Monitoring the plant water status
with terahertz waves
Dr. Gunter Urbasch
Experimental Semiconductor Physics
AG Martin Koch
Fachbereich Physik
Experimentelle Halbleiterphysik
Arbeitsgruppe M. Koch
Gunter Urbasch
Philipps-Universität Marburg – Fachbereich Physik
Experimentelle Halbleiterphysik
Arbeitsgruppe M. Koch
…(and several colleagues) at 4th Terahertz workshop on Elba (2012)
...thanx
Experimentelle Halbleiterphysik
Arbeitsgruppe M. Koch
main topics of interest
• ultrafast spectroscopy of semiconductor nanostructures
• development of semiconductor disk lasers
• terahertz systems and their applications
Electromagnetic TeraHertz (THz) radiation
visible
THz
radio waves micro waves
10
7
10
8
10
9
10
10
11
10
infrared
12
10
13
10
ultraviolet Röntgen
14
10
15
10
16
10
17
10
frequency (Hz)
high frequency microwaves  THz  extreme far infrared light
Electromagnetic THz radiation
visible
THz
radio waves micro waves
10
7
10
8
10
9
10
10
11
10
infrared
12
10
13
10
ultraviolet Röntgen
14
10
15
10
16
10
frequency (Hz)
frequency
0.1 THz - 10 THz
photon energy
0.4 meV - 41 meV
wavelength
3000 µm - 30 µm
wavenumber
3.3 cm-1
- 333 cm-1
17
10
water content
First demonstration of imaging THz spectroscopy
fresh leave
48 h after cutting
the leave
Bell Labs
Lucent Technologies
B.B. Hu and M.C. Nuss, Opt. Lett. 20(1995)1716
First THz monitoring of rehydration
30
Coleus
% transmission
Before watering
20
10
After
watering
0
1.00
0.95
Relative % transmission
0
8 hours
5
10 15 20 25 30 35 40 45 50
position (mm)
0.90
0.85
0.80
Bell Labs
0.75
Lucent Technologies
0.70
0.65
0
100
200
300
Time (minutes)
400
500
D. Mittleman et al., IEEE Journal of Selected
Topics in Quantum Electronics 2(1996)679
Water status of plants
Transmission
(%)
transmission (%)
90
Coffea arabica
measured
Messwerte
Trendlinie
60
30
0
5
10
15
20
Tage
nach
days
afterWassergabe
watering
• THz transmission - close-to-real-time information about hydration
and dry stress
• non-destructive
• water supply - intelligent irrigation strategies for plant breeding industry
• monitoring for selective cultivation
C. Jördens et al.
JBiologicalPhysics35(2009)255
…but not only in the lab
…plants live outside
Pulsed THz spectroscopy
Laboratory setup
THz time domain spectroscopy
(THz TDS)
beam splitter
femtosecond
laser pulse
sample
lens
lens
emitter
detector
delay line
THz TDS spectrometer
collimating & focusing
optics
substrate + lens
gating
fs pulse
photoconductive
antenna
(THz emitter)
THz
beam
y
photoconductive
gating
antenna
pulse
t
THz receiver
w/o tip
THz
field
with tip x70
x
0
near-field tip
sample
2
4
t (ps)
6
Imaging - raster scanning
sample
sample scanned through focus
for every pixel time transient collected and processed
integrated transmission over frequency window gives image pixel value
…typically image takes some time
Emission and detection of THz pulses
silicon lens
THz pulse
incoming THz pulse
Das Bild k ann zurzeit
nicht angezeigt
werden.
optical pulse
dipole antenna
Vb
Das Bild k ann zurzeit
nicht angezeigt
werden.
optical
pulse
I
dipole antenna
Fiber coupled THz antennas
Antenna chip
Electr.
Glass fiber contacts
Glue
N. Vieweg et al.
SPIE 6616, Munich (2007)
N. Krumbholz et al.
PolymerTesting28(2009)30
Harsh environment
…need for fiber based system
fs fiber laser
transmitter unit
sample
(leaf)
HDPE lenses
receiver unit
delay unit
Signal processing
Signal processing  fundamental
FFT
2,5
Referenz
reference
sample
Probe
2,0
Fotostrom
nA
current I I/ /nA
Amplitude
Einheit)
amplitude(willk.
/ arb. units
time domain (THz pulse)
1,5
1,0
0,5
0,0
-0,5
0
5
10
15
20
25
frequency domain
Frequenzbereich
10
Referenz
reference
sample
Probe
1
0,1
0,01
30
time
Zeitt /t ps
/ ps
0,0
0,5
1,0
1,5
2,0
2,5
3,0
Frequenz
frequencyf /f THz
/ THz
dielectric properties of sample
n  n  i
phase difference
d
    c 0 4 
↔ refractive index n(f)
ratio of amplitudes ↔ absorptions coefficient α(f)
Signal processing - advanced
software commercially available…The Tera Lyzer
• simultaneous extraction of thickness and optical parameters
• multi-layer analysis possible
• versatile time preprocessing features
THz transmission through water film
250
200
100
150
Transmission (%)
-1
Absorption coefficient (cm )
300
100
50
80
60
40
20
0
0
50
100
150
200
Water film thickness (m)
0
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
1,1
Frequency (THz)
cannot penetrate water film thicker than few 100 µm
attenuation depends on frequency
Monitoring the plant water status
Coffea arabica
refractive index
absorption
f = 0.3 THz
Monitoring the plant water status
Coffea arabica
10
20
30
0
40
10
50
20
60
30
70
Loss of water [ %]
( thermogravimetrically )
THz Transmission [%]
…inside the lab - works well
40
80
50
90
0
5
10
15
20
time without watering [d ]
C. Jördens et al.
JBiologicalPhysics35(2009)255
Drought stress
…of oilseed rape (Brassica napus L.)
0.20
moment
of
irrigation
0.30
0.35
0.40
0.50
2
light off
0.45
light on
Transmission
0.25
3
4
5
6
Days after last irrigation
7
Water content analysis based on a physical model
leaf consists of
dry plant material + water + air
effective medium theory (EMT)
optical properties result from volume contents
+ thickness + surface scattering
C. Jördens et al.
JBiologicalPhysics35(2009)255
nonlinear optimization to get water content
Lets go outside the lab!
computer controlled
all fiber based
THz TDS spectrometer
inside rack
with handheld sensor head
…outside the lab
…it works!
barley
100
rel. water content (grav. %)
95
EMT-model
gravimetric measurement
90
85
80
75
70
65
60
55
50
0
20
40
60
80
100
120
80
100
120
deviation (%)
drying time (min)
10
5
0
0
20
40
60
drying time (min)
Application of a further system inside greenhouse
Sub-THz system
…the idea of the chef
Sub-THz system
…and what comes out
60 GHz
emitter
light
source
35 GHz
emitter
35 GHz
detector
camera
60 GHz
detector
Sub-THz system
30
Wassergehalt / g
25
20
Kontrollgruppe
Stress
Null
Kontrollgr. (grav.)
Stress (grav.)
Null (grav.)
15
10
5
0
0
2
4
6
8
Zeit / Tage
10
12
water content of barley plants vs time
watered in different ways
red: sub-THz measurements
black: gravimetric measurements
14
16
Monitoring the plant water status with THz waves
• THz radiation senses through many opaque materials
• non-destructive and contactless sensing
• spatial resolution in the range of mm
• high sensitivity for water (inside plants)
• nearly ready for commercialization
Thank you for your attention!
Special thanks to Norman Born, Ralf Gente,
Michael Schwerdtfeger, and Nino Voß