WO2012154209A1

WO2012154209A1 - Highly unidirectional microcavity resonators

Info

Publication number: WO2012154209A1
Application number: PCT/US2011/062225
Authority: WO
Inventors: Qijie Wang; Nanfang Yu; Federico Capasso; Jan WIERSIG; Julia UNTERHINNINGHOFEN
Original assignee: President And Fellows Of Harvard College
Priority date: 2010-11-29
Filing date: 2011-11-28
Publication date: 2012-11-15

Abstract

A highly unidirectional microcavity resonator with low beam far-field divergence and high quality-factor (Q-factor) includes an elliptical shaped resonator with a notch at the boundary. The notch acts as a scatter that preserves high Q-factor whispering gallery modes. An in-plane beam divergence as small as 6 degrees has been demonstrated. The insensitivity of the beam divergence to the pumping current and to different notch sizes when the microcavity aspect ratio is optimized for high directionality demonstrates the robustness of this microcavity design.

Description

HIGHLY UNIDIRECTIONAL MICROCAVITY RESONATORS

GOVERNMENT SUPPORT

The invention was supported, in whole or in part, by Grant FA9550-08-1-0047 from the Air Force Office of Scientific Research. The United States Government has certain rights in the invention.

BACKGROUND

Optical microcavities can be designed to take advantage of total internal reflection, which results in resonators supporting whispering gallery modes (WGMs) with a high -factor. Whispering-gallery-mode resonators offer great promise for investigation in the physical sciences, and applications of these devices have spanned a wide range from novel laser sources and dynamic filters in communications to sensors.

The study of whispering gallery modes in the optical domain was first performed in liquid droplets, then in a spherical resonator [see J. M. Baer, US Patent No. 4,829,537 (1989)] and later in a micro-disk diode laser [see S. L. McCall, US Patent No. 5,343,490 (1994)], which may be the simplest structure of a semiconductor microcavity laser. A schematic diagram of a micro-disk resonator 11 [see S. T. Ho, US Patent, 5,825,799 (1998) and K. J. Vahala, US Patent No. 6,865,317 (2005)] is shown in FIG. 1 together with a fundamental whispering gallery mode 15 corresponding to a certain wavelength that is supported by the resonator. In such a system, light is confined in the microcavity 11 through total internal reflection of whispering gallery modes 15 where the incident angle of rays, χ, at the boundary is always larger than the critical angle, 6_C. And the light 15 is output coupled through tunneling and surface roughness scattering. Because of preservation of high- -factor whispering gallery modes 15, very-high- - factor microcavity resonators have been demonstrated. Later, very-high- -factor and low- threshold disk lasers with active gain medium have also been demonstrated based on the same design concept. The disadvantages of this device, which are also general problems of microcavities, are that the coupled output power is low and the far-field profile is isotropic.

To overcome these difficulties, one approach has been to use deformed optical microcavities to increase the directionality of emission and power collection efficiency [see A. D. Stone, et ai, US Patent No. 5,742,633 (1998) and A. D. Stone, et ai, US Patent No. 6,259,717 (2001)]. The most successful designs include quadrupolar-shaped lasers [see A. D. Stone, et ai, US Patent No. 5,742,633 (1998)], microcavity-supporting bow-tie modes [see F. Capasso, et ai, US Patent No. 6,134,257 (2000)] and spiral-shaped lasers [see R. M. Montgomery, US Patent No. 7,184,629 (2001)]. However, all deformed microcavities studied in the literature have the problem that the Q.-factor degrades significantly as the deformation increases; in most of the cases, WGMs are no longer supported in the cavity. Recently, a Limagon-shaped microcavity, as shown in FIG. 2 [see J. Wiersig, et ai, "Combining Directional Light Output and Ultralow Loss in Deformed Microdisks," 100 Phys. Rev. Lett. 033901 (2008)], was proposed to improve directionality while maintaining high Q.-factor modes and was soon demonstrated via simulation 17 and experimentally 19 in the mid-infrared spectral regions [see C. Yan, et ai, "Directional emission and universal far-field behavior from semiconductor lasers with limagon- shaped microcavity", 94 Appl. Phys. Lett. 251101 (2009)], as shown in FIG. 3. The far-field divergence angle of the main lobe of these devices, however, is about 20 or more degrees; and side lobes persist.

SUMMARY

A notched elliptical microcavity resonator and the use thereof is described herein.

