3.1.

Engineered Structures

Three samples were fabricated with different glass and polymer thickness ratios ($2:1$, $1:1$, and $1:2$) to investigate their stability and structural integrity.

All structures were highly optically transparent and stable except for (3), which is disintegrated almost a week later due to very large-internal stress in the deposited film. Therefore, this particular sample was not used for any studies reported hereafter. We attribute this effect to the larger thickness of the polymer layer leading to stress relaxation over time overcoming the equilibrium force provided by the thinner glass layer. As it is evident from Fig. 1, there is a strong-contrast difference in transmission electron microscopy (TEM) images of the glass and polymer layers, which can be seen to exist in a well organized and spatially very well-defined manner. The lighter shaded layers are of low-density polymer material whilst the darker layers are that of tellurite glass with a much higher comparative density. The deposited layers have a thickness error of $\pm 0.3\mathrm{nm}$, which illustrates the excellent deposition quality of the layers. However, the cross-sectional area homogeneity of the layers is limited to $\sim 100\times 100{\mathrm{nm}}^2$ due to particle inclusions. The formation of such particles within and its effects on the growth of the nanolayers is discussed in our previous publication.⁹

Fig. 1

TEM of a cross section of superlattice structure with glass to polymer ratio of (a) $2:1$ and (b) $1:1$

3.2.

Optical Transmittance and Fluorescence Properties

The engineered superlattice films show very high transparency as shown in Fig. 2(a). The optical transmittance was well above 93% within the wavelength range of 700 to 2000 nm. Transparency is one of the major challenges in the case of organic–inorganic hybrid materials, and this plasma-based deposition technique is shown to overcome this. Two sets of films were produced, one doped with ${\mathrm{Er}}^{3+}$ and the other doped with ${\mathrm{Eu}}^{3+}$ ions. The ${\mathrm{Eu}}^{3+}$ ions–doped film was excited using 532-nm laser and the emission was recorded in 560 to 700 nm range, and the fluorescence in this range useful for chemical and gas sensing applications where conventionally fluorescent dyes are used.^10,12 The most prominent peak of the ${\mathrm{Eu}}^{3+}$ ions in films is observed at 614 nm corresponding to the ${{}^5\mathrm{D}}_0$ to ${{}^7\mathrm{F}}_2$ transition is shown in the spectrum normalized to this peak in Fig. 2(b). The peak at 593 nm resulting from ${{}^5\mathrm{D}}_0$ to ${{}^7\mathrm{F}}_1$ transition of excited ${\mathrm{Eu}}^{3+}$ ions is magnetic dipole in nature while the ${{}^5\mathrm{D}}_0$ to ${{}^7\mathrm{F}}_2$ transition is electric dipole in nature. This electric dipole transition is known to be highly sensitive to local coordination environment. Therefore, we can attribute the relative increase in intensity of the 593-nm peak in the normalized spectrum to the change in local coordination environment of ${\mathrm{Eu}}^{3+}$ ions in the film comparing with the target glass, which affect the electric dipole transition (${{}^5\mathrm{D}}_0$ to ${{}^7\mathrm{F}}_2$). The relative change in the line shapes of the two spectra are also related to the above particular properties of the two different transitions involved. For erbium-doped films, the fluorescence spectra [Fig. 2(c)] were measured from the films upon 980-nm laser excitation. ${\mathrm{Er}}^{3+}$: ${{}^4\mathrm{I}}_{13/2}\to {{}^4\mathrm{I}}_{15/2}$ transition peaked at 1534 nm showed the almost exact line shape compared with the bulk glass and this spectrum is also shown to demonstrate absorption at 980 nm by the film. A small drop (0.2 ms) in the fluorescence lifetime for ${\mathrm{Er}}^{3+}$-doped superlattice in comparison to doped PT glass was observed in previous studies and discussed therein.⁹ On comparing the line shapes of the doped systems, we can say that there is better stoichiometric transfer of the ${\mathrm{Er}}^{3+}$-doped glass than ${\mathrm{Eu}}^{3+}$-doped glass in the PLD. It is almost impossible to record the absorption spectrum with characteristic absorption features of dopants because of the low-volume fraction of the dopant ions in the thin films. The absorption of the laser will produce heating in the superlattice film due the nonradiative relaxations mainly from ${\mathrm{Er}}^{3+}$: ${{}^4\mathrm{I}}_{11/2}$ to ${{}^4\mathrm{I}}_{13/2}$, and this aspect is utilized in the wavelength dependent deflection of the Si microcantilever in Sec. 4.2. The refractive index of the doped (${\mathrm{Er}}^{3+}$ and ${\mathrm{Eu}}^{3+}$) superlattice structures used here were 1.635 at 633 nm, while the doped target glasses were 1.666 and PDMS was 1.454.

