"Coupled Optical microcavities: an enhanced refractometric sensing configuration"

 Yun-Feng Xiao, Venkat Gaddam and Lan Yang

  Accepted by Optics Express

"Coupling whispering-gallery-mode microcavities with modal coupling mechanism"

  Yun-Feng Xiao, Bumki Min, Xiaoshun Jiang, Chun-Hua Dong, and Lan Yang

 Accepted by IEEE J. of Quantum Electronics

"Static Envelope Patterns in Composite Resonances Generated by Level Crossing in Optical Toroidal Microcavities"

Tal Carmon, Harald G.L.Schwefel, Lan Yang, Mark Oxborrow, A. Douglas Stone, and Kerry J. Vahala

Phy. Rev. Lett. 100, 103905 (2008)

We study level crossing in the optical whispering-gallery (WG) modes by using toroidal microcavities. Experimentally, we image the stationary envelope patterns of the composite optical modes that arise when WG modes of different wavelengths coincide in frequency. Numerically, we calculate crossings of levels that correspond with the observed degenerate modes, where our method takes into account the not perfectly transverse nature of their field polarizations. In addition, we analyze anticrossing with a large avoidance gap between modes of the same azimuthal number.

"Perturbative analytic theory of an ultrahigh-Q toroidal microcavity"

     Bumki Min,, Lan Yang, and  Kerry Vahala

     Phy. Rev. A. Vol. 76, p013823 (2007).

A perturbation theoretic approach is proposed as an efficient characterization tool for a tapered fiber coupled ultrahigh-quality factor (Q) toroidal microcavity with a small inverse aspect ratio. The Helmholtz equation with an assumption of quasi-TE/TM modes in local toroidal coordinates is solved via a power series expansion in terms of the inverse aspect ratio and the expanded eigenmode solutions are further manipulated iteratively to generate various characteristic metrics of the ultrahigh-Q toroidal microcavity coupled to a tapered fiber waveguide. Resonance wavelengths, free spectral ranges, cavity mode volumes, phase-matching conditions, and radiative Q factors are derived along with a mode characterization given by a characteristic equation. Calculated results are in excellent agreement with full vectorial finite-element simulations. The results are useful as a shortcut to avoid full numerical simulation, and also render intuitive insight into the modal properties of toroidal microcavities. The American Physical Society.

"Ultralow-threshold Yb³⁺:SiO₂ glass laser fabricated by the solgel process"

     Eric Ostby, Lan Yang, and  Kerry Vahala

     Opt. Lett. Vol. 32, p 2650 (2007).

A Yb-doped silica microcavity laser on a silicon chip is fabricated from a solgel thin film. The high-Q microtoroid cavity, which has a finesse of 10,000, is evanescently coupled to an optical fiber taper. We report a threshold of 1.8 μW absorbed power that is, to the best of our knowledge, the lowest published threshold to date for any Yb-doped laser. The effect of Yb³⁺ concentration on laser threshold is experimentally quantified. 2007 Optical Society of America

"Ultralow threshold on-chip microcavity nanocrystal quantum dot lasers"

     Zhaoyu Zhang, Lan Yang, Victor Liu, Ting Hong, Kerry Vahala, Axel Scherer

     App. Phy. Lett. Vol. 90, p 111118 (2007).

We describe the performance of submicron microdisk lasers fabricated within InGaP/InGaAlP quantum well material working at room temperature. The smallest lasers, with diameters of approximately 600  nm, feature ultrasmall mode volumes and exhibit single mode operation at low threshold powers. Their small cavity volumes of approximately 0.03  um³ enable microdisk lasers to be used as spectroscopic sources. Here the we demonstrate the fabrication and characterization of visible, monolithically fabricated, submicron microdisk lasers. 2007 American Institute of Physics

 "Ultralow threshold on-chip microcavity nanocrystal quantum dot lasers"

   Bumki Min, Sungjee Kim, Koichi Okamoto, Lan Yang, Axel Scherer, Harry Atwater and Kerry Vahala

    App. Phy. Lett. Vol. 89, p 191124 (2006).

