Lecture Juraj Darmo - IR-On

Transcription

TECHNISCHE
UNIVERSITAT
WIEN
VIENNA
UNIVERSITY OF
TECHNOLOGY
Time-Domain Spectroscopy of
THz Quantum Cascade Structures
J. Darmo
Photonics Institute
Vienna University of Technology, Vienna, Austria
IQCLSW 2012, 2.-6. Sept. 2012, Baden, Austria
Outline
1. Time domain THz spectroscopy
2. Interaction of THz-QCL and few-cycle THz pulse
-  theory
-  technical issues
3. THz-QCL data
-  transmission through QCL (gain)
-  role of design and waveguide
-  gain recovery (MIR QCL)
-  advanced concepts based on TDS
-  advances in electro-optic detection
4. Outlook
Acknowledgement for contribution
J. Kröll, M. Martl, D. Dietze,
D. Bachmann, W. Parz,
K. Unterrainer
Photonics Institute
Vienna University of Technology, Austria
M.A. Andrews, H. Detz,
P. Klang, W. Schrenk,
G. Strasser. E. Gornik
Institute of Solid-State Electronics
& ZMNS
Vienna University of Technology, Austria
S. Barbieri and C. Sirtori
University Paris Diderot, Paris 7, France
S.S. Dhillon, N. Jukam
Ecole Normale Superier, Paris, France
Q. Hu, D. Burghoff
MIT, Cambridge, U.S.A.
G. Scalari and J. Faist
Institute of Quantum Electronics
ETH Zurich, Switzerland
M. Kuhn, T. Elsaesser
MBI for Nonlinear Optics, Berlin, Germany
J. W. Cockburn
University of Sheffield, U.K.
M.S. Vitiello
Scuola Normale Superiore, Pisa, Italy
Funding: FWF (ADLIS,PLATON,QOCUS) & GMe Austria, EC (POISE,TERANOVA)
Brief history of THz-TDS
... triggered by the advent of femtosecond laser
emitters: photoconductive switches with ultrafast recovery time (Auston, LT-GaAs)
optical rectification and DFG in NLOC
– broadband coherent THz pulses
– typical average power ~ uW
detectors: photoconductive antenna (LT GaAs, Fe doped InP)
free space electro-optic detection (X.-C. Zhang)
- instant electric field vector can be measured
with time resolution as high as 10 fs
– gated detection: excellent SNR with respect to detected THz power
Terahertz time-domain spectroscopy applied to THz-QCL
(transmission measurement) … first attempts in 2005
J. Kroell et al., Nature 449, 698 (2007) TU Wien - SPWG, BTC design
N. Jukam (S.S. Dhillon) ENS Paris - SPWG, BTC & RPD design, latter DMWG
D. Burghoff (Q. Hu) MIT - DMWG, RPD & Co.
accessible information on - absorption/gain in active/passive THz waveguides
- real temperature in the THz-QCL active region
- gain dynamics, mode dynamics
THz time-domain spectroscopy
6.0
Amplitude
EO signal
4.0
2.0
0.0
-2.0
-4.0
100
80
60
40
20
0
0
-6.0
35
40
45
50
55
60
time delay (ps)
Free induction decay
of coherently excited H2O molecules
2
3
5
4
5
Frequency (THz)
100
80
60
40
20
0
0
1
2
3
Frequency (THz)
4
Amplitude
30
1
F.I.D. of coherently excited H2O molecules
Main pulse
First coherences
