Amorphous Silicon-Doped Titania Films for on-Chip Photonics

Article pubs.acs.org/journal/apchd5

Amorphous Silicon-Doped Titania Films for on-Chip Photonics Thomas Kornher,*,† Kangwei Xia,† Roman Kolesov,† Bruno Villa,‡ Stefan Lasse,† Cosmin S. Sandu,§ Estelle Wagner,§ Scott Harada,§ Giacomo Benvenuti,*,§ Hans-Werner Becker,∥ and Jörg Wrachtrup† †

3. Physikalisches Institut, Universität Stuttgart, 70569 Stuttgart, Germany Semiconductor Physics Group, Cavendish Laboratory, JJ Thomson Avenue, Cambridge, CB3 0HE, United Kingdom § 3D-OXIDES, 130 Rue Gustave Eiffel, Saint Genis Pouilly, 01630, France ∥ RUBION, Ruhr-Universität Bochum, 44780 Bochum, Germany ‡

ABSTRACT: High quality optical thin film materials form a basis for on-chip photonic micro- and nanodevices, where several photonic elements form an optical circuit. Their realization generally requires the thin film to have a higher refractive index than the substrate material. Here, we demonstrate a method of depositing amorphous TiO2 films doped with 25% Si on various substrates, a way of shaping these films into photonic elements, such as optical waveguides and resonators, and finally, the performance of these elements. The quality of the film is estimated by measuring thin film cavity Q-factors in excess of 105 at a wavelength of 790 nm, corresponding to low propagation losses of 5.1 db/cm. With a refractive index of n > 2, the film supports waveguiding on different substrates, such as quartz, YAG, and sapphire, and shows evanescent coupling to chromium ions embedded in YAG, presenting film−substrate interaction. Additional functionalization of the films by doping with optically active rare-earth ions such as erbium is also demonstrated. Thus, Si:TiO2 films allow for creation of high quality photonic elements, both passive and active, and also provide access to a broad range of substrates and emitters embedded therein. KEYWORDS: integrated optics devices, integrated optics materials, waveguides, thin film deposition and fabrication, ion implantation, rare-earth ions systems through evanescent light fields14,15 coupling to embedded emitters. Moreover, this approach leaves the substrate crystal untouched from processing and preserves the spectroscopic properties of embedded emitters.16 In the visible range, titanium dioxide (TiO2) features the highest refractive index out of a variety of transparent thin film materials, thus, allowing for waveguiding on the majority of transparent substrates. Additionally, it has a wide transparency range covering the whole visible and near-infrared spectral regions. Shaping the deposited TiO2 film into photonic elements can then be conveniently done by reactive ion etching (RIE).17 The deposited TiO2 films often tend to form nanocrystallites which lead to significant scattering and, therefore, to substantial optical losses.18,19 The best performance is typically shown by amorphous TiO2 films featuring a refractive index between 2.3 and 2.4.19−25 In order to preserve the amorphous composition of the film, special precautions have to be taken. In the present work, we report on Chemical Beam Vapor Deposition (CBVD) of high optical quality Si:TiO2 film whose amorphous state is preserved by doping with silicon.26 The amorphization of deposited TiO2 with Si doping by this technique was already reported.27

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anoscale fabrication of optical waveguide structures offers a wide range of opportunities spanning from on-chip photonic devices for optical networks to sensor applications. At the heart of such devices commonly lies a certain functionality that can be readily interfaced with other components and be included into sophisticated architectures, as it is standard with silicon-on-chip technology.1 Material systems hardly compatible with silicon photonics, such as YAG, YSO, or sapphire crystals, can host a large variety of emitters, foremost rare-earth elements, with applications ranging from quantum communication2−4 and quantum memories5,6 to lasing materials.7,8 Based on the idea of exploiting their potential in resonators, different approaches have been discussed.9−12 As a scalable approach, we propose these substrates to be enabled by a suitable waveguiding platform. Previously demonstrated photonic structures in these materials based on scalable femtosecond laser-writing share common drawbacks like a low refractive index contrast between waveguide and substrate.13 The corresponding increase of the minimum feature size of photonic elements by up to 2 orders of magnitude makes this technology unpractical for the field of cavity quantum electrodynamics (CQED) and nanoscale photonics in general. An alternative way to realize a waveguiding platform is the deposition of a high refractive index thin film on top of these substrate crystals, which then provides on-chip access to these © XXXX American Chemical Society

