THz Spectrometer Research Articles

Optical properties of DAST in the THz range

We report the far-infrared properties of the organic crystal DAST, a popular terahertz emitter, from 0.6-12 THz through use of a THz spectrometer incorporating air-plasma THz generation and electro-optic (EO) sampling in a poled EO polymer. We identify absorption features at 1.1, 3.1, 5.2, 7.1, 8.4, 11, and 12.3 THz and at 1.1, 1.3, 1.6, 2.2, 3, 5.2, 7.2, 9.6 and 11.7 THz for a-axis and b-axis polarized THz radiation respectively. These results allow for more accurate prediction of the optimum crystal thickness for broadband THz emission via optical rectification and difference frequency generation.

THz photomixing synthesizer based on a fiber frequency comb

A frequency doubled erbium doped modelocked fiber frequency comb is used to implement a THz photomixing synthesizer. The useful THz linewidth is in order of 150 kHz and has been assessed along with the frequency accuracy by spectroscopic measurements demonstrating a relative accuracy of 10(-8) at frequencies around 1 THz. The THz synthesizer is used to implement a THz spectrometer to study the rotational absorption spectrum of carbonyl sulfide (OCS). Measurement of the principal transitions between 813 GHz and 1283 GHz allowed the properties of the THz spectrometer to be compared with competing techniques, and demonstrates the potential of the THz photomixing synthesizer as an alternative means to explore the THz domain.

Recent Developments of an Opto-Electronic THz Spectrometer for High-Resolution Spectroscopy

A review is provided of sources and detectors that can be employed in the THz range before the description of an opto-electronic source of monochromatic THz radiation. The realized spectrometer has been applied to gas phase spectroscopy. Air-broadening coefficients of HCN are determined and the insensitivity of this technique to aerosols is demonstrated by the analysis of cigarette smoke. A multiple pass sample cell has been used to obtain a sensitivity improvement allowing transitions of the volatile organic compounds to be observed. A solution to the frequency metrology is presented and promises to yield accurate molecular line center measurements.

Optical and terahertz measurement techniques for flat-faced pharmaceutical tablets: a case study of gloss, surface roughness and bulk properties of starch acetate tablets

Surface and bulk properties of flat-faced starch acetate tablets were studied. For surface quality inspection optical coherence tomography and recently developed diffractive glossmeter were utilized. Both these optical devices together provide local information on surface roughness and gloss of a tablet over a measured area. The concepts of mean topography and mean gloss profile for surface quality of a tablet are introduced. It was observed that the surface quality of the tablet varies, and compression at high pressure may not guarantee a good surface quality of the tablet. Using novel statistical parameters for gloss and relevant surface roughness parameter, it is possible to get more comprehensive quantitative data on the surface condition of a tablet. THz spectrometer was utilized for detection of THz pulse delay in transmission measurement mode from the tablets. The delay time and thickness ratio of the tablet are consistent with the porosity of the tablet as a function of compression pressure. We suggest that the multimeasurement scheme using three different devices helps tablet makers to better assess bulk and surface quality of their products.

All-semiconductor laser driven terahertz time-domain spectrometer

We present a terahertz time-domain spectrometer pumped by an all-semiconductor femtosecond laser system which emits 600 fs long optical pulses at 830 nm. Standard LT-GaAs antennas are used to generate and coherently detect THz radiation with a bandwidth of 1.4 THz. The measured time traces are in good agreement with simulations based on the Drude–Lorentz model. Spectroscopic measurements on polymers with different additive contents as well as THz imaging were carried out to confirm the successful operation of the spectrometer. Our approach holds great potential for the development of cost-effective, rugged and portable THz spectrometers for a variety of applications.

High-resolution GaP Terahertz Spectrometer and Its Application to Detect Defects in Gamma-irradiated Glucose Crystal

We developed high-resolution GaP THz signal generator using Cr:Forsterite lasers with gratings as both a pump and a signal beam for difference-frequency generation. A line width of less than 500 MHz and a wide tunable frequency range (0.6–6.2 THz) provide sufficient resolution for measuring materials with sharp absorption bands using the generator as the light source for a THz spectrometer. This is suitable for materials such as gases or solid samples at low temperatures. We demonstrated the detection of defects in organic materials, as they appear as slight deviations in the absorption frequency in the THz region.

