Photonic crystal nanobeam cavities are valued for their small mode volume, CMOS compatibility, and high coupling efficiency-crucial features for various low-power photonic applications and quantum information processing. However, despite their potential, nanobeam cavities often suffer from low quality factors due to fabrication imperfections that create surface states and optical absorption. In this work, we demonstrate InP nanobeam cavities with up to 140% higher quality factors by applying a coating of Al2O3 via atomic layer deposition to terminate dangling bonds and reduce surface absorption. Additionally, changing the deposition thickness allows precise tuning of the cavity mode wavelength without compromising the quality factor. This Al2O3 atomic layer deposition approach holds great promise for optimizing nanobeam cavities that are well-suited for integration with a wide range of photonic applications.

Due to their compact size, nanobeam photonic crystal cavities have a remarkable ability to confine light both spatially and temporally, which makes them valuable for ultra-small lasers,1,2 nonlinear optics,3,4 sensing,5,6 quantum optics,7,8 and photonic integrated circuits.9,10 For example, nanobeam cavities utilizing III–V materials, such as InP, are extensively used for active nanophotonic devices that incorporate quantum dots and quantum wells.11,12 However, due to fabrication imperfections, nanobeam cavities often feature low quality factors. These imperfections are mainly due to dry-etching techniques that create surface roughness and absorption caused by the formation of surface states that act as recombination sites for free carriers.13–16 Such challenges emphasize the ongoing need for innovative approaches and techniques to address the complications of fabricating and preserving high quality factor nanobeam cavities.

Atomic layer deposition is a versatile thin-film deposition technique that features precise thickness control, uniformity along the surface normal,17 and the ability to modify material properties at the nanoscale.18 These properties have been harnessed in various applications, including refractive index sensing,19 microlens arrays,20 and resonance tuning of 2D photonic crystals.21,22 Additionally, the conformal film reduces surface roughness. As a result, atomic layer deposited films like Al2O3 have been shown to decrease propagation losses in waveguides,23 improve the performance of lasers,24 and provide surface passivation of III–V semiconductors25 and photodetectors,26 as well as improve the quality factor of silicon nanobeam cavities.27 These findings suggest that it would be a promising direction to explore atomic layer deposition as a passivation technique to enhance the quality factor of InP nanobeam cavities, though there has been a lack of experimental reports.

In this work, we demonstrate improved quality factors for an InP nanobeam cavity operating at telecom wavelengths through surface passivation by atomic layer deposition of Al2O3. We optimize the atomic layer deposition by sequentially adding 2-nm-thick coatings of Al2O3 at 150 °C on top of the InP nanobeam cavity. From this process, we determine the optimized layer thickness to be 6 nm, which improves the average quality factor by up to 140%. This enhancement is attributed to the reduction in out-of-plane scattering loss and surface absorption following the atomic layer deposition. In addition to improving the quality factor, we show that the Al2O3 coating can tune the resonant wavelength of the InP nanobeam cavity. This atomic layer deposition approach can improve the performance of InP nanobeam cavities for applications in both classical optoelectronics and quantum optics.

Figure 1(a) illustrates the nanobeam cavity used in this work. The nanobeam is designed to be an air-clad structure. We numerically optimized the design to achieve a cavity resonance at the telecom O-band with an optimal value of width w = 475 nm, lattice parameter a = 350 nm, and radius r = 92 nm. The cavity region is realized by gradually reducing the radius and lattice parameter of the four pairs of holes toward the center by a factor of 0.935. The left side (NL) consists of 13 holes that are fully reflective, and we varied the number of holes of the right side (NR) from five to eight holes that are partially reflective, which allows us to control the coupling strength of the cavity into the nanobeam waveguide.3 Using 3D finite-difference time-domain (FDTD) simulations, we calculated the theoretical quality factor for various outcoupling mirror numbers (NR) and observed the quality factor saturate at 9.60 × 10 5 for NR 13. Based on the simulated quality factor, we anticipate the critical coupling condition for the nanobeam cavity to fall between NR = 8 and NR = 10 (see the supplementary material, Note 1.1). Additionally, we calculated a quality factor of 3.65 × 10 4 and a mode volume of approximately V 0.55 ( λ / n ) 3 for NR = 8. Figure 1(b) presents the fundamental mode profile ( | E | 2) of the optimized nanobeam cavity at a wavelength of 1310 nm. The width of the nanobeam waveguide is adiabatically reduced from w = 475 to b = 150 nm over a 15 μm-long linear taper to enable outcoupling to the lensed fiber (see the supplementary material, Note 1.2). In Fig. 1(c), the 3D-FDTD calculated Gaussian far-field mode profile from the edge of the linear taper displays the directionality of the emission. The white dashed line within the far-field profile corresponds to a numerical aperture of 0.5, which represents a coupling efficiency of 90% to a lensed fiber.

