Ultrafast THz Spectroscopy and Nonlinear Optical Properties of Semiconductor Nanostructures Zhen-Yu ZHAO 17 July 2008 Laboratoire Pierre Aigrain - Ecole Normale Supérieure, Paris State Laboratory of Precise Spectroscopy - East China Normal University, Shanghai For 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.

Outline Section 1: Section 2: Development of THz Time Domain Spectroscopy (THz-TDS) Optical Rectification Micro-Photoconductive Emitter Application of THz - TDS Gain Measurement of Quantum Cascade Laser (QCL) Section 2: Nonlinear Optical Properties of AgCl Nanocrystals doped Tellurite Glasses Fabrication Characterization Nonlinear Optical Measurement In 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.

Section 1 Development & Application of THz Time Domain Spectroscopy Now, let’s start with section 1.

Application THz spectroscopy : Semiconductor nanostructures THz Radiation Section 1 THz 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↔10K Application THz spectroscopy : Semiconductor nanostructures http://www.thznetwork.org

THz Time domain Spectroscopy Section 1 Ti : sapphire laser τ BS M2 M5 M1 M3 M4 Emitter Electro-Optic Sampling S ZnTe λ/4 WP Balanced photodiodes Probe beam Pump beam Δτ The 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 Sampling THz emitter: Optical Rectification Photoconductive antenna

Optical Rectification Section 1 Laser pulse, Δτ :100fs, λ :800nm ZnTe crystals THz radiation Z Lens SHG & Transmitted beam FFT At 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.

Optical Rectification Section 1 Picarin Lens Z Teflon Bolometer Z Filter Lens PMT In 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 process Z Lens Photodiode

Optical Rectification Section 1 Nonlinear Optical Processes 1. Optical Rectification 2. Second Harmonic Generation ħω 2ħω Nonlinear Crystals Nonlinear Crystals 3. Two Photon Absorption 4. Free Carrier Absorption high state Conduction band In order to optimize the THz output of ZnTe crystal, we study the competition betwen optical rectification and other laser-induced nonlinear optical process. ħω 2ħω ħω β: TPA coefficient low state Valence band

Optical Rectification Section 1 [001] θ Laser polarization ZnTe X.-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)

Optical Rectification Section 1 Lens :f=4cm ZnTe Rotation Laser beam THz radiation 15mm BBO Two Color Experiments 15mm far from the focus so that there is no SHG

Interdigitated photoconductive antenna Section 1 + Laser pulse, Δτ :100fs, λ :800nm Hemisphere Si lens Consider 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 —

- + Interdigitated photoconductive antenna stripline gap1.5µm 500µm Electrods Opaques A. Dreyhaupt et al. Appl. Phys. Lett. 86 :121114 (2005) A. Dreyhaupt et al. Opt. Lett. 31 :1546 (2006) Nathan Jukam, UCSB

Interdigitated photoconductive emitter Section 1 Bias dependence

Interdigitated photoconductive emitter Section 1 Γ→L Intervalley scattering F-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)

Interdigitated photoconductive emitter Section 1 Optimization by change the exciting intensity

Interdigitated photoconductive emitter Section 1 Space Charge Screenings Effect Low Optical Flux High Optical Flux - - Space charge screening + — + — Bias Field Coulomb Field

Interdigitated photoconductive emitter Section 1 Temperature Dependence of THz emitter J. S.Blakemore, J. Appl. Phys. 53: R123-R181 (1982)

Comparison of 2 THz emitters Section 1 ZnTe Crystals Interdigitated Photoconductive antenna THz Amplitude 10V/cm 100V/cm Central Frequency 2THz 1THz~~1.4THz Bandwidth 0.5~~2.7THz 0~~3.5THz S/N 500~1000 5000~~10000

THz Quantum Cascade Laser Section 1 Concept Semiconductor Laser Quantum Cascade Laser ħω Why the casacade structure because of the wavelength and bandgap of materials Interband transition Inter-subband transition Milestone 1970 1980 1990 2000 1971 1994 2002 2004 2006 Years First Idea First QCL @ Bell Labs THz QCL 2.9 THz QCL 77k 1.9 THz QCL 95k

THz Quantum Cascade Laser Section 1 Bound to Continuum Active-injection Region of 2.9THz QCL Cascade structure:

THz Quantum Cascade Laser Section 1 Surface Plasmon Waveguide of 2.9 THz QCL 220µm Active Region Metal 220µm SI Substrate 12µm (a) Bottom n+ layer Metal Contact What is surface plasmon waveguide? Why this type of waveguide? MPQ-Paris VII

THz Quantum Cascade Laser Section 1 Characterization of 2.9 THz QCL V QCL Pyroelectric Detector A THz Collimation THz

THz Gain Measurement Zone Active Section 1 Ti : sapphire laser τ BS M2 ETHz FSEOS S A B C D Probe beam Pump beam Zone Active

THz Gain Measurement 2.9THz Section 1 Amplified THz transmission by gain of quantum cascade laser 2.9THz THz Gain at different injection current THz Gain at different temperature

THz Gain Measurement Gain Clamping Section 1 Threshold increase with the temperature growing The gain increase with the current when it below the threshold. However, the gain clampaged by the loss when the laser beyond the threshold.

