Ultra-high Q alumina optical microresonators in the UV and blue bands

UV and visible photonics enable applications ranging from spectroscopic sensing to communication and quantum information processing. Photonics structures in these wavelength regimes, however, tend to experience higher loss than their IR counterpart. Particularly in the near-UV band, on-chip optical microresonators have not yet achieved a quality factor beyond 1 million. Here we report ultra-low-loss photonic waveguides and resonators patterned from alumina thin films prepared by a highly scalable atomic layer deposition process. We demonstrate ultra high Q factor of 1.5$\,\times\,$10$^6$ at 390nm, a record value at UV bands, and 1.9$\,\times\,$10$^6$ at 488.5nm.


Introduction
UV and visible band integrated photonics has witnessed rapid progress in recent years.Applications such as atomic clocks [1], biochemical sensing [2,3], visible light communications [4], quantum sensing [5,6], quantum information processing based on trapped ions [7,8] and atoms [9,10], all call for UV and blue band photonic integrated circuits with high scalability and low loss.Yet, low-loss photonics at short wavelengths remains difficult to achieve as material absorption dramatically increases when photon energy approaches the bandgap of a material, and Rayleigh scattering scales as  −4 .One approach to reduce the loss at short wavelengths is to use very thin silicon nitride (Si 3 N 4 ) waveguides cladded with low loss silica [9,11] so that the propagation mode is weakly confined.In this way, absorption inherent to the Si 3 N 4 waveguide core can be diluted, while scattering loss induced by waveguide sidewall roughness is also reduced due to the reduced sidewall height.However, so far, devices employing Si 3 N 4 as the waveguide core still show strong absorption in the UV and blue bands.[12,13] Further reduction of waveguide loss at short wavelengths requires a new waveguide core material that demonstrates a large bandgap while still maintaining a higher refractive index than the low-loss cladding material, usually silica, to provide good confinement.One such candidate is AlN, which has a large bandgap of 6.2 eV.In our previous work, we employed single-crystalline AlN as waveguide core [14] and demonstrated a quality factor of 210 k at 390 nm, which was a significant advance for devices operating in the near-UV bands, but still below the state-of-the-art achieved by IR and near-visible optical resonators.
An alternative to AlN for short-wavelength passive photonic integrated platforms is amorphous alumina.Recent progresses in deposition methods have greatly improved the quality of amorphous alumina films, which now demonstrate a band gap that is comparable to bulk sapphire (7.0 -8.3 eV of ALD alumina [15,16] vs. 8.8 eV [17]).Many reports on amorphous alumina films deposited via either reactive sputtering or atomic layer deposition (ALD) also confirmed very low loss at short wavelengths (< 0.3 dB/cm at 405 nm) [18][19][20], and their compatibility with photonic platforms, either standalone [21] or combined with other materials [22].Recently, near-UV extended cavity diode lasers (ECDLs) were demonstrated by interfacing low-loss alumina waveguides with InGaN semiconductor amplifiers [23].Thanks to the amorphous microstructure of these alumina films, the associated deposition process also has no requirements for the lattice structure of the substrate on which it is grown, thus relaxing requirements for substrate material.Furthermore, both ALD and reactive sputtering process are CMOS compatible, paving the way for CMOS integration with amorphous alumina-based photonics.
In this letter, we leverage an industrial ALD process to grow alumina as the waveguide core.This highly scalable process is capable of providing uniform growth coverage to substrates over 20" in diameter and can coat hundreds of 4" wafers in a single batch.Because the absorption of UV and blue light is low in alumina, the propagation mode can be fully supported in the waveguide core, minimizing the scattering loss at top and bottom surfaces of the alumina film.With new waveguide core material and corresponding design principles, our resonators demonstrated ultra high Q of 1.5 × 10 6 at 390 nm and 1.9 × 10 6 at 488.5 nm.Those are the highest quality factors reported at corresponding wavelengths for resonators featuring high confinement design, including previously demonstrated alumina resonator [18,23].

Design and Simulation
We employ a shallow etch geometry to minimize sidewall scattering loss.The resonators are air-cladded to promote refractive index contrast with the alumina core, reducing the etch depth needed for confinement.The etch depth is optimized by simulating the radiation loss of waveguides subject to different etch depths in Lumerical.For ring resonators with 400 μm radius, it was found that radiation loss can be suppressed to less than 0.06 dB/cm at 488.5 nm and less than 0.001 dB/cm at 390 nm when the etch depth is greater than 80 nm out of 400 nm thick alumina film.During fabrication, the etch depth is targeted at 100 nm, deep enough so that the resonators are not radiation loss limited.The width of the waveguide is set to 4.5 μm so that the outer sidewall provides most of the confinement for the propagation mode, and the overlapping between propagation mode, particularly TE00 mode, and the inner sidewall is minimal, further reducing scattering loss.It should be noted that this wide waveguide allows for propagation of multiple propagation modes, however, as Fig. 2 shows, by optimizing bus-to-resonator coupling, the coupling to the higher order modes can be greatly suppressed.
We utilized an under-coupled point-coupling design to reduce coupling-induced loss and probe the intrinsic alumina resonator loss.The coupling geometry between straight bus waveguide and ring resonator is optimized by varying the bus waveguide width and the gap between the bus waveguide and the ring resonator.We use a combination of simulations with FIMMWAVE software and experimental data to optimize the parameters.The optimal coupling condition for 390 nm light is 0.65 μm wide bus waveguide and 1 μm gap between bus waveguide and ring resonator.For 488.5 nm light, the optimum coupling condition is 0.75 μm wide bus waveguide and 1.1 μm gap.We also fabricated and measured microrings with smaller gaps, which provide stronger coupling.

