IMPLANTATION OF IONS GENERATED BY LASER ABLATION

Abstract

A process for fabricating a substrate comprising a laser-induced plasma assisted modified layer, and a substrate comprising an ion-implanted layer. The process comprises ablating ions from a first target and a separate second target with incident radiation from a laser in the presence of a substrate whereby a quantity of ablated ions from the first target and the second target are separately implanted into the substrate. Ablated ions from the second target are implanted into the substrate amongst implanted ions from the first target. Ablated ions of the first target (e,g Erbium) are a different material compared to ablated ions of the second target (e.g. Ytterbium). The resulting ion-implanted layer may have a substantially uniform distribution of the implanted ions from both the first and second targets collectively, and may be at a significantly greater depth than previously possible, desirably to a well-defined and sharp boundary within the substrate.

Claims

1. A process for fabricating a substrate comprising an ion-implanted layer, the process comprising: ablating ions from a first target with incident radiation from a laser in the presence of a substrate whereby a quantity of ablated ions from the first target is implanted into the substrate; ablating ions from a second target with incident radiation from a laser in the presence of said substrate whereby a quantity of ablated ions from the second target is implanted into the substrate and amongst said implanted ions from the first target; wherein ablated ions of the first target are a different material not comprised amongst ablated ions of the second target.

2. A process according to claim 1 wherein said ablating of material from said second target is performed after said ablating of material from said first target.

3. (canceled)

4. (canceled)

5. A process according to claim 1 in which said ablating material from said first and second targets is repeated sequentially a plurality of times.

6. (canceled)

7. (canceled)

8. (canceled)

9. A process according to claim 1 in which said substrate is heated.

10. A process according to claim 9 in which said substrate is heated to a temperature less than the glass softening point/temperature, or not exceeding the temperature of crystallization, of the material of the substrate.

11. (canceled)

12. (canceled)

13. (canceled)

14. (canceled)

15. (canceled)

16. (canceled)

17. (canceled)

18. A process according to claim 1 wherein at least one of the first target and the second target comprises a glass comprising ions of a transition metal and said ablated material from the at least one of the first target and the second target comprises Lanthanide ions.

19. (canceled)

20. A process according to any of claim 18 in which a said ions of a transition metal is an ion from amongst the following: erbium, ytterbium, neodymium, praseodymium, holmium, cerium, yttrium, samarium, europium, gadolinium, terbium, dysprosium or lutetium, holmium.

21. A process according to claim 1 wherein at least one of the first target and the second target comprises a glass from amongst the following: tellurium-based glass, e-r-a chalcogenide-based glass, a germanium-based glass, a bismuth-based glass, a silicon-based glass, a phosphate glass.

22. (canceled)

23. A process according to claim 1 wherein the laser is a Femtosecond laser and the process includes ablating at least one of said first andiar said second target with said incident radiation comprising femtosecond laser pulses of peak intensity not less than the threshold laser ablation intensity of the target material.

24. (canceled)

25. A process according claim 1 wherein the substrate is a glass selected from: silica, silicate, phosphate, tellurite, tellurite derivatives, germanate, bismuthate and solgel route glasses, or an optical polymer.

26. A process according claim 1 wherein the substrate is a selected from: silicon, a composite substrate comprising a silica layer formed upon a silicon layer, a composite substrate comprising a silicon layer formed upon a layer of an insulator material.

27. A process according to claim 26 wherein the optical polymer is selected from: Poly(methyl methacrylate), polyvinyl alcohol, polyether ether ketone, polyethylene terephthalate, polyimide, polypropylene, polydimethylsiloxane (PDMS) and polytetrafluoroethylene.

28. A process according to claim 1 wherein said ion-implanted layer has a substantially uniform distribution of the implanted ions substantially from the surface of the substrate.

29. A process according to claim 1 wherein said ion-implanted layer has an implanted ion density of at least about 10.sup.15 ions cm.sup.3.

30. (canceled)

31. (canceled)

32. A substrate comprising an ion-implanted layer containing implanted ions which are at least one of: transition metal ions andier Lanthanide ions, mixed with different implanted ions which are at least one of: transition metal ions andief Lanthanide ions, wherein the implanted ion density is at least about 10.sup.15 ions cm.sup.3.

33. A substrate comprising a photo-luminescent ion-implanted layer containing implanted ions which are at least one of: transition metal ions and Lanthanide ions, mixed with different implanted ions which are at least one of: transition metal ions and/or Lanthanide ions, wherein the photo-luminescent ion-implanted layer has a photo-luminescence lifetime-density product of at least about 910.sup.12 seconds/cm.sup.3.

34. A substrate comprising a photo-luminescent ion-implanted layer containing implanted ions which are at least one of: transition metal ions and/or Lanthanide ions mixed with different implanted ions which are at least one of: transition metal ions and/or Lanthanide ions wherein the penetration depth of the implanted ions is at least one atomic layer.

35. A substrate according to any one of claims 32 to 34 wherein the extent of the ion implanted layer has a substantially uniform distribution of the implanted ions substantially from the surface of the substrate.

36. (canceled)

37. (canceled)

38. (canceled)

39. (canceled)

40. (canceled)

41. (canceled)

42. A waveguide comprising a substrate according to any one of claims 32 to 34.

43. An optical component comprising a substrate according to any one of claims 32 to 34 claim and providing an optical gain per unit length exceeding 5dB/cm.

