Radial Bragg ring resonator structure with high quality factor

[0001] The present invention relates generally to optical resonator devices and, more particularly, to a radial Bragg ring resonator structure with a high quality factor, Q.

[0002] Optical resonators comprise the central components of light source devices such as, for example, high-efficiency light emitting diodes, lasers, switches, and filters. In the field of computing, multi-core microprocessor architectures have been developed in order to mitigate increased power dissipation in high-performance computer chips. However, the bandwidth limitations for global electrical interconnections between various cores are rapidly becoming the major factor in restricting further scaling of total chip performance. One approach to resolving this interconnect bottleneck is to transmit and route signals in the optical domain, since optical signals can provide both immense aggregate bandwidth and large savings in on-chip dissipated power. As such, optical resonators are desirably integrated with integrated circuit (IC) device substrates. In fact, the field of integrated optics has expanded tremendously in recent years, and integrated optical device solutions are now being proposed for applications in a variety of fields including, for example, telecommunications, data communications, high performance computing, biological and chemical sensing, and radio frequency (RF) networks.

[0003] Specific solutions for optical resonators that are may be integrated on planar substrates (e.g., silicon) include structures such as, for example, linear resonators with distributed Bragg reflector mirrors, individual ring or disc resonators, photonic crystals and radial Bragg ring resonators. In particular, radial Bragg ring resonators, which are also known as circular grating resonators (CGRs) or “fingerprint” structures, have more recently been considered for applications in integrated optics such as lasing and all-optical switching. Radial Bragg ring resonators have a very small footprint of a few micrometers, which essentially corresponds to the smallest optical resonators possible. Thus, even at relatively low refractive index contrasts, radial Bragg ring resonators offer full two-dimensional light confinement, making them a very attractive candidate for future integrated photonic devices since they may be fabricated of any transparent (low absorption) material.

[0004] Two of the primary parameters of interest for optical resonators having a resonance at a certain wavelength are the quality factor, Q, and the effective mode volume, V_eff. With respect to both linear resonators having distributed Bragg reflector mirrors and disc resonators, a primary disadvantage thereof is the large mode volume of those resonators, wherein V_eff is on the order of hundreds of times the operating wavelength of light cubed (λ³). This in turn leads to low optical confinement, prevents dense areal integration and requires relatively high power to drive optically active material inside the optical resonator. Furthermore, integrated devices which harness cavity quantum electrodynamic effects such as the Smith-Purcell effect (e.g., single-photon sources) or the photon blockade regime (e.g., single-photon switches) are not possible as they require a large ratio of Q/V_eff.

[0005] With respect to photonic crystals, the primary drawback of this type of resonator is that it requires a large refractive index contrast between the material of the photonic crystal (such as GaAs or Si) and the surrounding material (such as air) in order to achieve a complete two-dimensional bandgap. This limits their use in terms of wavelengths (infrared), fabrication (suspended membranes) and materials (i.e., semiconductor materials which are not back end of line (BEOL)-compatible in their crystalline form).

[0006] In addition, the aforementioned linear resonators having distributed Bragg reflector mirrors, disc resonators, and photonic crystals all share the disadvantage that resonator modes with a dipole-like mode profile are not known. This, however, is required for optimum coupling to (for example) molecules, nanoparticles and quantum dots, which are potentially located in the resonator serving as absorbing, emitting or non-linear material. Finally, a primary drawback of conventionally designed radial Bragg ring resonator is the low quality factor Q (e.g., <10000) for devices having small mode volumes (e.g., on the order of a few λ³), which is caused by large vertical or out-of-plane losses.

[0007] In an exemplary embodiment, an optical resonator structure includes a substrate; a center disc formed on the substrate; a plurality of concentric grating rings surrounding the center disc, the concentric rings spaced apart from the center disc and from one another by regions of lower index of refraction material with respect thereto; and wherein spacing between the grating rings and the center disc is non-periodic such that a magnitude of a displacement distance of a given grating ring with respect to a λ/4 Bragg reflector geometry is largest for a first of the grating rings immediately adjacent the center disk and decreases in a radially outward direction.

[0008] In another embodiment, a method of forming an optical resonator structure includes forming a center disc formed on a substrate; forming a plurality of concentric grating rings surrounding the center disc, the concentric rings spaced apart from the center disc and from one another by regions of lower index of refraction material with respect thereto; and wherein spacing between the grating rings and the center disc is non-periodic such that a magnitude of a displacement distance of a given grating ring with respect to a λ/4 Bragg reflector geometry is largest for a first of the grating rings immediately adjacent the center disk and decreases in a radially outward direction.

