Ultrafast demagnetization of ferromagnetic films D. J. Hilton 1 , R. - PowerPoint PPT Presentation

Ultrafast demagnetization of ferromagnetic films D. J. Hilton 1 , R. D. Averitt 1 , C. A. Meserole 2 , G. L. Fisher 3 , D. J. Funk 2 , J. D. Thompson 1 , and A. J. Taylor 1 1 MST-10 , 2 DX-2, 3 NMT-16 Los Alamos National Laboratory, Los Alamos, NM 87545 Supported by AFRL/DELE, Kirtland AFB, Albuquerque, NM and Los Alamos, LDRD/DR Unclassified

Outline • Applications of Ultrafast THz • THz emission mechanisms – Current Surge – Optical rectification – THz emission from Metals – Ultrafast Demagnetization – Towards higher electric field sources • Ultrafast Demagnetization in Iron – History of ultrafast magnetization changes – Terahertz Emission Spectroscopy Unclassified

THz Time Domain Spectroscopy • Terahertz frequencies and THz -TDS (1 THz � 4 meV � 33 cm -1 � 300 µ m) • Material characterization and bistatic ranging Feasibility studies (RCS characterization) for spaced-based broadband radar (FEM at 300 GHz with >30 GHz bandwidth) Increased range resolution, detection of embedded materials • NDE of energetic materials Imaging of voids in plastic bonded explosives, THz spectroscopy of single crystal HMX • Sensor Negation THz frequencies are only 1 to 2 orders of magnitude faster that the frequencies of electronics. Unclassified

Terahertz time domain spectroscopy (THz-TDS) Ultrafast optical approach for generating pulses in an underutilized region of the EM spectrum (“THz Gap”) Photonics THz Microwave 3 mm – 10 micron Free Electron 100 GHz – 30 THz Maser 3.33 cm -1 – 1000 cm -1 • THz radiation (T-rays) consists of short pulse (< 1 ps), single- cycle, freely propagating FIR pulses generated via ultrafast optoelectronic techniques • Directional, focusable, broadband (0.1 to 3 THz) • The electric field (i.e. amplitude and phase) is directly obtained. • Time-gated technique enables very high SNR. • Easy access to the time and frequency response – both are useful in characterizing the THz response of materials. Unclassified

Metalized Plastics Transmission induced via disorder, not cracks in metal Unclassified

N 2 O absorption Spectra Metalized Metalized Plastic Plastic Laser Excitation Laser Excitation 1.5eV 100-fs 1.5eV 100-fs Trigger Pulse Gate Pulse To simulate an inflated metalized plastic balloon, we inserted sheets of balloon material in the beam path on either side of the gas cell . Unclassified

THz Spectroscopy of Explosives Are there spectroscopic signatures of these molecules at THz frequencies? HMX Done in collaboration with Dave Funk, Dan Hooks, and Jeff Barber, DX-2 Unclassified

THz Spectroscopy of Explosives Unclassified

Electrical Interactions Introduced picosecond THz transient in working transistor to cause it to switch off. 0.25 µ m V Gate ~3.3 V Gate Need E THz ~10 2 kV/cm inside channel to Source Drain disrupt the transistor E Channel ~130 kV/cm Channel Unclassified

Mode Locked Ti:Sapphire • ~100,000 frequencies locked into phase coherently result in a fs pulse. • Outputs 50 fs pulses at 80 MHz with a center wavelength of 800 nm (375 THz). • Can be amplified using Ti:S based amplifier to produce 30 mJ/pulse. • Higher pulse energies are possible. Unclassified

THz emission via fs excitation Mechanisms: • Current Surge - FIR dipole radiation from acceleration of photo- injected carriers in a surface depletion field • Pondermotive acceleration of electrons in a laser plasma • Optical Rectification - Difference Frequency Mixing – Bulk electric-dipole: χ (2) – Bulk electric-quadrupole/magnetic dipole: χ (Q) – Field Induced (Surface) electric-dipole: χ (3) , E d – Surface or bulk magnetization: χ (2) ( M ) • Ultrafast demagnetization—FIR dipole radiation from rapid demagnetization following the creation of a nonthermal electron distribution with a fs optical pulse. Unclassified

THz Generation Mechanism [ ] 2 d Optical Rectification ∝ χ * ( 2 ) E E E THz pump pump E THz ~ χ (2) |E pump | 2 2 dt dJ Photoconductive Switch E THz ∝ Pondermotive Acceleration dt 2 d M Ultrafast Demagnetization E THz ∝ 2 dt Unclassified

Auston switches • Photoconductive Emitters: Electrodes with V bias a several kV/cm bias across a gap. 1.5 eV, 100 fs • A fast surface current transient is initiated 800nm Pulse by photo-injecting carriers with an ultrashort laser pulse. Ensuing THz radiation temporally tracks the time derivative of the total surface current. • Peak output fields of ~ kV/cm • Emitters: GaAs, LT-GaAs, InP ~0.5 THz, • Radiated THz field saturates with fluence, 300 µ m, 4 meV F: E THz ~ E B F/F o /(1 +F/F o ) Unclassified