Description

DRAWINGS

[0139] FIG. 1 shows schematically the ablation, plasma production and the multi-ion implantation process.

[0140] FIG. 2A shows schematically the ablation, plasma production and the multi-ion implantation process according to a sequential ablation of two differently-doped target glasses.

[0141] FIG. 2B shows schematically the ablation, plasma production and the multi-ion implantation process according to a single ablation of one co-doped target glass.

[0142] FIG. 3a shows a view of an ion-implanted layer in a silica substrate made according to a sequential ablation of two differently-doped target glasses of FIG. 2A.

[0143] FIG. 3b shows a view of an ion-implanted layer in a silica substrate made according to a single ablation of one co-doped target glass of FIG. 2B.

[0144] FIG. 3c shows a view of an ion-implanted layer in a silica substrate of FIG. 3a, at higher magnification.

[0145] FIG. 4 shows an electron microscope image of an ion diffused layer within a silica glass, together with respective SAED patterns.

[0146] FIGS. 5 represents a schematic diagram of an ion-implanted silica later (top) and an EXD mapping from cross-sectional TEM analysis of an actual ion-implanted silica later (bottom), such as shown in FIG. 3a.

[0147] FIGS. 6 represents a PP-TOFMS profile of an ion-implanted silica later (bottom), such as shown in FIG. 5.

[0148] FIG. 7a shows atomic concentrations as a function of implantation depth of an ion-implanted silica later such as shown in FIG. 3a.

[0149] FIG. 7b shows atomic concentrations as a function of implantation depth of an ion-implanted silica later such as shown in FIG. 3b.

[0150] FIG. 8a shows the RBS spectra of an ion-implanted silica later such as shown in FIG. 3a.

[0151] FIG. 8b shows the RBS spectra of an ion-implanted silica later such as shown in FIG. 3b.

[0152] FIG. 9a shows the refractive index profile of an ion-implanted silica later such as shown in FIG. 3a.

[0153] FIG. 9b shows ellipsometric data from an ion-implanted silica later such as shown in FIG. 3b.

[0154] FIG. 9c shows light transmittance data from an ion-implanted silica later such as shown in FIG. 3a.

[0155] FIG. 10 shows the variation of density in a target glass (i.e. the ablated target glass) as a function of rare-earth ion concentration.

[0156] FIG. 11a shows the PL intensity spectrum for an ion-implanted silica later such as shown in FIGS. 3a, and 3b.

[0157] FIG. 11 b shows the PL profile for an ion-implanted silica later such as shown in FIGS. 3a, and 3b.

[0158] FIG. 12 shows the variation of PL lifetime of an ion-implanted silica later such as shown in FIGS. 3a (left-hand axis of graph) and of doped target glass, as a function of target glass dopant concentration.

[0159] FIG. 13 shows the variation in implanted ion layer depth/thickness (measured from the surface of a substrate e.g. waveguide containing the implanted layer) and refractive index of that layer, as a function of target glass dopant concentration;

[0160] FIG. 14 schematically shows an apparatus and procedure for implementing the manufacture of a substrate according to a preferred embodiment.

ABBREVIATIONS USED

[0161] HAADF high angle angular dark field elemental mapping [0162] NIR near infra red [0163] SEM Scanning electron microscopy [0164] TEM Transmission electron microscopy [0165] HRTEM High-resolution cross-sectional transmission electron microscopy [0166] RBS Rutherford back-scattering spectrometry [0167] SIP System in package [0168] DWDM Dense wavelength division multiplexing [0169] PL Photoluminescence lifetime [0170] ADTS Erbium-doped tellurite-modified silica [0171] SAED Selected area electron diffraction [0172] FWHM Full width at half maximum [0173] RETS Rare-earth doped tellurite modified silica

DETAILED DESCRIPTION

EXAMPLE

Implantation into Silica Glass

[0174] Highly rare-earth doped silicates have potential to provide advances in optical and photonic applications and devices. However, the limited solubility of rare-earth ions in silica hampers this.

[0175] The present invention, in preferred embodiments such as the embodiment described below, provides a novel method of producing a hybrid material comprising the integration of rare-earth doped (e.g. Lanthanide) silica substrate or chalcogenide glass substrate (e.g. a glass containing selenium or tellurium). This has been found to provide significantly high doping concentration of rare-earth dopant ions (e.g. Er.sup.3+ and Yb.sup.3+, for example, though not limited to these), without segregation. This has been validated by the provision of a remarkably high value of the standard metric: the lifetime-density product', of 1.4910.sup.19 s.cm.sup.3.

[0176] This embodiment illustrates the invention in terms of the sequential ablation of two individual rare-earth (Er.sup.3+/Yb.sup.3+) doped-tellurite glass targets. However, as discussed above, the invention applies equally to other dopant ions (not just Lanthanides), to other target glasses (not just tellurite glasses e.g. chalcogenide glass) and to other substrate materials (not just silica), and to equivalent non-sequential ablation of the two different target materials (provided that sequential arrival of ion plumes at the substrate is achieved, as discussed above).

[0177] As a means of better illustrating the remarkable, and unexpected, effects achieved by sequential ablation (or its equivalent) of different target materials, a tandem experiment is described for comparison in which a single co-doped Er.sup.3+-Yb.sup.3+-tellurite target glass was ablated alone for the ion-implantation of an otherwise identical silica substrate. The two-target sequential ablation processes accomplished a rare-earth concentration of 0.8910.sup.21 cm.sup.3, demonstrating an Er.sup.3+:.sup.4I.sub.13/2 PL lifetime of 12.9 ms. The twin-target sequential processes led to the formation of a cluster-less, continuous and homogeneous tellurite modified silicate layer, confirmed by cross-sectional transmission electron microscopy and plasma profiling time-of-flight mass spectrometry analysis.

