Terahertz Time-domain Spectrometer Research Articles

Correction of Optical Delay Line Errors in Terahertz Time-Domain Spectroscopy

One of the key elements of terahertz time-domain spectrometers is the optical delay line. Usually it consists of a motorized translation stage and a corner reflector mounted on its top. Errors in the positioning of the translation stage lead to various distortions of the measured waveform of terahertz pulses and, therefore, terahertz spectra. In this paper, the accuracy of position measurements is improved by using an optical encoder. Three types of systematic errors are found: Increasing and periodic offsets of the translation stage position, as well as a drift of its initial position in a series of consecutive measurements. The influence of the detected errors on the measured terahertz spectra is studied and correction methods are proposed.

Optical cryostat with sample rotating unit for polarization-sensitive terahertz and infrared spectroscopy

We developed an optical cryostat with a sample-rotation unit for polarization-sensitive measurement in terahertz (THz) and infrared (IR) ranges. The cryostat, in combination with two metal-grid polarizers, provides full control of mutual orientation of the sample’s crystallographic axes and the light polarization plane. Importantly, this control is realized in-situ , i.e., during the sample cooling–heating cycle. To demonstrate the abilities of the developed cryostat, we used it in combination with a laboratory-made THz time-domain spectrometer, for polarization-sensitive measurements of an orthoferrite (YFeO3) in the range of 5 to 50 cm − 1. These measurements revealed strong angular dependence of the sample transmission. The developed cryostat is capable for solving numerous demanding problems of THz and IR spectroscopy in condensed matter physics and materials science, biophysics, chemical, and pharmaceutical sciences.

Broadband dielectric spectroscopy from sub GHz to THz of hydrated lipid bilayer of DMPC.

In order to study the dynamics of a phospholipid and its hydration water, we measured complex dielectric spectra of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) from the sub-GHz to the THz frequency region with varying the temperature and hydration level of the sample. Spectra obtained from a vector network analyzer and two terahertz time-domain spectrometers are adjusted, which enables us to analyze the dielectric spectra from the sub-GHz region to the THz region by a model function. We confirmed a fast relaxational mode in the sub-THz region, which was suggested by the previous work which only used a THz spectrometer.

A Single-Resonant-Structure and Optically Transparent Broadband THz Metamaterial Absorber

Broadband THz metamaterial absorbers have important applications in terahertz modulation, imaging, and communication, but current designs usually involve complex nesting or stacking resonant structures. This paper reports a single-resonant-structure and optically transparent broadband terahertz metamaterial absorber. The absorber demonstrates > 80% broadband absorption in a wide frequency range of 0.4~1.04 THz, or 88.9% of the central frequency. The thickness of the absorber is only 1/15 of the wavelength corresponding to the lowest absorption frequency. The absorption spectrum has been experimentally verified with a reflective terahertz time-domain spectrometer. Further study shows that the absorber is insensitive to the polarization angle, and the absorbance decreases very little for incident angles less than 30°. The absorption spectrum can be finely tuned by varying the structure parameters.

Integration of a rapid scanning technique into THz time-domain spectrometers for nonlinear THz spectroscopy measurements.

We have implemented a rapid scanning technique into THz time-domain spectrometers using an oscillating frictionless delay line, especially adapted for nonlinear THz experiments. Thereby we were able to increase the dynamic range of THz measurements in the frequency range from 40 to 200 cm-1 by up to 24 dB and reduce the scanning time by up to a factor of 200. We report here test measurements on TDS-setups at repetition rates of 80 MHz and 5 kHz. The dynamic range exceeds 64 dB, which allows to record even small changes in the THz absorption upon optical excitation by a THz probe, covering the frequency range of the intermolecular modes and the phonon bands. We demonstrate the potential of this technique for optical-pump THz-probe experiments using a 70 μm thick high-resistivity silicon, excited by 400 nm, ∼50 fs pulses as a sample.

Temperature dependent optical and dielectric properties of liquid water studied by terahertz time-domain spectroscopy

Terahertz waves have found wide applications in the biological and medical areas. Since water is one of the most important ingredients in the biological systems, it is necessary to investigate its optical and dielectric properties in the terahertz range. These fundamental physical parameters play an important role in many applications, such as biological imaging and hyperthermia applications, and are temperature dependent. In order to shed more light on the property of temperature dependence, we performed a systematic investigation by changing the temperature of the water sample and using a terahertz time-domain spectrometer. The experimental results show that when the temperature increases from 275 K to 340 K, the refractive index and the complex dielectric constant of water increase noticeable, with the absorption coefficient increased observably. A good agreement is found between experimental data and the double Debye model.

