Arūnas Krotkus

Center for Physical Sciences & Technology,

Vilnius, Lithuania

• Introduction. THz optoelectronic devices.• GaBiAs: technology and main physical

characteristics.• THz time-domain system based on

GaBiAs components.• THz burst generation by optical mixing.• Conclusions. Other possible ultrafast

applications of GaBiAs components.

Vilnius:

Drs.: K. Bertulis, V. Pačebutas, R. Adomavičius.

PhD students: G. Molis, A. Bičiūnas.

Stockholm:

Prof. S. Marcinkevičius

Berkeley:

Prof. W. Walukiewicz, Dr. K.M. Yu.

Lithuanian Science & Study Foundation.

Lithuanian Science Council.

Visible Infra-red

Milli-metreTerahertz Ultra-

violetX RayMicro-

wave and Radio

Non-ionisingpenetratingpenetrating

spectroscopy

• Key properties:– Penetrates clothing, leather, paper, plastics, packaging

materials.– Materials identification using characteristic Terahertz

spectra.– 3-D imaging capability.– Non-ionizing - no damage to body or cells– Short wavelength – high resolution

• Biased photoconductor made from a material sensitive in the fs laser wavelength range;

• Contact metallization has the shape of a Hertzian dipole type antenna;

• THz pulse is emitted into free space.

fs laser pulseTHz output

thin SC layer

dc bias

)()()()( tqvtntPtj emem dt

tdjtE em

THz)(

)(

0,0 0,5 1,0 1,5 2,0

time (ps)

Photocurrent

Optical pulse

THz pulse

0,0 0,5 1,0 1,5 2,0

time (ps)

Photocurrent

Optical pulse

THz pulse

0,0 0,5 1,0 1,5 2,0

time (ps)

Photocurrent

Optical pulse

THz pulse

0,0 0,5 1,0 1,5 2,0

time (ps)

Photocurrent

Optical pulse

THz pulse

fs laserpulseTHz input

thin SC layer

• Electro-optical and photoconductive detection;

• Photoconductor made from material with ultrashort carrier lifetime is biased by the incoming THz pulse;

• By illuminating it at different time delays, different parts of THz pulse are sampled;

-2 -1 0 1 2 3-0.50

-0.25

0.00

0.25

0.50

0.75

1.00

Delay time, ps

Photocurrent

fs laserpulseTHz input

thin SC layer

• Electro-optical and photoconductive detection;

• Photoconductor made from material with ultrashort carrier lifetime is biased by the incoming THz pulse;

• By illuminating it at different time delays, different parts of THz pulse are sampled;

-2 -1 0 1 2 3-0.50

-0.25

0.00

0.25

0.50

0.75

1.00

Delay time, ps

Photocurrent

fs laserpulseTHz input

thin SC layer

• Electro-optical and photoconductive detection;

• Photoconductor made from material with ultrashort carrier lifetime is biased by the incoming THz pulse;

• By illuminating it at different time delays, different parts of THz pulse are sampled;

• Critical material parameter – carrier lifetime.

Photocurrent

-2 -1 0 1 2 3-0.50

-0.25

0.00

0.25

0.50

0.75

1.00

Delay time, ps

Femtosecondlaser

Lock-in amplifier

PC

Step motor driver

THz detector

Si lens

Polarizing beamsplitter

9

THz emitter

• Ultrafast electrical transients generated using femtosecond laser pulses;

• Their Fast-Fourier transform spectrum is shown on the right;

• Signal-to-noise ratios better than 60 dB easily achievable;

• Coherent detection – both amplitude and phase of different frequency components can be measured.

-10 0 10 20 30 40 50-0.50

-0.25

0.00

0.25

0.50

0.75

1.00

THz

field

am

plitu

de, a

.u.

Delay time, ps

0 1 2 3 4 5 6 710-710-610-510-410-310-210-1100101

Frequency, THz

Pow

er, a

.u.

photomixeron Si lens

tunable GaAs lasers1 2 780 nm (380 THz)

P1, λ1

P2, λ2

Superposition Intensity

Iphoto

THz

Energy bandgap – semiconductor should be photosensitive at the laser wavelength.

THz pulse emitters: dark resistance, electron mobility, breakdown field, lifetime (shorter that the laser pulse repetition period).

THz pulse detectors: electron trapping time, electron mobility, dark resistance.

