LA EVALUACIÓN DEL LIDERAZGO EN DIRECTIVOS DOCENTES

Amongst chalcogenide hosts, there has been a particular focus has on those containing Ga, such as Ga-La-S (GLS), Ge-Ga-S and Ge-Ga-Se [46, 117-123], since they have been shown to accept high Er3+ concentrations without clustering. For example, Schweizer et al. [117] studied the properties of the Er3+ doped GLS glass system for MIR applications. Tonchev et al.[118], reported 1550 nm photoluminescence (PL) decay lifetime around 1.5-2 ms with 975 nm pumping in (As2Se3)1-x(GaSe)x (with Ga

from 0 to 5 at%) bulk glass doped with Er2S3 (1% Er3+). They found that the PL decay

lifetime increased linearly with Ga concentration from about 1.5 ms to 2 ms with the addition of Ga up to 5%. Allen et al. [119] studied the photoluminescence characteristics of a series of Er3+ doped chalcogenide glasses and found that all samples exhibited lifetimes in the 1–4 ms range. The Ga-Ge-As-Se glass had the shortest lifetimes of 1–1.5 ms for 980 nm pumping, whilst Ga-Ge-Se and Ga-Ge-S samples had the highest values of 2–4 ms. A strong correlation between the Er3+ _{ion and Ga}

concentrations that affects the properties of Er3+ doped Ge-Ga-Se glasses was found in [120]. Kasap et al. examined the optical and photoluminescence properties of Er3+

doped Ge-Ga-S glasses of near stoichiometric compositions (Ge28Ga6.2S65.3:Er0.5 at%)

Ga-containing bulk glasses pose significant problems during the thin film deposition required to make waveguides. In particular, upon melting these glasses tend to phase separate into a non-volatile Ga-S(Se) phase leading to significant deviations from the starting composition in the film, typically the film being gallium poor. As a result, to the best of our knowledge there have been no reports of stable, high quality thin films or waveguides (loss of less than 1 dB/cm) made from Ga-containing chalcogenide glasses. Chalcohalide glasses are also reported with high rare-earth solubility and promising PL emissions in MIR [104, 124, 125], however, due to the lack of experience in chalcohalide film deposition and waveguide fabrication in our group, no research on chalcohalide was conducted in this thesis.

There have, however, been many demonstrations of high quality chalcogenide thin films and waveguides in other chalcogenide hosts such as As2S3, Ge-Sb-S and

Ge11.5As24Se64.5 (at%) [4, 11, 13, 14]. For example, loss values as low as 0.05 dB/cm at

1550 nm have been measured for un-doped 4 μm wide As2S3 rib waveguides etched into

2.5 μm thick films, whilst losses for highly nonlinear dispersion-engineered As2S3

waveguides with a nonlinearity, γ≈10 W−1_m−1 _{are now as low as 0.3 dB/cm [13].}

Recently, Ge11.5As24Se64.5 (at%) nanowires (585×575 nm) with extreme nonlinearity

coefficient of γ=130 W−1m−1 and moderate losses of ≈1.65 dB/cm were also reported [65]. Propagation loss as low as 0.42 dB/cm was demonstrated in submicron Ge23Sb7S70

(at%) waveguides (700×600 nm ribs with 290 nm etch depth) using chlorine plasma etching [14]. These materials, have to date, proven to be the most suitable for planar waveguides [4, 11]. However, doping of rare-earth ions into bulk glasses with these compositions is difficult because the absence of Ga means the solubility of rare-earth ions in such glasses is low. As2S3 bulk glass doped with Er2S3 to give 0.1 at% Er

concentration produced complex narrow line structures in the PL spectrum that are similar to those found in crystalline material such as Er3+:YAG [126], implying low erbium ion solubility in As2S3 bulk glass; while for the ternary Ge33As12Se55 (at%)

glass, erbium ion clustering was observed at concentrations above 0.2 wt% [127]. Unfortunately, useful waveguide based devices require ~1-5 at% doping levels in order to obtain the ~1dB/cm gain required for practical devices.

