Terahertz Wave Radiation Research Articles

Semiconductor Diode Lasers for Terahertz Technology

We discuss different concepts for generating terahertz (THz) radiation with semiconductor diode lasers. Photomixing enables the generation of continuous wave THz radiation by difference frequency generation of two lasers or of two-colour lasers. Pulsed THz radiation for time domain THz spectroscopy is generated with modelocked diode laser systems including amplification and chirp compression. Finally, we analyse the concept of quasi time domain spectroscopy based on broadband diode laser systems.

Principles and promise of Fabry–Perot resonators at terahertz frequencies

Fabry–Perot resonators have tremendous potential to enhance the sensitivity of spectroscopic systems at terahertz (THz) frequencies. Increasing sensitivity will be of benefit in compensating for the relatively low power of current high resolution continuous wave THz radiation techniques, and to fully express the potential of THz spectroscopy as source power increases. Improved sensitivities, and thus scanning speeds, will allow detailed studies of the complex vibration-rotation-tunneling dynamics that large molecules show at THz wavelengths, and will be especially important in studying more elusive, transient species such as those present in planetary atmospheres and the interstellar medium. Coupling radiation into the cavity presents unique challenges at THz frequencies, however, meaning that the cavity configurations common in neighboring frequency domains cannot simply be translated. Instead, novel constructions are needed. Here we present a resonator design in which wire-grid polarizers serve as the input and output coupling mirrors. Using this configuration, Q-factors of a few times 105 are achieved near 0.3 THz. To aid future investigations, the parameter space that limits the quality of the cavity is explored and paths to improved performance highlighted. Lastly, the performance of polarizer cavity-based Fourier transform (FT) THz spectrometers is discussed, in particular those design optimizations that should allow for the construction of THz instrumentation that rivals and eventually surpasses the sensitivities achieved with modern FT-microwave cavity spectrometers.

Detection of terahertz and sub-terahertz wave radiation based on hot-carrier effect in narrow-gap Hg1−xCdxTe

We have proposed and developed terahertz and subterahertz wave detector using hot-carrier effect in narrow-gap Hg1−xCdxTe. Epitaxial Hg1−xCdxTe-layers were integrated into dipole antennas. The response of Hg1−xCdxTe hot-electron bolometer was measured in the range of frequencies 0.037–1.58 THz, in the temperature region T = 68–300 K at various bias currents. A bolometer theoretical model was developed and the experimental results confirm the model main conclusions.

Terahertz-wave radiation emitted by intrinsic Josephson junctions

Radiation properties of terahertz (THz) electromagnetic waves emitted from stacks of 3 and 50 intrinsic Josephson junctions by flowing bias currents have been numerically investigated for high-frequency device applications. A novel multibranch structure in the resistive state was obtained for the current-voltage characteristics of the stacks. Each branch corresponded to an individual mode of a two-dimensional cavity resonance for Josephson plasma waves in the directions parallel and perpendicular to the c-axis of the stack. The electromagnetic field distribution in the stack exhibited standing-wave patterns not only for the in-phase mode between junctions but also for the other modes in different branches. Maximum radiation power was attained for the in-phase mode, and the corresponding bias point was measured for the 3- and 50-junction stacks. The peak power in the 50-junction stack was approximately 50 times larger than that in the 3-junction stack. The frequency spectra at maximum power exhibited a sharp main peak and a few harmonics. These properties are useful in designing a THz oscillator using intrinsic Josephson junctions.

Detection of terahertz and sub-terahertz wave radiation based on hot-carrier effect in narrow-gap Hg1−xCdxTe

AbstractWe have proposed and developed terahertz and subterahertz wave detector using hot-carrier effect in narrow-gap Hg1−xCdxTe. Epitaxial Hg1−xCdxTe-layers were integrated into dipole antennas. The response of Hg1−xCdxTe hot-electron bolometer was measured in the range of frequencies 0.037–1.58 THz, in the temperature region T = 68–300 K at various bias currents. A bolometer theoretical model was developed and the experimental results confirm the model main conclusions.

Generation of continuously tunable terahertz-wave radiation exploited by stimulated polariton scattering

The characteristics of a coherent tunable terahertz-wave source have been experimentally investigated in detail. By using a difference frequency generation as a result of stimulated Raman scattering by polaritons in MgO:LiNbO3 crystals, the signal wave was continuously tuned in the range of 1.069–1.075 μm, which corresponded to a terahertz-wave in the wide range from 227 to 104 μm (1.3 to 2.9 THz). The highest output signal pulse energy at the pumping level of 44 mJ/pulse was 1.8 mJ/pulse with 20 mW seeder injection, and the terahertz peak power was 139 mW at the wavelength of 143 μm. This source has the advantages of simple alignment, simplicity of tuning, and compactness that makes the device more attractive.

Technology Trends in Light Application and Visual Science

Three topics in light application and visual science are introduced in this report: Terahertz wave radiation from intrinsic Josephson junctions, recent trends of lithography for semiconductor device fabrication, and compact microwave discharge lamp.

Terahertz/Millimeter Wave Characterizations of Soils for Mine Detection: Transmission and Scattering

The detection of anti-personnel mines depends on the soil transmission and scattering at a given wavelength. Transmission spectra were measured over the range 90–4200 GHz for 19 soil samples that span a number of soil orders that have extensive worldwide distribution using a vector network analyzer (90–140 GHz) and a Fourier spectrometer (120–4200 GHz). Transmission drops to nearly zero for wavelengths shorter than the characteristic particle size of the sample as a consequence of scattering. This interpretation is supported by a fit to a standard scattering model with physically reasonable fitting parameters. Transmission spectra were also measured for various liquids (90–600 GHz) for possible index matching. These liquids were mixed with the soil sample and were found to reduce scattering and increase transmission through the soil at higher frequencies. This work is relevant to mine detection using terahertz and millimeter wave radiation for high resolution images through the soil.

