Germano-silicate glass fiber containing gold nanoparticles was developed by modified chemical vapor deposition and solution doping processes. Pumping with 488nm Argon ion laser, we firstly report on the visible to infrared photoluminescence of the gold nanoparticles embedded in the core of the germano-silicate fibers. The surface plasmon resonance absorption peak at 498.4nm and the visible to infrared photoluminescence over the range of 600nm~1560nm were found and explained according to the interband and intraband electronic transitions of Au atoms. The averaged quantum efficiencies of the photoluminescence at 833nm and 1536nm were estimated to be 5.75×10-8 and 2.01×10-9, respectively.
©2007 Optical Society of America
Noble-metal nanoparticles encapsulated in a dielectric matrix have attracted sustained interest over several centuries owing to their unusual optical and electrical properties. Metal-dielectric nanocompositions show nonlinear and fast optical response near the surface plamon resonance (SPR) frequency due to their enhanced third-order optical susceptibilities; thus they have been applied in all-optical switching devices [1–3], ultra-short pulse generation  and optical parametric amplification [5,6]. By the multiphoton-absorption-induced luminescence (MAIL), efficient luminescence from 100nm gold nanoparticles was presented to date  and the deeper understanding of the particle-size-dependent MAIL was performed , which can be used for biological labeling and imaging. The upcoming research interests in noble metal nanoparticles (NPs) in the field of photonics are putting forth a new branch of optics: nanophotonics, which focuses on the linear, nonlinear, photoluminescence and other novel optical properties found in the NPs and the materials containing these NPs.
Several approaches, such as the sol-gel process, metal dielectric co-sputtering deposition, metal-ion implantation into a dielectric matrix and pulsed laser deposition, have been used to prepare metal-dielectric nanocompositions [9–12]. Based on the previous work to fabricate the germano-silicate glass fiber incorporated with gold NPs  by using the modified chemical deposition (MCVD) and easily-performed solution doping processes, we provide another way to dope the noble metal NPs into the fiber-type waveguides, opening a new method to conduct research in the field of nanophotonics.
Having reported the ultrafast third-order nonlinearity of the gold NPs incorporated fiber , we firstly started to investigate the photoluminescence (PL) from these gold NPs embedded in the germano-silicate glass matrix. Visible PL from smooth gold surface was reported by Mooradian in 1969 and was found to be very inefficient (10-10) . In the present study, while remaining relative weak, the broadband visible to narrowband infrared PL from gold NPs embedded in the germano-silicate fiber was found and proven to be measurable at room temperature with the normal optical measurement systems. The averaged quantum efficiencies (QEs) of the PL at 833nm and 1536nm were estimated to be 5.75×10-8 and 2.01×10-9, respectively. Based on the present optical characteristics of the germano-silicate glass fiber containing gold NPs, novel fiber devices for nonlinear optical applications will be developed.
The fiber incorporated with gold NPs was fabricated in house using the MCVD and solution doping processes, which was detailedly described in Ref. 13 reported by our group. The doping solution was prepared by dissolving reagent grade Au(OH)3 powder (Aldrich Chem. Co. Inc., 99.9%) in HNO3 solution (Junsei Co., 70%) to obtain Au3+ concentration of 2.5mol%. After the deposition of the germano-silicate core layers in the silica glass tube by using the MCVD process, the porous deposition layers were soaked with the doping solution for two hours. Then the tube was sintered and sealed, and finally the preform was drawn into fibers with 125μm in diameter using the draw tower at 1900°C. The chemical reactions inside the silica glass tubes during the MCVD process are as follows :
Due to the boiling temperature as high as 2856°C , Au atoms and their clusters can survive from the MCVD process with the temperature up to 2350 °C. Since the structure of the fiber (6-8μm diameter core surrounded by 125μm diameter outside cladding) and the extremely low dopant concentration of ppm level make the gold NPs difficult to detect by direct measurement methods, such as Transmission Electron Microscopy (TEM) and X-ray diffraction (XRD), the cut-back method was used to measure the absorption spectrum of the optical fiber to confirm the formation and existence of the gold NPs in the core of fiber.
To obtain PL, the fiber was pumped with a 488nm Argon-ion laser at room temperature. Figure 1 illustrates the experiment setup in which the 488nm laser beam from Argon-ion laser was reflected by two mirrors and then coupled into one arm of a 3dB coupler by a collimator. At the same time, one power controller was used to measure the feed-back power from the other arm of the given coupler to determine the actual input power into the fiber incorporated with gold NPs under test. In order to get accurate luminescence signal, high sensitive optical spectrum analyzer (OSA) was used.
