Terahertz (THz) waves have been exploited for the non-contact measurements of thickness and refractive index, which has enormous industrial applicability. In this work, we demonstrate a 1.3-μm dual-mode laser (DML)-based continuous-wave THz system for the real-time measurement of a commercial indium-tin-oxide (ITO)-coated glass. The system is compact, cost-effective, and capable of performing broadband measurement within a second at the setting resolution of 1 GHz. The thickness of the glass and the sheet conductivity of the ITO film were successfully measured, and the measurements agree well with those of broadband pulse-based time domain spectroscopy and Hall measurement results.
© 2014 Optical Society of America
Terahertz time-domain spectroscopy (THz-TDS) has demonstrated various possibilities of THz waves as a non-contact diagnostic tool . Tomographic inspection techniques have been developed based on time-of-flight analysis [2–6], and the electrical properties of conductive thin films such as indium-tin oxide [7,8] and highly-doped semiconductor layers  have been measured as well. However, in spite of the usefulness and uniqueness of THz-TDS, its widespread industrial application has been hampered due to the price, size, and measurement time requirements.
Recently, photomixing techniques [10,11] have been adopted for continuous-wave (CW) THz spectroscopy to realize cost-effective and compact THz systems for industrial applications . Central to the technique are two frequency-offset continuous-wave lasers that are used to generate optical beating at THz frequencies [10–14]. By using semiconductor lasers, the system cost can be significantly reduced, and further improvement has been achieved by using dual-mode lasers (DMLs) [15,16]. The DMLs consist of two distributed feedback laser diodes (DFB LDs) of different emission wavelengths, sharing one optical cavity in common. The emission wavelengths of the DFB LDs are independently controlled by two micro-heaters monolithically integrated into each of the DFB laser diodes. This single-cavity design ensures co-polarized and collinear dual-mode emission, significantly simplifies the optical alignment, and reduces the number of required components. Moreover, the micro-heaters permit fast frequency tuning, leading to a cost-effective real-time system. Combined with the homodyne-detection method, a compact CW THz spectrometer has been implemented and demonstrated .
Relying on the homodyne technique, the CW THz spectroscopy provides broadband phase and amplitude spectral data from THz waves transmitted through the sample. This permits extraction of the complex refractive index of the sample under investigation. However, due to the 2π phase ambiguity, the thickness measurement is not as straightforward as the time-of-flight analysis that is typically adopted in the pulse-based THz-TDS system. To remove the phase ambiguity, time-domain measurements were performed at several THz frequencies [18–20]. However, this requires multiple scans of the mechanical delay line, diminishing the usefulness of this approach. Alternatively, a delay-fixed frequency scan generates an oscillatory output current, the phase analysis of which yields the sample thickness  and index , independently. In this technique, the frequency scanning speed determines the total measurement time, which is important for real-time applications. In addition, phase retardations due to the system should also be considered.
In this work, we implemented a cost-effective and compact real-time CW THz system based on a 1.3-μm DML. To demonstrate its applicability, we measured a commercial indium-tin-oxide (ITO)-coated glass for a plasma display panel (PDP). The thickness of the glass substrate and the sheet conductivity of the ITO layer were measured within one second, and the results are in good agreement with the conventional THz-TDS and Hall measurements. Our THz system can be applied to various kinds of industrial thin-films such as highly-doped semiconductor layers, copper indium gallium selenide (CIGS), conductive polymers, and ink-jet-printed conductive films.
2. Experimental setup and theoretical analysis
For a stable system operation, the linearity of frequency tuning and the spectral purity of the THz emission are critical. In the DML-based THz system [15–17], the THz emission frequency is continuously tuned by two monolithically integrated μ-heaters, one on each of the distributed feedback laser sections. We measured the linearity of mode beat frequency tuning as a function of the dissipated power at the μ-heaters, as shown in Fig. 1 (a).
To measure the DML mode beat frequency, the DML laser output was mixed with a commercial tunable laser to generate heterodyne down-converted beating signal. The down-converted beat signal is detected by a high-speed photodiode which is connected to an electrical spectrum analyzer. The mode beat frequency linearly depends on the dissipated power, as shown by the linear fitting to the experimental results (blue lines in Fig. 1(a)).
