Erbium silicate compound optical waveguide amplifier and laser [Invited]

Xingjun Wang; Peiqi Zhou; Yandong He; Zhiping Zhou

doi:10.1364/OME.8.002970

1. Introduction

Silicon (Si) photonics has attracted increasing attention and research effort in recent years because of its potential for low-cost integration using existing complementary metal–oxide–semiconductor (CMOS) technology and the small footprints of silicon photonic devices [1–4]. Silicon-based light source is the most important part of silicon-based photonic components. An off-chip light source displays high light-emitting efficiency and good temperature stability, but suffers from relatively large coupling losses between the off-chip laser and the Si chip and a high packaging expense. However, an on-chip light source could potentially achieve a higher integration density with a compact size, and display a better performance in terms of energy efficiency and energy proportionality. Despite these substantial advantages, the development of an on-chip light source on Si has seriously lagged behind that of other photonic components such as high-speed modulators and detectors because of the low emission efficiency of Si. Therefore, how to improve the luminous efficiency of Si is the key to the development of silicon-based light sources. A few promising candidates have been extensively researched, including porous Si [5], Si nanocrystals [6], Si Raman lasers [7], Erbium (Er)-related light sources [8–11], Germanium (Ge)-on-Si lasers [12–15] and III-V-based Si lasers [16–19]. In these methods, Er doped material is a more promising candidate for silicon-based light sources: its broad gain at important low loss, low-dispersion wavelengths covering the telecom C and L-bands (1525–1565 nm and 1565–1610 nm, respectively), long luminescence lifetime, high working speed, small signal crosstalk, small noise, and better CMOS technology compatibility, make it an excellent fit for silicon-based light sources. Er doped waveguide silicon-based light sources have been intensively researched over the last 20 years [20].

A key consideration in the design and realization of Er doped light sources is the choice of host material. Er³⁺ was firstly included as dopants in the host, resulting in a uniform Er³⁺ distribution throughout the medium. Many Er-doping host materials have been investigated for integrated optical applications. The Er-doping materials can be broadly divided into two categories: crystalline (or poly-crystalline) and amorphous hosts [20]. Crystalline materials such as dielectrics and semiconductors offer sharp emission lines, high peak cross sections, and high stability for narrow-band amplifier and laser applications. Amorphous hosts such as glasses and polymers exhibit broad emission spectra. The lack of periodicity in amorphous materials results in a multitude of local Er environments and the superposition of each of these transition spectra creates an overall broadened transition linewidth for the Er³⁺. What’s more, it was discovered that Si nanocrystals excite Er³⁺ efficiently, significantly increasing the light emission from Er in silicon oxide or silicon nitrides [21]. The study of Er doped silicon nanocrystalline (nc-Si) riched silicon oxide and nitride is currently a very active research field. Compared with the resonant absorption of a photon, Er³⁺ can have about two orders of magnitude higher excitation cross section with the sensitization of silicon nanocrystals, thus these material systems emit light efficiently. However, the optical gain is limited by the high carrier absorption of silicon nanocrystals. Above all, for doping host materials, it is usually difficult to reach high doping concentrations because of the limitation of Er solid solubility.

Instead of including Er³⁺ as dopants, another novel approach is to use Er silicate compound materials, which can get rid of the limitation of Er³⁺ solid solubility. Er silicates (Er₂SiO₅, Er₂Si₂O₇) have been attracting considerable attentions as materials for new light sources and optical amplifiers in silicon photonics. Such materials contain high density of Er³⁺ (~10²² cm⁻³) of 2–3 orders of magnitude higher than other Er³⁺-doped material (~10^16-20 cm⁻³) [22–28], as one of the constituent elements of the crystals and show strong emissions around 1.5μm due to the intra-4f transition, corresponding to the low-loss window of standard silica based optical amplifers. In 2004, Isshiki et al. [22] first fabricated Er silicate compounds (Er₂SiO₅) by a wet chemical method using ErCl₃ deposited on Si substrates. The fine photoluminescence (PL) structure due to Stark splitting with a full width at half-maximum as small as 7.0 nm (4 meV) at around 1.53 µm has been observed at room temperature. Then Priolo's group fabricated Er silicate by using radio frequency magnetron sputtering. Efficient room temperature photoluminescence (PL) at 1535 nm was obtained and up-conversion was limited to high optical pump powers [24]. They found that oxygen annealing can remove defects efficiently and enhance the PL intensity strongly [26]. Electroluminescence (EL) of Er silicate riched silicon oxide was obtained at around 1530 nm with a high forward bias of about 19 V [27]. However, such a high concentration in silicate results in upconversion due to near distances of Er³⁺ that limits the Er luminescence. Therefore, characterizing and controlling Er³⁺ distances in such Er silicates are necessary. To overcome this problem, Ytterbium (Yb) cations and Yttrium (Y) were added into the structure to dilute Er³⁺, which can substitute Er³⁺ in the silicate lattice and prevent neighboring Er³⁺ from causing upconversion [29–34]. Moreover, the Er concentrations of silicates can be easily continuously changed through Y or Yb co-doping, which is necessary to reduce the deleterious effects for 1.53 μm gain and luminescent efficiencies, such as the concentration quenching and up-conversions due to the neighboring Er³⁺. The Yb co-doped Er silicates have another advantage than the Y co-doping that is the sensitization effect of Yb for Er, which can enhance the excitation efficiency of Er. Accordingly, Suh et al. [30] used ion-beam sputter method to prepare Er-Y silicate thin film optical waveguide amplifier, which obtained ~60% of population inversion and 0.5 dB/cm net gain with a laser excitation wavelength of 1480 nm. Kimerling et al. [32] added Yb³⁺ to improve the luminescence properties of Er silicate compounds by means of RF-magnetron co-sputtering. These investigations showed that Er silicate has very good optical and electrical properties, and can be fabricated with compatible silicon fabrication technology, thus it is very promising for the application of silicon-based light sources. However, research teams have not yet produced the high-gain Er silicate compounds waveguide amplifiers the theories predict. Two main factors account for this difficulties: the large waveguide transmission losses and the high pump-power density requirement. So recent studies, taking into account these research results, have examined a novel single-crystal core-shell-structure Er silicate nanowire [35–40]. In 2015, Wang et al. [39] report a unique design of silicon and erbium silicate core-shell nanowires for high gain submicrometer optical amplification. Experimental results further demonstrate that an optimized core-shell nanowire can exhibit an excellent net gain up to 31 dB/mm. In 2017, Sun et al. [40] produced a single-crystal ECS nanowire that exhibited a large unit net gain of 124.5 dB/cm at 1532 nm, which was two orders of magnitude higher than existing reported values. By systematically tuning the core diameter and shell thickness, a large portion of the optical power can be selectively confined to the erbium silicate shell gain medium to enable a low loss waveguide and high gain optical amplifier.

