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Materials Science

Fluorescence Upconversion

Two-photon process leads to emission at shorter wavelengths

Fluorescence upconversion particles absorb light in the near infrared (NIR) and emit light in the visible range. These particles are currently in the focus of research because of their prospective application for, e.g., in vivo optical imaging, as they allow excitation in the NIR spectral range where light absorption and scattering from biological tissues is minimized. Another possible application are dye sensitized solar cells. A standard solar cell does not absorb in the near infrared spectral range, as the energy of the NIR light is too low for injecting electrons into the conducting band of the solar cells semiconducting material. A possible option to overcome this limitation, and thus to create solar cells with a higher light conversion efficiency, are dye sensitized solar cells in which upconversion particles are used to convert the NIR light into visible light, which then has enough energy to leap the band gap of the semiconducting material.

A classical spectrometer or fluorescence microscopy set-up is used to study upconversion. The sample is excited by a pulsed or cw laser and the fluorescence is collected in the usual 90° angle orientation. The emitted fluorescence is filtered by means of a monochromator or optical filters and detected using detectors with a large active area such as PMT detectors or Hybrid-PMTs. For lifetime measurements, either Time-Correlated Single Photon Counting (TCSPC) or Multi-Channel Scaling (MCS) is used for data acquisition.

Consequently the essential components of a spectrometer set-up are:

  • pulsed (and/or CW) excitation source
  • single photon sensitive detector
  • monochromator or optical filters
  • for lifetime measurements: TCSPC or MCS unit to measure the time between excitation and fluorescence emission

PicoQuant offers the following system that can be used for upconversion measurements:

FluoTime 300

FluoTime 300 - automated fluorescence spectrometerFully Automated High Performance Fluorescence Lifetime Spectrometer

The FluoTime 300 "EasyTau" is a fully automated, high performance fluorescence lifetime spectrometer with steady-state and phosphorescence option. It contains the complete optics and electronics for recording fluorescence decays by means of Time-Correlated Single Photon Counting (TCSPC) or Multichannel Scaling (MCS). The system is designed to be used with picosecond pulsed diode lasers, LEDs or Xenon lamps. Multiple detector options enable a large range of system configurations. With the FluoTime 300 decay times down to a few picoseconds can be resolved.

MicroTime 100

MicroTime 100 - upright time-resolved fluorescence microscopeUpright Time-resolved Fluorescence Microscope

The MicroTime 100 is an idea tool for the study of time-resolved photoluminescence of solid samples such as wafers, semiconductors or solar cells. The system is based on a conventional upright microscope body that permits easy access to a wide range of sample shapes and sizes. It can be supplied with either manual scanning or with a 2D piezo scanner with either µm or cm resolution. The flexible excitation subsystem is designed for picosecond pulsed laser diodes with wavelengths ranging from 375 to 900 nm. The MicroTime 100 can be configured with up to 4 detection channels that can be equipped with a variety of single-molecule sensitive detector types.

MicroTime 200

MicroTime 200 - time-resolved confocal fluorescence microscopeTime-resolved Confocal Fluorescence Microscope with Unique Single Molecule Sensitivity

The MicroTime 200 is high-end, versatile time-resolved confocal microscope for the quantification of molecular dynamics or properties as well as interaction studies in Material and Life Sciences. The modular design paradigm allows adapting the system to the individual researchers needs. The excitation subsystem consists of picosecond pulsed laser heads with wavelengths from 375 to 900 nm whose optical output power and repetition rates can be flexibly adjusted by the laser diver. The microscope is equipped with a high precision 2D piezo scanner and optionally with a PiFoc element for 3D scanning. The single molecule sensitive detection system can be configured with up to 4 channels that can be equipped with a variety of detector types.

The following core components are needed to build a system capable of fluorescence upconversion measurements, which are (partly) available from PicoQuant:

Fluorescence upconversion using NaYF4:Yb/Er

The left figure shows the steady state upconversion spectrum of NaYF4:Yb/Er solved in cylcohexane. The figure on the right displays time resolved measurement of the same sample of NaYF4:Yb/Er solved in cylcohexane. The excitation of the sample was performed in a burst mode, i.e. first multiple laser pulses are used to deposit energy into the sample and then the excitation is stopped long enough to capture the comparably slow decay of the sample. The analysis of the data reveals a single fluorescence lifetimes of 113 µs.


Sample courtesy of T. Nyokong, Rhodes University, South Africa