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Resolving transitions in the mesoscale domain configuration in VO2 using laser speckle pattern analysis.

Seal K, Sharoni A, Messman JM, Lokitz BS, Shaw RW, Schuller IK, Snijders PC, Ward TZ - Sci Rep (2014)

Bottom Line: The configuration and evolution of coexisting mesoscopic domains with contrasting material properties are critical in creating novel functionality through emergent physical properties.However, current approaches that map the domain structure involve either spatially resolved but protracted scanning probe experiments without real time information on the domain evolution, or time resolved spectroscopic experiments lacking domain-scale spatial resolution.Our straightforward analysis of laser speckle patterns across the first order phase transition of VO2 can be generalized to other systems with large scale phase separation and has potential as a powerful method with both spatial and temporal resolution to study phase separation in complex materials.

View Article: PubMed Central - PubMed

Affiliation: 1] Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA [2] Department of Physics &Astronomy, University of Tennessee, Knoxville, TN 37996, USA.

ABSTRACT
The configuration and evolution of coexisting mesoscopic domains with contrasting material properties are critical in creating novel functionality through emergent physical properties. However, current approaches that map the domain structure involve either spatially resolved but protracted scanning probe experiments without real time information on the domain evolution, or time resolved spectroscopic experiments lacking domain-scale spatial resolution. We demonstrate an elegant experimental technique that bridges these local and global methods, giving access to mesoscale information on domain formation and evolution at time scales orders of magnitude faster than current spatially resolved approaches. Our straightforward analysis of laser speckle patterns across the first order phase transition of VO2 can be generalized to other systems with large scale phase separation and has potential as a powerful method with both spatial and temporal resolution to study phase separation in complex materials.

No MeSH data available.


Related in: MedlinePlus

(a) Simplified diagram of experimental setup for speckle pattern collection. Polarized laser light is focused onto a sample using a lens and is reflected from the VO2 surface where green and blue represent regions of different dielectric values. The reflected diffuse (dashed) speckle pattern is sampled using a CCD, the specularly reflected light (solid) is not collected. The image gives an example of a typical speckle pattern under 800 nm illumination at 60°C. (b)–(g) Variance (b, c, d) and mean intensity (e, f, g) as a function of temperature plotted for three illumination wavelengths, 488 nm, 633 nm and 800 nm. Also plotted is the d.c. resistance. The relatively increased noise in the variance for the shorter wavelengths is due to the smaller number of pixels per speckle at these shorter wavelengths.
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f2: (a) Simplified diagram of experimental setup for speckle pattern collection. Polarized laser light is focused onto a sample using a lens and is reflected from the VO2 surface where green and blue represent regions of different dielectric values. The reflected diffuse (dashed) speckle pattern is sampled using a CCD, the specularly reflected light (solid) is not collected. The image gives an example of a typical speckle pattern under 800 nm illumination at 60°C. (b)–(g) Variance (b, c, d) and mean intensity (e, f, g) as a function of temperature plotted for three illumination wavelengths, 488 nm, 633 nm and 800 nm. Also plotted is the d.c. resistance. The relatively increased noise in the variance for the shorter wavelengths is due to the smaller number of pixels per speckle at these shorter wavelengths.

Mentions: Optical speckle patterns are a result of the interaction of laser light with the local optical features of a surface in terms of the spatial distribution of dielectric contrast. The speckle patterns from the VO2 surface show a variation of the average brightness, speckle position and image contrast as a function of temperature and wavelength. A simplified diagram of the experimental setup and an example of a typical speckle pattern is given in Fig. 2a. To quantify speckle pattern changes, the intensity statistics were calculated from each image and plotted as a function of temperature in Fig. 2(b–g). The statistical properties of the speckle show a transitional behavior close to the TMIT evident in the resistance data. The mean value of the speckle intensity MI = 〈I〉 and the variance V = (〈I2〉 − 〈I〉2) show the onset of the transition in the form of a significant change in magnitude within a temperature range of 57–60°C. As in the temperature dependent resistance, the first order phase transition produces a hysteresis in the MI and the variance although with a narrower width. The decrease in the MI is easily understood since the overall reflectivity is expected to increase across the transition due to the growth of the metallic phase volume which has higher reflectivity. This implies an increase in the principal reflected specular intensity and a corresponding decrease in the measured diffused speckle intensity. The transition in the optical measurements occurs at a higher temperature than the electronic transition on both heating and cooling. This can be attributed to the fact that in percolative systems, low resistance pathways can be present already at low metal domain coverages of 20%16. The optical response, where the effective lengthscale of the metallic domains needs to be on the order of the wavelength used15, requires a higher metal domain coverage for it to be sensitive to the domain configuration.