In this paper, we have summarized the Er silicate compound optical waveguide amplifier and laser from theory, design, simulation, preparation methods, process and device optimizations. Firstly, a more accurate and comprehensive theoretical model was established to design and simulate erbium silicate amplifier and laser. After theoretical analyses, the Er silicate compound films were prepared, and several optimizations were given to improve the luminescence properties. Then three kinds of waveguide amplifiers, the strip-loaded, slot, and hybrid-structure, were fabricated with attention to the material components and structure optimizations. The Er silicates light emitter was also fabricated to realize 1.53 μm electroluminescence. In addition, the single-crystal Er silicate nanowire was proposed, which can solve the difficulties of large transmission losses in Er silicate films because of its high single-crystal quality. This research not only provides support and guidance for the construction of high-performance Er-silicate amplifiers and lasers, but also indicates the great prospects for applications of Er silicate compound materials in scale-integrated light sources.

2. Theoretical analyses of Er silicate light sources

2.1 Theoretical models

The interesting optical properties of Er arise from the fact that they maintain an atomic-like energy structure when incorporated in a host material [20]. Er neutral form is is [Xe]4f¹²6s², which has a similar electronic structure to xenon (Xe). When incorporated in a host they are normally found in the trivalent oxidation state, whereby the weakly bound 6s² electrons and either the 4f electron are removed. The neutral form changes into [Xe]4f¹¹5s²5p²6s⁰ and becomes Er³⁺. In this case, The partially filled 4f electron shell is shielded by the larger-radius 5s and 5p orbitals, resulting in a localized electronic environment. In the optical communication, and the 1.53 μm wavelength corresponds to the 4f-4f transition. Figure 1 shows the multiple-energy-levels model of an Er-Yb co-doped system that uses 980 nm pumping. This model uses Er³⁺ and Yb³⁺ structures with five and two energy levels, respectively. The five Er³⁺ levels—⁴I_15/2, ⁴I_13/2, ⁴I_11/2, ⁴I_9/2, and ⁴F_7/2—are represented by N₁, N₂, N₃, N₄, and N₅, respectively. Likewise, the average populations of the two Yb³⁺ levels—²F_7/2 and ²F_5/2—are represented by $N_{1}^{Yb}$ and $N_{2}^{Yb}$ , respectively. The simulation involves two types of equation sets. The rate equation describes the population dynamics of Er and Yb ions on each level. The propagation equation describes the evolution of copropagating and contrapropagating components of signal, pump, and ASE along the waveguide. The rate and propagation equations serve primarily to form the theoretical foundation for waveguide amplifiers and lasers [41], [42].

Fig. 1 Energy-levels model of Er-Yb silicate system.

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This explanation of the energy-levels diagram in Fig. 1 supports writing the steady-state rate equations for the Er-Yb silicate system. Thus, the equations for Er³⁺ are

{\begin{cases} \frac{\partial N_{1}}{\partial t} = - R_{13} N_{1} - W_{12} N_{1} + W_{21} N_{2} + A_{21} N_{2} + C_{3} N_{3}^{2} - C_{14} N_{1} N_{4} - K_{t r} N_{2}^{Y b} N_{1} = 0 \\ \frac{\partial N_{2}}{\partial t} = W_{12} N_{1} - W_{21} N_{2} - A_{21} N_{2} + A_{32} N_{3} - 2 C_{2} N_{2}^{2} + 2 C_{14} N_{1} N_{4} = 0 \\ \frac{\partial N_{3}}{\partial t} = - R_{13} N_{1} - A_{32} N_{3} - 2 C_{3} N_{3}^{2} + A_{43} N_{4} + K_{t r} N_{2}^{Y b} N_{1} = 0 \\ \frac{\partial N_{4}}{\partial t} = - A_{43} N_{4} + C_{2} N_{2}^{2} + C_{3} N_{3}^{2} - C_{14} N_{1} N_{1} = 0 \\ N_{1} + N_{2} + N_{3} + N_{4} = N_{E r} \end{cases} .

The equations for Yb³⁺ are

{\begin{cases} \frac{\partial N_{1}^{Y b}}{\partial t} = - R_{12}^{Y b} N_{1}^{Y b} + R_{21}^{Y b} N_{2}^{Y b} + A_{21}^{Y b} N_{2}^{Y b} + K_{t r} N_{2}^{Y b} N_{1} = 0 \\ \frac{\partial N_{1}^{Y b}}{\partial t} = R_{12}^{Y b} N_{1}^{Y b} - R_{21}^{Y b} N_{2}^{Y b} - A_{21}^{Y b} N_{2}^{Y b} - K_{t r} N_{2}^{Y b} N_{1} = 0 \\ N_{1}^{Y b} + N_{2}^{Y b} = N_{Y b} \end{cases} .

Here, A_ij = 1/τ_ij, in which τ_ij represents the lifetime between levels i and j, describes the spontaneous emission and nonradiative relaxation probability. C₂ and C₃ are the first- and second-order cooperative upconversion coefficients, C₁₄ is the Er³⁺ cross-relaxation coefficient, K_tr is the Yb³⁺-to-Er³⁺ energy-transfer coefficient, and N_Er and N_Yb represent the Er³⁺ and Yb³⁺ concentrations, respectively. The stimulated emission and absorption transition rates of signal and pump wavelength, W_ij and R_ij, respectively, are given by

W_{12} = \frac{σ_{12} (υ_{s})}{A_{c} h υ_{s}} Γ_{s} P_{s} (z) + \sum_{j = 1}^{M} \frac{σ_{12} (υ_{j})}{A_{c} h υ_{j}} \times Γ_{s} [P_{A S E}^{+} (z, υ_{j}) + P_{A S E}^{-} (z, υ_{j})],

W_{21} = \frac{σ_{21} (υ_{s})}{A_{c} h υ_{s}} Γ_{s} P_{s} (z) + \sum_{j = 1}^{M} \frac{σ_{21} (υ_{j})}{A_{c} h υ_{j}} \times Γ_{s} [P_{A S E}^{+} (z, υ_{j}) + P_{A S E}^{-} (z, υ_{j})],

R_{3} = \frac{σ_{13} (υ_{p})}{A_{c} h υ_{s}} Γ_{p} P_{p} (z),

R_{12}^{Y b} = \frac{σ_{12}^{Y b} (υ_{p})}{A_{c} h υ_{p}} Γ_{p} P_{p} (z),

and

R_{21}^{Y b} = \frac{σ_{21}^{Y b} (υ_{p})}{A_{c} h υ_{p}} Γ_{p} P_{p} (z),

where A_c is the waveguide-core cross-sectional area, h is Planck’s constant, σ_ij (υ_s,p) represents the absorption and emission cross sections for Er³⁺ and Yb³⁺ between levels i and j for the pump and signal frequencies (υ_s,p), respectively. The amplified spontaneous emission (ASE) noise can be calculated by discretizing the Er³⁺ continuous absorption and emission spectra into M frequency slots having widths of Δυ_j and centers at frequencies υ_j. In these expressions, P_s (z), P_p (z), and P_ASE (z, υ_j) represent the signal, pump, and forward–backward ASE powers at the frequencies υ_s, υ_p, and υ_j, respectively. Also, Γ_s and Γ_p, representing the confinement factors at the pump and signal wavelengths.

