Raman thermometry can be used to determine the temperature of a material either by measuring the shift in Raman peak with respect to increase in temperature or by measuring the signal strengths of a particular Raman band at the Stokes (Is) and anti Stokes (Ias) positions and calculate the temperature based on Botzman distribution of the ground and the first excited state populations using the following formula:
Is/Ias =(νl −νph)4/( vl + νph)4 * exp(hcνph/ kT),
vl is laser frequency (532nm), vph is phonon frequency, and h, k are Planck 's and Boltzman 's constant.
I am using Horiba LabRam HR Evolution spectrometer to measure Raman spectra and then the temperature of superlattice system such as AlGaAs and InAlAs using Raman thermometry and I see discrepancies in the measuring temperature. I also measure the temperature using an independent means by a thermistor attached near the superlattice system.
What other factors do I need to consider to get right value of temperature by Raman thermometry?
A conclusive analysis without data is hard. Just a few general comments: Comparing Raman scattering intensities across a wide range of frequencies can be quite tricky. One important point to factor in is the frequency-dependent sensitivity of your spectrometer. To try and correct for this you can calibrate your instrument against a standard light source with a known intensity profile over a broad frequency range. Moreover, if you want to measure temperatures well below the energy of your phonon frequency your error bars will be incredibly big.
The space scale where you measure the temperature is quite different: a few microns for the Raman and a few millimeters for the thermistor. I would not be surprise if the Raman temperature is much higher.
I agree with Fernando, your measurement is local and the thermistor is measuring in a different region, both will tell you the same only if they are in an isothermal surface. Besides Raman thermography can be tricky because you can warm considerably the volume you are probing with the laser beam (one should always lower the laser intensity to see if the Raman peak is not shifting). Also, the Raman shift may be affected by other things apart of temperature like lattice stress.
Regards,
Julian
I agree with Julian, firstly temperature locally can be different than the average temperature plus in cases it is needed to separate temperature and stress contributions in the measurement (either by measuring two phonon modes and determining those two variables, or by finding a Raman modes which is just less stress sensitive and mainly contains temperature contributions). See e.g. Article A Review of Raman Thermography For Electronic and Opto-Elect...
Thank you all for sharing insight on my measurements. Special thanks to Martin for sending good review on Raman Thermography.
Just an additional consideration...
The answers about Raman scattering from the others are all great! But do we need to use the Raman method? How about these competing laser diagnostic techniques: Thermoreflectance and PL spectroscopy?
Thermoreflectance: This method is particularly useful for semiconductors like AlGaAs. It involves measuring the change in reflectivity of a material with temperature. A laser is used to heat the material, and the change in reflectivity is measured with another laser. This technique can provide high spatial resolution and is non-contact, making it suitable for delicate or small-scale structures.
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Photoluminescence Spectroscopy: This technique involves exciting the material with a laser and then measuring the emitted light. The energy (or wavelength) of the emitted light can be related to the bandgap of the material, which is temperature-dependent. This method can be used even if the material is opaque, as long as it is luminescent.
More thermometric methods across all phases can be seen in this review:
Chapter Laser‐Based Combustion Diagnostics
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Hi Robin, PL spectroscopy can be employed to determine the temperature as the bandgap changes but one needs to have InGaAs array detector for these measurement performed on superlattice system such as AlGaAs and InAlAs. I did not have InGaAs detector so could not use this method. And thermoreflectance could not be used either as quantum cascade laser (superlattice ) cannot be heated by an external laser in my experiment.
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