Continuous-wave Terahertz Radiation Research Articles

Single-Molecule Continuous-Wave Terahertz Rectification Spectroscopy and Microscopy

We report rectification spectroscopy (RS) for single molecules performed with continuous-wave terahertz (CW THz) radiation at the tunneling junction of a scanning tunneling microscope (STM) at 8 K. CW THz-RS serves as a new technique in single-molecule vibrational and magnetic excitation spectroscopy besides inelastic electron tunneling spectroscopy (IETS). By quantitatively studying IETS and THz RS, we show that CW THz induces a sinusoidal bias modulation with amplitude linearly dependent on the THz far-field amplitude. Such THz-induced bias modulation amplitude appears to be sensitive to the THz beam alignment but insensitive to variation in the tunneling gap far smaller than the THz wavelength.

Terahertz direct polarization detector based on integrated antenna-coupled AlGaN/GaN high-electron-mobility transistors.

The foundation for polarization-based terahertz applications is the acquisition of polarization information. To develop an all-electronic terahertz straightforward polarization detection system, in this paper, a terahertz polarization detector based on three antenna-coupled AlGaN/GaN high-electron-mobility transistors (HEMTs) on a single chip is designed and fabricated. The function of the direct polarization detector is proven by measuring the polarization angle of linearly polarized continuous-wave terahertz radiation at 216 GHz. The average deviation and maximum deviation of the measured polarization angle are 3.7 degrees and 10 degrees, respectively. The error comes mainly from the disturbance of the local terahertz field by the interference effect. Simulations locate the sources of interference and guide the further device design and packaging of such kind of direct polarization detectors.

Continuous-wave THz difference frequency generation based on kHz-linewidth dual-wavelength single-longitudinal-mode Er-doped fiber laser and DAST organic crystal

In this paper, a high-average-power continuous-wave terahertz (CW-THz) radiation based on difference frequency generation (DFG) in a 4-dimethylamino-N-methyl-4-stilbazolium-tosylate (DAST) organic crystal has been experimentally demonstrated. Under the jointly effects of a section of ∼4.8 m un-pumped EDF and two uniform fiber Bragg gratings (FBGs) with bandwidth of 0.078 nm and 0.09 nm in the ring fiber laser cavity, two ultra-narrow bandwidth self-tracking filters have been formed, which have paved the way for the formation of single-longitudinal-mode (SLM) operation. THz radiation of ∼1.48 THz is generated by collinear phase-matched DFG using a stable dual-wavelength single-longitudinal-mode Er-doped fiber laser (DWSLM-EDFL) and a DAST crystal. A maximum THz radiation with average output power as high as 68.8 μW has been obtained in the experiment. The highest optical-to-THz conversion efficiency is estimated to be ∼8.19 × 10−5.

Maximizing Performance of Quantum Cascade Laser-Pumped Molecular Lasers

Quantum cascade laser (QCL)-pumped molecular lasers (QPMLs) have recently been introduced as a new source of powerful (>1 mW), tunable (>1 THz), narrow-band (<10 kHz), continuous-wave terahertz radiation. The performance of these lasers depends critically on molecular collision physics, pump saturation, and on the design of the laser cavity. Using a validated three-level model that captures the essential collision and saturation behaviors of the QPML gas nitrous oxide (N2O),we explore how threshold pump power and output terahertz power depend on pump power, gas pressure, as well as on the diameter, length, and output-coupler transmissivity of a cylindrical cavity.The analysis indicates that maximum power occurs as pump saturation is minimized in a manner that depends much more sensitively on pressure than on cell diameter, length, or transmissivity. A near-optimal compact laser cavity can produce more than 10 mW of power tunable over frequencies above 1 THz when pumped by a multi-watt QCL.