Received: November 17, 2016

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DOI: 10.1021/acsphotonics.6b00919 ACS Photonics XXXX, XXX, XXX−XXX

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Even though this doping leads to a slight decrease in the refractive index of the film, in agreement with results achieved with other chemical vapor deposition techniques,28 the index is still high enough to support waveguiding on high-index optical crystals such as YAG and sapphire. On the other hand, Si doping keeps the TiO2 from forming nanocrystallites even at elevated temperatures up to 650 °C, resolving the inherent thermal instability of amorphous TiO2 films to some extent.29 The optical performance of the film was assessed by fabricating whispering gallery mode (WGM) cavities and measuring their Q-factor. The latter was in excess of 105. Evanescent coupling to fluorescent spots in the substrate material is shown, and also, additional functionality was added to the Si:TiO2 film by doping it with fluorescent rare-earth ions, such as erbium. This shows the potential of Si:TiO 2 nanophotonic structures to also act as active elements within photonic devices.

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DEPOSITION OF SI:TIO2 COMBINATORIAL FILM Oxide thin films were deposited by CBVD, as described in detail elsewhere.27 The CBVD process makes use of thermal decomposition of organometallic precursors on a heated substrate in a high vacuum environment (≤10−3 Pa). These precursors impinge upon the substrate as molecular beams and do not undergo any gas phase reaction. Deposition was conducted on epi-polished YAG crystals (CrysTec), sapphire and quartz, for functional characterization and on Si and glass wafers for material characterization. The liquid organometallic precursors used during the present investigation were titanium tetraisopropoxide (Ti(OiPr)4, CAS 546−68−9, evaporated from a reservoir at 32 °C) and tetrabutoxysilane (Si(OnBu)4, CAS 4766−57−8, evaporated from a reservoir at 65 °C). The Ti and Si precursor flows were homogeneously distributed on the substrate (the flow ratio of precursors was estimated as Si/ Ti = 1.1). The substrate temperature during the deposition was kept at 500 °C. Under these conditions, the Si doping in the film (defined as Si/(Si + Ti) ratio) was about 25% (as measured by SEM-EDX), and the growth rate was about 10 nm/min. Before starting the deposition, the chamber was pumped to a base pressure of 5 × 10−4 Pa. A liquid nitrogencooled cryo-panel helped to maintain a pressure below 2 × 10−3 Pa during the deposition. The morphology and the chemical composition of the thin films were investigated by SEM-EDX using a Merlin SEM and in TEM cross-section using a Tecnai Osiris microscope. From cross-sectional TEM images, we can estimate the average thickness of deposited films and their growth morphology. Figure 1 shows typical TEM images of a sample film on glass substrate. On 4 in. wafers, the thickness variation of films is ±2%. A high angle annular dark field image (Figure 1 a) shows a homogeneous compact film with relatively low roughness. The inserted selected area electron diffraction pattern together with the high resolution TEM image (Figure 1b) confirm the amorphous phase of the film. Further characterization of the unpatterned Si:TiO2 film of a thickness of 535 nm by ellipsometry (Variable Angle Spectroscopic Ellipsometer from J. A. Woollam Co.) yields the dispersion of the refractive index as shown in Figure 1c). Within the transparency window of the film starting roughly at 400 nm and extening all the way to the infrared, the refractive index ranges between 2.3 and 2.1.

Figure 1. Cross-sectional view by TEM of a Si:TiO2 film deposited on glass substrate. (a) HAADF-STEM image and SAED pattern. (b) HRTEM image with inserted FFT. (c) Refractive index for an unpatterened Si:TiO2 film on silica sample and for comparison the refractive index of bulk anatase TiO2.30 n and k denote the refractive index and the extinction coefficient, respectively. (d) Surface height profile of deposited film after annealing measured with AFM. Film surface roughness is approximately 3 nm.

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STRUCTURING SI:TIO2 FILMS For assessment of the optical quality of the film, we fabricated monolithic optical WGM resonators and tested the width of their resonances. The resonators were microrings and microdisks evanescently coupled to a nearby optical waveguide through which excitation light was supplied. The patterning was done by standard RIE with a 50 nm thick Ni mask defined by e-beam lithography. The e-beam lithography was performed on a double-layered positive PMMA E-beam resist, with a shorter polymer chain length in the bottom layer (200 K) than in the top layer (950 K). This facilitates an undercut of the resist mask due to different exposure sensitivities between the two layers and supports clean metal mask lift-off. RIE of Si:TiO2 was performed in an atmosphere of Ar/CF4/ O2 with the respective flow rates 4/16/3 sccm and the RF power being 100 W, resulting in 387 V bias voltage.31 The process pressure was 15 mTorr and the total time required to etch through 535 nm of Si:TiO2 was 24 min. After etching, the nickel mask was still present, indicating that the etching selectivity was better than 1:11. The residual nickel mask was removed by dissolving the metal in an aqueous 1 M solution of nitric acid. Structured Si:TiO2 films that underwent reactive ion B