Summary 1 Development of THz-TDS THz performance of ZnTe crystal. Section 1 Development of THz-TDS THz performance of ZnTe crystal. THz output of interdigitated photoconductive antenna Application of THz-TDS First measurement of Gain of 2.9 THz Quantum Cascade Laser

Section 2 Nonlinear Optical Properties of AgCl NCs doped Tellurite Glasses

Er+ Doped TeO2-Nb2O5-ZnO Glass Introduction Section 2 Photonic Glasses Tellurite Glasses Optical Switching Optical limiting Glass Χ(3) ~ 1014esu SiO2 2.4 46PbO–42Bi2O3–12Ga2O3 42 20Nb2O5–80TeO2 72 Er+ Doped TeO2-Nb2O5-ZnO Glass ------J. Lin et al. J. Non-Cryst. Solids 336 : 189–194 (2004) ------Y.Q. LI et al. J. Rare Earth 25 : 412 – 415 (2007) Nanocrystals doped Tellurite Glasses

Fabrication Melting: 80TeO2:20Nb2O5 & 1%wt AgCl powder Section 2 Melting: 80TeO2:20Nb2O5 & 1%wt AgCl powder 800°C / 15minutes. Quenching: Annealed at 300°C Thermal treatment: At 360°C 30min, 60min, 90min, 120min

Characterization Nanocrystals FESEM Image vs Termal treatment time Section 2 Nanocrystals FESEM Image vs Termal treatment time (a) 30min thermal treated (b) 60min thermal treated

Characterization Nanocrystals FESEM Image vs Termal treatment time Section 2 Nanocrystals FESEM Image vs Termal treatment time (c) 90min thermal treated (d) 120min thermal treated

Characterization Size distribution function vs thermal treatment time Section 2 Size distribution function vs thermal treatment time 12nm / 30 min 17nm / 60 min 26nm / 90 min 35nm / 120 min

Characterization Cl- Ag+ Jahn-Teller effect : Lattice deformation Section 2 Jahn-Teller effect : Lattice deformation Cl- Ag+ Reaction: 2Cl- →Cl2 + 2e- & 2Ag++2e-→2Ag Cl- colour center H. Vogelsang, Phys. Rev. B 61: 1847-1852 (2000)

Characterization Urbach law: Eg Eg Samples Eg (eV) undoped 3.1 0 min Section 2 Urbach law: Trapped state Samples Eg (eV) undoped 3.1 0 min 2.9 30 min 60 min 2.3 90 min 1.9 120 min 1.7 Eg Eg Bandgap of glass matrix Bandgap redshift of treated glass

Nonlinear Optical Properties Section 2 lens lens Powermeter 50% BS Attenutation Sample Powermeter

Nonlinear Optical Properties Section 2 Lock-In PC Ti :sapphire Laser M1 M2 L S D Z Open Aperture Z-scan β: 1GW/cm ~~ 1.8 GW/cm

Nonlinear Optical Properties Section 2 Lock-In M1 M2 M3 M4 M5 BS Δτ PC L S K1 K2 2K2-K1 2K1-K2 D Ti :sapphire Laser Samples Eg (eV) n0 χ(3) (10-13esu) n2 (10-11esu) undoped 3.1 2.0 7.2 1.2 0 min 2.9 2.1 7.3 1.3 30 min 2.6 2.2 7.8 1.4 60 min 2.3 14 90 min 1.9 11 120 min 1.7 9.5 1.6 DFWM experiment

Summary 2 Section 2 Nonlinear optical properties of AgCl NCs doped tellurite glass Samples were Prepared by Melt-Quenching and Thermal Treatment Methods Characterization with Microscopic and Spectroscopic Methods Nonlinear Optical Properties were Measured by Z-scan, Optical Limiting and DFWM

Conclusion Section 1: Section 2: Development of THz Time Domain Spectroscopy (THz-TDS) Competition OR TPA FCA, Azimuthal dependence Intervally scattering, Space charging screening, Electron mobility Application of THz – TDS Gain Measurement of 2.9THz QCL Section 2: Nonlinear Optical Properties of AgCl Nanocrystals doped Tellurite Glasses Fabrication Optical limiting performance and Two-photon absorption Enhancement of χ(3)

Acknowledgement THz group (LPA-ENS) Advisor: Jérôme Tignon Staff 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.