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Abstract
Three kinds of point defects, SiE’ center (≡ Si·), unrelaxed oxygen deficiency center (ODC (II)) and non-bridging oxygen hole center (≡ Si−O·, NBOHC), have been generated in hydroxyl fused silica by ultrashort pulsed laser irradiation. Hydroxyl is proved to be a decisive component for defect producing: NBOHC originates directly from hydroxyl; Hydroxyl facilitates the generation of SiE’ in an indirect way; No obvious relevance could be built between ODC (II) and hydroxyl. By improving hydroxyl content to 1000 ppm, NBOHC becomes the dominant defect species and its red luminescence is hence discernible to naked eye. Intended for application, high hydroxyl fused silica is screened out as the desired candidate, and NBOHC becomes the final interested defect. NBOHC’s intrinsic features of lifetime and temperature stability and extrinsic properties of laser condition dependence are specifically and systematically investigated. Prospective use of defect manipulation and fabrication in one-chip exploited for anti-counterfeiting and lab-on-a-chip is also discussed.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Fused silica, for its high spectral transparency, good thermal and mechanical stability, is an important optical material, now employed in modern communications, electronics technologies, as well as laser optics, etc [1–3]. In these applications, a hardness of fused silica against laser, gamma and particle radiations is of technological importance, as these irradiations can induce changes of densities and indices of refraction of fused silica [4], and can even generate optical absorption and luminescence bands in fused silica based bulk optical components [5]. It is worth noting that all of these radiation induced structural or optical changes are caused or at least initiated by the creation of point defects.
Point defects in fused silica have been thoroughly discussed by previous work. With the development of laser technology, especially with the increasing use of ultrashort pulsed laser, the focus has shifted from traditional gamma or particle-radiation induced point defects to high power pulsed laser induced ones in fused silica. A few reviews on this subject, contributed separately by D. L. Griscom and L. Skuja, have covered all optically active defects in fused silica [6–8]. And it has also been revealed that for every point defects ever found in gamma or particle-irradiated silica, there has been reported an analog in laser-irradiated glass.
The original aim for defects study is to prevent further generation of defects. However, with the ultrashort pulsed laser beam tightly focused, refractive index increase is induced in the damage spots or lines (in the scale of micro or sub-micrometer), which has enabled the preparation of functional photonic materials and devices in fused silica, etc [9–10]. In addition to the defects related refractive index, Watanabe et al found that damaged micro-region in silica features a strong emission of blue photoluminescence (PL), which can be used for the fluorescence readout of bits [11]. Sun et al also conducted an overall investigation on the generation and recombination of defects in silica induced by a near-infrared femtosecond laser [12]. However, the above mentioned point defects is created in dry fused silica, with hydroxyl concentration less than 10 ppm.
Hydroxyl is traditionally an unfavorable component for improving the optical performance of active silica glass. Hydroxyl groups are extremely active and capable of strong adsorptive interactions with many compounds, which leads to stationary phase instability and column deterioration [13]. For example, hydroxyl has to be reduced to improve its optical transmission of 157 nm. [14]. Despite its disadvantages, however, there is still report demonstrating that proper hydroxyl incorporation can enable a better durability to high-power laser [15]. Hydroxyl fused silica is also found to exhibit extrinsic red PL induced by femtosecond laser [16]. Then Kawashima et al fabricated invisible two-dimensional barcode also inside hydroxyl fused silica by femtosecond laser processing using a computer-generated hologram [17]. Since then, there is no more work particularly concentrating on ultrashort pulsed laser generated point defects in hydroxyl fused silica, let alone the application of its points defects.
In this letter, we focus on hydroxyl fused silica, and report a systematic research on the characteristics of point defects generated by an ultrashort pulsed laser. And by adjusting the concentration of hydroxyl, the behavior of ultrashort pulsed laser selectively destroying hydroxyl groups is monitored. Finally, in investigating the dependence of point defects generation on laser conditions, prospective use of defect manipulation and fabrication in one-chip exploited for anti-counterfeiting and lab-on-a-chip is also discussed.
