Presentation on theme: "Ultrafast THz Spectroscopy and Nonlinear Optical Properties of Semiconductor Nanostructures Zhen-Yu ZHAO 17 July 2008 Laboratoire Pierre Aigrain - Ecole."— Presentation transcript:
1 Ultrafast THz Spectroscopy and Nonlinear Optical Properties of Semiconductor Nanostructures Zhen-Yu ZHAO17 July 2008Laboratoire Pierre Aigrain - Ecole Normale Supérieure, ParisState Laboratory of Precise Spectroscopy - East China Normal University, ShanghaiFor the practical reason, I work on 2 different subject both are ultrafast spectroscopy of semiconductor nanosturctures.Both subjets concentrate on the ultrafast spectroscopy of semicondeuctor nanostructures.
2 Outline Section 1: Section 2: Development of THz Time Domain Spectroscopy (THz-TDS)Optical RectificationMicro-Photoconductive EmitterApplication of THz - TDSGain Measurement of Quantum Cascade Laser (QCL)Section 2:Nonlinear Optical Properties of AgCl Nanocrystals doped Tellurite GlassesFabricationCharacterizationNonlinear Optical MeasurementIn the section 1, I would like to present my work on the development and application of THz time domain spectroscopy. We tried 2 different THz emitter and compare their output performance respectively. Then, we use the photoconductive antenna to measure the gain of 2.9THz QCL.In the section 2. I’d like to present the nonlinear optical properites of AgCl NCs doped tellurite glasses.
3 Section 1 Development & Application of THz Time Domain Spectroscopy Now, let’s start with section 1.
4 Application THz spectroscopy : Semiconductor nanostructures THz RadiationSection 1THz radiation is 10 to 12Hz on electromagenetic spectr which locate on the gap betwen infrared radiation and microwave.For a long time, there is no proper source and detector in this electromagnetic region so that people call it THz gap.Now, there is new technique -----THz-TDS one can obtain sub-ps pulse to investigate the materials’ physical properties without Kramer-Kronig transformation at a high signal-noise-ratio. It has become a powerful tool to study the semiconductor nanostructure.1THz↔300μm↔1picosecond↔4.1meV↔10KApplication THz spectroscopy : Semiconductor nanostructures
5 THz Time domain Spectroscopy Section 1Ti : sapphire laserτBSM2M5M1M3M4EmitterElectro-Optic SamplingSZnTeλ/4WPBalanced photodiodesProbe beamPump beamΔτThe THz-TDS is base on a pump-probe optical setup.The pump beam drive the THz emitter and the probe beam pulse sampling the THz waveform by change the time delay.The probe beam turns to circular polarization and its 2 perpendicular components were equaly seperated by the wollastom prism into balance photodiode.THz pulse change the polarization because of the Pockel’s effect so that the 2 component is not equal.One can record the differece signal to describe the THz waveform.Free Space Electro-Optic SamplingTHz emitter:Optical RectificationPhotoconductive antenna
6 Optical Rectification Section 1Laser pulse,Δτ :100fs, λ :800nmZnTe crystalsTHz radiationZLensSHG & Transmitted beamFFTAt first, we tried to use optical rectification method to optimize the THz output performance.an appropriate crystal material is quickly electrically polarized at high optical intensities. This changing electrical polarization emits terahertz radiation.However, there are other nonlinear optical process occurs when the intense femtosecond laser pass through the ZnTe crystal.