[0016] Disclosed herein is an optical resonator having both a very high quality factor Q (e.g., >10000) and a very small effective mode volume V_eff (on the order of a few λ³). Briefly stated, the optical resonator embodiments include a center disc (e.g., circular or oval) and surrounding concentric, non-periodic gratings (e.g., circular or oval) which serve as radial Bragg reflectors. The spacing between the gratings and their widths are chosen appropriately such that the “mirror-like” effect nearest the central region is reduced. Although this adjustment with respect to ideal λ/4 reflectors results in a slight increase in V_eff, there is also a dramatic increase in Q (e.g., by up to 3 orders of magnitude) due to much smaller vertical losses as compared to conventional radial Bragg ring resonators. In addition, the disclosed resonator embodiments do not require a high-index contrast between the materials, and dipole-like mode profiles may be realized.

[0017] Referring initially to FIG. 1, there is shown a top view of a radial Bragg ring resonator structure 100 configured in accordance with an embodiment of the invention. The structure 100 includes a substrate 102, such as silicon dioxide (SiO₂) or silicon dioxide on silicon (silicon-on-insulator technology), for example. A center disc 104 (e.g., Si, GaAs, InP, ZnO, TiO₂, Ta₂O₅, HfO₂, Al₂O₃, diamond) is formed on the substrate 102, and may be circular or oval in shape for example. The center disc 104 is concentrically surrounded by a plurality of grating rings (i.e., Bragg gratings) 106 that are spaced apart from the center disc 104 and from one another by regions 108 of lower index material, such as air. Alternatively, the rings may be encapsulated in a transparent cladding material such as silicon oxide (SiO₂), or polymethyl (meth)acrylate (PMMA). In any event, the grating rings 106 have a higher index of refraction with respect to the surrounding in-plane material (e.g., air), and the grating rings 106 may, but need not be formed from the same material as the center disc 104 (e.g., Si, GaAs, InP, ZnO, TiO₂, Ta₂O₅, HfO₂, Al₂O₃, diamond). Moreover, the index contrast between the material of the grating rings 106 and the surrounding regions can be small, e.g., using tantalum oxide (Ta₂O₅) (n=2.1) or silicon (Si) (n=3.4) for the grating rings 106 and silicon dioxide (SiO₂) (n=1.46) or air (n=1) for the surrounding.

[0018] With certain existing radial Bragg ring resonator structures, the number, period and the width of the individual grating rings 106 are designed so as to achieve a high reflectivity to minimize the V_eff. The period and/or refractive index of the annular reflectors are mathematically adjusted such that they act as equivalent λ/4 reflectors (or some multiple thereof) and maximize light confinement at the centermost region of the resonator device. However, as indicated above, such resonator structures have a low quality factor Q because of the strong localization of the optical mode near the central region, which involves high vertical (out-of-plane) scattering losses.

[0019] Accordingly, the structure 100 of FIG. 1 overcomes this disadvantage by having the spacing or period between the center disc 104 and between adjacent grating rings 106 deviated from an “unperturbed,” ideal λ/4 geometry. The degree of this deviation is largest for the first ring nearest the center, and exponentially decreases in the radially outward direction. (In other embodiments as described in further detail below, this may be achieved by appropriately changing the period and/or the width of the grating rings.) In so doing, the otherwise strong localization proximate the center of the structure is relaxed, which leads to a decrease in the vertical losses (out-of-plane) of the resonator device. Although a result of this geometry is a slightly larger V_eff (e.g., increased by less than a factor of 2), for most applications this is tremendously outweighed by a substantial increase in the quality factor (e.g., up to three orders of magnitude, depending on the materials used for the grating rings and the surrounding).

[0020] Another aspect of the present embodiments is that due to the cylindrical symmetry, azimuthal orders, m, of zero or higher may be used. Hence, the spatial field profile can be fully circular symmetric (m=0), dipole-like (m=1) or multi-pole-like (m>1). For circular (not oval) configurations, the radius r₀ of the center disc 104 determines the resonance frequency ω_res of the resonator. The radii of “unperturbed” Bragg rings r_unperturbed may be calculated, for example, according to U.S. Pat. No. 7,099,549, the contents of which are incorporated herein in their entirety. As indicated above, the displacement distance, Δ, from each of the corresponding “unperturbed” or λ/4 Bragg rings decreases from the center to the outer region. In an exemplary embodiment, the displacement distance of each Bragg grating ring 106 from the unperturbed location depends on the Bragg ring number “p,” and in accordance with the following expression:
Δ(p)=Δ(p=1)*η^(p−1);  (Eq. 1)

