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Abstract
A mirror vibration induces a space rotation, and accordingly, phenomena in the time domain are converted to spatial distributions. This time-space conversion realizes high-speed measurements that exceed the operation limit of instruments. In this study, a time-resolved spectrometry was conducted by using slow optical devices, i.e., a continuous-wave laser diode, a CCD-based spectrometer (exposure time: 1 s), and a galvano-mirror (30 Hz). In comparison with pulsed lasers, the continuous-wave laser is advantageous for continuous integration of weak fluorescence. Experiments were conducted with fluorescent lanthanide solutions to examine the validity of this method.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Data conversion between the time (frequency) and space domains realizes fast measurements that exceed the response of optical devices or materials, e.g., signal detection at a rate of tera-bits per second [1,2], high-speed imaging at a frame time of 163 ns [3], and high-frequency modulation of a long-lasting phosphorescence [4]. Although the principles of these measurements differ with one another, their common concept, i.e., the time-space conversion, seems applicable to various technical fields. A possible application is a time-resolved spectral measurement, which is currently attracting interests in the fields of material or biomedical science [5–7].
The time-resolved spectroscopy has been conducted by using a variety of methods [8], e.g., a pump-probe method [9] and a step-scan method [10,11]. Although an attainable temporal resolution is 1 µs or shorter, the spectral measurements are generally time-consuming and lack reproducibility [9,12]. In addition, the time-resolved measurements usually require fast-response photodiodes or detector arrays [5,7,13,14] as well as high-speed electric devices like image intensifiers [6,15] or boxcar cells [8,16]. Recent progress in the nonlinear optical technology, e.g., ultrashort pulse emission, optical mixing, or super-continuum light generation, has promoted the research on spectrometry [6–9,13,16–20]. These techniques, however, further advance the complexity and expensiveness of the measurement system, since they generally use a femto-second laser and some other sophisticated instruments.
The time-space conversion method is useful for not only ultrafast measurements but also slower optical measurements. For example, a time-resolved spectral measurement was conducted in a microfluidic channel, in which a temporal change of the transmission spectrum corresponded to a spatial distribution toward the downstream [5]. A wavelength conversion of photonic signals was achieved at 1 MHz by rotating an upconversion phosphor on an optical disk, in which a bright-spot distribution on the disk corresponded to a series of signal pulses [4,21]. These time-space conversion methods, however, cannot be applied to other samples that are difficult to flow or rotate. Swinging a probe beam is more preferable than moving a sample. A spinning or vibrating mirror is usually used to swing a beam in a spectrometric system, e.g., a beam scan across a monochromator prism [22], a cavity mirror rotation in a interferometer [23], a rapid scan of diffracted beams on a focal plane array [24], and a delay control of pump and probe beams [25]. In these measurements, however, a mirror acts only as a gate or a synchronizer for instantaneous light detection, and hence, the “time-space conversion” is not a suitable term for them.
In this study, we made the best use of the time-space conversion principle to conduct time-resolved spectral measurements. Neither pulsed lasers nor fast-response electric devices was used in the optical system. Instead the system was constructed with ordinary slow instruments, i.e., a continuous-wave (CW) laser diode (LD), a CCD-based spectrometer with an integration time of 1 s, and a galvano-mirror that swung at 30 Hz. Usefulness of this system was confirmed by the spectral measurements of lanthanide phosphors, whose fluorescent characteristics needed the time-resolved analysis [15,26,27].