The propagation equations for the pump, signal, and forward–backward ASE powers are described by

{\begin{cases} \frac{d P_{p} (z)}{d z} = - Γ_{p} [σ_{13} N_{1} (z) + σ_{12}^{Y b} N_{1} (z) - σ_{21}^{Y b} N_{2}^{Y b} (z)] P_{p} (z) - α (υ_{p}) P_{p} (z) \\ \frac{d P_{p} (z)}{d z} = Γ_{s} [σ_{21} N_{2} (z) - σ_{12} N_{1} (z)] P_{s} (z) - α (υ_{s}) P_{s} (z) \\ \frac{d P_{p} (z)}{d z} = \pm Γ_{s} (υ_{j}) [σ_{21} (υ_{j}) N_{2} (z) - σ_{12} (υ_{j}) N_{1} (z)] P_{A S E}^{\mp} (z, υ_{j}) \pm α (υ_{s}) P_{A S E}^{\mp} (z, υ_{j}) \\ \pm m h υ_{j} Δ υ_{j} Γ_{s} (υ_{j}) σ_{21} (υ_{j}) N_{2} (z) (j = 1, 2, ..., M) \end{cases} .

where α (υ_s,p) are the propagation losses at the pump and signal wavelengths, respectively, L is the waveguide length, and m is the number of guided modes propagating at the signal wavelength. P_p0 and P_s0 are the input pump and signal power, respectively.

A combination of the rate and propagation equations allows the internal signal-optical gain G for the Er-Yb silicate nanowire amplifier to be written as

G (z) (d B) = 101 g [\frac{P_{s} (z)}{P_{s} (0)}] \sim σ_{12} N_{E r} Γ_{s} L .

Equation (9) shows that it is necessary to improve the concentration of Er³⁺ (N_Er), while developing the required small size (L) optical waveguide silicon-based light sources for on-chip integration. Due to the crystalline nature of Er silicates, they show almost no segregation of Er atoms and formation of defects even at such a high Er density in contrast to Er³⁺ doped Si-based materials, as mentioned above. Therefore, Er silicates are expected as small size and high optical gain waveguide amplifiers and lasers, which can satisfy the requirement and become the most promising candidate for integrated silicon-based light sources.

2.2 Design and simulation of Er silicate optical waveguide amplifier

Based on the theory model above, the Er silicate thin film material waveguide amplifier was analyzed, respectively. The basic waveguide structure is shown in Fig. 2. According to COMSOL simulation of the step–index waveguide, the maximum allowed waveguide size has been chosen for single-mode transmission with good energy confinement and convenient light input coupling. In addition, the corresponding parameters of Er silicate thin film are prepared according to our previous study of Er_xYb(Y)_2-xSiO₅ thin films [29, 31]. The CUC coefficient for different Er concentrations is estimated following Hehlen’s theoretical estimation [43]. It is also assumed that the propagation loss is set to 3 dB/cm, which could be the proper value of the thin film waveguide.

Fig. 2 Er silicate thin film waveguide structure.

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Table 1. summarizes the parameters used for modeling Er/Er-Yb silicate waveguide amplifier in the preceding calculations and analysis.

Table 1. Parameters of an Er/Er-Yb Silicate Waveguide Amplifier

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Figure 3 shows the signal gain of the Er silicate waveguide amplification as a function of waveguide propagation distance for a 1532 nm input signal pumping at different pump powers for a wavelength of 980 nm. The signal gain of Er silicate increased nearly linearly with the increase of waveguide transmission length before reaching a maximum value, then the gain began to decrease with further increases in transmission length, as shown in Fig. 3. These results support the conclusion that, for the given pump power, the maximum gain of the amplifier corresponds to an optimum pump length. When the value of L exceeded this optimized value, the gain dropped very quickly because, in this case, the nanowire completely absorbed the pump. Here, too, the remainder of the nanowire was not pumped but, rather, absorbed the amplified signal while there was no population inversion. As a result, the optimum pump length depended on the pump power and increased with increases in pump power. In contrast, the signal gain also could be enhanced further with an increase in pump power at the same transmission length because of enhanced population inversion. Figure 3 shows that the optimum pump length was nearly 1 mm at a pump power of 100 mW. Thus, the Er silicate waveguide should be fabricated to 1 mm. The gain, at this length, could reach over 10 dB. Furthermore, the gain improved to 16 dB at 1.3 mm when the pump power was increased to 120 mW.

Fig. 3 Signal net gain vs propagation distance for different input pump powers from 60 mW to 120 mW. The optimum pump length is 1 mm. The gain can reach 11 dB when N_Er = 1.6 × 10²² cm⁻³ and P_p = 100 mW.

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The gain of amplifiers may be improved further by introducing Yb³⁺ to form Er-Yb silicate materials in which the total concentration, pump light, and transmission length remain unchanged. This configuration presents two advantages. First, the introduction of Yb³⁺ dilutes the Er³⁺ concentration and suppresses the cooperative upconversion process of Er³⁺. Second, Yb functions as an effective sensitizer for Er³⁺ at the 980 nm excitation wavelength because the absorption cross section of Yb³⁺ to pump light is one order of magnitude higher than that of Er³⁺. Therefore, the introduction of Yb³⁺ increases the absorption efficiency of pump light. Both the Yb and Er concentrations and the ratio of the Yb and Er concentrations make important contributions to the amplifier gain optimization of Er-Yb silicates. Figure 4(a) shows that the gain first increases and then decreases with increases in the Yb:Er ratio. The inhibition of upconversion is not obvious when the Yb³⁺ concentration is too low, and the luminous efficiency of Er³⁺ is reduced when the Yb³⁺ concentration is too high. As a result, the ratio of Er³⁺ to Yb³⁺ must be set near an optimum value. The optimum Yb:Er ratio is about 2.3:1 for 1 mm waveguide.. The unit net gain at this level can reach 28.5 dB. In addition, Fig. 4(b) shows that the optimum Yb:Er ratio varies with the propagation distance. These variations occur because the absorption efficiency of waveguides to pump light differs for different components. The gain changes fastest with propagation distance (pump length), but the optimum pump length is also relatively short because the pump light is absorbed more quickly. It can be concluded that the optimum Yb:Er ratio depends on the waveguide length.

Fig. 4 Signal net gain vs Yb:Er ratio (a) and propagation distance (b). (a) The optimum Yb:Er ratio is 2.3:1. The gain can be improved to 28.5 dB, where L = 1 mm, P_p = 100 mW, and N_total = 1.6 × 10²² cm⁻³. (b) Signal net gain vs propagation distance at Yb:Er ratios from 1:0 to 1:6 at input pump power of 100 mW.

