Larger-Area Single-Mode Photonic Crystal Surface-Emitting Lasers Enabled by the Accidental Dirac-Point

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 FIG. 1A is a three-dimensional plot of an energy band structure for a two-dimensional photonic crystal characterized by a Dirac cone.  FIG. 1B is a three-dimensional plot of an energy band structure for a two-dimensional photonic crystal characterized by dispersion relations approaching a Dirac point.  FIG. 1C is a three-dimensional plot of an energy band structure for a two-dimensional photonic crystal characterized by a pair of energy bands with quadratic dispersion. FIG. 2 is a diagram of a surface-emitting laser that includes a photonic crystal whose band structure exhibits a Dirac point at or near the center of the Brillouin zone.  FIG. 3 is a diagram of the photonic crystal in the laser of FIG. 2 showing directions in which the electromagnetic wave propagates (within the plane of the photonic crystal; solid lines) and in which energy radiates (out of the plane of the photonic crystal; dashed line). FIG. 4A is a plot of a transverse-magnetic (TM) photonic band structure of a triangular array of dielectric rods (∈rod=12.5) in a high-contrast background material (∈bg=1).  FIG. 4B is a plot of a TM photonic band structure of a triangular array of dielectric rods (∈rod=12.5) in a low-contrast background material (∈bg=11).  FIG. 4C is a plot of the density of states (DOS) of the triangular array of dielectric rods (∈rod=12.5) in a high-contrast background material (∈bg=1). FIG. 5A is diagram of a finite-sized photonic crystal cavity that includes a triangular array of dielectric rods (∈rod=12:5) embedded in air with a period a and a dimension L=40a.  FIG. 5B is a plot of high out-of-plane quality factor band dispersions near Γ of three photonic crystals with different rod radii; the upper band has a linear dispersion and the lower bands have quadratic dispersions.  FIG. 5C is a plot of band-edge modes of a finite-sized photonic crystal cavity of L=300a with linear dispersion.  FIG. 5D is a plot of band-edge modes of a finite-sized photonic crystal cavity of L=40a with quadratic dispersion, with insets that show the mode profiles with the electric field pointing into the page.  FIG. 5E is a plot of analytical determinations (lines) and finite-difference time-domain (FDTD) calculations (circles) of the first band-edge mode spacing as a function of the cavity area for the photonic crystal cavities associated with the band dispersions plotted in FIG. 5B.  FIG. 5F is a plot of curve-fit determinations (lines) and FDTD calculations (circles) of the in-plane quality factor of the band-edge mode as a function of the cavity area for the photonic crystal cavities associated with the band dispersions plotted in FIG. 5B. FIG. 6A shows a unit-cell of a 0.3a thick, one-dimensional photonic crystal slab that includes alternating high (∈high) and low dielectric constant (∈low) materials whose width is tuned so that the bands are accidentally degenerate at Γ.  FIG. 6B is a plot of the band structure of the photonic crystal slab in FIG. 6A with ∈high=12.5 and ∈low=6.25.  FIG. 6C is a plot of out-of-plane quality factor versus wave vector for the photonic crystal slab in FIG. 6A with ∈high=12.5 and ∈low=6.25.  FIG. 6D is a plot of the band structure of the photonic crystal slab in FIG. 6A with ∈high=12.5 and ∈low=11. FIG. 7A illustrates a finite-sized, 0.3a thick, one-dimensional photonic crystal slab of dimension Lx comprising alternating layers of high (∈high=12.5) and low (∈low=6.25) dielectric constant materials.  FIG. 7B is a plot of the mode spacing between the first and second band-edge modes of the photonic crystal shown in FIG. 7A as a function of the photonic crystal slab dimension Lx.  FIG. 7C is a plot of in-plane quality factor (left axis) and out-of-plane quality factor (right axis) of the first band-edge mode of the photonic crystal shown in FIG. 7A as a function of the photonic crystal slab dimension Lx.  FIG. 7D is a plot of the total quality factor of the first and second band-edge modes of the photonic crystal shown in FIG. 7A as a function of the photonic crystal slab dimension Lx. FIG. 8A is a plot of the band structure (transverse electric (TE)-like modes near Γ) of a GaAs-based, two-dimensional photonic crystal slab on an AlAs (n=3) substrate.  FIG. 8B is a close-up of the plot shown in FIG. 8A.  FIG. 8C is a plot of out-of-plane quality factor versus wave vector for the photonic crystal slab whose band structure is plotted in FIG. 8B.
Categories
Inventors
Professor Marin Soljacic
Department of Physics, MIT
External Link (www.mit.edu)
Ling Lu
Materials Processing Center, MIT
External Link (www.mit.edu)
Song Liang Chua
Department of Electrical Engineering & Computer Science, MIT
Managed By
Jim Freedman
MIT Technology Licensing Officer - Chemicals, Instruments, Consumer Products
Patent Protection

Photonic crystal surface-emitting lasers enabled by an accidental dirac point

US Patent 8,902,946

Photonic crystal surface-emitting lasers enabled by an accidental dirac point

US Patent 9,065,249
Publications
Larger-area Single-mode Photonic Crystal Surface-emitting Lasers Enabled by an Accidental Dirac Point
Opt. Lett., Vol.39, p.2072, (2014)

Applications

Applications for this technology are currently found in telecommunications, spectroscopy, laser printing, biological tissue analysis, and meteorology.

Technology

This invention exhibits a photonic-crystal surface-emitting laser (PCSEL) with an accidental Dirac point. PCSELs include a gain medium electromagnetically coupled to a photonic crystal; however, in this invention, the energy band structure exhibits a Dirac cone of linear dispersion at the center of the photonic crystal's Brillouin zone. Because the Dirac point is at the Brillouin zone center, it is called an accidental Dirac point. This is of great importance because tuning the photonic crystal's band structure (e.g., by changing the photonic crystal's dimensions or refractive index) to exhibit an accidental Dirac point increases the photonic crystal's mode spacing by orders of magnitudes and reduces or eliminates the photonic crystal's distributed in-plane feedback. Thus, the photonic crystal can act as a resonator that supports single-mode output from the PCSEL over a larger area than is possible with conventional PCSELs, which have quadratic band edge dispersion. Because output power generally scales with output area, this increase in output area results in higher possible output powers.

Problem Addressed

Distributed feedback (DFB) lasers and vertical-cavity surface-emitting lasers (VCSELs) rely on one-dimensional feedback structures to provide relatively high-power, single-mode beams. Unfortunately, these lasers suffer from intrinsic drawbacks: DFB lasers and other edge-emitting sources tend to suffer from catastrophic optical damage at their facets, and the VCSELs' output powers are usually limited by their small cavity sizes.

On the other hand, conventional photonic-crystal surface-emitting lasers (PCSELs), have a higher functionality than the previously mentioned lasers.  However, the lasing areas of PCSELs are limited by two fundamental constraints. First, the mode spacing decreases as the cavity area increases, which promotes multi-mode lasing. Second, the distributed in-plane feedback localizes the lasing fields to individual sections, which promotes multi-area lasing. Since the output power scales with the lasing area, these constraints limit the maximum output power of a single-mode beam emitted by a PCSEL.

Advantages

  • Increased orders of magnitude in photonic crystal mode spacing
  • Reduction in the crystal's distributed in-plane feedback
  • Greater output power and output area