THz-radiation Power Research Articles

Development of High Frequency Gyrotrons and Application to High Power Thz Technologies

Development of high frequency cw gyrotrons (Gyrotron FU CW Series) as high power THz radiation sources was advanced in FIR FU on the basis of the success in the development and application of the previous Gyrotron FU series [1]. In this paper, we would introduce the development of high frequency gyrotrons in FIR FU and applications to high power THz technologies

The Fabrication of THz Emitting Mesas by Reactive Ion-Beam Etching of Superconducting Bi2212 with Multilayer Masks

Generation of powerful THz radiation from intrinsic Josephson Junctions (IJJs) of Bi2Sr2CaCu2O8+δ (Bi2212) may require mesas with large lateral dimensions. However, there are difficulties in fabrication of perfect rectangular mesas. The lateral angles of mesas should be close to 90 degrees to obtain IJJs with same planar dimensions for synchronization of IJJs. We patterned Ta/photoresist and photoresist/Ta/photoresist masks on Bi2212 and used selective ion etching to overcome the thick photoresist layer shading on the lateral dimension of mesa during the ion-beam etching. The reactive ion-beam etchings have been done with ion beams of Ar, N2 and O2, and we have obtained mesas about 1 μm with lateral angle of approximately 50 to 75°, which is better than the mesas fabricated with photoresist mask.

Observation of coherent terahertz edge radiation from compressed electron beams

Coherent radiation emitted from a compressed electron bunch as it traverses the sharp edge regions of a magnetic chicane has been investigated at the Brookhaven National Laboratory Accelerator Test Facility. Electron beam measurements using coherent transition radiation interferometry indicate a 100 fs rms bunch accompanied by distinct distortions in energy spectrum due to strong self-fields. These self-fields are manifested in emitted high power THz radiation, which displays signatures of the phenomenon known as coherent edge radiation. Radiation characterization studies undertaken include spectral analysis, far-field intensity distribution, polarization, and dependence on the electron bunch length. The observed aspects of the beam and radiation allow detailed comparisons with start-to-end simulations.

Terahertz-Radiation Photoconductive Antenna in Sputtered Zinc Oxide Thin Film

We report on terahertz (THz)-radiation generation from a photoconductive antenna fabricated on a zinc oxide (ZnO) thin film. This ZnO thin film was epitaxially grown on a sapphire single crystal by rf-planar magnetron sputtering. The THz-radiation power from the ZnO thin film did not saturate below 14-kV/cm-biased electric field.

Long-pulse optical pumping THz laser based on optical amplification

To obtain narrower linewidth and more intense THz radiation, long-pulse and high-peak power optical pumping source is attempted by performing a light amplification experiment in which a 6 W tunable continuous-wave CO2 seeding laser is amplified by a double-pass amplifier with TEA CO2 laser. The result of an amplification factor of 30 at 14 μs pulse duration （FWHM） is obtained experimentally. During the amplification, the output power of the amplified light increases with that of seeding light increasing, especially for the CO2 10P （20）line. Based on the optical pumping source of long pulse laser, a dynamic model for generating pulsed THz radiation is established, and the expression of saturation pumping intensity which yields THz radiation is given, thus the range of pumping intensity can be determined. In addition, by solving time-dependent equation of dynamic model, the expression of pumping energy absorbed by THz medium is deduced. Theoretical analysis shows further that the heavy molecules will be more appropriate for THz gain medium for long pulse pumping source, and no THz radiation can be generated due to the absence of population reversion below pumping intensity.

High-power beam-based coherently enhanced THz radiation source

We propose a compact Smith-Purcell radiation device that can potentially generate high average power THz radiation with high conversion efficiency. The source is based on a train of short electron bunches from an rf photoemission gun at an energy of a few MeV. Particle tracking simulation and analysis show that, with a beam current of 1 mA, it is feasible to generate hundreds of watts of narrow-band THz radiation at a repetition rate of 1 MHz.

The calculation of terahertz wave power radiated from GaAs photoconductive antenna

Based on the Larmor formula, a mathematic model is established to calculate the power of THz wave radiated form the photoconductive antenna. The power of THz waves radiated by GaAs photoconductive antenna under different experimental conditions was calculated by Monte Carlo simulations. The results show that the power of THz radiation can be improved by increasing the bias electric field or the energy of triggering light pulse, and large-aperture photoconductive antenna can emit THz wave with higher power compared with photoconductive antennas with small apertures because the former can carry more photo-generated carriers.

