Tunable Narrow-band Terahertz Research Articles

Dynamically tunable terahertz multi-band perfect absorber based on photosensitive silicon

A tunable narrowband terahertz absorber is proposed based on the photosensitive characteristics of silicon. When silicon is insulating without the pump beam, the absorber realizes three-frequency absorption at 0.731 THz, 1.145 THz, and 1.546 THz with absorptivity of 99.43%, 99.99%, and 99.98%, respectively. When the silicon is excited by the pump beam, it is conducting, and the absorber realizes double-frequency absorption at 0.852 THz, 1.536 THz, with 99.99% and 99.31%. The impedance matching theory explains the perfect absorption, and the electric field and surface current distributions are further discussed to elaborate the physical mechanisms. In addition, the effect of geometric parameters on the absorptivity is discussed. The absorber exhibits wide-angle absorption characteristics when light is polarized along the y-direction, and the absorptivity exhibits weak dependence on the polarization angle. The proposed absorber has promising applications in electromagnetic cloaking, narrow-band thermal radiation, and optoelectronic detection.

Coherent generation and control of tunable narrowband THz radiation from a laser-induced air-plasma filament.

We report on the proof-of-principle experiment of generating carrier-envelope phase (CEP)-controllable and frequency-tunable narrowband terahertz (THz) radiation from an air-plasma filament prescribed by the beat of a temporally stretched two-color laser pulse sequence. The pulse sequence was prepared by propagating the fundamental ultrafast laser pulse through a grating stretcher and Michelson interferometer with variable inter-arm delay. By partially frequency-doubling and focusing the pulse sequence, an air-plasma filament riding a beat note was created to radiate a THz wave with primary pulse characteristics (center frequency and CEP) under coherent control. To reproduce experimental results and elucidate complex nonlinear light-matter interaction, numerical simulation has been performed. This work demonstrates the feasibility of generating coherently controlled narrowband THz wave with high tunability in laser-induced air plasma.

Widely tunable electron bunch trains for the generation of high-power narrowband 1–10 THz radiation

Terahertz radiation enables the coherent excitation of many fundamental modes, such as molecular rotation, lattice vibration and spin precession. To excite the transient state of matter far out of equilibrium, high-power and tunable narrowband terahertz radiation sources have been in great demand for years. However, the terahertz source available at present cannot meet these tunability and power demands, leaving a large scientific gap that has yet to be fully explored. Here we convert the energy modulation induced by the nonlinear longitudinal space charge force to a density modulation and experimentally demonstrate the generation of electron bunch trains with modulation frequencies that are adjustable between 1 and 10 THz. The electron bunch trains can be directly used to produce tunable high-power narrowband terahertz radiation that fully covers the long-standing ‘terahertz gap’, which will open up many more possibilities in terahertz science. A tunable terahertz radiation pulse is demonstrated based on a linear accelerator. The emission frequency of this terahertz radiation is tunable from 1 to 10 THz by changing the bunching frequency of a 34 MeV electron beam. The pulse energy is at the submillijoule level.

Perspective on Terahertz Applications of Molecular Crystals

In this review, we present a survey on the use of molecular nonlinear crystals in the context of terahertz (THz) photonics. The fundamentals of nonlinear optics for converting optical and infrared radiation into THz radiation with the basic theory of femtosecond optical rectification and difference frequency generation are described. Various types of phase-matching conditions that can be achieved in molecular crystals are discussed. It is shown that one of the unique features of molecular crystals is the ability to generate tunable narrowband terahertz radiation using femtosecond lasers. We also provide a detailed description of the most commonly used and promising molecular crystals such as DAST, DSTMS, OH1, HMQ-TMS, DCMBI, and GUHP. This review also presents a description of recent publications which show the prospects of using molecular nonlinear optical crystals in THz photonics.

Resonance-enhanced spectral funneling in Fabry–Perot resonators with a temporal boundary mirror

Abstract A temporal boundary refers to a specific time at which the properties of an optical medium are abruptly changed. When light interacts with the temporal boundary, its spectral content can be redistributed due to the breaking of continuous time-translational symmetry of the medium where light resides. In this work, we use this principle to demonstrate, at terahertz (THz) frequencies, the resonance-enhanced spectral funneling of light coupled to a Fabry–Perot resonator with a temporal boundary mirror. To produce a temporal boundary effect, we abruptly increase the reflectance of a mirror constituting the Fabry–Perot resonator and, correspondingly, its quality factor in a step-like manner. The abrupt increase in the mirror reflectance leads to a trimming of the coupled THz pulse that causes the pulse to broaden in the spectral domain. Through this dynamic resonant process, the spectral contents of the input THz pulse are redistributed into the modal frequencies of the high-Q Fabry–Perot resonator formed after the temporal boundary. An energy conversion efficiency of up to 33% was recorded for funneling into the fundamental mode with a Fabry–Perot resonator exhibiting a sudden Q-factor change from 4.8 to 48. We anticipate that the proposed resonance-enhanced spectral funneling technique could be further utilized in the development of efficient mechanically tunable narrowband terahertz sources for diverse applications.