Various embodiments of the device and method may include some or all of the elements, features and steps described below.

The microcavity resonator includes a microcavity having an elliptical boundary and a notch defined in that elliptical boundary. Though the terms, "elliptical" is used herein, the boundary, of course, is not a "perfect" ellipse in view of the notch and, e.g., machining tolerances. Nevertheless, the characterization of an "elliptical boundary" is intended to cover shapes with these imperfections.

The microcavity can formed of a passive material and/or an active material, such as a semiconductor material. The elliptical boundary is centered about a short axis and a long axis, and the notch is located at the intersection of the elliptical boundary and the short axis. The notch can have a depth and an opening, both measuring, e.g., from 0.1 to 10 μιη (and in particular embodiments from 1 to 5 μιη). The microcavity resonator can also include electrically conductive contacts (formed, e.g., of gold or copper) on opposite sides of the microcavity and a substrate (formed, e.g., of indium phosphide or gallium arsenide) on which the microcavity is mounted. In various embodiments, the microcavity resonator can be a quantum cascade laser with quantum wells including a pair of group lll-V or ll-VI semiconductors; a diode laser; or an interband cascade laser.

A low-beam-divergence unidirectional emission with a high quality factor can be produced using the microcavity resonator by propagating light in a whispering gallery mode at a perimeter of an elliptical microcavity having a notch and scattering the light at the notch in a substantially unidirectional emission from the microcavity.

The substantially unidirectional emission can have a beam divergence no greater than 6°. The wavelength of the propagating light has a wavelength no greater than the depth and opening width of the notch and no more than a third of the dimensions of the microcavity; and that light can be generated by pumping electrical current into the microcavity. Most of the light striking the notch can be scattered out of the microcavity, and the spectral range of the emitted light can be from 300 nm to 50 μιη.

The notched elliptical microcavity resonator can be implemented with passive or active mediums. When a passive medium is used to construct the resonator, the passive medium acts as a passive resonator; and the light for generating whispering gallery modes can be coupled into the resonator externally. When an active medium is used, the notched elliptical microcavity resonator acts as a microcavity laser.

The notched elliptical microcavity resonator can include a resonator having an elliptical boundary and a notch defined in the elliptical boundary of the resonator. The notch can have a depth, d, for example, of at least 1 μιη; and the notch can be located at the intersection of the elliptical boundary and the short axis of the ellipse.

Embodiments of the passive microcavity resonator and microcavity laser can exhibit high output coupling and can produce a highly unidirectional emission from the cavity and high-quality-factor whispering gallery modes in the cavity. The produced emission can have a far-field profile with a single lobe and without side-band lobes. The effectiveness of the microcavity in producing the highly unidirectional emission can be relatively insensitive to variations in the size of the notch and drive currents, thereby demonstrating the robustness of this type of resonator. BRI EF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a microdisk laser; 6_C is the critical incident angle, and χ is the incident angle of the ray trajectory.

FIG. 2 shows the schematic structure of a limagon-shaped microcavity.

FIG. 3 shows the measured in-plane far-field beam divergence for the microcavity of

FIG. 2.

FIG. 4 is a schematic illustration of a notched microcavity laser.

FIGS. 5 and 6, respectively show the light rays in the whispering gallery mode in the microcavity of FIG. 4 and the rays reflected out of the microcavity by the notch.

FIG. 7 shows a fabricated notched ellipse resonator.

FIGS. 8 and 9 respectively show the electrical and optical characteristics of the microcavity resonator of FIG. 7.

FIG. 10 is a schematic of the notched ellipse resonator for far-field measurement.

FIG. 11 shows the measured two-dimensional far-field intensity distributions of a representative notched ellipse resonator with a minor radius, X = 80 μιτι; a major radius, Y = 96 μιτι and notch dimensions, o = 3 μιτι, d = 2 μιτι, at a pumping current of ~720 mA .

FIG. 12 shows the strong agreement of the experimental and simulated in-plane far- field intensity profiles in polar coordinates for the notched elliptical resonator.

FIG. 13 shows the measured far-field profiles of the notched ellipse resonator of FIG. 10 at different pumping currents in a sensitivity study.

FIG. 14 provides a comparison of the measured far-field profiles for notched ellipse resonators with slightly different notch sizes near the optimum o = 3 μιτι and d = 2 μιτι.

FIG. 15 shows light-voltage-current characteristics of a notched ellipse resonator in continuous wave operation.