Terahertz and Visible Probing of Particles Suspended in Air

The attenuation of air suspended particles is measured with a terahertz (THz) time-domain spectrometer. Simultaneously, the attenuation at a wavelength of 650 nm is probed with a laser diode. On the one hand, this dual measurement allows a direct assessment of the visibility evolution in the THz range compared to the visible range. On the other hand, this setup provides an estimation of the scattering strength and the density of particles. Using the Mie theory, the method is successfully applied to experimentally characterize the refractive index of sand grains and glass beads. The refractive indexes of sand grains and glass beads, average over the acquisitions, are 1.67 and 2.54, respectively. The estimation of the scattering properties of sand grains is crucial to evaluate the performance of THz systems to image through brownout clouds that are created by helicopter rotors when landing in arid areas.

High-Throughput and Low-Cost Terahertz All-Dielectric Resonators Made of Polymer/Ceramic Composite Particles

In this paper, we report a new and generic method for all-dielectric resonators operating at terahertz (THz) frequencies based on polymer/ceramic composite particles. In this method, the polymer solution is added into ceramic powders (strontium titanate) and prilling technology is employed to make them become polymer/ceramic composite micro particles. The prepared micro particles are then assembled as single-layer all-dielectric resonators. By using a THz time-domain spectrometer, we find that various Mie resonances can be induced in these all-dielectric resonators. It is also demonstrated that the permittivity of the composite particles can be improved by being sintered at proper temperatures, thereby provides a new way for ceramic all-dielectric resonators in THz range. This method, free from complex process, time-consuming, expensive equipment, and clean room, provides a generic method for high-throughput and low cost THz all-dielectric resonators, which is promising for developing potential applications in wide-bandwidth THz devices.

A 400-GHz High-Gain Quartz-Based Single Layered Folded Reflectarray Antenna for Terahertz Applications

Compact high-gain antennas are highly desired in the high-speed terahertz (THz) wireless system, especially for the space limited application, such as the high-speed inter link inside the high density wireless communication base station. To this end, a 400-GHz folded reflectarray (FRA) antenna with high gain, high aperture efficiency, and compact profile is proposed in this paper. It is composed of a feed source, a single-layered reflectarray using a lithography process on quartz, and a wire-grid polarizer implemented by the printed-circuit-board technology. A 3-D printed fixture is used to assemble all parts together. In order to design accurately the proposed antenna, the THz electromagnetic properties of the supporting dielectric materials are extracted by using a THz time-domain spectrometer system. Then, a single-layered phasing element, made up of a pair of orthogonally I-shaped structures with an open square ring, is proposed and designed based on the extracted material characteristics. Both phase compensation and polarization conversion can be realized by the proposed unit cell. A reflectarray is designed by using the proposed phasing element with the conventional array synthesizing theory, and a THz grid polarizer is designed with strips on a 0.127 mm Taconic TLY-5 substrate. The THz grid is placed in front of the THz feed and the reflectarray, which is fully reflective to the feed and transparent to the reflectarray. All components of the FRA antenna have been fabricated and assembled. Experiments show that the FRA prototype has a peak gain of 33.66 dBi at 400 GHz with an aperture efficiency of 33.65%, and a 3-dB gain bandwidth of 16% (357–421 GHz).

Evaluation of amplitude difference referencing technique with terahertz metasurfaces for sub-micron analytes sensing

Terahertz metasurfaces with subradiant modes that allow excitation of a high-quality factor are potential candidates for label-free sensing of thin-film biomolecules. Nevertheless, sensing sub-micron analytes are rather difficult due to the normalization method that has been used so far. Here, we evaluate an amplitude difference referencing technique that exploits the response of the uncoated metasurface structure as a reference. Amplitude difference percentage has been calculated by subtracting the frequency response of coated metasurface from the frequency response of the uncoated metasurface for different analyte thicknesses. To our surprise, we observe significantly large values of the amplitude difference that can be easily measured using conventional terahertz time-domain spectrometers. In future, this technique can be utilized to detect analytes with sub-micron thickness and could be exploited across the electromagnetic spectrum.

