We report ultra-narrow-linewidth erbium-doped aluminum oxide (Al2O3:Er3+) distributed feedback (DFB) lasers with a wavelength-insensitive silicon-compatible waveguide design. The waveguide consists of five silicon nitride (SiNx) segments buried under silicon dioxide (SiO2) with a layer Al2O3:Er3+ deposited on top. This design has a high confinement factor (> 85%) and a near perfect (> 98%) intensity overlap for an octave-spanning range across near infrared wavelengths (950–2000 nm). We compare the performance of DFB lasers in discrete quarter phase shifted (QPS) cavity and distributed phase shifted (DPS) cavity. Using QPS-DFB configuration, we obtain maximum output powers of 0.41 mW, 0.76 mW, and 0.47 mW at widely spaced wavelengths within both the C and L bands of the erbium gain spectrum (1536 nm, 1566 nm, and 1596 nm). In a DPS cavity, we achieve an order of magnitude improvement in maximum output power (5.43 mW) and a side mode suppression ratio (SMSR) of > 59.4 dB at an emission wavelength of 1565 nm. We observe an ultra-narrow linewidth of ΔνDPS = 5.3 ± 0.3 kHz for the DPS-DFB laser, as compared to ΔγQPS = 30.4 ± 1.1 kHz for the QPS-DFB laser, measured by a recirculating self-heterodyne delayed interferometer (RSHDI). Even narrower linewidth can be achieved by mechanical stabilization of the setup, increasing the pump absorption efficiency, increasing the output power, or enhancing the cavity Q.
One of the key challenges in the field of silicon photonics remains the development of compact integrated light sources. In one approach, rare-earth-doped glass microtoroid and microdisk lasers have been integrated on silicon and exhibit ultra-low thresholds. However, such resonator structures are isolated on the chip surface and require an external fiber to couple light to and from the cavity. Here, we review our recent work on monolithically integrated rare-earth-doped aluminum oxide microcavity lasers on silicon. The microlasers are enabled by a novel high-Q cavity design, which includes a co-integrated silicon nitride bus waveguide and a silicon dioxide trench filled with rare-earth-doped aluminum oxide. In passive (undoped) microresonators we measure internal quality factors as high as 3.8 × 105 at 0.98 µm and 5.7 × 105 at 1.5 µm. In ytterbium, erbium, and thulium-doped microcavities with diameters ranging from 80 to 200 µm we show lasing at 1.0, 1.5 and 1.9 µm, respectively. We observe sub-milliwatt lasing thresholds, approximately 10 times lower than previously demonstrated in monolithic rare-earth-doped lasers on silicon. The entire fabrication process, which includes post-processing deposition of the gain medium, is silicon-compatible and allows for integration with other silicon-based photonic devices. Applications of such rare earth microlasers in communications and sensing and recent design enhancements will be discussed.
A key challenge for silicon photonic systems is the development of compact on-chip light sources. Thulium-doped fiber and waveguide lasers have recently generated interest for their highly efficient emission around 1.8 μm, a wavelength range also of growing interest to silicon-chip based systems. Here, we report on highly compact and low-threshold thulium-doped microcavity lasers integrated with silicon-compatible silicon nitride bus waveguides. The 200-μmdiameter thulium microlasers are enabled by a novel high quality-factor (Q-factor) design, which includes two silicon nitride layers and a silicon dioxide trench filled with thulium-doped aluminum oxide. Similar, passive (undoped) microcavity structures exhibit Q-factors as high as 5.7 × 105 at 1550 nm. We show lasing around 1.8–1.9 μm in aluminum oxide microcavities doped with 2.5 × 1020 cm−3 thulium concentration and under resonant pumping around 1.6 μm. At optimized microcavity-waveguide gap, we observe laser thresholds as low as 773 μW and slope efficiencies as high as 23.5%. The entire fabrication process, including back-end deposition of the gain medium, is silicon-compatible and allows for co-integration with other silicon-based photonic devices for applications such as communications and sensing.
Optimization of the photonic bandgap in finite-height photonic crystal (PhC) slab structures requires high-fill-factor lattices. We present a method for fabrication of high-fill-factor PhC devices in silicon-on-insulator (SOI) substrates using electron-beam lithography and high-aspect-ratio reactive-ion etching (RIE). We achieve 8:1 aspect-ratio PhC structures with 60-nm vertical membrane walls using a custom deep reactive-ion etching process in a conventional low-end RIE with patterned resist as the only etch mask. We present examples of various PhC devices fabricated using this method including a high-efficiency coupling structure for PhC waveguides.
Terahertz electroluminescence was produced by intersubband transitions in silicongermanium quantum wells. The devices were grown by solid-source molecular-beam epitaxy on high-resistivity silicon substrates, and were fabricated by standard photolithography and processing techniques. Using FTIR spectroscopy at at temperature of 5 K, electroluminescence was observed around 9 THz. The emission was attributed to heavy-hole-to-light-hole transitions and demonstrates the potential for SiGe technology as terahertz emitters.