Optoelectronics with Carbon Nanotubes

3. Experimental set-up
The optical path for detecting emitted light is schematically described in Figure 3. The
sample was mounted on a sample stage with silver paint in an MRR vacuum chamber equipped
with four probes. The stage and the probes were electrically connected to an Agilent 4145C
parameter analyzer for applying voltages and measuring currents. The vacuum level in the
chamber was typically 3 to 5 × 10 -6 Torr.
Emitted light from samples was collected by a 10x, 20x or 50x Mitutoyo near-infrared
(NIR) objective lens mounted directly above the sapphire view port on top of the chamber. Light
then goes through a grating prism (GRISM) for spectroscopy, and/or through a linear polarizer
for polarized measurements. The field lens mounted before the camera is used to keep the light
path parallel through the GRISM and the polarizer. All the optical components are at ambient
temperature before the light enters the detector through the outer window. The short-pass filters
and the mercury-cadmium-telluride (MCT) chip inside the detector are cooled by liquid nitrogen.
For low-temperature measurements, a sample stage cooled directly by a small
continuous-flow liquid helium cryostat by MMR Technologies was used in conjunction with an
MMR K-20A Programmable Temperature Controller. The cold pad on the stage is equipped
with a resistor heater and a temperature sensor. Liquid helium is pushed by the pressure in the
dewar through a specially designed transfer line and into the cryostat. The sample stage can be
cooled to about 20 K, but when the probe needles (connected thermally to ambient temperature
through cables) are touching the sample, the temperature increases quickly and the practical low
temperature measurement limit is close to 90 K.
34