Generation of Tunable 10-mJ-Level Terahertz Pulses through Nonlinear Plasma Wakefield Modulation

High-density electron-bunch train--based tunable coherent terahertz radiation with a narrow spectral bandwidth is highly essential for many scientific applications. Here we propose a scheme to produce ultrahigh-peak-current (more than 10 kA) electron-bunch trains with tunable picosecond spacing for the generation of such terahertz sources. Passing through a plasma section and interacting with the self-excited nonlinear plasma wake, the beam gains a ``sawtooth'' energy modulation. By means of magnetic optics, it is then effectively converted into a beam density modulation forming microbunches with a high charge of up to a few nanocoulombs and a bunching factor as high as approximately $0.8$. Tunable narrowband terahertz radiation with energy on the order of 10 mJ can be generated, which we show in both theoretical analyses and simulations via this process. This scheme can be realized in accelerator facilities with repetition rates on the order of kilohertz or even submegahertz, which provides great potential in modern science and related technologies.

Tunable narrowband terahertz multichannel filter based on one-dimensional graphene-dielectric photonic crystal

In this work, low terahertz (THz) spectroscopic properties of defective one-dimensional graphene-dielectric photonic crystal (1D-GPC) structure are theoretically investigated and analyzed in detail. The 1D-GPC is composed of a periodic stack of graphene nanolayers separated by isotropic dielectric slabs (SiO2), including an air defect as a central layer. The study is carried out using the well known transfer matrix method. Our numerical simulations showed that the suggested structure exhibits comb-like resonant peaks in the first transmission band of the 1D-GPC. These peaks are narrow, fully separated from each other and their number density increases by increasing the number of periods of the structure. Tunability of the optical characteristics of the device is also explored, taking into account the dependencies to the incidence angle, width, thickness, and refractive index of the dielectric layers, and the chemical potential of graphene sheets. Our calculations reveal that these transmission channels exhibit tunable behaviors against the parameters mentioned above. The observed spectroscopic features of the proposed structure make it an ideal candidate to realize tunable narrowband multichannel filters.

Narrow-band and tunable intense terahertz pulses for mode-selective coherent phonon excitation

We generate frequency-tunable narrow-band intense fields in the terahertz (THz) range by optical rectification of a temporally modulated near-infrared laser pumping a nonlinear organic crystal. Carrier-frequency tunability between 0.5 and 6.5 THz is achieved by changing the modulation period of the laser pump. This tunable narrow-band THz source allows the selective coherent excitation of adjacent vibrational modes, which are demonstrated for two phonons with a frequency offset of 0.8 THz in single-crystal SrCu2(BO3)2. Our compact and scalable source enables an effective approach for the advanced manipulation of low-energy collective modes in condensed matter and has the potential to reveal the coupling of specific lattice vibrations with other degrees of freedom.

Continuously tunable narrow-band terahertz generation with a dielectric lined waveguide driven by short electron bunches

A continuously tunable narrow-band terahertz source based on Cherenkov wakefield generation in a dielectric lined waveguide driven by short sub-picosecond electron bunches has been experimentally demonstrated. A large tunability range of 0.55--0.95 THz was achieved by implementing the waveguide design as a planar structure with variable 0.15--1.10 mm gap and thin $25\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ dielectric layers. The source operates essentially in a single mode regime with higher order modes of negligible intensity. THz pulse energy was measured across the tunable range and compared with theory.

Over 100-THz bandwidth selective difference frequency generation at LaAlO3/SrTiO3 nanojunctions

The ability to combine continuously tunable narrow-band terahertz (THz) generation that can access both the far-infrared and mid-infrared regimes with nanometer-scale spatial resolution is highly promising for identifying underlying light-matter interactions and realizing selective control of rotational or vibrational resonances in nanoparticles or molecules. Here, we report selective difference frequency generation with over 100 THz bandwidth via femtosecond optical pulse shaping. The THz emission is generated at nanoscale junctions at the interface of LaAlO3/SrTiO3 (LAO/STO) that is defined by conductive atomic force microscope lithography, with the potential to perform THz spectroscopy on individual nanoparticles or molecules. Numerical simulation of the time-domain signal facilitates the identification of components that contribute to the THz generation. This ultra-wide-bandwidth tunable nanoscale coherent THz source transforms the LAO/STO interface into a promising platform for integrated lab-on-chip optoelectronic devices with various functionalities.