Device fabrication and results
The fabrication started with 4 μm of wet thermal oxide grown on silicon wafers.The test wafers were then coated at Entegris by applying a blanket layer of atomic layer deposition (ALD) amorphous alumina.The ALD deposition of alumina coating was performed by sequentially cycling of TMA / H 2 O with pulsing times between 0.05 s and 0.15 s and nitrogen purge times between 18 s and 20 s at temperatures between 180 • C and 250 • C, using a 20" diameter crossflow thermal ALD coatings system custom-built at Entegris in a class 10,000 clean room.The growth rate of this deposition recipe is approximately 1.1 Å / cycle.The thickness of the alumina coatings was measured to be a nominal 420 nm, determined by spectroscopic reflectometry using an Angstrom Sun SR300 system.The ring resonators and associated bus waveguides were defined with a 100kV electron-beam lithography system (Raith EBPG 5200+) with a negative FOx-16 resist.To mitigate electron charging effects due to the highly insulating alumina and silica layers, 200 nm of poly(4-styrene sulfonic acid) (PSSA) was spun on top of FOx-16 resist before 10 nm of gold was sputtered to provide grounding for stray electrons.The PSSA is water soluble to help the removal of gold after e-beam lithography.After the removal of gold by dipping in water, the chip was developed in 25 percent tetramethylammonium hydroxide (TMAH) developer.
The pattern was then transferred to the alumina layer using an Oxford PlasmaPro 100 Cobra Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE) system with a BCl 3 based etching recipe.Leftover FOx-16 resist was then removed from the chip by dipping the chip in 10:1 buffered oxide etch for 10 seconds.To further reduce the absorptive loss in alumina waveguide, the chip was annealed in the atmosphere at 500 • C (for 390nm chip) and 600 • C (for 488.5nm chip) for 5 hours to achieve the lowest loss and while avoiding crystallization, which has been reported to take place above 800 • C [24,25].
To characterize the ring resonators, we construct a sweeping blue/UV laser by frequency doubling a Ti-Sapphire laser (M2 SolsTiS, 700-1000 nm) to 390 nm and 488.5 nm.The Ti-Sapphire laser is locked to an external cavity, ensuring <50kHz linewidth and the wavelength of the laser is precisely determined by a 0.1 pm resolution wavemeter, or 0.05 pm resolution after frequency doubling.To create a sweeping ∼390 nm laser, the ∼780 nm pump laser from Ti:Sapphire laser is coupled to a lithium triborate (LBO) doubling crystal in a resonant cavity (M2 ECD-X).To create a sweeping ∼488.5 nm laser, ∼977 nm pump laser from Ti:Sapphire laser is sent through a Magnesium-doped Periodically Poled Lithium Niobate (MgO:PPLN, Covesion MSHG 976-0.5-30)crystal to frequency double to 488.5 nm.The MgO:PPLN crystal is put in an oven, whose temperature is adjusted as the frequency of the pump laser is scanned to maintained phase matching condition of the MgO:PPLN crystal for maximal frequency doubling efficiency.It should be noted that the output power from the Ti:Sapphire laser is wavelength dependent as the spacing of the étalon in the resonant cavity is being continuously tuned.The extended transmittance spectrum covering two FSRs is therefore stitched from four continuous scans around 390 nm (five around 488nm), with the étalon spacing being retuned for maximum power output at the beginning of each piecewise scan.
Fig. 2 shows the transmittance of the alumina ring resonator at ∼ 390 nm and ∼ 488.5 nm, respectively.For the extended transmittance spectrum, the pump Ti:Sapphire laser is scanned at 500 MHz/s, corresponding to a scan speed of 1 GHz/s after frequency doubling.For the zoomed in resonances depicted in the insets, to ensure high wavelength resolution, the pump Ti:Sapphire laser is scanned at 200 MHz/s (400 MHz/s after frequency doubling).Multiple sets of resonance peaks can be observed for both the 390 nm and 488.5 nm cases, as the 4.5 μm wide waveguide supports multiple TE transmission modes.Despite this, the coupling to TE00 mode is being optimized while the coupling to other modes are suppressed.For TE00 modes at 390 nm, the TE00 modes exhibit an FSR of 65.6 GHz, while the TE10 modes have an FSR of 66.3 GHz, as predicted by the 400 μm radius ring geometry.Even higher TE modes are not prominent.One of the TE00 mode resonance peaks demonstrates a loaded Q factor of 1.2 M, and has an extinction ratio of 2.4 dB.Using the formula  int = 2 L 1+ √ 10 −ER/10 (Here  int stands for intrinsic Q,  L stands for loaded Q, and ER stands for extinction ratio in dB.), for under-coupled conditions, we obtain an intrinsic Q of 1.5 M for this resonance.For TE modes at ∼ 488.5 nm, the FSR is 68.5 GHz for TE00 mode and 68.6 GHz for TE10 modes, with one of the TE00 mode resonance peaks demonstrating a high loaded Q of 1.4 M and has an extinction ratio of 3.2 dB, corresponding to a loaded Q of 1.9 M.
The current Q of our device is likely limited by the residual absorption of alumina and the scattering of the remaining alumina sidewall roughness as the radiation loss limited Q for the resonator is calculated to be beyond 10 10 for both TE00 and TE10 modes at wavelengths shorter than 500 nm.Since the modal absorption for TE00 and TE10 mode is the same, the Q difference between the two sets of resonances can be attributed to coupling loss and scattering loss.The waveguide is also capable of transmitting TM modes, however, the confinement of bus waveguide is weak for TM modes and the radiation loss limited Q for TM modes of ring resonator is calculated to be <10 M. Thus, we did not perform any further measurements of TM mode transmittance.
In Fig. 3, we compare the performance of our alumina ring resonator to other recent works on UV and blue band photonics.At wavelengths larger than 450 nm, low confinement Si 3 N 4 ring resonators with > 1.5 mm radii still hold the record for quality factors [9,11].At shorter wavelengths, absorption inherent to the Si 3 N 4 waveguide core would drastically impacts the performance of these devices.AlN was the star material for nanophotonic devices operating at UV-blue band, and progress in AlN film quality boosted the quality factor of AlN based resonators to up to 2.1 × 10 5 at 390 nm [14].Alumina film deposited with reactive sputtering and ALD boasts even larger bandgap compared to AlN and renewed the quality factor record to 4.7 × 10 5 at 405 nm [18,23].With ALD deposited alumina film and optimized geometry, our alumina ring resonator raises the quality factor record at UV band once again to 1.5 × 10 6 at 390 nm.
From the Q measurements, the propagation loss of the current ring resonator can be derived to be 0.84 dB/cm at 390 nm and 0.51 dB/cm at 488.5 nm based on the expression  = 4.343 × 2    int  , where   is the group refractive index obtained through   =  2  ring * FSR .The superior performance of the ring resonator in this paper can be attributed to the implementation of low-loss amorphous alumina as the waveguide's core material, as well as the ring geometry, which utilizes shallow etching to reduce the scattering loss.

