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We investigated liquid-sheet jets with controllable thickness for application to terahertz (THz) spectroscopy. Slit-type and colliding-jet nozzles were used to generate optically flat liquid jets. The thickness of the liquid sheet was determined precisely by spectral interference and THz time-domain-spectroscopy methods. By adjusting the collision angle of the colliding-jet nozzle, we could control the thickness of the liquid sheet from 50 to 120 μm.
© 2014 Optical Society of America
1. Introduction
Techniques to generate and detect terahertz (THz) light have become more sophisticated over the past decade, and owing to this, spectroscopy in the THz frequency range has become a common tool for material sciences [1]. A newly developed THz spectroscopy allows us to investigate the various motions of molecules and charged particles: plasma oscillations and lattice vibrations in solid systems [2], low-frequency vibrations in biological macromolecules [3], high-frequency rotations of small polar molecules in the gas phase [4], and fast dielectric relaxations and intermolecular vibrations in polar liquids [5, 6], and so on.
Photo-induced reactions are also an attractive target for study by THz spectroscopy. Optical-pump THz-probe time-resolved spectroscopy has been widely applied to the study of ultrafast carrier dynamics in semiconductors [7–9]. In the liquid phase, we know the variety of the transient phenomena in the THz frequency region induced by optical light: for example, the response of solvent molecules due to photo-induced modifications in the charge distribution of a solute molecule [10]. However, only a few reports have been proposed for the time-resolved THz spectroscopy in the liquid phase [10, 11], as far as we know. In conventional THz spectroscopy of solution samples, we measure the small change in the dielectric function of the solvent by dissolving the solute in this solvent. To study photo-induced reactions in solution samples, the solute molecules must be excited with high yield to obtain a significant change in the dielectric function, which is a bulk quantity. Therefore, an intense pump light is focused on the solution sample to obtain sufficiently high photon density. Unfortunately, irradiating liquid samples with such strong optical fields easily damages the optical windows of solution sample cells.
One of the methods to solve this problem is to perform windowless measurement, where the sample comprises a liquid jet. For spectroscopic use, the liquid jet must satisfy several requirements, the first of which is optical flatness. Liquid sprayed from a cylindrical nozzle into air usually forms a conically shaped jet, which is not suitable for spectroscopy. But, a liquid sheet is a good candidate for windowless spectroscopic measurements. By exploiting knowledge of fluid dynamics, extensive efforts have been made to obtain optically flat liquid sheets [12–20]. The second requirement is that the sheet thickness should be appropriate for spectroscopic studies. Because THz light is strongly absorbed by water (absorption coefficient is 250 cm−1 at 1 THz [21]), aqueous solutions require thin liquid sheets for spectroscopy in the transmittance geometry. On the other hand, to detect the small photo-induced transient changes in the dielectric function of the solution, the THz light must interact with the solution through a sufficiently long optical path. To satisfy both requirements, the thickness of aqueous samples must be 50–100 μm for THz experiments for the aqueous system [5, 22–24]. The third requirement is the controllability of the thickness. To better understand the spatial distribution of photo-excited molecules, scanning the sample thickness in a predetermined fashion is helpful.
In the present study, we developed an apparatus to produce stable liquid sheets with controllable thickness for use in THz spectroscopy. We employed a colliding-type jet nozzle to realize the thickness control which has been investigated in the research area of fluid dynamics [17, 18]. The colliding-type jet nozzle was evaluated and modified for the purpose of THz spectroscopy. Then, to evaluate how well the sheet thickness is controlled, we also developed a method to measure the sheet thickness. The paper is organized as follows: In Section 2, we describe the principle and technical details to produce liquid sheets and to measure sheet thickness. We summarize the experimental details of the liquid-sheet nozzles and the THz spectrometer in Section 3. In Section 4, we present the experimental results and discuss the controllability of sheet thickness. Finally, in Section 5, we conclude and provide prospective for future research.
