Surface plasmons offer the exciting possibility of improving the functionality of optical devices through the subwavelength manipulation of light. We show that surface plasmons can be used to shape the beams of edge-emitting semiconductor lasers and greatly reduce their large intrinsic beam divergence. Using quantum cascade lasers as a model system, we show that by defining a metallic subwavelength slit and a grating on their facet, a small beam divergence in the laser polarization direction can be achieved. Divergence angles as small as 2.48 8 8 8 8 are obtained, representing a reduction in beam spread by a factor of 25 compared with the original 9.9-mm-wavelength laser used. Despite having a patterned facet, our collimated lasers do not suffer significant reductions in output power ( 100 mW at room temperature). Plasmonic collimation provides a means of efficiently coupling the output of a variety of lasers into optical fibres and waveguides, or to collimate them for applications such as free-space communications, ranging and metrology.

Optical microcavities can be designed to take advantage of total internal reflection, which results in resonators supporting whispering-gallery modes (WGMs) with a high-quality factor (Q factor). One of the crucial problems of these devices for practical applications such as designing microcavity lasers, however, is that their emission is nondirectional due to their radial symmetry, in addition to their inefficient power output coupling. Here we report the design of elliptical resonators with a wavelength-size notch at the boundary, which support in-plane highly unidirectional laser emission from WGMs. The notch acts as a small scatterer such that the Q factor of the WGMs is still very high. Using midinfrared (λ ∼ 10 μm) injection quantum cascade lasers as a model system, an in-plane beam divergence as small as 6 deg with a peak optical power of ∼5 mW at room temperature has been demonstrated. The beam divergence is insensitive to the pumping current and to the notch geometry, demonstrating the robustness of this resonator design. The latter is scalable to the visible and the near infrared, thus opening the door to very low-threshold, highly unidirectional microcavity diode lasers.F ollowing the first description of the whispering-gallery mode (WGM) phenomenon in the acoustic regime by Lord Rayleigh in London's St Paul's Cathedral (1) and its subsequent analysis in terms of guided surface waves by Raman and Sutherland (2), its study was later extended to the radiofrequency (3) and optical domains (4) through the investigation of the ionosphere and solid spheres, respectively. WGMs were later investigated in liquid droplets (5) and microdisk diode lasers (6), opening a previously undescribed direction in photonics technology. WGM resonators offer great promise for investigation in the physical sciences (6-8), and applications of these devices have spanned a wide range from unique laser sources (9) and dynamic filters in communications (10) to sensors (11). One drawback, however, is that, in rotationally symmetric cavities (6, 12), WGMs can only be coupled out inefficiently and isotropically through scattering of evanescent waves by surface roughness or diffraction losses when the radius of curvature is comparable to the wavelength (9).Previously, this problem was addressed through evanescent coupling using prisms (13), in-plane waveguide (14), or tapered fibers (15). The technique of using tapered waveguide (16) for coupling high Q WGMs out of cavities is quite successful for the study of fundamental cavity physics; however, it requires careful alignment and the devices are relatively sensitive to mechanical vibrations or other variations in the surrounding environment, which limit its usage for practical applications such as achieving microcavity lasers with directional emission. Another approach is to break the rotational symmetry by using deformed optical microcavities to increase the directionality of emission and power collection efficiency (17, 18), which has the advantage of easy and robust fabrication. Th...

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The frequency-noise power spectral density of a room-temperature distributed-feedback quantum cascade laser emitting at λ = 4.36 μm has been measured. An intrinsic linewidth value of 260 Hz is retrieved, in reasonable agreement with theoretical calculations. A noise reduction of about a factor 200 in most of the frequency interval is also found, with respect to a cryogenic laser at the same wavelength. A quantitative treatment shows that it can be explained by a temperature-dependent mechanism governing the transport processes in resonant tunnelling devices. This confirms the predominant effect of the heterostructure in determining shape and magnitude of the frequency noise spectrum in QCLs.

We report experimental demonstration of directional light emission from limaçon-shaped microcavity semiconductor lasers. Quantum cascade lasers (QCLs) emitting at λ≈10 μm are used as a model system. Both ray optics and wave simulations show that for deformations in the range 0.37<ε<0.43, these microcavities support high quality-factor whispering gallerylike modes while having a directional far-field profile with a beam divergence θ∥≈30° in the plane of the cavity. The measured far-field profiles are in good agreement with simulations. While the measured spectra show a transition from whispering gallerylike modes to a more complex mode structure at higher pumping currents, the far field is insensitive to the pumping current demonstrating the predicted “universal far-field behavior” of this class of chaotic resonators. Due to their relatively high quality factor, our microcavity lasers display reduced threshold current densities compared to conventional ridge lasers with millimeter-long cavities. The performance of the limaçon-shaped QCLs is robust with respect to variations of the deformation near its optimum value of ε=0.40.