Intense Terahertz Excitation of Semiconductors

S.D. Ganichev

University of Regensburg

Why Intense Terahertz Excitation of Solids?

ω ~ characteristic energies in solids like: size-quantization energies of QWs, optical phonon energies, plasma oscillation energy, Landau levels, Rashba-Dresselhaus spin splitting of subbands etc.

THz range + high radiation intensity: new phenomena and various nonlinear effects whose characteristic features are basically different from the corresponding effects in the microwave or visible range.

ω < Eg: effects of carrier redistribution in momentum space and on the energy scale.

Molecular physics and biology: THz radiation does not pose an ionization hazard for biological tissue, many biological molecules exhibit vibrational and rotational modes at THz frequencies

~

THz range frequency: between 0.3 THz and 10 THz wavelengths λ: from 1000 µm to 30 µm photon energies ω: from 1 to 35 meV

Intense Terahertz Excitation of Semiconductors

Fills the gap between nonlinear optics/opto-electronics and microwave physics/transport phenomena

New regimes of electron transport, powerful tools for exploring non-linear dynamics in semiconductor nanostructures

Transition from semiclassical physics with a classical field amplitude to the fully quantized limit with photons

Novel methods of material characterization

Concepts of terahertz devices important for development ofterahertz technology

High-Power Terahertz Centers aimed to Semiconductor Physics

Ioffe-Institute/Regensburg Univ.molecular lasers, since 1980

High intensities nanosecond pulses0.15 - 75 THz, single lines

Univ. of California Santa Barbarafree electron laser, since 1988

Moderate intensities microsecond pulses0.15 - 5 THz, tunability

European User Center "FELIX"free electron laser, since 1994

High intensities picosecond pulses1 - 75 THz, tunability

German User Center Rossendorffree electron laser, THz: planned for 2006

High intensities sub-picosecond pulses3 - 60 THz, tunability

Regensburg High-Power Terahertz Center

0 200

t ( n s )

101

102

103

104

CH3F

D2O

NH3

NH3

NH3CO

2

Energy ( meV )

10 100 1000Wavelength ( µm )

Inte

nsity

( k

W/c

m2 )

1

10

Electric field ( kV

/ cm

)

100 10 1

CH3F

CH3F

NH3

Frequency (THz) 30 3 0.3

100Regensburg

Regensburg Terahertz Center

[S.D. Ganichev, et al, JETP Lett., 35, 368 (1982)]

• Intensity: up to 10 MW/cm2 • Pulse duration: ~ 100 ns• Wavelength: 25 µm - 2000 µm

Two high power THz laser systems

two 100 MW mid-infrared lasers, λ = 4.6 µm - 10.8 µm

stable cw-THz laser, λ = 30 µm - 2000 µm, ~100 mW power

60 Watt cw-CO2 laser, λ = 4.6 µm - 10.8 µm

2 kW CO2 Q-switch laser, λ = 4.6 µm - 10.8 µm, 300 ns pulses

THz Q-switch laser, λ = 30 µm - 2000 µm, 300 ns pulses, high stability

Microwave sources: 375 GHz (8mm) and 100 GHz (3.3 mm)

Intense Terahertz Excitationof Semiconductors atRegensburg Center

THz Tunneling Non-linear THz Optics

Hot ElectronsPhotocurrents

THz Device Physics

Spin phenomena

THz Material Physics Biology

THz Tunneling

Phonon assisted tunneling [Phys. Rev. Lett. 71, 3882 (1993)]

Tunneling time [Phys. Rev. Lett. 75, 1590 (1995)]

Drastical enhancement of tunneling in THz range [Phys. Rev. Lett. 80, 2409 (1998)]

Radiation pressure mediated tunneling [JETP. Lett. 44, 301 (1986)]

Spin dependent tunneling [Phys. Rev. B. Rap. Comm. 67, R201304 (2003)]

- Frequency times tunneling time is about unity, ωτ ~ 1- Large number of photons, much more photons per power in the THz-range - Resonance with energy levels in semiconductors- Quasi-optical methods result in a highly senitive detection - High intensities may result in essential ponderomotive action of light