2. Experimental methodology
2.1 Liquid-sheet nozzles
Liquid sheets with good optical surfaces have been extensively investigated, especially for thin flowing-jet streams of dye solution in dye-laser systems, which also require windowless free-flowing liquid jets. Several designs of liquid-sheet nozzles have been proposed, e.g., the slit-type nozzle [12–14], the wire-guided nozzle [15], the colliding-type nozzle [16–20], and so on. For this paper, we adopted and evaluated the slit- and colliding-type jet nozzles for THz spectroscopy.
The slit-type nozzle is the simplest of the liquid-sheet nozzles. The slit-type nozzle generates a liquid sheet directly through a rectangular-shaped orifice [12–14]. Watanabe and associates introduced the slit-type nozzle by using the two razor blades fixed at the end of a rectangular tapered tube. With this device, they demonstrated the ultrathin (3.5 μm) liquid ethylene-glycol sheets of extremely high optical quality [12]. The sheet thickness depends on the dimension of the rectangular slit, the viscosity of the liquid, and the distance and angle of the measurement position from the orifice. However, for the purpose of the thickness control in spectroscopy, it is difficult to adjust these parameters systematically.
The colliding-type jet nozzle produces a liquid sheet by the oblique impact of two cylindrical jets [16–19] or by the impact of a single cylindrical jet with a flat surface [20]. The available thickness range is 65−110 μm for the colliding-jet nozzle, and 5−20 μm for the colliding flat-plate nozzle. A significant advantage of the colliding-jet nozzle is that the sheet thickness can be easily controlled by adjusting the angle between the colliding jets or between the jet and plate. Therefore, the colliding-jet nozzle is the best candidate for use with optical-pump THz-probe time-resolved spectroscopy of solution samples.
Knowledge of fluid dynamics has been used to develop a theory of the physical properties of the liquid sheet produced by colliding-jet nozzles. Figure 1 shows a schematic diagram of a liquid-sheet formation. Two cylindrical jets with a diameter of 2R flow coplanarly at speed V and collide obliquely at an angle 2θ. This collision produces a flat sheet in the plane perpendicular to the plane defined by the two jets [16–18]. The sheet thickness h, which is one of the important parameters describing the liquid sheet, depends on the collision angle 2θ, the jet diameter 2R, and the distance r and azimuthal angle φ from the impingement point [16, 25–27]. From the laws of mass and energy conservation, the thickness h is inversely proportional to the distance r [16]:
where K(φ,θ,R) is the sheet-thickness parameter. For the present study, the liquid sheet was always measured on the line φ = 0 (the y axis in Fig. 1). However, the collision angle 2θ was varied to adjust the sheet thickness. Conservation of momentum causes the sheet thickness to decrease for larger collision angles [26, 27], and conservation of mass causes the sheet thickness to increase for larger jet diameter 2R [26, 27]. The sheet width W is also of practical importance to THz spectroscopy because THz light (wavelength is 0.3 mm at 1 THz) cannot be tightly focused because of diffraction limit. From the laws of conservation of mass and energy, the sheet width increases for greater jet velocity and collision angle [16, 25, 28, 29].
Fig. 1 Schematic of liquid sheet formed by two colliding jets with jet velocity V, jet diameter 2R, and collision angle 2θ. The polar coordinates r and φ indicate the point in the liquid sheet at which the THz light is focused.
2.2 Measurement of sheet thickness
For a non-rigid liquid sheet, optical noncontact methods must be used to measure the sheet thickness. One such method is the spectral interference, which is widely used to measure the thickness of the thin films. When white light impinges onto a film at an incident angle α, the light reflected from the front surface interferes with that reflected from the back surface [see Fig. 2(a)]. Therefore, the interference pattern appears in the spectrum of the reflected light. The thickness h of the film can be obtained from the wavelengths of adjacent peaks in the interference pattern as follows:
where λ1 and λ2 are the wavelength of any two adjacent peaks, and n1 and n2 are the refractive indices of the film at λ1 and λ2, respectively [12]. The interference method is limited by the wavelength resolution of the spectrometer and the spectral range of the white light, which determine the upper and lower limits of the measureable thickness, respectively. When we use a fiber-optic spectrometer with a spectral resolution of 2 nm and a halogen lamp with a spectral range of 450–700 nm, the measurable thickness range is from 0.5 to 60 μm for a water sheet with refractive index of n ~1.33 in our spectral range [30].Fig. 2 Schematics of methods used to measure thickness of liquid sheet. (a) Spectral interference method. (b) Optical time delay detection method with Michelson interferometer. (c) THz-TDS method.
