Terahertz-wave generation for industrial applications

Terahertz (THz) waves can pass through many packaging materials, enabling nondestructive and noninvasive inspection solutions in a range of industries.¹ We have been conducting a variety of research activities in this active field, including time-of-flight THz reflection tomography of multilayered thin coatings and metal-mesh sensor development to measure thin-film thicknesses or molecular bindings.

Since subpicosecond pulses can be measured directly with THz time-domain spectroscopy, a sample's multilayered structure may be mapped based on pulse echoes reflected by the individual layers.² This technique could be valuable in industrial settings because the unique transmission characteristics of THz waves enable inspection of multilayered paints or tablet coatings^3,4 that cannot be measured using optical coherence tomography.⁵ Instead, the common ~10μm thickness of paints in various industrial products requires development of higher-resolution THz tomography. One of the simplest and most effective ways to improve the axial tomographic-imaging resolution involves generating short broadband THz pulses using shorter optical pulses.

Practicality is also a critical issue. Most THz-tomography systems developed to date use titanium (Ti):sapphire lasers as pump light sources because they meet stringent specifications and are suitable for generating and detecting these pulses. However, for realistic applications, a more robust system that counteracts vibrations and temperature variations is preferable. Ultrashort-pulse fiber lasers would be strong candidates, except for the generally held belief that generating broadband ultrashort THz pulses with such devices would be difficult because their pulse width is greater than that of Ti:sapphire lasers. However, we have developed an all-fiber laser system that produces 17fs optical pulses at a wavelength of 1.56μm, and we managed to generate and detect broadband pulses from 0.1 to 27THz using DAST (4-dimethylamino-N-methyl-4-stilbazolium tosylate) as our nonlinear organic crystal.⁶ The resulting axial resolution is sufficient to measure 5μm-thick Teflon^® film. Figure 1 shows the reflection tomography for multilayered thin car-paint coatings.⁷

Figure 1. Time-of-flight terahertz (THz) reflection tomography of multilayered thin car paints. a.u.: Arbitrary units.

Photonic crystals (or metamaterials) have attracted a great deal of attention in recent years.⁸ They will likely be applied in many future optical devices because of their straightforward handling of electromagnetic waves. We proposed a sensing and imaging method using 2D photonic-crystal ‘metallic-mesh’ films in the THz regime.⁹ Their transmission spectrum is sensitively affected by the refractive index of the material inside and above the metallic-mesh openings. Using this to our advantage, we applied metallic-mesh sensors to detect small amounts of proteins as well as for label-free detection of antigen-antibody reactions.¹⁰ We built an experimental system to measure transmission spectra and capture 2D images using a THz-wave source that is tunable from 0.9 to 1.1THz. We placed samples in the focal region to measure their transmission properties. We also obtained 2D images of the transmittance by scanning a given sample using an X-Y stage. (The grid constant of our free-standing metallic mesh was 254μm.) We successfully detected 3.5μm-thick ultrathin polyethylene terephthalate (PET) films using this method (see Figure 2) by focusing on the dip in the metallic-mesh transmission spectrum (which is caused by a sudden change in transmittance).^11,12 We also detected differences of 1μm in ultrathin PET films based on 2D transmittance images. These thickness differences appeared as changes in the dip shifts of the metallic-mesh transmission properties. The films were held tightly to the metal surface by applying vacuum pressure from the rear. We normalized the transmittance to that of the metallic mesh without any film. Although they were placed on the same metallic mesh and, hence, should have behaved exactly the same, differences in transmittance appeared in the Y direction because the films did not fully adhere to the metallic-mesh surface.

Figure 2. Measurements of polyethylene terephthalate film thickness (from left to right: 3.5, 4.5, and 5.7μm) using a metal-mesh sensor.

We will continue our applied research in the THz regime, which also includes computed tomography for inspection of the soot distribution in ceramic filters,¹³ spectroscopic imaging for detection of illicit drugs in envelopes,^14,15 and development of a laser THz-emission microscope for semiconductor-device inspection.¹⁶ Although we still have a long way to go, applications of THz waves have started to make noticeable progress. Research into a large range of other applications for industrial quality testing is underway in many research groups around the world, and is already attracting much interest.