The linewidth of CW THz wave is important as well. It has been measured by heterodyne mixing with stable references such as gas laser  or frequency comb . In the DML, strong four wave mixing signals (FWM) are observed through entire tuning range, which suggests low phase noise and strong mode correlation between the two laser modes. Thus, the presence of FWM signal implies a narrow spectral linewidth of CW THz radiation generated by the photomixing technique . Furthermore, we measured the linewidth of the 1.3-μm DML  to roughly estimate the spectral purity of the CW THz radiation. A self-homodyne method with an optical delay of 25 μs  was adopted while maintaining the dual-mode operation. The optical linewidth was measured to be less than 10 MHz. The setting resolution and frequency stability of the DML are also measured by using the heterodyne beating signal with external reference laser as shown in Fig. 1(b). The setting resolution was below 80 MHz for a current variation of 5 μA in the high operating current regime. Furthermore, we did not observe any thermal drift of beating signal. The frequency tuning speed of DML is measured to be 3ms/100 GHz . The schematic and the delay-fixed broadband frequency-scan results of the DML-based CW THz system are shown in Fig. 2(a) and (b), respectively.
The 1.3-μm DML generates the optical beat signal in a single chip, which is the key to the low-cost compact THz emitter. The currents applied to the μ-heaters are represented by I1 and I2 in Fig. 2 (a). The optical beat signal from the DML is split with a fiber-optic 50:50 coupler, and the resulting two signals are sent to two separate photomixers based on In0.53Ga0.47As grown at low temperature, for the homodyne generation and detection of the CW THz waves. A three-turn log-spiral antenna was integrated with the photomixers, and the bias of the THz generating photomixer is modulated for lock-in detection. A mechanical delay line was not used, but the optical path difference was carefully adjusted for the measurements. An exemplary frequency-scanning movie file is included as supplemental data (Fig. 2(c)).
In the homodyne CW THz system, the detected current is proportional to the cosine of the phase difference between the THz wave (ETHz) and the optical beating signal (Eopt) incident on the receiving photomixer. ETHz and Eopt are expressed by [14, 25]:Eq. (1), the spectral response of the CW THz system is written as A(ω)eiφ(ω), which collectively includes the effects of antenna-integrated photomixers [26–28], optics including high-resistivity silicon lenses, and alignment accuracy. The rest of the terms in Eq. (1) describe the transmission of the sample including the effects of multiple reflections. The output current from the detector is a function of the phase difference between ETHz and Eopt. Neglecting the effects of the multiple reflections within the sample, the phase difference δ is expressed as:
Therefore, the output current oscillates when the path difference (ΔL) or the THz frequency is scanned. By scanning ΔL, the time-domain waveform is measured, and in this case, the 2π phase shift corresponds to the wavelength. More importantly, δ also depends linearly on the THz frequency, where the slope of this linear relationship is a function of ΔL and dsample, neglecting φ(ω). Therefore, scanning the frequency at a fixed path difference also produces an oscillatory output (Fig. 2(b)).
To estimate the speed of the THz system, we scanned the THz frequency by scanning I1 and I2 without a sample. The path difference was set to be about 8.5 mm, and the lock-in amplifier time constant was decreased as the scanning speed was progressively increased. The results are shown in Fig. 2(b) as a function of the time constant, which is set to be equal to the averaging time per point. The frequency difference between each point was set to be 1 GHz. At a time constant of 0.1 ms, a broadband spectral range of 0.8 THz was measured within 3 seconds without significant phase retardation or amplitude reduction compared to a time constant of 300 ms, as shown in Fig. 2(b). This indicates that the thermal tuning speed is fast enough for many practical applications. Although the signal-to-noise ratio deteriorates as the scanning speed is increased, it can be improved by using a high-efficiency THz photomixer [12,29,30].
From the frequency-scan result, the slope S = Δδ/Δω is deduced from the zero-crossing frequencies. By extracting the slope of the phase with (Ssample) and without (Sair) the sample, the thickness of the sample is obtained as:Eq. (4). However, to ensure the accuracy of the thickness measurement, we numerically calculated φ(ω) and corrected the measured phase spectra. In addition, by taking the local peaks of the frequency-scan results, a broadband transmittance is obtained. In this case, a π phase difference corresponds to the frequency spacing, expressed as
3. Results and discussion
A commercial PDP-glass (Asahi Glass PD-200) was measured by the DML-based CW THz system. The sample consists of glass substrate and an ITO layer coated on the surface, which was partially removed to measure the thickness and refractive index of the glass. Thicknesses of the glass (dglass) and the ITO layer (dITO) were 1.808 mm and 135 nm, respectively, as measured by a mechanical micrometer and an atomic force microscope.
