1. Introduction

Terahertz (THz) electromagnetic radiations are sandwiched between the microwave and the infrared, bridging the gap between electronics and optics. The typical THz frequency ranges from 0.1 THz to 10 THz, and the corresponding wavelength is from 30 μm to 3 mm [1, 2]. Due to its unique properties, THz has been finding invaluable potential in use of information and communications technology, astronomy, and homeland security [3, 4]. THz also provides powerful insight into material properties of biology specimens, chemistry drugs, and explosive goods that cannot be accessed otherwise. Despite it had been emerged for more than 3 decades and its radiometry had received much concerns; however, traceability of THz measurements was not available at all until the scientists in the Physikalisch-Technische Bundesanstalt (PTB) performed a traceable measurement of a THz radiometer against a cryogenic radiometer (CR) in 2009 [5].

The wavelength in THz region is hundreds of times larger than that of visible light. Almost all of the absorptive materials in visible light and infrared, such as graphite, gold black, black silicon, and 3M Velvet-coating, show a dramatically high reflectance in THz region [6], because they look as smooth as a mirror in THz wavelength. No suitable materials can be used as an effective absorber in THz radiometers, which makes THz radiometry challenging. The total measurement uncertainty of 7.3% in PTB comes mainly from the uncertainty of cavity absorptance of the CR in THz region [5, 7].

The scientists in the National Institute of Standards and Technology (NIST) also paid attention to THz radiometry. Lehman et al reported a vertically aligned carbon nanotube array (VANTA) as the absorber in THz radiometer in 2011 [8]. By increasing tube length, they found the reflectance decreases substantially. A specular reflectance of approximately 1% in 0.76 THz was observed for the 1.5 mm long VANTA [8]. However, their measurement only gave a single THz frequency of 0.76 THz. The THz radiometry in a boardband range other than 0.76 THz is still kept unsolved. Furthermore, the radiometers with VANTA as absorber have a long heat balance period, and thus part of the absorbed heat may be lost by re-emission during the balance period. Fabrication of VANTA demands a complex substrate preparation procedure and a precise control of time, temperature and air flow [8], which makes it difficultly accessible. Therefore, an easily fabricated, broadband, and high-absorbance absorber for terahertz radiometry is desired.

In this paper, we report a simple coating for THz radiometry. It has a broader bandwidth and a higher absorptance than VANTA. This coating lowers the demands of the tradeoff between the far infrared detector’s response time and the coating thickness [8]. An effective absorber with both short response time and thin coating thickness is obtained.

2. Reflection-type THz spectrometer for reflectance measurements

Terahertz time-domain spectroscopy (THz-TDS) [9] is a spectroscopic technique in which the spectral properties of a material are probed with ultrashort THz electric-field pulses. The spectrum and spectral phase are obtained with Fourier transform of the probed THz pulse waveforms. And then the sample material’s optical properties are deduced from the comparison of the spectra and phases of the input pulse and the propagated pulse. THz-TDS provides more information than conventional Fourier-transform spectroscopy, which is only sensitive to the amplitude [10]. For characterization of sample reflectance, we home made a reflection-type THz time-domain spectrometer. The schematic diagram is shown in Fig. 1.

 

Fig. 1 Schematic diagram of reflection-type THz spectrometer. HWP: Half-wave plate; PBS: Polarizing beam splitter; PCA: Photoconductive antenna; THz BS: THz beam splitter; ZnTe: zinc telluride crystal; QWP: Quarter-wave plate; HDPE lens: high density polyethylene lens.

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We use a femtosecond laser (Micra-5, Coherent) as a pump and detection source. The pulse width is 18 fs after a pulse compressor [11]. The pulse is split into two paths, one is used to pump a photoconductive antenna (PCA) to generate THz radiations, and the other is used to detect the THz amplitude with electro-optic sampling in a zinc telluride crystal (ZnTe). The THz electric filed changes the polarization characteristics of ZnTe, and then is detected with a balanced detector. The amplitude is read via a lock-in amplifier. By scanning the optical delay line in the detection path, a THz time-domain waveform is generated. The spectrum and spectral phase are obtained with Fourier transform technique.

We use a polished aluminum (Al) mirror as reference. The reflectance of Al is reported to be between 99.59% and 99.54% from 1.5 THz to 2.0 THz [12]. Considering the reflectance increases with the decrease of the frequency, we assumed the reflectance to be 99.60% from 0.1 THz to 2.0 THz. The spectral reflectance is obtained through the comparison of the detected THz pulse reflected from a sample with that reflected from Al. In this setup, vertical spectral reflectance can be measured. The diameter of the high density polyethylene (HDPE) lens is 50 mm and its focus length is 76.2 mm; therefore, the reflective radiations within a solid angle of 0.32 sr are collected.