2. Experiment
During the experiment, a 1 kHz regenerative amplified Ti: sapphire mode-locked laser system (Spectra Physics) was used with 800 nm wavelength. The pulse energy was controlled by using a variable optical attenuator (Thorlabs, NDC-1000C-4M). Linearly polarized laser beam was guided into a microscope and focused into the sample at a depth of 500 µm beneath the sample surface with a 10 times objective (focal length 20 mm). Each hydroxyl fused silica was pre-scanned a 1cm${\times} $1cm area by ultrashort pulsed laser for optical tests. If not specially mentioned, the ultrashort pulsed laser was operated at a pulse duration of 200 fs and laser power of 15 mw. An UV-VIS-NIR spectrophotometer (Perkin Elmer Lambda 1050) with a resolution of 0.05 nm was used to record the absorbance of the irradiated fused silica in the wavelength range of 200-800 nm. The fluorescence spectra and lifetime of the defects were measured with instrument FLSP920 (Edinburgh instruments Ltd., UK). The corresponding time and spectral resolution are 200 ns and 0.05 nm, respectively. Mid Fourier Transform IR (FT-IR) spectrum with a resolution of 0.09 cm−1 was recorded in by Nicolet 6700 FT-IR spectrometer. All experiments were performed at room temperature and under atmospheric pressure.
3. Results and discussion
3.1 Hydroxyl content characterization
Samples with three different hydroxyl concentration are used. The FT-IR spectrum is performed in the range of 4000−2000 cm−1 (Fig. 1). One absorption peak around 3670 cm−1 indicates the stretching mode of hydroxyl. The hydroxyl concentration is thus calculated according to Lambert-Beer law, ${\textrm {C}_{\textrm {OH}}} = \textrm{[}{{{{\textrm M}_{\textrm {OH}}}} \mathord{\left/ {\vphantom {{{M_{OH}}} {({{\mathrm{\varepsilon}} \cdot {\mathrm{\rho}} \cdot {\textrm d}} )}}} \right.} {({{\mathrm{\varepsilon}} \cdot {\mathrm{\rho}} \cdot {\textrm d}} )}}\textrm{]} \cdot {\textrm{lg}}({{{{\textrm I}_0}} \mathord{\left/ {\vphantom {{{{\textrm I}_0}} {\textrm I}}} \right.} {\textrm I}})$, where ε (77.5 L·mol−1 ·cm−1) and MOH (17g·mol−1) is the extinction coefficient and molar mass of hydroxyl, respectively. ρ (2.2 g ·cm−3) is the density of fused silica, and d is the thickness of the sample. I0 and I indicate the transmittance of the baseline and the absorption peak (∼3670 cm−1) of hydroxyl. The concentration of hydroxyl is in parts per million (ppm) by weight. The thickness of samples A, B and C is 2 mm, 2 mm, and 5 mm, respectively. Hydroxyl content corresponding to A, B and C is therefore calculated to be 1214 ± 145 ppm, 82 ± 2 ppm and 10 ± 1 ppm, respectively. And according to hydroxyl content, sample A-C is also denoted as high-OH, mid-OH and low-OH fused silica.
Fig. 1. FTIR spectra of high-OH (A), mid-OH (B) and low-OH (C) fused silica. The inset table illustrated the OH content of all the three samples.
3.2 Defects characterization in hydroxyl fused silica
PL-PLE experiment was firstly launched to illustrate all the possible fluorescence defects of laser irradiated hydroxyl fused silica. From Fig. 2(a), we can see that all the three samples exhibit two PL bands peaking at 290 nm (4.3 eV) and 468 nm (2.65 eV) corresponding to the excitation spectra locating at about 250 nm. It is now well accepted that the 250 nm (5.0 eV) excitation band accompanied by 2.65 and 4.3 eV PL band is a signature of unrelaxed oxygen deficiency center (ODC (II)) in fused silica [7,18,19]. Due to the conversion of ODC (I) to ODC (II), the two emission bands of 2.65 eV and 4.3 eV involve another excitation band at 7.6 eV, which is related to relaxed oxygen vacancies (ODC (I)) [20,21]. Both ODC (II) and ODC (I) are two distinct configurations of oxygen deficiency center. However, an alternative non-radiative pathway competes with the conversion process of ODC (I) to ODC (II), so the PL bands at 2.65 eV and 4.3 eV undergo a significant thermal quenching under excitation at 7.6 eV [21]. In addition to ODC (II), high and mid-OH fused silica also give rise to a distinct PL band peaking at 650 nm (1.9 eV) under the excitation of 268 nm (Fig. 2(b)). This PL band at 1.9 eV is attributed to the non-bridging oxygen hole center (≡ Si−O·, NBOHC), which is the most important oxygen-excess defect in silica [22]. However, low-OH fused silica exhibits no 1.9 eV PL band with the same excitation, which indicates that hydroxyl is the origin of NBOHC. In Fig. 2(b), we noticed another small PL band around 580 nm, which appears only in mid and low-OH fused silica. The origin of this PL band is still under investigation.
