Tensile-strained Ge/SiGe multiple quantum well microdisks
Xiaochi Chen, Colleen S. Fenrich, Muyu Xue, Ming-Yen Kao, Kai Zang, Ching-Ying Lu, Edward T. Fei, Yusi Chen, Yijie Huo, Theodore I. Kamins, and James S. Harris
An efficient monolithically integrated laser on Si remains the missing component to enable Si photonics. We discuss the design and fabrication of suspended and tensile-strained Ge/SiGe multiple quantum well microdisk resonators on Si for laser applications in Si photonics using an all-around SiNx stressor. An etch-stop technique in the Ge/SiGe system is demonstrated and allows the capability of removing the defective buffer layer as well as providing precise thickness control of the resonators. Photoluminescence and Raman spectroscopy indicate that we have achieved a biaxial tensile strain shift as high as 0.88% in the microdisk resonators by adding a high-stress SiNx layer. Optical gain calculations show that high positive net gain can be achieved in Ge quantum wells with 1% external biaxial tensile strain.
Photonics Research
  • Publication Date: Aug. 05, 2017
  • Vol. 5, Issue 6, 060000B7 (2017)
LED-pumped whispering-gallery laser
Simon J. Herr, Karsten Buse, and Ingo Breunig
Photonics Research
  • Publication Date: Oct. 11, 2017
  • Vol. 5, Issue 6, 06000B34 (2017)
Constructing the scattering matrix for optical microcavities as a nonlocal boundary value problem
Li Ge
We develop a numerical scheme to construct the scattering (S) matrix for optical microcavities, including the special cases with parity-time and other non-Hermitian symmetries. This scheme incorporates the explicit form of a nonlocal boundary condition, with the incident light represented by an inhomogeneous term. This approach resolves the artifact of a discontinuous normal derivative typically found in the R-matrix method. In addition, we show that, by excluding the aforementioned inhomogeneous term, the non-Hermitian Hamiltonian in our approach also determines the Periels–Kapur states, and it constitutes an alternative approach to derive the standard R-matrix result in this basis. Therefore, our scheme provides a convenient framework to explore the benefits of both approaches. We illustrate this boundary value problem using 1D and 2D scalar Helmholtz equations. The eigenvalues and poles of the S matrix calculated using our approach show good agreement with results obtained by other means.
Photonics Research
  • Publication Date: Sep. 17, 2017
  • Vol. 5, Issue 6, 06000B20 (2017)
Genetically optimized on-chip wideband ultracompact reflectors and Fabry–Perot cavities
Zejie Yu, Haoran Cui, and Xiankai Sun
Reflectors are an essential component for on-chip integrated photonics. Here, we propose a new method for designing reflectors on the prevalent thin-film-on-insulator platform by using genetic-algorithm optimization. In simulation, the designed reflector with a footprint of only 2.16 μm×2.16 μm can achieve ~97% reflectivity and 1 dB bandwidth as wide as 220 nm. The structure is composed of randomly distributed pixels and is highly robust against the inevitable corner rounding effect in device fabrication. In experiment, we fabricated on-chip Fabry–Perot (FP) cavities constructed from optimized reflectors. Those FP cavities have intrinsic quality factors of >2000 with the highest value beyond 4000 in a spectral width of 200 nm. The reflectivity fitted from the FP cavity resonances is >85% in the entire wavelength range of 1440–1640 nm and is beyond 95% at some wavelengths. The fabrication processes are CMOS compatible and require only one step of lithography and etch. The devices can be used as a standard module in integrated photonic circuitry for wide applications in on-chip semiconductor laser structures and optical signal processing.
Photonics Research
  • Publication Date: Sep. 07, 2017
  • Vol. 5, Issue 6, 06000B15 (2017)
Single-mode lasing via loss engineering in fiber-taper-coupled polymer bottle microresonators
Fuming Xie, Ni Yao, Wei Fang, Haifeng Wang, Fuxing Gu, and Songlin Zhuang
Due to the lack of mode selection capability, single whispering-gallery-mode (WGM) lasing is a challenge to achieve. In bottle microresonators, the highly nondegenerated WGMs are spatially well-separated along the long-axis direction and provide mode selection according to their axial mode numbers. In this work, we use a loss-engineering approach to suppress the higher-order WGMs and demonstrate single-mode lasing emission in small polymer bottle microresonators. The fiber tapers are not only used to couple pump light into the bottle microresonators to excite the WGMs but also to bring optical losses that are induced from the diameter mismatch between fiber tapers and microresonators. By adjusting the coupling positions, the diameters of fiber tapers, and the coupling angles, single fundamental-mode lasing is efficiently generated with side-mode suppression factors over 15 dB. Our loss-engineering approach is convenient just by moving the fiber taper and may find promising applications in miniature tunable single-mode lasers and sensors.
Photonics Research
  • Publication Date: Sep. 13, 2017
  • Vol. 5, Issue 6, 06000B29 (2017)
Universal single-mode lasing in fully chaotic two-dimensional microcavity lasers under continuous-wave operation with large pumping power [Invited]
Takahisa Harayama, Satoshi Sunada, and Susumu Shinohara
For a fully chaotic two-dimensional (2D) microcavity laser, we present a theory that guarantees both the existence of a stable single-mode lasing state and the nonexistence of a stable multimode lasing state, under the assumptions that the cavity size is much larger than the wavelength and the external pumping power is sufficiently large. It is theoretically shown that these universal spectral characteristics arise from the synergistic effect of two different kinds of nonlinearities: deformation of the cavity shape and mode interaction due to a lasing medium. Our theory is based on the linear stability analysis of stationary states for the Maxwell–Bloch equations and accounts for single-mode lasing phenomena observed in real and numerical experiments of fully chaotic 2D microcavity lasers.
Photonics Research
  • Publication Date: Oct. 24, 2017
  • Vol. 5, Issue 6, 06000B39 (2017)
Self-adjustment of a nonlinear lasing mode to a pumped area in a two-dimensional microcavity [Invited]
Yuta Kawashima, Susumu Shinohara, Satoshi Sunada, and Takahisa Harayama
We numerically performed wave dynamical simulations based on the Maxwell–Bloch (MB) model for a quadrupole-deformed microcavity laser with spatially selective pumping. We demonstrate the appearance of an asymmetric lasing mode whose spatial pattern violates both the x- and y-axes mirror symmetries of the cavity. Dynamical simulations revealed that a lasing mode consisting of a clockwise or counterclockwise rotating-wave component is a stable stationary solution of the MB model. From the results of a passive-cavity mode analysis, we interpret these asymmetric rotating-wave lasing modes by the locking of four nearly degenerate passive-cavity modes. For comparison, we carried out simulations for a uniform pumping case and found a different locking rule for the nearly degenerate modes. Our results demonstrate a nonlinear dynamical mechanism for the formation of a lasing mode that adjusts its pattern to a pumped area.
Photonics Research
  • Publication Date: Oct. 31, 2017
  • Vol. 5, Issue 6, 06000B47 (2017)
Optical microcavities: new understandings and developments
Li Ge, Liang Feng, and Harald G. L. Schwefel
Photonics Research
  • Publication Date: Nov. 15, 2017
  • Vol. 5, Issue 6, 06000OM1 (2017)