towards filling the high power thz gap - pte ttk...

Towards filling the high power THz gap

Daniel Dietze, Dominic Bachmann, Karl Unterrainer, Juraj Darmo

Institut für Photonik, Technische Universität Wien,

Vienna, Austria

Workshop on High-Field THz Science, University of Pecs, Pecs, 08.10.2012

Outline

I.  Motivation II.  Generation of strong THz pulses with Ti:sapphire

amplified system i.  semiconductor plasmon emitter ii.  large area photoconductive emitter iii.  optical rectification in GaP iv.  two color air plasma

III. Non-equilibrium dynamics in quantum well

Nonlinear Optics Requires Not Only High Electric Fields...

...but also a high SNR and a large bandwidth

•  state-of-the-art sources for intense THz pulses are: (with Ti:Sapphire pumping)

–  OR in LiNbO3 using a tilted pulse-front for phase matching Hirori et al., Appl. Phys. Lett. 98, 091106 (2011)

–  DFG in GaSe using two phase locked OPAs Sell et al., Opt. Lett. 33, 2767 (2008)

•  semiconductor ISBTs require intense THz pulses in the 2 to 5 THz

??? frequency tuning and scalling present emitters ???

f < 3 THz

f > 15 THz

High-power THz-TDS setup in Vienna

780nm pump / 1560nm probe configuration:

•  use all THz pulses for detection •  detection benefits from stability of fiber laser •  EOD is immune against stray light (InGaAs detectors are blind)

Reimann et al., Opt. Lett. 28, 471 (2003) Sell et al., Opt. Lett. 33, 2767 (2008)

Generation of single cycle THz pulses

C.  THz emission from dual-color plasma filaments Dietze et al., J. Opt. Soc. Am. B 26, 2016 (2009) Dietze et al., Appl. Phys. Lett. 100, 091113 (2012)

D.  OR in ZnTe (GaP) Dietze et al., Opt. Lett. 37, 1047 (2012)

B.  photoconductive emitter (matrix) D. Bachmann, Master‘s Thesis, 2012

A.  surface plasmon emitters Kersting et al., Phys. Rev. Lett. 79, 3038 (1997) Darmo et al., Appl. Phys. Lett. 86, 091113 (2002)

THz emission from semiconductor (surface)

•  use large area to circumvent saturation •  excited dipole is oriented perpendicular to surface •  center frequency given by carrier concentration •  „quasi-transmission“ geometry •  maximum fluence 30 – 50 µJ/cm2

( 4“ emitter usable size)

Kersting et al., Phys. Rev. Lett. 79, 3038 (1997) Darmo et al., Appl. Phys. Lett. 86, 091113 (2002)

THz Emission from Semiconductor Surfaces

15 µJ/cm2

20µJ/cm2

20 µJ/cm2

Dietze et al., unpublished

THz Emission from PCA Arrays

•  interdigitated electrodes, low bias voltage needed •  array of small antennas (8x8), large total area (15x15 mm2) but individual control

D. Bachmann, Master‘s Thesis, 2011

Dynamically Phase-Matched OR in GaP

•  collinear geometry / quasi-transmission scheme

•  velocity mismatch between NIR and THz, effect of strong NIR pump

Dietze et al., Opt. Lett. 37, 1047 (2012)

Dynamically Phase-Matched OR in GaP – Basic Idea

•  stimulated Raman scattering of strong pump pulse creates optical phonons (close to the Γ point), which decay into acoustic phonons at the zone boundary

Ushioda et al., Phys. Rev. B 8, 4634 (1973) TO, LO TA(X) + LA(X)

•  decay rate increases with increasing number of phonons

•  THz refractive index below the phonon line is modified and condition for zero is not fulfilled


THz Refractive Index in GaP – Effect of Polariton Decay


Tight Focusing •  short and dense plasma •  short interaction length •  NO intensity clamping •  high carrier / plasma density

•  microscopic polarization / current surge model

•  strong THz absorption in dense plasma

Loose Focusing

•  long plasma filament (~cm) •  long interaction length •  intensity clamping •  low plasma density (a few %)

•  four-wave mixing with χ(3) of air

•  extended THz source

THz Emission from Two-Color Plasmas

•  effectively no damage threshold; high optical pump powers possible

•  wide spectrum (0..70THz) Kim et al., Nat. Photonics 2, 605 (2008)

•  high peak field strengths (400kV/cm up to MV/cm) Bartel et al., Opt. Lett. 30, 2805 (2005), Dai et al., IEEE J. Sel. Top. Quantum Electron. 18, 183 (2011)