[0021] wherein η is a constant having a value between 0 and 1. With respect to the resonator structure 100 in FIG. 1, Bragg ring 1 corresponds to the ring 106 immediately surrounding the center disc 104, ring 2 is the ring immediately surrounding ring 1, and so on. Initially selected values of Δ(p=1) and the scaling factor η are different for each resonator configuration, depending upon (for example) thickness, index contrast, azimuthal order “m”, and central radius. For an unperturbed ring width of w(p) the inner radius is given by:
r_inner(p)=r_unperturbed(p)+Δ(p)−w(p)/2  (Eq. 2)

[0022] and the outer radius is given by:
r_outer(p)=r_unperturbed(p)=Δ(p)+w(p)/2.  (Eq. 3)

[0023] For large values of p, Δ(p) decreases exponentially, and thus the grating rings furthest from the center become periodic; i.e., r_unperturbed(p+1)=r_unperturbed(p)+a, where “a” is the periodicity of the gratings.

[0024] In the exemplary embodiment of FIG. 1, the high-Q resonator structure 100 features non-uniform grating ring spacing distances with constant widths of the circular (or oval) Bragg rings. By way of comparison, FIG. 2 is a top view of a radial Bragg ring resonator structure 200 configured in accordance with an alternative embodiment of the invention. In this embodiment, it will be noted that the resonator structure 200 has both a non-uniform spacing distance between grating rings 106 and non-uniform widths of the rings themselves.

[0025] Referring now to FIG. 3, there is shown a flow diagram illustrating a method 300 of designing a high-Q radial Bragg ring resonator in accordance with a further embodiment of the invention. As shown in block 302, a duty cycle, D, and a height, h, of the resonator structure is determined as a function of the radial Bragg grating periodicity “a” for a given target wavelength λ=a/ω_res (wherein ω_res is the resonance frequency of the resonator in units of c/a, where c is the speed of light). The calculations may be performed by solving Maxwell's equations in the frequency domain. The results for some exemplary configurations are illustrated in FIG. 4, which is a graph that plots possible configurations for the azimuthal order m=1 as a function of the height, h, and the duty cycle, D, (defined as a ratio between width of the surrounding material between the gratings and periodicity “a”). The height, h, is normalized to the periodicity a. The upper and the lower limits in FIG. 4 correspond to the configurations where the intended resonance frequency of the cavity ω_res is no longer inside the photonic bandgap of the radial Bragg rings. The configurations with the highest Q are those where ω_res is near the center of the bandgap, i.e., on the center line labeled “band gap center.” In the example depicted, the plot is suitable for Bragg rings made of Si (n=3.4) and where the surrounding is SiO₂ (n=1.46).

[0026] Referring again to FIG. 3, as shown in block 304, an optimum radius r₀ of the central region, the displacement of the first ring Δ(p=1) and the scaling factor η for the intended target wavelength λ are then determined. This may be done by solving Maxwell's equations in the time domain and calculating the Q factor of the optical resonator. After an initial guess for all three parameters, they are iteratively altered in order to achieve the maximum Q. For example, as shown in block 306 the duty cycle and height of the structure are varied to optimize Q. If at decision block 308, the Q is not yet optimized, then blocks 304 and 306 are repeated until the Q is optimized and the process exits. Alternatively, this optimization may be done using one or a combination of several algorithms, e.g., Monte Carlo, genetic algorithms and the like. Additionally, the widths of the radial Bragg rings may be changed ring-by-ring in order to further increase Q.

[0027] Finally, FIG. 5 is a graph that plots the quality factor Q of the radial Bragg ring resonator as a function of the shift Δs of the first ring (p=1), Δs=−Δ(p=1), in units of the periodicity, a. The parameters are the same as in FIG. 4, with the duty cycle D=0.33 and the height h=1.12*a. The radius of the center disc is r₀=1.62*a, and the scale factor is η=0.85. From the plot in FIG. 5, an exemplary optimum displacement of the first grating Δ(p=1) toward the center (with respect to an unperturbed λ/4 device) is −0.21*a, which results in Q=1.0×10⁶ and a mode volume of 9.9*a³ at ω_res=0.182 (the resonant wavelength is then λ=a/ω_res).

[0028] As will thus be appreciated, the present resonator embodiments differ from those structures that seek to attain optimum confinement of the optical mode inside the resonator. Such devices would only be ideal for an imaginary, vertically infinitely extended radial Bragg ring resonator with no vertical losses. However, for realistic devices which have a finite thickness (and are, for example, built on top of a substrate), the performance of ideal λ/4 reflectors or equivalent λ/4 reflectors will be dominated by vertical scattering losses, and therefore such resonators will suffer from having a low quality factor Q. In contrast, the present embodiments are directed to reducing vertical losses in practical devices with finite heights (thicknesses) by slightly relaxing V_eff, and in so doing, produce a significant increase in the Q-factor of the resonator.

[0029] While the invention has been described with reference to a preferred embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.