Jepsen, Peter Uhd

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Downloaded from orbit.dtu.dk on: Dec 19, 2017Ultrabroadband, time-resolved THz spectroscopy of disordered materialsJepsen, Peter UhdPublication date:2011Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Jepsen, P. U. (2011). Ultrabroadband, time-resolved THz spectroscopy of disordered materials [Sound/Visualproduction (digital)]. RIKEN Seminar, Sendai, Japan, 01/01/2011Peter Uhd Jepsen (puje@fotonik.dtu.dk)DTU Fotonik – Department of Photonics EngineeringTechnical University of DenmarkDK-2800 Kongens Lyngby, DenmarkUltrabroadband, time-resolved THz spectroscopy of disordered materialsRIKEN Seminar, Sendai, November 22nd 20112Outline• Single-cycle, ultrabroadband light at long wavelengths• Time-resolved THz spectroscopy• AC conductivity: Drude and non-Drude response– Silicon nanoparticles– Polymer solar cells• Adiabatic field compression to MV/cm field strengths3Early prediction of THz technology…“Get to the point man. What did you discover?”“Ah me, these excitable, highly strung Nordic races!” sighed Duval. “What we did was to make a type of low-powered radar set. Besides radiowaves of very high frequency, it used far infrared – all waves, in fact, which we were sure no creature could possibly see, however weird an eye it had.”…“We’ve proved that there is a large room behind that screen of yours. The screen is about three centimetres thick, and the space behind is at least ten meters across. We couldn’t detect any echo from the far wall, but we hardly expected to with the low power which was all we dared to use. However, we did get this.” He pushed across a piece of photographic paper on which was a single wavy line. In one spot was a kink like the autograph of a mild earthquake. “See that little kink?”“Yes, what is it?”“Only Karellen [the alien creature] […] If the resolution had been a bit better, we might even have calculated his size.”4Time-resolved Terahertz Spectroscopy (TRTS)Propagation directiontppPump pulseThin film photoexcitationamplitudephaseFFT5Extraction of spectroscopic dataTHz-TDS raw data:FFTTransmission amplitudeand phaseTransmission amplitude and phase are then used to calculate optical properties;Photoexcited charge carriers in semiconductors are best described by their conductivity:Thin film approximation: Tinkham equations for photoexcited carriersF. A. Hegmann, and K. P. H. Lui, Proc. SPIE 4643, 31 (2002)63D: Photoexcited GaAsCarriers in GaAs are well described by Drude model just picoseconds after photoinjection.05.12.2011Terahertz Technologies and Biophotonics7Time-dependent Drude parameters• Fit Drude model expression to data for increasing delay tpp• Time dependence of the Drude parameters• Example: semi-insulating GaAs excited @ 400 nmP. U. Jepsen, D. G. Cooke and M. Koch,Laser Photon. Rev. 5, 124-166 (2011).Time-dependent Drude parameters8 P. U. Jepsen, D. G. Cooke and M. Koch,Laser Photon. Rev. 5, 124-166 (2011).• Fit Drude model expression to data for increasing delay tpp• Time dependence of the Drude parameters• Example: semi-insulatingGaAs excited @ 400 nm• Parameter extraction reveals the(here: known) carrier dynamics• Instant rise of plasma frequency• Slower rise of scattering time• Slower rise of DC conductivityFDTD simulation of TRTS• In some situations the simpleFourier transform spectroscopyapproach is not sufficient tounderstand the THz-TDS data• Time-resolved spectroscopy neartpp = 0• Diffusion of charges in the sampleduring measurement• Master student Casper Larsencoded a TRTS experiment in FDTD• Thorough comparison betweenFDTD and experiment• GaAs, 800 and 400 nm excitation• Carrier dynamics explicitly includedin code• Full description of dispersion,diffraction, phase mismatch05.12.2011Terahertz Technologies and Biophotonics9 C. Larsen, D. G. Cooke and P. Uhd JepsenJ. Opt. Soc. Am. B 28, 1308 (2011)Simulation vs experiment05.12.2011Terahertz Technologies and Biophotonics101( , )pptσ ω2 ( , )pptσ ω7 µJ/cm2800 nm45 fsC. Larsen, D. G. Cooke and P. Uhd JepsenJ. Opt. Soc. Am. B 28, 1308 (2011)FDTD simulation tool available atwww.terahertz.dkAir Plasma Time-resolved THz SpectrometersampleHV @ 500 Hz2-colourplasmaBBOλ/2Black polyethylene(blocks residual pump)Sampling beam (350 mW)Generation beam (500 mW)BBOPMTLIALIA13 Hz13 HzinBlue filter1.5-1.7 W35 fs, 1 kHz25 mW 400 nmJ. Dai, X. Xie, X.