can possess both multichannel and tuning features, which could be of technical use in the terahertz wave communications system applications.

THz band-stop filter using metamaterials surfaced on LiNbO(3) sub-wavelength slab waveguide.

We designed and implemented periodic bar arrays metamaterials to select appropriate frequencies of terahertz (THz) waves propagating in a LiNbO(3) sub-wavelength waveguide. The spatial and temporal electric field profiles of the THz waves were recorded using a time-resolved phase-contrast imaging system. The metamaterials can operate as a band-stop filter to realize blocking back THz waves in a band range of 0.6-1.0 THz, while transparent transmission for the fundamental mode of the slab over a range of 0.3-0.6 THz.

Slowing and trapping THz waves system based on plasmonic graded period grating

Graded and partial graded period grating structures are proposed to trap and slow terahertz (THz) waves. Different frequencies of THz waves can be trapped at different positions along the graded grating. The exact trapping positions of every different frequency can be estimated from a fitting curve. The separation distance between trapping point of every 0.1 THz is about 1 ~ 2 mm, which can be applied in a THz waves spectrometer. An ultraslow THz waveguide can be realized by using a partial graded grating. The propagated speed of THz waves can be designed and tuned by adjusting the period of the grating. This structure offers the advantage of reducing the speed of the THz waves and propagation them over an ultra-wide spectral band. The physical trapping mechanism of the THz waves is attributed to the transformation from surface modes to cavity modes. The localized cavity modes at the trapping position have a saturated stable state. The extra energy will be reflected back along the graded grating.

Terahertz polarization beam splitter based on two photonic crystal cavities

A compact terahertz polarization beam splitter by combining line defects and two photonic crystal cavities is designed and numerically demonstrated. Both plane wave expansion method and finite-difference time-domain method are used to calculate and analyze the characteristics of the proposed device. The simulation results show that the designed polarization beam splitter can split TE and TM polarized terahertz wave into different propagation directions with high efficiency over the terahertz wave frequency range from 1.16THz to 1.19THz. The extinction ratios are found to be larger than 20dB for both TE and TM polarized terahertz wave. By using such a polarization beam splitter, the device size could be reduced significantly. This enables the polarization beam splitter to be used in terahertz wave system and terahertz wave integrated circuit fields.

Dual-frequency terahertz emission from splitting filaments induced by lens tilting in air

Dual-frequency terahertz radiation from air-plasma filaments produced with two-color lasers in air has been demonstrated experimentally. When a focusing lens is tilted for a few degrees, it is shown that the laser filament evolves from a single one to two sub-filaments. Two independent terahertz sources emitted from the sub-filaments with different frequencies and polarizations are identified, where the frequency of terahertz waves from the trailing sub-filament is higher than that from the leading sub-filament.

Theoretical and Experimental Studies of Terahertz Wave Propagation in Unmagnetized Plasma

The spacecraft will experience the well-known “blackout” problem when it re-entry into the Earth’s atmosphere, which results in communication failures between the spacecraft and the ground control center. It is important to study the properties of terahertz (THz) wave propagation in plasma for communications using THz wave is an alternative method for solving the blackout problem. The properties of THz wave propagation in plasma have been studied with the ZT-FDTD (Z-Transform Finite Difference Time Domain) method and experimental method in this paper. The dependence of plasma density, plasma collision frequency, thickness of plasma and THz wave frequency on the THz wave propagation are presented. The properties of 0.22THz electromagnetic wave propagation in plasma have been studied experimentally with shock tube, and the experimental results are in good agreement with the FDTD ones. Both the FDTD and experimental results indicate that communications using THz wave is an alternative and effective way to solve the blackout problem.

Terahertz electro-optical detection: optical phase or energy measurements

Models of different available single-photodetector electro-optic sampling schemes are considered from the unified theoretical position. It is taken into account that under electro-optic terahertz wave detection, not only do the polarization states of the femtosecond optical pulse components change but the modules of their amplitudes are also varied. As a result, information concerning the terahertz wave can be obtained not only using the conventional ellipsometry readout scheme (probe-phase sampling) but also by simple measuring of the induced power of the optical pulses (probe-energy sampling). Spectral sensitivities of both of these electro-optic sampling methods are calculated for the cases of nonlinear-optical crystals with zinc-blende symmetry (like ZnTe) and the crystals with one active component of the second-order optical susceptibility tensor (like PPLN). It is found that the ratio between spectral sensitivities of the pure probe-phase and pure probe-energy schemes is proportional to the ratio between the optical and terahertz wave frequencies in all types of the crystals. It is shown that the signal in the near-zero optical transmission point scheme, which is the best for terahertz imaging, has a mixed character. While the contribution of the probe-phase sensitivity appears to be due to spatially nonuniform residual birefringence of the nonlinear zinc-blende crystal, the probe-energy part of the sensitivity has a uniform distribution and can be increased by the angle misalignment of the optical polarization element.