Figure 2 shows a measured THz waveform reflected from an Al mirror as a reference and its Fourier-transformed spectrum. Figure 2(a) is the measured referred THz waveform. We perform Fourier transform on the measured THz waveform in Fig. 2(a), and then both the amplitude and the phase are gotten. Here, we merely use the amplitude of the spectrum, which is shown in Fig. 2(b). The scanning time-domain length is 70 ps, and the frequency resolution is 14 GHz. Figure 2 also shows a noise waveform and its Fourier-transform spectrum. From Fig. 2(b), we can see that this THz spectrometer has a desirable dynamic range from 0.1 THz to 2.0 THz. In the frequency beyond 2.0 THz, the referred spectrum approaches noise floor and there are some absorption peaks of the ZnTe crystal and the remanent water vapor. Therefore, we focused the measurement range on from 0.1 THz to 2.0 THz.

 

Fig. 2 Measured referred THz waveform and its Fourier-transformed spectrum. (a) Measured referred THz waveform and noise waveform, (b) referred THz spectrum and noise spectrum obtained with Fourier transform.

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With the THz spectrometer, we have measured some visible light absorptive materials, including graphite, home-made graphite paste, silicon carbide (SiC), and 3M Velvet-coating. The measured spectral reflectances of these materials are shown in Fig. 3.

 

Fig. 3 Measured THz reflectances of graphite, home-made graphite paste, SiC, and 3M Velvet-coating.

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Figure 3 demonstrated that most of the visible light and infrared absorptive materials, such as graphite, graphite paste, and 3M Velvet-coating, show dramatically high reflectances in THz region. The reflectance of graphite is more than 50% in spectral range from 0.1 THz to 2.0 THz. In comparison with graphite, the reflectances of SiC and 3M Velvet-coating are relatively low, the reflectances of which are nearly 20%.

The reflecting layer model indicates that the reflectance is related to the roughness of the upper surface and the inlet radiation wavelength. The specular Fresnel reflectance of the upper surface will be reduced an order of magnitude by exponential scattering loss when σ/λ > 0.12, where σ is the root mean square (RMS) roughness of the upper surface, and λ is the inlet wavelength [6]. Therefore, the reflectance may be greatly reduced by enlarging the roughness of the upper surface.

3. Coatings preparation and characterization

To enlarge the roughness of the absorptive sample, we mixed some SiC particles into the 3M Velvet-coating paint, and sprayed them on polished Al mirrors. The reason why we selected SiC and 3M Velvet-coating as two principal components of the mixture is that they have relative high absorption in THz region (see Fig. 3). The ratio of SiC particles and 3M Velvet-coating is approximately 1:5 by volume. Enlarging the ratio of SiC particles may produce more sediment, and reducing the ratio is not good for improving the roughness. Three scales of SiC particles are chosen, which are 150 μm, 200 μm and 300 μm, respectively. The thicknesses of the coatings for the three particles are 600 μm, 800 μm and 1.2 mm, respectively. We measured the spectral reflectance of the sprayed mixture coatings with the THz spectrometer (shown in Fig. 1). The measured spectral reflectances are shown in Fig. 4.

 

Fig. 4 Measured spectral reflectances of VANTA and three scales of SiC particles mixed coatings (plotted on logarithmic scale).

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For comparison, we have also measured the spectral reflectance of a 1.5 mm long VANTA (Beijing DK nano technology Co., LTD), and superimposed it on Fig. 4 together with the measured spectral reflectances of the three coatings. From Fig. 4, we can see that for the 150 μm SiC particles coating, the spectral reflectance is comparable to that of 1.5 mm long VANTA. However, the thickness of 150 μm SiC particles coating is 2.5 times thinner than that of VANTA. For the 200 μm SiC particles coating and 300 μm SiC particles coating, the reflectances are much lower than that of VANTA, especially in the frequency range below 0.75 THz. For the 300 μm SiC particles coating, the spectral reflectance is an order of magnitude lower than that of VANTA at frequency ranging from 0.05 THz to 0.95 THz. The measured reflectance is less than 0.3% ranging from 0.2 THz to 0.5 THz and less than 0.1% ranging from 0.5 THz to 2.0 THz for the 300 μm SiC particles mixed coating.

Figure 4 also demonstrated that there is a cutoff frequency of the spectral reflectance for this kind of coatings, and it decreases with the increase of the particles scale. The cutoff frequencies for 150 μm, 200 μm and 300 μm SiC particles mixture coatings are 0.45 THz, 0.25 THz and 0.05 THz, respectively. The spectral reflectance goes up dramatically in the frequency lower than the cutoff frequency and goes down sharply in the frequency higher than the cutoff frequency.