I n the accompanying drawings, like reference characters refer to the same or similar parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating particular principles, discussed below.

DETAI LED DESCRI PTION

The foregoing and other features and advantages of various aspects of the invention(s) will be apparent from the following, more-particular description of various concepts and specific embodiments within the broader bounds of the invention(s). Various aspects of the subject matter introduced above and discussed in greater detail below may be implemented in any of numerous ways, as the subject matter is not limited to any particular manner of implementation. Examples of specific implementations and applications are provided primarily for illustrative purposes.

Unless otherwise defined, used or characterized herein, terms that are used herein

(including technical and scientific terms) are to be interpreted as having a meaning that is consistent with their accepted meaning in the context of the relevant art and are not to be interpreted in an idealized or overly formal sense unless expressly so defined herein. For example, if a particular composition is referenced, the composition may be substantially, though not perfectly pure, as practical and imperfect realities may apply; e.g., the potential presence of at least trace impurities (e.g., at less than 1 or 2% by weight or volume) ca n be understood as being within the scope of the description; likewise, if a particular shape is referenced, the shape is intended to include imperfect variations from ideal shapes, e.g., due to manufacturing tolerances.

Spatially relative terms, such as "above," "upper," "beneath," "below," "lower," and the like, may be used herein for ease of description to describe the relationship of one element to another element, as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the apparatus in use or operation in addition to the orientation depicted in the figures. For example, if the apparatus in the figures is turned over, elements described as "below" or "beneath" other elements or features would then be oriented "above" the other elements or features. Thus, the exemplary term, "above," may encompass both an orientation of above and below. The apparatus may be otherwise oriented (e.g., rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

Further still, in this disclosure, when an element is referred to as being "on,"

"connected to" or "coupled to" another element, it may be directly on, connected or coupled to the other element or intervening elements may be present unless otherwise specified.

The terminology used herein is for the purpose of describing particular embodiments and is not intended to be limiting of exemplary embodiments. As used herein, the singular forms, "a," "an" and "the," are intended to include the plural forms as well, unless the context clearly indicates otherwise. Additionally, the terms, "includes," "including," "comprises" and "comprising," specify the presence of the stated elements or steps but do not preclude the presence or addition of one or more other elements or steps.

I n various embodiments, the resonator described herein is in the form of an elliptical microcavity structure with a wavelength-size notch at its boundary. An elliptical resonator was chosen because it possesses whispering gallery modes (WGMs) with very high Q.-factors for shapes with various long-to-short (Y/X) aspect ratios. A wavelength-size notch (i.e., comparable to the ratio of the light's vacuum wavelength to the refractive index of the microcavity composition) suitably defined on the boundary of the microcavity diffracts light towards the opposite boundary. FIG. 4 illustrates how the aspect ratio, Y/X, of the microcavity 12 was optimized to improve collimation. The notch 14 is located at the intersection of the short axis, X, and the microcavity boundary (point 0). I n order to achieve collimation of the majority of the light scattered by the notch 14, a well-known property of the ellipse is exploited; i.e., for any given refractive index, n > 1, one can find an ellipse 18 (i.e., an auxiliary ellipse) such that all incoming parallel rays are collected into one of its foci, meaning that all rays starting from that focal point are collimated. The arrows 16 indicate that light waves are scattered by the notch 14 at point 0, then collimated as a parallel beam in the far field by the right-hand-side boundary of the notched elliptical microcavity 12. The notch 14 is located at one of the foci of the auxiliary ellipse 18, but not at the focus of the elliptical microcavity 12. The Y/X ratio of the elliptical microcavity 12 is now chosen such that its boundary best approximates (i.e., over the largest possible angle, 2Θ) that of the auxiliary ellipse 18.