2. Optical system
Figure 1 shows the optical system for the measurements. A galvano-mirror with a reflection area of 35×55 mm2 (Cambridge Technology, 6650) vibrates sinusoidally between θ=−5° and 5° at 30 Hz, and accordingly, the angular velocity is dθ/dt = 800 deg/s or 14 rad/s at around θ=0°. The mirror swings the laser beam (396 nm, 100 mW) with the twice velocity, i.e., 28 rad/s. If a sample is placed at a distance of z = 0.3 m, therefore, the laser beam sweeps it at a linear velocity of 8.4 m/s. The sample is a fluorescent material, e.g., a phosphor or a dye solution. As Fig. 1(a) shows, the laser beam irradiates a position A on the sample, inducing a fluorescence emission at that point. The fluorescent rays are reflected by the mirror and focused by a lens to create an equi-magnification image at a point P. A probe fiber (core diameter: 400 µm) of a multichannel spectrometer (B&W tek, BTC112E) is fixed at this point to pick up the fluorescent rays. As the dotted lines show, an image of the position B is measurable when the fiber is moved to the corresponding point Q. Figure 1(b) shows the optical paths of the beams on the occasion of the mirror rotation. The laser beam moves a distance of d arriving at a position C. At the same time, the image point corresponding to A moves from P to Q (the dotted lines). Accordingly, the point P receives fluorescent rays that are emitted at C, i.e., the position of the laser irradiation. It follows that the fluorescence at the instance of excitation is continuously measureable at P regardless of the mirror angle θ. On the other hand, the point Q always monitors a position that is located at a distance d behind the laser beam. That is, a fluorescent spectrum with a fixed delay (afterglow) is continuously measureable at Q. If the distance is d = 1 mm, for example, the corresponding delay is 120 µs, since the sweep velocity is 8.4 m/s.
Fig. 1. Optical system for the time-resolved spectral measurement. (a) When the mirror is at the original position (θ=0°), the pump laser beam excites the position A of the phosphorescent sample. Fluorescence at the positions A and B are imaged at the points P and Q, respectively. (b) As the mirror rotates, the laser beam moves to the position C, and the position corresponding to P moves to C. An afterglow at the position A is imaged at the point Q. The fiber position determines the time after the excitation.
If the spectral data are taken successively as the fiber moves from Q to P and further, they will exhibit a symmetrical distribution with respect to P, since the mirror swings back and forth. That is, there are two points that correspond to the same delay. During a single sweep process, the fiber picks up the fluorescence continuously for a period of tm=2.4 ms, in which time the laser beam moves across the sample with a 20 mm width. Since the beam sweeps the sample 60 times (f = 30 Hz) during the exposure time of the spectrometer (1 s), the spectral data are integrated for 2ftm=144 ms in a single exposure period. Such a long integration time is difficult to attain with pulsed lasers. Note that the integration time is unaffected by improvement of the temporal resolution, since a rapid sweep increases the number of the repeated sweeps (2f) while it decreases the excitation duration (tm). This fact renders the CW laser excitation advantageous particularly on occasions of weak fluorescence.
The sweep velocity was measured by setting a mask (a stripe with a 1 mm period) at the sample position. The transmitted laser beam was collected by a lens and detected by a photodiode. As Fig. 2(a) shows, the beam intensity changed with a period of 120 µs in agreement with the prediction above (120 µs/mm or 8.4 m/s). Figure 2(b) shows the laser beam profile, which was measured at the sample position by moving a pinhole of 50 µm diameter. The mirror was fixed (no swing) during this measurement process. The measured profile fitted a Gaussian curve of 0.48 mm width. When the sweep velocity is 8.4 m/s, this width corresponds to a period of 58 µs. That is, if the moving beam is picked up at a fixed point, the measured pulse shape will be
where W0 is a suitable coefficient and tB is 29 µs. The pulse width (2tB), which affects the temporal resolution, decreases as the sweep velocity increases or the beam diameter decreases. In the current experiment, the beam diameter of 0.48 mm was attained by placing a lens (focal length: 500 mm) between the LD and the mirror (between the points L and M in Fig. 1). At the focal point the laser beam excited europium ions (Eu3+) in a sample solution. Figure 2(c) shows the energy levels of the Eu3+ ion and the electronic transitions that induce fluorescence at various wavelengths [27]. The lifetimes of the excited and fluorescent states are τe and τf, respectively.Fig. 2. (a) Laser beam intensity that was measured through a striped mask (1 mm period) to evaluate the sweep velocity at the sample surface (z = 0.3 m). (b) Laser beam profile at the sample position. (c) Electronic transitions in the Eu3+ ion that are induced by the pump beam irradiation [27].