We define the ratio of noise spectrum to referred spectrum as noise to signal ratio (NSR), and also plotted it on Fig. 4. The NSR goes up in high frequency region; therefore, all the reflectance curves go up correspondingly. This does not mean that the reflectances become corrupt in high frequency region. The measured spectral reflectance cannot be lower than NSR; therefore, NSR is a limit of reflectance measurement.

These coatings were sprayed on the Al mirrors; therefore, all of the THz radiations were reflected back or were diffused, and there were no transmitted radiations. The THz spectrometer collected a solid angle of 0.32 sr reflective radiations. Neglecting the diffuse radiations larger than 0.32 sr solid angle, the measured absorptances of these coatings are higher than 99% in a broad THz frequency range.

4. THz radiometer and heat-transfer analysis

Based on the reflectances measurement in Fig. 4, we assembled a THz radiometer with the 300 μm SiC particles mixed coating as absorber. The thickness of the coatings is 1.2 mm. The schematic of the detector was shown in Fig. 5. We used a thermopile as sensor, and glued its one ceramic surface on a heat sink. The mixed coating was sprayed on the other ceramic surface of the thermopile. The outlet voltage was read via a nanovolt meter (34420A, Agilent). We used a dome, which is made of polished Al, covered in front of the detector to collect the diffuse radiations. The diffuse radiations and thermal re-emission can be reflected back to the absorptive coating so as to improve the cavity absorptance. The inner diameter of the dome is 50 mm and its opening size is 15 mm.

 

Fig. 5 Schematic of THz radiometer.

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With the THz radiometer, the radiation from a back-ward wave oscillator (BWO) (SMW-81, ELVA-1) was characterized. The radiant frequency is tunable from 0.64 THz to 0.87 THz, and the maximum radiant power is 5 mW. We operated the BWO at 0.775 THz radiation, and applied a 10 mm diameter aperture to limit the beam size. No noticeable responsivity difference of the radiometer with and without the dome was observed. This verified the high-absorbance of this coating.

We also characterized the absorptance of the mixed coatings at 632.8 nm with an integrating sphere [13]. The measurement results showed that the absorptance of the mixed coating with the 300 μm SiC particles is 99.02% at 632.8 nm. Therefore, we performed a traceable measurement of the power responsivity to Chinese National Laser Prime Standard with a He-Ne laser operating at 632.8 nm wavelength. The power responsivity is 172 μV/mW.

We investigated the heat balance period of the THz radiometer with the 1.2 mm thick 300 μm SiC particles coating as absorber and that of a radiometer with a 1.2 mm long VANTA as absorber. The 1.2 mm long VANTA was also glued on a thermopile. The schematics of the two radiometers are shown in Fig. 6. The heat balance period (from 10% to 90% of the maximum) of the 1.2 mm long VANTA radiometer is 130 s, while that of the 1.2 mm thick SiC coating radiometer is 70 s. The reason for the difference of heat balance is in that the different structures of the two absorbers. Figure 6 shows the schematics of heat transfer in VANTA absorber and that in SiC particles coating absorber.

 

Fig. 6 Schematics of heat transfer in VANTA absorber and in SiC particles coating absorber of THz radiometer. (a) In VANTA absorber, (b) in SiC particles coating absorber.

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VANTA is an aggregation of a great number of carbon hollow tubes, and the gaps are full filled with air. One part of heat is conducted through the thin-walls of the carbon nanotubes (CNTs), and the others are lost by convection or by radiation. The heated air brings a part of heat when blowing from the gaps. And each of the thin, long and hollow tube re-emits heat when being radiated.

Heat transfer in SiC particles coating also contains conduction, convection and radiation. However, the coating consists of solid SiC particles, and the gaps are filled with 3M Velvet-coating paint; therefore, the thermal conductivity is higher than VANTA, and heat balance can be reached during a short time. The contact area of absorber with air is less than VANTA, and the lost heat is less correspondingly.

5. Conclusion

We developed and characterized a simple and high-absorbance coating for THz radiometry. This coating has high-absorbance in a board bandwidth of THz region. The measured spectral reflectance is less than 0.3% ranging from 0.2 THz to 0.5 THz and less than 0.1% ranging from 0.5 THz to 2.0 THz. The absorptive frequency range can be expanded by enlarging the scale of the mixed SiC particles. This broadband high-absorbance is helpful in linking different wavebands in THz absolute radiometry. This coating is easily fabricated. It can be sprayed in cavity shape and on a great majority of substrates. In comparison with VANTA, this coating has shorter responsivity time and thinner coating thickness; therefore, thermal re-emission is reduced and measurement period is shortened. This coating has high-absorbance both in THz region and in visible light; therefore, the power responsibility can be easily traceable to National Laser Power Standards. The applications of this coating for THz radiometry are helpful in filling the gap of THz metrology.