Widely tunable narrow-band coherent Terahertz radiation from an undulator at THU

There is anxious demand for intense widely tunable narrow-band Terahertz (THz) radiation in scientific research, which is regarded as a powerful tool for the coherent control of matter. We report the generation of widely tunable THz radiation from a planar permanent magnet undulator at Tsinghua University (THU). A relativistic electron beam is compressed by a magnetic chicane into sub-ps bunch length to excite THz radiation in the undulator coherently. The THz frequency can be tuned from 0.4 THz to 10 THz continuously with narrow-band spectrums when the undulator gap ranges from 23 mm to 75 mm. The measured pulse THz radiation energy from 220 pC bunch is 3.5 μJ at 1 THz and tens of μJ pulse energy (corresponding peak power of 10 MW) can be obtained when excited by 1 nC beam extrapolated from the property of coherent radiation. The experimental results agree well with theoretical predictions, which demonstrates a suitable THz source for the many applications that require intense and widely tunable THz sources.

Recent progress of tunable terahertz sources based on difference frequency generation

Terahertz technology has been developed rapidly in the past 30 years. Numerous applications in medicine, biology, agriculture, materials, security, communication and astronomy have been demonstrated. Terahertz sources can be divided into narrowband (monochromatic) source and broadband source according to their spectral characteristics. From a spectral perspective, coherent broadband and narrowband terahertz sources are mutually complementary, each having its own characteristics and scope of applications. Broadband terahertz sources can be used for quick access to the hybrid spectra of rotational and vibrational molecular fingerprints or imaging in a wider spectral range. Narrowband terahertz source with good spectral resolution and sensitivity, is suitable for pump-probe, fine structure resolution of molecular fingerprints and terahertz remote detection and imaging. Therefore, developing the tunable high peak power and narrowband terahertz sources is very important for the applications in the detection and identification of molecular fingerprints. The difference frequency generation is one of the most important techniques for obtaining widely tunable, high power and narrowband terahertz sources. In this review, the recent progress of tunable terahertz sources based on the difference frequency generation in the last five years is reviewed, including the two fields of optical laser-based difference frequency sources and quantum cascade laser-based difference frequency sources. For the former class, the experimental results from reports with different difference frequency sources and several typical nonlinear crystals are classified, and the corresponding experimental techniques and results are introduced. For terahertz wave generation, different optical difference frequency sources by a dual-wavelength laser, double laser, a laser and an optical parametric oscillator (OPO), the signal and idler waves of an OPO, and double OPOs are demonstrated in increasing their tunabilities. Significant progress has been made in the nonlinear crystals used to generate terahertz wave by the difference frequency process, for example, by improving the property of inorganic crystals with ion doping, taking advantage of waveguide and PPLN structures, and especially developing novel nonlinear organic crystals. For the quantum cascade laser-based difference frequency sources, the latest advances in the techniques of difference frequency generation and wavelength tunability are presented. GaAs-based terahertz quantum cascade lasers are powerful semiconductor THz sources but cryogenic cooling is still a necessity. Recently, difference frequency generation was combined with the mid-infrared quantum cascade laser technology, thus becoming a leading room temperature semiconductor source in the terahertz range. To improve the frequency tuning range in the difference frequency terahertz quantum cascade laser, wavelength tuning techniques of the inner cavity and the external cavity have been developed. The difference frequency generation quantum cascade terahertz laser source has been the only technique workable at room temperature for the quantum cascade laser so far, which opens the door for developing the compact and widely tunable room temperature terahertz sources.

A Tunable Terahertz Photonic Crystal Narrow-Band Filter

We theoretically propose and investigate a magnetically tunable narrow-band terahertz filter based on a triangular lattice silicon photonic crystal with a point and two line defects. The optical properties of the filter have been analyzed in detail. It is found that a single resonant peak with the central frequency of $\sim 1$ THz is existed in the transmission spectrum, which has a narrow full width at half maximum of <2 GHz. Moreover, under the control of an external magnetic field, transmission frequency and width of passband are adjustable, which reveals that the 2-D silicon photonic crystal waveguide with point and line defects can serve as a continuously tunable bandpass filter at the terahertz waveband.

Tunable narrow band difference frequency THz wave generation in DAST via dual seed PPLN OPG.

We report a widely tunable narrowband terahertz (THz) source via difference frequency generation (DFG). A narrowband THz source uses the output of dual seeded periodically poled lithium niobate (PPLN) optical parametric generators (OPG) combined in the nonlinear crystal 4-dimthylamino-N-methyl-4-stilbazolium-tosylate (DAST). We demonstrate a seamlessly tunable THZ output that tunes from 1.5 THz to 27 THz with a minimum bandwidth of 3.1 GHz. The effects of dispersive phase matching, two-photon absorption, and polarization were examined and compared to a power emission model that consisted of the current accepted parameters of DAST.

