Chapter 11Ultrafast MeasurementTechniques11.1Pump Probe Measurements11.1.1Non-Colinear Pump-Probe Measurement:Pump puiseTestdeviceChopperMode-locked Laser Beam spltterS(t)SlowdetectorProbe pulseFtLock-ln4AmnieTime delay between1fs<>0.15μmpumpandprobepulseS(t translation stageComputerscreenFigure 11.1: Non-colinear pump-probe setup with co-polarized pump-probebeams.Adapted from U. Keller.Figure 11.1 shows a non-colinear pump-probe measurement setup. To sup-press background light and low frequency noise of the probe beam the pump371
Chapter 11 Ultrafast Measurement Techniques 11.1 Pump Probe Measurements 11.1.1 Non-Colinear Pump-Probe Measurement: translation stage 1 fs <=> 0.15 µm Beam splitter S(t) Slow detector Time delay between pump and probe pulse t Computer screen Pump pulse Probe pulse Lens Test device Chopper Lock-In Amplifier S(t) t Mode-locked Laser Figure 11.1: Non-colinear pump-probe setup with co-polarized pump-probe beams. Adapted from U. Keller. Figure 11.1 shows a non-colinear pump-probe measurement setup. To suppress background light and low frequency noise of the probe beam the pump 371
372CHAPTER 11.ULTRAFAST MEASUREMENT TECHNIQUESbeam is chopped. Typical chopper frequencies of regular mechanical chop-pers are fch = 100Hz- 2kHz: Mechanical choppers up to 20kHz have beenbuilt. With acousto-optic modulators or electro-optic modulators chopperfreuqencies up to several hundred MHz are possible.Lets denote Sin = So+&S as the probe pulse energy,where So is theaverage value and dsa low frequency noise of the pulse source and S(t)is the probe signal transmitted through thetest device.Then thedetectedsignal transmitted through the test device can be written as(11.1)S(t) = T(P(t))SindT(Pom(t))= ToSin +dpwhereTois the transmission without pumppulse, Po is the pumppulse energyand m(t) the chopper modulation function. It is obvious that if the noise ofthe probe laser ss is of low freuqency, then the signal can be shifted awayfrom this noise floor by chosing an appropriately large chopper frequency inm(t). Ideally, the chopper frequency is chosen large enough to enable shotnoise limited detection.Sometimes the test devices or samples have a rough surface and pumplight scattered from the surface might hit the detector.This can be partiallysuppressed by orthogonal pump and probe polarizationThis is a standard technique to understand relaxation dynamics in con-densed matter, such as carrier relaxation processes in semiconductors forexample.11.1.2ColinearPump-ProbeMeasurement:Sometimes pump and probe pulses have to be collinear, for example whenpump probemeasurements of waveguide devices have to be performed. Thenpump and probe pulse, which might both be at the same center wavelengthhave to be made separable. This can be achieved by using orthogonal pumpand probe polarization as shown in Figure 11.2 or by chopping pump andprobe at different frequencies and detecting at the difference frequency, seeFigure 11.3
372 CHAPTER 11. ULTRAFAST MEASUREMENT TECHNIQUES beam is chopped. Typical chopper frequencies of regular mechanical choppers are fch = 100Hz − 2kHz . Mechanical choppers up to 20kHz have been built. With acousto-optic modulators or electro-optic modulators chopper freuqencies up to several hundred MHz are possible. Lets denote Sin = S0 + δS as the probe pulse energy, where S0 is the average value and δs a low frequency noise of the pulse source and S(t) is the probe signal transmitted through the test device. Then the detected signal transmitted through the test device can be written as S(t) = T(P(t))Sin (11.1) = T0Sin + dT dP (P0m(t)) where T0 is the transmission without pump pulse, P0 is the pump pulse energy and m(t) the chopper modulation function. It is obvious that if the noise of the probe laser δS is of low freuqency, then the signal can be shifted away from this noise floor by chosing an appropriately large chopper frequency in m(t). Ideally, the chopper frequency is chosen large enough to enable shot noise limited detection. Sometimes the test devices or samples have a rough surface and pump light scattered from the surface might hit the detector. This can be partially suppressed by orthogonal pump and probe polarization This is a standard technique to understand relaxation dynamics in condensed matter, such as carrier relaxation processes in semiconductors for example. 11.1.2 Colinear Pump-Probe Measurement: Sometimes pump and probe pulses have to be collinear, for example when pump probe measurements of waveguide devices have to be performed. Then pump and probe pulse, which might both be at the same center wavelength have to be made separable. This can be achieved by using orthogonal pump and probe polarization as shown in Figure 11.2 or by chopping pump and probe at different frequencies and detecting at the difference frequency, see Figure 11.3