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range of 770−810 nm with a mode-hop-free fine-tuning range of 30 GHz. The spectral width of the laser was below 1 MHz. Laser output was injected into one of the ends of the optical waveguide from the top and its frequency was swept while monitoring the scattered emission at the rim of the cavity. No additional measures were taken to further optimize the incoupling of light into the waveguide. We rely only on residual scattering of light into the waveguide at its end (on the order to 10%), which is then propagating in the waveguide mode. Despite this high insertion loss, it was found to be wellsuited for characterization purposes. The rim lit up once the laser was in resonance with one of the cavity modes due to residual scattering on the imperfections of the structured film (see Figure 4a). This scattering was detected as a function of

etching were annealed in air for 4 h at 500 °C in order to restore surface termination. However, annealing the film at temperatures above 800 °C leads to a visual change of the film, suggesting recrystallization. The film has a surface roughness of approximately 3 nm, obtained by atomic force microscopy (shown in Figure 1d). Upon annealing the film, neither a refractive index change nor a change in surface roughness was seen. A sample SEM image of the resulting structure with a WGM resonator radius of 5 μm and a waveguide length of 30 μm is shown in Figure 2. The waveguide is attached to a 100 μm2 pad with incoupling holes and the distance between the resonator and the waveguide is around 400 nm.

Figure 2. SEM image of a Si:TiO2 WGM resonator evanescently coupled to a straight optical waveguide.

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OPTICAL CHARACTERIZATION OF RESONATORS COUPLED TO WAVEGUIDES ON QUARTZ SUBSTRATE In the following, waveguides and resonators were fabricated out of Si:TiO2 thin films on quartz substrate with a refractive index 1.45 at 790 nm. Studies of the optical performance of waveguides and cavities were performed in a home-built confocal microscope with an additional ability of scanning the detection point in the vicinity of the position of the excitation laser focus.32 Its schematic is shown in Figure 3a. The excitation

Figure 4. (a) Camera image of a structure excited on resonance. The left end of the waveguide was used as input and the cavity mode is visualized by the residual scattering of the laser being in resonance. Tunable blue diode laser was used for visualization. (b) Typical mode spectrum of the studied resonator geometry between 780 and 800 nm obtained with a broadband light source. Mode width measurement is limited by the resolution of the spectrometer for the fundamental mode. (c) The spectral shape of the cavity mode at 787.98 nm. It was obtained by sweeping a single-mode laser through the cavity resonance. (d) Resonator model with calculated electric field profile of the fundamental 790 nm mode, used for FSR confirmation.

laser frequency to estimate the spectral width of the mode. The results of the spectral measurements are shown in Figures 4c. As the laser was finely swept around 787.89 nm wavelength, the narrowest line width for fabricated resonators was found to be 2.6 GHz, fitted with the Fano model.33 In this particular case, the gap between waveguide and resonator was large enough to keep the system in the undercoupled regime, in order to minimize waveguide damping. This spectral width of the resonance corresponds to a quality factor of Q ≈ 146000. For this specific resonator geometry with an outer resonator radius of R = 8 μm, a film thickness of 535 nm and a rim width of 2 μm, the mode spectrum was measured with a broadband light source in a wavelength region between 780 and 800 nm in order to extract the free spectral range (FSR) of ΔλFSR = 5.60 nm for the fundamental mode. Including the refractive index measurement, our corresponding finite element method (FEM) based simulation can confirm the measured FSR for the fundamental mode in this resonator geometry. The respective

Figure 3. Schematic diagram of the microscope. The sample is mounted on a 3D nanopositioner and can be moved through the laser focus. Light emission is collected through the same objective lens and sent through a 2F−2F telescope and a tip-tilt mirror onto the pinhole selecting the observation point on the sample. Single photon counting avalanche photodiode (APD) and grating spectrometer equipped with cooled CCD camera were used as detectors.

