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

 Objective:

In the following exercises students will excite Er³⁺ with a diode laser and obtain emission spectra. From the spectra, they 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:

Diode Laser Operation and Characteristics  

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.

CAUTION: THE DIODE 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.

• Begin by running the Ocean Optics S2000 control software. Make sure the correct spectrometer is installed and set the acquisition parameters.
• Switch on the ILX Lightwave LDX-3525 current source and set the diode current to 65 mA.  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?
• Zoom in on the laser peak. Turn on the Temperature Controller and set the resistance to 32kohms. Watch the laser wavelength change in the OO software. As the resistance increases, the temperature of the laser is getting colder. Does the direction of the wavelength shift make sense? Measure the wavelength of the laser once the temperature has stabilized (R=32kohms).
  

Er³⁺:YLiF₄ Emission Spectrum  

• Place the Er³⁺:YLiF₄ crystal so that the diode laser is focused into the crystal and the emitted light is observed through the spectrometer.
• 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.  Your report should have a graph of the emission spectrum.  Identify and label the transitions on the graph.
• Measure the peak intensity of three lines that are below and three lines that are above the laser energy. Since the laser is exciting levels within the ⁴F_9/2 manifold of states, choose one level that is above and one that is below the laser energy. The other four levels should come from different manifolds.  Repeat the measurement with the OD filters and combinations of the OD filters (0.15, 0.30, 0.60).     Be sure that if you change the integration time, you account for this in your data. Plot the log of the peak intensity emitted from each level as a function of the log of the excitation intensity. Because upconversion is a two-step process, the intensity dependence (or "power dependence") should be different than that of normal emission. Does your data confirm this? What is the power dependence for the lines?
• 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.
• Draw a block diagram of the experimental setup.

 Suggested reading (a step ahead): A new process, called quantum cutting, which can be considered the opposite of up-conversion is now being studied. Future mercury-free fluorescent lighting will likely be produced using this process.