Materials We Use: LPP & LCP

Liquid Crystal Polymer (LCP) Network

  • a.k.a., reactive mesogen
  • Remember: molecules align according to boundaries set by surface (see kayaks è)
  • Manifests birefringence that controls retardation
  • Our materials custom-developed by Merck / EMD, typical molecule above
  • See Kelly, J Mater Chem 5, 2047 (1995)

Photo-Alignment Layer

  • a.k.a., linearly polymerized photopolymer (LPP)
  • Remember: molecules cause alignment of LC to parallel/perpendicular to incident linearly polarized UV light; i.e., where the spatial pattern (wavefront) is set
  • Manifests alignment, but no optical effect since so thin (50 nm)
  • Our materials custom-developed by DIC / Dainippon Ink & Chemicals
  • See Chigrinov et al, Phys Rev E 68, 061702 (2003); M. Schadt et al, Nature 381, 212 (1996)

Using Liquid Crystal Polymers (LCP), Linear Photo Polymerization (LPP) and a variety of dopants, we are able to create three-dimensional patterns of liquid crystals captured within a thin film (2-4um).

Photo-Alignment Layer


Actual LCP Thickness of 2um

Actual LCP Thickness of 2umreactive LC photo alignment layer


“Standard” Thin-Film Fabrication Techniques

alignment layer processing


Underlying Technology: PGs

  • Polarization Gratings (PGs) are basis for IO technology
  • Polarized diffraction
  • Right & left circular
  • +/- 1st orders only
  • Highly efficient
  • Low leakage: <1%
  • Tunable operation
  • Broadband
  • Narrowband

 


Geometric vs Dynamic Phase

(The Dynamic) Phasedynamic-phase term

The “normal” phase delay.

For example,

  • Prisms
  • Lenses
  • Phase gratings
  • Spatial Light Modulators (computer generated holograms)

All optical path length difference

(The Geometric) Phase

(Reference, wave in vacuum)geometric phase term

(Isotropic media) “normal” phase delay called dynamic phase due to optical path length difference

(Anisotropic media)

“extra” phase delay

called geometric phase

 


Geometric Phase Hologram Theory

  • Phase encoded into orientation
  • Hologram, with primary, conjugate, and leakage waves
  • Spectral behavior depends on retardation not spatial profile
  • Features
    • around 100% efficient for most and interesting profiles
    • 1000s of radians, continuous phase profile
    • Max phase slope ~ π rad/µm (!)


Holographic Mastering

Traditional Holographic Setup

polarization holography setup

 

 

cylindrical lens rotation stage sample holder

Lens Recording

 

 

 

 


Direct-Write Mastering

  • For virtual elementshologram
  • Direct write laser lithography (3D)
  • Continuous scanning
  • Eqn phase à continuous OA orientation à continuous Geo Phase
  • Records only pure phase of object (not amplitude)
  • Key improvements over (and acknowledgements)
    • Nakajima 1976
    • Hasegawa 2002
    • Cai 2006
    • Fratz 2009

 

 


Replication Exposure (proximity lithography)

ReplicationReplication Exposure

  • Standard aligner w/ polarizer and birefringent Bragg PG master(mask)
  • Simplified replication tool w/ collimated UV LED/lamp/laser
  • Principle: the replica orientation angle is x2 of the master everywhere, i.e., grating period is halved
  • Delivered fluence (365 nm): 1-5 J/cm2
  • If intensity is 10-30 mW/cm2, exposure times can be few minutes