7 Optical Rectification Section 1Picarin LensZTeflonBolometerZFilterLensPMTIn the begining, we put the lens far from the ZnTe crystal and THz output increase.The diffraction effect will cause the THz output saturation when the spot-size is equal to the THz wavelength.Therefore, the THz Z-Hole must be originate from other nonlinear optical processZLensPhotodiode
8 Optical Rectification Section 1Nonlinear Optical Processes1. Optical Rectification2. Second Harmonic Generationħω2ħωNonlinear CrystalsNonlinear Crystals3. Two Photon Absorption4. Free Carrier Absorptionhigh stateConduction bandIn order to optimize the THz output of ZnTe crystal, we study the competition betwen optical rectification and other laser-induced nonlinear optical process.ħω2ħωħωβ: TPA coefficientlow stateValence band
9 Optical Rectification Section 1θLaser polarizationZnTeX.-C. Zhang et al. J. Opt. Soc. Am. B 18 : 823 (2001)D.C. Hutchings and B.S. Wherrett, J. Opt. Mod. 41: 1141 (1994)
10 Optical Rectification Section 1Lens :f=4cmZnTeRotationLaser beamTHz radiation15mmBBOTwo Color Experiments15mm far from the focus so that there is no SHG
11 Interdigitated photoconductive antenna Section 1+Laser pulse,Δτ :100fs, λ :800nmHemisphere Si lensConsider the competition between optical rectificationa and other nonlinear properties, we tried another THz emitter: photoconductive emitter.The semiconductor changes abruptly from being an insulator into being a conductor. This conduction leads to a sudden electrical current across a biased antenna patterned on the semiconductor. This changing current emits terahertz radiation.Conventional photoconductive antenna need hemi-sphere lens to collimating the THz output because of the diffraction limiting and need a larger bias voltage.Dreyhaup et al prupose the interdigitated structure antenna.We thanks Dr.Nathan Jukam to provide the antenna sample.Conventional Photoconductive antenna—
12 - + Interdigitated photoconductive antenna stripline gap1.5µm500µmElectrodsOpaquesA. Dreyhaupt et al. Appl. Phys. Lett. 86 : (2005)A. Dreyhaupt et al. Opt. Lett. 31 :1546 (2006)Nathan Jukam, UCSB
14 Interdigitated photoconductive emitter Section 1Γ→L Intervalley scatteringF-L intervalley scattering: the F valley electrons were scattered into L valley when the bias field is large enough.C. Ludwig and J. Kuhl, Appl. Phys. Lett. 69 (9), 1194 (1996)J.-H. Son, T. B. Norris, and J. F. Whitaker, J. Opt. Soc. Am. B 11, 2519 (1994)
15 Interdigitated photoconductive emitter Section 1Optimization by change the exciting intensity
19 THz Quantum Cascade Laser Section 1ConceptSemiconductor LaserQuantum Cascade LaserħωWhy the casacade structure because of the wavelength and bandgap of materialsInterband transitionInter-subband transitionMilestone197019801990200019711994200220042006YearsFirst IdeaFirst Bell LabsTHz QCL2.9 THz QCL 77k1.9 THz QCL 95k
20 THz Quantum Cascade Laser Section 1Bound to Continuum Active-injection Region of 2.9THz QCLCascade structure:
21 THz Quantum Cascade Laser Section 1Surface Plasmon Waveguide of 2.9 THz QCL220µmActiveRegionMetal 220µmSI Substrate12µm(a)Bottomn+ layerMetalContactWhat is surface plasmon waveguide? Why this type of waveguide?MPQ-Paris VII
23 THz Gain Measurement Zone Active Section 1 Ti : sapphire laser τ BS M2 ETHzFSEOSSABCDProbe beamPump beamZone Active
24 THz Gain Measurement 2.9THz Section 1Amplified THz transmission by gain of quantum cascade laser2.9THzTHz Gain at different injection currentTHz Gain at different temperature
25 THz Gain Measurement Gain Clamping Section 1 Threshold increase with the temperature growingThe gain increase with the current when it below the threshold. However, the gain clampaged by the loss when the laser beyond the threshold.
26 Summary 1 Development of THz-TDS THz performance of ZnTe crystal. Section 1Development of THz-TDSTHz performance of ZnTe crystal.THz output of interdigitated photoconductive antennaApplication of THz-TDSFirst measurement of Gain of 2.9 THz Quantum Cascade Laser
38 Summary 2Section 2Nonlinear optical properties of AgCl NCs doped tellurite glassSamples were Prepared by Melt-Quenching and Thermal Treatment MethodsCharacterization with Microscopic and Spectroscopic MethodsNonlinear Optical Properties were Measured by Z-scan, Optical Limiting and DFWM
39 Conclusion Section 1: Section 2: Development of THz Time Domain Spectroscopy (THz-TDS)Competition OR TPA FCA, Azimuthal dependenceIntervally scattering, Space charging screening, Electron mobilityApplication of THz – TDSGain Measurement of 2.9THz QCLSection 2:Nonlinear Optical Properties of AgCl Nanocrystals doped Tellurite GlassesFabricationOptical limiting performance and Two-photon absorptionEnhancement of χ(3)
40 Acknowledgement THz group (LPA-ENS) Advisor: Jérôme TignonStaff members: Sophie Hameau, Sukhdeep Dhillon et al.Postdoc: Nathan Jukam;Ph.D student: Dimitri Oustinov;Master student: Julien Amijo, Geog Dürr;Techniciens: Pascal Morfin, Phillipe Pace et al.Collaborators: Carlo Sirtori et al.Sun’s Group (East China Normal University)Co-Advisor: Zhenrong Sun,Staff members: Tianqing Jia, Xiaohua Yang et al.Collaborators: Jian Lin, et al.