Spectroscopy of thulium-doped tantalum pentoxide waveguides on silicon

The spectroscopic properties and laser operation of thulium-doped tantalum pentoxide (Tm:Ta2O5) waveguides are reported in this paper. Fluorescence ranging from 1600 nm to 2200 nm, corresponding to the F4 → H6 transition was observed from 3 wt% Tm:Ta2O5 waveguides pumped at a wavelength of 795 nm. Measurements of excited-state lifetime, the emission and absorption spectra, with subsequent calculation of the crosssections for the deposited films, reveal its potential as a gain medium. Laser operation at a wavelength of 1865 nm was obtained with feedback from the polished end faces alone, demonstrating gain of >9 dB/cm. © 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement


Introduction
Optical sources and amplifiers operating at wavelengths near 2 µm are important for applications from remote sensing and LIDAR [1], medical diagnostics and surgical systems [2], to free-space and optical fiber communications [3]. Thulium-doped crystals and glasses are of significant interest for these applications, potentially offering low noise, high efficiency, and high-power operation. They can be optically pumped in-band at ~1.6 µm or using a "two-for-one" cross-relaxation process by pumping at ∼800 nm, where high-power low-cost semiconductor laser diodes are readily available. Tm-doped silicate glass optical fiber devices exhibit high efficiency and high power (>1 kW CW [4]) and high gain (5.8 dB/cm [5]) leading to excellent performance as individual fiber components.
In comparison with fiber devices, integrated photonics offers the potential for enhanced functionality combined with a robust construction, good thermal management, and low-cost mass-production of complex optical circuits. Glass and crystalline waveguide lasers operating at ~2 µm have been demonstrated based on various fabrication methods, for example by: ionimplantation in Tm-doped germanate glasses [6], Ti-diffusion into Tm-doped LiNbO 3 [7], direct bonding of Tm:YAG and sapphire [8], pulsed-laser-deposition growth of crystalline Tm:Y 2 O 3 [9], and liquid-phase epitaxy in Tm-doped potassium double tungstates [9,10]. A combination of high Tm concentration and the well-confined waveguide structure leads to a high gain coefficient as required for compact integrated devices.
In the last ten years, silicon photonics has grown to become the favored option for the wide deployment of photonic circuit technology, harnessing silicon micro/nanofabrication capabilities. However, as silicon is an indirect bandgap semiconductor it is an inefficient light emitter, leading to intensive investigation into ways to integrate laser sources and amplifiers on a silicon platform such as flip-chip bonding, transfer printing and heterogeneous epitaxy of III-V materials [11]. An alternative approach is to integrate rare-earth-doped laser sources directly in a CMOS compatible material on the silicon platform, allowing for monolithic integration and wafer scale manufacturing. Integrated rare-earth-doped lasers at wavelengths from 1 to 1.6 µm have been demonstrated on silicon with rare-earth doping using ytterbium and erbium [12,13,14,15,16]. While such systems require optical pumping with a semiconductor light source rather than the preferred electrical pumping, they have potential for high efficiency, low noise and low thermal load [13].

Waveguide design and fabrication
Tm-doped tantalum pentoxide waveguides on oxidized silicon wafers were designed to enable spectroscopic measurements of absorption and emission spectra, fluorescence lifetime, and conduct studies of laser operation. The fabrication was based on processes developed for Er:Ta 2 O 5 waveguides on silicon [24] and ellipsometer data for refractive index from that work.

Waveguide geometry
While low-threshold lasing, and gain with low pump power, can be achieved with tightlyconfined "nanowire" waveguides [25], thicker slab and rib waveguides are more appropriate for spectroscopic measurements and determination of material properties because a larger proportion of the propagating mode is confined within the core material. Slab waveguides were fabricated for fluorescence measurements, while rib waveguides were produced for absorption measurements and laser action. The slab waveguides used were 2 µm thick and had a refractive index of 2.1±0.04 at 1.55 μm. Rib waveguides for lasing were designed, using COMSOL Multiphysics, for monomode operation at a wavelength of 1.85 µm.
Defining ribs in the 2-µm-thick Tm:Ta 2 O 5 film, with an etch-depth of 330 nm, demonstrated that waveguides of widths less than 3 µm were found to support the fundamental mode only. The design and mode intensity profile in the TE polarization for the 3-μm-wide waveguide are shown in Fig 2. The theoretical FW1/e 2 spot size at a wavelength of 1.866 μm was 4.6 μm in the x-direction and 1.1 μm in the y-direction, and at the pump wavelength of 795 nm the spot size was 3.8 μm by 1.0 μm. Rib waveguides of 20 µm width were used for absorption measurements, ensuring strong mode confinement within the core to give an accurate measurement for the doped material only. The larger waveguide also offers reasonable inputcoupling efficiency of white light from an SMF28 optical fiber.