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.

A dual-wavelength diode-end-pumped Nd:YAG laser operating simultaneously at 1319 and 1338nm

Based on the lasing threshold equation of a diode-end-pumped Nd:YAG laser, the threshold condition of simultaneous dual-wavelength lasing is determined and the relationship between the transmission ratios of the output mirror corresponding to the two wavelengths is calculated. A simultaneous dual-wavelength continuous wave (cw) or quasi-cw diode-end-pumped Nd:YAG laser operating at 1319nm and 1338nm has been demonstrated. A total cw output power of 6W was achieved at a pump power of 21.6W with a slope efficiency of 30%. And a total quasi-cw output power of 4.75W was obtained at a pump power of 21.6W and a pulse repetition rate of 50kHz, with a slope efficiency of 24.73%, and the corresponding pulse width was 55.05ns. Inserting a Brewster plate in the cavity and the highest linearly polarized wave output power was boosted up to 2.22W at a pulse repetition rate of 50kHz. The instability of the output power was less than 0.52% and the beam quality M2 factor was as low as 1.16 at the linearly polarized wave output power of 2.17W. The simultaneous dual-wavelength laser provides an experimental basis for generating highly coherent terahertz wave radiation of 3.23THz by nonlinear optical difference frequency method.

Generation of Terahertz radiation with two color semiconductor lasers

AbstractWe discuss and analyze concepts for the generation of tuneable continuous wave terahertz (THz) radiation with two color diode lasers. First, different geometries of two color lasers are reviewed. We show that the THz power of two color lasers in combination with external photomixers becomes sufficient for scanning THz imaging applications when optical amplification with a tapered amplifier is implemented. Then, the concept of direct emission of THz radiation out of a two‐color semiconductor laser is reviewed and the potential of this concept with respect to THz bandwidth and achievable THz power is critically analyzed.

Terahertz spectroscopy applied to the measurement of strengths and self-broadening coefficients for high- J lines of OCS

A spectrometer operating in the 100–2000 GHz range and allowing for absolute line strength measurements has been developed. The continuous wave terahertz radiation is generated by mixing two Ti:Sapphire laser beams in a vertically integrated low temperature grown GaAs (LTG-GaAs) photomixer. Pure rotational lines of 16O 12C 32S in the ground vibrational state have been considered for J values up to 90. Observed self-broadening parameters are in agreement with those deduced from infrared experiments. For the first time in the submillimeter range, absolute line strengths have been determined, allowing for a determination of the electric dipole moment in good agreement with the value previously obtained from Stark effect measurements.

Terahertz radiation from laser accelerated electron bunches

Coherent terahertz and millimeter wave radiation from laser accelerated electron bunches has been measured. The bunches were produced by tightly focusing (spot diameter ≈6 μm) a high peak power (up to 10 TW), ultra-short (⩾50 fs) laser pulse from a high repetition rate (10 Hz) laser system (0.8 μm), onto a high density (>1019 cm−3) pulsed gas jet of length ≈1.5 mm. As the electrons exit the plasma, coherent transition radiation is generated at the plasma-vacuum boundary for wavelengths long compared to the bunch length. Radiation in the 0.3–19 THz range and at 94 GHz has been measured and found to depend quadratically on the bunch charge. The measured radiated energy for two different collection angles is in good agreement with theory. Modeling indicates that optimization of this table-top source could provide more than 100 μJ/pulse. Together with intrinsic synchronization to the laser pulse, this will enable numerous applications requiring intense terahertz radiation. This radiation can also be used as a powerful tool for measuring the properties of laser accelerated bunches at the exit of the plasma accelerator. Preliminary spectral measurements indicates that bunches as short as 30–50 fs have been produced in these laser driven accelerators.

Intracavity second-harmonic and sum-frequency generation with a laser-diode-pumped multitransition-oscillation LiNdP_4O_12 laser

Efficient simultaneous intracavity second-harmonic and sum-frequency generation that is free from irregular large amplitude fluctuations (the so-called green problem) is achieved with a laser-diode-pumped LiNdP(4)O(12) (LNP) laser with a type II phase-matching KTiOPO(4) crystal. Observed frequency-mixing operations result from the simultaneous single-frequency oscillations on different transitions that arise from the wide fluorescence linewidth in a stoichiometric LNP laser that possesses inherently high Nd concentrations. Sum-frequency generation has been shown to arise from the interaction of oscillating longitudinal modes that belong to different transitions. A promising application to coherent terahertz-wave radiation by use of photomixers is proposed.

Terahertz Parametric Oscillator Using a Trapezoidal Nonlinear Optical Crystal.

A tunable terahertz-wave source has been realized by Optical Parametric Oscillator (OPO) utilizing the polaritonmode scattering of LiNbO3. This technique has several advantages such as wide tunability and compactsystem. In this paper, we demonstrate an efficient terahertz-wave radiation by using a new OPOconfigurationwith a trapezoidal LiNbO3 crystal. A tunable range of 130-310 μm has been achieved without a couplingdevice. This method reduces an influence of the propagation loss in the crystal, so that the shorter wavelengthoscillation is enhanced.