[0178] An exceptional intermixing of Er.sup.3+and Yb.sup.3+ ions extending to the pristine silica substrate is achieved via the sequential two-target ablation (therefore, sequential doping) method. Moreover the unique sequential ablation (doping) process achieved a 30% higher planar optical layer thickness and 35% longer fluorescence lifetime as compared to the co-doped single-target comparison sample.

[0179] These results indicate that the sequential ablation/doping approach is better than the single co-doped target process for fabricating thicker rare-earth-doped (e.g. Er.sup.3+-Yb.sup.3) lanthanide-modified silicate layers (e.g. tellurite glass, or chalcogenide glass etc., alternatively). It is postulated that this process enables the substrate matrix to maintain the larger distance between the dopant ions, which is highly preferable for lowering fluorescence quenching.

[0180] Rare-earth (RE) doped silicates have been widely explored as an amplifying media for the optical communication systems. For example, the intra-4f transition .sup.4I.sub.13/2.fwdarw..sup.4I.sub.15/2 in erbium (Er.sup.3+) ions matches the C+L-band communication wavelength (1530-1605 nm), which makes them attractive for optical amplifier and laser applications. In erbium-doped fiber amplifiers (EDFAs), long lengths of fibers (>5 m) are required to achieve 20 dB optical gain, this long length requirement is due to the limited solubility of rare-earth ions (e.g. Er.sup.3+) ions in silica. This low optical gain per unit length makes miniaturisation very difficult. Thus, loss-compensation is required. In this respect high-gain erbium-doped waveguide amplifiers are highly desirable. Erbium chloride incorporated silicates can have Er.sup.3+ ion density of up to 10.sup.22 cm.sup.3, however the metastable lifetime of the ion is much too short. This is due to the phenomena of concentration quenching, where the physical limits of ion-ion interactions are reached, assisting macroscopic energy transfer at the .sup.4I.sub.13/2 energy level.

[0181] The invention, as illustrated by the present non-limiting embodiment, enables rare-earth (e.g. Er.sup.3+) ions to be incorporated into a substrate (e.g. silica) at much higher concentrations (10.sup.21 cm.sup.3) without concentration quenching and shortening of the metastable lifetime at the .sup.4I.sub.13/2 energy level. In addition, the invention in its preferred embodiments is able to incorporate a different rare-earth ion (e.g. Yb.sup.3+) ion for enhancing pump absorption via the .sup.2F.sub.7/2.fwdarw..sup.2F.sub.5/2 transition over a short distance. This allows the efficient inversion of one species of rare-earth ion (e.g. Er.sup.3+ ions) via resonant energy transfer using the other species of rare-earth ion (e.g. from Yb.sup.3+:.sup.2F.sub.5/2.fwdarw.Er.sup.3+:.sup.4I.sub.13/2).

[0182] This assists in increasing the mean inter-atomic distance to minimize the ion-ion (e.g. Er.sup.3+-Er.sup.3+ ion) energy transfer interactions.

[0183] In the following embodiment, it is demonstrated that an unusual rare-earth ion (e.g. Yb.sup.3+and Er.sup.3+) mixing in the e.g. a silicate matrix is achieved according to a preferred embodiment of the invention, during fs laser-induced plasma assisted sequential doping process. This method enhances both the modified silicate layer thickness and PL characteristics, as compared to the doping of Er.sup.3+/Yb.sup.3+ ions using a single co-doped tellurite target for ablation. The unexpected existence of inter-layer mixing yields remarkable spectroscopic results.

[0184] Tellurite glass targets of the following molar percentage concentrations were used, with x=1, but other values of x are equally applicable:

[0185] Target (a): (80-x)TeO.sub.2-10ZnO-10Na.sub.2O-xEr.sub.2O.sub.3 (this is the Er-doped target of two targets);

[0186] Target (b): (80-x)TeO.sub.2-10ZnO-10Na.sub.2O-xYb.sub.2O.sub.3 (this is the Yb-doped target of two targets);

[0187] Target (c): (80-x-y)TeO.sub.2- 10ZnO-10Na.sub.2O-xEr.sub.2O.sub.3-yYb.sub.2O.sub.3 (the single Er/Yb co-doped target).

[0188] These were prepared using standard glass melting and quenching processes.

[0189] The fs laser plasma assisted doping process was carried out in a vacuum chamber under an oxygen (O.sub.2) atmosphere of 80 mTorr at an optimum processing temperature of 973 K.

