“Optical characterization of quantum dots entrained in microstructured optical fibers”, Kenith E. Meissner, Carvel Holton, William B. Spillman Jr., Physica E26(2005) 377-381.

Purpose:  To investigate the optical properties of a microstructured optical fiber infused with colloidal CdSe/ZnS quantum dots (QD).

 

Methods:

 

·      CdSe/ZnS quantum dots were synthesized using trioctylphosphine as the coordinating solvent and following established methods.  The resulting nanocrystals were precipitated using methanol and then suspended in heptane.  The suspension exhibited peak luminescence at approximately 573nm.

·      The optical fibers were created from standard, optical-fiber grade silica using a rod and tube fabrication process.  Cross section shows each fiber consists of thirty-six capillary holes in the cladding material forming a ‘web’ region surrounding a solid core.  Each core and capillary has a diameter of approximately 12.7microns and 14.6microns, respectively.

·      Optical fiber distal ends were immersed in the colloidal QD suspension with QD’s distributing throughout the capillaries by way of capillary effect. 

·      An argon laser (488nm) served as a high power pump laser to photoexcite the QD’s and a HeNe laser (594nm) served as a low power probe laser.

 

Key Findings:

 

·      Upon excitation by the high-power (10mW) argon laser, QD luminescence from the capillaries was captured and propagated down the core of the fiber as a result of tight evanescent field coupling and scattering.  Emission spectrum exhibits no phase shift from excitation of QD’s in suspension alone.

·      When the low power (20microW) laser is applied to the fiber core, images show light becomes coupled to both the core and capillary clad region of the fiber microstructure.

·      When both the probe and pump lasers are applied to the fiber, significantly more probe light is observed at the distal end, representing an optical gain.  This gain is represented by both the spectral output as well as a CCD camera image.

·      The experiment was repeated many times with the same optical gain result. 

·      The mechanism explaining the optical gain is unexplained and warrants further investigation.

 

Definitions:

 

Luminescence:  here the term is used broadly and refers to fluorescence – photons emitted (at a wavelength higher than the excitation wavelength) from a chemical substrate undergoing an internal energy transition resulting from excitation photons.