I n quantitative terms, let r( 0) be the distance between some point on the microcavity boundary 12 and point 0; and let R( 0) be the distance between a corresponding point on the auxiliary ellipse and point O. Maximizing the range of angles, Θ, where R( 0) - r( 0) is negligible [here we set \ R( 0)-r( 0) \ /R( 9) ~ 1%, corresponding to the accuracy of fabrication], we find ε ≡ Y/X \s ~1.2 for a refractive index, n_eff = 3.2, for the laser material used in the experiment described below. This optimal value, ε = 1.2, yields 20_max = 70°, thus, indeed, the majority of the light is collimated by the elliptical microcavity 12. An analytical expression can also be derived in the paraxial ray approximation for the optimal ε by starting rays at the notch, using Snell's law to find the far-field rays, and requiring that they be parallel. We find ε - (2-2/n_e//)^{1 2} = 1.17 for n_eff = 3.2, which agrees well with the numerical result of 1.2. FIG. 5 shows ray simulation of the collimation effect, wherein a number of rays 16 propagate from the position of the notch 14 with different outgoing angles, simulating a scattering process. The rays 16 travel inside the cavity 12 until they hit its boundary, upon which the rays 16 either are specularly reflected or, if the angle of incidence at the surface is smaller than the critical angle for total internal reflection, are refracted out of the microcavity 12. The solid rays 16 that leave the notch under relatively smaller outgoing angles, get collimated; the collimation is worse for higher outgoing angles (i.e., the outermost rays 16). The dash-dotted ray 20 leaves the notch 14 at a high outgoing angle and is re-launched into a whispering-gallery mode inside the cavity 12.

FIG. 6 shows ray simulation of whispering-gallery mode dynamics, with a magnified view of the dynamics in the region around the notch 14 projected from the main image. A single ray 15 is started at some position along the boundary of the microcavity 12 with an initial condition such that the angle of incidence is larger than the critical angle. The ray 15 is then specularly reflected many times along the boundary of the microcavity 12, corresponding to a whispering-gallery-like mode, until at some point the ray 15 hits the notch 14 and is reflected to the opposite boundary of the microcavity 12, where it is refracted out and leaves the microcavity 12 parallel to (or substa ntially parallel to) the axis due to the collimation effect.

The microcavity 12 can be any kind of passive or active microcavity and can be built using a large variety of semiconductor materials emitting laser light across the entire spectral range from the near ultraviolet (e.g., 300 nm) to the mid-infrared (e.g., 50 μιη), including the visible and near infrared wavelengths. The underlying layer structure of the microcavity laser can be (a) a quantum cascade laser, in which case the lasing mode is the transverse magnetic (TM) polarized mode, wherein the electric field is perpendicular to the plane of the device material layers; (b) a diode laser, where the lasing mode is preferentially a transverse electric (TE) polarized mode, wherein the electric field lies in the plane of the device material layers.; or (c) an interband cascade laser, which also operates preferentially on TE modes. The size of the microcavity ca n range from a few times the wavelength of the light in the microcavity material to much larger sizes.

I n one embodiment, the microcavity 12 is realized with an active gain medium of a quantum cascade laser (Q.CL) on a substrate. The substrate can be indium phosphide and/or gallium arsenide. The top and back electric contacts can be gold and/or copper. The active region of quantum cascade lasers is composed of quantum wells made of indium gallium arsenide / indium aluminum arsenide, gallium arsenide / aluminum gallium arsenide or other pairs of lll-V or ll-VI group semiconductors.

To demonstrate the concept, quantum cascade lasers are used as a model system. Notched elliptical quantum cascade lasers 12 with different dimensions and notch sizes were fabricated on a substrate 34 and tested in pulsed mode operation at room temperature.

Quantum cascade lasers were first fabricated with different cavity sizes, X = 50, 80, and 110 μιη, and with different axis ratios, ε - 1.0, 1.1, 1.15, 1.2, 1.25, 1.3, 1.5. The quantum cascade laser material is lattice-matched Ga₀.₄₇ln₀.53As/Al₀.₄8l no.52 s/lnP, as in C. Yan, et al., "Directional emission and universal far-field behavior from semiconductor lasers with limagon-shaped microcavity", 94 Appl. Phys. Lett. 251101 (2009), designed at an emission wavelength, /t~10 μιη. Additionally, elliptical quantum cascade lasers with different notch sizes near the optimum and significantly away from the optimum were also fabricated. Standard photolithography was used to define the contour of the laser cavity 12, shown in FIG. 7, which includes a magnified view of the region around the notch 14 at the bottom. The structure was etched through the gain medium 32 using inductively coupled plasma reactive ion etching to obtain vertical and smooth sidewalls. Then, the top 28 and back metal contacts were deposited. Both the top and bottom contacts comprise a dual-metal layer made of 10 nm titanium (Ti) and 200 nm gold (Au). The titanium layer is used for adhesion, so it is applied first to form a direct contact with the semiconductor device; then the thick gold layer is deposited. The magnified SEM image of the device sidewall shown in FIG. 7, including the cladding 30 and gain medium 32, shows a roughness of about 300 nm, which gives minor scattering in the mid-infrared wavelength range.