Fig. 2. PL-PLE mapping of laser induced (a) ODC (II) and (b) NBOHC in high-OH (A), mid-OH (B) and low-OH (C) fused silica.
We also notice that part of the 290 nm emission band of ODC (II) overlaps with the excitation spectrum of NBOHC (ranging from 240 to 320 nm), so PL band of ODC (II) at 290 nm may probably be reabsorbed by NBOHC. This could be taken as the reason for the observed lower emission intensity of ODC (II) in high-OH fused silica than in low-OH fused silica. By noting that the vertical scale of NBOHC emission intensity is increased about 4 times with respect to that of ODC (II) under the same excitation intensity, we deduce that NBOHC are created much more efficiently than ODC (II) in hydroxyl fused silica.
To further confirm the extrinsic defects, we also measured the absorbance prior to and after laser irradiation (Fig. 3). For high-OH fused silica, laser irradiation induces a great increase in the absorption band ranging from 190-350 nm comparative to pre-irradiated one. Laser exposed mid-OH fused silica shows similar absorption profile to high-OH fused silica while with lower intensity. In the broad laser induced absorption band, the relative narrow peak at 214 nm (5.8 eV) is due to SiE’ center. SiE’is the best known paramagnetic color center in irradiated silica glass, with its unpaired spin located in sp3-like orbital of 3-coordinated silicon atom [23]. Another shoulder peak around 268 nm is due to NBOHC [22,23]. As expected, NBOHC’s absorption region corresponds to its excitation band. Considering the existence of ODC (II) and its excitation band locating at about 250 nm, one can then reasonably assume that there may exists another low intensity absorption region of ODC (II) just around 250 nm. Due to inevitable wavelength overlap, we cannot clearly distinguish the absorption peak of ODC (II) (250 nm) from the two absorption band of SiE’ center (214 nm) and NBOHC (268 nm). Noteworthy, from the enlarged spectra, there is another weak band occurring at around 630 nm in high-OH fused silica. We assign this peak also to the presence of NBOHC [23]. Comparing the absorption bands all, we can come to three plain conclusions: First, part of SiE’ center and ODC (II) are intrinsic in all the three hydroxyl fused silica and later laser irradiation can enhance the density of the two defects; Second, NBOHC is totally extrinsic and originates from hydroxyl. Third, hydroxyl can also facilitate the generation of SiE’ defect center as SiE’ centers are created much more efficiently in high-OH fused silica than in low-OH fused silica. Considering the existence of Si-H indicated by the absorption band around 2250 cm−1 in Fig. 1, Si-H may also be a precursor of the SiE’ center due to a laser radiation-induced radiolysis process [24]. In addition, radiolysis of OH can also give rise to the generation of mobile species (H and H2). H and H2 can react with both NBOHC and SiE’ centers, which will influence the generated defects of NBOHC and SiE’ centers [25,26]. Therefore, laser induced point defect is a quite complicated process, which needs further investigation.
Fig. 3. Absorption spectra of laser irradiated high-OH (A), mid-OH (B), low-OH (C) and pre-irradiated high-OH (p-A) fused silica. The inset is enlarged spectra indicated by the black dashed box.
3.3 NBOHC properties in hydroxyl fused silica
From above discussion, we have observed that the generated three kinds of point defects show a dependence on hydroxyl concentration. When hydroxyl content is less than 10 ppm, only ODC (II) and SiE’ are detectable. By improving hydroxyl content, another luminescence center of NBOHC emerged. Meanwhile, nonluminous center SiE’ has also been enhanced. Luminescence intensity of NBOHC follows an increasing trend with growing hydroxyl concentration. Tested by application, only with hydroxyl content higher than 1000 ppm, the red luminescence of NBOHC can be discernible to the naked eye under the photostimulation of a lab used UV-254 nm light. That is, from application point, we need NBOHC to become the dominant defect species. So high-OH fused silica is the desired candidate.