Parasitic reflections
0
frequency
1.5
3.0
4.5
6.0
0
2
4
6
time (ps)
8
10
THz time-domain spectroscopy
applied to THz-QCLs
THz-TDS of THz QCL
NIR
80 MHz
THz
emitter
QCL
High repetition fs-system (80 MHz):
Ti:sapphire at 800 nm, < 80 fs pulses
EOC
Photoconductive THz emitter
Electro-optic detection:
-  gated detection
-  detection bandwidth of 7 THz
( with 300 um GaP)
double modulation used
Coupling optics
Electro-optic detection
photodetectors λ/2 plate & Wollaston prism electro-‐opFc crystal THz wave ellipFcally polarized NIR probe Δϕ ∝
2π
⋅ LEOC ⋅ r41 ⋅ n 3 ⋅ ETHz
λ
linearly polarized NIR probe response funcFon of single layer EOS exp[i ⋅ Δk (ω, Ω)⋅ L] − 1
Ω NIR
g (ω, Ω) =
(ω ) − nTHz
Δk (ω , Ω ) = ⋅ [ngroup
phase (Ω )]
i ⋅ Δk (ω , Ω)
c
n3
no ≈ n − ⋅ ETHz ⋅ r41
2
n3
ne ≈ n + ⋅ ETHz ⋅ r41
2
G.Gallot, J.Opt.Soc.Am. B16, 1204 (1999) P. Planken, J. Opt. Soc. Am. B18, 313 (2001) van der Valk, J.Opt.Soc.Am. B21, 622 (2004) Electro-optic detection
probe light intensity aYer polarizer, waveplate and PBS probe light intensity aYer two crossed polarizers I (φ0 , Δφ ) = I 0 sin (φ0 + Δφ )
I0
I (φ0 , Δφ ) = ⋅ [1 − cos(φ0 + Δφ )]
2
Taylor expansion ... 1
1
cos(φ0 + Δφ ) = cos(φ0 ) − sin (φ0 ) ⋅ Δφ − cos(φ0 ) ⋅ Δφ 2 + sin (φ0 ) ⋅ Δφ 3 + …
2
6
1
1
sin (φ0 + Δφ ) = sin (φ0 ) + cos(φ0 ) ⋅ Δφ − sin (φ0 ) ⋅ Δφ 2 − cos(φ0 ) ⋅ Δφ 3 + …
2
6
selected terms gives ... I (ϕ 0 , Δϕ ) ∝
Δϕ = Δϕ THz ⋅ cos ( Ωmod ⋅t )
I0
cos (ϕ 0 ) ⋅ Δϕ THz
2
... signal (instant THz ﬁeld) modulated at Ωmod
I (ϕ 0 , Δϕ ) ∝
I0
2
cos (ϕ 0 ) ⋅ Δϕ THz
4
... signal (instant THz power) modulated at 2 ⋅Ωmod
THz-TDS in QCL: Travel-wave amplification
I < Ith
THz QCL
spontaneous emission – few photons
with random timing and phase
injected photons – trigger photon emission
with correct timing and phase
I > Ith
THz QCL
stimulated emission – photons
with random timing and phase
injected photons – trigger photon emission
with correct timing and phase
(long/short coherence time)
I > Ith
THz QCL
stimulated emission –photons
with correct timing and phase with respect
to injected photons
(even mode-locked laser)
Time-domain response of gain medium
THz-QCL: two-level system
rin
rdep
b
hν
ω ab
a
rsp
ωa
rout
Density matrix
if (tobserve > 1/rin, 1/rout, 1/rdep ) then
interacting system
if (tobserve < 1/rin, 1/rout, 1/rdep) then
insulated system (standard)
⎛ ρaa
⎜
⎜⎝ ρba
ρaa , ρbb
ρab , ρba
ωb
ρab ⎞
⎟
ρbb ⎟⎠
~ population
~ coherence
Two-level system: time domain response
standard spectroscopy – sensitive to probe intensity changes
2,0
E(t)
E (~t )rabρab
el. field (a.u.)
1,5
el. field (a.u.)
15
10
5
0
2
E
∫ dt ~ ρaa − ρbb
1,0
0,5
0,0
-0,5
-1,0
-5
1
-10
2
3
4
5
6
7
8
9 10 11 12
time (ps)
-15
2
3
4
5
6
7
8
9 10 11 12
time (ps)
1,5
el. field (a.u.)
