Ultrafast Optical Characterization of Novel Mid-Infrared Nanoscale Structures

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1 NSF ERC MIRTHE Summer Symposium, Princeton University, June 15-16, 2015 Ultrafast Optical Characterization of Novel Mid-Infrared Nanoscale Structures Dr. Anthony M. Johnson, Director* Center for Advanced Studies in Photonics Research (CASPR) Professor of Physics Professor of Computer Science & Electrical Engineering University of Maryland Baltimore County (UMBC) 2002 President of the Optical Society of America (OSA) MIRTHE Deputy Director * Before January 1, 1995 Distinguished Member of Technical Staff Photonic Circuits Research Department, AT&T Bell Laboratories (now Alcatel-Lucent) Chair, Physics Dept., New Jersey Institute of Technology (NJIT)

2 The photogenerated carriers contribute to changes in the refractive index of the material and thus changes in reflectivity through a combination of mechanisms such as free-carrier absorption, bandfilling and band-gap renormalization. Depending on the sample structure and its quality, these mechanisms may occur on a time scale as short as several picoseconds. Time-Resolved Reflectivity of Mid-IR Materials -- UMBC-CASPR Robinson Kuis, Raymond Edziah, Elaine Lalanne, Anthony Johnson We will compare the quality of MOCVD and MBE-grown III-V and MBE grown II-VI materials by studying the carrier dynamics and surface quality of the materials. Sources: 10ps, 1064nm Nd:Vanadate 10ps, 1342nm Nd:Vanadate 2ps, Tunable Ti:Sapphire 130fs, Tunable Ti:Sapphire

3 Pump-Probe reflectivity and photoluminescence of superlattice structures w/ & w/o purge Pump-Probe Reflectivity Photoluminescence λ = 800nm MOCVD Growth w/o purge greatly reduces QC laser growth times - Choa Group, UMBC Growth of InAlAs/InGaAs superlattices (SL) w/ and w/o As-purge between layers An interruption or purging time creates a sharp interface and abrupt composition but increases the impurities at the interface W/O purging minimizes the impurities at the interface but the composition is not sharp Near-IR characterization shows improved quality for SL w/o purge

4 Intesity (a.u.) PL of sample A2430 PL of sample A2360 ZnCdSe samples Normalized Intensity λ pump = 395nm λ probe = 790nm A2360 t1 = 3.4 ps and t2 = 20 ps A2430 t1 = 47 ps and t2 = 390 ps Pump = 395 nm and 20 mw Probe = 790 nm and 0.5 mw Pulsewidth = 2 ps Wavelength(µm) Time Delay (ps) The pump-probe reflectivity measurement shows a dramatic change in the carrier lifetime between the two samples which is complemented by the PL measurement Sample A2430 (red) is of higher quality than A2360 (blue) as is indicated by its narrow PL width and the longer carrier lifetime(390 ps vs. 20 ps) MBE II-VI samples provided by the Maria Tamargo Group (CCNY)

5 NSF International Research and Education in Engineering (IREE) Supplement to MIRTHE Prof. Karl Unterrainer s Research Group, Photonics Institute, Vienna University of Technology March 31- April 29, 2007 Dr. Elaine Lalanne, Research Associate, CASPR, NSF MIRTHE, UMBC

6 Mid-IR fs Pulse Generation OPA λ:1-2 µm; 120 fs Rep. Rate: 250 khz Ti: sapphire Amplifier λ: nm; 160 fs Rep. Rate: 250 khz Ti: sapphire Laser λ: nm; 120 fs Rep. Rate: 76 MHz Retro- Reflector OPA Signal 1.1<λ<1.6 µm OPA Idler 1.6<λ<2.4 µm Dichroic Mirror FTIR CdSiP 2 vs. AgGaS 2 DFG Generation Drs. Zawilski and Schunemann, BAE Systems CdSiP 2 Variable delay AgGaS 2 / AgGaSe 2 Long-pass filter Mid-IR fs pulses AgGaS 2 Lens Parabolic Mirror ω DFG = ω signal - ω idler NSF ECS MRI: Development of Ultrafast Diagnostic Instrumentation for Mid-IR QCLs 6

7 The fs mid-ir system Mira & RegA OPA DFG

8 fs Mid-IR Pulsewidth Measurement Two-photon autocorrelation technique FWHM of 4.5 µm pulses measured by two-photon absorption in InGaAs Detector 1.0 InGaAs Detector Intensity (a.u.) τ p 120fs Mid-IR Pulses Delay (fs) Cross-correlation technique was also used to investigate the pulse width. 8

9 Mid-IR Pump-probe Setup OPA λ:1-2µm; 120fs Rep. Rate: 250kHz Ti: sapphire Amplifier λ: nm; 160fs Rep. Rate: 250kHz Ti: sapphire Laser λ: nm; 120fs Rep. Rate: 76MHz Half-wave plate Lens InSb Detector Retro- Reflector OPA Signal 1.1<λ<1.6µm OPA Idler 1.6<λ<2.4µm Dichroic Mirror Beam splitter Probe QCL AgGaS 2 Long-pass filter Pump Chopper lens Parabolic Mirror Lock-in Amplifier Generation of Mid-IR Femtosecond pulses: 3-12µm Pump-probe Setup