In this section, high-OH fused silica is the study object, and NBOHC is the interested defect. So we exclusively discuss the properties of NBOHC and its dependence on laser conditions. The PL decay curve of NBOHC follows well a single exponential function I(t) ∝ exp(-t/τ). PL lifetime is defined as the time when PL intensity decreases to e−1 fold of the initial intensity. This was produced by fitting curve in Fig. 4(a). The lifetime of NBOHC is 15 µs, and following the same way, the lifetime of ODC (II) at 290 nm and 468 nm are fitted to be 3 ns and 8 ms, respectively. NBOHC is stable at room temperature, however it degrades under high temperature condition. Based on the discussion of Fig. 3, we could use the absorption spectra to depict NBOHC’s temperature performance. Before absorption measurement, laser irradiated high-OH fused silica are heat annealed at each temperature for 2 h. As shown in Fig. 4(b), absorbance of high-OH fused silica are compared after heat annealing at 100 to 400 ${^{\circ}}{{\textrm C}}$. Below 100 ${^{\circ}}{{\textrm C}}$, NBOHC is relatively stable, and it begins a rapid degradation at 300 ${^{\circ}}{{\textrm C}}$, and nearly disappears at 400${\; }{^{\circ}}{{\textrm C}}$.
Fig. 4. NBOHC’s intrinsic properties of (a) lifetime and (b) thermal endurance, and the dependence of NBOHC’s PL intensity on (c) pulse duration and (d) laser power.
Above discussion concerning Fig. 2 and Fig. 3 reveal that hydroxyl concentration is the first important factor influencing the formation of NBOHC. We have testified that NBOHC itself is extrinsic, so laser pulse width and power are varied to explore how laser conditions make an impact on the generation of NBOHC. In Fig. 4(c), pulse duration is adjusted from 180 to 390 fs while keeping laser power at 9 mw. Before 300 fs, there is no obvious intensity change. With increasing of pulse duration more than 300 fs, NBOHC emission intensity decreases gradually. Considering the laser induced visible damage, laser power is controlled at a low level so that the scanned region still keeps transparent in the sunlight. By keeping pulse duration at 253 fs, NBOHC PL intensity follows an increase trend with laser power below 20 mw (Fig. 4(d)).
3.4 Applications of NBOHC in high-OH fused silica
In section 3.3, we have discussed when laser power is below 20 mw, laser irradiated zone appears still transparent and the written pattern is invisible by naked eye under sunlight. This capability opens the possibility of anti-counterfeiting. We fabricate a barcode in high-OH fused silica with a laser power of 15 mw and pulse duration of 200 fs. As expected, no laser irradiated signs can be seen in the sunshine (Fig. 5(a)). With excitation of UV light (central wavelength at 254 nm), a vivid red barcode clearly appears (Fig. 5(b)). Apart from barcode, any other anti-counterfeiting pattern can also be written in high-OH fused silica, including characters, two-dimension code, geometric designs, etc. This technique provides advantages over the conventional anti-counterfeiting methods. First, the defects related to red PL is invisible under white light and stable at all seasons. So it is effective against unauthorized production. Besides, laser spatially selective machining enables the encoding of two and three-dimensional copyright patterns, hence it is difficult to duplicate.
Fig. 5. High-OH fused silica with a barcode fabricated in it (a) under sunlight and (b) UV-254 nm exposure.
In addition to anti-counterfeiting, NBOHC featured PL in fused silica can also find applications in lab-on-a-chip. This potential use is favored by three key factors: First, the luminescence of NBOHC is around 650 nm, and this emission band is widely applied for biological and biomedical purposes, especially in skin phototherapy [27]. Hence it is a novel light source for detection and analysis of biomedical assays. Second, fused silica is a very attractive substrate due to its unmatched optical transparency and outstanding mechanical, chemical and thermal resistance. It is now one of the most important high-performance microchip materials used for scientific research [28]. Third, ultrafast laser direct writing technique provides a way for producing 3D micro-optics in fused silica. Sophisticated functional units with integrated modules can be fabricated in one chip.
