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Spectrometer Application ReportBrillouin Spectroscopy of Nb​2​O​5 ThinFilms​
March 20, 2020
For questions, contact ​hyperfine@lightmachinery.com

A. Thin Film of Niobium Pentoxide on Glass or Germanium Substrate
The Brillouin frequency shift of a thin film of niobium pentoxide (Nb​2​O​5​) deposited on both aglass or a germanium substrate was measured using a 20X microscope objective. Thethickness of the film in both cases is 5 μm, which corresponds to ~ 20 % of theexcitation/collection focal volume depth of the objective (~ 25 μm along the z-axis). Thespectra in Figures 1 and 2 were obtained using 25 mW of laser power at the sample.
The measured frequency shift of ​Nb​2​O​5 is​49.84 ​± 0.05 GHz on g​lass substrate and 49.21 ​±​0.03 GHz on germanium, representing a difference of ​0.63 GHz. This ​ΔGHz ​suggests theinfluence of substrate on ​the Brillouin frequency shifts of thin films. More work is required tounderstand and characterize this potential influence. T​he substrate also appears to impact theBrillouin ​signal ​intensity of the ​Nb​2​O​5 ​layer​. Interestingly, ​its amplitude is about 5 times larger
on germanium than on glass substrate. Future exploratory research might thus include testing
the effect of substrate reflectivity on the Brillouin signal intensities of thin films. Since thegermanium substrate is more reflective than the glass substrate, a small pump laser signal
can be observed in Figure 2. Also interesting in Figure 2 is the presence of unknown Brillouinpeaks at low frequency shift (~ 4.9 GHz). Impurities associated with the deposition method ofNb​2​O​5 ongermanium might explain the presence of these peaks. Further investigation is​required.

Figure 1:​ a) Brillouin frequency shift spectrum of a thin film of ​Nb​2​O​5​ deposited on a glass substrate. b) Raw sensor image.
The time exposure was set to 20 seconds and the number of averages to 5.

2

Figure 2: a) Brillouin frequency shift spectrum of a thin film of Nb2O5 deposited on a germanium substrate. b) Raw sensor
image. The time exposure was set to 20 seconds and the number of averages to 5.

Specifications

SampleSystem• Material: Nb​2​O​5​ layer on germanium substrate(​not a single crystal​)• Pump wavelength: 532 nm• Thickness of Nb​2​O​5​: 5 μm

• Excitation/collection with 20X objective• The estimated Brillouin frequency shift for bulk
Nb​2​O​5 isaround 47.3 GHz (using a speed of​sound of 5311 m/s, a refractive index of 2.37,and a pump wavelength of 532 nm).

• FWHM of the instrument response: nominal 0.9GHz• Power at sample: 2​ 5 mW• Repeatability of Brillouin shift: highly sample
and exposure dependent; < 10 MHz is possible

B. Thin Film of Niobium Pentoxide on Germanium Substrate using a lower noisedetector
The same experience was performed with a lower noise camera in order to acquire theBrillouin signal of the germanium substrate. Figure 3b) shows the raw sensor image, where thestrong peaks are attributed to the Nb2O5 layer, while the faint diagonal lines correspond to theBrillouin signal of the germanium substrate. This signal is difficult to distinguish in the3unwrapped spectrum because it is shaped as a weak uniform pedestal. There are nowell-defined Brillouin peaks from the germanium presumably because it is not single
crystalline and also because of the strong absorption at that wavelength (532 nm). We also
notice the presence of the unknown peaks at ~ 4.9 GHz, as discussed in section A).

Figure 3: a) Brillouin frequency shift spectrum of a thin film of Nb2O5 deposited on a germanium substrate, acquired withthe lower noise camera. b) Raw sensor image. The time exposure was set to 10 seconds and the number of averages to 20.

Specifications

SampleSystem• Material: Nb​2​O​5​ layer on germanium substrate(​not a single crystal​)• Camera: 0.7 e- rms read out noise sCMOS
cooled to -5°C

• Thickness of Nb​2​O​5​: 5 μm

• Pump wavelength: 532 nm• The estimated Brillouin frequency shift for bulk
Nb​2​O​5 isaround 47.3 GHz (using a speed of​sound of 5311 m/s, a refractive index of 2.37,
and a pump wavelength of 532 nm).

• Power at sample: 2​ 5 mW• Excitation/collection with 20X objective• FWHM of the instrument response: nominal 0.9GHz• Repeatability of Brillouin shift: highly sample
and exposure dependent; < 10 MHz is possible4