•  choice of focusing separates two different regimes:

Dual Frequency Mixing and Phase Compensation

•  fundamental and second-harmonic pulses have to be combined with –  correct relative phase –  correct polarization –  as high temporal overlap as possible

Cook & Hochstrasser, Opt. Lett. 25, 1210 (2000). Dietze et al., J. Opt. Soc. Am. B 26, 2016 (2009)

Dietze et al., Appl. Phys. Lett. 100, 091113 (2011)

Dai et al., Phys. Rev. Lett. 103, 023001 (2009)

Imaging of an Extended Line Source to the EO Detector

•  use ABCD matrix formalism to propagate the THz pulses through the optical setup –  assume THz pulses are generated and propagate as Gaussian beams (FWM!) –  take into account the extended spectrum and the finite aperture of the optics (2 inch) –  otherwise conceptionally similar to Zhong et al., Appl. Phys. Lett. 88, 261103 (2006)

•  total filament length 95mm •  effective filament length 25mm •  detected signal is aperture limited

filament THz

z 0 -L ABCD

EOD


Only 1/3 of filament contributes!

filament THz

ABCD EOD

waveguide

Effect of SWG on THz Pulses

•  waveguide leads to 1.4 times higher peak-to-peak amplitude •  pulse length is barely influenced (SWG acts as oversized waveguide)

•  increased peak electric field AND larger beam waist:


detected THz pulse energy with waveguide is

4.3 times higher than without waveguide!

Influence of Waveguide Parameters

Plate Separation

•  for 2 mm plate separation: SWG is impedance matched to free space = NO reflection losses

•  for larger separations: negligible effects SWG = oversized waveguide

•  for smaller separations: increased diffraction losses

Waveguide Position

•  optimal distance = geometric focus

•  positive shift: lower signal due to smaller overlap with filament and shift of image spot

•  negative shift: oscillatory behavior due to interference between WG emission and residual filament emission Dietze et al., Appl. Phys. Lett. 100, 091113 (2011)

THz Induced Non-Equilibrium Carrier Dynamics...

...in multi-level quantum systems

•  transmission of intense single-cycle THz pulses through a multiple QW sample in waveguide geometry

•  many nonlinear effects may be observable (Rabi flopping, Autler- Townes effect, parametric processes, saturation of absorption, EIT, etc.)

...requires high fields, broad spectrum and high SNR!

•  use dynamic phase-matched emission from GaP crystal •  use depletion-modulation technique to selectively

measure the electronic contribution to the transmitted THz signal

Heyman et al., Appl. Phys. Lett. 72, 644 (1998)

THz Field Dependent Modulation Signal

•  THz field incident on front facet estimated from optical pump power and Fresnel coefficients of cryostat window

ETHz = 0.8kV/cm ETHz = 4.5kV/cm

ETHz = 6.8kV/cm ETHz = 12kV/cm

THz Field Dependent...

...modulation spectra •  in-phase THz component •  refractive index modification

...absorption spectra •  out-of-phase THz component •  QW absorption

pedestal & spectrally broad feature

relative absorption strength of transitions

red shift of first transition

Comparison to 1d FDTD Model

•  simulate THz pulse propagation through an ensemble of N-level quantum systems •  solve coupled set of Maxwell and von Neumann equation for density matrix using

a weakly coupled scheme and a predictor-corrector step •  use transmitted signal as input; optimize QW parameters for 1W pump power

THz Nonlinear Refractive Index...

...created by THz four-wave mixing in QW (self-phase modulation)

pedestals broad feature

exp. sim.

THz Field Dependent Absorption Strength...

...as consequence of fast (coherent) population transfer

exp. sim.

THz Induced Undressing of Collective Excitations...

...so far only observed with free electron lasers Craig et al., Phys. Rev. Lett. 76, 2382 (1996)

•  red shift is absent in simulation results (collective effect)

•  THz photons couple to intersubband plasmon rather than to intersubband transition •  transition frequency experiences a blue shift (depolarization shift)

•  depolarization shift proportional to population difference N1 – N2

•  fast population transfer destroys the depolarization shift and thereby the coupling to collective excitations

exp. sim.

Acknowledgements

Photonics Institute, VUT: –  Audrius Pugzlys, Stefan Roither, and Andrius Baltuska,

Institute of Solid State Electronics, VUT:

–  Hermann Detz, Max Andrews, Werner Schrenk, and Gottfried Strasser

Macalester College, MN, USA: –  James Heyman

UC Santa Barbara, CA, USA: –  A. C. Gossard, K. Campman

towards filling the high power thz gap - pte ttk...

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