FIG. 1.

(a) The tapered nanobeam InP photonic crystal cavity design with width w, lattice parameter a, and hole radius r. The cavity region is realized by tapering and shifting four pairs of holes by a factor of 0.935, toward the center. NL and NR are the total number of holes (including the tapered four holes) on the left and right sides of the nanobeam, respectively. (b) The 3D-FDTD simulated fundamental mode profile ( | E | 2) of the nanobeam cavity, and (c) the far-field mode profile calculated from the nanobeam taper edge with width b. The white dashed line in the far-field profile corresponds to a numerical aperture of 0.5 of the objective lens.

FIG. 1.

(a) The tapered nanobeam InP photonic crystal cavity design with width w, lattice parameter a, and hole radius r. The cavity region is realized by tapering and shifting four pairs of holes by a factor of 0.935, toward the center. NL and NR are the total number of holes (including the tapered four holes) on the left and right sides of the nanobeam, respectively. (b) The 3D-FDTD simulated fundamental mode profile ( | E | 2) of the nanobeam cavity, and (c) the far-field mode profile calculated from the nanobeam taper edge with width b. The white dashed line in the far-field profile corresponds to a numerical aperture of 0.5 of the objective lens.

Close modal

To fabricate the nanobeam cavity, we utilized a two-layer etching process.28,29 Initially, we applied a 170 nm silicon nitride film on a 280 nm thick InP membrane with InAs quantum dots as an etching mask using plasma-enhanced chemical vapor deposition. Subsequently, we employed electron beam lithography with fluorine-based reactive ion etching to pattern the nanobeam cavities onto the silicon nitride layer. Next, a chlorine-based reactive ion etching process was employed to transfer this pattern onto the InP layer. Finally, to make the nanobeam cavities suspended from a 30 × 30 μm2 square pad, as shown in Fig. 2(a), we use chemical wet etching of a 2-μm-thick InAlAs sacrificial layer utilizing a mixture of H2O, HCl (37%), and H2O2 (30%) in a 3:1:2 ratio. To ensure that the nanobeam structures do not bend or collapse on the substrate, they were immersed in isopropyl alcohol and dried using a critical point drier after the wet etching process. We note that the InAs quantum dots have a low density of 10 μm−2, which may contribute additional losses beyond the native losses of the InP substrate.

FIG. 2.

(a) and (b) SEM images of an array of nanobeam cavities after (a) wet etching and (b) transfer printing. (c) Zoomed-in view of a nanobeam cavity after transfer printing and 2 nm atomic layer deposition of Al2O3. (d) Schematic diagram of the reflectance spectra measurement setup for the InP nanobeam cavity (NBC) array; BS: beam splitter.

FIG. 2.

(a) and (b) SEM images of an array of nanobeam cavities after (a) wet etching and (b) transfer printing. (c) Zoomed-in view of a nanobeam cavity after transfer printing and 2 nm atomic layer deposition of Al2O3. (d) Schematic diagram of the reflectance spectra measurement setup for the InP nanobeam cavity (NBC) array; BS: beam splitter.