[0190] FIGS. 1A, 1B and 1C schematically show the ablation of the tellurite target (a) listed above, and it is to be understood that it applies equally to the Yb-doped target (b) defined above in which the illustrated Er ions are replaced by Yb ions, and it applies equally to the Er/Yb co-doped target (c) defined above in which the illustrated Er ions are joined by Yb ions. In particular, referring to FIG. 1A and 1B, multi-ion implantation into silica glass 4 was produced via femtosecond pulsed laser ablation of the relevant glass target. A femtosecond pulsed laser 1 was used to ablate the glass target 2 thereby generating an expanding plasma plume 3 consisting of multiple metal ions (multi-ion). The glass target produces multiple ions of Te, Zn, Na and Er, and/or Yb, which diffuse into the silica glass substrate 4 under certain process conditions, as shown in FIG. 1C. The silica glass substrate was coupled to a heater chamber 5 arranged to heat the substrate to a desired temperature. The laser source 1 (e.g. Ti-Sapphire laser) was used with a pulse duration of 100 fs, operation wavelength of 800 nm, a pulse repetition rate of 1 kHz and a single pulse energy of 50 J and peak intensity of 110.sup.13 W/cm.sup.2 was used to ablate the targets. The silica glass substrate 4, measuring 30201 mm (widthlengththickness), was positioned 70 mm above and parallel to the tellurite target glass 2 for receiving the ablated high energy plasma plumes 3.

[0191] The substrate was heated during this process to a temperature of about 973K (below the glass transition temperature and below the softening point/temperature of silica).

[0192] The schematic diagram of FIGS. 2A and 2B show the procedural differences in the fabrication of different Er.sup.3+-Yb.sup.3+-tellurite modified silica layers using: (FIG. 2A), the sequential ablation and doping of Er.sup.3+and Yb.sup.3+tellurites into silica using a two separate target glasses (7, 8) target (a) 6 and target (b) 7, and; (FIG. 2B) the single-step, single-target process of ablating one target glass 8 co-doped with Er.sup.3+and Yb.sup.3+and defined above as target (c).

[0193] The sequential-ablation approach (FIG. 2A) was used to prepare a first ion-implanted test substrate Sample 1 (S1 hereafter). This was done by the initial ablation of the target (a) glass 6 first, using the laser 1 for continuous period of two hours. It is to be understood that a different time period may be used, such as little as about a minute, or a up to five minutes, or up to 30 minutes, or up to 60 minutes, or more. After that, substantially without pause (a few seconds or so to allow target change-over) the ablation process was continued using the laser 1 but this time targeting the second target (b) glass 7 for the following 2 hours without changing the ambient conditions. It is to be understood that a different time period may be used, such as little as about a minute, or a up to five minutes, or up to 30 minutes, or up to 60 minutes, or more.

[0194] Separately, under otherwise materially identical conditions, the single-ablation approach (FIG. 2B) was used to prepare a second ion-implanted test substrate Sample 2 (S2 hereafter). This sample (S2) used only the Er/Yb, co-doped target 8, and was continuously ablated by the laser 1 for a period of four hours under the identical process conditions.

Structural CharacterisationMethods

[0195] The structural characterization of the two resulting substrate samples S1 and S2 was done using transmission electron microscopy (TEM) FEI Tecnai TF20 field emission gun (FEG) TEM (200 kV) fitted with a high angle annular dark field (HAADF) detector; a Gatan SC600 Onus CCD camera (Gatan Inc., Pleasanton, Calif.); and an Oxford Instruments 80mm.sup.2 X-max SDD energy dispersive X-ray spectroscopy (EDX) detector (Oxford Instruments plc., Abingdon, UK). Rutherford Backscattering Spectrometry (RBS) is a powerful technique for non-destructive compositional analysis of thin layers and nanostructured materials.

[0196] In this work, RBS analysis was performed in a scattering chamber with a two-axis goniometer connected to a 5 MV EG-2R Van de Graaff accelerator. The 2820 keV .sup.4He.sup.+ analyzing ion beam was collimated with two sets of four-sector slits to the spot size of 0.50.5 mm.sup.2, while the beam divergence was kept below 0.06. The beam current was measured by a transmission Faraday cup. Backscattering spectra were detected using an ORTEC surface barrier detector mounted in Cornell geometry (i.e. detector under the beam, with vertical sample rotation axis) at a scattering angle of =165.

[0197] An electron source installed in the sample chamber was used to avoid ion beam-induced charging of the insulating glass samples. In the spectrum recording, a blind detector with a thin Aluminium reflection layer on top was used to avoid light-induced background counts originating from the electron source and from ion beam-induced luminescence of the glass samples. The energy resolution of the detection system was 20 keV. Note, in this work the depth resolution of RBS at the sample surface is about 3 nm, and a poorer value can be considered as a function of depth. Spectra were recorded for sample tilt angles of 7 and 45. The RBX spectrum simulation has been performed to achieve best fits simultaneously for both tilt angles. The effective thickness of the modified layers was estimated supposing an atomic layer density of silicon dioxide, N=6.510.sup.22 atom cm.sup.3. This value can be calculated from silica glass density measurements resulting in 2.2g/cm.sup.3. The relative elemental concentrations in the samples were obtained using plasma profiling time of flight mass spectrometry (PP-TOFMS) by Horiba Scientific, France. PP-TOFMS combines a glow discharge (GD) plasma and an orthogonal time of flight mass spectrometer. The plasma is created in ultra-pure Argon between a grounded electrode and the sample which is powered from its back with a pulsed 13.56MHz RF voltage. The plasma ensures fast and uniform material removal over a 4 mm diameter area from the surface to the bulk and parallel excitation/ionization of the sputtered species. The ultrafast and full mass coverage TOFMS detection is adapted to the fast erosion rate (up to tens of nm/s) of the plasma. Time of Flight (TOF) records a full and continuous mass spectrum in every 30 s, thereby providing constant monitoring of all the species throughout the depth profile. An instantaneous semi-quantification giving elemental atomic concentrations can be obtained by calculation of Ion Beam Ratios (IBR). The IBR approach is commonly used in glow discharge mass spectrometry (GDMS) and is based on calculating the ratio of ion current for any one isotope with respect to the total ion current except the signal rising from the plasma gas and using the assumption that this ratio is representative of the atomic concentration of that isotope in the sample. A Metricon model 2010 prism coupler at 1550 nm was used to measure the planar optical layer thickness and refractive indices on the silica substrate. The amplitude ratio (IP) and phase difference (A) of the complex reflectance ratios for light polarized parallel and perpendicular to the plane of incidence was measured using a Woollam M-2000D1 rotating compensator spectroscopic ellipsometer. Acquisition of the transmission spectra for these samples was done using PerkinElmer LAMBDA 950 UV/Vis/NIR spectrophotometer. Standard excitation and emission fluorescence of the silicate planar waveguide samples were obtained using an Edinburgh Instruments FLS920 series of spectrometer fitted with a liquid nitrogen cooled photomultiplier tube (PMT) near-infrared (NIR) detector, operating in the 700-1700 nm wavelength range and a JDSU pump laser source operating at a wavelength of 980 nm, respectively. The fluorescence lifetime was also calculated using a time resolved fluorescence spectra, whereby the laser source was pulsed with a 100ms period and a pulse width of 10 s.