laser was focused onto the sample with a high numerical aperture (NA) objective lens (Olympus 0.95 × 50). This results in tight focusing of the light on the sample surface with the focal spot size λ/(2NA) = 0.53λ. For characterizing fabricated Si:TiO2 resonators, the width of the cavity resonance was studied with a single frequency tunable diode laser. The laser could be tuned coarsely over the C


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resonator model and the electric field profile of the mode is shown in Figure 4d. With the FSR measurement around 790 nm, the group index of 2.18 was estimated by ng ≈ λ2/(2πR × ΔλFSR). Based on the quality factor measurement and the group index estimation, propagation losses α in fabricated structures of 5.1 dB/cm around 790 nm were estimated by α = 2πng/Qλ. Table 1 compares different TiO2-based thin film waveguiding

almost degenerate modes, indicating that both TE and TM modes are supported in the resonator. Depending on the acquisition position on the cavity rim, one of these two modes provides the more prominent signal. With extracted line widths of Δν636.5nm = 34 Ghz, Δν805.1nm = 6.4 Ghz, Δν1013.3nm = 5.9 Ghz, corresponding to quality factors of Q636.5nm = 14000, Q805.1nm = 58000, and Q1013.3nm = 50000, these fabricated resonators show deteriorated quality compared to the lowest loss measured in this material at 790 nm shown above. Still, for an assessment of the film over a broader spectral region, the measurements provide sufficient insight. Based on the modeled FSR for each respective wavelength, the propagation loss was estimated to be α636.5nm = 68.6 dB/cm, α805.1nm = 12.5 dB/cm, and α1013.3nm = 11.6 dB/cm. With the gap size range on the sample ensuring coupling for all three wavelength regimes, the propagation loss measurement for 1000 nm suggests waveguide damping. This reasoning is supported by observations of scattered light intensities on light-propagating waveguides when compared to cavities on resonance, as shown in Figures 5d or 4a. The absence of scattered light on the waveguide at 1000 nm indicates surface roughness and material scattering to play a minor role for propagation loss, but rather absorption or waveguide damping. Propagation loss measurements in the range of 635 nm point to residual material absorption for this spectral range and therefore requires a more detailed investigation.

Table 1. Comparison of Propagation Loss in dB/cm in TiO2Based Thin Film Waveguiding Structures for Different Film Deposition Techniques film details

loss at 800 nm

RF magnetron sputtering laser molecular beam epitaxy atomic layer deposition reactive DC sputtering CBVD

9 57 5.6 5.1

loss at 1550 nm 4 2.4 1.2

ref 23 18 22 20 this work

structures based on their propagation loss. The doped Si:TiO2 film reaches benchmark propagation losses at 800 nm as a trade-off for a doping-dependent decrease of the refractive index. For a broader characterization of the material, the propagation loss and quality factors of fabricated photonic structures was measured at various wavelengths. Throughout this experiment, we changed the gap size between waveguide and resonator over a range of 250 nm for the specific geometry shown in Figure 2. For their characterization, the abovementioned diode laser working at 790 nm and two other narrow-band diode lasers were utilized. The second diode laser had a mode-hop-free fine-tuning range of 80 GHz and was used for measurements in the range of 632−640 nm. The third, home-built diode laser in Littrow configuration with a modehop-free fine-tuning range of 130 GHz was used in the range of 970−1070 nm. Typical spectral shapes of cavity modes for these three wavelength regimes are given in Figure 5a−c. The measured mode peaks were fitted with the Fano model in order to extract their line widths. Figure 5b displays two spectrally

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OPTICAL CHARACTERIZATION OF PHOTONIC STRUCTURES ON SAPPHIRE AND YAG SUBSTRATES The application of high index thin films for realization of a scalable photonic platform for rare-earth emitters in favorable host crystals was previously discussed. To that end, Si:TiO2 films were deposited on two substrate crystals, namely, sapphire and YAG with refractive indices of 1.76 and 1.82, respectively. The film was again shaped into resonators and waveguides, their optical performance and additionally film−substrate interaction were studied. In the case of sapphire, as depicted in Figure 6a, the fabricated cavities are coupled to two

Figure 6. (a) Microscope image of the fabricated add-drop filter out of Si:TiO2 film on sapphire. Resonator radius is 7 μm. (b) Spectral shape of a cavity mode at 785 nm of the add-drop filter geometry on sapphire.