The optical time delay detection has also been widely applied with the Michelson interferometer to measure the film thickness [see Fig. 2(b)]. When a film is inserted in one arm of the interferometer, the light passing through this arm is delayed by in time, where c is the velocity of light in vacuum and n is the refractive index of the film. This method, however, requires the film surface be extremely stable and flat, which is not appropriate for the flowing-liquid sheet.
Here, we used THz time-domain spectroscopy (TDS) to measure the sheet thickness. When the THz pulse passes through a film, the phase delay and the attenuation of intensity are detected in the transmitted THz pulse [Fig. 2(c)]. When we measure the temporal profile of a THz pulse with and without the film [ES(t) and ER(t), respectively], the complex transmission spectrum is
The functions and are the Fourier transformed spectra of the temporal profiles ER(t) and ES(t), respectively. The quantity is the complex refractive index of the film, where and are the refractive index and the extinction coefficient, respectively. N is the number of the multiple reflections [31]. When we know the spectra of and , for example from the literature or different measurements, the sheet thickness h can be determined from the observed by using Eq. (3).The THz-TDS method is suitable for measuring the thickness of a flowing liquid sheet. Because the wavelength of THz light is of the order of mm, the surface quality of the liquid sheet is not crucial, unlike the case for optical time-delay measurements. THz-TDS is limited by the absorbance of the liquid sheet and the temporal resolution of the measurement which determine the upper and lower limits of the measureable thickness, respectively. When the temporal profile of the transmitted THz pulse is measured with a temporal resolution of 50 fs, the measurable thickness range is from 15 to 370 μm for water, which has a refractive index of n ~2.1 and an absorbance of 250 cm−1 at 1 THz [21]. In the present study, spectral interference and the THz-TDS methods are used in a complementary manner to determine the sheet thickness over a wide range.
3. Experimental Apparatus
3.1 Liquid-sheet system
The colliding-jet-type liquid-sheet system is shown schematically in Fig. 3. The sample liquid was continuously circulated by a peristaltic pump through silicon tubes. The liquid that was pumped from the reservoir passed through a pulsation damper and a flow meter, and was divided into two identical branches to form the two colliding-liquid jets. The flow rate was precisely controlled by flow-control valves on each nozzle branch. Stainless-steel tubes 100 mm long with 1 mm inner diameter were used as colliding-jet injectors. Two stable cylindrical-jet streams emerged from each tube and collided with each other to produce a leaf-shaped liquid sheet. The liquid was returned to the reservoir through a funnel and a silicon tube.
Fig. 3 Schematic of colliding-jet nozzle for THz spectroscopy. R indicates the liquid reservoir, P is the peristaltic pump, D is the pulsation damper, FM is the flow meter, Va is the flow control valve, RS is the rotation stage, and ST is stainless-steel tube.
In addition, a slit-type nozzle (Alignment System Co.) [12] with a 0.2 mm × 0.4 mm orifice was used to produce a thin liquid sheet. For this nozzle, we used the same circulation system as that for the colliding jet.
