Design for a tunable, largeangle beamsteering device

1
Design for a tunable, large-angle beamsteering
device

• Peter Bermel
• Advisor J.D. Joannopoulos
• Collaborators Charles Tapalian, Jason Langseth

2
Outline

• Introduction
• Refractive index modulation
• Slow light
• Design for enhancing phase modulation
• Beam-steering applications
• Future directions
• Conclusions

3
Refractive index modulation

• Definition changing refractive index of material
  in controlled fashion
• Motivation controlling phase, amplitude of light
• Potential devices
• Beam steering devices
• Optical phased arrays
• Materials
• Electro-optic materials (e.g., organic-doped
  solgels, PLZT)

4
Refractive index modulation

• Two ways to enhance index modulation
• Material choice (limited capabilities)
• Structural design
• Structural approach necessary to achieve complete
  control of interference (i.e., Df2p)
• comes about from slowing down light (using PhCs)

5
Photonic crystals slow light

• Periodic dielectric media reflect certain
  wavelengths and transmit others
• Can slow down light through weakly coupled
  cavities, e.g., periodic defects in a 1D photonic
  crystal

6
Photonic crystals slow light
? Total savings O((vG/c)2)
7
Structural enhancement

• Single element design
• Non-linear material in middle
• Controlled by transparent oxide electrodes
• Photonic crystal enhancing non-linear effects

8
Structural enhancement

• Can calculate exact phase shift for PhC cavity
• For large Q, simplifies to
• Expect polarization-insensitive enhancement of
  index modulation that can be increased up to p
  (maximum phase shift through resonance)
• For m resonances, maximum phase shift is mp

9
Structural enhancement

• For full phase control, use two resonances
• Shift indices of non-linear material only to make
  all half-maxima coincide
• Each point at half-maximum will be p/2 out of
  phase with the next

10
Conceptual vision

• Photonic Crystal array
• Contain non-linear materials driven by
  transparent electrodes
• Placed between PhCs to slow down light, enhancing
  phase modulation
• Interference between devices can lead to true 2p
  solid angle beamsteering

11
Conceptual vision

• Advantages of photonic crystal beam steering
• Large angle steering only requires single stage
• Switchable at very high speed (110 GHz)
• Polarization-insensitive
• Common aperture device

12
Beam-steering

• Simulated structure
• Dual resonances
• 3 exterior, 1 interior photonic crystal bilayers
  of indices n2.87 and n1.8
• Index change of 8 required (can be decreased by
  increasing Q)

13
Beam-steering

• Behavior with non-linearities turned off

• No beamsteering occurs

14
Beam-steering

• Behavior with non-linearities turned on

• Beam is steered to an angle of 52º!

15
Beam-steering

• Can switch between straight and large-angle
  propagation using non-linear materials photonic
  crystals

Straight propagation with non-linearities off
52 degree propagation with non-linearities on
16
Future directions

• Experimental work on photonic crystal beam
  steering devices
• Fabrication
• Characterization
• Full 3-D modeling of experimental systems
• Common aperture showing that the reciprocal beam
  is directed at the same angle
• Modulation of multiple, discrete wavelengths
• Constant phase modulation from any angle or
  polarization

17
Conclusions

• Refractive index effects are often too weak to
  achieve maximum phase modulation
• Slowing down light can amplify these effects to
  useful levels
• Can design practical, large-angle beam-steering
  devices
• Prototype beam-steering devices based on this
  design can be fabricated and characterized in the
  laboratory