Optical Spectroscopy III
Er³⁺:YLiF₄ Emission Spectrum and Up-conversion

 Objective:

In the following exercises students will obtain the absorption spectrum for Er³⁺:YLiF₄ .  Then, Er³⁺ will be excited by a laser and the emission spectrum obtained. From the spectra, students will identify energy levels important in the visible emission of trivalent erbium. Among the observed emission lines, several are due to up-conversion. Students identify these emission lines and the excitation pathways, which enable this effect.

Background:

Inexpensive compact semiconductor diode lasers are readily available, which supply high intensities of near infrared light. A common technological problem is that semiconductors seldom emit in the visible region of the spectrum. One solution to this problem is the use of an up-converting material. That is, a material which absorbs photons of a certain energy and emits photons with higher energy. The details of this process will be made apparent in the following exercise.

Procedure:

 Er³⁺:YLiF₄ Absorption and Emission Spectra

The following procedure will guide you through the measurement of the emission of Erbium in a crystal host, the identification of energy levels, and the analysis of linear and nonlinear absorption processes.

Absorption

• Begin by running the Ocean Optics S2000 control software. Make sure the correct spectrometer is installed and set the acquisition parameters.  Subtract the background.  Take a reference spectrum of the tungsten filament light source.
• Insert the Er³⁺:YLiF₄ crystal in the cuvette holder and obtain the absorption spectrum of the crystal.
• Identify and label the transitions on the graph using the energy level diagram below.

Emission

• Have the instructor turn on the Ti:Sapphire laser.   Place a white card in the beam so that the spectrometer detects some of the scattered laser light.  Note the spectral width and wavelength of the laser.  Is the laser narrower in line width than the spectrometer resolution?

CAUTION: THE LASER EMITS INTENSE RADIATION AND EXPOSURE TO THE EYES AND SKIN MUST BE AVOIDED. PAY PARTICULAR ATTENTION TO REFLECTIONS OF THE LASER BEAM, AS THESE ARE SOMETIMES DIFFICULT TO PREDICT.  WEAR IR SAFETY GOGGLES.

• Insert the Er³⁺:YLiF₄ crystal in the cuvette holder such that the laser is focused into the crystal and the emitted light is observed perpendicularly through the spectrometer. Use a beam block to stop the transmitted laser beam.
• Record the emission spectrum of the sample. Notice whether there is any emission of photons with energies greater than those used to excite the sample.  You may need to increase the sensitivity of your graph in order to see the weak transitions.  Identify and label the transitions on the graph.
• Measure the peak intensity of one line originating from each of the J-levels. For peaks at higher energy than the excitation energy, measure the peaks of all lines.  Repeat the measurement with the OD filters and combinations of the OD filters (.15, .30, .60).   Plot the peak intensity emitted from each level as a function of the excitation intensity.  Be sure that if you change the integration time, you account for this in your data.
• What differences are there between the emissions from the various levels? Use your data and the Er³⁺ energy level diagram to explain the mechanism behind the up-converted emission.  It is a multi-level process but any explanation in which energy is not conserved will be considered laughable.