Low-repetition-rate pulsed lasers are excellent sources for two-photon excitation of samples frozen in low-temperature glassy solvents. Unfortunately, similar efforts to combine synchronously-pumped dye laser excitation with normal cryostats and sample cells have consistently failed. The continuous nature of the laser creates a weak refractive index gradient which causes the beam to wander randomly in space. We have found that a fiber-optic probe directly frozen in the glassy sample permits two-photon excited fluorescence spectra to be obtained. This arrangement is stable because the fiber restricts beam-induced refractive index changes to harmless expansion. Studies of fluxional fluorophores indicate that the viscosity at the fiber tip is not significantly changed under conditions of average power known to cause beam wander in ordinary cryostat cells.

Fiber-Optic Probe for Collection of Two-Photon Excited Fluorescence Spectra in Low-Temperature Glassy Solvents

Photoluminescence (PL) is an optical emission induced by photon absorption in a material where electrons are excited from the ground state to excited states, then relax to the lowest excited states and recombine radiatively. The PL emission provides vital information on bandgap energy, material purity and crystal quality. In this project, a PL characterization system, also capable of electroluminescence (EL) measurements, was designed and assem- bled to measure optoelectronic device structures with the capabilities of recording PL or EL spectra as well as micrometer-resolved PL or EL maps on device structures or active components.In order to realize the system with the above functions, an optical setup and a monochro- mator were used to achieve micrometer-range resolution and reasonable signal-to-noise ra- tio. A hardware control platform needed to be designed and assembled to control the precise movement of the sample stage and monochromator as well as for acquiring the signal. A PC-based control software were developed for fully automatic measurements . Furthermore, adequate alignment and calibration methods had to be developed to tune the optical path, monochromator and control program.The setup employs the basic ideas of confocal microscopy, with the parallel laser input focused on the sample surface with a spot diameter of 0.78 μm. A Czerny Turner diffraction grating based monochromator is used to measure PL emissions. A 532 nm laser diode and an InGaAs or Si detector are applied in the system for spectral range of detection of at least 850- 1600 nm, i.e. covering the important data and tele communication bands. The project builds on a platform containing EasyDrivers, an Anduino Uno micro-controller and a Labview based operation software, together working with an amplifier circuit for stepper motors actuation and signal acquisition. Finally, different quantum well samples were measured, showing that the wavelength accuracy and resolution as well as the program flexibility meet the specifications of the setup.

Setup for Micro Photo- and Electro-Luminescence of Optoelectronic Device Structures

This thesis presents a novel method to quantify the localised carrier collection efficiency of thin film solar cells by integrating a theoretical model with data from a newly developed combined measurement system capable of measuring spectrally-resolved photoluminescence (PL), time-resolved photoluminescence (TRPL) and transient photocurrent decay (TPCD) at the same spot on the solar cell. This combined measurement approach allows for the identification and further understanding of the limiting factors in the carrier collection efficiency of solar cells. [Continues.]

Combined optical and electrical characterisation of minority carrier lifetime in solar cells

We describe a novel photoluminescence measurement apparatus which incorporates an optical fiber for carrying both the excitation beam and the luminescence from the sample. The advantages of the optical fiber makes the system very versatile and suitable for sensitive measurements. The collection efficiency of the setup is demonstrated to be the same as that of a conventional setup. Several photoluminescence spectra of III‐V compounds recorded using the apparatus are also included to demonstrate its applicability over a wide wavelength range (0.8–2.0 μm).

A. Bandyopadhyay

Papers

Photoluminescence measurement system using fiber optics