Controlling electron beam induced defects for low-loss photonic integrated circuits

Abstract

Devices, systems, and methods are provided. In a first example embodiment, a device is provided. The device includes a substrate, and an optical waveguide. The substrate includes silicon dioxide. The substrate has a first surface and a second surface. The optical waveguide is disposed on the first surface of the substrate. The substrate includes a first region extending between the optical waveguide and the second surface of the substrate. The first region of the substrate includes a plurality of unpaired electron defects. In addition, a method of attenuating and optical signal with aspects of the device is provided. Additionally, a method of manufacturing the device is provided. A method of modulating an optical property of an aspect of the device is also provided.

Description

CONTROLLING ELECTRON BEAM INDUCED DEFECTS FOR LOW-LOSSPHOTONIC INTEGRATED CIRCUITSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 63 / 544,840, filed October 19, 2023, the contents of which is hereby incorporated by reference in their entirety.BACKGROUND

[0002] Integrated photonics promises scalable, cost-effective solutions to challenges in domains ranging from classical communications to quantum information processing. A critical step towards realizing this promise is the development of low-loss and stable integrated photonic components. The fabrication of these elements often involves exposure to energetic electrons, the effects of which are poorly understood. Exposure to energetic electrons, such as via an electron beam, may lead to the generation of optically active defects in an integrated photonic platform / device, such as a platform / device making use of a lithium niobate on insulator (LNOI) material. Such defects may impact the optical characteristics of such platforms / devices. There is therefore a need for techniques and systems that modulate active defects in an integrated photonic platform / device in order to modify the optical properties of such platforms / devices.BRIEF SUMMARY

[0003] According to some embodiments of the present disclosure, devices, systems, and methods are provided. In a first example embodiment, a device is provided. The device includes a substrate, and an optical waveguide. The substrate includes silicon dioxide. The substrate has afirst surface and a second surface. The optical waveguide is disposed on the first surface of the substrate. The substrate includes a first region extending between the optical waveguide and the second surface of the substrate. The first region of the substrate includes a plurality of unpaired electron defects.

[0004] In a second example embodiment, a method of attenuating and optical signal is provided.A device is provided. The device includes a substrate and an optical waveguide. The substrate includes silicon dioxide. The substrate has a first surface and a second surface. The the optical waveguide is disposed on the first surface of the substrate. The substrate includes a first region extending between the optical waveguide and the second surface of the substrate. The first region of the substrate includes a plurality of unpaired electron defects. The plurality of unpaired electron defects has a characteristic absorption band. An optical signal having a wavelength within the characteristic absorption band is directed through the optical waveguide, thereby attenuating the optical signal.

[0005] In a third example embodiment, a method of manufacturing a device is provided. A substrate is provided. The substrate includes silicon dioxide. The substrate has a first surface and a second surface. The substrate includes a first region extending between the first surface and the second surface. An optical waveguide is provided. The optical waveguide is disposed on the first surface of the substrate abutting the first region. The first region of the substrate is exposed to a dose of electron-beam radiation to produce a plurality of unpaired-electron defects in the first region.

[0006] In a fourth example embodiment, a method of modulating an optical property of a device is provided. A device is provided. The device includes a substrate that includes includes silicon dioxide. The substrate includes a first region. The first region includes a plurality of unpairedelectron defects. The device is annealed at or above a predetermined temperature, thereby reducing the plurality of unpaired-electron defects to a target density.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] A detailed description of various aspects, features, and embodiments of the subject matter described herein is provided with reference to the accompanying drawings, which are briefly described below. The drawings are illustrative and are not necessarily drawn to scale, with some components and features being exaggerated for clarity. The drawings illustrate various aspects and features of the present subject matter and may illustrate one or more embodiment(s) or example(s) of the present subject matter in whole or in part.

[0008] FIGS. 1A and IB show electron induced optical loss in photonic integrated circuits according to various embodiments of the present disclosure.

[0009] FIGS. 2A-2E show optically-active oxide defects generated by low-energy electron irradiation according to various embodiments of the present disclosure.

[0010] FIGS. 3A and 3B show the generation and suppression of electron-beam induced loss in LNOI photonics according to various embodiments of the present disclosure.

[0011] FIGS. 4A and 4B show giant and broadband optical loss generated by electron-beam exposure according to various embodiments of the present disclosure.

[0012] FIGS. 5A-5D show the suppression of low-frequency instability in LNOI photonics generated by e-beam irradiation according to various embodiments of the present disclosure.

[0013] FIG. 6 shows plots depicting 180 dB / cm reversible on-chip propagation loss according to various embodiments of the present disclosure.

[0014] FIG. 7 shows a plot of on-chip propagation loss due to electron beam exposure during electron evaporation metal deposition according to various embodiments of the present disclosure.

[0015] FIG. 8 shows a plot of EO stability in annealed LNOI modulators after 1 month according to various embodiments of the present disclosure.