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2.3 Design and simulation of Er silicate waveguide laser

Here, using the optimization results of amplifiers, we design and simulate a small-size, low-threshold Er–Yb silicate compound laser. The design parameters of effective structures, such as Fabry–Perot (F–P) resonators and distributed Bragg reflector/distributed feedback (DBR/DFB) microcavities, are critical for obtaining effective optical feedback, which is the key to lasing. The Er-Yb silicate laser can be designed and simulated using the aforementioned optimization results for determining Er-Yb silicate amplifier parameters.

The schematic configuration of the Er-Yb silicate waveguide laser is designed with F–P resonator, the schematic configuration is shown in Fig. 5. The resonator is considered to be a single waveguide attached to dielectric mirrors that exhibit partial reflectance at the signal and pump wavelengths, the 1532 nm and 980 nm schemes, respectively. The pump is injected from the right side, and laser output occurs on the left side. The cavity has been modeled to be double-pass. Thus, P⁺ and P⁻, the copropagating and contrapropagating components for signal and pump in the resonator, respectively, represent the overall forward and backward transmission of the optical evolution.

Fig. 5 Schematic configuration of the Er-Yb silicate waveguide laser.

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The propagation equations must be changed in two directions because of the copropagation and contrapropagation for signal and pump in the resonator. Thus,

{\begin{cases} \frac{d P_{p}^{\pm} (z)}{d z} = \pm Γ_{p} [σ_{13} N_{1} (z) + σ_{12}^{Y b} N_{1}^{Y b} (z) - σ_{21}^{Y b} N_{2}^{Y b} (z)] P_{p}^{\pm} (z) \pm α (υ_{p}) P_{p}^{\pm} (z) \\ \frac{d P_{p}^{\pm} (z)}{d z} = \pm Γ_{s} [σ_{21} N_{2} (z) - σ_{12} N_{1} (z)] P_{s}^{\pm} (z) \pm α (υ_{s}) P_{s}^{\pm} (z) \\ \frac{d P_{A S E}^{\pm} (z, υ_{j})}{d z} = \pm Γ_{s} (υ_{j}) [σ_{21} (υ_{j}) N_{2} (z) - σ_{12} (υ_{j}) N_{1} (z)] P_{A S E}^{\mp} (z, υ_{j}) \pm α (υ_{s}) P_{A S E}^{\mp} (z, υ_{j}) \\ \pm m h υ_{j} Δ υ_{j} Γ_{s} (υ_{j}) σ_{21} (υ_{j}) N_{2} (z) (j = 1, 2, ..., M) \end{cases} .

The Er-Yb silicate waveguide was pumped with a wavelength of 980 nm at an Er–Yb total concentration N_total of 1.62 × 10²² cm⁻³. The Yb:Er ratio was set at 2.3:1. The cavity was formed by using a high reflector (T_1p = 92% at 980 nm, R_1s = 99.8% at 1532 nm) attached to the entrance end-face and a partially reflective output coupler (R_2p = 99.8% at 980 nm, R_2s = 95% at 1532 nm) at the other end [44]. The stable Er-Yb silicate laser output was considered at λ_s = 1532 nm.

Figure 6(a) shows the output laser power of the Er-Yb silicate waveguide laser as a function of cavity length for different pump powers. It indicates that, in a steady laser oscillation, the light wave exhibits no power loss during the round trip. Thus, for settled R₁, R₂, and Γ, a threshold cavity length can be found. Here, the threshold resonator length is about 18 μm, which permits reducing the size of the laser and supports scale-integration requirements. Figure 6(a) also indicates that the output laser power increases with increases in resonant cavity lengths shorter than the optimum cavity length; at longer cavity lengths, the output signal power tends to decrease. Longer resonant cavity lengths lead to insufficient pumping distances, thus causing the absorption of the signal light. Therefore, the optimum resonant cavity length increases with increases in pump power. The output signal power can reach 30 mW at a 450 μm cavity length with a pump power of 100 mW; the power conversion efficiency in this configuration approaches 30%.

Fig. 6 Laser power vs (a) cavity lengths at varying pump powers (20–100 mW). The optimum cavity lengths were approximately 180 μm, 250 μm, 325 μm, 400 μm, and 450 μm for pump powers of 20 mW, 40 mW, 60 mW, 80 mW, and 100 mW, respectively. (b) various optimum cavity lengths. The inset in (b) shows an expanded view of the red-dashed region for pump powers 10–30 mW.

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Figure 6(b) illustrates the results from applying the optimum resonant cavity length reported above to the Er-Yb silicate waveguide laser. Thus, a threshold pump power producing oscillation can be found. The laser output power increases with increases in the resonator length, but the threshold pump power of the laser also increases. As the inset to Fig. 6(b) shows, the threshold pump power reached approximately 15 mW, 17 mW, 21 mW, 23 mW, and 25 mW for resonator lengths under 180 μm, 250 μm, 325 μm, 400 μm, and 450 μm, respectively. The laser power-conversion efficiency and threshold pump power of this configuration, considered together, are competitive. The need for compromises will arise when design requirements are set: smaller-size lasers can be used to reduce the input pump power, and larger-size lasers can be used to improve the power conversion efficiency.

3. Preparation of Er silicate thin film

For Er silicate compound film waveguide, the thin film growth methods typically involve deposition on oxidized silicon wafers (the oxide layer on the silicon providing the lower cladding layer) or other substrates. It potentially allows for integration with other devices on the silicon substrate and fabrication of devices over a large area. These methods have been utilized for depositing Er silicate waveguide films include the sol-gel method [29–31], RF-sputtering [24–26, 34] and pulsed laser deposition (PLD). For sol-gel method, Er_xY_2-xSiO₅ and Er_xYb_2-xSiO₅ films for (x = 0 to 2) were fabricated by using a mixture of Er-O, Y-O (Yb-O) and Si-O sol solutions. The sol solutions were first spin-coated on Si (100) SiO₂ substrates, then dried in air and baked in Ar. This procedure was repeated to adjust the film thickness. Finally, the coated surface of the samples were covered with a polished surface of another Si (100) wafer, and then sintered in the well heated furnace in Ar atmosphere. For RF-sputtering, the Er-Yb/Y silicate thin films were deposited in a ultra high vacuum magnetron sputtering on (100) c-Si substrates by the co-depositing from Er-, Yb/Y- and Si- oxide targets. By properly changing the powers supplied to the three targets, several films can have different ratio between the two involved REs but keeping always fixed their sum. And for PLD, the Er-Yb/Y silicate thin films were deposited in a PLD system on c-Si substrates by pulsed laser ablation on the Er-, Yb/Y- and Si- oxide targets. The films’ stoichiometry can be adjusted by the laser power.

4. Luminescence optimization of Er silicate films

Pure Er silicate with a high concentration always results in upconversion and quenching problems, so Y and Yb cations are usually added to the erbium silicate structure to substitute for Er³⁺ in the silicate lattice and to prevent neighboring Er³⁺ from causing upconversion and quenching. Therefore, several analyses and optimization were given to improve the luminescence properties of Er-Yb/Y silicate thin films after the preparation, which include the PL intensity and luminescence lifetimes.