3.2 THz spectrometer
Figure 4 shows the experimental setup for THz-TDS, which is similar to that reported previously [5, 9]. A 1 kHz laser pulse train centered at 800 nm with a pulse duration of 50 fs from Ti:Sapphire regenerative and multi-pass amplifiers was used for generation and detection of the THz pulse. The THz pulse was generated by optical rectification in a magnesium-doped LiNbO3 prism with pulse-front control [32]. The THz pulse was focused onto the liquid sheet, and the transmitted THz pulse was measured by electro-optic (EO) sampling in a ZnTe crystal to obtain the THz waveform. The detectable frequency of the THz pulse ranged from 0.2 to 2.4 THz.
The liquid sheet was positioned at the focal plane of the THz light. To ensure that the focal spot of the THz pulse was smaller than the width of the liquid sheet, a 2-mm-diameter pinhole was placed just before the liquid sheet. The point where the two jets were colliding was determined to be the position of minimal THz transmittance. The distance r and azimuthal angle φ of the measurement positions were measured with respect to this point. To eliminate the absorption of THz light by water vapor, the THz spectrometer was filled with dry air. All measurements were performed at room temperature (23 °C–25 °C).
The sheet thickness was also measured by using the spectral interference method, as indicated in Fig. 4. White light from a halogen lamp (Sigma Koki Co., LS-LHA) irradiated the liquid sheet at an incident angle of 22.5°, and was reflected into a fiber spectrometer (Ocean Optics Co., USB4000) to obtain the interference spectra. The spot size of the white light was 1 mm in diameter on the liquid sheet.
4. Results and discussion
4.1. Generation of liquid sheet
Figure 5 shows photographs of a liquid sheet of water produced by the slit-type nozzle at various flow rates. The liquid sheet with the larger surface was produced by the larger flow rate. As seen in Fig. 5(a), the sheet width at small flow rate (V = 40 ml/min.) was 2 mm, which is not large enough to accommodate the diameter of the THz beam at the focal point. However, at large flow rate (V = 90 ml/min.), the liquid sheet started to crack at the area indicated by the dotted circle in Fig. 5(c). This is the upper limit of flow rate for a stable liquid sheet. Thus, for the THz spectroscopy, we used a flow rate between 50 and 80 ml/min. for the slit-type nozzle.
Fig. 5 Photographs of liquid sheet of water produced by slit-type nozzle with flow rates of (a) 40, (b) 60, and (c) 90 ml/min. The dotted circle indicates the cracked zone of the liquid sheet.
Next, we have shown photographs in Figs. 6(a)–6(d) of the liquid sheet of water produced by the colliding-jet nozzle. Larger collision angle and flow rate led to a larger liquid sheet. This result is consistent with predictions from mass and energy conservation (see Sec. 2.1). However, as observed in Fig. 6(d), an upper limit of collision angle and flow rate exists beyond which the liquid sheet cracks. For the jet to be useful for spectroscopy, the available range of collision angle and flow rate was determined based on considerations similar to those for the slit-type nozzle [see Fig. 6(i)]. Finally, Figs. 6(e)-6(h) show photographs of ethylene-glycol liquid sheets produced by the colliding-jet nozzle. Compared with the water sheet, a larger stable sheet was obtained with ethylene-glycol, which indicates that high-viscosity liquids can provide good liquid sheets for THz spectroscopy.
Fig. 6 Photographs of liquid sheets produced by colliding-jet nozzle. Panels (a)−(d) show liquid sheets for water and panels (e)−(h) show those for ethylene-glycol at various flow rates and collision angles. (i) Region of acceptable flow rate and collision angle for THz spectroscopy of aqueous solutions.