[0016] FIG. 9 is a flow chart depicting a method of manufacturing a device.DETAILED DESCRIPTION

[0017] Presented herein are techniques and systems for the controlled generation and annihilation of optically active defects in a characteristic integrated photonic platform / device, such as a platform / device making use of a lithium niobate on insulator (LNOI) material. . Defects may be activated during low-energy (1-100 keV) electron irradiation, and can increase propagation loss by as much as 180 dB / cm at visible to near-infrared wavelengths due to thermal oxide defect absorption. This loss can subsequently be fully reversed by low-temperature annealing. Electron irradiation may give rise to up to 3 -orders of magnitude enhancement in DC electrical conductivity, which contributes to low-frequency instability in electro-optic (EO) LNOI devices. These results demonstrate a route towards universal low-loss and frequency stable LNOI photonic circuits and beyond.

[0018] Free electrons are ubiquitous in integrated photonics fabrication. Standard lithography, metrology, and deposition steps involve exposure of materials under process to electron beams. Free electrons are ubiquitous in the fabrication of micro- and nano-scale devices. Groundbreaking demonstrations in fields ranging from quantum information science to condensed matter physics have relied on devices fabricated using electron beam systems.

[0019] In photonic integrated circuits (PICs), electron beam techniques continue to enable novel devices and functionalities. The widespread adoption of electron-assisted fabrication techniques arises from the fact that they form the basis of standard lithography, metrology, and deposition techniques.

[0020] Waveguide layers written with electron beam lithography, device metallization using physical vapor deposition, and scanning electron microscopy all involve exposing materials to free electrons with energies ranging from 1-100 keV, as shown in FIG. 1A. Materials also often encounter secondary electrons during plasma etching of waveguides and deposition of cladding layers, albeit typically at lower energies (0.01-1 keV). Despite the prevalence of energetic free space electrons, little is known about how they influence the performance of PICs.

[0021] Irradiation effects on an exemplary PIC platform, lithium niobate on insulator (LNOI), are explored herein. LNOI has emerged as an exciting platform for integrated photonics due to its combination of ultra-low optical loss at telecommunication wavelengths, sizeable electrooptic (EO) coefficient and %(2)nonlinearity, and wide transparency window. These properties make it desirable for applications including classical and quantum communication, computing, sensing, and more. While significant progress has been made in demonstrating advanced functionality in LNOI circuits for these fields, challenges for the platform still remain. One notable barrier is that of large on-chip losses observed in the active regions of LNOI modulators operating at visible-to-near-infrared (VNIR) wavelengths, which are over an order of magnitude higher than the state-of-the-art at telecommunications wavelengths. The exact cause of this is poorly understood. While it is often attributed to metal absorption or the increased Rayleigh scattering from sidewall roughness, even waveguides with reduced overlap between the fundamental optical mode and waveguide sidewalls report significant modulator loss. Beyondoptical loss, a major limitation of LNOI observed at all wavelengths is that of low-frequency electro-optic instability. Its presence in both bulk and LNOI devices has been traditionally ascribed to charge carrier migration leading to the screening of sufficiently slow varying applied electric fields. Low-frequency switching and DC phase biasing in amplitude and IQ modulators all suffer from this effect, thus requiring many devices to utilize power-inefficient thermal tuners for biasing. Unlocking the full potential of LNOI devices therefore requires a pathway towards suppressing EO instability.

[0022] The effects of electron-beam irradiation on LNOI photonic devices are presented herein. Such electron-beam irradiation may lead to the generation of optically active defects in the thermally grown oxide isolation layer. These defects may affect other leading semiconductor on insulator platforms, including silicon nitride (SiN) and silicon carbide (SiC) on insulator. There is evidence that defects may be generated in the LN layer itself. Together, the observed defects may dominate optical losses at VNIR wavelengths, such as what is shown in FIG. IB. The observed defects may also exacerbate low-frequency instability in LNOI devices. These challenges may be addressed using low-temperature annealing to deterministically eliminate defects, enabling both low-loss VNIR LNOI photonic circuits and a significant enhancement in low-frequency EO performance.

[0023] Referring now to FIG. 1, FIG. 1A shows standard micro- and nano-fabrication processes, such as electron-beam lithography (top), electron- beam evaporation (left), and scanning electron beam microscopy (right) that may result in exposure of photonic integrated circuits to low- energy (1-100 keV) free electrons. FIG. IB shows insertion losses of lithium niobate on insulator optical waveguides with increasing propagation length. In FIG. IB, the propagation loss (where z = 730 nm) increases from <1 dB / cm (dark grey) to ~20 dB / cm (light grey) afterexposure to 50 keV electrons with a dose of 2000 / zC / cm2. This dose may be typical for high resolution lithography when using resists such as hydrogen silsesquioxane.ResultsElectron-beam induced defects in LNOI

[0024] By studying photoluminescence from both untreated and exposed LNOI samples, the impact of electron irradiation was characterized. In the experiments described herein, the LNOI (LN / SiO2 / Si, 300 nm / 2 um / 500 «m) was irradiated with a 50 keV electron source, and the exposure dose was set taking into account proximity effects (see the Methods section below for details). This electron energy is typical of high-speed electron lithography tools (Elionix HS50) used in the process. However, the following results in lower / higher energy systems were also observed. Representative LNOI photoluminescence spectra before and after exposure are shown in FIG. 2A. Before irradiation, only a single luminescence band was observed centered at ~960 nm (1.3 eV), which was assigned to the silicon layer. After being weakly irradiated (600 zC / cm2), the sample spectrum evolved to display a new, red-detuned luminescence band centered at ~650 nm (1.9 eV). This band appeared attributed to non-bridging oxygen hole centers (NBOHC) generated in the thermally grown amorphous SiO21ayer. To test this claim, the exposed samples were annealed at a low temperature (300 °C) for 12 hours. Previous work had identified that above 150°C, NBOHC defects are annealed out after several hours of heat treatment. The low temperature heat treatment, at a low temperature (300 °C), fully suppresses the 1.9 eV band.This supported the claim. Measurement of the band intensity as a function of irradiation dose revealed that the intensity exhibits a non-linear dependence on dose, suggesting a saturationbehavior also consistent with prior dose-dependent cathodoluminescence studies of NBOHC defects, as shown in the graph in FIG. 2B.