Conclusion
In conclusion, we demonstrate ultra-high-Q UV and blue band ring resonators featuring low loss ALD alumina as the waveguide core, and optimized geometry which sees propagation mode strongly confined within the alumina waveguide core.This work pushes the intrinsic Q record of high confinement ring resonators to 1.5 M at 390 nm and 1.9 M at 488.5 nm, corresponding to a propagation loss of only 0.84 dB/cm and 0.51 dB/cm respectively.Our results present an important solution in terms of material choice and waveguide design to achieve low-loss integrated photonics in UV and blue band.Polycrystalline AlN [25, 26, 27]   Single-crystalline AlN [14, 28]   Thin Si3N4 [9, 11]   Al2O3 [18, 23]   Fig. 3. Comparison of  int vs. wavelength (300 -500 nm) for recent reports on nanophotonic devices using different materials and geometries.At > 450 nm wavelengths, low confinement Si 3 N 4 ring resonators with > 1.5 mm radius still hold the record for quality factors [9,11].At shorter wavelengths, improvements in waveguide core materials from polycrystalline AlN [26][27][28] to single-crystalline AlN [14,29] to alumina [18,23] allow for a boost in quality factor records.

Fig. 1 .
Fig. 1.(a) Schematics of ALD chamber capable of processing hundreds of 4" wafers at the same time.(b) Microscope image of patterned ring resonators and interconnection waveguides.Inset: Bus waveguide and ring resonator coupling region.(c) SEM image of cleaved waveguide facet and EDS analysis revealing the elemental mapping of (d) Al (e) Si.

Fig. 2 .
Fig. 2. Transmittance spectrum of alumina ring resonators at (a) 390 nm and (b) 488.5 nm showing two sets of transmitted TE modes.Insets top left: Simulated TE00 propagation modes in ring resonators at 390 nm and 488.5 nm respectively.Insets bottom left: Ring resonators under test at 390 nm and 488.5 nm respectively.Insets right: Zoomed-in views of TE00 resonances demonstrating the highest loaded and intrinsic Q of 1.2 M / 1.5 M at 390 nm and 1.4 M / 1.9 M at 488.5 nm respectively. L stands for loaded Q, ER stands for extinction ratio in dB and  int stands for intrinsic Q.

Funding.
This work is funded in part by the Office of Naval Research (ONR) grant N00014-20-1-2693.The materials used in this work is developed under the support of Department of Energy under grant No. DE-SC0019406.