This study presents advancements in the synthesis and application of Bi2Se3, a topological van der Waals crystal, for terahertz (THz) detection. High-quality β-Bi2Se3 crystals were synthesized using the Selenium Vapor-Induced Supersaturated Solution Method (SVI-SSM), ensuring stoichiometric integrity. Heterostructures of Bi2Se3 were constructed using hot transfer methods, leading to the fabrication of a rectenna THz detector. Utilizing a 0.95-THz injection-seeded THz parametric generator (is-TPG) as the light source for pulsed-continuous wave (CW) THz waves, THz detection experiments were conducted. The results demonstrated a fast response time of 200 ps and a sensitivity of 40 mV/W with the THz detector, which was maintained even under zero bias conditions. These findings lay the foundation for developing passive THz detection systems with minimal energy consufmption, holding significant promise for advancing THz communication technology.
High-brightness sub-terahertz (THz)-wave source has been required for potential THz-wave nondestructive testing. We have succeeded in developing a ubiquitous THz-wave source with a peak intensity of about 200W at 0.3 THz which is on the level of a gyrotron, aimed to robot installation. The high-brightness THz-wave emission is based on a cascaded backward optical parametric down-conversion up to third-order Stokes radiation and a threshold reduction of 63% by an injection-seeding to the idler beam. This remarkable THz-wave source expands the horizon of THz-wave research and applications.
This paper reviews recent advancements in the research of THz detection by an asymmetric dual-grating gate structure epitaxial-graphene-channel field effect transistor (ADGG-EG-FETs). We designed and fabricated ADGG-EGFET for plasmonic (PL) detection, and it performed a high sensitivity and fast response to irradiated THz with 0.95 THz. The behavior of measured dependence on gate bias voltage cannot be explained only by the PL effect. We found such a phenomenon as a new current-driven phototermoelectric (PTE) detection assisted by electrostatic carrier drift/diffusion under the application of DC drain biases. Furthermore, we analyze the response speed of our fabricated detector to reveal the transition point between PL and PTE detection mechanisms. The minimum output pulse width was ~190 ps when one ADGG bias was at the Dirac voltage (i.e., charge neutrality point) to promote the PL detection, whereas the pulse width was ~200 ps when both ADGG biases were at well-doped levels to promote the PTE detection. Compared with the input pulse width of 155 ps, the intrinsic response time of the detector was estimated to be 10 ps for the PL and 20 ps for the PTE detection. This can be quantitatively explained by the characteristic relaxation times of the momentum relaxation for the PL, and the energy relaxation of the hot electrons by optical-phonon emission for the PTE detection. These results indicate that the ADGG-EG-FETs THz detectors are promising for applications in 6G to 7G-class THz wireless communication systems.
A highly sensitive terahertz parametric up-conversion detector based on KTiOPO4(KTP) crystal pumped by 1064nm laser was demonstrated in this paper. THz wave was generated in KTP crystal with a terahertz parametric oscillator (TPO), which can generate THz wave from 1.17-5.98 THz by varying the phase-matching angle between the pump and Stokes wave inside KTP crystal. THz wave and pump wave were mixed in KTP crystal to generate up-conversion signal based on stimulated parametric scattering. The up-conversion signal was amplified in another two KTP crystals based on non-collinear and collinear phase matching to improve detection sensitivity. Spectrometer and photodiode were used to measure the wavelength and pulse energy of up-conversion signal respectively. The detectable THz frequency range was 4.26-4.50 THz and 4.80-4.92 THz. The minimum detectable energy of 250 pJ was realized with dynamic range of 32 dB at 4.40 THz, and the minimum detectable energy at 4.85 THz was 9.4 pJ with dynamic range of 48 dB. All experiments were carried out under pump threshold conditions of spontaneous parametric noise generation. Compared with LiNbO3 crystal, the parametric up-conversion detection based on KTP crystal can realize high frequency range (>3 THz) THz wave detection, filling in the gaps for high-frequency detection.
