Miniaturized solid state lasers are useful components for applications ranging from optical communication to on-chip spectroscopy. Desirable device characteristics include single mode lasing with low pump power, high speed direct modulation, high radiation efﬁciency, directional emission, and small footprint for dense on-chip integration. Lasers with small optical mode volume have the particular advantage of enhanced spontaneous emission which helps reduce lasing threshold and increases modulation speed. To this end, there has been much research effort to reduce the size of lasers to (sub)wavelength scale. Photon conﬁnement using periodic structures such as Bragg gratings and photonic crystals has received much attention due to their high cavity Quality factor. However, dielectric periodic structures necessarily require the device size to be several times the wavelength in one or more dimensions. Metaloptic and plasmonic cavities have recently been of particular interest. Due to their dispersive dielectric function whose real part is negative in the near-infrared (NIR) and visible spectrum, metals such as gold and silver can be used to overcome the diffraction limit and conﬁne electromagnetic energy to volumes much smaller than is possible in purely dielectric systems. Yet metals also present considerable optical loss that worsens as wavelength decreases from NIR to visible. As a result, most subwavelength metallic cavities have room temperature Q factors of below 100 and so can only lase in cryogenic temperatures.
We analyzed metal-clad disk cavities designed for nanolasers in the visible red spectrum (670 nm) with subwavelength device size and mode volume. Adding a metal cladding suppresses radiation loss and supports low order modes with room temperature Q of 200 to 300, making it possible to lase without the need for cryogenic cooling. Threshold gain calculations showed that room temperature lasing is possible using multiple GaInP/AlGaInP quantum wells as the gain medium. Placing a planar metal reﬂector under the cavity can enhance radiation and extraction efﬁciencies or increase the Q, without incurring additional metallic absorption loss. Non-degenerate single mode operation can be achieved by shrinking device size, retaining only the TE011 mode, and thereby increase the spontaneous emission coupling factor β. The TE011 mode can have a Q of 230, a mode volume of 0.004 µm2 with a 400x400 nm2 footprint. Its radiation efﬁciency can be tuned to as high as > 0.5 and the surface emission extraction efﬁciency to 0.2. We use ﬁnite-difference time-domain (FDTD) simulation to analyze the disk cavities’ relative loss mechanisms, namely material absorption and radiation loss, and evaluate spontaneous emission enhancement, radiation efﬁciency, and the threshold gain to assess the possibility of room temperature lasing. We showed that the far-ﬁeld radiation characteristics are strongly affected by the devices’ immediate surroundings, such as changes in metal cladding thickness, even as the resonant mode proﬁle, frequency, and Q remain the same. When the metal cladding is 1 mm thick, light radiates upward with a distinct intensity maximum at 45°; when the cladding is 100 nm thick, the emitted light spreads in a near-horizontal direction.
We expect similar techniques as these can be used to control the radiation of micro- and nanoscale laser cavities of other geometries. These properties need to be taken into account when designing isolated lasers with directional emission or a coupled laser array. The metal-clad disk laser resonators can sustain reasonable Q-factors while the dielectric disk is in contact with silver on all but the top surface; thus, they have a very effective heat sink and allow much freedom in designing electrical contact for current-injection operation. For example, the dielectric disk can be designed to have a vertical p-i-n doping proﬁle. Top and bottom electrical contact can be established using silver or ITO. Alternatively, a radial doping proﬁle can be used. The design proposed in this article presents useful devices for applications that require small, densely in integrated on-chip light source, such as telecommunication and lab-on-chip spectroscopy.
- Huang, J., Kim, S.-H. & Scherer, A. (2010). Design of a Surface-Emitting, Subwavelength Metal-Clad Disk Laser in the Visible Spectrum. Optics Express, 18(19), 19581.