[0025] The NBOHC identification was completed by performing confocal resonant Raman spectroscopy, as shown in FIG. 2C. Under excitation with a 325 nm HeCd laser, a distinct Raman feature at 896 cm1appeared. This feature has previously been identified as a vibronic mode of the NBOHC defect arising from the Si-0 stretching vibration. Comparison of spectra measured on irradiated and non-irradiated regions additionally revealed that this feature only appears in regions exposed to electrons. As a final demonstration of the correlation between electron exposure and red luminescence, an electron beam was rastered on the surface of a thermally grown SiO2on Si sample to map out an institution logo, as shown in FIG. 2D.Confocal photoluminescence mapping was then performed to generate a spatial intensity map of the red luminescence. The institution logo is clearly visible, indicating the highly deterministic nature in which NBOHC defects can be generated under low-energy electron beam exposure.

[0026] NBOHCs may be observed here in both exposed LNOI and amorphous SiO2on Si wafers. This suggests that, in general, photonic platforms relying on SiO2isolation layers (amorphous or crystalline) may be sensitive to these defects. To prove this, the measurements were repeated on 3C silicon carbide-on-quartz and silicon nitride-on-oxide wafers from different sources, and the NBOHC luminescence band, such as what is shown in FIG. 2E, was observed. Because the NBOHC defect may exhibit an absorption band at wavelengths slightly blueshifted from its emission band, it appears that integrated photonic devices utilizing SiO2cladding layers may be susceptible to absorption losses at VNIR wavelengths driven by these defects.Photoluminescence evidence for other oxide defects was also found. These other defectsincluded, for example, oxygen-deficiency centers and / or ODC, after irradiation, and these defects exhibited luminescence / absorption in the blue band (~400 nm, 3 eV).

[0027] Referring now to FIG. 2, FIG. 2A shows photo luminescence spectra of LNOI wafers. FIG. 2A, shows that a red luminescence band centered at 650 nm (~1.9 eV) appears after electron irradiation. This band may be attributed to non-bridging oxygen hole centers (NBOHC) generated in the amorphous SiChlayer. The luminescence may be quenched after low temperature annealing (300 °C). FIG. 2B shows the intensity of the NBOHC luminescence band versus exposure dose. The oscillations in spectra may arise from thin-film interference effects in the LN / SiO2 / Si stack. FIG. 2Cshows the resonant excitation (325 nm) Raman spectroscopy of an electron-exposed LNOI wafer. In FIG. 2C, the exposed (light grey) spectrum displays the 896 cm1NBOHC vibrational mode, which is shaded for clarity. In FIG. 2C, after annealing (dark grey), this mode is suppressed. FIG. 2D shows a normalized confocal photoluminescence map of an SiO2 / Si die patterned using electron-beam lithography. FIG. 2E shows photoluminescence spectra of thermally grown amorphous SiO2, LN, SiC, and SiN on insulator (T. Oxide, LNOI, SiCOI, SiNOI) after electron irradiation with a dose of 3 x 104 CI cm2. In sub-figures a, b, and e of FIG. 2E„ the fringes observed in spectra arise from Fabry -Perot interference stemming from the material stacks.Impact of electron-beam irradiation on PICs

[0028] Fabrication steps for PICs may involve electron exposure during mask lithography, deposition of metal for thermal and / or EO functionality (e.g. electron bean evaporation), and characterization of devices under scanning electron microscopes. Understanding how electronirradiation affects the final device performance may therefore be essential. The way in which optical propagation loss in LNOI circuits is influenced by electron exposure is described herein.

