Investigation of Lasers and Play-Doh Diego Alcala, John Arko, Rachel Bennet, Kyle Kingsley Abstract: Ever since George Lucas described Luke Skywalker’s lightsaber as “a green laser beam” which sprang forth from an electronic sword hilt, people have been fascinated by the properties of lasers. For instance, laser light does not change color when pointed at a colored surface; green light is green whether reflected from a white wall or a red fire engine. However, there is an instance when a green laser may break this rule: when shone upon pink Play-Doh. The goal of this investigation is to find a reasonable explanation for the phenomenon. In other words... Analysis and Conclusions: Although the PMT spectrum (Fig. 5) appears to indicate a spike at 562 nm, within the wavelength range of the yellow light produced from the green laser pen, these results are inconclusive. Multiple attempts at recreating the spectrum resulted only in similar noise graphs. Since the light appears on other colors of Play-Doh besides pink (namely purple and orange), the team suspected that a red dye in the Play-Doh was the culprit. Investigation revealed that the green laser pen is a neodymium: YAG (Nd:YAG) laser. The YAG (yttrium aluminum garnet) lases at a wavelength of 1064 nm. The laser pen uses a frequency doubling crystal such as potassium titanyl phosphate to shorten the wavelength to the 532 nm shown in Fig. 2. Nd:YAG lasers are often used for laser pumping. This process involves using a higher frequency laser light to excite the atoms of some target. When these atoms release the excess energy, a lower frequency of light is emitted. Dye pumping uses a small amount of dye as the target. Not all dyes will exhibit this behavior, which explains why this phenomenon is not observed when shining the laser on other colored surfaces. Investigation into Rhodamine dyes courtesy of Exciton Co.’s website revealed two good candidates: Rhodamine-590 and Rhodamine-560. Both have emission ranges which encompassed the yellow light observed, and both have a red appearance consistent with the red tint of the Play-Doh. The failure of the spectrometer to collect spectra of the emitted wavelengths still remains an enormous challenge to the theory. If a spectrum were collected, a few finer specifications on the green laser would be necessary to pinpoint the specific dye(s) present in the Play-Doh. How does it go from this… …to this? Figure 1: Polarizer Setup for Analysis of Laser Light Procedure: A SPEX 1250M spectrometer was used to analyze the specific wavelengths of light present before and after shining the laser through the Play-Doh. Amid concerns about the intensity of the laser light and potential damage to the detectors, the light was dimmed using a pair of polarizers set at 60° to one another (Fig. 1). The setup was used to obtain a preliminary spectrum of the green laser (Fig. 2) and the violet laser (Fig. 3) with the CCD (charge-coupled device) detector. This spectrum confirmed that the lasers were monochromatic, and thus the differently colored lights were not inherent in the lasers. To collect the colored light from the Play-Doh, the team applied a pea-sized amount of Play-Doh to the bottom of a glass dish as a thin film. (Fig. 4) The laser could then be set on the other side of the dish and shone through the Play-Doh, still producing the characteristic yellow light from the green laser. Since this light is too dim for analysis by the CCD, the team switched to the PMT (photomultiplier tube) detector. The PMT is generally better at detecting lower light levels. Figures 2 and 3: CCD Spectra of Green and Violet Lasers Figure 4: Setup for Analysis of Play-Doh Figure 5: PMT Spectrum of Light from Play-Doh Acknowledgments: Dr. Richard Dietz, UNC Physics; Dr. Richard W. Schwenz, UNC Chemistry & Biochemistry; Exciton Corporation, Dayton, OH