Person:

Yu, Nanfang

We report the experimental realization of deformed microcavity quantum cascade lasers (QCLs) with a Limaçon-shaped chaotic resonator. Directional light emission with a beam divergence of (\theta_{|} \approx 33^{\circ}) from QCLs emitting at λ ≈ 10µm was obtained in the plane of the cavity for deformations in the range 0.37 < ε < 0.43. An excellent agreement between measured and calculated far-field profiles was found. Both simulations and experiments show that the Limaçon-shaped microcavity preserves whispering gallery-like modes with high Q-factors for low deformations (ε < 0.50). In addition, while the measured spectra show a transition from whispering gallery-like modes to a more complex mode structure at higher pumping currents, we observed ‘universal far-field behavior’ for different intracavity mode distributions in the Limaçon microcavity, which can be explained by the distribution of unstable manifolds in ray optics simulations. Furthermore, the performance of the deformed microcavity lasers is robust with respect to variations of the deformation near its optimum value ε = 0.40, which implies that this structure reduces the requirements on photolithography fabrication. The successful realization of these microcavity lasers may lead to applications in optoelectronics.

Multibeam emission and spatial wavelength demultiplexing in semiconductor lasers by patterning their facets with plasmonic structures is reported. Specifically, a single-wavelength laser was made to emit beams in two directions by defining on its facet two metallic gratings with different periods. The output of a dual-color laser was spatially separated according to wavelength by using a single metallic grating. The designs can be integrated with a broad range of active or passive optical components for applications such as interferometry and demultiplexing.

Surface plasmons have found a broad range of applications in photonic devices at visible and near-infrared wavelengths. In contrast, longer-wavelength surface electromagnetic waves, known as Sommerfeld or Zenneck waves, are characterized by poor confinement to surfaces and are therefore difficult to control using conventional metallo-dielectric plasmonic structures. However, patterning the surface with subwavelength periodic features can markedly reduce the asymptotic surface plasmon frequency, leading to ‘spoof’ surface plasmons with subwavelength confinement at infrared wavelengths and beyond, which mimic surface plasmons at much shorter wavelengths. We demonstrate that by directly sculpting designer spoof surface plasmon structures that tailor the dispersion of terahertz surface plasmon polaritons on the highly doped semiconductor facets of terahertz quantum cascade lasers, the performance of the lasers can be markedly enhanced. Using a simple one-dimensional grating design, the beam divergence of the lasers was reduced from ∼180◦ to ∼10◦ , the directivity was improved by over 10 decibels and the power collection efficiency was increased by a factor of about six compared with the original unpatterned devices. We achieve these improvements without compromising high-temperature performance of the lasers.

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.

We report experimental demonstration of directional light emission from limaçon-shaped microcavity semiconductor lasers. Quantum cascade lasers (QCLs) emitting at (\lambda \approx 10 \mu m) are used as a model system. Both ray optics and wave simulations show that for deformations in the range (0.37< \epsilon <0.43), these microcavities support high quality-factor whispering gallerylike modes while having a directional far-field profile with a beam divergence (\theta \approx 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 ( \epsilon = 0.40).

The field of plasmonics-in which surface plasmon resonances of metals are used to manipulate light at the sub-wavelength scale-is transforming our understanding of nanophotonics and integrated optics. Now, researchers are harnessing the power of plasmonics, paving the way to wavefront engineering of laser beams.

The authors reported the plasmonic control of semiconductor laser polarization by means of metallic gratings and subwavelength apertures patterned on the laser emission facet. An integrated plasmonic polarizer can project the polarization of a semiconductor laser onto other directions. By designing a facet with two orthogonal grating-aperture structures, a polarization state consisting of a superposition of a linearly and right-circularly polarized light was demonstrated in a quantum cascade laser; a first step toward a circularly polarized laser.

Systematic measurements of the photoluminescence lifetime of the 1.54 μm transition of erbium implanted at different energies in SiO2 films with different metallic overlayers are reported. The lifetime shows a strong reduction up to a factor of 20 with decreasing distance between the erbium and the metal overlayer. The reduction of lifetime is mainly due to a near-field interaction between the erbium ions and the metal overlayers through generation of surface plasmon polaritons at the metal/SiO2 interface and direct generation of heat in the metal. These experiments combined with rigorous theoretical modeling demonstrate that a high degree of control over the radiative properties of erbium can be achieved in erbium-implanted materials in a wide range of implantation energies. The experiments also allow us to determine the radiative efficiency of erbium in bulk SiO2.

Quantum cascade lasers are a unique laboratory for studying nonlinear laser dynamics because of their high intracavity intensity, strong intersubband optical nonlinearity, and an unusual combination of relaxation time scales. Here we investigate the nonlinear coupling between the transverse modes of quantum cascade lasers. We present evidence for stable phase coherence of multiple transverse modes over a large range of injection currents. We explain the phase coherence by a four-wave mixing interaction originating from the strong optical nonlinearity of the gain transition. The phase-locking conditions predicted by theory are supported by spectral data and both near- and far-field mode measurements.

This paper reviews recent work on device applications of optical antennas. Localized surface plasmon resonances of gold nanorod antennas resting on a silica glass substrate were modeled by finite difference time-domain simulations. A single gold nanorod of length 150 or 550 nm resonantly generates enhanced near fields when illuminated with light of 830 nm wavelength. A pair of these nanorods gives higher field enhancements due to capacitive coupling between them. Bowtie antennas that consist of a pair of triangular gold particles offer the best near-field confinement and enhancement. Plasmonic laser antennas based on the coupled nanorod antenna design were fabricated by focused ion beam lithography on the facet of a semiconductor laser diode operating at a wavelength of 830 nm. An optical spot size of few tens of nanometers was measured by apertureless near-field optical microscope. We have extended our work on plasmonic antenna into mid-infrared (mid-IR) wavelengths by implementing resonant nanorod and bowtie antennas on the facets of various quantum cascade lasers. Experiments show that this mid-IR device can provide an optical intensity confinement 70 times higher than that would be achieved with diffraction limited optics. Near-field intensities ~ 1 GW/cm2 were estimated for both near-infrared and mid-IR plasmonic antennas. A fiber device that takes advantage of plasmonic resonances of gold nanorod arrays providing a high density of optical ldquohot spotsrdquo is proposed. Results of a systematic theoretical and experimental study of the reflection spectra of these arrays fabricated on a silica glass substrate are also presented. The family of these proof-of-concept plasmonic devices that we present here can be potentially useful in many applications including near-field optical microscopes, high-density optical data storage, surface enhanced Raman spectroscopy, heat-assisted magnetic recording, and spatially resolved absorption spectroscopy.