[0029] The propagation losses of devices after each of the primary steps in a fabrication cycle were measured. FIG. 3A illustrates these measurement stages. The experiment shown in FIG. 3 began with the propagation loss of an uncladded LN waveguide immediately after fabrication, and without any heat treatment (see Methods section below for further details on the waveguide fabrication steps). Afterwards, the sample was annealed (520 °C, 2 hrs) under an oxygen environment. This post-fabrication heat treatment has been shown to reduce the material loss, previously attributed to damage induced during the ion-slicing process. The oxygen environment was hypothesized to help prevent lithium out-diffusion. These steps were followed with a low- dose electron beam exposure and a low-temperature ambient anneal (300 °C, 12 hrs). The electron beam dose was chosen to mimic the typical dose required for commonly used electronbeam resists. Similarly, the second, lower anneal temperature was chosen for its compatibility with metallized devices found in active LNOI photonic circuits. Propagation losses were extracted by measuring the insertion loss of waveguides with increasing length, and fitting to extract the optical loss per unit length. All devices were measured at an input wavelength of = 730 nm. The results of this experiment are provided in FIG. 3B. After fabrication, the extracted propagation loss was nearly 20 dB / cm, similar to large loss values reported in recent works covering LNOI devices fabricated using electron beam lithography and operating at comparable wavelengths. The measured loss was attributed predominantly to defect absorption generated by the electron exposure (~2000 / iCI cm2) encountered during the crosslinking of the hydrogen silsesquioxane (HSQ) mask. This device was then annealed (520 °C) and propagation losses approaching 0.1 dB / cm were measured. This value was better than the best reported propagationlosses at this wavelength in thin-film lithium niobate. Following the second, lower dose (600 / iCI cm2) electron irradiation step, the propagation loss increased again to 10 dB / cm. This verified that electron beam exposure itself was inducing losses in the photonic integrated circuit. The low-loss performance was recovered after a 300 °C annealing step, proving the repeatable nature of the electron-induced loss. The behavior shown in FIG. 3B is observed in measurements made on over 50 devices fabricated on distinct wafers, each obtained over the course of approximately one year.

[0030] Referring now to FIG. 3, FIG. 3A shows a diagram of the processing steps at which propagation losses of LNOI waveguides are measured ( = 730 nm). In FIG. 3A, waveguides are patterned and etched using electron beam lithography (2000 / iCI cm2) and reactive ion etching (Ar+) (top left portion), annealed at 520 °C (top right portion), re-exposed to electrons at a low- dose (600 / iCI cm2) (bottom left portion), and re-annealed at 300 °C (bottom right portion). FIG. 3B shows measured insertion loss as a function of device length, along with the extracted propagation loss (dB / cm), after each of the processing steps depicted in FIG. 3A. FIG. 3B shows that propagation loss is high after initial fabrication (~20 dB / cm) and when directly irradiated by an electron beam (~10 dB / cm), but losses return to low values (< 1 dB / cm) after both high (520 °C, dark grey line showing < 1 dB / cm) and low (300 °C, light grey line showing < 1 dB / cm) temperature annealing.

[0031] The dependence of optical loss on electron dose was examined by an experiment in which 18 low-loss devices were fabricated and subsequently irradiated each with increasing dosage of an electron beam. FIG. 4A presents the measured propagation loss of each device before and after irradiation. In the dosage range of ~103to 105 / iC! cm2, irradiated devices displayed a non-linear dependence on dose. While the dose was increased by only two orders ofmagnitude, the propagation loss spans 16 orders of magnitude. A maximum loss of nearly -180 dB / cm was measured at a corresponding dosage of ~7 104 / iCI cm2. At higher doses (> 105), the optical loss inflected before saturating. All losses can be fully reversed upon low-temperature annealing, as can be seen in FIG. 6. The optical loss over a broadband wavelength window was also probed by fabricating dosed (500 / iCI cm2) devices spanning / . = 600-1680 nm. The maximum loss was observed in the 600-700 nm range, in agreement with previous measurements on the NBOHC absorption band. However, elevated optical loss outside of this region, extending from 700 nm to 1680 nm, was also found. This broadband optical loss was hypothesized to arise from oxygen vacancy centers (F and F+ centers) generated in LN during electron exposure. In prior work on F / F+ centers in LN, a weak intensity and optically broadband absorption spectrum was observed in congruent LN crystals after low energy (< 300 keV) electron irradiation. These oxygen based defects in LN may also be suppressed by low-temperature annealing under temperatures (70-350 °C), as used in various embodiments presented herein.

[0032] Large losses may be observable without directly rastering an electron beam on a waveguide. For example, during metallization of electrodes for active LNOI devices, electron beam evaporation is often used. This process may generate secondary electrons which impinge onto the sample, inducing defects, and may potentially generate the large losses observed in previous visible LNOI works on active photonics (see FIG. 7 for data on electron evaporation deposition induced loss).

[0033] Referring now to FIG. 4, FIG. 4 A shows insertion loss (z = 730 nm) as a function of electron-beam dose (dark grey data points). In FIG. 4A, the light grey data points correspond to the insertion loss of each device before electron exposure. As shown, it appears that the propagation loss achieves a maximum of approximately 180 dB / cm at ~ 7 x 104 fiC / cm2, anddisplays saturation at higher doses. FIG. 4A shows propagation loss after electron-beam exposure (~ 500 CI cm2) as a function of wavelength ( = 0.6 - 1.68 / / m). Although the effect of e-beam exposure on optical loss may be most pronounced at shorter wavelengths due to absorption from NBOHC defects in the SiChlayer, increased losses may be observed even at near-infrared and telecommunication wavelengths.Impact of electron irradiation on instability in LNOI active photonics

[0034] The material LNOI is valued due to the extensive EO functionality it supports. Common devices using this material include phase and intensity modulators, both exploiting the material’s strong Pockels effect. It has long been known that such devices exhibit bias drift during their operation. During application of a voltage to LN, free charge carriers can mobilize and screen the applied electric field, thereby attenuating the material’s EO response. This process may commonly be referred to as EO relaxation. This behavior has historically been attributed to the crystal’s inhomogeneous charge composition. However, recent studies suggest that modifications to the crystal surface environment may drive the strong EO relaxation observed in LNOI. As described herein, the conductive states generated in the cladding during electron irradiation may be responsible for the EO relaxation observed in LNOI.