Terahertz laser diode using field emitter arrays

High-power and broad-tunable terahertz wave generating source on chip, which has broad application prospects, remains an unachieved goal of researchers after decades of pursuing. Here, we propose a concept of terahertz laser diode, which is a dc-biased self-excited microelectronic laser oscillator using collectively modulated free electrons emitted by field-emitter-arrays as the active medium and using an inherent Fabry-P\'erot interferometer formed by port reflections as the frequency selector. It is sub-millimeter to centimeter in size and can generate continuous-wave coherent terahertz radiation with several watts of power per square millimeter. Its frequency can be tuned to cover a wide terahertz band by changing the cathode-to-anode distance and the bias voltage. It has the potential to outperform the conventional free-electron or solid-state terahertz devices in terms of portability, tunability, and output power, signifying a promising class of tunable and high-power terahertz source on chip.

Frequency-domain optical coherence tomography system in the terahertz region

In this study, a frequency-domain optical coherence tomography (FD-OCT) system in the terahertz frequency range was developed. To generate tunable continuous-wave terahertz radiation, the outputs of two near-infrared external cavity diode lasers were mixed in an antenna-coupled photo-mixer. Using the FD-OCT system, tomographic profiling of the front and back surfaces of a high-resistivity silicon plate was realized.

Lens-less fiber coupling of a 1550-nm mode-locked fiber laser light on a low-temperature-grown GaAs photoconductive antenna

A fiber-coupled photoconductive antenna (PCA) is a powerful tool for portable terahertz (THz) systems using a compact 1550-nm mode-locked Er:fiber laser with a fiber output port. However, a low-temperature-grown GaAs (LTG-GaAs) PCA could not be used for this purpose due to the need for wavelength conversion of the 1550-nm light, regardless of the good characteristics for PCA. In this article, we achieved the fiber coupling of the 1550-nm mode-locked fiber laser light on a bowtie-shaped LTG-GaAs PCA detector without the need for wavelength conversion. While the two-step photo-absorption mediated by midgap states in the LTG-GaAs PCA makes it possible to use the 1550-nm light, the similarity of the size between the PCA gap spacing and the fiber core diameter enables the direct contact coupling between the fiber output tip and the PCA gap without any optical components. The developed lens-less fiber-coupled LTG-GaAs PCA detector was effectively applied for the absolute frequency measurement of continuous-wave THz radiation based on the photo-carrier THz frequency comb. The combination of the lens-less fiber-coupled LTG-GaAs PCA with the compact 1550-nm fiber laser will be useful for the portable apparatus for the absolute frequency measurement of practical CW-THz sources and other applications.

Waveguided Approach for Difference Frequency Generation of Broadly-Tunable Continuous-Wave Terahertz Radiation

The 1–10 terahertz (THz) spectral window is emerging as a key region for plenty of applications, requiring not yet available continuous-wave room-temperature THz spectrometers with high spectral purity and ultra-broad tunability. In this regard, the spectral features of stabilized telecom sources can actually be transferred to the THz range by difference frequency generation, considering that the width of the accessible THz spectrum generally scales with the area involved in the nonlinear interaction. For this reason, in this paper we extensively discuss the role of Lithium Niobate (LN) channel-waveguides in the experimental accomplishment of a room-temperature continuous wave (CW) spectrometer, with μW-range power levels and a spectral coverage of up to 7.5 THz. To this purpose, and looking for further improvements, a thought characterization of specially-designed LN waveguides is presented, whilst discussing its nonlinear efficiency and its unprecedented capability to handle high optical power (107 W/cm2), on the basis of a three-wave-mixing theoretical model.