Optical and Electrical Characterization of Pure PMMA for Terahertz Wide-band Metamaterial Absorbers

The characteristics of relative permittivity and reflectance for poly (methyl methacrylate) (PMMA) films prepared by spin-coating were investigated and an excellent wideband terahertz (THz) metamaterial absorber (MA) had been fabricated by employing PMMA film as the dielectric layer. XRD and AFM indicated that all the PMMA films with thicknesses from 11 to 17 μm were amorphous and extremely smooth with roughness about 0.203 nm. Raman results demonstrated that vibration strength of the same covalent bonds tends to be consistent with an increase in film thickness. The dual beam laser interferometer (DBLI) was employed to study the influence of applied frequency and thickness on the relative permittivity, and the results exhibited dielectric constants increased from 2.88 to 4.04 with the thickness increasing from 9 to 14 μm. And the dielectric constants for PMMA films with a certain thickness (8 μm) gradually declined when the applied frequency increased from 100 to 105 Hz; besides, the dielectric constant in THz band was forecasted approximately 2.5. The terahertz time domain spectrometer (THz-TDS) results revealed that the THz reflectance was of gradual degradation from 80 to 58% at 3.2 THz, as the thickness increased from 11 to 17 μm. In addition, the UV-VIS spectrum showed that thickness had little effect on reflectance and transmittance in visible region, leading to the almost same Eg (energy gap) about 3.7 eV. At last, by employing PMMA film as the dielectric layer, a highly wideband metamaterial absorber with absorption > 80% from 4.1 to 7.4 THz was fabricated.

Simultaneous acquisition of complex transmittance and birefringence with two counter-rotating, circularly polarized THz pulses.

Here we demonstrate simultaneous measurements of the complex transmittance and birefringence using left- and right-handed circularly polarized terahertz (THz) pulses. We change the polarization of the THz pulses periodically by modulating the polarization of the pump pulses directed onto a ZnTe (111) crystal, and we convert linear to circular polarization using a broadband THz quarter-wave retarder. By integrating the alternating-emission system with the polarization-sensitive terahertz time-domain spectrometer, we are able to obtain the electric-field vector of the transmitted terahertz pulses for both the left- and right-handed circular polarizations. Utilizing this technique, we are able to measure simultaneously the frequency-dependent complex refractive indices (real and imaginary parts) and the orientations of the slow and fast axes of birefringent materials, a quartz disc and a barium borate crystal, in a single temporal sweep.

A 0.1 THz low-loss 3D printed hollow waveguide

Bottlenecks in the application of terahertz (THz) technology are high transmission loss and expensive functional devices. Therefore, the paper proposes a novel low-loss THz hollow waveguide and demonstrates its design, fabrication, and characterization. Firstly, the waveguide’s material is characterized by a THz time domain spectrometer. Secondly, the influences of five different design parameters on the transmission loss coefficient are simulated and analyzed. Combined with anti-resonant waveguide theory, an optimal low-loss hollow waveguide (theoretical loss is 0.009 cm−1 at 0.1 THz) is proposed. Thirdly, the waveguide is fabricated by 3D printing technology. Its transmission loss (=0.015 cm−1) and beam divergence angle (=6.8°) are measured respectively. These results prove the effectiveness of the waveguide and will bring potential applications in THz remote transmission and imaging.

Optical Properties of KTP Crystals and Their Potential for Terahertz Generation

High nonlinearity, wide transparency range and optical quality allowed potassium titanyl phosphate (KTiOPO4, KTP) crystals to be used in a wide range of nonlinear applications. The success of KTP usage in the visible and infrared (IR) ranges drives interest in applying it at longer wavelengths, that is, in the terahertz (THz) range. We use THz optical properties of KTP crystals measured by terahertz time-domain spectrometer (THz-TDS) and refractive index approximated in the form of Sellmeier equations to investigate KTP application possibilities for IR-to-THz and THz-to-THz wave conversion. As a result, phase matching for s − f → f and s − f → s types of difference frequency generation (DFG) of Ti:Sapphire laser (at the wavelengths of 0.65, 0.8 and 1.1 µm) is found possible, as well as second harmonic generation (SHG) of THz waves by f + s→f type of interaction in the XZ principle plane of the crystal. Terahertz wave generation by phase-matched parametric processes in KTP demonstrates evident advantages in comparison with that of widely used MgO-doped LiNbO3 crystals.

Full electro-optic terahertz time-domain spectrometer for polarimetric studies.