[0035] The relationship between EO relaxation and electron-beam induced defects was investigated via experiments by first fabricating uncladded LNOI Mach-Zehnder (MZM) modulators using a standard process flow outlined in the Methods section below. These devices encountered low-energy electron irradiation during lithographic patterning of the electrodes (~50 keV), as well as during electronbeam evaporation of the metal contacts (< 10 keV). This is typical for the fabrication and / or post-fabrication of such devices. The MZM half-wave voltage (Er) was measured over a broadfrequency range, spanning 102to 105Hz, which extended well beyond the window over which EO instability is typically observed.

[0036] The experimental setup is shown in the inset of FIG. 5A. As shown in FIG. 5A, prior to annealing, the MZM displayed a flat response down to 101Hz. At driving frequencies lower than this, a marked increase in JA occurred. This response indicated the onset of screening via charge mobilization at time scales on the order of I O1to 10° s. This suppression of EO response at relatively high frequencies may make pure electro-optic biasing more challenging, requiring fast feedback operating beyond Hz speeds. Furthermore, if the screening is driven by charge relaxation, feedback may become unfeasible over longer timescales. After low-temperature annealing of the modulator, however, the response became flat over the entirety of the measured frequency range, thus indicating the suppression of charge-carrier effects. This significantly improved performance, which was stable over at least a month timescale in an ambient environment (see FIG. 8).

[0037] To understand this response, finite element simulations (COM- SOL) were performed and the frequency dependent EO response of the device was simulated (see Methods section, below, for further details). By incorporating a finite conductivity in the buried oxide layer (CTBOX ~ 4 / 107S / m) and LN layer (<TLN ~ 3 / 109S / m), the response of electron irradiated devices may be qualitatively reproduced, as shown in FIG. 5A. The impact of BOX conductivity can be understood by viewing the simulated voltage distribution at different frequencies, as shown in FIG. 5B. At high frequencies above the dielectric relaxation timescale of the buried oxide layer, the applied electric field may be uniformly distributed throughout the LN / SiO2 stack. At low frequencies (< 1 Hz), an image charge effect created by screening in the buried oxide decreasesthe electric field around the waveguide, thereby suppressing the EO response and driving the observed increase in Vn.

[0038] To support these findings, dark conductivity measurements were performed on additional LNOI phase modulator devices irradiated with electron beams. Dark DC conductivity (cdark) was measured before and after electron exposure, as shown in FIG. 5C. After irradiation, c dark was measured to be nearly 3 orders of magnitude larger (dose = 5 mC / cm2, cdark ~ 9 x 104S / m) than than before exposure. This dose is roughly an order of magnitude larger than that used in fabrication of the MZM, as measured in FIG. 5A.

[0039] To highlight whether the LN or SiCh cladding contributes to this conductivity enhancement, dark conductivity on uncladded LNOI phase modulators was measured before and after removing the underlying oxide. The removal of the underlying irradiated SiO2 caused c dark to return back to pre-exposure values, suggesting that charge migration may occur in the SiO2 or at the interface, as can be seen in the FIG. 5B. These results are interpreted in light of the findings on NBOHC defect formation in the SiO2 isolation layer. One hypothesis may be that electron irradiation generates conductive states in SiO2, which substantially contributes to charge relaxation during EO biasing of LNOI modulators. It is emphasized, however, that conductivity in the LN bulk or surface generated by electron exposure may also give rise to this effect.Finally, the observations suggest that this effect has wavelength broadband impact, and that LNOI devices operating at visible, near-infrared, or telecommunication bands may all be susceptible to reduced EO performance due to electron exposure.

[0040] To fully contextualize the significance of this change in stability after annealing, feedback electronics were implemented on annealed MZMs for active stabilization, as shown in FIG. 5D. It was determined that MZMs may easily be stabilized and locked to an arbitrary point(null) without encountering the fast intensity transients that would typically be driven by shorttimescale charge mobilization.

[0041] Referring now to FIG. 5, FIG. 5A shows the measured half-wave voltage of a Mach- Zehnder modulator as a function of frequency before and after annealing. As shown in FIG. 5A, before annealing, a significant roll-off of the electro-optic response is observed at Hz and sub-Hz frequencies, as commonly observed in LNOI devices. This roll-off window may be referred to as the dielectric relaxation window. Measurements are consistent with a finite element simulation of the gate region using electrical conductivity material parameters of 4 x 10 7 S / m for the buried oxide and 3 x 10 9 S / m for the LN layer. After annealing, a flat response is observed down to 102Hz. FIG. 5B (Left) shows the voltage distribution within the gate region predicted by the simulation used in the subplot shown in FIG. 5A. The distributions are shown at frequencies of 10^ Hz (top) and 10 1 Hz (bottom). FIG. 5C shows dark current- voltage curves across a LNOI device (electrode gap, g = 5 pm) for various exposure doses (0, 5, 50 mC / cm2). A significant increase in conductivity is observed when dosed. FIG. 5D (Top) shows a Mach-Zhender modulator operated with feedback electronics. FIG. 5D (Bottom) shows that the Mach-Zhender modulator device is biased to the null point (grey curve, left axis) and the feedback electronics stabilize the device without encountering fast EO instabilities (black curve, right axis).