Photoelectric Properties of InAs/GaAs Quantum Dot Photoconductive Antenna Wafers

In this paper, the study of the photoconductivity in self-assembled InAs/GaAs quantum dot photoconductive antenna in the wavelength region between 1140 nm and 1250 nm at temperatures ranging from 13 to 400 K is reported. These antennas are aimed to work in conjunction with quantum dot semiconductor lasers to effectively generate pulsed and continuous wave terahertz radiation. For the efficient operation, laser wavelengths providing the highest photocurrent should be determined. To study the interband photoconductivity of quantum dot photoconductive antennas, at room and cryogenic temperatures, we employed a broadly-tunable InAs/GaAs quantum dot based laser providing a coherent pump with power exceeding 20 mW over a 100 nm tunability range. The quantum dot antenna structure revealed sharp temperature-dependent photoconductivity peaks in the vicinity of wavelengths, corresponding to the ground and excited states of InAs/GaAs quantum dots. The ground state photoconductivity peak vanishes with a temperature drop, whereas the excited state peak persists. We associate this effect with different mechanisms of photoexcited carriers extraction from quantum dots.

Photo-conductive detection of continuous THz waves via manipulated ultrafast process in nanostructures

Time-domain and frequency-domain terahertz (THz) spectroscopy systems often use materials fabricated with exotic and expensive methods that intentionally introduce defects to meet short carrier lifetime requirements. In this study, we demonstrate the development of a nano-photomixer that meets response speed requirements without using defect-incorporated, low-temperature-grown (LTG) semiconductors. Instead, we utilized a thin InGaAs layer grown on a semi-insulating InP substrate by metal-organic chemical vapor deposition (MOCVD) combined with nano-electrodes to manipulate local ultrafast photo-carrier dynamics via a carefully designed field-enhancement and plasmon effect. The developed nano-structured photomixer can detect continuous-wave THz radiation up to a frequency of 2 THz with a peak carrier collection efficiency of 5%, which is approximately 10 times better than the reference efficiency of 0.4%. The better efficiency results from the high carrier mobility of the MOCVD-grown InGaAs thin layer with the coincidence of near-field and plasmon-field distributions in the nano-structure. Our result not only provides a generally applicable methodology for manipulating ultrafast carrier dynamics by means of nano-photonic techniques to break the trade-off relation between the carrier lifetime and mobility in typical LTG semiconductors but also contributes to mass-producible photo-conductive THz detectors to facilitate the widespread application of THz technology.

Rattling of Oxygen Ions in a Sub-Nanometer-Sized Cage Converts Terahertz Radiation to Visible Light.

A simple and robust approach to visualization of continuous wave terahertz (CW-THz) light would open up opportunities to couple physical phenomena that occur at fundamentally different energy scales. Here we demonstrate how nanoscale cages of Ca12Al14O33 crystal enable conversion of CW-THz radiation to visible light. These crystallographic cages are partially occupied with weakly bonded oxygen ions and give rise to a narrow conduction band that can be populated with localized, yet mobile electrons. CW-THz light excites a nearly stand-alone rattling motion of the encaged oxygen species, which promotes electron transfer from them to the neighboring vacant cages. When the power of CW-THz light reaches tens of watts, the coupling between forced rattling in the confined space, electronic excitation and ionization of oxygen species, and corresponding recombination processes result in emission of bright visible light.

Разработка материалов и фотопроводящих антенн на их основе для генерации и детектирования импульсного и непрерывного терагерцового (ТГц) излучения

Разработка материалов и фотопроводящих антенн на их основе для генерации и детектирования импульсного и непрерывного терагерцового (ТГц) излучения

Measurement of absolute frequency of continuous-wave terahertz radiation in real time using a free-running, dual-wavelength mode-locked, erbium-doped fibre laser

A single, free-running, dual-wavelength mode-locked, erbium-doped fibre laser was exploited to measure the absolute frequency of continuous-wave terahertz (CW-THz) radiation in real time using dual THz combs of photo-carriers (dual PC-THz combs). Two independent mode-locked laser beams with different wavelengths and different repetition frequencies were generated from this laser and were used to generate dual PC-THz combs having different frequency spacings in photoconductive antennae. Based on the dual PC-THz combs, the absolute frequency of CW-THz radiation was determined with a relative precision of 1.2 × 10−9 and a relative accuracy of 1.4 × 10−9 at a sampling rate of 100 Hz. Real-time determination of the absolute frequency of CW-THz radiation varying over a few tens of GHz was also demonstrated. Use of a single dual-wavelength mode-locked fibre laser, in place of dual mode-locked lasers, greatly reduced the size, complexity, and cost of the measurement system while maintaining the real-time capability and high measurement precision.

