2.2. THz spectroscopy
Measurements with THz-TDS were conducted to obtain the optical constants (i.e., the absorption coefficient α(ω) (ω: frequency) and refractive index n(ω)) of a sample [12–14,35,36]. The same two photoconductive antennas were used as an emitter and a detector of THz electric waves. Temporal waveforms of the THz electric field were measured over ~50 ps with a temporal resolution of ~0.1 ps.
The dual cell used had two sample chambers with the same dimensions. One was filled with the reverse micellar solution with water for a sample signal, and the other was filled with the solution with no water for a reference signal. The dual cell was fixed in a custom-made stainless-steel cryostat, and its temperature was controlled to within approximately ±1 K using nitrogen gas flow from a liquid nitrogen vessel. Hence, the two solutions had the same temperature. The temporal waveforms of the THz electric fields transmitted through the two solutions were alternately measured using a computercontrolled stage on which the cryostat was fixed, and the measurement was repeated eight times. The measurement system was placed in a nitrogen purge box to eliminate the absorption of the THz wave by water vapor.
The absorption coefficient and refractive index of water in the reverse micellar solution were derived as follows: (1) the temporal waveforms of the THz electric fields transmitted through the two solutions were converted to frequency-domain data (i.e., the intensity: Is(ω) and Ir(ω), phase: ϕs (ω) and ϕr (ω); s: sample, and r: reference) using Fourier transform, (2) the absorption coefficient was derived from α(ω) = d−1ln[Ir(ω)/Is(ω)], where d is the optical path length of the cell (1 cmin the present study), and (3) the refractive index was derived with the equation ns(ω) − nr(ω) = c(ωd)−1(ϕs (ω) −ϕr (ω)), where ns(ω) and nr(ω) are the refractive indices of the sample and reference solutions, respectively, and c is the speed of light. The AOT molecules are self-assembled, and a spherical reverse micelle is formed even if there is almost no water in the solution [5]. The two solutions in the dual cell had the same concentration of AOT. Therefore, we assumed that the contribution of AOT molecules to the optical constants is canceled out in their derivation [13]. The volume fraction of isooctane was not considerably different between the two solutions (the difference = ~0.03) owing to the same concentration of AOT and the small amount of water in the sample solution, and the absorption and phase shifts of the THz electric wave by isooctane are very small because of its nonpolar nature. Therefore, the effect of isooctane on the optical constants was considered negligible.
We examined the baselines of the absorption coefficient and refractive index from THz-TDS using the temperature-controlled dual cell. To that end, the sample and reference cells were filled with the same AOT reverse micellar solution with an AOT concentration of 0.1 M, and the temperature-dependent measurement of THz-TDS was conducted in the 260 K–296 K temperature range. The results are depicted in Fig. 2, with the optical constants of water in the reverse micellar solution with w0 = 35 at 295 K. Fig. 2(a) shows that the baselines for the absorption coefficient at the temperatures examined are flat and below ±0.05 in the frequency range between 0.1 THz and 1.2 THz. On the other hand, the baselines for the refractive index are flat and approximately unity, shifted only by ~0.003 in Fig. 2(b) in the same frequency range. Thus, the experimental system of THz-TDS used is found to work for temperature-dependent measurements of the optical constants, and we analyze the optical constants in this THz frequency range (=0.1–1.2 THz).
2.2 THz กวัด มี THz TDS ได้ดำเนินการเพื่อขอรับคงแสง (เช่น ค่าสัมประสิทธิ์การดูดซึมα(ω) (ω: ความถี่) และ n(ω)) ดรรชนีหักของตัวอย่าง [12-14,35,36] ส่วน photoconductive สองเดียวกันถูกใช้เป็นตัวส่งตัวและเครื่องตรวจจับคลื่นไฟฟ้า THz ขมับ waveforms ของสนามไฟฟ้า THz มีวัดกว่า ps ~ 50 ด้วยความละเอียดที่ขมับของ ~0.1 psเซลล์คู่ที่ใช้ได้สองห้องตัวอย่าง มีมิติเดียวกัน หนึ่งก็เต็มไป ด้วยโซลูชัน micellar ย้อนด้วยน้ำตัวอย่างสัญญาณ และอื่น ๆ ก็เต็มไป ด้วยการแก้ปัญหาน้ำไม่ได้สำหรับสัญญาณอ้างอิง เซลล์คู่ที่ถาวรใน cryostat เหล็กสแตนเลสการเลือก และมีควบคุมอุณหภูมิภายในประมาณ ±1 K ใช้กระแสก๊าซไนโตรเจนจากเรือไนโตรเจนเหลว ดังนั้น โซลูชั่นสองมีอุณหภูมิเดียวกัน Waveforms ขมับฟิลด์ไฟฟ้า THz ส่งผ่านโซลูชั่นสองได้มาระหว่างวัดใช้ระยะ computercontrolled ที่ cryostat ไม่ถาวร และวัดถูกทำซ้ำ 8 ครั้ง ตัวถูกวางในกล่องล้างข้อมูลไนโตรเจนเพื่อกำจัดการดูดซึมของคลื่น THz โดยไอน้ำThe absorption coefficient and refractive index of water in the reverse micellar solution were derived as follows: (1) the temporal waveforms of the THz electric fields transmitted through the two solutions were converted to frequency-domain data (i.e., the intensity: Is(ω) and Ir(ω), phase: ϕs (ω) and ϕr (ω); s: sample, and r: reference) using Fourier transform, (2) the absorption coefficient was derived from α(ω) = d−1ln[Ir(ω)/Is(ω)], where d is the optical path length of the cell (1 cmin the present study), and (3) the refractive index was derived with the equation ns(ω) − nr(ω) = c(ωd)−1(ϕs (ω) −ϕr (ω)), where ns(ω) and nr(ω) are the refractive indices of the sample and reference solutions, respectively, and c is the speed of light. The AOT molecules are self-assembled, and a spherical reverse micelle is formed even if there is almost no water in the solution [5]. The two solutions in the dual cell had the same concentration of AOT. Therefore, we assumed that the contribution of AOT molecules to the optical constants is canceled out in their derivation [13]. The volume fraction of isooctane was not considerably different between the two solutions (the difference = ~0.03) owing to the same concentration of AOT and the small amount of water in the sample solution, and the absorption and phase shifts of the THz electric wave by isooctane are very small because of its nonpolar nature. Therefore, the effect of isooctane on the optical constants was considered negligible.We examined the baselines of the absorption coefficient and refractive index from THz-TDS using the temperature-controlled dual cell. To that end, the sample and reference cells were filled with the same AOT reverse micellar solution with an AOT concentration of 0.1 M, and the temperature-dependent measurement of THz-TDS was conducted in the 260 K–296 K temperature range. The results are depicted in Fig. 2, with the optical constants of water in the reverse micellar solution with w0 = 35 at 295 K. Fig. 2(a) shows that the baselines for the absorption coefficient at the temperatures examined are flat and below ±0.05 in the frequency range between 0.1 THz and 1.2 THz. On the other hand, the baselines for the refractive index are flat and approximately unity, shifted only by ~0.003 in Fig. 2(b) in the same frequency range. Thus, the experimental system of THz-TDS used is found to work for temperature-dependent measurements of the optical constants, and we analyze the optical constants in this THz frequency range (=0.1–1.2 THz).
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