1
1,0
0,5
0,0
-0,5
-1,0
-1,5
1
2
3
4
5
6
7
time (ps)
8
9 10 11 12
Two-level system: time domain response
16
4
reference
2-level system
3
x20
2
1
0
0.0
1.0
2.0
3.0
0.5ω
0.67ω
0.8ω
12
amplitude
spectral density
5
4.0
frequency (THz)
1.05ω
1.3ω
4
0
0.0
5.0
ω
8
1.0
2.0
3.0
4.0
5.0
frequency (THz)
Two response components
-  instant & resonant
-  stronger for higher frequency overlap
-  effect of strong driving field
(Rabi oscillations)
amplitude
30
E0
1.5E0
3E0
5E0
10E0
20
10
0
0.0
1.0
2.0
3.0
4.0
5.0
frequency (THz)
6.0
Technical issues of THz-TDS
THz waveguides for QCL
Coupling of THz waves into laser waveguide
THz optics
THz pulse
aperture
THz-QCL chip
+ leaky laser cavity
- low coupling efficiency (<10%)
ω1
ω2
NIR pulse
THz-emitter chip
THz-QCL chip
- coupled cavities
+ high coupling efficiency (>60%)
Optics: Coupling of THz Waves into WG
HR-Si lens
THz-QCL
Towards integrated
THz Coupling
THz emitter
Approach
Waveguide Emitter (WGE)
Facet-Excitation on THz QCLs
M. Martl et al., IRMMW-THz 2008
S.S. Dhillon et al., APL 96, 061107 (2010)
SI-GaAs
Emitter section on THz QCLs
THz QCL
Micro Chip Emitter (MCE)
M. Martl et al., unpublished
SI-GaAs
•  semi-insulating GaAs
•  broadband THz pulse
generation at the facet of a
waveguide
- QW structure of THz QCL
- waferbonding-technique
- WG thickness defined by AR
- THz emitter (section A)
- active QCL (section B)
M. Martl et al., Opt. Expr. 19, 733 (2011)
D. Burghoff et al., APL 98, 061112 (2011)
J. Maysonnave et al., Opt. Lett. 37, 731 (2012)
Gain in THz-QCL
THz QCL – Design & Parameters
Design:
•  GaAs/AlGaAs system
•  band-to-continuum
•  transition energy:
2.0, 2.9 THz and 3.4 THz
Waveguide:
•  surface plasmon
(n+-GaAs buried confinement
layer)
•  ridge type
(110 – 200 um wide)
S: Barbieri et al., APL85(10), 2004
Ch. Worral et al., OE14(1), 2005
B. Williams et al., EL40, 2004
band structure
THz QCL – THz pulse transmission (2.9 THz)
0
100
50
0
-50
-100
10
2
ETHz (V/cm)
(a)
6
8
10
Reference pulse waveform
Eoff(t)
(b)
5
4
Δ E(t)=Eon(t)-Eoff(t)
Modulation observed
0
ETHz (arb. units)
-5
Comparison of experimental
data and model (red line)
(c)
-5
0
5
0
2
4
6
8
10
time delay (ps)
12
J. Kroell et al., Nature 449, 698 (2007)
Gain spectrum: current dependence
0
2
4
6
8
10
0
10
J=45 A/cm
5
2
3
4
4
2
2
0
-5
0
-10
10
J=113 A/cm
5
4
2
2
0
-5
0
-10
10
J=170 A/cm
5
4
2
2
0
-5
spectral intensity (arb. units)
THz electric field (arb. units)
1
0
-10
0
2
4
6
8
time delay (ps)
10
0
1
2
3
4
frequency (THz)
Gain peak region: 2.9 THz-QCL
100
150
200
250
spectral intensity
30
50
(a)
laser losses
20
WG loss
10
0
8
(b)
6
120
model
100
80
60
40
II.
I.
20
III.
0
0
100
200
300
400
500
current
4
2
0
6
25
(c)
20
15
4
10
2
5
0
0
50
100
150
200
2
current density (A/cm )
0
250
I. 
diff. resistance (Ω )
laser bias (V)
THz output power
(arb. units)
single pass gain
(1/cm)
0
Cascades aligned: gain
increases with current
I.  spatial hole burning – reduced
gain slope
III. gradual disalignment of
cascades
Probing THz-QCL with DMWG
Mini-broadband
THz-QCL
THz-QCL
with RPD design:
spectral gain
Mini-broadband
THz-QCL
THz-QCL
with RPD design:
gain
Mini-broadband
THz-QCL
THz-QCL
with RPD design:
gain
Mini-broadband
THz-QCL
THz-QCL
RPD design:
gain curve
Time-domain response of MIR-QCL
amplitude (arb. units)
MIR QCL: time domain analysis
0%
1
200 % of Jth
x 50
0
measurement
-1
0.0
amplitude (arb. units)
100 %
3.8
4.0
4.2
time (ps)
0%
1
100 %
4.4
4.6
200 % of Jth
x 50
good agreement with the response
function of a 2D waveguide calculation
for the TM00 mode.