[0042] Referring now to FIG. 6, FIG. 6 shows plots depicting 180 dB / cm reversible on-chip propagation loss. In particular, FIG. 6 shows on-chip insertion loss (A. = 730 nm) before (left plot) and after electron irradiation with increasing dose (middle plot). As described herein, the loss, which reached values as large as 180 dB / cm, was fully reversible after low-temperature annealing (right plot). The loss was extracted using the cutbackmethod. The anneal temperature was 300 °C. All three plots share the same y- and x-axis values.

[0043] Referring now to FIG. 7, FIG. 7 shows a plot of on-chip propagation loss due to electron beam exposure during electron evaporation metal deposition. In particular, FIG. 7 shows on-chip transmission loss (X = 730 nm) at various steps during sample preparation. The LNOI waveguide was initially fabricated (etched, cladded, annealed) and then the lithography for electrodes was performed (SPR-220, 405 nm UV exposure, MIBK developer). The propagation loss was measured at this point and found to be ~ 0.6 dB / cm (light grey). The device waws subsequently metallized using electron-beam evaporation (10 / 400 nm, Ti / Au). The propagation loss measured after metallization and liftoff was ~ 2.8 dB / cm. After annealing, the loss increased back to ~ 0.7 dB / cm. Data is plotted relative to the fit y-intercept.

[0044] With continued reference to FIG. 7, Electron exposure during electron evaporation of common metals is demonstrated by measuring device propagation loss directly before and after metallization. A phase modulator device was fabricated using the fabrication steps outlined in the Methods section herein (see “Device Fabrication” section). The LNOI device was etched, cladded, and annealed (520 °C, 2 hr, oxygen environment). Afterwards, the electrode design was patterned on the device using photolithography (405 nm laser, SPR-220 resist, MIBK developer). Once the pattern was developed, the on-chip propagation loss was measured using the cutback technique. The loss obtained was ~ 0.6 dB / cm, as shown in FIG. 7. The device then underwent metallization using electron beam evaporation (10 / 400 nm, Ti / Au), and the electrodes were completed using liftoff (NMP solvent, 80 °C). The measured propagation loss after this step was ~ 2.8 dB / cm. This loss was not caused by metal absorption, as the electrode gap was 5 pm, and the corresponding simulated absorption loss was less than 106dB / cm at this wavelength. Thisdetermination was further confirmed by performing a low temperature annealing step (300 °C, 12 hrs) and re-measuring the propagation loss afterwards. The loss returned to the pre-electron exposure value within the error of the grating coupler measurement setup (±0.2 dB / cm due to variation in fiber-to-chip coupling efficiency from grating to grating).

[0045] Referring now to FIG. 8, FIG. 8 shows a plot of EO stability in annealed LNOI modulators after 1 month. In particular, FIG. 8 shows a plot of the half-wave voltage of an annealed Mach-Zehnder Modulator as a function of frequency 1 day after annealing and 1 month after annealing. The electro-optic response remained flat after 1 month of the sample sitting in an ambient environment.Discussion

[0046] The results of the experiments described herein indicate that defects generated during electron exposure can significantly impact the performance of integrated photonic devices. The findings furthermore draw attention to the utility of PIC devices in radiation abundant environments (e.g., in space systems). The observation of these defects in various photonic platforms on both buried oxide and quartz wafers after irradiation indicates the potential for this to impact SiN, SiC, and other photonic platforms utilizing various silica isolation layers. The strong absorption and ability to locally and deterministically write defects suggests that these may find utility in on-chip filters and attenuators. Additionally, what is described herein focuses on defects generated through electron irradiation, studies indicate that high power UV radiation (355 nm) and beyond can also drive defect formation (NBOHC, ODC, and more) in fused silica. Comparable wave-lengths may be used in photolithography systems (DUV), indicating the potential for these processing steps to also generate NBOHC defects during PIC circuitfabrication. Finally, as described herein, the performance reduction of LNOI devices fabricated using electron beam lithography can be reversed by low-temperature annealing. This may provide a clear pathway towards visible to near-infrared devices with low optical loss and enhanced EO stability.MethodsDevice fabrication

[0047] All LNOI measurements were performed on thin film wafers from NanoLN (300 nm LN / 2 pm SiO2 / 500 pm Si). Integrated photonic devices were fabricated by first patterning waveguide layers using electron-beam lithography (HSQ, Elionix ELS-F125), and etching ~180 nm using reactive ion etching (Ar+). This was then followed by re-deposition cleaning (RCA SC-1). The photonic devices presented herein were uncladded unless otherwise noted. In this way, the surface and / or interface effects potentially induced by chemical vapor deposition of cladding materials was eliminated. During post-fabrication electron beam exposure, the LN and underlying SiChfilm were directly exposed to 50 keV electrons by rastering the beam over the bare waveguides. The exposure dose (pC / cm2) was set while taking into account proximity effects using commercial software (TRACER).