THz detection • Photoconductive receivers : - Based on the same principle of emitters. - THz E-field is used to bias a photo-gated detector, typically radiation damaged silicon-on-sapphire (SOS) or LT-GaAs. - Detector response time of SOS, τ r < 0.5 ps. • Electro-optic sampling - Based on detection of polarization rotation Pockels effect in a χ (3) material (ZnTe). - E-field from THz beam is used to rotate the polarization of an optical gate beam via electrooptic effect. - Detection bandwidth is limited by the group velocity mismatch between THz beam and optical beam. Unclassified

THz emission via fs excitation Mechanisms: • Current Surge - FIR dipole radiation from acceleration of photo- injected carriers in a surface depletion field • Pondermotive acceleration of electrons in a laser plasma • Optical Rectification - Difference Frequency Mixing – Bulk electric-dipole: χ (2) – Bulk electric-quadrupole/magnetic dipole: χ (Q) – Field Induced (Surface) electric-dipole: χ (3) , E d – Surface or bulk magnetization: χ (2) ( M ) • Ultrafast demagnetization—FIR dipole radiation from rapid demagnetization following the creation of a nonthermal electron distribution with a fs optical pulse. Unclassified

Ponderomotive Acceleration of electrons • PULSE code self-consistently propagates a pulse through ionizable media: Axial force completes full ±z oscillation Transverse force is always outward E x (x) E x (z) F pond (x) F pond (z) x,y z E x (z)cos(kz-wt) n i t n • THz radiation frequency linked to dominant laser gradient e i d ) Force from v x xB y in axial direction x a r n g i n d Transverse force from v x xB z o l e i t i f o - m E x e - r e y s b a l n m e o e s r f ( e n c o r • Laser E x and B y,z fields accelerate free electrons: i o t c F e r i d - x � � − π − ⎡ ⎤ 7 2 ( ) 2 dE dE dE 10 e e ∇ = + + � 2 x x x F I W/cm xE ˆ yE ˆ zE ˆ ⎢ ⎥ ω ω pond laser x x x 2 2 ⎣ ⎦ mc 4 m dx dy d z • Radiated field from ponderomotively accelerated electrons: � � � � ( ) � � � ⎧ ⎫ ⎡ ⎤ ⎡ ⎤ × × × × β � n n F / m ⎪ ⎪ n ( n ) ⎣ ⎦ e e ⎢ ⎥ pond = � pond ⎨ ⎬ E T Hz ⎢ ⎥ c r c r ⎪ ⎪ ⎣ ⎦ ⎩ ⎭ ret ret Unclassified

THz emission via fs excitation Mechanisms: • Current Surge - FIR dipole radiation from acceleration of photo- injected carriers in a surface depletion field • Pondermotive acceleration of electrons in a laser plasma • Optical Rectification - Difference Frequency Mixing – Bulk electric-dipole: χ (2) – Bulk electric-quadrupole/magnetic dipole: χ (Q) – Field Induced (Surface) electric-dipole: χ (3) , E d – Surface or bulk magnetization: χ (2) ( M ) • Ultrafast demagnetization—FIR dipole radiation from rapid demagnetization following the creation of a nonthermal electron distribution with a fs optical pulse. Unclassified

Difference Frequency Generation • Uses a special class of materials with strong nonlinearities, χ (2) , which result in transfer of energy from one frequency to another by generating a far IR dipole in material. [ ] ∂ 2 ( ) ( ) ( ) ν = χ ν ν ( ) * 2 E E E ∂ 3 1 2 2 t • Common materials: ZnTe, LiNbO 3 , BBO, KTP, KDP, AgGaSe, AgGaS, GaSe , etc. Unclassified

Optical Rectification Special case of DFG using the bandwidth of the Ti:S pulse to generate the THz pulse. THz bandwidth is limited by Ti:S pulse bandwidth. Unclassified

THz generation via optical rectification θ ∂ ∂ 2 2 ( ) P Ω ∝ = χ ω ω rad ( 2 ) * E E ( ) E ( ) ∂ ∂ i ijk j 1 k 2 2 2 t t φ Azimuth Incidence � Bulk : electric dipole E THz ~ cos 2( φ - φ ο ) electric quadrupole/magnetic dipole χ ( 2) : All of these are E THz ~ cos 4( φ - φ ο )sin θ “instantaneous” � Surface : electric dipole nonlinearity: processes and do not E THz (p,p) and E THz (s,p) ~ sin θ limit the emission E THz (p,s) and E THz (s,s) ~ 0 bandwidth. � Magnetic nonlinearity , χ ( 2) ( M ) : E THz ~ cos( φ - φ ο ) + A cos3( φ – φ 1 ) | ω 2 χ ( 2) ( M ) | ~ 10 -12 esu ref: Phys. Rev. B 48, 8607 (1993). Unclassified

THz generation mechanism: Optical rectification •Optical Rectification: Characterized χ (2) by nonlinear optical difference Crystal 1.5 eV, 100 fs frequency mixing : 800nm Pulse ∂ ∂ 2 2 ( ) P Ω ∝ = χ ω ω rad ( 2 ) * E E ( ) E ( ) ∂ ∂ i ijk j 1 k 2 2 2 t t •Peak field outputs of 10 - 100 V/cm •Emitters: GaAs, InP, DAST, ZnTe, LiNbO 3 , LiTaO 3 , GaSe ZnTe Unclassified