Interestingly, there is strong evidence that some films can incorporate larger amounts of rare-earth ions compared with bulk samples when prepared by physical vapour deposition methods. This is most probably due to the fact that films are created in strongly non-equilibrium conditions by condensing a vapour onto a cold substrate.

This means that single isolated rare-earth atoms (or ions) are immediately incorporated into the film thereby inhibiting clustering. This contrasts sharply with the situation used to create a bulk glass where the dopant has to be soluble in the molten host. Lyubin et al. [128] reported that co-thermal evaporation of erbium with As2S3 produced films with

Er3+ concentration as high as 4 at% without any signs of clustering and led to strong PL emission under Ar+ laser excitation at 514 nm. Vigreux-Bercovici et al. [129] also reported sputtering of a 3% Er3+:As2S3 composite target to produce a thin film that had

1.5 μm transition lifetime of 4 ms. Whilst this is encouraging, the problem of etching rare-earth ions doped films remains and hence it is sometimes attractive to introduce the dopant after waveguide fabrication using ion implantation. In this approach a chalcogenide host that makes the most stable low loss waveguides can be employed. Fick et al. observed a strong Er3+ emission at 1.54 μm from erbium ion implanted As2S3

and As24S38Se38 (at%) films with a lifetime of 2.3 ms [130]. In Ivanova’s work [131],

the PL properties of Ge-S-Ga films ion implanted with relatively low energy (320 keV) ions at different fluences was investigated. They reported that the PL efficiency reduced with increasing Er3+ concentration and that thermal annealing at 230 °C approximately doubled the PL efficiency at all doses.

Whilst rare-earth ion doped chalcogenide bulk glasses have been intensively investigated, e.g. [119, 120, 122, 127, 132], as noted above, at the outset of this work there have been only five demonstrations of gain or lasing in chalcogenide glasses. Despite the many rare-earth elements added in bulk glass and the rich vein of possible transitions at longer wavelengths, there have since been no further demonstrations of amplifiers or lasers. A brief collation of representative measured NIR and MIR emissions of various rare-earth ion doped chalcogenide glasses and fibres are shown in Table 1.3.

Table 1.3 Collation of the MIR emission of rare-earth ion doped chalcogenide glasses and fibres.

Dopant(s) Host glass Excitation / µm

Emission / µm Transition Reference

Er3+ _As 2S3 1.48 1.54 4I13/2→4I15/2 [133] GLS 0.66/0.81 2.0/2.75/3.6/4.5 [117] GeGaSbS 0.804 4.3-4.8 4_I 9/2→4I11/2 [134] Ho3+ _{GeGaAsS/Se 0.9 1.6} 5_I 5→5I7 [135] GeAsS 0.905 1.2/2.0/2.9 5_I 6→5I8 /5I7→5I8 /5_I 6→5I7 [136] GLS 0.76/0.9/2 1.2/1.25/1.67/2/2.2/2. 9/3.9/4.9 [137] Tm3+ _{GeAsS 0.698/0.8 1.2/1.4/1.8} 1_H 5→3H6 /3_H 4→3F4 /3_F 4→3H6 [138] GeGaS-CsI 0.8 1.48/1.8/2.3/2.8 [139] GLS 0.7/2 3.8/5.38 3_H 5→3F4 / 3_F 2,3→3H4 [48] Dy3+ _GeAsS GeGaS 0.808 1.33/1.75/2.9/4.38/5. 27 (No spectrum) [140]

GeGaS 0.798 2.9 (Tm3+_/Dy3+ _co-

doped) 6_H 13/2→6H15/2 [141] GaSbS 1.32 2.95/3.59/4.17/4.4 [142] ‘selenide’ ~4.5 6_H 11/2→6H13/2 [103] GeAsGaSe 1.3 3.0/3.2/4.5/5.5/7.6 [54] Pr3+ _{‘selenide’ 3.5-5.5 [41]} GeAsGaSe 2 3.4/4.0/4.8/5.2/7 [54] Tb3+ _{GeAsGaSe 1.97 3.1/4.7/4.8/7.5/10.5 [54]} GLS 2 4.8/8.1(No spectrum) 7_F 5→7F6 /7F4→3F5 [48]