A reference THz spectrum (Eair(ω)) was measured without a sample, and the waves transmitted through the glass (Eglass(ω)) and the ITO-coated glass (EITO(ω)) were measured as well. To suppress the noise, the lock-in time constant was set to be 300 ms, which implies a pure measurement time of about 4 min for a scanning range of 0.8 THz. To confirm the accuracy, an independent THz-TDS measurement was performed (not fully shown in this work) and compared with results from the CW THz system. The experimental setup and measurement results are shown in Fig. 3 (a) and (b), respectively. The DML module, shown in the inset of Fig. 3(a) and the photomixer modules realize a compact THz system. Due to the absorption in the glass substrate, the transmitted spectra, Eglass(ω) and EITO(ω), are obtained for frequencies below approximately 700 GHz. For the CW THz spectroscopy, the delay line is set to a position of 8.5 mm away from the zero delay. This results in a frequency spacing of 17.64 GHz for Eair(ω), which is more than 10 times of the frequency spacing of 1 GHz in the measurements to result in a clean oscillatory signal.
From the envelopes of results in Fig. 3(b), the amplitude spectrum of the transmitted THz waves are obtained, as shown in Fig. 4(a). In addition, from the zero-crossing frequencies, the phase spectra were deduced, as shown in Fig. 4(b), we denote the phase spectra as δair(ω), δglass(ω), and δITO(ω), corresponding to the amplitude spectra Eair(ω), Eglass(ω) and EITO(ω), respectively. Note that since δglass(ω) and δITO(ω) were almost identical, only δITO(ω) was plotted. As can be seen in Fig. 4(b), the experimental phase difference curves deviate from purely linear function, which stems from the frequency dependence of φ(ω) and multiple reflections in the glass. From Eair(ω), φ(ω) was calculated and commonly used for phase correction of the experimental phase spectra, and the resulting φ(ω) is shown in Fig. 4(b). Multiple reflections are also considered in Eq. (1), but no significant effect was observed in this case. The corrected phase spectra are shown as solid lines in Fig. 4(b), showing good linearity. From the slopes of δair(ω) and δITO(ω), dglass was calculated by Eq. (4), to be 1.868 mm and 1.886 mm for the time constant of 300 ms and 10 ms, respectively. The index of glass, nglass(ω), was available from the independent THz-TDS measurement. Note that, the glass is highly dissipative at frequencies below approximately 0.7 THz, and the conductivity of the ITO is spectrally flat over the THz region [7, 8]. Therefore, we restricted the spectral range of the measurement to be from 0.35 to 0.5 THz and adjusted the frequency step. The measurement time is reduced as well, and becomes less than one second for a time constant of 10 ms. This is a significant advantage over the THz-TDS technique, for which the measurement time is independent of the measurement bandwidth.Eq. (6), EITO is obtained by solving the two-layer transmission problem, and after being divided by Eglass, the effect of the substrate is canceled out . Thus, the glass thickness does not appear in the Eq. (6). The refractive index of the ITO layer is obtained by measuring Eglass(ω) and EITO(ω). And the sample thickness can be obtained from δITO(ω) because the phase delay imparted by the thin ITO layer is negligible, as shown in Fig. 3(b) and Fig. 4(b). Therefore, Eglass(ω) is accurately calculated from Eair(ω) and δITO(ω) without actual measurements. This is important because measuring Eglass(ω) requires a bare glass substrate without an ITO layer, which is not always available in practical situations. From nITO(ω), the sheet conductivity of the ITO film is calculated by using the relation εITO = nITO2 = εr,ITO + iσITO/(ωε0), and the results are shown in Fig. 5(b). The results accurately agree with the independent THz-TDS results for a time constant of 300 ms and also agree well with the Hall measurement, as shown in Figs. 5(a) and (b). At a time constant of less than 10 ms, the measurement time was also reduced to less than a second. Due to the increased noise, the results show undulations, which need to be improved further. However, it is important that the DML and the photomixer speed are fast enough for real-time measurement of the ITO glass. The noise should be suppressed, and in the future, the lock-in amplifier will be replaced by an ordinary data acquisition board to further reduce the measurement time. In an array configuration, our THz system could be used for real-time monitoring of ITO-coated PDP glass in a production line.
In conclusion, we implemented a CW THz spectrometer based on a 1.3-μm dual-mode laser in a homodyne detection scheme. The fast frequency tuning of the DML enables broadband measurements in one second without using a mechanical delay line. From phase analysis of the CW spectrometer, the substrate thickness and the sheet conductivity of a commercial ITO-coated glass were successfully measured. Due to the potentially low-cost and compactness of the system, our results demonstrate the possibility for widespread industrial applications of THz technology.
This work was partly supported by the IT R&D program of MOTIE/KEIT [10045238, Development of the portable scanner for THz imaging and spectroscopy], Joint Research Projects of ISTK, the Public Welfare & Safety Research Program through the National Research Foundation of Korea (NRF) Technology (NRF-2010-0020822), and Nano Material Technology Development Program through the NRF of Korea (NRF-2012M3A7B4035095).
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