Experimental research on spectrum and imaging of continuous-wave terahertz radiation based on interferometry**Project supported by the National Natural Science Foundation of China (Grant Nos. 61377109 and 11374007).

A system for measuring terahertz spectrum is proposed based on optical interferometer theory, and is experimentally demonstrated by using a backward-wave oscillator as the terahertz source. A high-resolution, high-precision interferometer is constructed by using a pyroelectric detector and a chopper. The results show that the spectral resolution is better than 1 GHz and the relative error of frequency is less than 3%. The terahertz energy density distribution is calculated by an inverse Fourier transform and tested to verify the feasibility of the interferometric approach. Two kinds of carbon-fiber composites are imaged. The results confirm that the interferometer is useful for transmission imaging of materials with different thickness values.

High-speed frequency-domain terahertz coherence tomography.

High-speed frequency-domain terahertz (THz) coherence tomography is demonstrated using frequency sweeping of continuous-wave THz radiation and beam steering. For axial scanning, THz frequency sweeping with a kHz sweep rate and a THz sweep range is executed using THz photomixing with an optical beat source consisting of a wavelength-swept laser and a distributed feedback laser diode. During the frequency sweep, frequency-domain THz interferograms are measured using coherent homodyne detection employing signal averaging for noise reduction and used as axial-scan data via fast Fourier transform. Axial-scan data are acquired while scanning a transverse range of 100 × 100 mm(2) by use of a THz beam scanner with moving neither sample nor THz transmitter/receiver unit. It takes 100 s to acquire axial-scan data for 100 × 100 points with 5 averaged traces at a sweep rate of 1 kHz. THz tomographic images of a glass fiber reinforced polymer sample with artificial internal defects are presented, acquired using the tomography system.

Real-Time Determination of Absolute Frequency in Continuous-Wave Terahertz Radiation with a Photocarrier Terahertz Frequency Comb Induced by an Unstabilized Femtosecond Laser

A practical method for the absolute frequency measurement of continuous-wave terahertz (CW-THz) radiation uses a photocarrier terahertz frequency comb (PC-THz comb) because of its ability to realize real-time, precise measurement without the need for cryogenic cooling. However, the requirement for precise stabilization of the repetition frequency (f rep) and/or use of dual femtosecond lasers hinders its practical use. In this article, based on the fact that an equal interval between PC-THz comb modes is always maintained regardless of the fluctuation in f rep, the PC-THz comb induced by an unstabilized laser was used to determine the absolute frequency f THz of CW-THz radiation. Using an f rep-free-running PC-THz comb, the f THz of the frequency-fixed or frequency-fluctuated active frequency multiplier chain CW-THz source was determined at a measurement rate of 10 Hz with a relative accuracy of 8.2 × 10−13 and a relative precision of 8.8 × 10−12 to a rubidium frequency standard. Furthermore, f THz was correctly determined even when fluctuating over a range of 20 GHz. The proposed method enables the use of any commercial femtosecond laser for the absolute frequency measurement of CW-THz radiation.