Waveguide fabrication
Tm:Ta 2 O 5 films, 2 μm thick, were deposited by radio-frequency (RF) sputtering on top of a 2.5-μm-thick thermal oxide layer on 4" silicon wafers. The 150mm diameter pressed ceramic sputtering target was made from powder or Ta 2 O 5 with a nominal 3 wt% of Tm 2 O 3.
Deposition was performed at a pressure of 10 mTorr in an oxygen (5 sccm) and argon (20 sccm) atmosphere, at an RF power of 300W, resulting in a deposition rate of ~3.33 nm/min. After deposition, the wafers were annealed in an oxygen atmosphere to reduce oxygen deficiency and stress in the film. Annealing times of 2 and 12 hours with temperatures between 500°C to 650°C were used, to study the respective influence on fluorescence lifetime and intensity. The composition of the annealed films was measured using EDX to verify the Tm concentration, which was found to be (1.1±0.1) × 10 21 Tm ions/cm 3 . Rib waveguides ranging from 2 to 20 μm in width were fabricated on wafers annealed at 650°C for 12 hours. The waveguides were patterned using conventional photolithography and etched to a depth of 330 nm by Ar ion-beam milling, using optimized parameters determined for Er:Ta 2 O 5 waveguides [24], and then annealed in oxygen at 650°C for a further 2 hours.
The wafers were diced and the end-facets were mechanically polished to optical quality to yield chips of length 4.5 mm to 10 mm.

Spectroscopic properties of Tm:Ta 2 O 5
In order to assess the potential of Tm:Ta 2 O 5 for gain and lasing, and to provide parameters for input to simulations, accurate measurement of material properties was required. The excitedstate lifetime, emission spectrum and cross-section, and the absorption spectrum and crosssection were experimentally determined for the fabricated Tm:Ta 2 O 5 waveguides as described below.

Excited-state lifetime measurements
Fluorescence measurements were performed on the 2-μm-thick Tm:Ta 2 O 5 slab waveguides using the apparatus shown in Fig. 3. The optimum pump wavelength had previously been determined to be 795 nm, where the highest fluorescence power was achieved [26]. Output from a Ti:sapphire laser, tuned to the pump wavelength of 795 nm, was mechanically chopped at 170 Hz and then end-fire coupled into the slab waveguide using an aspheric lens (Lens 3). Fluorescence was collected at 90° to the plane of the waveguide using a proximitycoupled fiber with a 1-mm-diameter core (Thorlabs SM2000), after which the light was collimated with an aspheric lens (Thorlabs C230TMD-C), passed through a long-pass filter to remove pump light (Schott RG1000, cut-off 1 µm), before being focused onto an InGaAs detector (DET10D/M) by a second aspheric lens (Thorlabs A260TM-C). The collected fluorescence signal was recorded on an oscilloscope and its decay after the pumping pulse is shown on a logarithmic scale in Fig. 4a. The effect of annealing temperature on the collected fluorescence power and excited-state lifetime were determined, as shown in Fig. 4. The highest fluorescence power and longest lifetime were attained with the sample annealed at 650°C for 12 hours. Fitting of the fluorescence decay to a single exponential yielded a lifetime of 477 ± 40 μs for this sample, with negligible residuals.
As with erbium-doped Ta 2 O 5 waveguides [24], it is believed that replenishing of oxygen during annealing reduces non-radiative transitions and improves the efficiency of the emission from the excited state.

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Conclusion
The spectroscopic characteristics of Tm:Ta 2 O 5 waveguides on silicon were investigated in this paper. A broad emission from 1450 to 2100 nm was observed when pumped at 795 nm and the corresponding excited-state lifetime was measured to be (477±40) μs. The absorption spectrum of Tm:Ta 2 O 5 from 600 to 2000 nm was measured, and the peak absorption crosssection at 792 nm was found to be (5.0±0.6) × 10 -21 cm 2 and at 1682 nm it was (4.1±0.4) × 10 -21 cm 2 . The peak emission cross-section was found to be (5.7±0.7) × 10 -21 cm 2 at 1772 nm, significantly higher than silicate glasses. The potential as a laser material was evaluated by pumping the waveguides at 795 nm and achieving lasing from the Fresnel reflection of the polished end facets alone. These results confirm that a gain of at least 9 dB/cm has been achieved. While low pump power threshold and high slope efficiency for efficient operation will require further optimization of the materials and waveguide properties including background loss and Tm concentration, and an optimized laser cavity. Tm:Ta 2 O 5 is a promising material for realizing integrated lasers and amplifiers compatible with silicon photonics.

Funding
This work was supported by the UK EPSRC Programme Grant, Silicon Photonics for Future Systems EP/L00044X/1, by Amy Tong's EPSRC studentship 1513767, and for J. I. Mackenzie by EP/N018281/1 and EP/P027644/1.