Chemically synthesized nanocrystal, CdSe/ZnS (core/shell), quantum dots are coated on the surface of an ultrahigh-Q toroidal microcavity and the lasing is observed at room and liquid nitrogen temperature by pulsed excitation of quantum dots, either through tapered fiber or free space. Use of a tapered fiber coupling substantially lowered the threshold energy when compared with the case of free space excitation. The reason for the threshold reduction is attributed to the efficient delivery of pump pulses to the active gain region of the toroidal microcavity. Further threshold reduction was possible by quantum dot surface-coverage control. By decreasing the quantum dot numbers on the surface of the cavity, the threshold energy is further decreased down to 9.9 fJ.  2006 American Institute of Physics.

"Transmission characteristics of a Fabry-Perot etalon-microtoroid resonator coupled systme"

    Wei Liang, Lan Yang, Joyce Poon, Yanyi Huang, Kerry J. Vahala, and Amnon Yariv

    Optics Lett. Vol. 31 (2006).

The transmission spectra of a Fabry-Perot etalon coupled to a microtoroid resonator are studied theoretically and experimentally. The resonance line shapes depend strongly on the resonance wavelength detuning and coupling strength between the two resonators. A wide variety of line shapes, ranging from a single to triple peaks, symmetric to asymmetric Fano-like peaks, and notches were predicted and observed experimentally. The capability to modify the spectral line shapes by tuning the coupling between or losses of two resonators may find applications in optical filtering, switching, sensing, and dispersion engineering.    2006 Optical Society of America.

"Controlled transition between parametric and Raman oscillations in ultra-high-Q silica toroidal microcavities"

    Bumki. Min, Lan Yang, and Kerry J. Vahala

    App. Phy. Lett., Vol. 87, p 181109 (2005).

A controllable and reversible transition between parametric and Raman oscillations in an ultrahigh-Q silica toroidal microcavity is experimentally demonstrated and theoretically analyzed. By direct change of cavity loading and indirect adjustment of frequency detuning, parametric and/or Raman oscillation can be accessed selectively without modification of cavity geometry in a toroidal microcavity with a large enough aspect ratio. Based on an effective cavity gain theory, this transition is analyzed in terms of cavity loading and frequency detuning leading to a better understanding of the combined effects of parametric and Raman processes in silica microcavities.  2005 American Institute of Physics.

"Erbium-doped and Raman microlasers on a silicon chip fabricated by the sol-gel process"

       Lan Yang, Tal Carmon, B. Min, S. M. Spillane, and Kerry J. Vahala

      App. Phy. Lett., Vol. 86, p 091114, 2005.

We report high-Q sol-gel microresonators on silicon chips, fabricated directly from a sol-gel layer deposited onto a silicon substrate. Quality factors as high as 2.5 X 10(7) at 1561 nm were obtained in toroidal microcavities formed of silica sol-gel, which allowed Raman lasing at absorbed pump powers below 1 mW. Additionally, Er3+-doped microlasers were fabricated from Er3+-doped sol-gel layers with control of the laser dynamics possible by varying the erbium concentration of the starting sol-gel material. Continuous lasing with a threshold of 660 dW for erbium-doped microlaser was also obtained.  2005 American Institute of Physics.

"Feedback control of ultra-high-Q microcavities: application to micro-Raman lasers and microparametric oscillators"

    Tal Carmon, T. J. Kippenberg, Lan Yang, H. Rokhsari, S. M. Spillane, and Kerry J. Vahala

    Opt. Express, Vol. 13, No. 9, p 3558, 2005.

We demonstrate locking of an on-chip, high-Q toroidal-cavity to a pump laser using two, distinct methods: coupled power stabilization and wavelength locking of pump laser to the microcavity. In addition to improvements in operation of previously demonstrated micro-Raman and micro-OPO lasers, these techniques have enabled observation of a continuous, cascaded nonlinear process in which photons generated by optical parametric oscillations (OPO) function as a pump for Raman lasing. Dynamical behavior of the feedback control systems is also shown including the interplay between the control loop and the thermal nonlinearity. The demonstrated stabilization loop is essential for studying generation of nonclassical states using a microcavity optical parametric oscillator. 2005 Optical Society of America.

"Temporal behavior of radiation-pressure-induced RF-oscillation of an optical microcavity photon mode"

   Tal Carmon, H. Rokhsari, Lan Yang, T. J. Kippenberg, and Kerry J. Vahala

   Phys. Rev. Lett., Vol. 94, p 223902, 2005.