Tunneling in high frequency alternating fields and tunneling in static fields assisted by high

frequency radiation

THz Tunneling

Radiation pressure mediated tunneling

Ref

lect

ion

1

λ = 27 µmp λ (µm)

THz

n-GaAs R

semitransparent metal electrode

10 100 1000

Wavelength (µm)

λp

λ > λp

λ < λp

plasma reflection

n-GaAs

n-GaAs

bΦ

0 x0 x* x

metalTHz Ph

otoc

ondu

ctiv

ity

0

1

2

THz

THz Tunneling

Drastic enhancement of tunneling in THz range

Quasy-static tunneling High-frequency tunneling Multi-photon transitions

ωτ

100

10

10 1 2 3

P(ω

) /

P(ω

=0)

Ge:Hg AlGaAs:Te

E

ε

ωτ < 1Field frequency

ωτ >> 1

n ω

I

ωτ > 1

E ( ω )

ionization of deep impuritiesω << εb

THz frequencies are aboutreciprocal tunneling time

Multi-photon excitation beyond the perturbation limit [JETP. Lett. 37, 901 (1983)]

Spin sensitive bleaching in QWs [Phys. Rev. Lett. 88, 057401 (2002)]

Terahertz absorption saturation [Semiconductors 21, 615 (1987)]

Near-field effects [phys. stat. sol. a 175, 289 (1999)]

Linear-circular dicroism of multi-photon absorption [Phys. Solid State 35, 104 (1993)]

Multi-photon vibronic excitation [Phys. Rev. B Rapid Comm. 52, R8617 (1995)]

- Large number of photons per watt- Transition from semiclassical to fully quantized limit- Resonances with vibrational and rotational modes at- Free electron absorption

Non-linear THz Optics

THz: multi-photon excitation beyond the perturbation limit

Parameter of non-linearity:

η > 1 : regime of fully developed nonlinearity

∝ n-photons n-photons

m-photons

n-photon absorption K ( n ) n - 1 ; ∆ ε = n ω ∝

ε

...

.

...

.

n ω

k

K ( n ) I∝ K ( n - 1 ) ω 3

η =

Multi-photon excitation in the perturbation limit

Absorption oscillates with light intensity (Bessel functions)

Non-linear THz Optics

Circular photogalvanic effect in QWs [Phys. Rev. Lett. 86, 4358 (2001)

Spin-galvanic effect [Nature (London) 417, 153 (2002)]

Magneto-gyrotropic effects [submitted to J. Cond. Matter (2005)]

Spin orientation by current (inverse spin-galvanic effect) [cond-mat (2004)]

Monopolar spin relaxation in QWs [Phys. Rev. Lett. 88, 057401 (2002)]

- Gyrotropic properties of low dimensional electron gas- Angular momentum of circular polarized radiation- Monopolar spin orientation due to small photon energy- Resonances with energy levels in quantum well structures

Spin phenomena

Circular photogalvanic effect

Sy

jx

2DEG

e

ey

σ+

kx0

e2

e1

jx

ε

kx+

Spin phenomena

Circular photogalvanic effect

Sy

jx

2DEG

e

ey

σ+

kx0

e2

e1

jx

ε

kx+

kx0

jx ε

Spin-galvanic effect

Spin phenomena

Circular photogalvanic effect

Sy

jx

2DEG

e

eyBxjx

2DEG

THz radiat ion

σ+

kx0

e2

e1

jx

ε

kx+

kx0

jx ε

Spin-galvanic effect Magneto-gyrotropic effects

ε

k0

j

∆ε = gµBB

.. ... .... ....... ...

. .

.

τε1 τε2<

...... .