A semiconductor based pump‐probe system and its application to THz spectroscopy

AbstractIn this work a description of a semiconductor laser based system is presented that can be utilised in many pump‐probe experiments and in particular as the basis for a THz spectrometer. There are two methods described, one which has a stepping time delay function and the other which has a sweep time delay function. These two methods are achieved in the first case by a manipulation of the relative gain switching of two semiconductor lasers and in the second case by a gain switching of two semiconductor lasers with slightly different RF frequencies. The time delay by cross‐correlating the two gain switched laser outputs on a silicon APD via two photon absorption is demonstrated. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Consequence of a Defect on the Terahertz Spectra of l-Asparagine Monohydrate

Abstract We measured the terahertz vibrational spectra of a solid solution of l-asparagine and l-aspartic acid monohydrate at 0.5 to 6.0 THz using a GaP Raman THz spectrometer. The absorption peaks at below 2.5 THz clearly showed intensity changes reflecting the change in hydrogen bonding between molecules. The peaks in the higher frequency region were indirectly affected by intermolecular bonding, which resulted in broadening.

FREQUENCY-TUNABLE TERAHERTZ-WAVE GENERATION FROM GaP USING Cr:FORSTERITE LASERS

We have constructed THz spectrometers using the widely frequency-tunable THz-wave generated from GaP crystal pumped at 1.2 /spl mu/m region using two Cr:forsterite lasers and compared with 1 /spl mu/m using a YAG laser and an optical parametric oscillator (OPO). The systems have sufficient resolution for observation of solids and liquids. We have measured the transmittance spectra of amino acids and saccharide.

Measurements of 14NH 3 in the ν2 = 1 state by a solid-state, photomixing, THz spectrometer, and a simultaneous analysis of the microwave, terahertz, and infrared transitions between the ground and ν2 inversion–rotation levels

In this paper, we report 30 THz measurements of ammonia in its excited, ν 2 = 1, inversion state. These measurements include transition frequencies and pressure shifts (where significant enough to be observed). Included in the data are two forbidden and three ro-inversion transitions, as well as 18 transitions never directly measured before. The measurements were made with an all-solid-state, diode-laser, difference-frequency spectrometer. Using an innovative laser-frequency locking scheme, this spectrometer provided accurately determined and continuously tuned THz-frequencies without requiring accurate knowledge of the absolute laser frequencies. The details of the spectrometer’s frequency calibration is discussed. A global analysis of NH 3 based on the available ground state, ν 2 = 1 state, and ν 2-band transitions was carried out, and the resulting set of recommended molecular effective-Hamiltonian parameters for ammonia is presented. In addition, calculated center frequencies and intensities for all possible transitions up to J = 20 between rotation, inversion, and vibration levels in the ground and ν 2 = 1 states are included as supplementary material.

High-resolution THz spectrometer with kHz scan rates

We demonstrate a rapid scanning high-resolution THz spectrometer capable of acquiring THz field transients with 1 ns duration without mechanical delay line. The THz spectrometer is based on two 1-GHz Ti:sapphire femtosecond lasers which are linked with a fixed repetition rate difference in order to perform high-speed asynchronous optical sampling. One laser drives a high-efficiency large-area GaAs based THz emitter, the other laser is used for electro-optic detection of the emitted THz-field. At a scan rate of 9 kHz a time resolution of 230 fs is accomplished. High-resolution spectra from 50 GHz up to 3 THz are obtained and water absorption lines with a width of 11 GHz are observed. The use of femtosecond lasers with 1 GHz repetition rate is essential to obtain rapid scanning and high time-resolution at the same time.

GaP Raman Terahertz high accuracy spectrometer and its application to detect organic and inorganic crystalline defects.

One of the most important uses of THz spectrometry is to detect defects in molecular structure or in crystals efficiently. We applied GaP Raman THz (GRT) spectrometer to detect and evaluate defects in inorganic and organic materials. High THz-wave absorption due to high defect density of GaSe crystal lowered the efficiency of THz wave generation, when the crystal is used as nonlinear material for DFG (Difference Frequency Generation). Defects in organic molecules could be observed as changes in frequency, intensities of the absorption, and broadenings of the spectra.