Close modal

In order to achieve suspended nanobeam cavities with direct access to lensed fiber coupling, we used transfer print lithography to transfer the nanobeam cavity array to the edge of a silicon carrier wafer. This transfer was performed using a polydimethylsiloxane stamp with dimensions of 30 × 30 × 40 μm3. Figure 2(b) displays a scanning electron microscopy (SEM) image illustrating the transferred nanobeam cavity arrays with the square pad positioned on the edge of the silicon carrier chip. Figure 2(c) shows the zoomed-in nanobeam cavity array after transfer printing and 2 nm atomic layer deposition.

We first characterized the fabricated nanobeams prior to Al2O3 deposition using the experimental setup illustrated in Fig. 2(d). In this arrangement, we employed a broadband stabilized tungsten-halogen lamp, which we directed into the tapered side of the suspended nanobeams using a lensed fiber with an objective lens of 0.5 numerical aperture. The reflected light from the nanobeam cavities was collected using a 90:10 beam splitter and directed toward a monochromator equipped with an InGaAs detector array. We determined the coupling efficiency by calculating the ratio of the total reflected and injected optical power between the nanobeam cavity and the objective lens, yielding a value of 55%, which is lower than the FDTD simulated coupling efficiency of 90% (see the supplementary material, Note 2.1). The disparity between the measured and simulated efficiencies can be attributed to factors such as imperfect angular alignment of the transferred nanobeams with the optical axis of the objective lens, scattering losses due to surface roughness of the tapered out-coupler, and mismatch of the numerical aperture between the nanobeam taper and the lensed fiber.

We employed atomic layer deposition to passivate the surface of the fabricated InP nanobeam cavities. This process initiates chemical reactions at the sample surface and is designed to be self-limiting, which means the film thickness can be accurately controlled by counting the number of deposition cycles.17 In our study, we employed an Al2O3 atomic layer deposition process, utilizing trimethylaluminum [Al2(CH3)6] and water (H2O) as precursors for each cycle.30 We performed 20 cycles for each deposition step, resulting in an Al2O3 with a thickness of 2 nm. The atomic layer deposition process was conducted at an operating temperature of 150 °C to ensure uniform deposition. The use of Al2O3, characterized by its low refractive index (1.75 at 1.31 μm), makes it a valuable tool for tailoring the optical properties of photonic devices.27 

We measured the reflectance spectra of a nanobeam cavity with NR = 7 after increasing steps of atomic layer deposition, as shown in Fig. 3(a). The normalized reflectance spectra demonstrate a reduction in linewidth for Al2O3 thicknesses of 2, 4, 6, 8, and 10 nm compared to the nanobeam cavity prior to atomic layer deposition. We calculated the quality factor values by fitting the reflection spectra using the Lorentzian function (see the supplementary material, Note 2.2). Applying a similar quality factor analysis to 30 cavities as a function of the Al2O3 thickness, we plotted the average quality factor in Fig. 3(b). The quality factor initially increases, reaching an average maximum quality factor of 4235 when the Al2O3 thickness is 6 nm. However, thicker Al2O3 layers lead to a reduced quality factor, with a significant decrease observed for thicknesses exceeding 10 nm. This trend is similar to the FDTD simulations, where the quality factor is highly dependent on the hole radius and decreases as the radius decreases (see the supplementary material, Note 2.3). Additionally, the resonance wavelength of the cavity is also impacted by the Al2O3 thickness, which is a consistent increase in wavelength [Fig. 3(c)]. We believe that the initial decrease in the cavity wavelength is due to the removal of native oxide31,32 in the atomic layer deposition chamber, resulting in an average reduction of 7 ± 2.6 nm (refer to the supplementary material, Note 2.4). From these measurements, we show that atomic layer deposition can tune the cavity over a 10.5 nm wavelength range without degradation to the cavity quality factor.

FIG. 3.

(a) Normalized reflectance spectra of a nanobeam cavity with NR = 7 for different thicknesses of Al2O3 using atomic layer deposition. The black curve, representing spectra without ALD, is blue-shifted by 1 nm to prevent overlap. (b) Average quality factors of 30 nanobeam cavities with NR = 7 for different atomic layer deposition thicknesses, with the average quality factor indicated by the red horizontal line, and (c) average wavelength shifts from the cavity resonance wavelength.

FIG. 3.

