F9180: Diagnostické metody 2 Time-Correlated Single Photon Counting Methods Tomáš Hoder Doporučená literatura: [1] D.V.O’Connor and D.Phillips - Time-correlated Single Photon Counting, 1984 [2] W.Demtroeder - Atoms, Molecules and Photons, 2006 [3] W.R.Ware – Techniques of pulse fluorometry Time-Resolved Fluorescence Spectroscopy in Biochemistry and Biology (NATO ASI Series A: Life Sciences) vol 69, ed R.B.Cundall and R.E.Dale (New York: Plenum), 1983 [4] K.V.Kozlov et al. 2001 Spatio-temporally resolved spectroscopic diagnostics of the barrier discharge in air at atmospheric pressure, J.Phys.D:Appl.Phys. 34 3164 [5] W.Becker 2007 Advanced Time-correlated Single Photon Counting Techniques [6] W.Becker 2014 TCSPC Handbook Overview •TC-SPC technique/idea •Time scales, where we can use it, where not •Light emission, fluorescence, quenching •Sensitivity, StNR, time resolution, PMT transit •Use in plasma-physics and synchronization •Selected examples for gas discharge quantitative spectroscopy • • • • TC-SPC technique SYNC-Signal: •represents shape of the full light pulse •giving a time reference START STOP Delay single photon accumulation MAIN-Signal: •random single photons •spatially resolved •spectrally resolved micro-discharge first counted photon time relative time information •triggering possible on stochastically appearing events in time as the time reference is set on the discharge itself >>> cross-correlation spectroscopy TC-SPC basics Becker 2006 Advanced TCSPC techniques Instead of having problems with slow analogue PMT signal … TC-SPC obtains light intensity by counting pulses as a digital units in subsequent time channels: §free of gain noise of PMT §free of electronic noise of accidental signals to PMT §high signal-to-noise ratio (to PMT background counting rate) §higher time-resolution (Transit Time Spread << Single Photoelectron Response/Transit time) Basics TC-SPC solves: •Problems of triggering/synchro – recording of irregular emission events •Problems of time resolution – no limitation by transit times or SER (single electron response) of detectors •Problems of sensitivity – statistical principles behind the accumulative recordings • Convolution -> cross-correlation Screen Shot 2016-01-06 at 09.23.20.png wiki (δ-fce) Relevant integral transformations: Screen Shot 2016-01-05 at 16.15.12.png wiki Different time scales and appropriate light detection techniques Gated cameras: •Time resolution from seconds to usually 2ns (new models down to 200 or even 50ps) •Almost impossible to synchronize to time-irregular emission events (random shot is the time consuming solution) •Sensitivity of the recording is given by the StNR of given device. Usually the noise increases linearly with number of accumulation cycles. Weak signals are not easy to record. • EEDF relaxation ps … radiative lifetimes 10ns … effective quenching at 1atm ns … relaxation of chemical kinetics ns … streamer structure 10ps … Different time scales and appropriate light detection techniques Start-stop TCSPC: •Time resolution from hundreds of nanoseconds to 10ps •Possible to synchronize to time-irregular emission events with the same high resolution in time •Sensitivity of the recording is given by the StNR of given synchronization arrangement. Poisson statistics is the limiting mechanism. •Limited for high-frequency repetition emission events. Limited by the speed of electronics of the counter dealing with high-frequency input. • Time-to-amplitude converter … first time 1961 by Koechlin (Thesis, Uni Paris) Different time scales and appropriate light detection techniques Reversed start-stop TCSPC: •Time resolution from hundreds of nanoseconds to 10ps •Possible to synchronize to time-irregular emission events •Sensitivity of the recording is given by the StNR of given synchronization arrangement. Poisson statistics is the limiting mechanism. •No limitation for high-frequency repetition emission events. Input processing only for the “main” signal. • SYNC-Signal: •represents shape of the full light pulse •giving a time reference START STOP Delay MAIN-Signal: •random single photons •spatially resolved •spectrally resolved micro-discharge Different time scales and appropriate light detection techniques Streak cameras: •Time resolution down to units of picoseconds (some new models down to hundreds of femtoseconds) •Sensitivity of the recording comparable to the TC-SPC 3775Figure.png Different time scales and appropriate light detection techniques Pump-and-probe technique: •Time resolution down to femtoseconds using femtosecond laser pulses •Possible to synchronize due to the synchronous generation of the fluorescence by the pumping laser pulse. • Light emission, fluorescence The main reason and use of TCSPC is in fluorescence lifetime imaging microscopy >>> proto ted lifetime, je to potřeba i ve fyzice plazmatu Light emission, fluorescence Light emission, fluorescence Radiative lifetime measurement (j = n) (k = n) Effective lifetime and quenching Effective lifetime and quenching Dilecce et al. 2010 J.Phys.D TC-SPC technique SYNC-Signal: •represents shape of the full light pulse •giving a