1 Characterization of TPB optical properties Enrico Nichelatti UTTMAT-OTT Optical Devices Lab ENEA – C.R. Casaccia Rome, Italy TPB Workshop - MIT 10/19/2012

2 Outline Hemispheric spectral measurements Correction for optically-active samples Photoluminescence and photoexcitation spectra Results for TPB layers* *samples prepared at INFN-LNGS (AQ) Italy TPB Workshop - MIT 10/19/2012

3 Hemispheric optical reflectance R and transmittance T (and absorbance A) incident light hemispheric reflectance R hemispheric transmittance T sample R + T + A = 1 photoluminescence PL TPB Workshop - MIT 10/19/2012

4 PROBLEM! Measurements of R and T (and A) of an optically-active sample are heavily affected by unwanted Stokes-shifted PL which is excited by the probe beam. In a typical spectrophotometer, while the impinging probe beam is filtered in wavelength, the reflected (transmitted) beam is detected with no spectral filtering. Is there a way to “clean” measured photometric spectra of TPB from PL? TPB Workshop - MIT 10/19/2012

5 integrating sphere TPB Photoluminescence affects the measurement of R Let us insert a suitable low-pass filter… … no more detected photoluminescence! Of course, some data elaboration is needed… TPB Workshop - MIT 10/19/2012

6 A suitable low-pass filter is needed… For TPB, range 2 spans from well below λ = 250 nm (lower measurement limit) up to λ ~ 380 nm. Range 3 is roughly 380 nm < λ < 420 nm. TPB Workshop - MIT 10/19/2012

7 Spectrophotometer Lambda 950 PERKIN ELMER UV/VIS/NIR, compatible with optical cryostat cooled at LNT Spectrofluorometer Fluorolog 3 ISA JOBIN YVON-SPEX Mod.FL3-11 compatible with optical cryostat cooled at LNT ENEA, C.R. Frascati, V. E. Fermi 45, Frascati (Rome) Italy, TPB Workshop - MIT 10/19/2012

8 TPB (167  g/cm 2 ) over glass slab VM2000 foil PL spectra of the TPB sample (167 µg/cm 2 ) excited at 295 nm, 348 nm and 380 nm. PLE spectra of the TPB sample (167 µg/cm 2 ) measured at 422 nm and 445 nm. PL spectra of the VM2000 foil. TPB Workshop - MIT 10/19/2012 ENEA, C.R. Frascati,

TPB Worshop - MIT 10/19/20129 Sorgente Monocromato re di ingresso Modulo portacampione Monocromator e di uscita Controller PMPM Spectrofluorometer equipped with a Xenon lamp Fluorolog 3 ISA JOBIN YVON- SPEX Mod.FL3-11 Ozone-free Xe lamp (450W) emission spectrum

10 Measurement correction (reflectance): low-pass filter requirements as measured spectrum (without filter) spectrum with filter placed between sample and detector evaluation of true sample reflectance LEGEND TPB Workshop - MIT 10/19/2012

11 Spectrophotometer and integrating sphere Perkin-Elmer Lambda 19 (dual-beam UV-Vis spectrophotometer) Perkin-Elmer mod. B (15 cm diameter integrating sphere) TPB Workshop - MIT 10/19/2012

12 УФС  2low-pass filter УФС  2 low-pass filter VM2000 naked foil TPB (372  g/cm 2 ) over VM2000 foil TPB layer over VM2000: null transmittance over all the examined range. High R at λ > 420 nm could be due to TPB or VM2000 or an intermediate situation. Low R at λ < 380 nm, lower than R of VM2000, can be ascribed to low R of TPB itself and high absorbance A of it. TPB Workshop - MIT 10/19/2012 VM2000 naked foil

13 glass slab polystyrene-diluted TPB over glass slab TPB (356  g/cm 2 ) over glass slab The plots demonstrate that TPB has an absorption band at about 350 nm (3.54 eV). The further absorption cutoff at λ < 300 nm for the polystyrene-diluted TPB is ascribed to the glass substrate. polystyrene over glass TPB Workshop - MIT 10/19/2012

Hemispheric T and R for various TPB surface densities of TPB layer on glass. Comparison of spectral properties of TPB layers on glass as a function of surface density Direct T of the glass substrate and of various TPB surface densities (  g/cm 2 ) of TPB layer on glass. TPB over glass slab glass slab

15 Conclusions An ad hoc procedure has been introduced to optically characterize optically-active samples such as TPB layers. The procedure allows correcting systematic errors introduced in the hemispheric T and R spectral measurements by (unwanted) photoluminescence. Results for TPB layers on different substrates, including PL ad PE spectra, have been shown. They confirm that TPB has an absorption band centred at about 350 nm. Quantitative measurements indicate that UV-VIS reflectance increases with surface density of TPB for samples evaporated on glass in the investigated surface densities range. TPB Workshop - MIT 10/19/2012 THANK YOU FOR YOUR ATTENTION