Characteristics Of Terahertz Radiation Research Articles

Terahertz Photoconductive Antenna Based on a Topological Insulator Nanofilm

In this study, the efficient generation of terahertz radiation by a dipole photoconductive antenna, based on a thin island film of a topological insulator, was experimentally demonstrated. The performance of the Bi1.9Sb0.1Te2Se antenna was shown to be no worse than those of a semiconductor photoconductive antenna, which is an order of magnitude thicker. The current–voltage characteristics were studied for the photo and dark currents in Bi1.9Sb0.1Te2Se. The possible mechanisms for generating terahertz waves were analyzed by comparing the characteristics of terahertz radiation of an electrically biased and unbiased topological insulator.

Generation of Terahertz Radiation under Interaction of Counterpropagating Laser Pulses in Underdense Plasma

Generation of terahertz waves under interaction of two counterpropagating laser pulses with different frequencies in underdense plasma is analyzed. Spectral, angular, and energy characteristics of terahertz radiation are investigated as functions of the frequency difference of the laser pulses. It is shown that, due to the frequency difference, in addition to the line at the doubled plasma frequency [L.M. Gorbunov, A.A. Frolov, JETP 98, 527 (2004)], a peak near the plasma frequency appears in the emission spectrum. The total energy of the terahertz signal is calculated, and the condition under which emission at the plasma frequency dominates is found. It is shown that the energy of radiation at the plasma frequency reaches its maximum value when the frequency difference of the laser pulses is close to the plasma frequency.

Spectral characteristics of terahertz radiation from plasmonic photomixers.

We present a comprehensive analysis of spectral characteristics of terahertz radiation from plasmonic photomixers. We fabricate plasmonic photomixer prototypes with plasmonic contact electrode gratings on a low temperature grown GaAs substrate and characterize the spectral properties of the generated terahertz radiation by use of a heterodyne detection scheme. Our analysis shows that linewidth, stability, and frequency tuning range of the generated terahertz radiation are directly determined by linewidth, stability, and wavelength tuning range of optical pump beam and not affected by device geometry, substrate properties, optical pump power level and other operational settings. Our study indicates the crucial role of optical sources in realizing high performance terahertz spectroscopy and wireless communication systems based on plasmonic photomixers.

Study on characteristics of terahertz radiation in the surface and far field generated by large-aperture photoconductive antennas

The time-domain characteristics of terahertz radiations emitted from a biased large-aperture photoconductive antenna trigged by an ultra-short optical pulse are studied. Succinct explicit expressions in the surface field and in the far field for the emitted THz radiation fields are deduced based on the well-known current-surge model. The expressions indicate that the THz radiations in the surface and far fields are mainly affected by the incident light intensity, the incident light wavelength, the carrier relaxation time, the reflectivity illuminated at the photoconductor, the carrier lifetime, and the incident light pulse width. The characteristics of THz radiation in the surface and far fields are discussed, and the phenomena of changes in the radiation peak values, the radiation pulse width, and the rising edges are explained in details. Especially the changes in surface fields are explained with the increasing of the ratio of luminous flux. The numerical results indicate that the emitted THz radiation intensity can be increased by enhancing the ratio of luminous flux, prolonging the carrier lifetime, or reducing the incident light pulse width. To avoid the shielding effect of the radiation field, an effectively method is presented that the intense of the incident laser pulse could be divided into multiple beams with low energy to trigger the photoconductive antenna, thus the THz radiation power can be further increased significantly.

Terahertz radiation from InN by femtosecond optical pulses of different wavelengths

The characteristics of terahertz radiation from an n-type InN excited by femtosecond laser pulses tunable from 750 to 840 nm are experimentally studied. Terahertz emission from InN is closely bound up with the Dember effect. Terahertz emission can be interpreted as being emitted from accelerated photo-carriers excited by a femtosecond pulse in Dember field. Terahertz radiation from InN shows a strong dependence on excitation wavelength. Results show that under laser pulse excitation with a different center wavelength, the terahertz radiation shows different characteristics, such as radiation intensity, radiation efficiency, and spectrum width. This work will be propitious to the development of terahertz time-domain spectrum technology and the optimization of experiment system, as well as being a reference for conducting research on terahertz emission with higher radiation intensity and efficiency.