We have demonstrated injection-seeded backward terahertz (THz)-wave parametric oscillators (BW-TPOs) based on a slant-strip-type periodically poled lithium niobate (PPLN) crystal with two different poling periods. The BW-TPOs were pumped by sub-nanosecond pump pulses at 1064 nm and designed for generating backward-propagating THz waves around 0.30 and 0.46 THz with PPLN poling periods of 53 and 35 μm, respectively. As a result of an optical injection seeding for the forward-propagating idler wavelength in the BW-TPO process, we achieved over a 1000-fold enhancement in backward-propagating THz-wave output energy, a 63% reduction of the oscillation threshold, and long-term stable operation compared to the unseeded case. Furthermore, we demonstrated that the oscillation frequency of backward-propagating THz waves is continuously tunable in the range of 0.27–0.35 and 0.41–0.52 THz for the poling periods of 53 and 35 μm, respectively, by angle tuning of the PPLN crystal in parallel with seed wavelength tuning. Using the developed injection-seeded BW-TPOs, we also performed the THz-wave imaging test in transmission geometry for various materials, including glass, wood, and liquids.
We have developed injection-seeded terahertz (THz)-wave parametric generators (is-TPGs), which have potential properties to THz applications because of high output, widely tunability and room temperature operation with compactness. Recently, the remarkable results are a 100 kHz repetition-rate is-TPG which marks a thousandfold enhancement, and a rapidly frequency-switching is-TPG using a multi-furcated Nd:YAG microchip laser with 11-GHz frequency separation. In addition, we demonstrated a security application with a is-TPG, which is a real-time screening system as passengers gate based on THz-wave spectroscopic detection.
We have developed a thin THz-wave planar lens based on the phase-patterned Fresnel zone plate (FZP) concept to obtain a high transmittance and short focal length in free space. The FZP lens was designed for focusing THz waves at 1.0 THz (λ=300 μm) with a transmittance of more than 80% and a focal length of 24 mm (80λ). The developed FZP lens was made of polymer BCB as a flexible film substrate with concentric zones of metamaterial-based phase shifter patterns with a subwavelength thickness of approximately 48 μm (0.16λ). To obtain the THz-wave phase retardation of π/2 compared to the naked polymer substrate, we employed the metamaterial unit structure consisting of double-layer unsplit ring resonators (USRRs) with a 32-μm distance between the two layers. The experimental result confirms that the FZP lens creates a focus by constructive interference of incident THz waves through concentric zones of metamaterial-patterned and un-patterned regions. By using a narrowband THz-wave beam from an injection-seeded THz-wave parametric generator, the measured focus spot size of 0.57 mm at full width at half maximum was obtained at the designed frequency of 1.0 THz. Using this FZP lens, the THz-wave imaging test in transmission geometry has also been demonstrated.
We identified eight nonlinear crystals enabling THz emission from quadratic phase-matched Difference-Frequency-Generation: YCOB, BNA, LBO, CSP, AGS, CdSe, ZnO and GaP. For all these crystals, we performed Time-Domain Spectroscopy in the same conditions to determine their absorption spectra in polarized light as well as their principal refractive indices as a function of wavelength in the 0.5-2.0 THz range. By combining previous data with the Sellmeier equations valid in their visible and infrared transparency ranges, we calculated the coherence length of Difference-Frequency-Generation associated to all possible configurations of polarization and found interesting and complementary phase-matching conditions in the eight studied crystals.
The development of reliable, high-power, frequency-tunable terahertz (THz)-wave sources is crucial for a wide variety of applications, such as spectroscopy, imaging, and sensing. In order to generate frequency-tunable THz waves at room temperature, one of the most promising methods is a wavelength conversion in nonlinear optical crystals. Here, we present our recent results on high-power, widely-tunable, frequency-agile THz-wave sources based on nonlinear parametric processes in MgO:LiNbO3 crystals. By changing the noncollinear phase-matching condition in MgO:LiNbO3, the tunability of sub-nanosecond-pumped injection-seeded THz-wave parametric generators (is-TPGs) covers the 3.65-octave frequency range from 0.37 THz up to 4.65 THz. The monochromatic THz-wave output from is-TPGs is greater than 10 kW peak power with the linewidth of approximately 3 GHz and the stability of 1%. These is-TPG systems are reliable and promising high-power tunable THz-wave sources for frequency-domain spectroscopic measurements towards THz sensing and detector sensitivity calibration.