[0048] During electron exposure, nm-thick layers of amorphous carbon may be deposited on the sample due to the interaction of surface hydrocarbon contaminants with the primary electron beam. Carbon contamination was ruled out as the cause of photoluminescence / Raman features and optical loss by performing piranha cleaning (H2O2:H2SO4, 1 :3 ratio) of samples before measurements.Measurement of frequency-dependent Vn

[0049] A variable frequency function generator produced a triangular waveform that is directed to the MZM electrodes using a 50 Q ground-signal-ground probe (40A-GSG-100-F). A CW laser ( / . = 730) was grating coupled to the MZM and out-coupled to an avalanche photodetector (APD410A). The detector output was monitored on an electronic oscilloscope, from which the Vn is extracted.Measurement of electro-optic stabilization

[0050] To perform electro-optic stabilization of MZM modulators, an error signal was extracted by applying a continuous 2 KHz weak sinusoidal dither to the MZM and then demodulating using a lock-in amplifier. This error signal was then fed-back using standard PID control software to fix the modulator’s bias point at the minimum transmission point.Finite element simulation of electric field screening

[0051] A 2D finite element simulation in COMSOL was used to study the effect of conductivity in the buried oxide on the EO response of the devices described herein. The LN and buried oxide layers were treated as simple materials defined by a frequency-independent permittivity (LN = (29, 44, 44) for Z, X and Y crystal axes and £BOX = 4.2) and conductivity (OLN ~ 3 x 109S / m and OBOX ~ 4 x 107S / m). For simplicity, negligible boundary resistance between materials was also assumed. Because the electric field was nearly uniform within the waveguide region for the device geometry, the electric field strength along the Z crystal axis in the center of the LN waveguide was used as a proxy for V -1. The full device geometry including the correct widths of electrodes was modeled in order to produce a valid prediction for the material conductivities.

[0052] Referring now to FIG. 9, method 900 of manufacturing a device, at step 902, a substrate is provided. ’The substrate includes silicon dioxide. The substrate has a first surface and asecond surface. The substrate includes a first region extending between the first surface and the second surface. At step 904, an optical waveguide is provided. The optical waveguide is disposed on the first surface of the substrate abutting the first region. At step 906, the first region of the substrate is exposed to a dose of electron-beam radiation to produce a plurality of unpaired-electron defects in the first region.

[0053] Although the components of various embodiments described herein may be described as being implemented using lithium niobate, it is to be understood that any electro-optic crystal may be used to implement such components. Examples of such electro-optic crystals may include lithium niobate, lithium tantalite, potassium titanyl, phosphate, P-barium borate, barium titanate and / or the like.

[0054] The present disclosure may be embodied as a device, a system, a method, and / or a computer program product. The computer program product may include a computer readable storage medium (or media) having computer readable program instructions thereon for causing a processor to carry out aspects of the present disclosure.

[0055] The computer readable storage medium can be a tangible device that can retain and store instructions for use by an instruction execution device. The computer readable storage medium may be, for example, but is not limited to, an electronic storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. A non-exhaustive list of more specific examples of the computer readable storage medium includes the following: a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), a static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such as punch-cards or raised structures in a groove having instructions recorded thereon, and any suitable combination of the foregoing. A computer readable storage medium, as used herein, is not to be construed as being transitory signals per se, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through a waveguide or other transmission media (e.g., light pulses passing through a fiber-optic cable), or electrical signals transmitted through a wire.

[0056] Computer readable program instructions described herein can be downloaded to respective computing / processing devices from a computer readable storage medium or to an external computer or external storage device via a network, for example, the Internet, a local area network, a wide area network and / or a wireless network. The network may comprise copper transmission cables, optical transmission fibers, wireless transmission, routers, firewalls, switches, gateway computers and / or edge servers. A network adapter card or network interface in each computing / processing device receives computer readable program instructions from the network and forwards the computer readable program instructions for storage in a computer readable storage medium within the respective computing / processing device.

[0057] Computer readable program instructions for carrying out operations of the present disclosure may be assembler instructions, instruction-set-architecture (ISA) instructions, machine instructions, machine dependent instructions, microcode, firmware instructions, state-setting data, or either source code or object code written in any combination of one or more programming languages, including an object oriented programming language such as Smalltalk, C++ or the like, and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The computer readable program instructions may execute entirely on the user’s computer, partly on the user’s computer, as astand-alone software package, partly on the user’s computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user’s computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). In some embodiments, electronic circuitry including, for example, programmable logic circuitry, field-programmable gate arrays (FPGA), or programmable logic arrays (PLA) may execute the computer readable program instructions by utilizing state information of the computer readable program instructions to personalize the electronic circuitry, in order to perform aspects of the present disclosure.

[0058] Aspects of the present disclosure are described herein with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the disclosure. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer readable program instructions.

[0059] These computer readable program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions / acts specified in the flowchart and / or block diagram block or blocks. These computer readable program instructions may also be stored in a computer readable storage medium that can direct a computer, a programmable data processing apparatus, and / or other devices to function in a particular manner, such that the computer readable storage medium havinginstructions stored therein comprises an article of manufacture including instructions which implement aspects of the function / act specified in the flowchart and / or block diagram block or blocks.

[0060] The computer readable program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other device to cause a series of operational steps to be performed on the computer, other programmable apparatus or other device to produce a computer implemented process, such that the instructions which execute on the computer, other programmable apparatus, or other device implement the functions / acts specified in the flowchart and / or block diagram block or blocks.

