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%).
Crystals
The crystals such as silicon, crystal quartz, and sapphire are important for THz optics production.
1 High Resistivity Float Zone Silicon (HRFZ-Si)
Besides synthetic diamond high resistivity sillicon is the only isotropic crystalline material suitable for the extremely wide range from NIR (1.2 µm) to MM (1000 µm) waves and more. In comparison with diamond it is rather cheaper to grow and machine. Moreover it may have considerably bigger dimensions that allows manufacturing the elements of fast-developing THz electronics based on that. For THz applications we offer High Resistivity Float Zone Silicon (HRFZ-Si) maintaining 50-54% transmission to 1000 µm (and for longer wavelengths up to 3000 and even to 8000 microns).
Fig.1 Transmission and reflection of HRFZ-Si 5.0 mm-thick sample in THz range.
HRFZ-Si has low losses in THz range. As follows from Fig. 2 the THz waveform of HRFZ-Si is similar to the THz waveform of air. That indicates the lack of HRFZ-Si absorption.
Fig.2 The THz signals transmitted through air and HRFZ-Si.(*)
The complex dielectric permittivity of silicon depends on its conductivity (i.e. free-carrier concentration). Figure 3 shows the dielectric permittivity of silicon at 1 THz with different impurity concentration. For low impurity concentration the dielectric permittivity is almost a real value, which is approximately equal to the high frequency dielectric permittivity. As a level of impurity concentration increases the real part of the dielectric constant becomes a negative value and its imaginary part can't be considered negligible anymore. The dielectric permittivity presents its complex nature and silicon becomes lost to THz wave. Loss tangent can be calculated using the following formula: tanδ=1/(ω*εv*ε0*R), where ω - circular frequency, εv - dielectric constant of vacuum (8.85*10-12 F/m), ε0 - dielectric constant of silicon (11.67), and R - specific resistance. For example, loss tangent of HRFZ-Si with resistivity 10 kOhm*cm at 1 THz is 1.54*10-5.
Fig.3 Real (solid, ε1) and imaginary (dashed, ε2) part of dielectric permittivity of n-type silicon with different impurity concentration at 1 THz.(**)
More about general characteristics of Silicon as well as transmission spectrum within NIR and MIR range can be found in the chapter Silicon.
2 Crystal quartz
One of the best materials for wavelengths above 50 µm is z-cut crystal quartz. It is important that z-cut crystal quartz windows are transparent in the visible range allowing easy adjustment with HeNe laser, do not change the state of light polarization, and can be cooled down below the λ-point of liquid helium.
Fig.4 Transmission and reflection of crystal quartz 1.0 mm-thick sample.
Due to quite big dispersion (please see the table below) lenses made of crystal quartz will have different focal lengths at visible and far infrared ranges. It should be taken into account if you are going to use such lenses for optical systems alignment:
λ,µm no ne 0.589 1.544 1.553 6.0 1.32 1.33 10.0 2.663 2.571 30.0 2.5 2.959 100.0 2.132 2.176 200.0 2.117 2.159 333.3 2.113 2.156
Crystal quartz is birefringent material that should be noted if the polarization of radiation is important. We use x-cut material to produce λ/2 and λ/4 waveplates for usage at THz wavelengths.
More about general properties of crystal quartz as well as transmission spectrum within UV and visible range, you can find at chapter Synthetic Crystal Quartz.
3 Sapphire
Sapphire like crystalline quartz is transparent in THz region as well as in visible one. Samples of various crystallographic orientations and thickness were measured. As can be seen from below spectra transmission doesn't depend on crystal orientation within measure of inaccuracy. For measured samples with thicknesses from 1 to 5 mm transmission lower 600 µm strongly depends on sample thickness. The transmission approaches to saturation at shorter wavelengths for thinner samples.
Fig. 5 Transmission and reflection of sapphire samples with different thickness.
Like HRFZ-Silicon, sapphire also can be used for manufacturing of photoconductive antennas for THz because of similar refractive index value in THz.