4.2. THz spectroscopy with liquid-sheet system for water
All the measurements described below were performed with a liquid sheet of water. The flow rate was fixed to 60−70 ml/min. for the slit-type jet nozzle. For the colliding-jet nozzles, we adjusted the flow rate to obtain a stable liquid sheet. Note that the flow rate does not significantly affect the sheet thickness [17]. Figures 7(a) and 7(b) show waveforms of transmitted THz pulses in the absence and presence of the liquid sheet provided by the slit-type nozzle, respectively. The data reproducibility of the THz waveforms was quite well over 1-hour measurement which indicates the stability of the liquid sheet and the THz light source was enough for the THz spectroscopy of the liquid sheet. With the liquid sheet present, in addition to amplitude attenuation, the waveform was clearly modulated at longer time delays [see Fig. 7(b)]. Figures 7(d) and 7(e) show the power spectra obtained by Fourier transform of the waveforms shown in Figs. 7(a) and 7(b), respectively. Several sharp absorption dips appear in the power spectrum [Fig. 7(e)] for the THz pulse transmitted through the liquid sheet. Based on the literature data, these peaks can be assigned to the rotational absorption lines of water molecules [33, 34]. This result indicates that water vapor generated by the impact of the returning liquid on the water reservoir would significantly disturb the precision of the THz spectroscopy measurements.
Fig. 7 Influence of liquid sheet of water on THz waveforms and spectra. Panels (a) and (b) show the THz waveforms measured in the absence and presence of the liquid sheet of water, respectively, for the water reservoir placed directly below the nozzle. (c) THz waveform transmitted through the liquid sheet of water when the reservoir was placed far from the spectrometer. Panels (d), (e), and (f) show power spectra obtained by Fourier transformed power spectra obtained from the waveforms (a), (b), and (c), respectively. Vertical dotted lines in panels (d), (e), and (f) indicate the rotational absorption lines of water molecules [33, 34].
To reduce the influence of water vapor, we displaced the reservoir from below the nozzle to a zone outside the THz spectrometer, as indicated in the left inset of Fig. 7(c). The liquid sheet was transported to the reservoir via the funnel and silicon tube. Figures 7(c) and 7(f) show the waveform and the power spectrum of the THz pulse transmitted through the liquid sheet for this reservoir configuration. The absorption owing to the water vapor was significantly suppressed despite the presence of the liquid sheet.
4.3. Measurement of sheet thickness
We first evaluated the spectral interference method of measuring sheet thickness. Figure 8(a) shows the interference spectrum of the white light reflected by a liquid sheet produced from the slit-type nozzle. The white light was reflected 16 mm downstream from the nozzle orifice. An interference pattern was clearly observed, and Eq. (2) was used to obtain the sheet thickness h. By using the average of the thicknesses calculated from the spacing between all adjacent peaks, we determined the sheet thickness to be 5.7 μm. However, as seen in Fig. 8(b), no interference pattern appears in the reflection spectrum taken from the liquid sheet produced by the colliding-jet nozzle. This spectrum was taken 5 mm downstream from the impingement point and the collision angle was 60°. This result indicates that the thickness of the liquid sheet from the colliding-jet nozzle is greater than the upper limit of the thickness measurable by the spectral interference method with our setup.
Fig. 8 Thickness measurement of liquid sheet of water by spectral interference method. (a) Interference spectrum of white light reflected from a liquid sheet of water produced by the slit-type nozzle. The white light was reflected 16 mm downstream from the nozzle orifice. (b) Spectrum of white light reflected by liquid sheet produced from the colliding-jet nozzle. The white light was reflected 5 mm downstream from the impingement point, and the jets collided at an angle of 60°.
Next, the THz-TDS method was used to measure sheet thickness. Figures 9(a) and 9(b) show waveforms of THz pulses transmitted through the liquid sheet produced by the slit-type and colliding-jet nozzles, respectively. The dotted and solid curves show the waveforms ER(t) and ES(t) measured in the absence and presence of the liquid sheets, respectively. The measurement points of the liquid sheet were the same as those in the spectral interference measurement. A phase delay and the amplitude attenuation clearly appear for both liquid sheets. The power transmittance T(ν) = ||2 and the phase shift ϕ(ν) = arg] were calculated from the observed waveforms ER(t) and ES(t) and are shown in Figs. 9(c) and 9(d) for the slit-type and colliding-jet nozzles, respectively. The filled and open circles indicate the power transmittance and the phase shift, respectively. The observed data were fitted by Eq. (3) with the values of n(ν) and k(ν) taken from the literature [21] and reproduced with the thicknesses h of 5.4 and 117.9 μm for the slit-type nozzle and the colliding-jet nozzle, respectively, as shown by the solid lines in Figs. 9(c) and 9(d), respectively.