[0061] The flowchart and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present disclosure. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of instructions, which comprises one or more executable instructions for implementing the specified logical function(s). In some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and / or flowchart illustration, and combinations of blocks in the block diagrams and / or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts or carry out combinations of special purpose hardware and computer instructions.

[0062] The descriptions of the various embodiments of the present disclosure have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.

[0063] While the disclosed subject matter is described herein in terms of certain preferred embodiments, those skilled in the art will recognize that various modifications and improvements may be made to the disclosed subject matter without departing from the scope thereof. Moreover, although individual features of one embodiment of the disclosed subject matter may be discussed herein or shown in the drawings of the one embodiment and not in other embodiments, it should be apparent that individual features of one embodiment may be combined with one or more features of another embodiment or features from a plurality of embodiments.

[0064] In addition to the specific embodiments claimed below, the disclosed subject matter is also directed to other embodiments having any other possible combination of the dependent features claimed below and those disclosed above. As such, the particular features presented in the dependent claims and disclosed above can be combined with each other in other manners within the scope of the disclosed subject matter such that the disclosed subject matter should be recognized as also specifically directed to other embodiments having any other possible combinations. Thus, the foregoing description of specific embodiments of the disclosed subjectmatter has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosed subject matter to those embodiments disclosed.

[0065] It will be apparent to those skilled in the art that various modifications and variations can be made in the method and system of the disclosed subject matter without departing from the spirit or scope of the disclosed subject matter. Thus, it is intended that the disclosed subject matter include modifications and variations that are within the scope of the appended claims and their equivalents.

Claims

CLAIMSWhat is claimed is:

1. A device, comprising: a substrate comprising silicon dioxide, the substrate having a first surface and a second surface; and an optical waveguide, the optical waveguide being disposed on the first surface of the substrate, wherein the substrate comprises a first region extending between the optical waveguide and the second surface of the substrate, the first region of the substrate comprises a plurality of unpaired electron defects.

2. The device of Claim 1, wherein the plurality of unpaired electron defects comprises nonbridging oxygen hole centers.

3. The device of Claim 1, wherein the first region of the substrate further comprises a plurality of oxygen-deficiency centers.

4. The device of Claim 1, wherein the substrate comprises quartz.

5. The device of Claim 1, wherein the optical waveguide has a higher index of refraction than the substrate.

6. The device of Claim 1, wherein the optical waveguide comprises a semiconductor.

7. The device of Claim 6, wherein the semiconductor comprises lithium niobate, silicon carbide, silicon dioxide, or silicon nitride.

8. The device of Claim 1, further comprising: a carrier disposed on the second surface of the substrate.

9. The device of Claim 8, wherein the carrier comprises lithium niobate, silicon, quartz, silicon dioxide, or sapphire.

10. The device of Claim 1, wherein the plurality of unpaired electron defects has a characteristic absorption band.

11. The device of Claim 10, wherein the characteristic absorption band is centered between 600nm and 700nm.

12. The device of Claim 11, wherein the absorption band is centered at 650nm.

13. The device of Claim 1, wherein the optical waveguide is unclad.

14. The device of Claim 1 wherein the first surface is substantially parallel to the second surface.

15. The device of Claim 1, wherein the optical waveguide is a linear waveguide, a waveguide with a bend, or a tapered waveguide.

16. The device of Claim 1, wherein the optical waveguide has a ratio of photoconductivity to dark DC conductivity less than IO'217. The device of Claim 1, wherein the optical waveguide has a dark DC conductivity greater than 104S / m.

18. A method of attenuating an optical signal, the method comprising: providing a device, comprising: a substrate comprising silicon dioxide, the substrate having a first surface and a second surface; and an optical waveguide, the optical waveguide being disposed on the first surface of the substrate, whereinthe substrate comprises a first region extending between the optical waveguide and the second surface of the substrate, the first region of the substrate comprises a plurality of unpaired electron defects, the plurality of unpaired electron defects has a characteristic absorption band; and directing an optical signal having a wavelength within the characteristic absorption band through the optical waveguide, thereby attenuating the optical signal.

19. A method of manufacturing a device, the method comprising: providing a substrate comprising silicon dioxide, the substrate having a first surface and a second surface, the substrate comprising a first region extending between the first surface and the second surface; providing an optical waveguide disposed on the first surface of the substrate abutting the first region; and exposing the first region of the substrate to a dose of electron-beam radiation to produce a plurality of unpaired-electron defects in the first region.

20. The method of Claim 19, wherein the dose is at least 104micro Coulomb per square centimeter.

21. The method of claim 19, wherein the dose is from about 103to about 106micro Coulomb per square centimeter.

22. The method of Claim 19, wherein the dose is about 7 104micro Coulomb per square centimeter.

23. The method of Claim 19, further comprising: annealing the substrate, thereby reducing the plurality of unpaired-electron defects to a target density.

24. A method of modulating an optical property of a device, the optical property being determined by a plurality of unpaired-electron defects, the method comprising: providing a device comprising a substrate comprising silicon dioxide, the substrate comprising a first region, the first region comprising a plurality of unpaired electron defects; and annealing the device at or above a predetermined temperature, thereby reducing the plurality of unpaired-electron defects to a target density.

25. The method of Claim 24, wherein the predetermined temperature is from about 70°C to about 520°C.

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