Structural CharacterisationResults

[0198] The high resolution TEM cross-sectional images of the samples S1 and S2, as prepared by focused ion beam (FIB) lithography is shown in FIGS. 3(a) and 3(b), respectively. Both the sequential-ablation process (FIG. 2A) and the single-step ablation process (FIG. 2B), described above, result in a homogenous metastable state of rare-earth doped tellurite modified silica (RETS) in the substrate.

[0199] A sharp interface between the RETS and pristine silica is obtained through the sequential-ablation process (FIG. 2A) in particular, as is shown in FIG. 3(c) as a high magnification of FIG. 3(a) using a high angle annular dark field (HAADF) cross-sectional image of S1 showing the interface of pristine silica and RETS.

[0200] The accompanying selected area electron diffraction (SAED) patterns (10, 11) are shown in FIG. 4. These illustrate SAED patterns generated from the modified RETS layer and pristine silica region from sample S1 (item 9 of FIG. 4). The larger diffraction ring radius of the modified layer implies higher density compared to the pristine silica layer. This demonstrates a typical amorphous nature of both the RETS region and the pristine substrate, respectively, showing a lack of long-range order in the atomic lattice, and exhibiting diffuse ring diffraction patterns without any discrete reflections.

[0201] Note the difference in the sizes of diffraction rings in FIG. 4 for the implanted RETS layer 10 and the pristine silica region 11, respectively. It can be clearly seen that the ring radius of the SAED pattern from the RETS layer is larger than in the pristine silica region. It is known that the radius of the ring is inversely proportional to the corresponding inter-atomic spacing in the sample. The average spatial distribution of atoms calculated from the ring radius for the modified site of sample S1 is 1.35 Angstrom, 27% shorter than the pristine silica, measuring 1.86 Angstrom. This indicates that the RETS layer is more closely packed, and therefore of greater density, than the base silica layer. The dense packing is due to the modification of the silica network with heavier tellurium and rare earth ions.

[0202] The RETS layer formed by the sequential-ablation approach (FIG. 2A) was further analyzed using energy dispersive X-ray spectroscopy (EDX). Mapping scans of a 1.4 m thick sample S1 are illustrated in FIG. 5. This reveals, quite remarkably and against all expectation, the generation of a substantially uniform and homogenous mixture of Er.sup.3+and Yb.sup.3+ ions extending from the surface of the substrate to a relatively sharp internal edge within the substrate defining the bottom of the modified layer. The distribution of various elements (Si, Te, Zn, Na, Yb and Er) in the RETS layer is indicated. Homogenous mixing of Er and Yb is achieved in spite of the sequential ablation of the target glasses in question and the resulting sequential doping of the substrate with ions therefrom.

[0203] FIG. 6 shows a PP-TOFMS profile of sample S1. Ion beam ratio of the elements in the sample indicates the uniform distribution of Er and Yb within the substrate. In this regard, sample S1 was characterized by plasma profiling time of flight mass spectrometry (PP-TOFMS) for semi-quantification of the elemental concentrations.

[0204] It can be seen from FIG. 6 that the tellurite compounds along with rare earth elements are mixed well with silica and the modified layer extends to the pristine silica region.

[0205] A silica rich RETS layer with homogeneously distributed Er and Yb is obtained, contrary to what one would expect of sequential ablation.

[0206] Based on the EDX mapping and the PP-TOFMS findings, and without being bound by theory, it is postulated the process can be assumed to be controlled by both highly energetic ions in the laser plasma and the high process temperature, 973K (below the glass transition temperature and below the softening point/temperature of silica), that initiates an interfacial reaction between the ablated ions of the target glass and the silica substrate. Above 873K, the alkali metals in the implanted into the silica substrate (Te and Na) attack the silica substrate, enabling the regular dissolution of silica throughout the process. This is thought to result in the formation of a well-defined metastable homogeneous modified ion-implanted layer. A similar phenomenon is also postulated for the subsequently-ablated target material, in which significant intermixing between the implanted ions of the second target (Yb.sup.3+doped) and of the first target (Er.sup.3+doped) arises. Consequently a uniform distribution of Er.sup.3+and Yb.sup.3+ ions may arise in the form of a single modified layer within the silica substrate, such as can be seen from the elemental maps in FIG. 5.