Fig. 2

(a) Representative transmittance spectrum of a glass–polymer superlattice structure (glass to polymer ratio of $2:1$) in comparison with the glass only film. Fluorescence spectrum of superlattice samples doped with (b) ${\mathrm{Eu}}^{3+}$ ions and (c) ${\mathrm{Er}}^{3+}$ ions.

4.1.

Europium Rare Earth Ion-Based Sensor

Recent advances in polymer waveguide technologies have demonstrated the potential to cost-effectively integrate optical waveguides onto standard printed circuit boards (PCBs) for use in high-speed board-level optical interconnects.^13–15 However, the futher integration of optical-sensing elements onto these hybrid opto-electronic (OE) PCBs can also enable the formation of low-cost versatile optical sensors suitable for use in a variety of sensing applications.¹⁶ The selective deposition of a glass–polymer superlattice on such polymer-based devices constitutes a promising alternative in generating the required fluorescence for the sensor operation. Initial work toward this direction has been undertaken, by fabricating a waveguide sensor with a deposited glass–polymer superlattice layer.

The sensor was realized by stacking a superlattice layer with selective ${\mathrm{Eu}}^{3+}$ ion doping in the glass layer, onto air-exposed channel polymer waveguides fabricated onto a standard PCB substrate. The waveguides were fabricated from siloxane polymer materials with standard photolithography techniques on a FR4 substrate. These polymer materials, OE-4140 (core) and OE-4141 (cladding) developed by Dow Corning, have been engineered to withstand the high-temperature environments in excess of 300°C, such as that are required for solder reflow and PCB lamination. The FR4 substrate constitutes a low-cost glass epoxy substrate, widely used in the electronics industry. The waveguides are designed to have a large cross section of $\sim 50\times 50{\mu \text{m}}^2$ in order to allow sensor assembly with relaxed alignment tolerances and low-cost tools such as pick-and-place machines.^17,18 The fabricated sample measures $30\times 20{\mathrm{mm}}^2$ and has a 10-mm long air-exposed section in order to allow the deposition of the superlattice onto the waveguides. The superlattice layers were deposited onto the waveguide sample using the PLD method discussed above. The deposition of the superlattice layer onto the exposed waveguides enables evanescensent wave coupling of the pump laser to the superlattice containing the ${\mathrm{Eu}}^{3+}$ ions and subsequently the generation of a red fluorescence signal, which is required for the sensor operation. A schematic of the fabricated device and images of parts of the sample are shown in Fig. 3. The propagation losses of the superlattice layer on silica measured at 633 nm was $\sim 1\mathrm{dB}/\mathrm{cm}$ while the losses for polymer waveguides are below $0.5\mathrm{dB}/\mathrm{cm}$. The propagation losses are mainly contributed by the surface roughness of the film, which is typically around 90 nm for a 1-μm-thick film. Particle inclusions discussed before will also contribute to the Rayleigh scattering at the measurement wavelength.

Fig. 3

(a) Schematic of the printed circuit board-integrated polymer waveguide sensor with the deposited superlattice layer and (b and c) images of the fabricated sample: (b) top view of the sample edge where the deposited superlattice layer is clearly visible and (c) coated polymer waveguides in the air-exposed sensing area.