0
calculation
-1
0.0
3.8
4.0
4.2
4.4
4.6
time (ps)
W. Parz et al, APL 93, 091105 (2008)
0
MIR QCL: gain curve
0.5
2
0
40
0.0
s
-40
as
se
-120
0
50
100 %
200 %
100
150
current (% of JTh)
24
26
28
frequency (THz)
3p
-80
50 % of JTh
-100
22
-1
gain (cm )
gain after
3 passes
-200
s
s
1 pa
0
0
gain (cm-1)
4
optical power (arb. u.)
voltage (V)
6
100 150 200
current (% of IThreshold)
8
1.0
50
200
- gain clamps above threshold
- stark shift to red when full gain is
present, in agreement with calculations
- no stark shift when gain builds up
- sign for individual alignment of the
cascades
W. Parz et al, APL 93, 091105 (2008)
Comparison of gain curves
Mini-broadband
THz-QCL
THz-QCL
gain bandwidth
comparison
N.Jukam et al., APL94 251108 (2009)
RPD + SPWG
N. Jukam et al., APL93 101115 (2008)
BTC + SPWG
M.Martl et al., OE19 733 (2011)
RPD + DMWG
D. Burghoff et al., APL98 061112 (2011)
RPD + DMWG
Mini-broadband
THz-QCL
gain curveTHz-QCL
comparison
N.Jukam et al., APL94 251108 (2009)
RPD + SPWG
N. Jukam et al., APL93 101115 (2008)
BTC + SPWG
M.Martl et al., OE19 733 (2011)
RPD + DMWG
D. Burghoff et al., APL98 061112 (2011)
RPD + DMWG
Active region temperature
Mini-broadband
THz-QCL
THz-QCL
with RPD design:
gain Tmax
Device Real Temperature
RPD
M.S. Vitiello et al., APL86 111115 (2005)
BTC
DMWG
SPWG
BTC
DMWG
SPWG
Gain dynamics
Pump/probe spectroscopy of MIR QCL
•  pump
& probe electric field strength: >6 kVcm-1
Kühn et. al, APL 93, 151106 (2008)
W. Parz et al, APL 93, 091105 (2008)
0,4
FFT
Power (arb. u.)
Transmission change ΔT / T0
0,2
0,0
0,8
1,2
1,6
2,0
frequency (THz)
0 mA
150 mA
400 mA
650 mA
0,6
0,4
0,2
0,0
4
2
0
0
2
4
6
Delay τ (ps)
8
10
Time shift (fs)
Phase shift (rad)
-0,2
•  below threshold relaxation
time constant ~3 ps
•  above threshold relaxation
time constant <1 ps
•  observation of relaxation
oscillations @ 0.8 THz
Kühn et. al, APL 93, 151106 (2008)
W. Parz et al, APL 93, 091105 (2008)
injection barrier
extraction barrier
1
1 THz
25 THz
2, g
1
2, g
~ 50 ps gain recovery Fme for MIR QCL from pulse autocorrelaFon (V.M. Gkortsas et al., OE18 13616 (2010)) ~ 50 ps gain recovery Fme for THz QCL from autocorrelaFon of photocurrent pulses (FEL) for BTC QCL @ 3.2 THz (P.R. Green et al., PRB80 075303 (2009)) H. Choi et al, phys. stat. sol. (c) 5, 225 (2008), APL 92, 122114 (2008), Phys. Rev. Le^. 100, 167401 (2008) ~ ps recovery, mulFcomponent Origin of gain oscillation:
injector/active region wave packet oscillation
F. Eickemeyer et al., APL76 3254 (2000) F. Eickemeyer et al., APL79, 165 (2001) F. Eickemeyer et al., PRL89 047402 (2002) -‐ ﬁrst p/p results on MIR QCS Kühn et. al, APL 93, 151106 (2008)
W. Parz et al, APL 93, 091105 (2008)
Advanced concepts based on TDS
Ultrafast gain switching
•  Ultrafast
gain switching of QCL
through Integrated Auston Switch
•  THz pulse amplification
Auston Switch OFF
Clamped Gain
Auston Switch ON
Unclamped Gain
1,5 1
1,0
5
3
7
0,5
0,0
-0,5
-1,0
-1,5
-2,0
0
20
40
60
80 100 120 140
Time (ps)
2,0
1,5
1,0
0,5
0,0
-0,5
-1,0
-1,5
-2,0
1
3
5
7
Electric Field (V/cm)
Electric field (V/cm)
2,0
0
20 40 60 80 100 120 140
Time (ps)
