Terahertz Emitter Modeling

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Terahertz Emitter Modeling

• Jacob Gagnon, MIT

Advisor Prof. Ram-Mohan (WPI, QSA) Mentor Dr.
Paul Sotirelis (HPC) Partner Jonathan Moussa
(WPI)
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The Terahertz Project Overview (2-3 yrs)

• Terahertz emission (UMass, Italy) solid state
  device emitting radiation at terahertz
  frequencies in all directions
• Optimize emitter design for specific applications
• Resonant Cavity Focuses the radiation, waves
  reinforces each other for higher output
• Detection

Me
UMass
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Why Build a Terahertz Laser?

• Pathogen Detection (shine laser, excites bonds
  into high energy vibrational and rotational
  states, detection)
• Communications
• Airport Security (clothes, cardboard)
• Medical Applications (tooth decay)

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My Tasks for the Summer

• Matrix library conversion
• Physics of terahertz lasers and emitters
• Derivation of governing equations
• Computer model of a terahertz emitter
• Optimal emitter design for specific applications

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How do you Build a Terahertz Emitter?
AlGaAs
GaAs
AlGaAs
GaAs
AlGaAs
Electrons are free in two of the dimensions

• Thickness of layer -gt thickness of well
• Material -gt barrier and well heights
• Wave Function and Energy Level Engineering
• FEM Generality

2D
In 2D and 3D, energy levels arent flat, instead
electrons fill up the energy states of the
parabola.
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Emitter Design Ideal Case

• 3 well, Multi stage (3 wells repeated), device
  under voltage
• Current injected to the left
• Self-consistency (e-e interaction, doping)

• Operation of emitter
• 3 to 2 photon emission each stage
• 2 to 1 interface phonon
• Tunneling to next stage (1 to 3)

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Emitter Design Collision Processes

• e-e scattering
• Confined phonons
• Interface phonons
• Photon emission (an electron emitting a photon)
  and gain (total of photons emitted in the
  system minus total absorbed)
• Bulk optical phonons (vibrations of the system as
  a whole colliding with an electron)

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Lifetime calculation
What is e-e Scattering?
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1
2
1

• The of scattering events per second
• Find matrix element ( of scattering events with
  specific momentum and spin)
• Apply Fermi blocking effects (likelihood state is
  free vs occupied)
• Sum over all momentum, spin, location
• Apply energy conservation
• Apply momentum conversation
• 7 dimensional integral

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e-e Results
S(kI 0)
n1 1011, n2 x 1011
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What is Confined Phonon Scattering?
AlGaAs
GaAs
AlGaAs
GaAs
AlGaAs

• Sinusoidal lattice vibrations confined in a layer
  colliding with an electron
• Find of scattering events per second
• Compared results (Rudin and mine)
• Use of infinite barrier (kI 0)

Width of layer
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Interface Scattering

• Find of scattering events per second
• Decaying and growing oscillations emanating from
  interface
• What an interface mode looks like

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Rate Equations
of scattering events for e-e, confined phonons,
interface phonons, and photons
Gain
Injector current
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Optimization

• Constrained multiple objective genetic algorithm
• Maximize gain, minimize threshold current,
  minimize e-e collisions
• Pareto dominance
• Initial population of individuals
• Crossover, mutation, mating, niches
• Iterate till convergence

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Future directions

• Other physical processes
• Optimization for specific applications
• Resonator design
• Detector
• Parallelization
• Matrix library (matrix operations)
• Numerical Integration (break up loops)