The notched elliptical quantum cascade lasers were electric pumped and tested in pulsed mode at room temperature with 125-ns current pulses at 80-kHz repetition rate. The tested quantum cascade laser was then mounted at the center of a motorized rotation stage with 0.5° resolution, and a mid-infrared mercury-cadmium-telluride detector positioned 10 cm away from the notched elliptical quantum cascade laser was scanned to measure the output of the laser. Power measurements were carried out with a calibrated power meter.

FIG. 8 presents the light output power versus current ( .-/) characteristics of this device.

FIG. 9 presents the emission spectra of the notched elliptical quantum cascade lasers measured at different pumping currents (i.e., at 520 mA, 750 mA and 1000 mA). The laser operates in single mode at a wavelength, λ, of ~ 10 μιη near the threshold current (i.e., 520 mA). At a pumping current of 750 mA, two sets of optical modes, respectively indicated by arrows 24 and 26, appear. At the even-higher pumping current of 1000 mA, several additional modes appear, indicated by arrows 22, corresponding to lower Q.-factor models. The average mode spacing of each set is approximately 5.80 cm^"1, which agrees very well with the calculated value of 5.85 cm ¹ for whispering-gallery-mode spacing, thereby verifying the notion that the demonstrated notched elliptical microcavity does support high-Q.-factor whispering gallery modes in the cavity.

A schematic illustration and a two-dimensional far field of the device are displayed in

FIGS. 10 and 11, respectively. A full-wave-half-maximum (FWHM) beam-divergence angle of 6 degrees in the plane of the laser cavity is demonstrated, as shown in FIG. 11, which is much narrower than the divergence of 40 degrees from Fabry-Perot ridge quantum cascade lasers. Excellent agreement is observed between the experimental 36 and simulated 38 far-field intensity profiles in FIG. 12. All of the far-field profiles 720, 820 and 920 are essentially the same at different pumping currents from 720 mA to 820 mA to 920 mA, respectively, as shown in FIG. 13. The far-field profiles 40, 42 and 44 shown in FIG. 14 are insensitive to variations of the notch size from 2 μιη to 4 μιη, a deviation well within fabrication uncertainties. Far-field profile 40 is for a notch size of o = 3 μιη, d = 2 μιη; far-field profile 42 is for a notch size of o = 3 μιη, d = 3 μιη; and far-field profile 44 is for a notch size of o = 4 μιη, οί = 3 μίη (where o represents the opening width of the notch at the edge of the microcavity 12, while d represents the depth of the notch measured inward and orthogonal to the width, o, as shown in FIG. 7).

I n the end, performance of the notched elliptical resonator device was also

demonstrated in continuous wave (cw) operation above liquid nitrogen cooling temperature (>80K). The voltage and optical power versus current for one representative device with X = 50 μιη and Y = 60 μιη (o = 3 μιη, d = 2 μιη) operated in cw at 80K are presented in FIG. 15. A threshold current density of approximately 1 kA/cm² and a collected optical output power of 1.2 mW were demonstrated. The inset of FIG. 15 shows the single-mode cw emission spectrum of the device at 80K operated at l_pump = ~130 mA. A laser emission with a wave number ~1042 cm ¹ was observed. Our simulations demonstrate that the far-field profile of TE-polarized modes is also highly directional, implying that the proposed concept is broadly applicable also to diode lasers operating in the near infrared, visible and ultraviolet spectra regions. The Q.-factors of the whispering gallery modes are still very high in these structures for the same wavelength-to-size ratio. At relatively short wavelengths (e.g., λ ~ 1 μιη), free carrier absorption is negligible compared to the mid-infrared and optical losses are small (~ 0.5 cm^"1), limited by the sidewall roughness of the cavity, and material absorption. Sirnivasan Kartik, et ai, "Optical loss and lasing characteristics of high-quality-factor AIGaAs microdisk resonators with embedded quantum dots," Appl. Phys. Lett. 86:151106 (2005). These qualities lead to a much smaller - factor degradation than that at mid-IR wavelengths; optical loss is ~15.6 cm ¹ for the notched elliptical quantum cascade lasers). Thus, notched-elliptical resonators may be advantageously used for low-threshold, highly directional microcavity diode lasers.

