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Broadband quantum cascade lasers and superluminescence QCLEDs

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Broadband quantum cascade lasers and superluminescence QCLEDs Powered By Docstoc
					Broadband quantum cascade lasers
   and superluminescence LEDs
     Wing Ng1, Evgeny Zibik1, Luke Wilson1,
Mark Hopkinson2, Kristian Groom2, John Cockburn1

1Deptof Physics and Astronomy, University of Sheffield, UK
  2 National Centre for III-V Technologies, Sheffield, UK


                       MIOMD-VII Lancaster
                     12th-14th September 2005
                Outline
•   Introduction
•   Sample design
•   Laser characterisation
•   Superluminescence QCLED
•   Summary




Wing Ng          MIOMD-VII Lancaster      2
               12th-14th September 2005
                Introduction
• Develop broadband mid-IR source for medical
   imaging applications.
• Species specific optical coherence tomography
   for tissue engineering
• In collaboration with the medical physics group
   at UCL, UK
• Need short coherence length to give higher
   resolution → Broadband source
• Two approaches:
   i) Multi-wavelengths QCL
  ii) Superluminescence QCLED

Wing Ng             MIOMD-VII Lancaster             3
                  12th-14th September 2005
             Broadband QCL
• Emission wavelengths can be tailored by
  band-structure engineering
• For intersubband transitions, emissions from
  different wavelengths cannot be absorbed by
  another active region

  Interband transitions          Intersubband transitions




                                               E
Wing Ng               MIOMD-VII Lancaster
                    12th-14th September 2005       kx,y     4
                                                  Sample design
•     QCL with 36 active regions
      designed to emit from 6 µm to
      8µm.                                                                            59 kV/cm
•     AlInAs/InGaAs lattice matched
      to InP grown by MBE
•     11 different wavelength active                                                             7.09 µm
                                                                                                 active region
      regions within the single                                  3'
      structure
                                                                 2'
                      8.0                                        1'
    Wavelength (µm)




                                                                                                                     3
                      7.5

                      7.0                                             6.9 µm                                         2
                                                                      active region   Injector                       1
                      6.5

                      6.0
                            0   5   10 15 20 25 30 35 40
                                    Active region index
      Wing Ng                                               MIOMD-VII Lancaster                                  5
                                                          12th-14th September 2005
                                                       Low temperature operation
                 8
                                                                              4.25A, 5x Ith                                      16
                 7
                                                                               3.4A, 4x Ith                                      14                                  5xIth
                 6
Intensity (au)




                                                                                                                                 12




                                                                                                         Lasing intensity (au)
                 5                                                             2.55A, 3x Ith
                                                                                                                                 10
                 4
                 3                                                             1.7A, 2x Ith                                      8

                 2                                                                                                               6
                                                                              0.94A, 1.1x Ith
                 1                                                                                                               4
                                                                              0.8A, sub-threshold
                 0                                                                                                               2
                                         6.0    6.5     7.0     7.5     8.0     8.5             9.0
                                                                                                                                 0
                                                      Wavelength (µm)                                                                 6.0   6.5    7.0         7.5      8.0
                                         14
                                                                                                                                             Wavelength (µm)
                                                                                      16
                                         12

                                                                                                         • Jth ~1.5kA/cm2 at 80K
                     Light output (au)




                                         10                                           14
                                                                                           Voltage (V)




                                         8
                                                                                                         • Very broad
                                                                                      12
                                         6
                                                                                      10
                                         4
                                                                                      8
                                                                                                           spontaneous
                                         2

                                         0                                           6
                                                                                                           emission spectrum
                                          0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

                 Wing Ng                                 Current (A)              MIOMD-VII Lancaster                                                                         6
                                                                                12th-14th September 2005
                             Room temperature operation
                                                                                                                            2
                                                                 (24µm x 2mm ridge): Ith~1.8A, Jth~3.8kA/cm
Light output (au)




                                                                                                           3A




                                                             Intensity (au)
                                                                                                            2.2A



                                                                                                      2A

                    0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0                       6.8   7.0   7.2   7.4   7.6       7.8   8.0
                                 Current (A)                                          Wavelength (µm)

        • Jth at room temperature (3.8kA/cm2) is very comparable
          with our single wavelength lasers (Jth~2.9kA/cm2)
        • At 2.2A, lasing peaks are observed at ~6.9, 7.2, 7.45 and
          7.6µm.
       Wing Ng                                   MIOMD-VII Lancaster                                                    7
                                               12th-14th September 2005
    Superluminescence QCLEDs
                                            Wet etched facet
• Same material as used
  in making lasers
• Wet etched one facet
  and one mirror like
  cleaved facet
• The wet etched facet
  reduce the optical
  feedback and the device
  gives out amplified
  spontaneous emission Cleaved facet
 Wing Ng           MIOMD-VII Lancaster                    8
                 12th-14th September 2005
                                Light-current (LI) characteristics
                                                                                     70
                    16