3. Experiments
The performance of this system was examined with a Eu3+ solution, which exhibited peculiar fluorescent characteristics in our previous experiment; e.g., the fluorescent intensity became 80-fold stronger in polyethylene glycol (PEG) than water [27]. A sample for the first experiment was an aqueous solution of EuCl3 with a concentration of 1 mol/l (1 M). The solution was put in a glass cell with a 20 mm width and 1 mm thickness. Figure 3(a) shows the fluorescent spectrum that was measured at the point corresponding to the irradiation position, i.e., the point P (0.0 mm or 0 µs). The highest peak appeared at 592 nm and some other peaks were visible around it. As Fig. 3(b) shows, the spectra at ± 0.2 mm (the solid and dotted lines, overlapping) exhibited afterglow peaks of 24 µs after excitation. Figures 3(c)−3(f) also show the overlapping spectra that corresponded to a delay of 48−120 µs. The peaks shrank gradually as time passed.
Fig. 3. Fluorescent spectra that were measured at different points. The corresponding delay is shown in the figure. The sample is an aqueous solution of EuCl3 (1 M).
Next, the sample solution was diluted to 0.01 M to examine the traceability of weak fluorescence. As Fig. 4 shows, measured spectra were similar to those of the original solution (Fig. 3) in spite of a lower signal-to-noise ratio. Figure 5 shows the decay curves of the fluorescent peaks, which are normalized with reference to the peak height at 0 µs. Although the fluorescence of the dilute solution (▴) was unmeasurable in the long-delay range, their decay process was close to that of the original solution (○).
Fig. 4. Time-resolved fluorescent spectra of an aqueous solution with a low Eu3+ concentration (0.01 M).
Fig. 5. Decay curves of the fluorescent peaks at (a) 592, (b) 615, or (c) 698 nm wavelength. The peak heights were taken from the spectra that were exemplified in Figs. 3 (○) and 4 (▴). The fitting curves were drawn by assuming suitable lifetimes shown in the figure.
The fitting curves (the solid lines) in Fig. 5 were drawn in the following manner. As Fig. 2(c) shows, electrons are first excited to the 5L6 state and then transit to the 5D0 state. The lifetimes of these states are τe and τf. If the excitation takes place instantaneously at t = 0, the electron population of the 5D0 state is expressed as
As mentioned earlier, the fluorescence intensity of Eu3+ increases notably in the PEG solution. Figure 6(a) shows the fluorescent spectra of Eu3+ (0.01 M) in PEG (molecular weight: 300). In comparison with the aqueous solution of the same concentration (Fig. 4), the peak height at 613−615 nm is about 30-fold higher, whereas the peak height at 592 nm is unchanged. In addition, a strong emission is visible even at 120 or 240 µs after excitation (the thick or dotted lines). This result contrasts with those of the aqueous solutions (Figs. 3 and 4).
Fig. 6. (a) Fluorescent spectra of the PEG solution that were measured at 0 (the thin line), 120 (the thick line), or 240 µs (the dotted line) after excitation. (b), (c) Decay curves of the fluorescent peaks at 592 or 613 nm. The triangles show the data that were taken in the slow-sweep process (1.7 m/s). The fitting curves were drawn by assuming the denoted lifetimes.
The circles in Figs. 6(b) and 6(c) show the fluorescence decay at 592 or 613 nm. The triangles show the data that were measured with a slow sweep velocity (1.7 m/s) to confirm the reliability of the measurements; i.e., the mirror swung between θ=−1° and 1° at 30 Hz. Although the probe fiber was moved with the same step as before (0.1 mm), the corresponding delay step became 5 times longer due to the slow sweep. The overlap of these two data (○ and ▴) verified that the measurement was unaffected by the sweep velocity. The fitting curves were drawn by assuming τe=4 µs and τf=200 ± 10 µs in Eqs. (1)−(3). The plateau in the range below 50 µs, which was not visible with the aqueous solutions (Fig. 5), seemed to be induced by the longer lifetime of the 5L6 state (τe). The lifetime of the 5D0 state (τf) also extended threefold in comparison with the aqueous solution.
Finally, experiments were conducted with mixed solvents of PEG and water. As the spectra in Fig. 7(a) show, the 613-nm peak became higher than the 592-nm peak when the PEG ratio exceeded 90%. This result agreed with those of ordinary spectral measurements [27]. Figures 7(b) and 7(c) show the decay curves of the two peaks. When the PEG ratio was 90% (●), the evaluated lifetimes were τe=3 µs and τf=65 µs for both 592- and 613-nm peaks. These values were the same as those of the aqueous solution (Fig. 5). The curve fitting for the 95% solvent (○) yielded the lifetimes of τe=3 µs and τf=80 µs.
