Far-Infrared and terahertz (THz) radiation is the spectral region at the long wavelength end of the infrared and the short end of the microwave region, also known as the sub-millimeter wavelength spectral region. Recently, there has been an explosion of interest in THz applications as the radiation generated can penetrate various materials. THz light is commonly used in materials science, security, pharmaceutical com- pound analysis, biomedical imaging, superconducting materials, astronomy, and particle physics research.
Sciencetech is a pioneer in THz spectroscopy, having supplied its SPS-200 Far Infrared FTIR for over 10 years to the sub-millimeter wavelength research communities in astronomy and particle physics. Recently Sciencetech has upgraded its SPS-200 into the superior commercial grade SPS-300/SPS-400 THz spectral analyzer and spectrometer. The European Space Agency, which used the SPS-300 to calibrate the detectors for its Planck/Herschel Mission to analyze the origins of the universe, deserve our thanks for their support in making this possible.
The SPS-300/SPS-400 is Sciencetech’s latest modular polarizing Fourier Transform Spectrometer (FTS), designed specifically to operate in the far infrared or THz spectral region (operating at wavelengths from 5μm to 5000μm, 0.06 to 60 THz or 2cm-1 to 2000cm-1). Standard FTS and FTIR systems are based on the Michelson configuration; however, the SPS-300/SPS-400 can also function using the Martin-Puplett configuration, which provides polarization division for a significantly wider band pass when measuring normal transmissions, and has improved signal to noise for circular and linear dichroism measurements.
The modularity of the SPS-300/SPS-400 allows a higher degree of versatility when compared to other similar systems. Our internal source, external input and dual output ports enable the user to configure the SPS-300/SPS-400 as a far-infrared/THz light source spectral analyzer (by attaching an external bolometer detector) or as a materials transmission and reflectance spectrometer (with an external light source, a sample chamber and detector).
The SPS-300/SPS-400 also comes with both a 125mm wire grid polarizing beam splitter and a Mylar beam splitter. The grid polarizer beam splitter allows the user to operate in polarizing Martin-Puplett mode, covering a wider range than the standard Michelson mode which uses the Mylar beam splitter.
Standard Hardware Features
SPS-300 Included Components
Host PC and Software
A PC with flat screen monitor is supplied as the host computer with the SPS-300/SPS-400 control software fully installed and operational. A copy of the software is also provided on CD. The SPS-300/SPS-400 control software is a LabView based application that operates on a Windows PC. An executable version of the software is supplied so the user does not require the LabView development environment. Although Sciencetech only supports the Windows version of the SPS-300/SPS-400 control software, a Mac-OS version is also available through a third party vendor.
Modular LabView VI Structure
Users may add application specific LabView virtual instruments (VI’s) without recompiling the SPS-300/SPS-400 software. This is useful for experiment automation where scanning operation needs to be integrated with external features (sample chambers, light sources, temperature controllers, magnetic field sweeps, etc.)
Main Software Features
•”Step and Scan” operation
•Data acquisition and automatic log-keeper
•Data stored in ASCII format text files for easy importation into other data analysis software
•Multitasking for simultaneous scan and analysis
Data Processing and Manipulation
•Phase correction by Mertz-Forman method
•Apodizations: Bessel, boxcar, cosine, triangular
•Digital filtering of the rapid scan signal
•Add, subtract, normalize, multiply (interfero grams or spectra)
•Average and standard deviation (interferograms or spectra)
•Fitting of data by user-specified functions: functions are analytically specified and new functions are automatically saved on the hard drive for easy retrieval
•Fast Scan Mode: display of interferogram, spectrum and the relative sigma of the spectrum
•Step and Integrate Mode: display of interfero gram and contents of lock-in amplifier buffer
•Display of spectra, interferograms, ratios, fit results and other data
•Print selected data
Scanning and Processing Modes
The SPS-300/SPS-400 generates an interferogram by taking multiple detector readings as a mirror translates across a known distance. In the standard Step and Integrate Mode, the mirror stops at a prescribed position for each detector reading. By stopping, the detector can take long readings for a better signal-to-noise ratio, and the relationship between the detector reading and mirror position is known to a precise degree. The software is capable of simultaneous scanning and data analysis. The SPS-300/SPS- 400 software runs in its own Window and is undisturbed by other Windows applications running in the background. Automated data acquisition can be achieved with simple command scripts, useful for organizing unattended automated measurements.