                                                           Optical Peak Power (µW)
                              Laser                                                  60                                        15
                    14
                              LED sf 1.7mm                                           50                            10K
Light output (au)




                    12
                    10                                                               40                                        10




                                                                                                                                    Bias (V)
                    8                                                                30
                    6                                                                                              LED sf 1mm
                                                                                     20
                                                                                                                   LED sf 1.7mm 5
                    4                                                                                              LED sf 2.5mm
                                                                                     10
                    2
                                                                                     0
                    0                                                                     0   2000   4000   6000     8000 10000
                         0   2000 4000 6000 8000 10000                                                                2
                                                   2
                               Curent density (A/cm )                                          Current Density (A/cm )


• Superlinear characteristics in LI curves
• Longer devices gives higher output power
• Peak power ~40µW for the 2.5mm device
Wing Ng                                           MIOMD-VII Lancaster                                                           9
                                                12th-14th September 2005
                                       LI characteristics
                                                                                             VN389 single facet
                  50
                           VN389 single facet                                      0.3       XL ridge, 1.7mm-long




                                                              Average Power (µW)
                           XL ridge, 1.7mm-long                                              T= 10K, 25 kHz
                  40       100ns, 50 kHz                                                     Pulse width:
Peak power (µW)




                           Temperature:                                            0.2           100ns
                  30           10 K                                                              200ns
                               80 K                                                              300ns
                  20           120 K                                                             400ns
                               150 K
                                                                                   0.1           500ns
                  10

                   0                                                               0.0
                       0    1     2    3      4    5     6                               0    1     2    3      4   5    6
                                   Current (A)                                                       Current (A)

                  • Peak power of ~10µW at 150K
                  • Increasing average power as duty cycle
                    increases
                  Wing Ng                           MIOMD-VII Lancaster                                             10
                                                  12th-14th September 2005
                                           SLED spectra
                       Vn389 XL (57µm x 1.7mm) 10K spectra                              Vn389 XL (57µm x 1.7mm) 10K spectra
                 1.6   with 100ns 25kHz pulses                                               with 300ns 25kHz pulses
                                                                                1.2
                 1.4                              2A                            1.0
                 1.2
Intensity (au)




                                                               Intensity (au)
                                                  3A
                 1.0           (/20)              4A                            0.8                                     2A
                                                                                                                        3A
                 0.8                              5A                            0.6                                     4A
                 0.6                              5.3A
                                                                                0.4                                     5A
                 0.4
                                                                                0.2
                 0.2
                 0.0                                                            0.0
                       5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0                                     5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5
                              Wavelength (µm)                                                      Wavelength (µm)

                 • Emissions are observed from 6                                                  2
                   to 8µm for the 100ns 25kHz

                                                                                      FWHM (µm)
                   pulses and from 6 to 8.5µm for
                   300ns 25kHz pulses                                                             1
                                                                                                          100ns
                 • Continuous emission from 5.7                                                           300ns
                   to 8.5µm                                                                       0
                                                                                                      0   2        4         6
                   Wing Ng                          MIOMD-VII Lancaster                                                          11
                                                                                                          Current (A)
                                                  12th-14th September 2005
                                  Coherence length
                               Single faset (XL):
                 0.4
                              ~15µm
                                   50ns pulse, 50KHz, 5.5A
                                                                  • Information about the
                 0.3
                                      Coherence length ~30µm        coherence length can
Intensity (au)




                 0.2
                 0.1
                                                                    be obtained from the
                 0.0
                                                                    interferogram
                 -0.1                                             • Coherence length can
                 -0.2
                                                                    also be estimated
                 -0.3
                    2300    2350         2400          2450         from the spectrum
                           Mirror position (µm)
                                                                    ~20µm

                    Lc= (2ln2/π)(λ2/∆λ)
                 Wing Ng                            MIOMD-VII Lancaster               12
                                                  12th-14th September 2005
                 Summary
• Broadband laser sample:
  - Room temperature operation at 4 wavelengths
  - Emits continuously between 6 to 6.8µm at low
  temperature
• Single cleaved facet devices
  -Show superluminescence characteristics
  -Broad emission spectra from 6 to 8µm
• Coherence length was estimated to be ~30µm
  for SLED
Wing Ng             MIOMD-VII Lancaster        13
                  12th-14th September 2005

				
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