Terahertz time-domain spectrometer with module heads coupled to photonic crystal fiber

We constructed optical-fiber-based THz time-domain spectrometers (THz TDSs) with standard single-mode fibers (SSMFs) and large-mode-area photonic crystal fibers (LMA PCFs) and compared those to THz waves and spectra. The optical fibers are used for guiding optical pulses from ultra-fast lasers to a THz emitter and detector. The LMA-PCF-based THz TDS exhibits increased bandwidth from 1 to 2 THz and increased field amplitude by a factor of four compared with the SSMF-based THz TDS under the relatively higher excitation power of ultra-fast lasers. This improvement results from LMA PCFs that are suitable for high-power transmission without introducing nonlinear effects. We also fabricated compact THz emitter and detector module heads, which are connected with LMA PCFs. The LMA-PCF-based THz TDS had THz radiation power and bandwidth comparable with those obtained by a conventional THz TDS with a free-space optical arrangement.

Generation of terahertz radiation using zinc oxide as photoconductive material excited by ultraviolet pulses

Terahertz (THz) radiation generated from photoconductive antenna fabricated on a single crystal zinc oxide (ZnO) is presented. The THz-radiation power is saturated at bias voltages above 800V∕cm and the obtained spectrum extends up to 1 THz. Moreover, ZnO is found to be highly transparent in the visible, near-infrared, mid-infrared and THz frequency regions. The results depicted here will categorically unravel the prospects of using ZnO as a material for integrated active optics.

Three-dimensional carrier-dynamics simulation of terahertz emission from photoconductive switches

A semi-classical Monte Carlo model for studying three-dimensional carrier dynamics in photoconductive switches is presented. The model was used to simulate the process of photoexcitation in GaAs-based photoconductive antennas illuminated with pulses typical of mode-locked Ti:Sapphire lasers. We analyzed the power and frequency bandwidth of THz radiation emitted from these devices as a function of bias voltage, pump pulse duration and pump pulse location. We show that the mechanisms limiting the THz power emitted from photoconductive switches fall into two regimes: when illuminated with short duration (<40 fs) laser pulses the energy distribution of the Gaussian pulses constrains the emitted power, while for long (>40 fs) pulses, screening is the primary power-limiting mechanism. A discussion of the dynamics of bias field screening in the gap region is presented. The emitted terahertz power was found to be enhanced when the exciting laser pulse was in close proximity to the anode of the photoconductive emitter, in agreement with experimental results. We show that this enhancement arises from the electric field distribution within the emitter combined with a difference in the mobilities of electrons and holes.

Terahertz radiation from InAs with various surface orientations under magnetic field irradiated with femtosecond optical pulses at different wavelengths

We present the magnetic-field dependence of terahertz (THz)-radiation power from femtosecond-laser-irradiated InAs with various surface orientations. Under 800 nm optical excitation, the magnetic field that provides the maximum THz-radiation power is found to be affected by the surface orientation, and InAs (111) exhibits it at lower magnetic fields than that of the other surfaces. In contrast, under 1560 nm excitation, the dependence on the surface orientation almost disappeared, and saturation is observed at a much smaller magnetic field than that in the 800 nm excitation case. Additionally, from the results of magnetic-field dependence up to 14 T, the shift of the peak in the THz-radiation spectrum toward lower frequency is confirmed, depending on the magnetic field applied, which is possibly induced by the emergence of a magnetoplasma effect.

Study of THz radiation from semiconductor surfaces excited by femtosecond laser pulses under laser illumination

The effect of continuous-wave (cw) laser illumination on THz radiation from semiconductor surfaces such as n- and p-type GaAs, semi-insulating (SI-) GaAs and SI-InP has been studied. The THz radiation power for SI-InP decreases under cw laser illumination, which is explained by the screening of the surface electric field by the photogenerated carriers. On the other hand, an increase of 40% of the THz radiation power is observed for SI-GaAs by cw laser illumination with 167 mW. The origin of this increase is discussed in relation to the metastable defect and temperature rise in SI-GaAs.