Second-order nonlinear optical wavelength-conversion has been attractive for generating terahertz (THz) wave with high peak-power and for THz-wave detection with high sensitivity. Over 50 kilowatt peak-power THz-wave radiation and sensitive THz-wave detection down to several tens of atto-joule using LiNbO3 or 4-dimethylamino-N’-methyl-4’- stilbazolium tosylate (DAST) crystals have been demonstrated. LiNbO3 crystal is an eminent nonlinear crystal for converting wavelengths between THz wave and near infrared (NIR) at frequency range from 1 to 3 THz. Mixing THz wave with an intense NIR pump beam in the LiNbO3 provides generation of a signal light at a different frequency because of efficient figure of merit. Additionally, sensitivity of up-conversion detection working at room temperature is more than that of cryogenically cooled THz detector. Here, we report on a sensitive THz-wave detection based on novel design using a slant-stripe-type periodically poled Mg doped lithium niobate (PPMgLN) for practical use. The efficient scheme that two optical waves, the pump and up-conversion signal beams, propagate collinearly in the PPMgLN to achieve effective parametric amplification for the signal beam was designed. Minimum THz-wave detection was achieved down to energy about 100 aJ at the frequency of 1.6 THz. The result leads to a novel THz detector based on fiber and integrated optics with high sensitivity, robustness, and easy handling. The nonlinear optical up-conversion detection is promising and broadening THz horizons.
We study theoretically and experimentally the plasmonic THz detection by the asymmetric dual-grating-gate HEMT at room temperature without source-to-drain bias. We derive the analytical expressions of photocurrents due to the plasmonic drag and ratchet effects, and we discuss about their frequency dependences. We also compare the theory to the experimentally obtained frequency dependence. It is demonstrated that they agree qualitatively well.
Sensitive terahertz (THz)-wave sensor at room temperature is crucial for most applications such as 2-dimensional realtime
imaging and nonlinear phenomena in semiconductors caused by multi-photon absorption, light-induced ionization,
and saturated absorption. LiNbO3 is a promising material for frequency up- and down-conversion because of its high
nonlinearity and high resistance to optical damage. In this report, we propose a slant-stripe-type periodically poled Mg
doped LiNbO3 (PPMgLN) crystal for the construction of a practical THz detector. The PPMgLN solves compromised
optical design and low coupling efficiency between THz and infrared (IR) pump beam due to imperfect dichroic coupler.
The effective coupling of both pump beam and THz-wave into identical interaction region of up-conversion device
promotes the THz detector in practical use. The phase-matched-condition in slant-stripe-type PPMgLN was designed to
offer collinear propagation of two optical waves, the pump and up-conversion signal beams, because of efficient
frequency conversion. The phase-mached-condition was calculated and a slant-stripe-type PPMgLN with an angle (α) of
20° and a grating period (Λ) of 29.0 μm was used in this experiment. A minimum detectable energy of 0.3 pJ/pulse at the frequency of 1.6 THz was achieved with the pump energy of 1.8 mJ/pulse in room temperature. The dynamic range of the incident THz-wave energy of 60 dB was demonstrated. Further improving for the sensitivity using longer interaction
length in a PPMgLN crystal was also investigated.
Terahertz imaging has attracted a lot of interests for more than 10 years. But real time, high sensitive, low cost THz imaging in room temperature, which is widely needed by fields such as biology, biomedicine and homeland security, has not been fully developed yet. A lot of approaches have been reported on electro-optic (E-O) imaging and THz focal plane arrays with photoconductive antenna or micro-bolometer integrated. In this paper, we report high sensitive realtime THz image at 60 frames per second (fps) employing a commercial infrared camera, using nonlinear optical frequency up-conversion technology. In this system, a flash-lamp pumped nanosecond pulse green laser is used to pump two optical parametric oscillator systems with potassium titanyl phosphate crystals (KTP-OPO). One system with dual KTP crystals is used to generate infrared laser for the pumping of THz difference frequency generation (DFG) in a 4- Dimethylamino-N-Methyl-4-Stilbazolium Tosylate (DAST) crystal. The other one is for generation of pumping laser for THz frequency up-conversion in a second DAST crystal. The THz frequency can be tuned continuously from a few THz to less than 30 THz by controlling the angle of KTP crystals. The frequency up-converted image in infrared region is recorded by a commercial infrared camera working at 60 Hz. Images and videos are presented to show the feasibility of this technique and the real-time ability. Comparison with a general micro-bolometer THz camera shows the high sensitivity of this technique.