-C. Zhang PRL 97,103903 (2006)ABCD:Experiment in reflection• 400 nm pump – THz probe• F400nm = 800 μJ/cm2samplefs opticalpumpTHz probeSimultaneous acquisition of Eref(t) and –ΔE(t)is SiO2 substrate index Reflection coefficient:Spectrometer performance1 – 11 THzSiO2 substrate limits useful bandwidth to 1 – 11 THz.(5 times spectroscopy to date)05.12.2011Terahertz Technologies and Biophotonics14Drude and non-Drude conductivity• Drude model: Mean free path                  (Fermi velocity)• What could happen if carriers are confined to• Enhanced backscattering• Can be phenomenologically modelled by the Smith generalization • Generalized current response function j(t). Truncate to first order• Kramers-Kronig compatible (causal response function)• Parameter c: degree of backscattering• Parameter c: reflection coeffient at boundary• Parameter c: first-order Taylor coefficient of generalized impulse responseN. V. Smith, Phys. Rev. B. 64, 1 5106 (2001)H. Nemec et al, Phys. Rev. B 79, 115309 (2009)Conductivity in disordered mediaelectrodes barriersVgrainsdc measurement ac measurementSmaller conducting grains can contribute more to conductivity at higher ω.dc conductivity is determined bythe highest barrier in conduction pathSilicon nanocrystals in glassanneal1100 oC20 nmHigh-resolution TEMVariable NP density:200 nm / 1 micron SiOx on SiO2 substratesAnnealed at 1100 C, 1 atm 95% N2 + 5% H2⇒Thermal decomposition of non-stoichiometric oxide: Si nanocrystals in SiO2x = 1.0 (31% silicon)Low bandwidth measurementsD. Cooke et al., Phys. Rev. B 73, 193311 (2006).L. Titova et al., Phys. Rev. B 83, 085403 (2011).  poly-nc Si Si-NCsin glass τ = 26 ± 1 fsc = -0.83 ± 0.01τ = 17.6 ± 0.5 fsc = -0.983 ± 0.001Fused silica substrate cuts off transmission above 3 THz. Reflection measurements inter-connected Si grains (~20 nm)BulkDisorderωcBulkDisorderωcConductivity dynamics: SiOx=0.2Drude behavior at early timesDrude-like behavior at early times!Carriers have yet to “feel” their confinement.Partially localizedLong range conduction limited by SiO2 barrierst = 0.5 ps t = 2.0 ps t = 100 psEvolution of conductivity: from Drude to partially localized responseSiOx=0.6 (51% silicon)Agreement with previous workt = +52 psSiOx = 0.6 (51% silicon)Reflection at particle boundary: R = 85%Particle diameter = 10 nm(courtesy of F. Hegmann group, Univ. Alberta, CA)Monte Carlo parameters:Excellent agreement with both low bandwidth work and Monte Carlo simulationsRandom sampling and no scaling of dataRdPlastic Photovoltaics• Solution processed• Lightweight• Cheap to manufacture• Compatible with large scale processing• Fundamental excitation thought to be an exciton.• Exciton EB > kBT• Device performance depends on charge generation through exciton dissociation.• Ultrafast charge generation mechanism is a topic of huge debate, fueled by transient absorption and fs flourescenceupconversion.• THz spectroscopy can contribute significantly hereP3HT/PCBM roll-to-roll processed films[6,6]-Phenyl-C61 Butyric acid Methyl esterPoly-3-HexylThiopheneMeasure on a real device-ready film!polyethylene800 nmP3HT/PCBM1:1 conc.400 nm photoexcitationFluence = 570 μJ/cm2Differential THz scans at early timesReference and differential spectra:Reference pulse:Differential pulses:Non-zero real conductivity is signature of mobile charges (delocalized polarons)Mobile charges created within 75 fs of excitation.