Stories
Slash Boxes
Comments

SoylentNews is people

posted by janrinok on Tuesday January 30 2018, @03:08AM   Printer-friendly
from the colour-me-surprised dept.

A new way of modifying the dipole moment of cholesteric liquid crystals allows for researchers to select between the different band-edge modes experimentally for the first time.

Since lasers were first developed, the demand for more adaptable lasers has only increased. Chiral nematic liquid crystals (CLCs) are an emerging class of lasing devices that are poised to shape how lasers are used in the future because of their low thresholds, ease of fabrication, and ability to be tuned across wider swaths of the electromagnetic spectrum. New work on how to select band-edge modes in these devices, which determine the lasing energy, may shine light on how lasers of the future will be tuned.

The laser cavities are formed of a chiral nematic liquid crystal doped with a fluorescent dye. The liquid crystal creates a photonic bandgap in the laser cavity. An international team of researchers demonstrated a technique that allows the laser to electrically switch emission between the long- and short-wavelength edges of the photonic bandgap simply by applying a voltage of 20 V. They report their work this week in Applied Physics Letters, from AIP Publishing.

"Our contribution is to find a way to change the orientation of the transition dipole moment of the gain medium [the fluorescent dye] in the CLC structure and achieve mode selection between long- and short-wavelength edges without tuning the position of the photonic bandgap," said Chun-Ta Wang, an author of the paper. "We also demonstrated a polymer-stabilized CLC system, which improved the laser's stability, lasing performance and threshold voltage."

CLC lasers work through a collection of liquid crystals that self-assemble into helix-shaped patterns, which then act as the laser's cavity. These helices are chiral, meaning they corkscrew in the same direction, which allows them to be tuned across a wide range of wavelengths. While many lasers, like the laser diodes used in DVD players, are fixed at one color, many CLC lasers can be tuned to multiple colors in the visible light spectrum and beyond.

Chun-Ta Wang, Chun-Wei Chen, Tzu-Hsuan Yang, Inge Nys, Cheng-Chang Li, Tsung-Hsien Lin, Kristiaan Neyts and Jeroen Beeckman. Electrically assisted bandedge mode selection of photonic crystal lasing in chiral nematic liquid crystals. Applied Physics Letters Jan. 22, 2018 (DOI: 10.1063/1.5010880.


Original Submission

 
This discussion has been archived. No new comments can be posted.
Display Options Threshold/Breakthrough Mark All as Read Mark All as Unread
The Fine Print: The following comments are owned by whoever posted them. We are not responsible for them in any way.
  • (Score: 1, Informative) by Anonymous Coward on Tuesday January 30 2018, @03:15PM

    by Anonymous Coward on Tuesday January 30 2018, @03:15PM (#630387)

    Here is the first part from the paper's introduction section:

    CLCs are a particular class of liquid crystals in which the molecules self-assemble into one-dimensional helices, typically formed by mixing a nematic liquid crystal with a chiral agent. The concentration and twisting power of the chiral agent determine the helical pitch (p). Such a periodic chiral structure can be regarded as a one-dimensional photonic crystal for a circularly polarized probe with the same handedness as that of the structure. The associated photonic bandgap (PBG) is located at λc = nc ⋅ p with a bandwidth Δλ = Δn ⋅ p, where nc and Δn are the average refractive index and birefringence of the CLC, respectively. Resonances occur at the two bandedges: long wavelength edge (LWE) and short wavelength edge (SWE); the corresponding wavelengths are λLWE= ne ⋅ p and λSWE = no ⋅ p, respectively, where ne is the extraordinary refractive index and no is the ordinary refractive index. At the bandedge, the group velocity of light approaches zero, and hence, the density of states is significantly enhanced.4,5 In the presence of a gain medium, laser action can occur at the photonic bandedge(s) through optical excitation—usually termed the bandedge lasing.

    As you can see, the helical pitch factors into the width of the bandgap, and hence the wavelengths emitted (since they depend upon the bandgap edges), so a useful way to tune the wavelengths you want is to change the helical pitch (or, how tightly the helixes are wound). The paper mentions that one can do this using electric fields, heat, light, and mechanical stress.

    You can get lasing from either edge of the bandgap, and what this particular paper is about is how you can pick which edge of the bandgap you want to use, as well as to switch between them.

    Starting Score:    0  points
    Moderation   +1  
       Informative=1, Total=1
    Extra 'Informative' Modifier   0  

    Total Score:   1