UV Fluorescence from Integrating Spheres Measurement and Theory

Transcription

1 UV Fluorescence from Integrating Spheres Measurement and Theory Ping-Shine Shaw, Zhigang Li, Uwe Arp, and Keith R. Lykke National Institute of Standards and Technology, Gaithersburg, MD, USA

2 Outline: 1. UV fluorescence measurement using lasers. 2. Determination of the total spectral fluorescence yield of an integrating sphere. 3. Theory of fluorescence from an integrating sphere.

3 Integrating Sphere for UV Application - irradiance calibration of deuterium lamps using synchrotron radiation at NIST SURF III Integrating sphere Calculable source standard Temperature Stabilized PMT Deuterium lamp Monochromator X-Y-Z translation stage Facility for Irradiance Calibration Using Synchrotrons (FICUS)

4 Problem with integrating sphere in the UV - fluorescence 1.2 Relative throughput of an integrating sphere measured with monochromatic and polychromatic light Relative throughput with monochromatic light with polychromatic light from deuterium lamp Wavelength (nm)

5 UV Characterization of integrating spheres Laser Induced Fluorescence (LIF) Integrating sphere Tunable UV laser Temperature Stabilized PMT Monochromator

6 UV induced fluorescence from typical PTFE integrating spheres excited by 22 nm laser.24.2 Normalized signal Sintered PTFE #3 Sintered PTFE #2 Sintered PTFE #1. Pressed PTFE Wavelength (nm)

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8 Sintered PTFE integrating spheres exposed to diesel gas exhaust With 22nm excitation laser Signal 1.E+ 1.E-1 1.E-2 1.E-3 before exposed baked for 19 h baked for 68 h 1.E-4 1.E Wavelength (nm)

9 Raw data from laser induced fluorescence measurement of integrating sphere 1.E+ 1.E-1 Normalized signal 1.E-2 1.E-3 1.E-4 1.E-5 1.E-6 1.E-7 1.E-8 1.E nm 268 nm 245 nm 221 nm 211 nm 25 nm Wavelength / nm P.S. Shaw, Z. Li, U. Arp, and K.R. Lykke Applied Optics, Vol. 46, p (27)

10 The total spectral fluorescence yield Define the total spectral fluorescence yield of an integrating sphere as f λ, total ( λ f, λ ) = Φ λ, fl ( λ Φ f, λ ) Laser λ Fluorescence λ f Φ : power of the primary radiation λ entering the integrating sphere. Φ λ,fl (λ f, λ ) : the total spectral power of fluorescence excited.

11 Measuring total spectral fluorescence yield 1. System response calibration Φ λ,cal (λ) Spectrolon Integrating sphere Calibrated UV source Temperature Stabilized PMT Signal S cal (λ) System response R s ( λ) = S Φ cal ( λ), ( λ) λ cal Monochromator

12 Measuring total spectral fluorescence yield 2. Relative laser power determination from in-band slit scattering function Normalized signal 1.E+ 1.E-1 1.E-2 1.E-3 Normalized signal nm laser inband Fit with Hg lamp result Wavelength / nm 1.E Wavelength (nm) Relative laser power Φ = in band s S( λ) dλ R ( λ )

13 Measuring total spectral fluorescence yield 3. Relative spectral fluorescence power determination Normalized signal 1.E+ 1.E-1 1.E-2 1.E-3 Normalized signal Wavelength / nm Fluorescence excited by 25 nm laser Fitted background from 295 nm laser 1.E Wavelength (nm) Spectral fluorescence yield f λ, total ( λ, λ ) = [ S( λ) S ( λ) ] s bg R ( λ) in band s S( λ ) dλ i R ( λ ) i

14 Total spectral fluorescence yield of an integrating sphere Spectral fluorescence yield x1 4 / nm nm 211 nm 221 nm 245 nm 268 nm Wavelength / nm P.S. Shaw U. Arp and K.R. Lykke, to be published in Metrologia

15 Theory of fluorescence from integrating spheres Laser λ Fluorescence λ f P.S. Shaw and Z. Li, Applied Optics, Vol. 47, page 3962 (28)

16 Basic integrating sphere throughput formula 2 nd ref. Throughput of an integrating sphere can be calculated by summing contribution from all reflections inside the sphere as Φ in (λ ) 3 rd ref. 1 st ref. Φ Φ out in ( λ ) ( λ ) = ρ( λ ) αa 1 ρ( λ )(1 r A r ) Φ out (λ ) A r : total port area α : exit port area / total port area ρ(λ) : diffuse reflectance of the inner coating in hemispherical illumination geometry.

17 Calculated integrating sphere throughput as a function of reflectance 1. Throughput Reflectance A r = 5% A r = 1% A r =.2%

18 Calculation of the fluorescence excited by multiple reflection of the incident radiation Φ in (λ ) By summing fluorescence from all reflections inside the sphere, the fluorescence yield is Ψ Φ out in ( λ ) = ( λ ) f ( λ, λ ) ρ( λ ) αa f r [(1 ρ( λ )(1 A )][ 1 ρ( λ )(1 A )] f f f r r f(λ f, λ ) : fluorescence yield at λ f for a plaque excited by λ. Φ out (λ ) Ψ out (λ f ) Define a fluorescence gain factor, κ, as the ratio of the fluorescence emerging from an integrating sphere to that of a flat sample excited by the same incident beam. κ = ρ( λ ) αa f r [ 1 ρ( λ )(1 A )][(1 ρ( λ )(1 A )] f r r

19 Calculated fluorescence gain of an integrating sphere ρ(λ ) ρ(λ.95 f ) Total port area = 1% High fluorescence gain of an integrating sphere caused by multiple reflection of exciting radiation

20 Calculated fluorescence gain as a function of port area 3 Fluorescence gain ρ(λ )=.99 ρ(λ )=.98 ρ(λ )= A r

21 Theoretical expression for the measured total spectral fluorescence yield of an integrating sphere Spectral fluorescence yield x1 4 / nm nm 211 nm 221 nm 245 nm 268 nm Wavelength / nm Total spectral fluorescence yield f λ, total ( λ, λ ) f = f ( λ, λ ) f 1 [ 1 ρ( λ )(1 A )] r

22 Conclusion Integrating sphere fluorescence is mainly caused by contamination of the wall coating. Total fluorescence yield of an integrating sphere can be derived from the measurement result of laser induced fluorescence. Integrating sphere fluorescence has been modeled theoretically. Fluorescence from an integrating sphere is enhanced because of multiple reflection of the incident beam. Integrating spheres can potentially be used as sensitive contamination detection devices.