We report on ultrahigh sensitive, broadband terahertz (THz) detectors based on asymmetric double-grating-gate (A-DGG)
high electron mobility transistors, demonstrating a record responsivity of 2.2 kV/W at 1 THz with a superior low
noise equivalent power of 15 pW/√Hz using InGaAs/InAlAs/InP material systems. When THz radiation is absorbed
strong THz photocurrent is first generated by the nonlinearity of the plasmon modes resonantly excited in undepleted
portions of the 2D electron channel under the high-biased sub-grating of the A-DGG, then the THz photovoltaic response
is read out at high-impedance parts of 2D channel under the other sub-grating biased at the level close to the threshold.
Extraordinary enhancement by more than two orders of magnitude of the responsivity is verified with respect to that for
a symmetric DGG structure.
Sensitive water concentration mapping in thin animal tissue samples has been demonstrated using tunable
monochromatic THz-wave parametric source. A novel sample preparation approach is performed to effectively preserve
tissue freshness at room temperature. The time course results show the sample characteristic of water content and
distribution can be well measured and excellently repeated in 70minutes with a standard deviation of less than 1%. These
results suggest the method of water volume concentration and distribution measurement using THz-wave has good
stability with proper sample preparation, which has great potential in the fields of medical and biological diagnosis.
We have developed ultra-widely tunable THz-wave source using organic nonlinear optical crystals such as 4-
dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) and N-benzyl-2-methyl-4-nitroaniline (BNA). The THz-wave
difference frequency generation using these crystals covers the ultra-widely tunable range of 0.1-40 THz with frequency
agility. Collaborating with Furukawa Co. Ltd., we used the progressive, frequency-agile THz-wave source for industrial
applications and produced a sensitive, non-destructive method for examining carrier-density and electrical properties of
semiconductors. This method presents novel possibilities for use in the semiconductor industry.
Terahertz-frequency (THz) waves have shown potential for a wide range of applications. We have developed tunable
THz-wave sources using nonlinear optical crystals, which have several advantages, including frequency agility, wide
tunability, high output, and high coherency. We found that the organic nonlinear crystal of
4-dimethylamino-N-methyl-4-stilbazolium-tosylate (DAST) had particular potential for ultra-wide THz-wave generation
from sub-THz to mid-infrared frequencies. Using DAST, we manufactured a coherent, tunable source (1-40 THz) with
frequency agility. Moreover, we demonstrated THz-wave detection through up-conversion using DAST or MgO:LiNbO3
nonlinear optical crystals, which provided a fast response, high sensitivity, and room-temperature operation.
Widely tunable terahertz (THz) -wave generation using difference frequency generation (DFG) in an organic N-Benzyl-2-methyl-4-nitroaniline (BNA) crystal was demonstrated. An organic nonlinear optical (NLO) BNA crystal is one of the
promising materials for efficient and strong THz-wave generation because of its potential to have a sufficiently large
enough second-order optical nonlinearity. Large and high quality single crystals of BNA (Φ8×30mm) were successfully
grown by a vertical Bridgman method. The NLO coefficient d33 of BNA crystal is about 230pm/V. It is the largest value reported for any yellow-colored NLO materials. BNA has low refractive index dispersion between the optical and THz-wave
region, therefore the colinear phase matching condition of the Type0 configuration is satisfied by using 0.7~1μm
band pump wavelength. So, we developed a near-infrared dual-wavelength pump source for BNA-DFG. Two KTiOPO4
(KTP) crystals were mounted on galvano scanners inside a double-pass optical parametric oscillator (OPO). It is pumped using a frequency-doubled Nd:YAG laser (532 nm, 8 ns, 100 Hz). The signal wave of the KTP-OPO output was controlled independently and rapidly using a galvano scanner. We successfully generated THz-wave using organic BNA
crystal. The THz-wave generation range is from 0.1 to 15THz, while the pumping dual-wavelength is controlled in the 0.8-0.9μm range.