Fig. 7. (a) Fluorescent spectra that were measured at the moment of excitation. The solvents of the samples were mixtures of PEG (90 or 95 vol%) and water. (b), (c) Decay curves of the peaks at 592 or 613 nm. The fitting curves were drawn by assuming τe=3 and τf=65 µs for the 90% solution and τe=3 and τf=80 µs for the 95% solution.
4. Discussion
Figures 4(a) and 6(a) show that the peak at 613−615 nm becomes 30-fold higher in PEG than water. On the other hand, Figs. 5(b) and 6(c) show that the lifetime of the 5D0 state (τf) is threefold longer in PEG than water. The ordinary spectral measurement integrates spectra of different delays in a long exposure period, and hence, the measured fluorescence intensity is influenced by both the instantaneous intensity and the lifetime. The 80-fold fluorescence enhancement at 613 nm, which was attained in the ordinary spectral measurements of the previous experiment [27], was caused by mainly the increase of the instantaneous emission intensity (30-fold) and partly the lifetime extension of the 5D0 state (threefold). Regarding the peak at 592 nm, only threefold enhancement was attained in the ordinary spectral measurements [27]. This fact agreed with the result of the current experiment; i.e., PEG caused no notable change in the instantaneous emission at 592 nm while extended the lifetime from 65 to 200 µs. These findings are useful to elucidate the fluorescence mechanism or the interaction of the Eu3+ ions with the surrounding fields [27].
The experiments above demonstrated the usefulness of the current measurement method for analyses of fluorescent processes with a 10‒100 µs lifetime. The accurate temporal resolution has to be evaluated by the use of fluorescent dyes with a short lifetime (nano-seconds). Physical parameters that determine the temporal resolution are the sweep velocity, the laser beam diameter, and the probe fiber diameter. The sweep velocity is readily raiseable by extension of the distance (z) between the mirror and the sample. Doing so, however, enlarges the instrument size. We are currently designing the multi-reflection system to extend the optical path. More effective improvement is attainable by substitution of the galvano-mirror with a spinning polygon-mirror. In our recent experiment, an angular frequency of ω=3,000 rad/s (500 round/s) has been attained, which realizes a sweep velocity of v = 2ωz = 2,000 m/s at z = 0.3 m. This velocity is 200-fold higher than that of the galvano-mirror, and hence, the temporal resolution will reach the 100 ns range. Reduction of the laser beam diameter or the probe fiber diameter will further improve the temporal resolution. Regarding the sensitivity, one should note that the sample width is an important factor. Since the sample sweep time or the data integration time increases with the sample width, the signal intensity of a dark phosphor can be enhanced by using a wide sample. By contrast a sample volume can be reduced if the sample material yields a bright fluorescence. We are currently conducting experiments to create a fast, sensitive, compact spectrometer through these modifications.
5. Conclusion
The time-space conversion with a galvano-mirror provides a useful method for conducting time-resolved spectral measurements. In comparison with pulsed lasers, a CW laser is advantageous for weak fluorescence measurements, since the continuous pumping allows a long integration time for the data collection. This integration time is independent of the sweep velocity due to the continuity of the time-space conversion process, and hence, a high-speed measurement is achievable with no significant sensitivity reduction. A rapid spectral change in the nanosecond range is possibly measurable by the use of a high-frequency galvano-mirror or a spinning polygon-mirror.
Appendix
If the lifetime of the excited state is short enough (τe≈0), the second term in Eq. (2) vanishes, and hence, the fluorescence exhibits an exponential decay, as shown in Fig. 8(a). If τe is not negligible, the curve starts from zero, as shown in Fig. 8(b), and rises as electrons transit from the excited state to the fluorescent state successively. The population of the fluorescent state becomes the maximum when the incoming and outgoing electrons balance in number. In the current experiment, however, the number of electrons in the fluorescent state is not zero at t = 0 s, i.e., the moment at which the beam center passes the observation point, since the outer portion of the beam (0.48 mm diameter) excites that position before t = 0 s. This effect is taken into account in the integral of Eq. (3). Figure 8(c) shows the decay curve that is calculated by Eq. (3) with the following parameters, i.e., τe=3 µs, τf=65 µs, and tB=29 µs.
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