4.1 Optimization of Er-Y silicate films

ErxY2-xSiO5 films have been fabricated on Si substrates by the sol-gel method [29]. Figure 7(a) shows the PL spectra of Er_xY_2-xSiO₅ (x = 0-2) films at the wavelength pump of 654 nm. It can be seen that the same PL shape with main peak of 1.528 μm, corresponding to the typical PL spectrum of Er₂SiO₅ phase, was observed for Er_xY_2-xSiO₅ films at the different Er³⁺ concentrations. It indicated that local atomic structure of Er³⁺ is similar for all samples. However, PL intensity has a significant change at the different Er³⁺ concentrations. PL intensity first increased ~30 times with the decrease of Er³⁺ concentrations from 25 at.% (x = 2.0) to 1.25 at.% (x = 0.1), and then decreased slightly when the Er³⁺ concentrations were further decreased to 0.5 at.% (x = 0.04). Figure 7(b) shows the integrated PL intensity from 1400 to 1700 nm and decay time dependent on x value for the Er_xY_2-xSiO₅ films. Decay time was increased about 100 times when Er³⁺ concentrations were decreased from 23.75 at. % to 1.25 at. %. The two factors, crystal structure, and radiative and nonradiative transition rates were considered to study this phenomenon.

Fig. 7 (a) PL spectra of Er_xY_2-xSiO₅ films (x = 0-2) at the wavelength pump of 654 nm. (b) Integrated PL intensity from 1400 to 1700 nm and decay time dependent on x value for the Er_xY_2-xSiO₅ films.

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Figure 8 shows the decay time of Er_xY_2-xSiO₅ films. For high Er³⁺ concentrations of 20 at.% and 23.75 at.% (x = 1.6 and 1.9), fast decay time (~20 μs) was observed, and slow decay time (~2 ms) appeared for low Er³⁺ concentration of 1.25 at.% (x = 0.1). The results may suggest that the incorporation of upconversion results in the fast decay observed in pure Er₂SiO₅. Y additions fully separate Er³⁺ and weaken upconversion of Er³⁺ for Er₂SiO₅, which lead to the decrease of nonradiative transition rate. It shows that (~30 times) increase of PL intensity can be explained partly due to increase of decay time.

Fig. 8 Decay time of Er_xY_2-xSiO₅ films

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The enhanced Er³⁺ luminescence about 30 times of Er₂SiO₅ by optimizing Y addition concentration were obtained at the pump wavelength of 654 nm. Decreased upconversion and nonradiative transition through Y addition are two main reasons for enhanced Er³⁺ luminescence. The optimized Er³⁺ concentration of 1.25 at.% (x = 0.1) was obtained to get above 10 dB gain for the Er_xY_2-xSiO₅ films.

4.2 Optimization of Er-Yb silicate films

Er_2-xYb_xSiO₅ films have also been fabricated on Si substrates by the sol-gel method [31]. Figure 9(a) shows the PL spectra of Er_2-xYb_xSiO₅ (x = 0-2) films on Si(100) substrates at the wavelength pump of 654 nm. It can be seen that the typical PL spectrum with main peak at 1.528 µm of Er₂SiO₅ phase was observed without Yb additions (x = 0) [8]. The PL spectra have no significant change with the increase of Yb concentration to 2.5 at.%(x = 0.2) compared with that of Er₂SiO₅ phase. However, the peak intensity at 1.528 µm becomes weak, and another two strong peaks at 1.535 µm and 1.545 µm appear when the Yb concentration was further increased to above 12.5% (x = 1.0). It shows that the local environment of Er³⁺ has been changed compared with that of samples having low Yb concentrations. The 1.53 µm integrated PL intensity is similar for the different Yb concentrations at 654 nm pump wavelength. In order to study the effect of Yb on Er, the 980 nm wavelength laser was used as pump source. Figure 9(b) shows the 1.53 µm integrated PL intensity of Er_2-xYb_xSiO₅ films on SiO₂/Si substrates and Si(100) substrates as a function of Yb concentration at 980nm and 654 nm pump wavelengths. PL intensity by pumping at 980 nm has a significant increase than that by pumping at 654 nm. The above 10 times enhanced PL intensity for Er_2-xYb_xSiO₅ film on Si substrate was obtained with the Yb concentration increased to 23.75 at.% (x = 1.9), and then decreased slightly when the Yb concentration was further increased to 24.5 at.% (x = 1.96). At the base of above 10 times enhancement, another 20 times enhanced PL intensity for the Er_2-xYb_xSiO₅ (x = 1.9) film on SiO₂/Si substrate was observed compared with that on Si substrate by pumping at 980 nm.

Fig. 9 (a) PL spectra of Er_2-xYb_xSiO₅ (x = 0-2) films on Si(100) substrates at the wavelength pump of 654nm, (b) 1.53µm integrated PL intensity of Er_2-xYb_xSiO₅ films on SiO₂/Si substrates and Si(100) substrates as a function of Yb concentration at 980nm and 654 nm pump wavelengths.

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Figure 10 shows the decay time of Er_2-xYb_xSiO₅ (x = 0-2) films on SiO₂/Si substrates and Si substrates. For pure Er₂SiO₅ phase on Si substrate, fast decay time of ~20 μs was observed. The slow decay time of ~0.7 ms, was observed for Er_2-xYb_xSiO₅ films on SiO₂/Si substrate for high Yb concentration of 18.75 at.% (x = 1.5). With the further increase of Yb concentration to 23.75 at.% (x = 1.9) and 24.5 at.% (x = 1.96), decay time become more longer ~1.8 ms and ~3.5 ms, above 100 times of pure Er₂SiO₅. The smaller amount of Er and higher amount of O supplied from the SiO₂ underneath layer compared with Er₂SiO₅ may result in the longer decay time. The enhancement of decay time with increasing the composition of Yb additions can be explained by the decrease in the number of nonradiative decay channels, which may involved reduction of concentration quenching of Er³⁺ at high Yb concentrations. Energy transfers among Er³⁺ become very efficient when the distances of neighboring Er³⁺ get to be smaller. The energy finally dissipates to quenching centers such as –OH when they meet with an excited Er³⁺.

Fig. 10 Decay time of Er_2-xYb_xSiO₅ films on SiO₂/Si substrates and Si substrates.

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Above 200 times enhanced PL intensity was observed for the Er_0.1Yb_1.9SiO₅ film on SiO₂/Si substrate by pumping at 980nm compared with pure Er₂SiO₅ film on Si substrate at 654 nm. It can conclude that above two orders of magnitude enhanced PL for Er_0.1Yb_1.9SiO₅ film may be due to the higher radiative transition rate, and all Er ions are optically active for Er_0.1Yb_1.9SiO₅ film.

