Two-Dimensional Photonic Crystal Defect Lasers by murplelake80

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									Two-Dimensional Photonic Crystal
        Defect Lasers



               Po-Tsung Lee
        National Chiao Tung University
    Institute of Electro-Optical Engineering




                  Sep. 4, 2003
                     Outline
• Introduction of Photonic Crystals
      ♦ Two-Dimensional Photonic Crystals
      ♦ Photonic Band Structures & Bandgap
• Defect Laser Cavity Structure
      ♦ Microcavity & epitaxial structures
      ♦ Defect cavity resonant modes
• Fabrication
• Characterization
      ♦ Pulsed lasing at and above room temperature
      ♦ Optical losses investigation
      ♦ Lithographic Tuning
      ♦ Thermal analysis – towards CW operation
• Conclusion
Photonic Crystals
Two-Dimensional Photonic Crystals

         1     H r  ω2 H r
    ∇ ×  r ∇ × ( ) = 2 ( )
        ε ( )       c


                Air Hole
                                      ky
y                High Index
                                  Κ
                              Μ
                  Material
                                           kx
                                      Γ
      x
   Band Structures for Two-Dimensional
            Photonic Crystals

• Triangular lattice: air holes in a dielectric substrate
• Plane-wave expansion method: 225 plane waves used

       TE modes                            TM modes
Two-Dimensional Photonic Crystal Slab



                z
                    y




                        x

          d
        Band Structure for Two-Dimensional
              Photonic Crystal Slab *
            For even modes: TE polarized at the mid-plane of the slab


                                  leaky modes




                                                      guided modes
                                                                         light line
                                                 photonic band gap
        ky
    Κ
Μ
                    kx
        Γ


                         * provided by Woo Jun Kim using finite element calculation
       Microcavity Structure Illustration
  • In-plane localization: 2D photonic crystal
  • Vertical confinement: a thin InGaAsP slab waveguide


                                    2D photonic crystal
High-index slab
                                             Defect cavity
              Epitaxial Structure

Electric Field Profile              60nm InP cap

                             57.5nm unstrained InGaAsP
                            4 compressively strained
                                  InGaAsP QWs
                               (Eλ~1.55µm) with
             224nm           3 unstrained InGaAsP
            membrane          barriers (Eg~1.22µm)

                             57.5nm unstrained InGaAsP


                               InP buffer & substrate
   3D FDTD Simulation *



Amplitude Plot of Hz Component of the EM field




                                                 * provided by Wan Kuang
                   Fabrication
   Nanometer scale fabrication technology


PMMA PMMA               • Deposit mask: SiNx, Cr, Au
Cr/Au Cr/Au             • Spin coat PMMA
SiNx   SiNx
  Epitaxial layers
                        • Electron beam lithography
   InP substrate        • Ar+ milling
                        • CF4 RIE etch
                   Fabrication


Cr/Au                            • CH4/H2/Ar ECR etch
SiNx
Epitaxial               layers
                                  • Chemical etch:
        InP substrate            HCl:H2O = 4:1 @ 00C
     Fabrication
1. Electron beam lithography
2.   Ar+ milling
3.   CF4 RIE etch
4.   CH4/H2/Ar ECR etch
After Selective Wet Etch of InP
                  HCl chemical etching of InP

                                                   (100) InP

                                                        (100)


                                                                 (01-1)
                                                   (0-1-1)



                                                   θ1 = 54.7¢X
                                                   θ2 = 90¢X (92~95¢X )
                                                   θ3 = 95¢X

From L. A. Coldren, K. Furuya, and B. I. Miller,   θ4 = 140¢X or 40¢X
    J. Electrochem. Soc. 130, 1918 (1983).
Photonic Crystal Suspended Membrane

                  V-shaped undercut trench
                  passing through an array
                  of cavities along <0,1,-1>

             _
         <011>
 Photonic Crystal Defect Cavity
• Hexagonal defect cavity: 19 missing holes



                                          _
                                      <011>
            Photonic Crystal Lasers
♦ Below threshold: many resonant modes
♦ Above threshold: side-mode suppression ratio = 20dB

         lasing mode above threshold (λ=1617nm)

                           Spectrum below threshold
       Photonic Crystal Lasers
• Room temperature operation under pulse excitation
• Optical pump source: 860nm top-emitting VCSEL
• Threshold incident pump power = 2.4 mW

lasing wavelength = 1647nm
♦ Pulse repetition rate of the
pumping VCSEL = 0.5 MHz
        Photonic Crystal Lasers


• Lasing observed for “a” between 490 and 560 nm &
“r/a” between 0.21 and 0.28

• Lowest threshold pump power obtained: 0.5 mW
(the corresponding quality factor Q = 1200)

