THz Materials
Terahertz (THz) radiation is located in the spectral region ~0.1-10 THz (~3 mm - 30 μm, 3 cm-1 - 300 cm-1) between the microwave and mid infrared range of the electromagnetic spectrum.
In comparison with visible or infrared waves, THz radiation can penetrate into organic materials such as skin, plastics, cloth, or paper products. Because of low photon energy involved, it does not cause any damage associated with ionizing radiation ( e.g. X-rays). THz waves do not penetrate into metals. These properties can be used in process (e.g. drugs manufacturing) and quality control as well as in THz imaging. It is also of great current interest for such applications as safety control, packaging inspection, semiconductor characterization, chemical composition analysis, and biomedical investigations, with great promise for spectroscopy, defense imaging, and security applications.
Traditionally for THz applications we use High Resistivity Float Zone Silicon (HRFZ-Si) as it is the most investigated substance for operating within this range and has a good transmission performance. In parallel with this material we have been investigating other materials which also can be utilized in THz range.
Below you can see transmission spectra and other characteristics of materials we use for THz optics production. Measurements in THz region were made at ABB FTIR spectrometer Bomem DA3 and Bruker IFS 125HR (measure of inaccuracy is 2-3% below 100 µm and 4-5% over 100 µm). Measurements in near infrared range were made at Perkin Elmer “Lambda- 9” (measure of inaccuracy < 0.5%).
Polymers
Among large variety of available polymers there are some of excellent terahertz transparencies with relatively low reflectivity. The best materials in this sense are TPX (polymethylpentene), polyethylene (PE), polypropylene (PP), and polytetrafluoroethylene (PTFE or Teflon). At longer wavelengths, the transmission of these polymers is structureless and flat. Going to shorter wavelengths, mainly below 200 µm, characteristic bands of intrinsic vibrations appear and scattering due to inhomogeneities increases. Polymers generally become increasingly opaque at shorter wavelengths.
1 Polymethylpentene (TPX)
TPX is the lightest of all known polymers. It is optically transparent in UV, visible, and THz ranges, what for example allows using a HeNe laser beam for alignment. Index of refraction is ~1.46 and is relatively independent on wavelength:
λ, µm n 0.633 1.463 24 1.4568 60 1.4559 300 1.46 667 1.46 1000 1.4650 3191 1.466
Losses are very low up to mm-wavelengths. TPX has excellent heat resistance and is highly resistant to most organic and inorganic commercial chemicals.
Fig. 6 Transmission of TPX 2 mm-thick sample. THz region.
Fig. 7 Transmission of TPX 2 mm-thick sample. NIR&MIR regions.
Fig. 8 Transmission of TPX 2 mm-thick sample. UV&VIS&NIR ranges.
Typical properties of TPX
Density, g/cm3 0.83 Tensile strength 4100 psi ~28.3 MPaTensile modulus 280000 psi ~1930.5 MPa Tensile elongation at break, % 10 Flexural strength 6100 psi 42.1 MPaFlexural modulus 210000 psi 1447.8 MPa Heat deflection temperature, °C 100 Melting temperature, °F/°C 464/240 Water absorption (ASTM-D 1228), % <0.01 Moisture permeability (thk 25 µm, 40C, 90%RH), g/m2*24h 110 Oxygen permeability (thk 100 µm), cm3/m2*d*MPa 120000
TPX is a hard solid material which can be mechanically shaped into various optical components like lenses and windows. Also specifically TPX is used in CO2 laser pumped molecular lasers as output window because it is transparent in the whole terahertz range and totally suppresses the ~10 µm pump radiation. Also TPX windows are useed in cryostats as "cold" windows. The THz transparency of TPX does not change in dependence on temperature. Temperature coefficient of refractive index is 3.0*10-4 K-1 (for the range 8-120 K).
Fig. 9 Temperature dependence of refractive index.(***)
In comparison with other materials being used for operating in THz range TPX shows excellent optical properties and for example can be good substitution for Picarin (Tsurupica) lenses. In addition TPX is cheaper and commercially available in opposite to Picarin.
Fig. 10 Transmission of 2 mm-thick samples of TPX, Picarin, and HDPE.
2 Polyethylene (PE)
PE is light elastic crystallizing material. It can be heated up to 110°C and cooled down to -45 ÷ -120°C depending on grade. PE has good dielectric characteristics, chemical resistance, and radioresistance. Contrariwise, it is unstable to UV-radiation, fats, and oils. PE is biologically inert, is easy to be processed. Density (23°C) is 0.91-0.925 g/cm3. Tensile flow limit (23°C) is 8-13 MPa. Modulus of elasticity (23°C) is 118 - 350 MPa. Refractive index is ~1.54 and is rather equal within wide wavelength region. Usually high-density polyethylene (HDPE) is used for component's production. Besides quite thick lenses and windows, thin HDPE films are used for THz polarizers. In addition, we use HDPE as the window for Golay cells.
Fig. 11 Transmission of 2 mm-thick HDPE sample. THz region.
Fig. 12 Transmission of 2 mm-thick HDPE sample. NIR&MIR region.Fig. 13 Transmission of 2 mm-thick HDPE sample. VIS&NIR region.
Unfortunately, HDPE transmission in visible region is very poor, thus it can't be used for adjustment of optical systems.
Should notice that THz transmission of HDPE doesn't depend on temperature that allows using HDPE windows in cryostats. Temperature coefficient of refractive index is 6.2*10-4 K-1 (for the range 8-120 K).
Fig. 14 Temperature dependence of refractive index (***)
3 Polytetrafluoroethylene (PTFE, Teflon, in Russian - Ftoroplast)
PTFE is a white solid at room temperature, with a density of about 2.2 g/cm3. Its melting point is 327°C, though its properties remain at a useful level over a wide temperature range of -73°C to 204°C. Refractive index is ~1.43 within wide wavelength region.
Fig. 15 Transmission of PTFE film ~0.1 mm-thick. THz region.Fig. 16 Transmission of PTFE film ~0.1 mm-thick. NIR&MIR region.
Due to good transmissionin the range 1-7 µm PTFE films are used for manufacturing of IR polarizers. First cost of such polarizers is lower than for crystalline ones. It is advantageous for their mass application in IR sensors using polarized radiation.
Typical properties of PTFE
Tensile strength 3900 psi ~26.7 MPa Tensile modulus (psi) 80000 psi ~551.6 MPaTensile elongation at break (%) 300 Flexural strength (psi) No break Flexural modulus (psi) 72000 Compressive strength (psi) 3500 Compressive modulus (psi) 70000