Optical mixers: electron and hole trapping times, dark resistance.

Advantages:

* Complex optical pumping scheme;* Cannot be made much smaller and cheaper.

Disadvantages:

* Mature laser technology (700-800 nm);* LTG GaAs – material sub-ps lifetimes, high resistivity, electron mobility, and with energy bandgap matching the laser photon energy.

Diode laser bar=808 nm

Ti:sapphire

=800 nm

Nd:YAG laser

=1064 nm

2 harmonics

=532 nm

Advantages:

• Semiconductor material similar to LTG GaAs but sensitive to longer wavelengths required;• Longer wavelength means narrower energy bandgap and smaller dark resistivities.

Disadvantages:

• 1 m or 1.55 m wavelength range solid-state or fiber laser are already commercially available; • Directly pumped by diode laser bars;• Compact, efficient, cheaper.

Diode laser bar KLM laser

GaSbxAs1-x: LTG (at 170oC) on GaAs substrates. x=0.4 (~1.4 m): dark resistivity ~ 104 ·cm.

InxGa1-xAs:

LTG (at 200oC) on GaAs. x=0.25 (~1.05 m); dark resistivity 8·103 ·cm, lifetime ~7 ps.

LTG (at 180oC) on InP substrates. x=0.53 (~1.5 m): free electron density >1018 cm-3, lifetime ~2 ps;

LTG, Be-doped. x=0.53 (~1.5 m): free electron density 5·1016 cm-3, lifetime ~2 ps;

heavy ion implanted (Au+, Br+, or Fe+): dark resistivity< 10 ·cm, electron lifetimes 0.5÷2 ps.

In0.53Ga0.47AsErAs islandsBe doping

In0.53Ga0.47AsErAs islandsBe doping

InGaAs – photoconductive layer;

Be- doping – for compensation of the residual donors;

ErAs –electron trapping nanoclusters.

F. Ospald, APL., 92, 131117 (2008)

InGaAs – photoconductive layer;

Be- doping – for compensation of the residual donors;

InAlAs – LTG electron trapping layer.

B. Sartorius, Opt.Expr., 16, 9565 (2008)

1995 – 2003. Low-temperature MBE grown GaAs; heavy ion implanted GaAs for ultrafast optoelectronic applications;

Search of a short-lifetime material with a low mismatch to GaAs.

2003. Papers from T. Tiedje and Kyoto groups on GaBixAs1-x grown by MBE. x=3.1% (S.Tixier et.al, APL) and x=4.5% (M. Yoshimoto et.al., Jpn.J. Appl. Phys.). Growth temperatures were 380oC and 350oC, respectively.

2005. Starting MBE growth at even lower substrate temperatures 240-330oC.

240 260 280 300 320 3400,5

0,6

0,7

0,8

0,9

1,0

1,1

1,2

1,3

1,4

1,5

g, eV

Tgrowth, oC

0.10

0.05

0

x

Soviet –made MBE machine (ШТАТ).

As4 source.

x=5 % (K. Bertulis et.al. APL,2006).

x=8.5% (V. Pacebutaset.al., Sem.Sc.Technol., 2007).

x=11 % (V. Pacebutaset.al., J. Mater,Sc.., 2008)

Bandgap versus the growth temperature for 30 GaBiAsgrowth runs.

Rutherford Back-Scattering.

W. Walukiewicz, Lawrence Livermore Nat. Lab.

X-ray diffraction.

800 1000 1200 1400 1600

0,0

0,2

0,4

0,6

0,8

1,0

Wavelength, nm

Phot

ocon

duct

ivity

, a.u

.

GaBi9 GaBi10 GaBi12

Photoconductivity spectra at room temperature.

800 1000 1200 1400 16000

1x104

2x104

3x104

4x104

5x104

Bi9 Bi13 Bi14

Abs

orpt

ion

coef

ficei

ent,

cm-1

Wavelength, nm

Optical absorption spectra at room temperature.

0,8 0,9 1,0 1,1 1,2-2

0

2

4

T/T

R/R

Eg = 0.954 eV ± 3meV = 120 mev ± 3meV

Eg = 0.959 eV ± 3meV = 103mev ± 3meV

GaBiAs #36A 300 K

R

/R,

T/T

(a.u

.)

Energy (eV)

Exp. Fit

ga)

Photomodulated transmittance (T/T) and photomodulatedreflectance (R/R) spectra for two GaBiAs samples measuredat room temperature.