N. Jukam. et al. Nature Phot. 3, 715 (2009).
(Integrated) Injection seeding
gain switching of QCL
•  Injection seeding of QCL –
Detection of QCL emission using
coherent techniques
8
8
4
0
-4
RF OFF DC bias above threshold
-8
0
Field (V/cm)
Field (V/cm)
Field (V/cm)
•  Ultrafast
100
200
Time (ps)
300
400
4
0
-4
-8
925
926
Time (ps)
927
928
8
4
0
-4
RF ON DC bias below threshold
-8
0
100
200
300
400
500
600
700
800
900
1000
Time (ps)
D. Oustinov et al., Nature Comm. DOI 10.1038 /ncomms (2010)
J. Maysonnave et al., OL37, 4 (2012)
Sampling of mode-locked THz QCL
1 Δfr
ZnTe
QCL
frep1~10GHz
l/4
l/2
1/frep2
Δfr = frep1 − n ⋅ frep2
fs-laser
frep2 ~ 100MHz
Down-converted THz spectrum
Sampled pulse train
-30
-40
-50
10
20
30
Frequency (MHz)
40
3mm ridge – surface plasmon
Electric field (arb. units)
QCL phase-locked
to fs-laser rep rate
Schematic of the asynchronous sampling setup
1/frep1
Measured
Computed
-100
-50
0
50
100
Time (ps)
Barbieri et al. Nature Photon. 5, 306 (2011)
Advances in electro-optic detection
EOD: quasi phase matching
A.
B.
C.
vTHz
@ t1
GaP (110)
n′′ = n0 −
n′ = n0
ph
2 3
n0 r41 ETHz
2
vNIRg
vTHzph
@ t2
GaP (110)
n′′ = n0 −
n′ = n0
2 3
n0 r41 ETHz
2
vNIRg
vTHzph
@ t2
GaP (110)
GaP (-1-10)
n′′ = n0 +
n′ = n0
2 3
n0 r41 ETHz
2
vNIRg
-‐  sensiFvity of the EOS can be increased by an increase of interacFon length without compromise if the mismatching between probe and THz pulses is handled J. Darmo et al. Elerctron Lett. 46, 788 (2010)
response funcFon of ... ... periodically inverted crystalline orientaFon of individual layers of EOS g (ω, Ω) =
exp[i ⋅ Δk (ω, Ω)⋅ L] − 1
⋅ {1 − exp[i ⋅ Δk (ω, Ω)⋅ L] + exp[i ⋅ Δk (ω, Ω)⋅ 2L] − exp[i ⋅ Δk (ω, Ω)⋅ NL]}
i ⋅ Δk (ω, Ω)
... or exp[i ⋅ Δk (ω , Ω)⋅ L] − 1 1 − (− 1) ⋅ exp[i ⋅ Δk (ω , Ω)⋅ (N + 1)⋅ L]
g (ω , Ω) =
×
i ⋅ Δk (ω , Ω)
1 + exp[i ⋅ Δk (ω , Ω)⋅ L]
N +1
single layer responce ‚‚enhancement‘‘ factor (frequency dependent) J. Darmo et al. Elerctron Lett. 46, 788 (2010)
photodetectors l/2 plate & Wollaston prism electro-‐opFc crystal ellipFcally polarized NIR probe linearly polarized NIR probe EO signal (arb. units)
THz wave 16
a/
14
L = 1.5 mm
L = 0.3 mm
L = 5x0.3 mm
12
10
8
6
4
2
0
0
1
2
3
4
5
frequency (THz)
J. Darmo et al. Elerctron Lett. 46, 788 (2010)
Outlook
þ gain saturation and gain recovery dynamics (design impact)
þ
gain degradation mechanisms
(temperature, heterogeneous active region)
þ broadband THz laser/amplifier
þ
waveguide dispersion, mode locking
þ
mode-locked QCL for remote sensing
þ
large area surface emitting QCLs

Lecture Juraj Darmo - IR-On

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