The another Er-Yb silcate structure phase, α-(Yb_1-xEr_x)₂Si₂O₇ thin films on Si substrates were synthesized by magnetron co-sputtering [34]. Figure 11(a) shows some traces at different Er contents taken at low pump flux of 1.6 × 10¹⁹ cm^-²s^-¹. In the same figure the decay curve in absence of Yb (in Y-Er disilicates) is also reported for a particular N_Er, 1.6 at. %. It is interesting to note that the Er³⁺ lifetime depends only on Er³⁺ content and is not affected by the presence of Yb (or Y). The lifetime values, τ_{1, Er}, have been evaluated for all the samples by single exponential fits of the decay curves and reported in the right hand scale of Fig. 11(b) (blue line and open squares). It is evident that by decreasing N_Er, τ_1,Er varies between 0.3 ms and 5.6 ms. The same trend is observed in the case of α-(Yb_1-xEr_x)₂Si₂O₇ samples (black open triangles). Therefore the τ_{1, Er} behavior can be associated only to the Er-Er interactions not involving Yb³⁺ ions. For the highest N_Er, the very short lifetime can be justified by the occurrence of concentration quenching between Er³⁺ owing to the short mean Er-Er distance. This phenomenon consists in a resonant energy transfer from one excited Er³⁺ at the first excited level to a nearby Er³⁺ in the ground state. Hence energy travels along the sample by eventually being lost when a quenching center is encountered. By increasing N_Yb and, as a consequence, by decreasing N_Er the deleterious Er-Er interactions are reduced. In this α-(Yb_1-xEr_x)₂Si₂O₇ host the good Yb-Er coupling and the reduced back-transfer from Er³⁺ to Yb³⁺ are guaranteed for the whole N_Er range under investigation. In Fig. 11(b) data on the PL intensity at 1535 nm as a function Yb and Er contents are also summarized (left hand scale). A very strong PL emission has been already demonstrated from Er³⁺ in Er₂Si₂O₇ (N_Yb = 0 at.%), because all Er³⁺ are optically active. By introducing just 2 at.% of Yb, PL intensity at 1535 nm already increases by a factor of three though the emitting Er³⁺ are reducing from 18 at.% to 16.5 at.%. This is due to the occurrence of mediated excitation from Yb³⁺ as already demonstrated. The PL intensity continues to increase by further increasing N_Yb.

Fig. 11 (a) PL decays recorded at 1535 nm for α-(Yb_1-x-Er_x)₂Si₂O₇ at different N_Er. (b) PL intensity (left hand scale) and lifetime (right hand scale) at 1535 nm as a function of N_Yb (bottom scale) in α-(Yb_1-x-Er_x)₂Si₂O₇. For comparison the decay times (black open triangles) at 1535 nm in α-(Yb_1-x-Er_x)₂Si₂O₇ as a function of N_Er (top scale) have been reported. The blue line is a guide for the eye.

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The highest photoluminescence emission at 1535 nm is obtained as a result of the best compromise between the number of Yb donors (16.4 at.%) and Er acceptors (1.6 at.%), for which a high population of the first excited state is reached.

5. Fabrication of Er silicate optical waveguide amplifiers

The Er silicate optical waveguide amplifiers were fabricated based on above optimized materials. Suh et al. have demonstrated fabrication of single-phase, polycrystalline ErxY2-xSiO5 thin film waveguide amplifier using reactive ion beam sputter deposition followed by rapid thermal anneal [30]. Figure 12 shows a scanning electron microscope (SEM) image of the fabricated waveguide prior to polishing. The silicate layer can be easily discerned by its rough surface due to the polycrystalline structure of the film. Shown in the inset is the calculated TE-mode profile, indicating that the waveguide is single-mode.

Fig. 12 The SEM image of “low-Er” waveguide prior to polishing. The scale bar represents 1 µm. The inset shows the calculated TE-mode-profile of the waveguide.

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The gain characteristics of fabricated waveguides are summarized in Fig. 13(a) and 13(b) that show the pump-power dependent signal enhancement of “low-Er” and “high-Er” waveguide at 1529 nm, respectively. Also shown as the inset are the transmission spectra of the waveguides at zero and maximum pump powers. In case of the “low-Er” waveguide, the signal enhancement at 1529 nm is 2.9 dB, which is larger than the Er absorption loss of 2.5 dB expected at that wavelength and indicates an internal optical gain of 0.4 dB. Furthermore, a clear peak in the transmission spectrum is observed at 1529 nm, confirming that population inversion has been achieved. The level of population inversion is estimated to be ~0.6, which indicates that most of the incorporated Er atoms are optically active. In case of the “high-Er” waveguide, the signal enhancement saturates at a value of 9.8 dB, corresponding to internal optical loss of 1.0 dB. Furthermore, no transmission peak peak at 1529 nm can be observed, confirming that population inversion did not occur. Based on the internal optical loss of 1.0 dB, the level of population inversion is estimated to be ~0.45.

Fig. 13 The gain characteristics of (a) “low-Er” and (b) “high-Er” waveguide at 1529 nm. Also shown as the inset are the transmission spectra of the waveguides at zero and maximum pump powers. Population inversion is achieved for “low-Er” waveguide, but not for the “high-Er” waveguide.

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We fabricated the three kinds of waveguide amplifiers, the strip-loaded Er-Yb/Y silicates waveguide [45], hybrid SiN_x Er-Yb/Y silicates [46], and slot Er-Yb/Y silicates waveguide amplifiers [47]. Figure 14(a) shows the scanning electron microscopy (SEM) image of Er-Yb silicate strip loaded waveguide amplifier. For this structure, the pumping light at 1480 nm and signal light at 1530 nm can be well confined in the active layer. The inset shows the calculated fundamental TE-mode profile of the signal light source. The calculated core-mode overlap is approximately 0.35. Figure 14(b) shows designed structure diagram and SEM image of hybrid Si₃N₄ Er-Yb/Y silicate waveguide amplifier. For this structure, the pumping light at 1480nm and signal light at 1530nm can be well confined in the interface of Si₃N₄ strip and silicate film. With half of the power transmitted in the Si₃N₄ strip, the propagation loss in the hybrid waveguide can be reduced dramatically. Figure 14(c) shows the cross-section image of the fabricated slot waveguide by scanning electron microscope. The bottom and upper layers are c-Si and α-Si, which have similar index of about 3.45. In this structure, low-index silicate material is sandwiched by high-index silicon to form high-index-contrast configuration. Due to the continuity of normal electric displacement (D = εE) at dielectric interface, the electric field for a TM-like mode can be enhanced by a factor of n_si²/n_slot² in the low-index slot region. Furthermore, this structure can provide relatively high electric field confinement, which is expected to produce high modal gain. For active material embedded in the slot region, the concentrated electric field can promote the light-matter interaction.

Fig. 14 SEM micrograph profile of (a) Er_0.1Yb_1.9SiO₅ strip loaded waveguide amplifier, (b) hybrid Si₃N₄ Er_0.2Yb_1.8SiO₅ silicate waveguide amplifier, (c) Er_0.1Yb_1.9SiO₅ slot waveguide amplifier Inset: calculated fundamental TE-mode profile of fabricated waveguide.