0,8 0,9 1,0 1,1 1,2-0,5

0,0

0,5

1,0

R/R

T/T

Eg = 0.901 eV ± 3meV = 94mev ± 3meV

Eg = 0.898 eV ± 4 meV = 78 mev ± 4meV

R/R

, T

/T (a

.u.)

Energy (eV)

GaBiAs #35A 300 K

b)

900 1000 1100 1200 1300 1400 1500

0,0

0,2

0,4

0,6

0,8

1,0

PL

inte

nsity

, a.u

.

Wavelengthnm

Tg=330 oC

Tg=280 oC

GaBiAs layers with the carrier lifetime shorter than 10 ps. PL measured by frequency up-conversion, single-photon counting technique.

Prof. S. Marcinkevičius lab., KTH, Stockholm.

0,8 0,9 1,0 1,1 1,2 1,3 1,4

-1

0

1

2

3

4

5

6

7

Inte

nsity

(arb

. u.)

Energy (eV)

non-annealed 400 °C 500 °C 600 °C

GaBiAs-9, PL (473 nm), 300K 2009-12-10

Lazeriolinija

GaBiAs layer with a long carrier lifetime (~200 ps non-annealed). PL measured by a standard cw technique.

Complex annealing behavior.

0 2 4 6 8 10 12

0,7

0,8

0,9

1,0

1,1

1,2

1,3

1,4

1,5

1,55 m

1,3 m

Ban

dgap

, eV

GaBi part, %

1,05 m

With the addition of Bi, the energy bandgap decreases at a rate of -62 meV/%Bi.

Other material parameters:

Conductivity – p-type.

Hole concentration ~1015cm-3,

Hole mobility 20-200 cm2/Vs (decreasing in layers with a larger GaBi part).

Resistivity – typically 100’s of ·cmfor nominally undoped layers; >104

·cm in Si-compensated GaBiAs.

V. Pacebutas et.al., Sem.Sc.Technol., 2007

Optical pump – THz probe experiment. THz pulse absorption caused by non-equilibrium electrons is measured at different time delays with respect to the sample photoexcitation.

Optically induced change in THz pulse transmittance is proportional to

ln(I/I0)=-iL=-(4ni/l0)L~dc~N

Both the electron mobility and their lifetime can be determined.

0 100 200 300 400 500 600

1,0

1,5

2,0

2,5

3,0

3,5 A B C

Po, mW

, p

s

0 10 20 30 40

0

1

2

3

4

400 mW

200 mW

TTH

z, a

.u.

Delay, ps

100 mW

Lifetime varies from less than 1 ps to more than 200 ps. Electron mobility as determined from the amplitude of the induced THz absorption >2000 cm2/Vs.

Electron lifetime is dependent on the photoexcitation level. Trap filling effect.

Best fit with a single trap model obtained for the capture cross-section of 4.5·10-13 cm2 and the trap density of 3·1016 cm-3 (sample A) and 4·1016 cm-3

(samples B and C).V. Pačebutas et.al., pss(c), (2009)

As-antisite. Point defect leadingto a deep level in the band gap.M.Kaminska, E.Weber, (1990)

The main carrier trapping center in LTG GaAs.

Electron capture cross-section by AsGa in GaAs– 10-13 cm2

A. Krotkus et.al. IEE Proc. Optoelectron.,

In LTG GaAs the lifetime increases after annealing because the majority of the excess As atoms precipitate into nm size clusters.

0,8 0,9 1,0 1,1 1,2 1,3 1,4

-1

0

1

2

3

4

5

6

7

Inte

nsity

(arb

. u.)

Energy (eV)

non-annealed 400 °C 500 °C 600 °C

GaBiAs-9, PL (473 nm), 300K 2009-12-10

Lazeriolinija

Photoluminescence in GaBiAs layer with a long carrier lifetime (~200 psnon-annealed). PL measured by a standard cw technique.

Complex annealing behavior.Additional PL band at ~1.3 m (Bi-clusters?)

THz probe measurement.

Lifetime in as-grown layer is ~200 ps, it slightly increases after anneal at 400 and 500oC, and drops to <30 ps at T=600oC.

0 100 200 300 400

0,1

1

T/

T. a

.u.

Delay, ps

600oC 500oC 400oC non-annealed

-8 -6 -4 -2 0 2 4 6 8

-0,25

0,00

0,25

0,50

0,75

1,00

THz

field

am

plitu

de, a

.u.