...

e1(-1/2)

e1(+1/2)

Spin phenomena

Photon drag effect [JETP. Lett. 35, 368 (1982)]

Linear photogalvanic effect (quantum ratchet) [Appl. Phys. Lett. 77, 3146 (2000)]

Plasma reflection induced photocurrents

in Schottky-diode [JETP Lett. 62, 53 (1995)]

- Effects of carrier redistribution in momentum space and on the energy scale ( ω < Eg)- Ponderomotive action of radiation- Angular momentum of circular polarized radiation- Quantum ratchet effects

Photocurrents

Microwave induced pattern formation [Nature (London) 397, 398 (1999)]

Light impact ionization [JETP Lett. 40, 948 (1984)]

Nonlinear absorption due to light

impact ionization [Appl. Phys. Lett. 64, 1977 (1994)]

Heating and cooling of electron gas in bulk materials [JETP Lett. 38, 448 (1983)]

Heating of 2D electron gas [JETP Lett. 48, 269 (1988)]

THz heating of LO phonons

in δ-doped GaAs [Solid State Communic. 97, 827 (1996)]

- Photon energies can be smaller than optical phonon energy- Effective free electron absorption- Contactless application of high fields- Interplay of phonon emission and photon absorption

Hot Electrons

Light impact ionization ( )

ω = 3meV ω = 224 meV

ωτp >> 1

0 20 40 60 80norm

aliz

ed n

umbe

r of

ge

nera

ted

carr

iers

n-InSbT = 78 K

102

10-1

101

100

10-2

10-3

10-4

E -2 (10-8, V -2*cm2)

∆p = εE(ω)/ω

p0

heating is caused by collisions

E(ω)

Hot Electrons

λ = 385 µm

λ = 152 µm

λ = 90 µm

Spin photocurrents (spin-orbit coupling in band structure, spin relaxation)

[Phys. Rev. Lett. 92, 256601 (2004)]

Saturation spectroscopy (spin and energy relaxtion)

bulk materials: [Semiconductors 16, 179 (1982)]

quantum wells: [J. Appl. Phys. 96, 420 (2004)]

Terahertz phonon assisted tunneling (impurities characterization)

[Phys. Rev. B Rap. Comm. 63, R201204 (2001)]

[Phys. Rev. B 61, 10361 (2000)]

[J. Appl. Phys. 87, 3843 (2000)]

[Phys. Rev. B Rap. Comm. 55, R9243 (1997)]

Light impact ionization (impurities parameters)

[JETP 63, 256 (1986)]

THz µ-photoconductive spectroscopy (relaxation)

[JETP 70, 1138 (1990)]

THz Material Physics

THz laser technique for semiconductor physics [JETP Lett. 35, 368 (1982)]

Photon drag detectors for THz radiation [Tech. Phys. Lett. 11, 20 (1985)]

µ-photoconductive detectors [Tech. Phys. Lett. 11, 377 (1985)]

Polarization detector [Tech. Phys. Lett. 14, 580 (1988)]

Plasma-reflection tunnel diodes [Tech. Phys. Lett. 15, 290 (1989)]

Generation of ps-THz laser pulses [Int. J. Infrared & Milimeter Waves 11, 851 (1990)]

Detector of radiation helicity [Int. J. Infrared & Millimeter Waves 24, 847 (2003)]

Spectral range 2 µm - 2000 µm (75 - 0.15 THz)

THz Device Physics

Tunneling processes induced by terahertz fields,

[J. Biological Physics, 29, 327 (2003)]

Dental tissue analysis and treatment

by interaction with terahertz radiation,

[Int. J. Infrared & Millimeter Waves 21, 407 (2000)]

- THz radiation does not pose an ionization hazard

for biological tissue

- Biological molecules exhibit vibrational and

rotational modes at THz (characteristic fingerprints)

- T-rays can easily penetrate and image inside

most dielectric materials

- Tunneling in proteins induced by contactless

applied THz radiation

Biology

Dental tissue analysis and treatment by interaction with terahertz radiation

0.0

0.5

1.0IIR

= 21 MW/cm2

1090107010501030

Frequency ν ( cm-1 )

ν3

(c)ν

3

(b) ν3

(a)

Ligh

t Int

ensi

ty (

arb

. uni

ts )

1075 1080 1085 10900

5

10

15

20

ν3

(a)

Frequency ν ( cm-1

)Pla

sma

Thr

esho

ld (

MW

/cm

2 )

Detection

Resonance with the spectral position of the PO4 stretch mode

Resonance laser ablation

Biology

RussiaA.F. Ioffe Institute, St. Petersburg

Polytechnical University, St. Petersburg Institute of Radioelectronic, MoscowInstitute of Semiconductor Physics, Novosibirsk

GermanyUniversität Regensburg

Walter Schottky Institute Universität HannoverUniversität GießenTU MünchenUniversität DortmundUniversität WürzburgTU BraunschweigUniversität Bayreuth

USAUniversity of California, BerkeleyNaval Research Lab., WashingtonUniversity of New York, BuffaloUniversity of New York, RochesterUniversity of PurdueM.I.T.

