1. Thermal diffusivity measurement on Nb by
Femtosecond optical pump-probe spectroscopy
Md Mahmudur Rahman
Ph.D. student
Advisor
Dr. Hani Elsayed-Ali
Department of Electrical and Computer Engineering
Old Dominion University
Presented by
Ganapati Myneni
ISOHIM/ODU-AARC
TTC2019 Vancouver February 6, 2019

2. Thermal diffusivity2K α2K ~ k/ρC = 2333 cm2 s-1
Temperature and magnetic field dependence of heat capacity of superconducting large grain Niobium
S B Roy et al (unpublished)
During cavity operation heat is deposited in the sc layer of ~60 nm τ (1.5 GHz) ~ 6.6×10-10 s
Thermal diffusivity2K α2K ~ k/ρC = 2333 cm2 s-1
Jlab/JSA LLC Provisional patent January 2016

3. Examples of time scales in solids
Process
Time scale
Electron –Electron interaction
10-16 to sec
Electrons interacting with atomic vibration (Phonons)
10-13 to sec
10 August 2019

4. Pump-probe technique hνprobe Energy En e- hνpump EF
e from below the Fermi level (EF) knocked out into an intermediate state with pump pulse.
Probe pulse interrogate the system.
The time scale of the e to decay back to equilibrium can be measured.
hνprobe En
Energy e-
hνpump EF
10 August 2019

5. Pump-probe setup
10 August 2019

6. Spatial difference converts into time delay between pump and probe beam
Liner stage
Time delay, 𝜏= 2𝑑 𝑐
c = speed of light
d = distance varied
(1 mm ~ 6.66 ps )
(1 µm ~ 6.66 fs )
Pump
Detector
Probe d
The beam path difference between pump and probe beam is varied
10 August 2019

7. Thermal conductivity measurement of metallic thin film
Ingot Nb at room temperature
K = 55 W/mK
Heat equation
𝐶 𝜕𝑇 𝑥,𝑡 𝜕𝑡 =−𝐾 𝜕 2 𝑇 𝑥,𝑡 𝜕𝑥 2 +𝑆
C = Heat capacity (J/m3 . K)
K = Thermal conductivity (W/m.K)
S = Source term
10 August 2019

8. Low temperature measurements started
After using 5pt FFT filer