Delay, ps

GaBi26 1-2 InGaAs L-2GaBiAs

SI GaAs

Au

0 1 2 3 4 510-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

GaBi26 1-2 InGaAs L-1

Frequency, THz

Pow

er

The width of the gap – 15 m.Detected THz pulse amplitude is 5 times and S/N ratio 100 times larger than when detector made from LTG InGaAs layer is used. THz emitter is p-type InAs.

G. Molis et.al., Electron. Lett. 43,190-191 (2007)

GaBiAsAu

SI GaAs

The geometry of the antenna is similar to one used for THz detectors, except for mesa-etching of GaBiAs layer area between the microstriplines.

GaBiAs doping with Si used for residual p-type conduction compensation.

Dark resistance of the emitter >100 M. Breakdown field larger than 50 kV/cm.

0 10 20 30 40 50 60 70 800

2

4

6

8

10

12

14

16

THz

pow

er,

W

U, V

10mW 20mW 30mW

GaBi 24B 2d

A Golay Cell is a type of detector mainly used for FIR spectroscopy. It consists of a small metal cylinder which is closed by a blackened metal plate at one end and by a flexible metalized diaphragm at the other. The cylinder is filled with Xe and then sealed.

Measurements of THz power as functions of the bias voltage and the optical power.

Optical-to-THz conversion efficiency obtained was up to 710-4, much better than ~10-5 reached with other similar systems.

.

20 dB-bandwidth of THz-TDS system as a function of the optical pulse duration. Solid lines – calculations for different carrier lifetimes in the detector.

The Yb:fiber oscillator operated at a repetition rate of 45 MHz; an average output power of ~8 mW and ~11 mWwas used for photoexcitation of the THz emitter and detector respectively.

0 1 2 3 4 5 6 7

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

Pow

er, a

.u.

Frequency, THz

Dry N2

0 1 2 3 4 5 6 7

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

Pow

er, a

.u.

Frequency, THz

Ambient air

Yb:KGW laser, 1030 nm, 70 fs.

Spectral width ~5 THz, S/N ratio ~70 dB.

Average fs laser power of ~ 10 mW is sufficient for activating both the emitter and the detector.

f0t

f

t

Stretched chirped pulse Splitting in two equal parts

Variable delay between two parts

Photoconductive antennaTHz radiation

Fs laser pulse

-10 0 10 20 30 40 50 60 70

-0,0010

-0,0005

0,0000

0,0005

0,0010

THz

field

am

plitu

de

Delay, ps

t=1,8 ps, f=0,26 THz

-30 -20 -10 0 10 20 30 40 50-0,0006

-0,0004

-0,0002

0,0000

0,0002

0,0004

0,0006

Delay, ps

THz

field

am

plitu

de

0.6 THz

0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0

1E-11

1E-10

1E-9

1E-8

Pow

er, a

.u.

f. THz

Emitter – Hertzian dipole, length 70m, resonance ~0.5 THz.

Yb:KGW laser pulses stretched to 30 ps, average optical power in both arms – 25 mW, bias voltage 30 V.

Maximum frequency observed 1.3 THz.

-4 -2 0 2 41,0

1,2

1,4

1,6

1,8

T

z/z0

0.0424 µJ 0.106 µJ 0.318 µJ 0.53 µJ 0.53i0.274 GW/cm2

0.318i0.274 GW/cm2

0.106i0.249 GW/cm2

0.0424i0.249 GW/cm2

Open aperture Z-scan measurement. Large optical bleaching effect with relatively low saturation intensity.

Possible applications in saturableabsorbers for mode-locked lasers and all-optical switches.

p- GaBiAsSI-GaAs

Uni-travelling carrier photodiode.

Response time limited by the electron sweep-out through the collection layer. Bandwidths >1THz demonstrated.

Dilute GaBiAs due to its large electron mobility in a material with sub-picosecond carrier lifetimes is a prospective material for ultrafast optoelectronic application in the wavelength range of from 1 m to 1.5 m.

THz time-domain-spectroscopy system with components manufactured from GaBiAs and activated by femtosecond pulses of compact 1-mm wavelength laser was demonstrated.

This material shows great prospects in other ultrafast device applications, such as cw THz optical mixers, semiconductingsaturable absorbers, and all-optical switches.