(a) Normalized reflectance spectra of a nanobeam cavity with NR = 7 for different thicknesses of Al2O3 using atomic layer deposition. The black curve, representing spectra without ALD, is blue-shifted by 1 nm to prevent overlap. (b) Average quality factors of 30 nanobeam cavities with NR = 7 for different atomic layer deposition thicknesses, with the average quality factor indicated by the red horizontal line, and (c) average wavelength shifts from the cavity resonance wavelength.

Close modal

We next measure the reflectance spectra of the nanobeam cavities as a function of the number of holes in the output coupling mirror (NR), which controls the output coupling strength. Figures 4(a) and 4(b) display the normalized reflectance spectra before [Fig. 4(a)] and after [Fig. 4(b)] deposition of a 6 nm thick Al2O3 coating for different values of NR, ranging from 5 to 8. As expected, after atomic layer deposition, we observed decreased linewidth in the full width at half maximum ( Δ λ), as shown in Fig. 4(b). Additionally, we find that the cavity reflectance minima strongly depend on NR, both with and without Al2O3 deposition. At NR = 6, the reflectance dip drops to nearly zero, indicating a near-critical coupling condition.33 This critical coupling NR number is lower than the simulated critical coupling NR due to the experimental “cold cavity” quality factor being much lower than the simulated value, primarily because of scattering and material losses. We also observe a clear transition between the over- and under-coupled regimes at NR = 5 and NR  7, respectively. Different coupling regions are of interest for various applications; the under-coupling region is particularly significant for characterizing low power optical frequency microcombs,34,35 while critically coupled cavities are essential in nonlinear optics and optical sensing.33 We also note that the reflectance minima for NR = 6 increases after atomic layer deposition, indicating that the device was brought even closer to the critical coupling regime, likely due to reduced cavity loss by the surface passivation process.

FIG. 4.

Normalized reflectance measurements spectra along with Lorentzian fits for a range cavity hole numbers of outcoupling mirror (NR) from 5 to 8 (a) Before ALD and (b) After ALD. The measurements are in blue circles, while the red dashed curve represents the Lorentzian fit. (c) The corresponding measured quality factor for the same nanobeam cavities for different NR before (blue) and after (red) atomic layer deposition (ALD) of a 6 nm coating of Al2O3. (d) Calculated Q | | and Q from Eq. (1) values before (blue) and after (red) the atomic layer deposition treatment. (e) Average percentage change in the quality factor (Q) across 20 samples for NR values ranging from 5 to 8. Purple, green, and gray color schemes correspond to the respective over-coupled, critically coupled, and under-coupled regions.

FIG. 4.

Normalized reflectance measurements spectra along with Lorentzian fits for a range cavity hole numbers of outcoupling mirror (NR) from 5 to 8 (a) Before ALD and (b) After ALD. The measurements are in blue circles, while the red dashed curve represents the Lorentzian fit. (c) The corresponding measured quality factor for the same nanobeam cavities for different NR before (blue) and after (red) atomic layer deposition (ALD) of a 6 nm coating of Al2O3. (d) Calculated Q | | and Q from Eq. (1) values before (blue) and after (red) the atomic layer deposition treatment. (e) Average percentage change in the quality factor (Q) across 20 samples for NR values ranging from 5 to 8. Purple, green, and gray color schemes correspond to the respective over-coupled, critically coupled, and under-coupled regions.

Close modal

We also investigate the cavity quality factor as a function of NR, as shown in Fig. 4(c). The measured quality factor is defined as Q = λ Δ λ, where the resonance wavelength λ and linewidth Δ λ are determined from the Lorentzian fit to the reflection spectra in Figs. 4(a) and 4(b). The blue curve in Fig. 4(c) illustrates the experimentally calculated quality factor before the atomic layer deposition process, while the red curve shows the quality factor after the deposition of 6 nm of Al2O3. From this curve, we observe an enhancement in the quality factor for all NR values. This enhancement is also more prominent in the under-coupled regions (NR  7) than in the over-coupled (NR = 5) and critically coupled regions (NR = 6). For example, the most significant improvement in the quality factor after the atomic layer deposition treatment is observed for NR = 8, with a 155% increase.