Excitation fluence dependence of terahertz radiation mechanism from femtosecond-laser-irradiated InAs under magnetic field

The excitation fluence and magnetic field dependence of terahertz (THz) radiation power from InAs is investigated. At low excitation fluence, an enhancement of the THz-radiation power is observed independent of the magnetic-field direction. As the excitation fluence is increased, a crossover of the terahertz radiation mechanism is observed. At excitation fluence above this crossover, the radiation power is either enhanced or reduced depending on the magnetic-field direction. These results are explained by considering the different THz-radiation mechanisms from the InAs surface with or without photoexcited carrier screening.

Significant enhancement of terahertz radiation from InSb by use of a compact fiber laser and an external magnetic field

We investigated the magnetic-field dependence of terahertz (THz) radiation power from InSb. Significant enhancement of THz-radiation power is observed by using a compact fiber laser that delivered 100 fs optical pulses at a center wavelength of 1560 nm. Additionally, applying external magnetic fields dramatically enhanced the THz-radiation power. THz-radiation power reaches a maximum value at around 1.2 T, and its enhancement factor exceeds 100. From an applications viewpoint, this is a significant finding for practical light source design, since it is easily achieved by using a compact fiber laser and a conventional magnet.

Very high power THz radiation at Jefferson Lab

We report the production of high power (20 W average, ∼1 MW peak) broadband THz light based on coherent emission from relativistic electrons. We describe the source, presenting theoretical calculations and their experimental verification. For clarity we compare this source with that based on ultrafast laser techniques, and in fact the radiation has qualities closely analogous to those produced by such sources, namely that it is spatially coherent, and comprises short duration pulses with transform-limited spectral content. In contrast to conventional THz radiation, however, the intensity is many orders of magnitude greater due to the relativistic enhancement.

Intense THz Radiation from Semiconductors in a Magnetic Field and Its Application.

We have generated enhanced THz-radiation from a femtosecond laser irradiating semiconductor in a magnetic field with a shallow incidence angle. The average power of THz-radiation has reached approximately 100 μW. Using intense THz radiation, a simple, noncontact, simultaneous method is applied to measure thickness and water content of black ink films independently from frequency-dependent and frequency-independent absorption characteristics of black ink films.

Development of Intense and Compact THz-Radiation Source Using Femtosecond-Laser Irradiated InAs Emitter in a High Magnetic Field

THz-radiation power from femto second pulse ir ra di ated InAs is found to be sat u rated at the mag netic field around 3T. We have found this sat u ra tion mag netic field dras ti cally de pends on geo met ri c al lay out and in ter est ing mag netic field de pend ence of the cen ter fre quency for the THz ra di a tion. Ad di tionally, a com pact THz-radiation source us ing an InAs semi con duc tor is dem on strated with a turn-key femto second fi ber la ser as an ex ci ta tion source and a newly de signed 2-T per ma nent mag net. By us ing a newly de signed mag netic cir cuit, a com pact per ma nent mag net which can ex ceed the remanence mag netic field was ob tained. This com pact source is suf fi ciently in tense for ab sorp tion spec tros copy.

Intense THz radiation from femtosecond laser pulses irradiated InAs in a strong magnetic field

THz-radiation power from femtosecond pulse irradiated InAs is found to be saturated at the high magnetic field around 3 T. We have also observed interesting dependence of the frequency and the spectral width of the radiation.

Saturation of THz-radiation power from femtosecond-laser-irradiated InAs in a high magnetic field

THz-radiation power from femtosecond-pulse-irradiated InAs is found to be saturated at the magnetic field around 3 T. Additionally, we find that this saturation magnetic field strongly depends on geometrical layout. Interesting magnetic-field dependence of the center frequency for THz radiation is also observed.

Intense THz-radiation from semiconductors under magnetic field irradiated with femtosecond laser pulses

The THz-radiation power from InAs reaches sub-mW level in a 1.7-T magnetic field irradiated with femtosecond laser pulses of 1.5-W average power. The THz-radiation power is related almost quadratically both to the magnetic field and to the excitation laser power. Furthermore, the THz-radiation spectrum is found to be controlled by the excitation pulsewidth, chirp direction of the excitation pulse, and the magnetic field.