3. Results and discussion
3.1 Linear absorptive optical properties
Figure 2 shows the absorption spectrum of the germano-silicate glass fiber incorporated with gold NPs. The absorption peak appeared in the range of 495~560nm, which was due to the SPR of the gold NPs embedded in the fiber core and is the indirect evidence of the existence of gold NPs. The SPR peak of the fiber was found to locate centered at 498.4nm (2.48eV), which is shorter than the SPR peak wavelengths found in the other gold NPs incorporated materials [16,17]. This blue shift of the present fiber may be due to denser structure of the core and the subsequent increase of the ligand field .
3.2 Broadband visible to narrowband infrared photoluminescence
The PL of the fiber was obtained by pumping with the 488nm Argon-ion laser at the power of 0 ~ 90 mW and shown in Fig. 3(a). The broadband visible PLs in the vicinity of 833nm (1.48eV) to the narrowband infrared PL centering 1536nm (0.81eV) were found. Note that the infrared PL at 1536nm is the first time to report in the field of fiber optics.
where h is Plank’s constant, c is the light speed in vacuum, λ is the wavelength, n is the refractive index at a given wavelength (assuming that n is constant for all the wavelength from 400~1650nm in this estimation), I is the intensity of the light wave in arbitrary unit, and the suffixes of ‘out’ and ‘ex’ represents the case for the output emission and excitation input signal, respectively. The averaged QE of the PL at 833nm (1.48eV) and 1536nm (0.81eV) were estimated to be 5.75×10-8 and 2.01×10-9, respectively. As shown in Fig. 3(b), with the increase of the input pumping power, the QEs were found to decrease due to the extremely low concentration of gold NPs embedded in the fiber, which means that the number of the gold NPs is not enough to absorb all the input pumping energy as expected. It is important to note that the visible PL intensity at 833nm is two orders of magnitude larger than that of the bulk gold metal .
To explain the PL from the gold NPs in the core of the fiber, the electronic structure of Au atoms was considered. In the case of noble metals, in particular gold, the optical properties are due to 5d (valence) and 6sp (conduction) electrons . The outermost d and s electrons of the constituent atoms must be treated together leading to 6 bands: 5 of them are fairly flat and lie a few eV below the Fermi level, they are usually denoted as d bands, the 6th one being almost free-electron-like, i.e. roughly parabolic with an effective mass very close to that of a free electron as shown in Fig. 4. This last band is known as the conduction band or sp band where there is an opening electronic band gap of 1.3eV (950nm) between the highest occupied (molecular) orbital and the lowest unoccupied orbital (HOMO-LUMO gap) . The results in Fig. 3(a) suggest that the luminescence located at 833nm (1.48eV) cannot originate from a totally excited state across the HOMO-LUMO gap at 1.3eV. Instead, the radiative recombination between a higher excited state and the ground state has to be concluded. On the other hand, the luminescence maximum of the infrared luminescence shown in Fig. 3(a) occurs at lower energies of 0.81eV (1536nm) than the onset of absorption (HOMO-LUMO gap). Furthermore, the fact that the PL intensity and QEs of the two bands are independent of the excitation power as shown in the insets of Fig. 3(b) indicates that the excitation involves a one-photon process in both cases.
As for the gold NPs embedded in the germano-silicate glass matrix, an excitation at 488nm (2.53eV) from the Argon-ion laser may lead to the excitation of d-band electrons into the sp-conduction band (interband transition) and a radiative recombination is followed by an initial electronic relaxation bringing about the visible luminescence, judging from the fact that the luminescence between 629nm (1.97eV) and 1200nm (1.03eV) was independent of the excitation wavelength. The high energy luminescence band, therefore, corresponds to the recombination of the excited electron from higher excited states in the sp-band with the hole in the lower lying d-band (interband transition) as illustrated in the schematic model in Fig. 4. By the same token, the lower energy luminescence band in the vicinity of 1536nm (0.81eV) can then be assigned to be the relaxed radiative recombination across the HOMO-LUMO gap at 1.3eV within the sp-conduction band (intraband transition). Host-materials characterized ligand fields are responsible for the tiny differences between our results and the previous reports [22–24].
In summary, the germano-silicate glass fiber incorporated with gold NPs was successfully fabricated by using MCVD technique and solution doping process. At room temperature, weak but distinct broadband visible to narrowband infrared PL was found upon excitation by the 488nm Argon-ion laser. The QEs of the PL at 833nm and 1536nm were estimated to be 5.75×10-8 and 2.01×10-9, respectively. Interband transition between the sp and d-band and intraband transition across the HOMO-LUMO electronic gap inside the gold NPs are responsible for the observed PL from the gold NPs incorporated germano-silicate fiber.
This research was partially supported by Korea Science and Engineering Foundation (KOSEF) through grant No.R01-2004-000-10846-0, by the National Core Research Center (NCRC) for Hybrid Materials Solution of Pusan National University, and by BK-21 Information Technology Project, Ministry of Education and Human Resources Development, Republic of Korea.
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