UKUniversity of Surrey, GuildfordHeriot-Watt University, Edinburgh

The NetherlandsEU THz-center "FELIX"

ÖsterreichUniversität LinzUniversität Wien

Japan Tohoku University

FranceUniversity of Toulouse

IsraelTechnion, HaifaBar-Ilanh University

Czech RepublicInstitute of Physics, Prague

UkraineInstitute of Semiconductor Physics, Kiev

IrelandTrinity College, Dublin

Intense Terahertz Excitationof Semiconductors atRegensburg Center

Current projects

- Spingalvanischer Effekt

- Magnetogyrotroper Effekt im Halbleiternanostrukturen

- Monopolare Spinorientierung und Spinrelaxation mit nichtlinearer

Intersubband-Spektroskopie

- Spinphotoströme in Halbleiternanostrukturen

- Terahertz-Nichtlinearitäten in Halbleiter-Nanostrukturen

- Spin dependent tunneling

- Detection of radiation helicity

- Optical and nonlinear properties of nanostructures

Intense Terahertz Excitationof Semiconductors atRegensburg Center

Topics: spin-phenomena, tunneling,

detection principles, light impacz ionization

Tasks for future Intense Terahertz Excitationof Semiconductors atRegensburg Center

(submitted proposals)

- Infrared excited spin photocurrent in SiGe quantum structures

- Spinphotoströme in Quantentrögen und niederdimensionalen

lateralen Halbleiter-Übergittern

- Rein elektrische Spininjektion durch Spin-Bahn-Wechselwirkung in

gyrotropischen Nanostrukturen

- Rashba/Dresselhaus Spinaufspaltung in Quantentrögen

- Dental Tissue Analysis and Treatment by Interaction with Terahertz

Laser Radiation

Topics: spin-phenomena, tunneling,

medicine, characterization of QWs

Tasks for future Intense Terahertz Excitationof Semiconductors atRegensburg Center

(first steps)

- Quantum ratchets

- Zero resistance states at high frequencies

- Helicity dependent currents in carbon nanotubes

- Tunneling controled chirality of molecules

- Light impact ionization prozesses in QWs

- THz tunneling in proteins and molecules

- Fully developed multi-photon transitions in QWs

- Dynamic of spin photocurrents

- Near field spectroscopy of nanostructures

Topics: molecular physics, tunneling, quantum transport

detection principles, near-field effects,

multiphoton processes, light impacz ionization

ratchet effects

Summary Intense Terahertz Excitationof Semiconductors atRegensburg Center

- S.D. Ganichev, and W. Prettl, Intense terahertz excitation of semiconductors Oxford University Press , pp. 1 - 380 scheduled 2005.- S.D. Ganichev, in series ''Advances in Solid State Physics'', B. Kramer (Ed.) (Springer-Verlag Berlin-Heidelberg) Vol. 43, pp. 427-442 (2003).- S.D. Ganichev, and W. Prettl, J. Phys.: Condens. Matter 15, R935 (2003).- S.D. Ganichev, I.N. Yassievich, and W. Prettl, J. Phys.: Condens. Matter 14, R1263 (2002).- S.D. Ganichev, Physica B 273-274, 737 (1999).- S.D. Ganichev et al, in Best of Soviet Semiconductor Physics and Technology (1989-1990), ed. by M. Levinstein and M. Shur (World Scientific, Singapore), 567 (1995).

http://www.physik.uni-regensburg.de/forschung/ganichev/ Reviews:

- novel field in investigations of solids.

- new phenomena and variety of nonlinear effects whose charactersitic features are basically different from the corresponding effects in the microwave range as well as in the range of visible light.

- provide powerful tools for material characterization and for development of THz technology