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Such three kinds of waveguide amplifiers’ characteristics are shown in Fig. 15. Figure 15(a) shows the 1533nm signal light enhancement as a function of pump power for the strip loaded waveguide amplifier. Despite large coupling and propagation loss, a 5.5 dB signal enhancement was observed in the as-fabricated waveguides at the pump power of 372 mW. The curve shows that the signal enhancement keeps linearly increasing even at the maximum pump power. With higher pump power or pumping at forward and backward of the waveguide simultaneously, the signal enhancement can be further increased. To characterize the enhancement for different wavelength, an enhancement spectrum in the range of 1515 nm–1580 nm for the fixed pump power of 372 mW was shown in the inset of Fig. 15(a). The PL peak and absorption dip reasonably appear at the same wavelength of 1533 nm. For the wavelength with little absorption such as 1580 nm, the enhancement effect is relatively small.

Fig. 15 The 1532 nm signal light enhancement as a function of pump power for (a) strip loaded waveguide amplifier, and (b) hybrid Si₃N₄ silicate waveguide amplifier. (c) The enhancement spectrum at pump power of 372 mW for the slot waveguide amplifier.

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Figure 15(b) shows the 1532 nm signal light enhancement as a function of pump power for the hybrid Si₃N₄ waveguide amplifier. Despite large coupling loss, at the pump power of 372 mW, signal enhancement of 5.5 dB/cm and 10.1 dB/cm was observed in the fabricated waveguides annealed at 750 °C and 1000 °C respectively. With higher annealing temperature, the signal enhancement of the waveguide became higher because more Er³⁺ ions were activated at high temperature. Given higher pump power or pump at forward and backward of waveguide simultaneously, the signal enhancement can be further increased. To characterize the enhancement for different wavelength, an enhancement spectrum range of 1520 nm-1580 nm for fixed pump power of 372 mW was shown in the inset of Fig. 15(b).

Figure 15(c) gives the enhancement spectrum at pump power of 372 mW for the slot waveguide amplifier. From this figure, it can be clearly seen that the enhancement peak appears at 1.53 μm, which corresponds to the PL peak and absorption dip. The high-frequency oscillation noise comes from the Fabry-Perot (F-P) cavity effect, which is formed by the reflection of waveguide facets. Therefore, no net gain is obtained.

6. Fabrication of Er silicate light-emitting diodes

An efficient electrical-pumped silicon-based light source compatible with silicon technology is a key component for the rapid developing silicon photonics [5]. The Er silicate compounds provide much higher optically-active Er concentrations than the conventional Er-doped silicon based materials due to the stoichiometric nature [11, 27, 45–47]. Recently, we have demonstrated extraordinary PL efficiency and optical amplification at 1.53 μm in Er-Yb silicate [29, 31, 45–48]. In contrast to the intense investigation on the PL and optical amplification of this material system, EL is difficult to obtain due to the insulating nature of the silicate compound which prevents efficient current injection. One possible way to get EL directly from Er silicate materials is to use the metal-insulator-semiconductor (MIS) structure, which has been successfully used in the Er-doped silicon dioxide light emitting devices [49–51]. Based on this, the MIS light emitting device containing Er-Yb silicate thin film as optical active material was fabricated using the standard silicon technology [52]. The schematic cross section view of the device structure is shown in Fig. 16(a). The direct 1.53μm EL from the Er-Yb silicate was observed the first time at an ultralow current density of 0.3 mA/cm² due to the direct impact excitation of Er³⁺ by hot electrons produced in the silicate. Figure 16(b) shows the Er-Yb silicate MIS device’s I-V curve measured on a 1 mm² ITO electrode. The current conduction mechanism of the device was revealed to be the Fowler-Nordheim (FN) tunneling. Figure 16(c) shows the EL spectra of the device, measured using current pulse with different amplitude from 1.5 μA to 5 μA. The EL spectra profiles are identical to the 1.53 μm PL spectra. This indicates that the EL is due to the radiative transition of Er³⁺ from ⁴I_13/2 to ⁴I_15/2 level. The EL spectra are peaked at 1531 nm and the spectra structure is due to the Stark splitting of the referred Er ions’ energy levels. No saturation of EL intensity was observed, which means the EL intensity can be further increased.

Fig. 16 (a) The schematic cross section view of the device structure. (b) The Er-Yb silicate MIS device’s I-V curve measured on a 1mm² ITO electrode. (c) The EL spectra of the device at varying current pulse amplitudes (1.5–5 μA).

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Although Er silicates have nice luminescence properties, research teams have not yet produced the high-gain optical-waveguide amplifiers and lasers the theories predict. In principle, so-called quenching and energy transfer upconversion effects caused by such high Er³⁺ concentration result in the reduction of the photoactive Er³⁺ population and the loss of the pump, ultimately affect the luminous efficiency. And in technology, two main technical factors account for this difficulty. The first is the presence of large waveguide transmission losses up to 8 dB/cm [45]. The Er–Yb/Y silicate thin films usually assume the polycrystalline state after annealing, which results in large light-transmission scattering losses. Side-wall roughness also is generated in the waveguide etching process. The second factor is that current pump lasers have difficulty in producing the high pump-power density required for high-density Er³⁺ population inversion.

7. Er silicate nanowire optical waveguide amplifier

Systematic studies, taking into account the factors of Er silicate thin films mentioned above, have examined a novel single-crystal core-shell-structure erbium chloride silicate (ECS) nanowire [35]– [40]. Pan et al. [35] first adopted a bottom-up chemical-vapor-deposition method to prepare single-crystal ECS nanowires. These nanowires, because of their single-crystal characteristics, were almost free of defects. Furthermore, the nanowire structure did not require waveguide etching. Therefore, transmission losses could be reduced greatly. In contrast, these ECS nanowire samples presented the longest fluorescence lifetimes of all Er³⁺ compound materials with Er³⁺ concentrations higher than 10²² cm⁻³ [36]. Such lifetimes provided significantly lower required pumping densities to achieve population inversion and high gain. In 2017, Sun et al. [40] produced a single-crystal ECS nanowire that exhibited a large unit net gain of 124.5 dB/cm at 1532 nm, which was two orders of magnitude higher than existing reported values. Figure 17(a) shows the scanning electron microscopy (SEM) image of such an as-grown ECS sample. Such micron sized ECS wires with single-crystalline material quality form natural waveguides with relatively low propagation loss and are essential in demonstrating the large net material gain in this research. Figure 17(b) shows the power-dependent signal enhancement at 1,532 nm of a single ECS nanowire (56.2 µm in length and 1 µm in diameter). The probe wavelength is chosen at the PL peak wavelength of ECS nanowires. To operate in the small-signal regime, the launched probe power (prior to coupling) was set to -15 dBm. A maximum value of 5.7 ± 0.3 dB at launched pump power of 75.6 mW was obtained, which corresponds to 1,014 ± 53 dB cm^–1 averaged signal enhancement. Here, the 0.3 dB uncertainty originates from the signal fluctuation of the optical spectrum analyser. The signal enhancement value shown here is nearly two orders of magnitude larger than that obtained from other erbium-based materials. It is concluded that the significant improvement of this value is mainly attributed to the high erbium concentration and single-crystalline material quality.