We have demonstrated a quasi-monolithic THz-wave parametric oscillator (TPO) to confer more stability, a lower
threshold, and more compact size on THz-wave generating devices. In this report, we describe narrow linewidth
generation in a quasi-monolithic TPO. The cavity configuration was designed so that the noncollinear phase-matching
condition was satisfied in the crystal. A 5 mol% MgO:LiNbO3 crystal within dimensions of 15 mm × 20 mm with three
surfaces for total reflection was used as a nonlinear optical crystal. The quasi-monolithic TPO in a ring-cavity
configuration consisted of a nonlinear optical crystal and a super-mirror that reflected the idler beam (&lgr; > ca. 1067 nm)
and transmitted the pump beam (1064 nm). We obtained narrow oscillation linewidth of < 760 MHz at 1.6 THz of THz-wave
radiation. The low threshold of the oscillation was around 5.4 mJ/pulse.
We report the experimental results of generation and coherent detection of narrow linewidth tunable terahertz radiation at room
temperature utilizing a difference frequency generation as a result of stimulated scattering in the nonlinear crystal of MgO:LiNbO3.
The terahertz radiation was generated from an all-solid-state tunable injection-seeded THz-wave parametric generator (is-TPG), which
emits the monochromatic THz-wave over a wide tunable frequency range from 0.6 THz to 2.4 THz with the linewidth of narrower
than 100 MHz. Mixing of terahertz radiation (frequency &ohgr;T ) with a near-infrared intense pump pulse (frequency &ohgr;P ) results in the
excitation and amplification of the difference frequency component with frequency &ohgr;i =&ohgr;P -&ohgr;T, which is detected with a InGaAsbased photodiode. We demonstrate this method a fast response and very sensitive THz-wave detection running at room temperature,
which is at least three of magnitude faster and two of magnitude more sensitive than a typical Liquid-He cooling Si bolometer for
detecting the quasi-cw THz-wave beam. This detection technique is possible for coherent detection, it can measure the THz electric
field, not only the intensity. As a result, the phase information is preserved, the real and imaginary parts of a sample's dielectric
function may be determined simultaneously with this detection.
We carried out real-time measurement of gas density using monochromatic terahertz waves. The THz-wave absorbance is useful to measure the density of a gas having a characteristic spectrum in the THz region. We used the ring cavity THz-wave parametric oscillator (ring-TPO) as a monochromatic tunable THz-wave source. One can change the oscillation frequency of ring-TPO with a rotating galvano mirror forming the ring cavity. The frequency can be changed by synchronization with a repeating pump-pulse of 500 Hz. We demonstrated real-time measurement of the gas density in R-22, which had some spectral structure in THz frequency region. The gas density in the sample cell was changed by controlling the pressure to lower than 1 atm. When the gas density in the cell was the most tenuous, the maximum sensitivity was about 5%, which was limited by the fluctuation of THz-wave intensity.
We developed a fast data-acquisition rate terahertz (THz)–wave spectrometer based on an all-solid-state achromatically injection-seeded THz-wave parametric generator (is-TPG). It takes only seconds to smoothly sweep the frequency range from 0.9 THz to 2.4 THz. This system has been used successfully in THz-wave absorption spectrum measurement with high brightness, a fast data-acquisition rate, and ease of handling.
A new type of flexible THz-wave hollow waveguide was developed using ferroelectric polymer PVDF as the cladding material of the waveguide. The transmission efficiency of the PVDF waveguide was better than a metal coated hollow waveguide with the same size and structure as the PVDF waveguide. We also report the tilt-angle dependencies of the transmission efficiency of the hollow PVDF and metal waveguides. The polarization dependency of the PVDF waveguide was better than that of a metal waveguide.
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