time reference START STOP Delay single photon accumulation MAIN-Signal: •random single photons •spatially resolved •spectrally resolved micro-discharge first counted photon time relative time information •triggering possible on stochastically appearing events in time as the time reference is set on the discharge itself >>> cross-correlation spectroscopy Convolution >>> cross-correlation Screen Shot 2016-01-06 at 09.23.20.png wiki (δ-fce) TCSPC statistics scheme PMT_2-1 NE zi wi NA TAC Ni=NA i-th channel Ni ND NA number of counts (anode pulses) in i-th interval TC-SPC statistics basics 1 Photoelectrons generated by impinged photons on the cathode with given quantum efficiency. The probability of emission of l photoelectrons in the i-th interval is given by the Poisson distribution. The Taylor series of the exponential function is (1-wi+wi2/2+…), we take the first two. TCSPC statistics scheme PMT_2-1 NE zi wi NA TAC Ni=NA i-th channel Ni ND NA number of counts in i-th interval TC-SPC statistics basics 2 After developing to Taylor series, as shown before: And it follows: After a large number of excitation pulses NE, the number of anode pulses NA in the i-th interval. Therefore the number of anode pulses NA is proportional to the intensity of the fluorescence at time ti. TCSPC statistics scheme PMT_2-1 NE zi wi NA TAC Ni=NA i-th channel Ni ND NA number of counts in i-th interval TC-SPC statistics basics 3 Relation of NA to number of Counts in the i-th channel Ni: Because the TAC detects only the first photon in given time interval for a given excitation cycle, NA is not the number of counts in the i-th channel Ni. The true relation is given left. Consequently the count in channel i is a measure of the fluorescence intensity at time ti. TC-SPC statistics basics 4 Screen Shot 2016-01-05 at 21.31.39.png PILE-UP effect Other issues to be aware of •Color effect (consequence of photoeffect) •Afterpulsing (consequence of PMT setup) •Ultra-short reflections •… • Sensitivity and precision •The effect of PMT noise is greatly reduced by the mode of TAC operation >>> enhanced Signal-to-Noise ratio (up to 100x noise reduction) •Noise due to the dark counts on PMT (cooling, background subtraction …) • •Noise due to the counting error, number of counts in each channel I(ti) follows a Poisson distribution with a standard deviation σi given by σi=(I(ti))1/2 •It follows that to have 5% precision in the number of Ni counts in i-th channel, where the the curve decayed to 1% of its maximum value, one has: 0.05=1/σi =1/(Ni)1/2 and Ni is 400, that means one has to measure 40000 counts in maximum •Signal-to-Noise ratio is given as well by the Poisson distribution and is equal to the standard deviation: •Dynamic range (ratio between the largest and smallest value of measured quantity): for ICCD typically 1000:1, streak 10000:1, for TCSPC usually 100000:1 and more • Comparison Screen Shot 2016-01-06 at 14.57.56.png Hamamatsu News 2009 PMT and MCP structure PMT_2-1 Gain of up to 108 PMT resolution, transit time Becker 2006 Advanced TCSPC techniques Leading edge discrimination CFD pulse-height-induced timing jitter avoided PMT resolution, transit time Becker 2006 Advanced TCSPC techniques PMT transit time spread getImage.xqy.jpeg PMT transit Lakowicz 2006 Principles of fluorescence spectroscopy TC-SPC review Becker 2006 Advanced TCSPC techniques light intensity > 0.01 to 0.1 photons per signal period à Pile-Up Problem Relatively slow recording speed and long data acquisition times à high repetition rates and low dead time (approx. 100 ns; i.e. 107 photons/s) Use in plasma-physics and signal synchronization •Short discharges with high repetition: • rf discharges, barrier discharges, Trichel • pulsing corona, self-pulsing • sparks •Synchronization via light pulse, current pulse, laser excitation or TTL of applied voltage waveform • Kinetic scheme dependent example •Streamer discharges generated in atmospheric pressure air •Spectra is dominated by the second positive system of molecular nitrogen •Relatively weak bands of first negative system are present as well Kinetic scheme dependent example •Streamer discharges generated in atmospheric pressure air •Spectra is dominated by the second positive system of molecular nitrogen •Relatively weak bands of first negative system are present as well Kinetic scheme dependent example •Streamer discharges generated in atmospheric pressure air Trichel pulse corona •Breakdown in negative corona Trichel pulse Setup for corona and calibration Calibration •Streamer discharges generated in atmospheric pressure air Trichel pulse electric field SPS FNS Limits for TCSPC on streamers Quantum mechanics based example Obradovic 2008 APL Quantum mechanics based example Kuraica 1997 APL RF discharge in helium at 1atm Navratil 2015 TCSPC results Quantum mechanics based example Navratil 2015 TCSPC results Summary •The principles and technical realization of the TCSPC technique were introduced •The measured fluorescence from the atomic/molecular transitions was followed back to its origin •Selected examples of quantitative high-resolution spectroscopy were presented