Fig. 17 (a) SEM image of an as-grown ECS sample showing long and large wires with high material quality. The wire in the center of the image has a length of 106 μm and diameter of ~800 nm. (b) Spatial-averaged signal enhancement at 1,532 nm as a function of launched pump laser output power, obtained from a single ECS nanowire. A signal enhancement value of 1,014 dB cm⁻¹ was obtained at the maximum pumping level, as marked by the red dashed line.

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We have demonstrated the near-infrared up-conversion lasing in Er-Y chloride silicate nanowires when pumped by 1476 nm laser at room temperature [53]. The single-crystal Er-Y chloride silicate nanowires were grown by the chemical vapor deposition method, using ErCl3 micro beads mixed with YCl3 micro powder and silicon powder according to the proportion of atomic.

Figure 18(a) shows the spectra of the nanowire around 980 nm pumped by 1480 nm laser on 1 mW, 6 mW, 15 mW, respectively. As previously discussed, the nanowires are highly ordered and crystalline, as a result, obvious multi-peaks with high signal to noise ratio in the spectra are observed even at very low pump power (1 mW). A two-order CUC process and a four energy state model was proposed and demonstrated to explain 980 nm up-conversion lasing mechanism, as shown in the inset. The dependence of the integrated intensity on the pump power at different wavelengths is presented in Fig. 18(b). A clear threshold for 985 nm peak was observed at a launched average pump power of approximately 7 mW, shown in left inset. The superb linear relationship above threshold between emission intensity and pump power were presented. Figure 18(c) presents the spectra of 980 nm band at different measurement temperature pumped by space coupling at a maximum pump power of 225 μW. Well separated sharp emission lines within the visible and infrared band have a linewidth of only 0.25 nm at 77 K.

Fig. 18 (a) Spectra at 950–1035 nm band under different pump power and the corresponding four energy state model (inset). (b) The dependence of the integrated intensity on the pump power; (i) Magnified view around the threshold point. (ii) Nonlinear response of laser output power with increasing pump power. (c) Spectra of the nanowire around 979 nm in different measurement temperature and the dependence of emission intensity and linewidth of the 979.1 nm peak to the temperature (inset).

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The Er/Y ratio in the nanowires was further varied to analyze the 980 nm up-conversion emission mechanism and a blue shift of the strongest peak in the band of the nanowires with higher ratio of Er/Y was observed. These 980 nm lasing properties of Er-Y chloride silicate nanowires presented above pave a new way of utilizing up-conversion mechanism in Er-Y nanowire to achieve tunable near-infrared laser and indicate its potential in future application in nanoscale optoelectronic devices operating at near-infrared wavelength.

8. Conclusion

In conclusion, the Er silicates as novel light source materials, which that can provide high Er concentrations and refractive index, have been studied for compact Si based on-chip light sources. A more accurate and systematic theoretical model of Er/Er-Yb silicate was established to inform the design and simulation of silicon-based waveguide amplifiers and lasers, which provided support and guidance for the preparation of high-performance Er-silicate light sources. Then the different Er-Yb/Y silicate compound films with the optimized their luminescence have been obtained for Y and Yb co-doped Er silicates compared with that of pure Er silicate. Based on the optimized materials, several kinds of waveguide structure, the strip-loaded, slot and hybrid Er-Yb/Y silicates waveguides have been fabricated, and the optical amplification and internal optical gain were observed in these waveguide structures. 1.53 μm electroluminescence in Er-Yb silicates was also realized using hot carriers’ impact excitations of Er³⁺. A novel single-crystal Er silicate nanowire was also proposed, which can solve the difficulties of large transmission losses in Er silicate films because of its high single-crystal quality. To sum up, the actual realization of silicon based light sources using these materials still needs more research: on the one hand, the preparation of high quality single-crystal Er silicate thin films that can further reduced transmission losses is the key, and on the other hand, the Er silicate nanowires need to be further lengthened. In the future, the promising on-chip light sources realized with Er silicates remain to be seen.

Funding

National Natural Science Foundation of China (NSFC) (No. 61635001).

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Parameters
Er³⁺ concentration	$N_{E r}$	$(0.23 - 1.6) \times 10^{22} {cm}^{- 3}$
Yb³⁺ concentration	$N_{Y b}$	$(0 - 1.37) \times 10^{22} {cm}^{- 3}$
signal wavelength	$λ_{s}$	1532 nm
pump wavelength	$λ_{p}$	980 nm
Er³⁺ emission cross section at 1532 nm	$σ_{21}$	$0.654 \times 10^{- 20} {cm}^{2}$
Er³⁺ absorption cross section at 1532 nm	$σ_{12}$	$0.654 \times 10^{- 20} {cm}^{2}$
Er³⁺ absorption cross section at 980 nm	$σ_{13}$	$2 \times 10^{- 21} {cm}^{2}$
Yb³⁺ emission cross section at 980 nm	$σ_{21}^{Yb}$	$1.2 \times 10^{- 20} {cm}^{2}$
Yb³⁺ absorption cross section at 980 nm	$σ_{12}^{Yb}$	$1.2 \times 10^{- 20} {cm}^{2}$
Er³⁺ emission lifetime at ⁴I_13/2 level	$τ_{21}$	1.5 ms
Er³⁺ lifetime at ⁴I_11/2 level	$τ_{32}$	10 μs
Er³⁺ lifetime at ⁴I_9/2 level	$τ_{43}$	10 μs
Yb³⁺ emission lifetime at ²F_5/2 level	$τ_{21}^{Yb}$	2 ms
cooperative upconversion coefficient of Er³⁺	$C$	${(N_{E r} / 10^{20})}^{0.5} \times 10^{- 18} {cm}^{3} / s$
Yb³⁺-to-Er³⁺ energy-transfer coefficient	$K_{tr}$	$4 \times 10^{- 17} {cm}^{3} / s$
propagation loss	$α (ν_{s, p})$	3 dB/cm

Erbium silicate compound optical waveguide amplifier and laser [Invited]

Abstract

1. Introduction

2. Theoretical analyses of Er silicate light sources

2.1 Theoretical models

2.2 Design and simulation of Er silicate optical waveguide amplifier

2.3 Design and simulation of Er silicate waveguide laser

3. Preparation of Er silicate thin film

4. Luminescence optimization of Er silicate films

4.1 Optimization of Er-Y silicate films

4.2 Optimization of Er-Yb silicate films

5. Fabrication of Er silicate optical waveguide amplifiers

6. Fabrication of Er silicate light-emitting diodes

7. Er silicate nanowire optical waveguide amplifier

8. Conclusion

Funding

References and links

Cited By

Figures (18)

Tables (1)

Equations (10)

Optical Materials Express