[0207] The evidence implies that even though the Er.sup.3+ and Yb.sup.3+ doped tellurites were ablated sequentially, interfacial reactions and intermixing of the implanted ions help the Yb.sup.3+ doped ions to extend down to the modified-pristine silica boundary, suggesting that over a period of time (e.g. two hours in this example), sufficient ion mobility may be occurring during the process at a substrate temperature maintained at 973K. Such novel nanoscale layer formation, without silica precipitation, is not possible with conventional fabrication methods, and this represents a step-change in materials production.

[0208] RBS spectra of both samples, S1 and S2, were taken to estimate the atomic composition for O, Si, Na, Zn, Te, and Er/Yb. The sample spectra for S1 and S2, along with their simulations are shown in FIGS. 8(a) and 8(b), respectively. The RBS spectra of the implanted layer/SiO.sub.2 substrate were measured at two different sample tilt angles of 7 (open dots) and 45 (solid dots), respectively. The corresponding RBX simulations are also shown (underlying solid lines: 12, 13, 14 and 15). Surface spectrum edges for Er, Te, Zn, Si, Na and O are indicated.

[0209] RBS cannot distinguish between Er and Yb due to their very similar energy threshold, so the peak corresponding to Er in the RBS spectra indicates both the Yb and Er content together. In FIG. 8(a), the different height of the spectra at channel numbers above 250 is primarily due to the different Er/Yb content. For S2 nearly double height of the Er shoulder can be observed than for S1, revealing about two times higher Er/Yb concentration. The spectrum S1 is significantly wider as it can be observed for the Zn, Te, and Er components (Na and O overlap with the substrate SiO.sub.2 signal). This is the consequence of the thicker modified silica layer (i.e. ion-implanted) for sample S1. The atomic composition and evaluated effective layer thicknesses are summarized in Table 1.

[0210] The depth profiles with atomic concentrations of various elements was evaluated from these RBS spectra using a simple two layer model, and these are represented in FIGS. 7(a) and 7(b) for S1 and S2, respectively. The effective thicknesses (depth) of the implanted layers are given in nm, and are recalculated from thicknesses given in atom/cm.sup.2 (provided by the RBS analysis) assuming the atomic density of silica, respectively, and evaluated from 2.82 MeV He.sup.+ RBS spectra using a simple two layer model.

TABLE-US-00001 TABLE 1 Parameters used in the RBX simulation of the measured RBS spectra. Implanted layer Implanted Transition composition (at. %) layer eff. layer eff. Er thickness thickness Sample Si O (+Yb) Te Zn Na (nm) (nm) S1 18 56.9 1.45 3.5 6.4 13.7 820 ~185 S2 17 60 2.8 3.6 5.8 10.8 633 153

[0211] The effective doping concentration of Er/Yb in sample S1 and S2 was measured to be 1.45 at.% (0.8910.sup.21 cm.sup.3) and 2.8 at. % (1.6310.sup.21 cm.sup.3), respectively. Notably, these are exceptionally high rare-earth doping concentrations in pure silica, without rare-earth clustering. It is observed that the rare-earth content in sample S1 is approximately 50% that of Sample S2. This is related to the individual target ablation times (2 Hrs., each of two targets) in the preparation of S1 which is exactly half of the co-doped target ablation time (4 Hrs. for ne target) for the preparation of S2. The amount of rare-earth ions in S1 can be improved by increasing the processing time that will further help to increase the thickness of RETS layer whereby maintaining the larger spacing between the doped ions.

Optical characterization

[0212] The RETS layers were analyzed for their planar optical layer characteristics using the prism coupler. The layer thickness and refractive indices were obtained at a wavelength of 1550 nm. The planar optical layer thickness for S1 was 1.01 m, while for S2 it was 0.76 m with refractive indices of 1.592 and 1.610, respectively. Thus, for a similar overall processing time, a 30% higher thickness is observed with the two-target sequential approach (FIG. 2A) than the single co-doped rare-earth target case (FIG. 2B), while the refractive index of the sole target process is greater. FIG. 9(a) indicates the typical refractive index profile of 1.4 m RETS layer fabricated through the femtosecond (fs) laser plasma assisted process according to the two-target sequential approach (FIG. 2A). The data reveals the formation of a homogenous layer of step index waveguide with a refractive index of 1.62 on silica at a wavelength of 1550 nm.

[0213] The refractive index (n) and extinction coefficient (k) profiles have been further calculated using multi-layer optical models using the ellipsometric data. The optical properties of each sub-layer were calculated using the Cauchy dispersion of n=A+B/.sup.2+C/.sup.4 (where A denotes the wavelength and A, B and C denote the Cauchy parameters) and the Urbach dispersion of k=De.sup.E (where E and D are the Urbach parameters).

[0214] The optical properties of the substrates were measured on the polished backside of the samples. The parameters of B and C were fixed at the values of the substrate, whereas parameter E was fixed at the value of the highest absorption surface layer, fitting the UV part of the spectrum with the smallest penetration depth. Consequently, only parameters A and D were fitted, resulting in acceptably low uncertainties. Typical uncertainties of A and D are 10.sup.3 and 10.sup.1 in the near surface and bottom interface regions, respectively. FIG. 9(b) shows typical measured and fitted ellipsometric spectra and the depth profiles of the optical properties (in the inset) calculated from the fit. In FIG. 9(b) measured (black dots) and fitted (solid lines) ellipsometric spectra are shown, these measurements were taken at an angle of incidence of 70 for sample S2. The inset shows the depth profiles of the refractive index (n, solid curve) and extinction coefficient (k, dotted line) at the wavelength of 1550 nm for S1 and S2, calculated from the fit.