waveguides simultaneously in an add-drop filter geometry, providing the option to detect the cavity resonances on the output of the dropping waveguide, while sending light through the bus waveguide. Figure 6b shows a Fano fitted resonator mode with a spectral width of 9.2 GHz at 785 nm, corresponding to a quality factor of Qsapphire = 41500. A YAG crystal (Y3Al5O12) was doped with chromium by ion implantation through a perforated copper mask with an average hole diameter of ≈400 nm.34 With an energy of 100 keV and a fluence of 1013 cm−2, Cr ions end up in the YAG crystal in a

Figure 5. Spectral shape of cavity modes at three different wavelengths: (a) 636.5, (b) 805.1, and (c) 1013.3 nm. They were obtained by sweeping a single-mode laser through the cavity resonance. (d) Camera image of 1013 nm light incoupled into the waveguide and on resonance with the cavity. Note that no light scattering on the waveguide is visible for this wavelength. D


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Figure 7. (a) Coupling efficiency of a dipole against the dipole depth. Inset: Schematic of the simulation setup with a single dipole placed at a certain depth. The refractive indices for the simulation are n1 = 1.0, n2 = 2.4, and n3 = 1.82. (b) Spectrum of fluorescence signal of implanted Cr spots. (c) Standard confocal microscope scan with overlapping excitation and observation point. Bright spots represent fluorescence of Cr implanted spots in YAG, two of them underneath a fabricated Si:TiO2 waveguide, highlighted in red. A resonator structure was placed too far from the waveguide, so that no coupling was achieved. (d) Tip-tilt mirror scan with excitation point kept at a stationary position to couple light into the fabricated Si:TiO2 waveguide (left end, large bright spot due to residual chromium impurities in YAG). Both images were taken with 640 nm long-pass optical filter to reject laser light and collect only fluorescent signal. The two bright spots at around 18 and 23 μm on the horizontal axis (horizontal axis corresponds to the propagation axis) represent fluorescence of Cr implanted spots, evanescently excited by light traveling through the waveguide. Additionally, the outcoupling point on the waveguide shows Cr fluorescence. The rest of the tip-tilt mirror scan is rendered dark, since no light coupling to resonators was detected.

depth of ≈55 nm with a straggle of 22 nm according to SRIM simulations.35 A Si:TiO2 film was subsequently deposited and shaped into waveguides resonators in order to study waveguiding and evanescent coupling of waveguided light to shallow implanted Cr-doped spots. Figure 7a36 shows the efficiency of the evanescent coupling of an emitter to the waveguide depending on the emitter depth. In the corresponding FEM simulation, the coupling efficiency was defined as the ratio of power coupled to the waveguiding structure to the total emitted power by the emitter. Even though our chromium implantation was slightly too deep, we expect to see evanescent coupling to the chromium ions belonging to the straggling tail toward the crystal surface. Based on a too large gap size, fabricated resonators showed no coupling to waveguides for this experiment, and so, light only propagated through waveguides. Cr fluoresence of implanted spots was detected close to 700 nm under excitation with 600 nm light (tunable Rhodamine 6G dye laser Coherent 599 was used as the excitation source), as shown in the confocal scan in Figure 7c. Incoupled light traveling within the waveguide was able to evanescently excite Cr implanted spots, as shown in the tilting mirror scan in Figure 7d. Here, the excitation laser position was kept stationary, coupling light into the left end of the waveguide. By scanning the point of detection with the tilting mirror, not only the excitation laser position yields signal, but also implanted Cr spots light up, which are embedded below the waveguide extending to the right. The origin of these fluorescent spots is confirmed to be chromium emission, as indicated by the spectrum of their fluorescence signal shown in Figure 7b. The excitation laser position shows the strongest signal due to residual chomium impurities in YAG. Additionally, the outcoupling point on the waveguide shows Cr fluorescence, picked up along the whole length of the waveguide due to the evanescent excitation, including the two implanted spots and

background from residual chromium. This experiment confirms the evanescent coupling of light between fabricated waveguide and emitters in the substrate. In combination with coupled optical resonators, this film−substrate interaction has the potential to facilitate CQED experiments with rare-earth ion doped crystals.16