In describing embodiments of the invention, specific terminology is used for the sake of clarity. For the purpose of description, specific terms are intended to at least include technical and functional equivalents that operate in a similar manner to accomplish a similar result. Additionally, in some instances where a particular embodiment of the invention includes a plurality of system elements or method steps, those elements or steps may be replaced with a single element or step; likewise, a single element or step may be replaced with a plurality of elements or steps that serve the same purpose. Further, where parameters for various properties are specified herein for embodiments of the invention, those parameters can be adjusted up or down by l/100 ^h, l/50 ^h, l/20 ^h, l/10 ^h, l/5 ^h, l/3^rd, 1/2, 3/4 ^h, etc. (or up by a factor of 2, 5, 10, etc.), or by rounded-off approximations thereof, unless otherwise specified. Moreover, while this invention has been shown and described with references to particular embodiments thereof, those skilled in the art will understand that various substitutions and alterations in form and details may be made therein without departing from the scope of the invention. Further still, other aspects, functions and advantages are also within the scope of the invention; and all embodiments of the invention need not necessarily achieve all of the advantages or possess all of the characteristics described above. Additionally, steps, elements and features discussed herein in connection with one embodiment can likewise be used in conjunction with other embodiments. The contents of references, including reference texts, journal articles, patents, patent applications, etc., cited throughout the text are hereby incorporated by reference in their entirety; and appropriate components, steps, and characterizations from these references optionally may or may not be included in

embodiments of this invention. Still further, the components and steps identified in the Background section are integral to this disclosure and can be used in conjunction with or substituted for components and steps described elsewhere in the disclosure within the scope of the invention. In method claims, where stages are recited in a particular order— with or without sequenced prefacing characters added for ease of reference— the stages are not to be interpreted as being temporally limited to the order in which they are recited unless otherwise specified or implied by the terms and phrasing.

Claims

CLAIMS What is claimed is:

1. A highly unidirectional microcavity resonator comprising

a microcavity having an elliptical boundary; and

a notch defined in the elliptical boundary of the resonator.

2. The highly unidirectional microcavity resonator of claim 1, wherein the microcavity comprises a medium selected from passive materials, active materials, and

combinations thereof.

3. The microcavity resonator of claim 1, wherein the notch has a depth, d, and an opening width, o, both measuring between 0 and the size of the microcavity resonator.

4. The microcavity resonator of claim 1, wherein the notch has a depth, d, and an opening width, o, both measuring from 0.1 to 10 μιτι.

5. The microcavity resonator of claim 1, wherein the microcavity resonator supports an electromagnetic wave that covers the whole electromagnetic spectrum.

6. The microcavity resonator of claim 1, wherein the elliptical boundary is centered about a short axis and a long axis, and the notch is located at an intersection of the elliptical boundary and the short axis.

7. The microcavity resonator of claim 1, wherein the microcavity comprises an active region comprising a semiconductor.

8. The microcavity resonator of claim 7, wherein the microcavity resonator is a quantum cascade laser.

9. The microcavity resonator of claim 8, wherein the semiconductor includes quantum wells having a composition selected a pair of group lll-V or ll-VI semiconductors.

10. The microcavity resonator of claim 1, wherein the microcavity resonator is a diode laser.

11. The microcavity resonator of claim 1, wherein the microcavity resonator is an interband cascade laser.

12. The microcavity resonator of claim 1, further comprising electrically conductive

contacts on opposite sides of the microcavity.

13. The microcavity resonator of claim 1, further comprising a substrate selected from

indium phosphide and gallium arsenide, wherein the microcavity is mounted on the substrate.

14. A method for producing a low-beam-divergence unidirectional emission with a high quality factor, comprising:

propagating light in a whispering gallery mode at a perimeter of an elliptical microcavity having a notch; and

scattering the light at the notch in a substantially unidirectional emission from the microcavity.

15. The method of claim 14, wherein the substantially unidirectional emission has a beam divergence no greater than 6°.

16. The method of claim 14, wherein the notch has a depth, d, and an opening width, o, both at least as great as the wavelength of the light propagating in the whispering gallery mode at the perimeter of the elliptical microcavity.

17. The method of claim 14, further comprising generating the light by pumping electrical current into the microcavity.

18. The method of claim 14, wherein a majority of the propagating light that strikes the notch is scattered out of the microcavity in the substantially unidirectional emission.

9. The method of claim 14, wherein the microcavity comprises a semiconductor, and wherein the semiconductor emits the light across the whole electromagnetic wave spectral range.