What is an H-PDLC?

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The Brief Explanation:

H-PDLC stands for Holographically Formed, Polymer Dispersed Liquid Crystal.  This is a thin film of polymer and liquid crystal that has been treated with a split laser to form a reflective Bragg grating, creating simple and cost effective high quality optical filters, beam steerers, and reflectors for display applications.

The Thorough Explanation:

Liquid Crystals

Liquid crystals are a group of materials that are suspended in a phase between solid and liquid.  These materials flow like liquids but maintain some orientational order like solids.  They are used often in display technology because of their electro-optical properties:

  1. Liquid crytsals are birefringent, meaning they have a refraction index that is dependent on the angle and polarization of liqht.  When light reaches a liquid crystal molecule from one direction it is allowed to pass through, but from another direction the light is scattered.
  2. By applying an electric field we can create a dipole moment in the crystals.  The crystals then orient along the direction of the field.  When the field ceases, the crystals return to their relax and random state.
These two properties combined give us the ability to control the amount and direction of light through a material by reorienting the liquid crystals at different angles.
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Polymer-Dispersed Liquid Crystals

The addition of "Polymer-Dispersed" to the title simply means that we mix these liquid crystals into a polymer (plastic).  This is to provide a structure for the liquid crystals and comes into play during holography.

Holographically Formed, Polymer-Dispersed Liquid Crystals

When you think of holography you might bring to mind the holograms so well known by Star Wars and Tupac Shakur.  But at its basic form holography is used by intersecting two beams of light to form a pattern.  In our experiments we place our mixtures of polymer and liquid crystal onto glass slides.  These slides are then oriented on an optical table alongside mirrors and a laser.  The beam for the laser is split into two identical beams.  When these beams rejoin at the polymer/liquid crystal interface, they interact and interfere with each other.  Where the beams interfere constructively the laser cures the polymer, solidifying the polymer in those areas.  Where the beams interfere deconstructively the photons are blocked, leaving dark spots of liquid.  The liquid crystals flow into these pockets that are uniformly spaced depending on the angle of the beams.

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This dotted pattern is called a Bragg grating, and is useful because it determines the wavelength (color) of light that is reflected by the sample.  When the sample is at rest these droplets of liquid crystal are randomly oriented.  Because of this, the refractive index varies greatly across the H-PDLC, scattering light depending on the wavelength designed.  However, by applying an electric field across the sample the liquid crystals all align in one direction that has been engineered to create a refractive index equal to that of the polymer.  In this situation light is allowed through the now-transparent sample.

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Why Study H-PDLCs?

Current displays using liquid crystal technology are designed in such a way that a backlight is required (think of the light that comes from your computer monitor or TV).  This light feeds on a huge portion of battery life, and so is inconvenient in situations that require long-lasting devices.

Kindles and other e-readers use technology that allows for ambient light to be reflected from their displays.  This is done through e-ink, in which white and black charged particles float to the top or the bottom of the display depending on the electric field.  This technology has the limitation that the "switching time" (the time it takes to turn the page) is too long to be of use in fast display requirements such as video.

Therefore, the H-PDLC technology can be developed to create a reflective, low-battery display that takes advantage of ambient light but maintains fast switching capability.  In addition H-PDLCs have great potential in creating fiber optic signal filtration and control, inexpensive spectrometers, dynamically reconfigurable contact photolithography masking layers, and optically switchable mirrors.