A 532-nm laser signal was lanuched into the waveguides and the europium ions in the superlattice layers were evanescently excited. Propagation of the broad amplified spontaneous emission (ASE) in the wavelength range of 570 to 650 nm is shown in Fig. 4(a) for a 532-nm laser power of 10 mW. The red color emission upon exciting the superlattice film surface is observed by the camera is shown in Fig. 4(b) while the light collected at the waveguide output clearly shows the ASE emission from the ${\mathrm{Eu}}^{3+}$ ions [Fig. 4(c)]. The ASE spectrum observed is broadened comparing with that of fluorescence spectrum reported in Fig. 2(b), which is attributed mainly to the recording of the weak evanascent signal at lower resolution. The real-time monitoring of the received fluorescense can provide information about the presence of red-absorbing chemical species in the sensor vicinity demonstrated using dye-doped waveguide systems.¹⁰ The experiments to demonstrate gas sensing are currently in progress and the integrated system requires further improvements in terms of ${\mathrm{Eu}}^{3+}$ doping concentration, transmission loss, and film thickness for optimizing the sensitivity.

Fig. 4

(a) Experimental setup for the characterization of the fabricated sample, (b) recorded image from top camera showing red fluorescence from the waveguide and (c) recorded fluorescence at the waveguide output.

4.2.

Erbium Rare Earth Ion-Based Sensor

The erbium ion–based sensor was designed to function as highly sensitive thermal bimorphs for unique applications, such as uncooled IR imaging devices, biological analyte microsensor, high-sensitivity thermal sensor etc. In a related experiment the ${\mathrm{Er}}^{3+}$ ions–doped glass–polymer superlattice was stacked on a Si microcantilever, the optical image of the cantilever before and after coating is shown in Fig. 5. Due to the internal stress which is compressive in nature the cantilever bends downwards when cooled from the growth temperature down to room temperature, and with heating it deflects upward. In order to monitor the nanoscale deflection in response to low-heating temperatures a modified atomic force microscope (AFM) was configured so as to detect the deflection at a nanoscale level. The usual AFM tip-cantilever was replaced by a Si microcantilever coated with the superlattice structure directly mounted on the sample stage. The specific doping of erbium ions in glass layers enables the cantilever to absorb 980-nm laser radiation and heat the superlattice because nonradiative relaxation of ${\mathrm{Er}}^{3+}$ ions in the glass layer from the ${{}^4\mathrm{I}}_{11/2}$ level. The continuous wave (CW) 980-nm laser differential heating of the superlattice and silicon leads to the deflection of the bimorph cantilever. A much lower power 633-nm laser was focused on the cantilever to track the deflection; the schematic of the set up is shown in Fig. 6.

Fig. 5

Si microcantilever (a) before and after (b) deposition of glass–polymer material.

Fig. 6

Schematic of the experimental set up for measuring cantilever deflection using an atomic force microscope.

The deflection corresponding to the increase in temperature was extracted using the measured AFM and optical microscope data. A linear fit [Fig. 7(a)] was obtained for the deflection angle versus temperature graph, the slope of which was calculated to be $0.5\mathrm{nm}/\mathrm{mK}$. The corresponding images of the deflected cantilevers at selected temperatures are shown in Fig. 7(b). In this figure, we can see that the length of the cantilever under focus increases with temperature due to the upward deflection. An advantage of this biomorph methodology comparing with previous reports relying on just polymers¹¹ is that we can design the polymer–glass interlayer thickness so as to obtain a well-controlled internal stress that can provide different levels of sensitivity. Moreover, it is possible to use the setup for monitoring wavelength dependent absorption and corresponding effects in the deflection by choosing a laser with different wavelengths or different lattice periods and materials. Since the superlattice layer can be made absorptive at wavelengths quite different from that of the underlying silicon we plan to exploit the selective heating of the glass–polymer superlattice layer for mid-IR and long wavelength detection. An array of such cantilevers can be produced on large silicon substrates for wavelength specific imaging in the long-infrared wavelength range.

Fig. 7

(a) Graph of Si microcantilever deflection versus temperature, (b) optical microscope images of deflected cantilever at selected temperatures darker regions are progressively bend downward.