Characteristics of terahertz radiation emitted from the intrinsic Josephson junctions in high-Tc superconductor Bi2Sr2CaCu2O8+δ

Characteristic features of terahertz radiation emitted from a rectangular mesa comprised stacked intrinsic Josephson junctions in the layered high-Tc superconductor Bi2Sr2CaCu2O8+δ are reported. The emission with the total power of ∼5 μW at 0.63 THz shows a characteristic spatial distribution reflecting the rectangular shape. The stability of the emission was measured for at least 20 min. and the power fluctuation was found to be less than 4%. Furthermore, the radiation is highly polarized linearly with a ratio of 50:1. All features, including the spectral purity narrower than 4 GHz at full width at half maximum, promise a stable, coherent, and continuous terahertz source for various applications.

Frequency-dependent characteristics of terahertz radiation on the enamel and dentin of human tooth

The accurate terahertz (THz) electrical and optical properties of biomaterials are required as the biological applications of THz electromagnetic waves were recently increased. The frequency-dependent characteristics of enamel and dentin of human tooth were measured in this study through THz time-domain spectroscopy. The power absorptions of enamel and dentin were found to be similar while the index of refraction of enamel was found to be bigger than that of dentin. The power absorption results of enamel and dentin were also compared with those of the wet samples.

The characteristics of terahertz radiation from InAs irradiated with femtosecond optical pulses of different wavelengths

The characteristics of terahertz (THz) radiation from the surface of an (100)p-InAs crystal excited by fs laser pulses of different wavelengths (from 750 to 850nm) are investigated experimentally. The terahertz radiation can be interpreted as being emited from accelerated photo-carriers in the Dember field and be detected by using the free-space electro-optic sampling method. A (110) ZnTe crystal was used as detector. The results of experiment show that the Dember field in the surface of InAs, concentration of excited carriers, intervalley scattering and the concentration of excited carriers of different states, all changed when the wavelength of excited pulse changed, so the radiant efficiency and the effective spectral width of terahertz wave were different. This investigation will be useful to the measure of time domain spectrum of samples and the optimization of experimental system. It also gives better physieal insight into the ultra-fast process of terahertz wave radiation from InAs excited by femtosecond pulses.

Photoinduced phase transition inPr0.7Ca0.3MnO3thin films probed via ultrafast switching characteristics of terahertz radiation

Picosecond electrical transients have been identified as switching characteristics of terahertz radiation from voltage-biased ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ thin films excited by femtosecond laser pulses. Despite the fact that the total time-averaged photocurrent exhibits a hysteresis in response to the application of the bias electric field ${E}_{\text{bias}}$, we find the considerable attenuation of terahertz radiation. Not the removal of ${E}_{\text{bias}}$ but rather the sweep of temperature is confirmed to be indispensable for the recovery of terahertz radiation. Our findings can be attributed to the clear manifestation of the intrinsic and persistent photoinduced phase transition in ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$, the instability of which by a removal of ${E}_{\text{bias}}$ can be explained by the reconstruction of metallic patches within the charge-ordered insulating phase.

Study of the surface and far fields of terahertz radiation generated by large-aperture photoconductive antennas

We have studied analytically the temporal characteristics of terahertz radiation emitted from a biased large-aperture photoconductive antenna triggered by an ultrashort optical pulse. We have included the effects of the finite lifetime and transient mobility dynamics of photogenerated carriers in the analysis. Succinct explicit expressions are obtained for the emitted radiation in the surface field and in the far field. The dependence of the waveforms of the radiated field on the fluence and duration of triggering optical pulse, carrier relaxation time and carrier lifetime are discussed in detail using the obtained expressions.