11.1.PUMPPROBEMEASUREMENTS373ProbepulseTestdevicePump puiseChopperLensPBSMode-Locked LaserPBSMZO入/2-plateOS(t)SlowdetectorLock-InAmplifierS(t)Figure 11.2: Colinear pump-probe with orthogonally polarized pump andprobe beams.Adapted from U. Keller.ProbepulseTestdevicePumppulseChopperfLensMLLS(t)SlowVtldetectorLock-InChopperf2Amplifieratff-f21S(t)Figure11.3:Colinearpump probewith choppingof pumpandprobe andlock-indetectionat thedifferencefrequency.Adapted from U. Keller
11.1. PUMP PROBE MEASUREMENTS 373 t Pump pulse Probe pulse Lens Test device S(t) Slow detector Chopper Lock-In Amplifier S(t) t PBS PBS λ/2-plate Mode-Locked Laser Figure 11.2: Colinear pump-probe with orthogonally polarized pump and probe beams. Adapted from U. Keller. MLL Pump pulse Probe pulse Lens Test device S(t) Slow detector S(t) t Chopper f1 Chopper f2 Lock-In Amplifier at f - f 1 2 Figure 11.3: Colinear pump probe with chopping of pump and probe and lock-in detection at the difference frequency. Adapted from U. Keller
374CHAPTER11.ULTRAFASTMEASUREMENTTECHNIQUES11.1.3Heterodyne Pump ProbeThe lock-in detection is greatly improved if the difference frequency at whichthe detection occurs can be chosen higher and the signal can be filtered muchbetter using a heterodyne receiver.This is shown in Figure 1l.4, whereAOM's are used to prepare a probe and reference pulse shiftet by 39 and 40MHz respectively. The pump beam is chopped at 1kHz. After the test devicetheprobeand referencepulseare overlayed with each other by delaying thereference pulse in a Michelson-Interferometer.The beat note at 1MHz isdownconverted to base band witha receiver.IotnnueePumppChopperMLLAOMT>>t+39 MhzAOM+40MHzReferenceProbeS(t)Slowdetector[1MHzReceiverLock-InAmplifierSftFigure 11.4:Colinear pump probe measurement with parallel polarizationand largedifferencefrequency.Adapted from U.Keller.If a AM or FM receiver is used and the interferometers generating thereference and probe pulse are interferometerically stable, both amplitude andphase nonlinearities can be detected with high signal to noise
374 CHAPTER 11. ULTRAFAST MEASUREMENT TECHNIQUES 11.1.3 Heterodyne Pump Probe The lock-in detection is greatly improved if the difference frequency at which the detection occurs can be chosen higher and the signal can be filtered much better using a heterodyne receiver. This is shown in Figure 11.4, where AOM’s are used to prepare a probe and reference pulse shiftet by 39 and 40 MHz respectively. The pump beam is chopped at 1kHz. After the test device the probe and reference pulse are overlayed with each other by delaying the reference pulse in a Michelson-Interferometer. The beat note at 1MHz is downconverted to base band with a receiver. Chopper t Pump pulse Probe pulse Test device S(t) Slow detector S(t) t Lock-In Amplifier AOM +39 Mhz AOM Probe +40 MHz Reference MLL T>>t Reference 1 MHz Receiver Figure 11.4: Colinear pump probe measurement with parallel polarization and large difference frequency. Adapted from U. Keller. If a AM or FM receiver is used and the interferometers generating the reference and probe pulse are interferometerically stable, both amplitude and phase nonlinearities can be detected with high signal to noise
11.1.PUMPPROBEMEASUREMENTS375TestdeviceProbepulsePumppulseReferChopperMLLLPZTAOMT>>t +39 MhzAOM+4oMHzReferenceProbeS(t)Slow.detector1 MHz HamRadioReciverAM or FMLock-InAmplifierS(t)Figure 11.5: Heterodyne pump probe using AM and FM receiver to detectamplitude and phase nonlinearities.Adapted from U.Keller
11.1. PUMP PROBE MEASUREMENTS 375 Chopper t Pump pulse Probe pulse Test device S(t) Slow detector S(t) t Lock-In Amplifier AOM +39 Mhz AOM Probe +40 MHz Reference MLL T>>t Reference 1 MHz Ham Radio Reciver AM or FM PZT Figure 11.5: Heterodyne pump probe using AM and FM receiver to detect amplitude and phase nonlinearities. Adapted from U. Keller