In Fig. 6(a), we designed a miniaturized biological analytical system in high-OH fused silica. The lower part is a standard lab-on-a-chip module. It encompasses elemental set of components, such as electrode arrays, separation columns, and reservoirs, etc. The upper layer is the micron-sized defect arrays. Under the illumination of UV light, NBOHC gives out stabilized red luminescence. This red light is therefore coupled to each corresponding underneath reservoir cell. Other additional component, for example, sensors and detectors, can also be integrated on-chip. Based on this platform, optical detection experiments such as absorbance or fluorescence analysis can be achieved. Figure 6(b) is a real arrays of micron-sized defects featured PL.
Fig. 6. (a) A schematic lab-on-chip system and (b) a fabricated arrays of micron-sized defects of NBOHC featuring red PL based on high-OH fused silica.
4. Conclusion
High, middle and low hydroxyl fused silica are subject to ultrashort pulsed laser irradiation. Three different kinds of point defects-SiE’, ODC (II) and NBOHC are generated. Ultrashort laser pulses are proved to selectively destroy strained Si-O bonds and hydroxyl groups, thus enhancing the generation of SiE’ and NBOHC. NBOHC is a luminous defect center, and its luminescence intensity can be regulated in a large range depending on hydroxyl content, laser power and pulse duration. Optimized by laser condition, high-OH fused silica provides as the extraordinary host for NBOHC, and NBOHC can give stable red photoluminescence. Based on ultrashort pulsed laser fabrication and defect manipulation technique, anti-counterfeiting and lab-on-a-chip applications are advocated on high-OH fused silica. Other prospective applications are still to be found.
Funding
National Key Research and Development Program of China (2016YFB1102402); National Natural Science Foundation of China (61675214).
Disclosures
The authors declare no conflicts of interest.
References
1. R. D. Maurer, “Glass fibers for optical communications,” Proc. IEEE 61(4), 452–462 (1973). [CrossRef]
2. C. W. Gwyn, “Model for radiation-induced charge trapping and annealing in the oxide layer of MOS devices,” J. Appl. Phys. 40(12), 4886–4892 (1969). [CrossRef]
3. S. Fahr, T. Clausnitzer, E.-B. Kley, and A. Tünnermann, “Reflective diffractive beam splitter for laser interferometers,” Appl. Opt. 46(24), 6092–6095 (2007). [CrossRef]
4. M. Rothschild, D. J. Ehrlich, and D. C. Shaver, “Effects of excimer laser irradiation on the transmission, index of refraction, and density of ultraviolet grade fused silica,” Appl. Phys. Lett. 55(13), 1276–1278 (1989). [CrossRef]
5. S. G. Demos, M. Staggs, and M. R. Kozlowski, “Investigation of processes leading to damage growth in optical materials for large-aperture lasers,” Appl. Opt. 41(18), 3628–3633 (2002). [CrossRef]
6. D. L. Griscom, “Optical properties and structure of defects in silica glass,” J. Ceram. Soc. Jpn 99(1154), 923–942 (1991). [CrossRef]
7. L. Skuja, “Optically active oxygen-deficiency-related centers in amorphous silicon dioxide,” J. Non-Cryst. Solids 239(1-3), 16–48 (1998). [CrossRef]
8. L. Skuja, H. Hosono, and M. Hirano, “Laser-induced color centers in silica,” Proc. SPIE 4347, 155 (2001). [CrossRef]
9. K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a femtosecond laser,” Opt. Lett. 21(21), 1729–1731 (1996). [CrossRef]
10. J. Zhang, M. Gecevičius, M. Beresna, and P. G. Kazansky, “Seemingly unlimited lifetime data storage in nanostructured glass,” Phys. Rev. Lett. 112(3), 033901 (2014). [CrossRef]
11. M. Watanabe, S. Juodkazis, H.-B. Sun, S. Matsuo, H. Misawa, M. Miwa, and R. Kaneko, “Transmission and photoluminescence images of three-dimensional memory in vitreous silica,” Appl. Phys. Lett. 74(26), 3957–3959 (1999). [CrossRef]