We present a terahertz (THz) time-domain spectrometer dedicated to polarimetric studies. THz pulses are generated through optical rectification in a ⟨111⟩-cut cubic crystal. The ⟨111⟩ crystal cut produces a THz polarization state similar to that of the exciting laser beam. Detection of the THz pulses is performed by electro-optic sampling in a similar crystal, and a dual detection scheme allows us to measure simultaneously the two polarization components of the THz beam. We experimentally illustrate that the polarization of the THz beam can be adjusted by adjusting the laser polarization. This technique alleviates the need for a polarization controller at THz frequencies.

Machinable ceramic for high performance and compact THz optical components

The properties of Shapal Hi-M Soft, an aluminum nitride based ceramic, as a material for THz optical components are investigated and compared with other ceramic materials. Shapal is a low-cost and machinable ceramic with a high-refractive index and low losses at THz frequencies: n = 2.65, α = 0.4 cm−1. To demonstrate the shaping capabilities, a Fresnel lens is fabricated with a micro-milling system. Additionally, a prism as an example of a bulk component is demonstrated. Both a THz time domain spectrometer and a vector network analyzer (0.75–1.1 THz) are used for the optical characterization and the absorption coefficient is precisely obtained in the VNA frequency range.

Ultraviolet Laser Lithography of Titania Photonic Crystals for Terahertz-Wave Modulation.

Three-dimensional (3D) microphotonic crystals with a diamond structure composed of titania microlattices were fabricated using ultraviolet laser lithography, and the bandgap properties in the terahertz (THz) electromagnetic-wave frequency region were investigated. An acrylic resin paste with titania fine particle dispersions was used as the raw material for additive manufacturing. By scanning a spread paste surface with an ultraviolet laser beam, two-dimensional solid patterns were dewaxed and sintered. Subsequently, 3D structures with a relative density of 97% were created via layer lamination and joining. A titania diamond lattice with a lattice constant density of 240 µm was obtained. The properties of the electromagnetic wave were measured using a THz time-domain spectrometer. In the transmission spectra for the Γ-X <100> direction, a forbidden band was observed from 0.26 THz to 0.44 THz. The frequency range of the bandgap agreed well with calculated results obtained using the plane–wave expansion method. Additionally, results of a simulation via transmission-line modeling indicated that a localized mode can be obtained by introducing a plane defect between twinned diamond lattice structures.

Classification and identification of molecules through factor analysis method based on terahertz spectroscopy

By means of factor analysis approach, a method of molecule classification is built based on the measured terahertz absorption spectra of the molecules. A data matrix can be obtained by sampling the absorption spectra at different frequency points. The data matrix is then decomposed into the product of two matrices: a weight matrix and a characteristic matrix. By using the K-means clustering to deal with the weight matrix, these molecules can be classified. A group of samples (spirobenzopyran, indole, styrene derivatives and inorganic salts) has been prepared, and measured via a terahertz time-domain spectrometer. These samples are classified with 75% accuracy compared to that directly classified via their molecular formulas.

Accurate Measurement of Absolute Terahertz Power Using Broadband Calorimeter

This paper presents a highly sensitive terahertz (THz) calorimeter developed using a magnetically loaded epoxy as a broadband absorber. The reflection loss of the absorber, which has a pyramidally textured surface, is less than 0.04, as determined using a THz time-domain spectrometer and a vector network analyzer. The THz calorimeter successfully enabled the measurement of the absolute THz power from a photomixer at microwatt levels at room temperature. The measurement uncertainties at a 95% confidence level were 6.2% for 13 μW at 300 GHz and 5.6% for 1.5 μW at 1 THz, respectively. Details of the evaluation and uncertainty analyses are also presented.

Non-destructive plasma frequency measurement for a semiconductor thin film using broadband surface plasmon polaritons

Measurement of the plasma frequency of a semiconductor film using broadband surface plasmon is demonstrated in this paper. We theoretically deduce a formula about the relation between plasma frequency and characteristic surface plasmon frequency. The characteristic surface plasmon frequency can be captured from the cut-off frequency of the transmission spectra of the broadband surface plasmon, which is used to measure the plasma frequency indirectly. The plasma frequencies of an intrinsic indium antimonide with and without optical illuminance are measured with a THz time-domain spectrometer at room temperature. The experimental measured plasma frequencies fit well with theoretical and simulation results. Compared with other methods, the proposed method has a special advantage on measuring the plasma frequency for a thin semiconductor film coated on other materials.