Temperature%20behaviour%20of%20threshold%20on%20broad%20area%20Quantum%20Dot-in-a-Well%20laser%20diodes

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Temperature behaviour of threshold on broad area
Quantum Dot-in-a-Well laser diodes
By Bhavin Bijlani
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Why use quantum dots?

• The gain of a laser active region, is
  proportional to its density-of-states function
  (DOS).
• In bulk (a), layered (b) and wire (c) materials,
  there are always states populated which do not
  contribute to gain. These are parasitic states
  and contribute to inefficiency.
• In quantum dot (d) materials, the DOS is a set of
  discrete states. Theory predicts this type of
  material is ideal for the gain region of a laser
  because fewer parasitic states are occupied.

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Ideal quantum dot lasers
From theory, it is predicted that using quantum
dots as a laser gain material has many beneficial
properties.

• If the energy separation between the ground and
  first excited state is large enough, then all the
  dots will have ground state population.
• Excited states are parasitic to ground state
  lasing. If an electron in an excited state emits
  radiatively, the photon would not be at the
  correct lasing frequency and would contribute to
  inefficiency.

Excited States
Ground State
Simplified Quantum Dot potential profile
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Ideal quantum dot lasers

• The threshold current is very low and wont vary
  with temperature because the excited state would
  not become populated. This is again assuming a
  large energy separation.
• The differential efficiency approaches the
  internal quantum efficiency as dot density
  increases. It is thus possible to have very high
  differential efficiency QD lasers.

Threshold Current
Slope is the differential efficiency
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Dot-in-a-well lasers

• For a quantum dot (QD) to capture an injected
  electron, the electron energy and confined state
  energy must be close to one another. Also, the
  spatial wavefunction of the electron must cover a
  significant portion of the dot. This is not
  always likely and causes typical QD lasers to
  deviate from the ideal.
• To remove this requirement, the concept of
  placing QDs within a quantum well (QW) was
  devised. The QW initially captures the electron,
  confining it within its boundaries. Then, the
  electron is captured and localized further by the
  QDs.

Example DWELL TEM image taken by a group at
University of Sheffield. These are InAs QDs in
InGaAs wells. Materials Science and Engineering C
25 (2005) 779 783
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Material and Band structure

• The lasers studied were Quantum-Dot-in-a-Well
  (DWELL) Broad area lasers. InAs quantum dots
  (QD) are placed within InAlGaAs quantum wells
  (QW), grown by molecular beam epitaxy onto InP.

Simplified layer profile
Simplified band structure
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Threshold characterization

• The temperature dependence of laser threshold
  between two temperatures is usually defined by
  the characteristic temperature, T0. This term is
  defined by the equation below.
• A larger T0 signifies a weak dependence of
  threshold on temperature. Conversely, a small T0
  signifies a strong variation of the threshold
  current with temperature.
• Typical InGaAsP quantum well lasers have room
  temperature (RT) T0 values around 60 K. GaAs
  quantum well lasers can have RT T0 values well
  over 100 K.

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Threshold characterization

• A pulsed current source drives the DWELL laser
  and simultaneously measures the power output. A
  temperature controller sets the temperature of a
  cooling chuck just below the laser while a
  computer collects the data.

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Characteristic Temperatures

• We have determined the temperature dependence of
  the laser threshold in the temperature range
  between 15 ºC and 40 ºC. The characteristic
  temperature, To, was determined for five cavity
  lengths ranging from 500 um to 2 mm.

Characteristic Temperature T0 (K) Characteristic Temperature T0 (K) Characteristic Temperature T0 (K) Characteristic Temperature T0 (K)
15 - 30 C 15 - 30 C 30 - 40 C 30 - 40 C
Cavity Length 1.0 µs pulses 0.5 µs pulses 1.0 µs pulses 0.5 µs pulses
0.50 mm 62.3 3.3 57.7 3.0 56.5 4.4 56.7 4.5
0.75 mm 60.2 3.1 60.0 3.1 56.2 4.3 57.1 4.7
1.00 mm 62.5 3.1 63.2 3.2 59.1 4.8 58.5 4.5
1.50 mm     58.1 1.7 ? from 15 to 40C 
2.00 mm 64.0 3.2 59.9 2.9 54.8 4.0 58.3 4.2
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Luminescence-current curves
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Threshold versus Temperature
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Summary

• We present the benefits of the Quantum-Dot-in-a-we
  ll structure as a coherent light source. By
  using InP as a substrate, long wavelength
  emission is possible (? 1.6 µm).
• The characterization of the threshold dependence
  on temperature reveals T0 values 60 K between
  15 C and 40 C.
• These values are close to performance of other
  long wavelength InP lasers.
• More spectroscopic studies of the dots and lasers
  are needed to refine the performance towards
  ideal behaviour.