12. H.-B. Sun, S. Juodkazis, M. Watanabe, S. Matsuo, H. Misawa, and J. Nishii, “Generation and recombination of defects in vitreous silica induced by irradiation with a near-infrared femtosecond laser,” J. Phys. Chem. B 104(15), 3450–3455 (2000). [CrossRef]
13. B. W. Wright, P. A. Peaden, M. L. Lee, and G. M. Booth, “Determination of surface hydroxyl concentration on glass and fused silica capillary columns,” Chromatographia 15(9), 584–586 (1982). [CrossRef]
14. Y. Ikuta, S. Kikugawa, M. Hirano, and H. Hosono, “Defect formation and structural alternation in modified SiO2 glasses by irradiation with F2 laser or ArF excimer laser,” J. Vac. Sci. Technol., B: Microelectron. Process. Phenom. 18(6), 2891–2895 (2000). [CrossRef]
15. S. Liu, S. P. Zheng, K. Yang, and D. P. Chen, “Radiation-induced change of OH content in Yb-doped silica glass,” Chin. Opt. Lett. 13(6), 060602 (2015). [CrossRef]
16. J. B. Qiu, A. Makishima, T. Uchino, and Y. Kawamoto, “Ultrashort-pulse-laser-induced fine structure in synthetic fused silicas,” Proc. SPIE 5350, 281–288 (2004). [CrossRef]
17. H. Kawashima, M. Yamaji, J. Suzuki, and S. Tanaka, “Invisible two-dimensional barcode fabrication inside a synthetic fused silica by femtosecond laser processing using a computer-generated hologram,” Proc. SPIE 7925, 79251C (2011). [CrossRef]
18. H. Imai, K. Arai, H. Imagawa, H. Hosono, and Y. Abe, “Two types of oxygen deficient centers in synthetic silica glass,” Phys. Rev. B 38(17), 12772–12775 (1988). [CrossRef]
19. S. Girard, A. Alessi, N. Richard, L. Martin-Samos, V. De Michele, L. Giacomazzi, S. Agnello, D. D. Francesca, A. Morana, B. Winkler, I. Reghioua, P. Paillet, M. Cannas, T. Robin, A. Boukenter, and Y. Ouerdane, “Overview of radiation induced point defects in silica-based optical fibers,”,” Rev. Phys. 4, 100032 (2019). [CrossRef]
20. D. Donadio, M. Bernasconi, and M. Boero, “Ab initio simulations of photoinduced interconversions of oxygen deficient centers in amorphous silica,” Phys. Rev. Lett. 87(19), 195504 (2001). [CrossRef]
21. S. Agnello, R. Boscaino, M. Cannas, F. M. Gelardi, M. Leone, and B. Boizot, “Competitive relaxation processes of oxygen deficient centers in silica,” Phys. Rev. B 67(3), 033202 (2003). [CrossRef]
22. L. Skuja, “The origin of the intrinsic 1.9 eV luminescence band in glassy SiO2,” J. Non-Cryst. Solids 179, 51–69 (1994). [CrossRef]
23. L. Skuja, H. Hosono, and M. Hirano, “Laser-induced color centers in silica,” Proc. SPIE 4347, 155 (2001). [CrossRef]
24. F. Messina and M. Cannas, “Photochemical generation of E’ centre from Si-H in amorphous SiO2 under pulsed ultraviolet laser radiation,” J. Phys.: Condens. Matter 18(43), 9967–9973 (2006). [CrossRef]
25. K. Kajihara, L. Skuja, M. Hirano, and H. Hosono, “In situ observation of the formation, diffusion, and reactions of hydrogenous species in F2-laser-irradiated SiO2 glass using a pump-and-probe technique,” Phys. Rev. B 74(9), 094202 (2006). [CrossRef]
26. F. Messina and M. Cannas, “Character of the reaction between molecular hydrogen and a silicon dangling bond in amorphous SiO2,” J. Phys. Chem. C 111(18), 6663–6667 (2007). [CrossRef]
27. P. Avci, A. Gupta, M. Sadasivam, D. Vecchio, Z. Pam, N. Pam, and M. R. Hamblin, “Low-level laser (light) therapy (LLLT) in skin: stimulating, healing, restoring,” Semin. Cutan. Med. Surg. 32(1), 41–52 (2013).
28. A. Hibara, T. Saito, H.-B. Kim, M. Tokeshi, T. Ooi, M. Nakao, and T. Kitamori, “Nanochannels on a fused-silica microchip and liquid properties investigation by time-resolved fluorescence measurements,” Anal. Chem. 74(24), 6170–6176 (2002). [CrossRef]