10 Experimental Setup Intensity (a.u.) QCL (A785) 13.8V QCL (A785) Lasing fs MIR Pulses Coupling and collecting fs Mid-IR beam in and out of QCL s waveguide setup QCL (room temperature, pulsed mode, 160ns, 250kHz, 5%duty cycle) Sync. 160ns pulsed bias Wavelength (µm) 120fs Mid-IR 10

11 Quantum Cascade Laser Band Structure tunneling phonon scattering Energy (ev) Wells: InGaAs Barriers: AlInAs inj. τ u l Injection barrier u.l. ω l.l. τ t Khurgin, J. B. et al. APL 2009

12 Resonant fs Pump-Probe Transmission in an Active QCL Signal (Normalized) Signal (Normalized) Signal (Normalized) No ND filter Delay (ps) (a) 0.3 ND filter attenuating pump&probe's power down to 50% Delay (ps) (b) 0.7 ND filter attenuating pump&probe's power down to 20% Delay (ps) (c) Ultrafast gain depletion u.l. l.l. Ultrafast gain recovery Resonant LO phonon scattering Slower gain recovery Electron transport through the injector region and tunneling into the u.l. of the next period 12

13 Ultrafast diagnostic instrumentation for mid-ir materials (a) Second Harmonic Generation (SHG) in QCLs Pumped By Femtosecond mid-ir Pulses µm fs Mid-IR Pulses SHG (TE) SHG (TM) SHG TM originates from ISB transitions in the QCL Intensity (a.u.) Wavelength (µm) Upper to lower curve the spectra of fs mid-ir pulses, TE pumped and TM pumped SHG signal when the QCL is biased just below lasing threshold at room temperature SHG TE is due to a bulk χ (2) nonlinearity Linear-to-nonlinear power conversion efficiency (SHG TM ) ~ 2 µw/w 2 - not optimized for SHG coherence length ~20µm for a 3mm pathlength QCL The SHG signal can also be used to measure group velocity dispersion and carrier dynamics in QCLs S. Liu, et al., SHG in QCLs pumped by fs mid-ir pulses, Appl. Phys. Lett. 99, (2011)

14 CO 2 H 2 O Theoretical estimate n 2 ~ 10-9 cm 2 /W R. Paiella, F. Capasso, C. Gmachl, D. Sivco, J. Baillargeon, A. Hutchinson, et al., Self- Modelocking of Quantum Cascade Lasers with Giant Ultrafast Optical Spectra of mid-ir fs pulses at 4.7 µm Nonlinearities, Science, 290, 1739 (2000). transmitted through the QCL. 0 db Our experimental result n 2 ~ 8 x 10-9 cm 2 /W and 17 db normalized incident optical intensities correspond to Silica glass fibers n 2 ~ cm 2 /W 1.7x10 10 W/cm 2 and 3.4x10 8 W/cm 2, respectively. CO 2 and H 2 O absorption bands limit spectra

15 Near-IR pump, Mid-IR Probe Setup ~1.38 μm ~1.95 μm ~4.72 μm InGaAs/InAlAs QCL Polarization Superlattice growth direction TM TE

16 Pump-probe measurement with near-ir pump below the bandgap λ pump = ~1.95 μm, pump pulse energy, ~700 pj Polarization selective transmission modulation ps level recovery lifetime τ = ~3.0 ps ~ 52 GHz ΔT=-2.6 % Zero TE transmission consistent with forbidden intersubband transition Intersubband transition assisted transmission modulation

17 In Progress Christi Madsen Group (TAMU) As 2 S 3 /LiNbO 3 waveguides fs signal (1.3µm) & idler (1.9µm) DFG: ω 1 -ω 2 ~ 4.5µm; FWM: 2ω 2 - ω 1 ~ 3.5µm White light generation Mid-IR supercontinuum? Vytran GPX 3400 Glass Processing System, Kuis (UMBC) Mid-IR couplers, tapers, supercontinuum generation using chalcogenide glass fibers 100µm optical access hole 200µm X 200µm QCD fs mid-ir pump-probe reflectivity (~ 5µm) will be used to measure the electron lifetime (~ 1ps) of broadband II-VI quantum cascade detectors (QCDs) produced by the Gmachl group (Princeton) and the Tamargo group (CCNY)

18 Research Associates and Graduate Students that made this research possible! Dr. Elaine Lalanne (PhD 2003) Dr. Robinson Kuis (PhD 2009) Dr. Raymond Edziah (PhD 2010) Dr. Aboubakar Traore (PhD 2011) Dr. Sheng Liu (PhD 2011) Dr. Hong Cai (PhD 2014) Ms. Shelly Watts (MS 2009) Mr. Akil Word-Daniels (MS 2009) Mr. Jared Dixon (MS 2014) Mr. Victor Torres Mr. Paul Burkins Mr. Isaac Basaldua

19 The Ultrafast Optics and Optoelectronics Group