Generating periodic terahertz structures in a relativistic electron beam through frequency down-conversion of optical lasers

We report generation of density modulation at terahertz (THz) frequencies in a relativistic electron beam through laser modulation of the beam longitudinal phase space. We show that by modulating the energy distribution of the beam with two lasers, density modulation at the difference frequency of the two lasers can be generated after the beam passes through a chicane. In this experiment, density modulation around 10 THz was generated by down-converting the frequencies of an 800 nm laser and a 1550 nm laser. The central frequency of the density modulation can be tuned by varying the laser wavelengths, beam energy chirp, or momentum compaction of the chicane. This technique can be applied to accelerator-based light sources for generation of coherent THz radiation and marks a significant advance toward tunable narrow band THz sources.

Observation of coherently enhanced tunable narrow-band terahertz transition radiation from a relativistic sub-picosecond electron bunch train

We experimentally demonstrate the production of narrow-band (δf/f≈20% at f≈0.5 THz) transition radiation with tunable frequency over [0.37, 0.86] THz. The radiation is produced as a train of sub-picosecond relativistic electron bunches transits at the vacuum-aluminum interface of an aluminum converter screen. The bunch train is generated via a transverse-to-longitudinal phase space exchange technique. We also show a possible application of modulated beams to extend the dynamical range of a popular bunch length diagnostic technique based on the spectral analysis of coherent radiation.

Femtosecond optical pulse shaping for tunable terahertz pulse generation

We report on the generation of phase-locked terahertz pulse pairs and tunable narrow-band terahertz pulses by means of optical pulse shaping combining a liquid crystal spatial light filter and optical rectification in ZnTe. They are generated through simple sinusoidal and/or triangular phase modulations and do not require any optimization algorithm. The corresponding spectra are tunable between 0.5 and 2.5 THz with a spectral bandwidth as narrow as 140 GHz.

Numerical modeling of a table-top tunable Smith–Purcell terahertz free-electron laser operating in the super-radiant regime

Terahertz (THz) radiation occupies a very large portion of the electromagnetic spectrum and has generated much recent interest due to its ability to penetrate deep into many organic materials without the damage associated with ionizing radiation such as x-rays. One path for generating copious amount of tunable narrow-band THz radiation is based on the Smith–Purcell free-electron laser (SPFEL) effect. In this paper we propose a simple concept for a compact two-stage tunable SPFEL operating in the super-radiant regime capable of radiating at the fundamental bunching frequency. We demonstrate its capabilities and performances using the conformal finite-difference time-domain electromagnetic solver VORPAL.

Enhanced tunable narrow-band THz emission from laser-modulated electron beams

We propose and analyze a scheme to generate enhanced narrow-band terahertz (THz) radiation through down-conversion of the frequency of optical lasers using laser-modulated electron beams. In the scheme the electron beam is first energy modulated by two lasers with wave numbers ${k}_{1}$ and ${k}_{2}$, respectively. After passing through a dispersion section, the energy modulation is converted to density modulation. Because of the nonlinear conversion process, the beam will have density modulation at wave number $k=n{k}_{1}+m{k}_{2}$, where $n$ and $m$ are positive or negative integers. By properly choosing the parameters for the lasers and dispersion section, one can generate density modulation at THz frequency in the beam using optical lasers. This density-modulated beam can be used to generate powerful narrow-band THz radiation. Since the THz radiation is in tight synchronization with the lasers, it should provide a high temporal resolution for the optical-pump THz-probe experiments. The central frequency of the THz radiation can be easily tuned by varying the wavelength of the two lasers and the energy chirp of the electron beam. The proposed scheme is in principle able to generate intense narrow-band THz radiation covering the whole THz range and offers a promising way towards the tunable intense narrow-band THz sources.

Terahertz Radiation-Band Engineering Through Spatial Beam-Shaping

We propose a terahertz (THz)-band engineering technique based on generation of specifically designed time-domain radiation. The desired time-domain radiation is formed by combining the time-delayed outputs of an array of photoconductive antennas. We have employed this technique to generate tunable narrowband THz radiation. By using an array of identical photoconductive antennas with identical spacings, we have demonstrated a 12.5-mW output peak-power over 0.05-THz bandwidth and a tuning range of 0.65-1.4 THz. Considering the 2.5-THz bandwidth of each individual photoconductive antenna, 50 times higher power efficiency is achieved as a result of radiation-band shaping.