• Pulse widths up to 200 ns (5 % duty cycle)
                         Scaling Property
                              ω 
                                      2
           1
        ∇×
           ε (r ) ∇ × H (r ) =   H (r )
                             
                             c
                                                                    1
                                                             ∇' =       ∇
                                                                    α
                         Scaling factor α
                                                             ε ' (r ) = ε (α r )
                               r' = α r
                                                             H ' (r ) = H (α r )

                          α ω               ω' 
                                 2                2
   1
∇×
   ε ' (r ) ∇ × H ' (r ) = 
                                 H ' (r ) =   H ' (r )
                            c             c                           ω '= α ω
       Multi-Wavelength Arrays
 31 laser cavities with lattice constants
varied from 490 nm to 550 nm (r/a fixed)




                          2 nm increments in lattice
                          constants between adjacent
                          cavities
 Lithographic Tuning of Wavelength

• a = 498 ~ 514 nm
• Average wavelength   • One mode is tuned over 80 nm
spacing is 4.6 nm
Laser Array with Varying Number of
      Photonic Crystal Periods
• The number of lattice periods varied along y direction
in increments of 2 periods between 1 and 11 periods.


                                  5 periods


             y

                                  7 periods


                       x


                                  9 periods
Input Power vs. Output Power Characteristics

          11 periods:                 For 9 periods:
          λ   lasing   = 1550 nm      λ   lasing   = 1550 nm
          Pth = 1.53 mW               Pth = 2.74 mW




          For 7 periods:              For 5 periods:
          λ   lasing   = 1473 nm      λlasing = 1458 nm
          Pth = 4.77 mW               Pth = 6.12 mW
Estimation of the Trend of the
    Cavity Quality Factor Q


                   g p (n ) = g 0[1 + λn( )]
                                         n
                                         n0

                          Q = ωτp

                           1     2πn 1
                  Γgth =      =(    )( )
                         υgτp     λ Q
     Losses Investigated by Varying Number
           of Photonic Crystal Periods

                                      Ispon ∝ B × n × V
                                                      2

Itotal = Ispon + Iauger + Isr         Iauger ∝ C × n3 × V
                                      Isr ∝ υs × n × A

          Threshold    Contribution    Contribution
            pump        from Auger     from surface
           power      recombination   recombination
           0.5 mW         23 %            45 %
          1.53 mW         42 %            26 %
          2.74 mW         51 %            19 %
          4.77 mW         59 %            13 %
          6.12 mW         62 %            12 %
     Lasing Above Room Temperature
• Lasing observed for substrate temperatures up to 500C



                                    dλ          dL      dn
                                         dT =    dT +    dT
                                       λ         L       n


                                   ♦ Thermal expansion of
                                   the photonic crystal lattice
                                   ♦ Refractive index change
                                   of the semiconductor
Lasing Above Room Temperature

           • Optical pump condition:
             20 ns pulse width (1% duty cycle)
           • T0 = 37.7 K

               Substrate Threshold incident
             temperature   pump power
                 200C           3.2mW
                 350C           5.3mW
                 500C           7.4mW
       Suspended Membrane Structure

• Under typical pulsed                 • Temperature increase ~ 110 K
optical pumping conditions


           Finite Element Simulation
       Placing the two-dimensional photonic
       crystal membrane on sapphire should
          improve the laser performance


Two-dimensional
photonic crystal
membrane




     Sapphire
     substrate
                 Towards CW Operation:
                 On a Sapphire Substrate
• Under typical pulsed optical   • Under CW optical pumping
pumping conditions               conditions
• Temperature increase ~ 4 K     • Temperature increase ~ 120 K
             Band Structure for Two-Dimensional
             Photonic Crystal Slab on Sapphire *



                              leaky modes

                                                                 light line




        ky
    Κ
Μ
                 kx
        Γ
                      * provided by Woo Jun Kim using finite element calculation
                 Microdisk CW Operation
                 On a Sapphire Substrate

                                        • Lasing wavelength = 1507 nm
                                        • Threshold pump power = 0.5 mW




                               Output Power (a.u.)
Microdisk bonded to sapphire
        • CW lasing




                                                     CW Input Power (mW)
                       Conclusion

• Realization of VCSEL-pumped photonic crystal lasers at
and above room temperature
   ♦ Standard photonic crystal fabrication procedure is developed
   ♦ Pulse widths up to 200 ns (5 % duty cycle)
   ♦ Q as high as 1200 (0.5 mW threshold pump power)
   ♦ Substrate temperatures up to 50 0C
   ♦ Wavelength tuning by lithographically varying the lattice
   constant

   ♦ Losses study by varying the number of lattice periods

								
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