The SPS-300/SPS-400 is used as a spectral analyzer by mounting an external THz light source at the external input port, and an external THz detector at the condensing beam output port.
Sciencetech provides multiple detector options: a room temperature pyro-electric detector, a room temperature DTGS detector, and a helium cooled bolometer. A user can also choose to attach a third party detector to the output port.
Materials Transmission / Reflectance Spectrometer
The SPS-300/SPS-400 is used as a materials transmission/reflectance spectrometer by mounting an external sample chamber at the collimated beam output port and an external THz detector at the sample chamber detector output port.
For example, the SPS403 sample chamber and detector allows both transmission and reflectance measurements. The source is the broadband Hg arc lamp inside to the SPS-300/SPS-400 body.
As well, other types of broadband THz sources such as a globar or Gunn oscillator can be mated to the external input light port. Please note that the SPS-300/SPS-400 base system does not include these additions.
THz Circular/Linear Dichrosim Spectrometer
In polarizing mode operation, the SPS-300/SPS-400 can also be used for transmission, circular dichroism, and linear dichroism investigations with a good signal-to-noise ratio. The measurement of vibrational circular dichroism (VCD) below ~600 cm-1 is a modern experimental challenge.
Polarization-division interferometry is considered to be the most efficient approach for these measurements. A crucial component for successful applications of this interferometry is the beam splitter that divides the incoming beam according to polarization.
The Sciencetech large area FIR grid polarizers consisting of metallic lines deposited on a mylar substrate are effective beam splitters for the 500cm-1~ 2.0 cm-1 frequency range.
The SPS-300/SPS-400 Spectral Range (both Standard and High Resolution Models) is 2 to 2000cm-1 (5µm to 5mm or 60GHz to 60 THz). Although the SPS-300/SPS-400 optics can cover a spectral range from 2 to 2000cm-1, the practical range is generally limited by the source and detector combination used. For example, the 2cm-1 low wavenumber spectral range cutoff can only be obtained with a high powered source such as that of a synchrotron entering through the external light source port and a 1.7K helium cooled bolometer detector. Using the internal Hg arc lamp provided, the low wavenumber spectral range cutoff is limited to approximately 4cm-1 using the bolometer. This low wavenumber cutoff increases to about 8cm-1 with a 4K helium cooled bolometer and the internal Hg arc lamp.
At the high wavenumber end, the grid spacing of the polarizer is the limiting factor. Since Sciencetech uses a close pitch 4µm grid polarizer, compared to other polarizers of 12.5µm or 25µm pitch, the SPS-300/SPS-400 is capable of reaching 2000cm-1 if the detector and source allow. More commonly the cutoff is closer to 1000cm-1.
Standard: 0.12cm-1 with 5cm translation stage High: 0.020cm-1 with 30cm translation stage
The resolution quoted for the SPS-300/SPS-400 is not the theoretical limit as determined by the length of the roof mirror translating stage, quoted by some manufacturers. Abrupt stopping of data collection at the ends of the interferogram produces oscillations in the Fourier transformed spectrum (that look like “feet” at the sides of strong lines). Sciencetech uses an “apodization” function which multiplies the interferogram by a factor that goes to zero smoothly at the scan ends – but this degrades the resolution somewhat. Although there are several alternatives, we quote the resolution for Bessel apodization, which is degraded by 1.904 from the theoretical limit. In general, resolution is given by:
Resolution = A/(2*2*0.80*L) [cm-1] ,
where L = stage length [cm] , A = Apodization factor,
0.80 is the zero path location factor (zero is 20% from the translation stage end position)
Four different apodizations can be selected on the SPS-300/SPS-400 software, each with its own factor affecting the resolution:
Bessel: A = 1.904 Boxcar: A = 1.207 Cosine: A = 2.000 Triangular:A = 1.772
The recommended apodization for the very far infrared and THz domains is the Bessel function.
Large 100mm beam optics coated for maximum infrared reflection, Fast f/2.35 off-axis paraboloid condensers for high light throughput, 90°±3” roof mirrors, Martin-Puplett optical configuration.
Stainless steel housing capable of maintaining 10-3 Torr.
Internal 75W Hg arc lamp (water cooled) with external DC stabilized power supply and igniter.
Motorized alignment for roof mirror (tilt, rotary shear, lateral shear) to fine tune interference at the beam splitter.
Polarizer Beam Splitter:
2μm lines with 4μm pitch aluminum on Mylar substrate.