To gain a better understanding of why the quality factor exhibits greater improvement with higher NR (the under-coupled regime), we perform numerical modeling. The quality factor of the cavity is given by the following equation:36,37
1 Q = 1 Q | | + 1 Q ,
(1)
where Q | | is the quality factor for the decay rate into the nanobeam, and Q is the quality factor for the out-of-nanobeam decay rate. We assume that reducing the number of holes in the output-coupling mirror decreases Q | | but does not significantly affect Q , which is dominated by scattering and material losses.37,38 Therefore, we first compute Q utilizing reflection (R) at the cavity resonance before and after atomic layer deposition for NR = 8, which can be expressed as R = ( Q / Q ) 2 using a coupled-mode theory38 (see the supplementary material, Note 3.1). Then, we determine Q | | using Eq. (1). The graphical representation in Fig. 4(d) illustrates the calculated values of Q | | and Q with respect to NR. As anticipated, the plot reveals an upward trend in Q | | as NR increases. Q | | is high in regions characterized by under-coupling, indicating that the overall Q factor is approximately ( Q Q ).37–39 Consequently, after atomic layer deposition, the total quality factor (Q) is higher, particularly in the under-coupled regions.

We perform a similar analysis for 20 samples for each value of NR before and after 6 nm atomic layer deposition. Figure 4(e) plots the average change in total quality factor after atomic layer deposition as a function of the output coupling mirror (NR). The different colors purple, green, and gray correspond to coupling regions that are over-coupled, critically coupled, and under-coupled, respectively. As expected, the lowest average improvement in the quality factor is 40% for NR = 5, while the most significant enhancement is observed for NR = 8, with an average increase in 140%.

In conclusion, we have demonstrated atomic layer deposition of Al2O3 as a valuable technique for significantly enhancing the quality factors of InP nanobeam cavities and tuning the wavelength. We also investigated the impact of coupling regions on quality factor improvements, revealing a 140% average increase in the under-coupling region. Furthermore, we demonstrated that atomic layer deposition can tune the wavelength of the cavity by over 10.5 nm without any degradation to the quality factor. Despite the potential of atomic layer deposition-based passivation to decrease surface roughness and minimize scattering losses at the Al2O3–air interface, a considerable loss may persist after Al2O3 deposition. A deeper exploration of the quality factor improvement through atomic layer deposition, considering various designs and material systems, will provide valuable insight. Additionally, surface treatment of nanobeam cavities using sulfur passivation15 before atomic layer deposition could further improve the quality factor. Our findings hold promise to improve the light–matter interaction of InP nanobeam cavities with individual emitters and tune the cavity wavelength to match that of the emitters. Atomic layer deposition passivated cavities could also enable low-threshold laser by reducing the nonradiative surface recombination rate.40 

See the supplementary material for figures, simulation methods, and theory.

This work was supported by the National Science Foundation (Grant Nos. OMA1936314 and OMA2120757), AFOSR (Grant No. FA23862014072), the U.S. Department of Defense (Contract No. H98230-19-D-003/008), the Office of Naval Research (No. N000142012551), and the Maryland-ARL Quantum Partnership (No. W911NF1920181). This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE's National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. DOE or the United States Government.

The authors have no conflicts to disclose.

Mohammad Habibur Rahaman: Conceptualization (lead); Data curation (lead); Formal analysis (lead); Investigation (lead); Resources (equal); Writing – original draft (lead); Writing – review & editing (equal). Chang-Min Lee: Conceptualization (supporting); Data curation (supporting); Resources (supporting); Writing – review & editing (supporting). Mustafa Atabey Buyukkaya: Conceptualization (supporting); Methodology (supporting). Samuel Harper: Resources (supporting). Fariba Islam: Resources (supporting). Sadhvikas Addamane: Resources (supporting), Funding acquisition (supporting). Edo Waks: Conceptualization (supporting); Funding acquisition (lead); Methodology (supporting); Project administration (lead); Supervision (lead); Writing – original draft (supporting); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Supplementary Material