Fig. 9 Thickness measurement of liquid sheet by THz-TDS method. Panels (a) and (b) show the waveforms of THz pulses transmitted through a liquid sheet produced by the slit-type and colliding-jet nozzles, respectively. The measurement point was 16 mm downstream from the nozzle orifice for the slit-type nozzle and 5 mm downstream from the impingement point for the colliding-jet nozzle, where jets collided at an angle of 60°. The dotted and solid lines show the waveforms measured in the absence and presence of a liquid sheet, respectively. Panels (c) and (d) show the power transmittance T(ν) (filled circles) and the phase shift ϕ(ν) (open circles) calculated from the waveforms measured for the liquid sheets produced by the slit-type and colliding-jet nozzles, respectively. The black solid line shows the results of fitting Eq. (3) to the observed spectra.
For the slit-type nozzle, the thickness 5.4 μm is very close to the 5.7 μm thickness obtained by the interference method. This result indicates that the THz-TDS method is a reliable way to determine the thickness of liquid sheets. Note that in the previous section we introduced a lower limit of 15 μm for the measureable thickness based on the temporal resolution of the EO sampling pulse. The thickness of 5.4 μm, however, is much less than that of the roughly estimated lower limit of 15 μm. This is because of the strong absorption of THz light by water (250 cm−1 at 1 THz). The amplitude attenuation of the transmitted THz light is also sensitive to sheet thickness. Therefore, the thickness of an ultrathin liquid sheet can also be determined by THz TDS for a liquid with strong THz absorbance. Undoubtedly, the thickness of a thick liquid sheet such as that produced by the colliding-jet nozzle can also be determined by this method, as shown in Fig. 9(d).
4.4 Control of sheet thickness
To control the thickness of the liquid sheet, we varied the collision angle of the colliding-jet nozzle within the range where a stable liquid surface forms, as discussed in Sec. 4.1. Figure 10(a) shows the colliding angle dependence of the THz waveforms transmitted through the liquid sheet 5 mm downstream from the impingement point. The measurements shown in this section were always performed on the line with the azimuthal angle φ = 0. Larger collision angles yield smaller phase delay and smaller amplitude attenuation, which indicate a thinner sheet. We determined the sheet thickness to be 117.9, 102.5, 84.5, and 59.8 μm for the collision angles of 60°, 80°, 100°, and 120°, respectively. Figure 10(b) shows sheet thickness as a function of collision angle. The dependence of the thickness on collision angle is consistent with that expected from the considerations of conservation of momentum discussed in Sec. 2.1.
Fig. 10 (a) THz waveforms as a function of collision angle for THz pulse transmitted through a liquid sheet produced by the colliding-jet sheet. The measurement point was 5 mm downstream from the impingement point and the collision angles 2θ were 60° (red), 80° (orange), 100° (green), and 120° (blue). The gray dotted line indicates the THz waveforms in the absence of the liquid sheet. (b) The sheet thickness as a function of the collision angle 2θ.
Figure 11(a) shows the measurement position dependence of the THz waveforms transmitted through the liquid sheet produced at the colliding angle of 80°. Measurements farther from the impingement point yield smaller phase delay and smaller amplitude attenuation in the THz waveforms, indicating a thinner sheet. We determined the sheet thickness to be 102.5, 76.5, and 57.2 μm at 5.0, 7.5, and 10.0 mm downstream from the impingement point, respectively. Figure 11(b) shows the sheet thickness as a function of distance from the impingement point. The observed dependence of sheet thickness on distance is consistent with Eq. (1), which is derived from the laws of conservation of mass and energy. Consequently, the controllable thickness range of our colliding-jet nozzle to produce the stable liquid sheet for the THz spectroscopy is from 50 to 120 μm. Note that the absorbance of the THz light by the liquid sheet depends on the sheet thickness relating to the measurement position. Therefore, the finite size of the THz spot on the liquid sheet acts as one of the origins of the error in the measured transmittance spectra. To reduce the error due to the finite spot size, we have to measure the spectra at the large distance r from the impingement point, since the measurement position dependence of the sheet thickness is small at the large distance r as shown in Fig. 11(b) and Eq. (1).