[0215] As seen from the above optical characterization of the RETS layer, sample S1 compared to the sample S2 is higher in thickness and lower in refractive index with similar k values. The drop in thickness and the increased refractive index can be ascribed to the rare-earth ion concentration in the tellurite target glass and hence in the implanted film. The relative concentrations of rare-earth ions is higher as a result of the co-doped single target process (FIG. 2B) as compared to the result of the dual-target sequential ablation process (FIG. 2A).

[0216] The replacement of Tellurium (Te) with heavier rare-earth atoms gives rise to the density of the target glass, measured using Accupyc 1330 Pycnometer, as indicated in FIG. 10. It can be assumed that the ablation rates for the dense materials are comparatively lower, at a fixed (e.g. fs) laser pulse energy, leading to a drop in the level of ablated materials reaching the surface of the silica substrate. Again, the reduced Te (alkali metal) content in the ablated material may slow down the interfacial reactions with silica and its dissolution, thereby decreasing the rate of formation of the RETS layer and, hence, the RETS layer thickness drops. The addition of rare-earth ions and tellurite ions results in a denser packing of ions in the silica network as shown by the SAED pattern and the refractive index, which is directly linked to the material density. Sample S1 contains a lower rare-earth content (1.45 at. %) as compared to that of S2 (2.8 at. %). The increase of highly polarized trivalent Er.sup.3+ ions may generate more non-bridging oxygen in the glass network, and the polarizability of the material may thus increase, thereby increasing the refractive index.

[0217] The UV-VIS-NIR transmittance of each sample was obtained and is presented in FIG. 9(c). The variations of transmittance with wavelength were comparable for the samples, giving sample S1 the upper value of 95% in the C-band wavelength range. The changes in the transmittance are minimal which can be attributed to the differences in their refractive index and the absorption characteristics of the RETS layer. The optical absorption characteristics are greatly affected with the oxygen bond strength related to the structural changes in the glass system with the addition of Er.sup.3+ ions. It is obvious that the bond strength depends on the concentration and the oxide state of the glass network modifier ions.

[0218] The steady state photoluminescence (PL) emission characteristics were obtained by exciting the glass samples S1 and S2 with a pump source at 30 mW of output power. Under the same experimental conditions, the emission spectral range was scanned from 1400-1700 nm with a resolution of 0.5 nm. The PL spectra for both samples, corresponding to the .sup.4I.sub.13/2.fwdarw..sup.4I.sub.15/2 transition in Er.sup.3+ ion, are presented in FIG. 11(a).

[0219] The PL emission intensity appears comparable between the two processing strategies, with lower peak intensity for the sample S2. The lifetime of the sample S1 was 12.94 ms, while that of sample S2 was 9.16 ms as shown in FIG. 11(b). The reduced PL intensity and lifetime in sample S2 are likely to be due to the relatively higher concentration of Er.sup.3+ ions, leading to concentration quenching of the .sup.4I.sub.13/2.fwdarw..sup.4I.sub.15/2 transition. It is postulated that the increased concentrations of rare-earth ions decrease the ion spacing to an extent where dipole-dipole interactions between the Er.sup.3+ ions are prominent and energy migration occurs. Furthermore the higher erbium concentration may create a relatively larger polar environment, validated by the higher refractive index of the sample S2. The fluorescence lifetime may tend to be shorter in the more polar environment as the system possibly tries to stabilize as quickly as possible from the excited level by radiative emissions.

[0220] The variation of PL lifetime with respect to the Er.sup.3+ ions concentration in the Er-doped target glass is shown in FIG. 12. This shows the measured PL lifetime of the .sup.4I.sub.13/2-.sup.4I.sub.15/2 transition of Er.sup.3+ ions within the RETS (left-hand axis of graph) produced according to the sequential ablation method (FIG. 2A) in which one of the target glasses is an erbium-doped tellurite target glass (the other being a Yb-doped target glass). The PL lifetime of that target glass is also shown (right-hand axis of graph) as a function of Er.sup.3+ ion concentration in that target glass. The lifetime for the RETS layer reduces with increasing Er.sup.3+ ions concentration within the Er-doped target glass. According to FIG. 11, a 0.125 mol.% Er.sup.3+ concentration in the Er-doped target glass results in a PL lifetime in the ion-implanted substrate of 13.2 ms while concentration in the Er-doped target glass of 1.25 mol. % results in a PL lifetime in the ion-implanted substrate of around 10ms. It is postulated that the marginal lifetime decrease is due to the increase in the concentration of Er.sup.3+ ions which reduces the average spacing between the erbium-erbium ions. Consequently, it is postulated that the electric-dipole interactions become more pronounced, facilitating energy transfer between Er.sup.3+ ions which contributes to the reduction in fluorescence lifetime. It is also observed, as shown in FIG. 13, that the effective thickness (right-hand axis of graph) of the ion-implanted RETS layer decreases with increasing Er.sup.3+ ion concentration in the Er-doped target glass, whereas its refractive index (left-hand axis of graph) increases, with increasing Er.sup.3+ ion concentration in the Er-doped target glass.