Terahertz wave generation from hyper-Raman lines in two-level quantum systems driven by two-color lasers

Based on spatial-temporal symmetry breaking mechanism, we propose a novel scheme for terahertz (THz) wave generation from hyper-Raman lines associated with the 0th harmonic (a particular even harmonic) in a two-level quantum system driven by two-color laser fields. With the help of analysis of quasi-energy, the frequency of THz wave can be tuned by changing the field amplitude of the driving laser. By optimizing the parameters of the laser fields, we are able to obtain arbitrary frequency radiation in the THz regime with appreciable strength (as strong as the typical harmonics). Our proposal can be realized in experiment in view of the recent experimental progress of even-harmonics generation by two-color laser fields.

Real propagation speed of the ultraslow plasmonic THz waveguide

A graded metallic grating structure can not only work as a wave trapping system, but also work as an ultraslow terahertz (THz) waveguide. The depth of the grating waveguide is partial graded and partial fixed. The real propagation speed of such a system is calculated. Different frequencies of THz waves can be propagated at a designed propagation speed, even close to zero.

Terahertz waveguide emitters in photonic crystal fiber form

The phase-matching condition for broadband terahertz (THz) wave generation based on optical rectification requires that the group velocity of the optical pump beam be equal to the phase velocity of the THz wave. The design of GaP THz waveguide emitters in the form of photonic crystal fibers (PCFs) for a pump source of wavelength 1040 nm is reported. By analogy with a circular waveguide emitter, we show how the phase-matched THz wave frequency can be tuned widely by the air hole pitch and finely tuned by the air hole size. In addition, a single THz wave mode can be guided in the endlessly single-mode regime of the fiber waveguide. The layers of air holes in the PCF design not only allow tunability of the generated THz radiation but also make small emitters easy to handle.

Studies of terahertz wave propagation in non-magnetized plasma

The space crafts, such as spaceship, space shuttle and so on, will face a famous blackout problem when they re-enter the earth's atmosphere at high velocity. The plasma sheath severely affects the propagation of the incident electromagnetic wave and causes severe electromagnetic wave attenuation, which results in the communication failures between the space crafts and the ground control center. It is important to study the properties of terahertz wave propagation in non-magnetized plasma, for using the terahertz wave communication is an alternative method to solve the reentry blackout problem. The terahertz wave propagation characteristics in non-magnetized plasma are studied in this paper. The variations of the terahertz wave propagation properties with the terahertz wave frequency, plasma density, plasma collision frequency and the thickness of the plasma are acquired. Some new phenomena are found i.e., there appears periodic oscillation on the reflectivity curve with the increase of terahertz wave frequency, and the period of oscillation is 0.03THz. The oscillation amplitude increases with the increase of terahertz frequency, decreases with the increase of plasma density, and increases with the increase of plasma collision frequency. The oscillation is attributed to the multiple reflections of the electromagnetic wave at the interfaces at z=0 and z=-d. The terahertz wave propagation properties in plasma are studied experimentally with the shock tube, and the experimental results match well with the theoretical ones. Both the theoretical and experimental results indicate that using terahertz wave communication is an effective way to solve the reentry blackout problem.

Harmonic Generation of Externally Applied Signal in a Biased Photoconductive Terahertz-Wave Emitter

A generation effect of higher harmonics for an externally applied signal in a photoconductive (PC) terahertz (THz)-wave emitter has been found. This effect is applicable to accurately measuring for frequencies of THz waves. This paper describes reasons why higher harmonics are generated in a PC device. The dependence of the photoconductance on the applied voltage in the PC device consists of a flat range and a negative slant range, and one sharply bending point is then formed at the boundary between the flat and slant ranges. When the PC device is irradiated by two laser beams with slightly different optical frequencies, the photoconductance is strongly modulated at the optical beat frequency in the THz region by photomixing the two laser beams. As a result, three bending points are formed in the average photoconductance (introduced as the average of the temporal photoconductance varying at the THz frequency). The slants comprised of the three bending points are different from each other. When the variation range of the applied voltage driven by the signal input on the biased voltage covers the voltage of one of the bending points, the photoconductance (or the average photoconductance in optical beating) varies along the different two slopes, the resultant temporal photocurrent is largely distorted, and then the harmonics of the signal input are generated in the photocurrent. The following features are clarified: (1) the harmonics of the signal input are generated by appropriately adjusting the bias voltage and the amplitude of the signal input, regardless of the presence/absence of optical beating; (2) the efficiency of the harmonic generation is about 10-4-10-5; and (3) the harmonics over 35th order with almost flat amplitudes (-3.8dB/octave) are generated.