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DOPING OF SI:TIO2 RESONATORS WITH ERBIUM Another way of adding functionality to the thin film, specifically to the fabricated thin film resonators, is by optical activation with fluorescent species. We have chosen rare-earth (RE) doping due to robust optical properties of RE ions in most crystalline and glassy transparent media. RE doping can be performed by ion implantation with high yield of fluorescent species.34 In our experiments, we chose to work with erbium as it is known for its strong upconverted fluorescence in the visible when excited in the infrared.37−40 In order to detect green upconverted fluorescence from erbium ions, a laser was tuned to λ ≈ 800 nm, where erbium exhibits strong absorption due to the 4I15/2−4I9/2 transition as a first excitation step. After nonradiatively decaying to 4I13/2 or 4I11/2, a second 800 nm excitation step populates either 4F3/2 or 2H11/2, causing green emission to originate from transitions between 2H11/2 or 4S3/2 and the ground state 4I15/2, as depicted in Figure 8a.37 The spectrum of the upconverted emission, shown in Figure 8b, is in good agreement with other works on erbium in glassy environment.41,42 For sample preparation, we implanted Si:TiO2 film on quartz with erbium ions of 2 MeV energy and with a fluence of 1014 cm−2. According to SRIM simulations, this leads to an erbium doping inside the Si:TiO2 film in a depth of ≈350 ± 78 nm, corresponding to a maximum local density of 5 × 1018 cm−3.35 This depth is close to the center of the strucuted film, where erbium ions are efficiently excited by the propagated light. Immediately after the E


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RE species can be fabricated into active optical elements. This can be exploited for creating on-chip lasers.

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CONCLUSION We have demonstrated a method of depositing low loss high index Si:TiO2 films on different substrates such as quartz, sapphire, and YAG. For most substrates, the refractive index of the film is high enough to support waveguiding and also evanescently excite shallow fluorescent centers within the substrate material. We have also shown a way of structuring the film to form on-chip photonic elements such as waveguides and resonators. The low propagation loss of 5.1 db/cm based on the Q-factor of fabricated resonators (1.5 × 105 at 800 nm) underlines the potential for CQED application in connection with rare-earth ion doped crystals for example. Finally, due to the increased thermal stability of this film when compared to TiO2, we could demonstrate further optical functionalization of the film by doping with fluorescent rare-earth species (erbium). These results show how the silicon doped titania film can be applied to on-chip photonics in various ways.

Figure 8. (a) Energy-level diagram for the Er3+ ion and its two-step upconversion with 800 nm excitation. NR denotes nonradiative relaxation. (b) Spectrum of upconverted fluorescence of erbium in Si:TiO2 film. Excitation wavelength is 785 nm. (c) Cavity mode visualized by upconverted fluorescence. The excitation laser is in resonance with one of the infrared cavity modes. (d) Spectrum of erbium upconverted fluorescence collected at the end of the waveguide. The smooth background is due to the fluorescence of the waveguide. The sharp peaks correspond to resonances of the cavity.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Thomas Kornher: 0000-0002-9117-6866

implantation, the appearance of the film changed from pink to gray, probably due to implantation-induced damage. At this point, no upconverted fluorescence from Er3+ ions was detected. Postimplantation annealing in air at 500 °C for 4 h healed out the implantation damage and at the same time activated erbium emission. After annealing, the film restored its original color. No change in surface roughness was detected. The film was shaped to form whispering gallery mode cavities with 5 μm radius coupled to straight waveguides as described above. The upconversion resonances of Er3+ in glassy environment are broad (≈10 nm); therefore, several infrared cavity modes could lead to upconverted fluorescence. Once the infrared excitation laser is tuned in resonance with one such mode, green fluorescence on the rim of the WGM cavity lights up (see Figure 8c, the excitation laser is filtered out). The spectrum of the fluorescence collected at one of the waveguide ends contains the mode structure as shown in Figure 8d. This indicates that not only the excitation infrared light can be coupled into the resonator, but also the green emission of erbium ions can be extracted from it. The widths of the cavity resonances are limited by the resolution of the spectrometer and amount to 140 GHz. Based on the resonance width of 34 GHz obtained in the 637 nm range and λ2 dependence of the scattering losses, one would expect the width of the green resonances to be 46 GHz. However, we had no means to confirm this value and, therefore, operate with the width given by the spectrometer resolution. The quoted above 140 GHz width corresponds to the Q-factor of 4000. This latter value is the lower limit of the quality factor of the cavities in the green range. The above results show that after implantation into Si:TiO2 film erbium ions retain their optical properties and, at the same time, the film retains its waveguiding capabilities allowing for high quality photonic structures in the green range. These two facts together indicate that the film activated with fluorescent

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank the CIME-EPFL team for their TEM investigation facilities. The authors wish to acknowledge the FEDER (Fonds Européen de Développement Economique et Régional) for financing the Nanobium project through the Interreg IVA programme. The work was also financed by ERC SQUTEC, EU-SIQS SFB TR21, and DFG KO4999/1-1.

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