[0221] In this way, the PL lifetime, refractive index and layer ion-implanted layer thickness may each be controlled as desired according to the appropriate choice of Er.sup.3+ ion concentration in the Er-doped target glass used in the according to the sequential ablation method (FIG. 2A) in which one of the target glasses is an erbium-doped tellurite target glass (the other being a differently-doped, e.g.Yb-doped, target glass).

Discussion

[0222] In summary, in a preferred and illustrative embodiment of the present invention, it has been shown that high doping levels of rare-earth ions (Yb.sup.3+ sensitized Er.sup.3+ ions) are achieved in silica without crystallization. This was achieved using laser-induced plasma assisted hybrid integration of ions from two separate/different rare-earth ion enriched tellurite target glasses into a silica substrate. The highly energetic ions in the laser-induced plasma enabled the formation of a metastable RETS layer. It is postulated that an appropriate substrate temperature further supports or enhances this process. The higher oxygen content in the RETS network promotes stable oxide formation, which is desirable for improved fluorescence efficiency. Substantial rare-earth ion doping concentrations of 0.8910.sup.21 atoms/cm.sup.3, and a very high PL lifetime-density product of 1.4910.sup.19 s.cm.sup.3.

[0223] The distinctive interlayer mixing accomplished the effective sensitization of rare-earth ions (e.g. sensitization of Er.sup.3+ with Yb.sup.3+ ions). This is in spite of expectation to the contrary due to the sequential nature of the target ablation used to generate these ions from two different and separate ablation targets. Indeed, the two-target sequential approach enhanced the RETS layer characteristics still further. In particular, lower rare-earth concentrations of the target glass enabled a thicker RETS layer formation on silica for a given processing time. This enables production of a homogeneous distribution of doped ions in a substrate, with larger average spacing thereby permitting higher doping concentration without significant fluorescence quenching. The methodology also surpasses the comparative co-doped single target ablation approach with a 35% longer PL lifetime which is highly beneficial for EDSWs. The method provides substantially homogeneously doped step index planar optical layers with high index contrast and enhanced thicknesses on a substrate platform that strongly supports the development of loss compensated photonic integrated circuits.

[0224] It is to be understood that the method of producing the implanted substrate, in other embodiments of the invention, is not limited to the sequential ablation process illustrated in FIG. 2A, and may be implemented by a process of simultaneous ablation of two different target materials. An example is shown schematically FIG. 14 in which the ablating of material from a second target 25 is performed substantially simultaneously with the ablating of material from a first target 24. The method includes, in this example, sequentially: obstructing firstly the plume of ions 29 from the second target 25 to the substrate in a first position 21 adjacent the first target 24, and subsequently obstructing the plume of ions 28 from the first target 24 when the substrate is moved to a second position 22 adjacent the second target 25. Thus, sequentially, the plume of ions 28 from the first target 24 reaches the substrate 21 before subsequently the plume of ions 29 from the second target 25 reaches the substrate. The obstruction takes the form of a static barrier 30 (31) partitioning and separating the first target material 24 from the second target material 25.

[0225] The static barrier 30, in effect, periodically is caused to block the path of an ion plume moving towards the substrate from the second target material and then from the first target material. It intercepts one ion plume or the other as the substrate itself is moved to be positioned 21 above the first material (but not the second one) and then positioned 22 above the second target material (but not the first one). At each of these positions of the substrate, one of the two target materials is behind an obstruction (e.g. the static barrier) while the other is not. Finally, after implantation of ions from the second target is completed, the substrate may be conveyed from its second position 22 to a subsequent position 23 for further processing or completion. In this way, control of the sequence of arrival of ion plumes from the two different targets is able to be achieved and controlled. In particular, this example may be implemented as a conveyor 32 arrangement carrying multiple substrates, spaced-apart so that the conveyer may intermittently stop to place any one substrate above one of the two target materials, with the other target material being obstructed by the barrier.

[0226] The conveyor 32 may then move substrate forward to a position over the next target material. The two target materials are continuously ablated during the whole period such that each substrate receives a plume of ablated ions separately and in sequence. The process may be conducted in one vacuum chamber where the temperature can be more easily controlled. The conveyor may carry many substrates which are conveyed, one after the other to positions over the first target material, then the second target material, so as to receive ion plumes from them sequentially. All the while, both target materials may be undergoing laser ablation continuously and simultaneously.

[0227] In further embodiments, the ablating of material from the second target is performed after the ablating of material from the first target material, the method including sequentially obstructing first the quantity of ions from the first target material and subsequently obstructing the quantity of ions from the second target material so that sequentially the quantity of ions from the first target material reaches the substrate before the subsequent quantity of ions from the second target material. In yet further embodiments, the ablating of material from the second target material is performed substantially after the ablating of material from the first target material, the method including positioning the substrate closer to the first target material than the second target material so that sequentially the quantity of ions from the first target material reaches the substrate before the subsequent quantity of ions from the second target material. Consequently, the sequenced arrival, at the substrate, of separate, of successive plumes of target materials, may be enabled either with simultaneous ablation of the targets or with successive ablation of targets.

[0228] The embodiments and examples of the invention descried above and provided to aid an understanding of the invention and are not intended to limit the scope of the invention, such as may be defined by the claims. It is to be understood that variants, modifications and equivalents of any one or more element of an embodiment, such as would be readily apparent to the skilled person, is within the scope of the invention such as may be defined by the claims.