Dendrimer Dipole Excitation: A New Mechanism for Terahertz Generation

An electro-optic dendrimer (EO dendrimer) material has been designed for high power terahertz generation. An ordinary poly (amidoamine organosilicon) (PAMAMOS) dendrimer was modified via doping and poling to generate a high electro-optic coefficient nanomaterial. Measured EO coefficient varied from ~130 pm/V at 633 nm to ~90 pm/V at 1553 nm. An emitter designed from this EO dendrimer generates milliwatts of continuous wave (CW) terahertz radiation (T-ray) when pumped by a CW laser of suitable wavelength. The mechanism termed as the dendrimer dipole excitation (DDE) works via excitation of the dipole population generated by the doping process. The doping protocol also generates a distribution of dipole moments, as opposed to fixed dipoles in the lattice of a crystalline material; thus, when excited by a suitable pump laser, these dipoles radiate a broadband frequency that range from 0.1 THz to ~ 30 THz. A terahertz time-domain spectrometer (TeraSpectra) was designed with this DDE terahertz source. As a test of the spectrometer functionality, a standard polyethylene calibration was conducted. It was found that TeraSpectra reproduces several known absorbance peaks of polyethylene. It also produces additional absorbance peaks not observed before. It is surmised that the ultra-high sensitivity of T-ray enables observation and discovery of additional absorbance peaks that are not visible via other spectroscopy such as visible, UV, FTIR or Raman.

Dual-Wavelength Erbium-Doped Fiber Laser to Generate Terahertz Radiation Using Photonic Crystal Fiber

We demonstrate a widely tunable dual-wavelength erbium-doped fiber laser that used a 10-cm photonic crystal fiber as a Mach–Zehnder interferometer to filter out the switchable dual-wavelength signals so as to generate tunable continuous-wave (CW) terahertz (THz) radiation. Each single longitudinal mode wavelength could be independently tuned by using a tunable band-pass filter and polarization controller (PC). The wavelength fine-tuning was achieved via adjustments to the PC, and the resulting dual-wavelength output had a side-mode suppression ratio (SMSR) of more than 30 dB. The wavelength spacing of 7, 11.4, and 21.2 nm, corresponding to 0.9, 1.4, and 2.66 THz radiation, respectively, was tuned in order to obtain CW THz radiation. This CW THz radiation was produced by means of a stable dual-wavelength fiber laser performing as the optical beat source, together with a DAST crystal-based photomixer. High-sensitivity thermal sensors calibrated for THz radiation were able to continuously detect the emitted CW THz radiation.

Cyclotron-resonance-assisted photocurrents in surface states of a three-dimensional topological insulator based on a strained high-mobility HgTe film

We report on the observation of cyclotron resonance induced photocurrents, excited by continuous wave terahertz radiation, in a 3D topological insulator (TI) based on an 80 nm strained HgTe film. The analysis of the photocurrent formation is supported by complimentary measurements of magneto-transport and radiation transmission. We demonstrate that the photocurrent is generated in the topologically protected surface states. Studying the resonance response in a gated sample we examined the behavior of the photocurrent, which enables us to extract the mobility and the cyclotron mass as a function of the Fermi energy. For high gate voltages we also detected cyclotron resonance (CR) of bulk carriers, with a mass about two times larger than that obtained for the surface states. The origin of the CR assisted photocurrent is discussed in terms of asymmetric scattering of TI surface carriers in the momentum space. Furthermore, we show that studying the photocurrent in gated samples provides a sensitive method to probe the effective masses and the mobility of 2D Dirac surface states, when the Fermi level lies in the bulk energy gap or even in the conduction band.

Two-dimensional tomographic terahertz imaging by homodyne self-mixing.

We realize a compact two-dimensional tomographic terahertz imaging experiment involving only one photoconductive antenna (PCA) simultaneously serving as a transmitter and receiver of the terahertz radiation. A hollow-core Teflon cylinder filled with α-Lactose monohydrate powder is studied at two terahertz frequencies, far away and at a specific absorption line of the powder. This sample is placed between the antenna and a chopper wheel, which serves as back reflector of the terahertz radiation into the PCA. Amplitude and phase information of the continuous-wave (CW) terahertz radiation are extracted from the measured homodyne self-mixing (HSM) signal after interaction with the cylinder. The influence of refraction is studied by modeling the set-up utilizing ZEMAX and is discussed by means of the measured 1D projections. The tomographic reconstruction by using the Simultaneous Algebraic Reconstruction Technique (SART) allows to identify both object geometry and α-Lactose filling.