GaP THz wave generator and THz spectrometer using Cr:Forsterite lasers

We have developed a type of THz wave generator that uses Cr:Forsterite lasers as the pump and signal sources for difference frequency generation in GaP (Cr:F source system). We confirmed the generation of THz waves in the frequency range from 0.3to7.5THz, which is just similar to that obtained using the THz wave generator previously developed utilizing yttrium aluminum garnet and optical parametric oscillator (OPO) lasers (OPO source system). A peak output power of 100mW was obtained from 1.2to5THz when the power of the two input beams was 3mJ each, similar to the OPO source system. A wide measurable frequency range from below 0.6THz to over 6THz was obtained by using the Cr:F source system as the light source of a spectrometer, which has the merits of simple structure, easy maintenance, and low cost compared with the OPO source system. Although the linewidth of the Cr:F source system is greater than that of the OPO source system, the THz spectrometer still has sufficient resolution for measuring solids or liquids at room temperature.

New p-Ge THz laser spectrometer for the study of solutions: THz absorption spectroscopy of water

We present the development of a high power, tunable far-infrared p-germanium laser spectrometer for the study of dissolved biomolecules in the THz range. As a first application we report on the measurement of the absorption coefficient α for liquid water in the frequency range from 81to96cm−1. Using the p-Ge laser spectrometer we were able to penetrate through layers of up to 100μm thickness. We discuss the advantages and the limitations of this THz spectrometer. We present an analysis of the experimental data based on a χ2 test to provide an objective procedure to minimize the influence of systematic effects, for example of interference due to multiple reflections within the sample chamber. The measured absorption coefficient α lies between (410±6) and (490±6)cm−1 at 81 and 96cm−1, respectively.

A comparative study of THz spectra

We have constructed two types of THz spectrometers using the frequency-tunable THz wave generators based on difference-frequency generation (DFG) in GaP crystal. The OPO-source system is for high accuracy measurement to detect fine structures in THz absorption spectra. The Cr:F-source system has merit in simple structure, easy maintenance and low-cost, but still has sufficient resolution for room temperature observation of solids or liquids. Comparing the absorption spectra of saccharides measured with these spectrometers and those measured with FT-IR or TDS, our spectrometers have superiority in frequency range and resolution, enough to record sub peaks, which can be analyzed as structural defects in organic compounds. (Contributed by Jun-ichi NISHIZAWA, M.J.A.)

Distortion and uncertainty in chirped pulse THz

A mathematical model of the behaviour of chirped pulse THz spectrometers is used to describe the response of the instrument to long and short pulses of high or low intensity. It is found that the output can be ambiguous because similar outputs can arise from dissimilar inputs.

Optimization of photoconducting receivers for THz spectroscopy

We have studied the sensitivity and noise of optically gated dipole receivers made from ion implanted Si and GaAs in an optimized time domain THz spectrometer. The spectrometer uses a room temperature, dc biased, semi-insulating GaAs stripline source capable of generating up to 30 µW average power. The 10% amplitude system bandwidth for 10 µm (50 µm) dipole receivers is 3 THz (1.5 THz). A dynamic range of 4 × 105 Hz−1/2 is achieved using a 10 µm dipole GaAs receiver and 2 × 106 Hz−1/2 using a 50 µm dipole for a total laser power of 110 mW and THz beam power of 20 µW. The dynamic range achieved with comparable silicon receivers is a factor of 2 smaller.

Processing and characterization of high Jc NbN superconducting tunnel junctions for THz analog circuits and RSFQ

A generic NbN Superconducting Tunnel Junctions (STJ) technology has been developed using conventional substrates (Si and SOI-SIMOX) for making THz spectrometers including SIS receivers and RSFQ logic gates. NbN/MgO/NbN junctions with area of 1 /spl mu/m/sup 2/, Jc of 10 kA/cm/sup 2/ and low sub-gap leakage current (Vm>25 mV) are currently obtained from room temperature sputtered multilayers followed by a post-annealing at 250/spl deg/C. Using a thin MgO buffer layer deposited underneath the NbN electrodes, ensures lower NbN surface resistance values (Rs=7 /spl mu//spl Omega/) at 10 GHz and 4 K. Epitaxial NbN [100] films on MgO [100] with high gap frequency (1.4 THz) have also been achieved under the same deposition conditions at room temperature. The NbN SIS has shown good I-V photon induced steps when LO pumped at 300 GHz. We have developed an 8 levels Al/NbN multilayer process for making 1.5 THz SIS mixers (including Al antennas) on Si membranes patterned in SOI-SIMOX. Using the planarization techniques developed at the Si-MOS CEA-LETI Facility, we have also demonstrated on the possibility of extending our NbN technology to high level RSFQ circuit integration with 0.5 /spl mu/m/sup 2/ junction area, made on large area substrates (up to 8 inches).