(“Hot” exciton dissociation likely)Then: re-trapping of charges (~120 fs)Later: 1/3 free charges2/3 trapped chargesFree chargesfirst 120 fsSub-100 fs THz conductivityD. G. Cooke, F. Krebs, P. U. Jepsen, submitted (2011)Field enhancement in waveguidesChallenge: Get high enough THz field strength for NLO at THz frequenciesHere: Adiabatic compression of the field in a PPWGPerform NLOexperiments at theWG tip!Iwaszczuk, Zhang, Jepsen, in preparationField compression0 µm20 µm (λ/15)100 µm (λ/3)500 µm (1.5λ)In contact: no light through the tipSub-λ gaps:significant field at the tip>λ gaps: very high transmissionK. Iwaszczuk et al, in preparationMeasurement of the THz field strength• THz-enhanced SHG (χ(3) process)• Calibrated measurement of fieldstrengthK. Iwaszczuk et al, Appl. Phys. Lett. 99, 071113 (2011).Calibration of E-field measurement( )( ) ( )( ) ( )2 2 232 2THz bias THz biasI I E E E Eω ωχ  ∝ + + ( )22 2THz bias THzI E E Eω ∝ +2 / 2bias THzdI dE Eω =SHG in the presence of a THz field and a static field:Modulate the THz field, use lock-in detection:Modulate bias voltage, changeof SH gives THz field strength:                                                                                                                                                                                                                                                0,01 0,1 11E-51E-41E-3  Y =-2,78648E-5+0,00486 X measurement Linear fitSH [arb. units]ETHz2 [arb. units]K. Iwaszczuk et al, in preparationResults: ETHz = 1.4 MV/cm at WG tip-0.5 0.0 0.5 1.00.00.51.01.52.0  output gap: 60 µmEpeak = 0.6 MV/cmSH ~ E2  [arb. units]Position in waveguide [mm]K. Iwaszczuk et al, in preparation (2011)• Good agreement with full vectorial simulations• >1.4 MV/cm obtained with 20 µm gap• 100 kV/cm input field strengthConclusions05.12.2011Terahertz Technologies and Biophotonics31• THz spectroscopy is a good way to measure photoconductivity• Complex conductivity spectrum is measured• Conductivity models can be tested• Disordered systems show clear non-Drude behaviour• Ultrafast time resolution is important to reveal detailed dynamicsAcknowledgements:Krzysztof Iwaszczuk (PhD student, DTU Fotonik)Casper Larsen (Master student, DTU Fotonik)Prof. X.-C. Zhang (Rensselaer Polytechnic Institute)Funding:Carlsberg FoundationDanish Defence Acquisition and Logistics OrganizationH. C. Ørsted FoundationNew THz journal32New THz journal from IEEE!Each issue features interview articles with THzpioneers – March issue will host an interview withNobel Laureate Bob Wilson.Get a technical paper in the same issue!Submissions at http://mc.manuscriptcentral.com/ieee-thzUpcoming conferences33Submission Deadline: December 5thSubmission opens:January 16th 2012Submission deadline:February 17th 2012http://www.myeos.org/events/tst2012http://www.cleoconference.org/05.12.2011Terahertz Technologies and Biophotonics34Simultaneous data acquisition• ”Normal” THz time-domain spectroscopy– record reference data set (”sample OUT”, or ”optical pump OFF”)– then record sample data set (”sample IN”, or ”optical pump ON”)• Alternative scheme for time-resolved pump-probe measurements:– Record reference and sample data simultaneously– minimizes influence of drift, power fluctuation, pointing stability issues, etc.K. Iwaszczuk et al., Opt. Express 17, 21969 (2009)05.12.2011Terahertz Technologies and Biophotonics35is a constant which depends onthe experiment (specifically the Fourier componentsat 2f and 3f of the PD signal in theEO sampling system)Dual lock-in scheme for data acquisitionchopped at 333 Hz (2x167 Hz)chopped at 500 Hz (3x167 Hz)K. Iwaszczuk et al., Opt. Express 17, 21969 (2009)05.12.2011Terahertz Technologies and Biophotonics36Simultaneous vs sequential data recordingSequential:- Drude model cannotreproduce the spectra- caused by minute(10-20 fs) jitter- Experiment was placedin a room with poortemperature stabilitySimultaneous:- Drude model describesthe spectra well- no effect of jitter- identical experimentalconditions- Scan time reducedby factor 2K. Iwaszczuk et al., Opt. Express 17, 21969 (2009)

Ultrabroadband, time-resolved THz spectroscopy of disordered materials

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Ultrabroadband, time-resolved THz spectroscopy of disordered materials

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