We analyze experimentally and theoretically mechanical oscillation within an optical cavity stimulated by the pressure of circulating optical radiation. The resulting radio frequency cavity vibrations (phonon mode) cause modulation of the incident, continuous-wave (cw) input pump beam. Furthermore, with increasing cw pump power, an evolution from sinusoidal modulation to random oscillations is observed in the pump power coupled from the resonator. The temporal evolution with pump power is studied, and agreement was found with theory. In addition to applications in quantum optomechanics, the present work suggests that radiation-pressure-induced effects can establish a practical limit for the miniaturization of optical silica microcavities.

"Dynamical thermal behavior and thermal self-stability of microcavities"

     Tal Carmon, Lan Yang, and Kerry J. Vahala

     Opt. Express, Vol. 12, No. 20, P 4742, 2004.

As stability and continuous operation are important for almost any use of a microcavity, we demonstrate here experimentally and theoretically a self-stable equilibrium solution for a pump-microcavity system. In this stable equilibrium, intensity- and wavelength-perturbations cause a small thermal resonant-drift that is enough to compensate for the perturbation (noises); consequently the cavity stays warm and loaded as perturbations are self compensated. We also compare here, our theoretical prediction for the thermal line broadening (and for the wavelength hysteretic response) to experimental results.  2004 Optical Society of America.

"Replica-molded high-Q polmer microresonators"

       Andrea Martin, Deniz Armani, Lan Yang and Kerry J. Vahala

       Opt. Lett., Vol. 29, No. 6, p533, 2004.

Ultrahigh-Q microtoroids on a chip are applied as replication masters to demonstrate replica-molded high-Q microresonator arrays. Replica Q factors are nearly material loss limited, affirming the integrity of the replication process, and are as high as 5 X 10(6), or nearly a factor of 40 greater than previous polymer-based devices. Because the molding process is nondestructive, both the master and the molds can be reused. Additionally, by using a novel optical polymer (Vicast), we demonstrate storage of high-Q microresonators in the mold for weeks, providing a method to preserve the whispering-gallery Q factor.  2004 Optical Society of America.

"Erbium-implanted High-Q silica toroidal microcavity laser on a silicon chip"

     Bumki Min, T. J. Kippenberg, Lan Yang, K. J. Vahala, J. Kalkman, A. Polman

     Phy. Rev. A., Vol. 70, No. 3, p033803, 2004.

Lasing from an erbium-doped high-Q silica toroidal microcavity coupled to a tapered optical fiber is demonstrated and analyzed. Average erbium ion concentrations were in the range 0.009-0.09 at. %, and a threshold power as low as 4.5 muW and an output lasing power as high as 39.4 muW are obtained from toroidal cavities with major diameters in the range 25-80 mum. Controlling lasing wavelength in a discrete way at each whispering-gallery mode was possible by changing the cavity loading, i.e., the distance between the tapered optical fiber and the microcavity. Analytic formulas predicting threshold power and differential slope efficiency are derived and their dependence on cavity loading, erbium ion concentration, and Q factor is analyzed. It is shown that the experimental results are in good agreement with the derived formulas.

 "Fiber-coupled Erbium microlasers on a chip"

       Lan Yang, Deniz Armani and Kerry J. Vahala

      App. Phys. Lett., Vol. 83, No. 5, p 825, 2003.

An erbium-doped, toroid-shaped microlaser fabricated on a silicon chip is described and characterized. Erbium-doped sol-gel films are applied to the surface of a silica toroidal microresonator to create the microcavity lasers. Highly confined whispering gallery modes make possible single-mode and ultralow threshold microlasers.  2003 American Institute of Physics.

"Gain functionalization of a silica microresonators"

      Lan Yang, and Kerry J. Vahala

      Opt. Lett., Vol. 24, No. 8, p592, 2003.

Erbium-doped solgel films are applied to the surface of silica microspheres to create low-threshold microcavity lasers. This gain functionalization can be applied by use of a number of different dopants, thereby extending the wavelength range of this class of device. Also, by varying the doping concentration and thickness of the applied solgel layer, one can vary the laser dynamics so that both continuous-wave, and pulsating modes of operation are possible.  2003 Optical Society of America.