Fig. 11 (a) THz waveform as a function of measurement position for THz pulse transmitted through liquid sheet produced by colliding-jet sheet with collision angle 2θ = 80°. The measurement positions were 5.0 (red), 7.5 (green), and 10.0 (blue) mm downstream from the impingement point. The gray dotted line indicates the THz waveforms in the absence of the liquid sheet. (b) Sheet thickness as a function of distance r from impingement point.
To evaluate our colliding-jet nozzle, we compare the sheet thickness obtained in the present study with that reported from a previous study [17]. Choo and Kang extensively studied the colliding-jet nozzle and measured the sheet thickness by using the imaging of the monochromatic-light interference [17]. Their measurements provide a sheet-thickness parameter K that appears in Eq. (1) but cannot directly determine the thickness h. Therefore, we use the sheet thickness parameter K in this discussion. Figure 12 show the dimensionless sheet-thickness parameter K/R2 as a function of collision angle. The result for K/R2 obtained in the present study is in good quantitative agreement with that obtained in the previous study. In Fig. 12, the theoretical prediction for the sheet-thickness parameter is shown as the solid line [18]. Although the theoretical models qualitatively reproduced the experimental results, the theory overestimates the thickness by 40%−110% [18]. As pointed out in Ref [18], a sophisticated model that includes the effect of gravity and the physical properties of the liquid such as surface tension and viscosity is required to bridge the gap between theory and experiment.
Fig. 12 Dimensionless sheet-thickness parameter K/R2 as a function of collision angle. The filled squares and open diamonds are the experimental data from the present study and from the previous study by Choo and Kang [17], respectively. The solid line shows the result of the theoretical model proposed by Choo and Kang [18].
5. Conclusion
For use in THz spectroscopy, we developed techniques for generating liquid sheets from slit-type and colliding-jet nozzles and for measuring the sheet thickness. We obtained the THz waveform and spectrum of THz pulses transmitted through a liquid sheet of water with minimal influence from water vapor. We also evaluated the thickness of liquid sheets by using both the spectral interference and the THz-TDS methods. Finally, we varied the thickness of the liquid sheet from 50 to 120 μm by adjusting the collision angle of the colliding-jet nozzle.
One of the most attractive and challenging subjects to which THz spectroscopy may be applied is to probe transient phenomena induced by optical light. We thus plan to apply the liquid-sheet system to study transient phenomena in solutions by using optical-pump THz-probe spectroscopy. For this purpose, the colliding-jet nozzle must undergo a few improvements. First, the nozzle shape should be modified to obtain a more stable liquid sheet. In the present study, we used the stainless-steel tubes with no special adaptation of the tube ends. Second, the size of the liquid circulation system must be reduced. In the current system, the solution sample has to be prepared in a volume of 200 mL, which is too large to make solutions of expensive solutes such as biomolecules. We believe that the present liquid sheet nozzle will be one of the fundamental techniques for such applications.
Acknowledgments
The authors would like to thank Dr. Hironori Ohba at the Japan Atomic Energy Agency for indispensable advice regarding the idea of the liquid-sheet jet nozzle. We also thank Dr. Ryuji Itakura for constructing the Ti:Sapphire laser system as the pumping light for the THz light source. We are grateful for financial support from the Ministry of Education, Science, Culture, and Sports (MEXT) of Japan through Grants-in-Aid (No. 22750022 and No. 24750027), and to the program of Consortium for Photon Science and Technology (CPhoST) funded by the Special Coordination Funds for Promoting Science and Technology commissioned by MEXT.
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