Resolving transitions in the mesoscale domain configuration in VO2 using laser speckle pattern analysis.

Seal K, Sharoni A, Messman JM, Lokitz BS, Shaw RW, Schuller IK, Snijders PC, Ward TZ - Sci Rep (2014)

(a) Simplified diagram of experimental setup for speckle pattern collection. Polarized laser light is focused onto a sample using a lens and is reflected from the VO2 surface where green and blue represent regions of different dielectric values. The reflected diffuse (dashed) speckle pattern is sampled using a CCD, the specularly reflected light (solid) is not collected. The image gives an example of a typical speckle pattern under 800 nm illumination at 60°C. (b)–(g) Variance (b, c, d) and mean intensity (e, f, g) as a function of temperature plotted for three illumination wavelengths, 488 nm, 633 nm and 800 nm. Also plotted is the d.c. resistance. The relatively increased noise in the variance for the shorter wavelengths is due to the smaller number of pixels per speckle at these shorter wavelengths.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4151099&req=5

f2: (a) Simplified diagram of experimental setup for speckle pattern collection. Polarized laser light is focused onto a sample using a lens and is reflected from the VO2 surface where green and blue represent regions of different dielectric values. The reflected diffuse (dashed) speckle pattern is sampled using a CCD, the specularly reflected light (solid) is not collected. The image gives an example of a typical speckle pattern under 800 nm illumination at 60°C. (b)–(g) Variance (b, c, d) and mean intensity (e, f, g) as a function of temperature plotted for three illumination wavelengths, 488 nm, 633 nm and 800 nm. Also plotted is the d.c. resistance. The relatively increased noise in the variance for the shorter wavelengths is due to the smaller number of pixels per speckle at these shorter wavelengths.
Mentions: Optical speckle patterns are a result of the interaction of laser light with the local optical features of a surface in terms of the spatial distribution of dielectric contrast. The speckle patterns from the VO2 surface show a variation of the average brightness, speckle position and image contrast as a function of temperature and wavelength. A simplified diagram of the experimental setup and an example of a typical speckle pattern is given in Fig. 2a. To quantify speckle pattern changes, the intensity statistics were calculated from each image and plotted as a function of temperature in Fig. 2(b–g). The statistical properties of the speckle show a transitional behavior close to the TMIT evident in the resistance data. The mean value of the speckle intensity MI = 〈I〉 and the variance V = (〈I2〉 − 〈I〉2) show the onset of the transition in the form of a significant change in magnitude within a temperature range of 57–60°C. As in the temperature dependent resistance, the first order phase transition produces a hysteresis in the MI and the variance although with a narrower width. The decrease in the MI is easily understood since the overall reflectivity is expected to increase across the transition due to the growth of the metallic phase volume which has higher reflectivity. This implies an increase in the principal reflected specular intensity and a corresponding decrease in the measured diffused speckle intensity. The transition in the optical measurements occurs at a higher temperature than the electronic transition on both heating and cooling. This can be attributed to the fact that in percolative systems, low resistance pathways can be present already at low metal domain coverages of 20%16. The optical response, where the effective lengthscale of the metallic domains needs to be on the order of the wavelength used15, requires a higher metal domain coverage for it to be sensitive to the domain configuration.

Bottom Line: The configuration and evolution of coexisting mesoscopic domains with contrasting material properties are critical in creating novel functionality through emergent physical properties.However, current approaches that map the domain structure involve either spatially resolved but protracted scanning probe experiments without real time information on the domain evolution, or time resolved spectroscopic experiments lacking domain-scale spatial resolution.Our straightforward analysis of laser speckle patterns across the first order phase transition of VO2 can be generalized to other systems with large scale phase separation and has potential as a powerful method with both spatial and temporal resolution to study phase separation in complex materials.

View Article: PubMed Central - PubMed

Affiliation: 1] Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA [2] Department of Physics &Astronomy, University of Tennessee, Knoxville, TN 37996, USA.

ABSTRACT
The configuration and evolution of coexisting mesoscopic domains with contrasting material properties are critical in creating novel functionality through emergent physical properties. However, current approaches that map the domain structure involve either spatially resolved but protracted scanning probe experiments without real time information on the domain evolution, or time resolved spectroscopic experiments lacking domain-scale spatial resolution. We demonstrate an elegant experimental technique that bridges these local and global methods, giving access to mesoscale information on domain formation and evolution at time scales orders of magnitude faster than current spatially resolved approaches. Our straightforward analysis of laser speckle patterns across the first order phase transition of VO2 can be generalized to other systems with large scale phase separation and has potential as a powerful method with both spatial and temporal resolution to study phase separation in complex materials.

No MeSH data available.


Related in: MedlinePlus