Practical: how to measure ultrafast spin and charge currents - PowerPoint PPT Presentation

Practical: how to measure ultrafast spin and charge currents Terahertz pulse Tobias Kampfrath Terahertz Physics Group Freie Universität Berlin and Fritz Haber Institute/MPG Berlin, Germany

Heat-driven currents: the Seebeck effect Metal film Thomas Seebeck (1821): A temperature gradient drives an electron current - - Ken-ichi Uchida (2008): - In ferromagnets, the Seebeck current is spin-dependent Hot Cold Temperature gradient

Spin-dependent Seebeck effect (SDSE) ← and → electrons have very different Fe film transport properties Uchida, Saitoh et al ., Nature (2008) Bauer, Saitoh, Wees, Nature Mat (2013)

Spin-dependent Seebeck effect (SDSE) ← and → electrons have very different Fe film transport properties Uchida, Saitoh et al ., Nature (2008) Bauer, Saitoh, Wees, Nature Mat (2013) ⇑ Spin-polarized current Detection with the inverse spin Hall effect Hot Cold Temperature gradient

Inverse spin Hall effect (ISHE) Heavy Fe film metal Spin-orbit coupling deflects electrons ⇑ Transverse charge current A ⇑ Spin-to-charge (S2C conversion Saitoh et al ., APL (2006 ) How can we induce an Hot Cold imbalance as fast as possible? Temperature gradient

Inverse spin Hall effect (ISHE) Heavy Fe film metal A fs pump pulse Technical challenge: ƒ Electric detection has cutoff at <50 GHz ƒ But expect bandwidth >10 THz

Inverse spin Hall effect (ISHE) Heavy Fe film metal Emission of electromagnetic pulse (~1 THz) Kampfrath, Battiato, Münzenberg et al ., Nature Nanotech. (2013) fs pump pulse ⇑ Measure THz emission from photoexcited FM|NM bilayers polycrystalline films (labs of M. Kläui and M. Münzenberg) Samples: from Ti:sapphire oscillator (10 fs, 800 nm, 2.5 nJ) Pump pulses: How can we detect the THz pulse?

THz detection: electro-optic sampling Delay σ Sampling THz pulse field E THz ( t ) Nonlinear- optical Wu, Zhang, crystal APL (1995) Electro-optic effect : Change in refractive index × E THz ( t ) ⇑ Crystal becomes birefringent Scan ellipticity of sampling pulse vs σ ⇑ Get THz electric field E THz ∋σ( A look in the lab…

Simple THz emission setup in the lab Optical pump beam Spintronic sample Si Probe Parabolic mirror beam Electrooptic crystal for sampling of the THz electric field To detection of probe ellipticity

Typical THz waveforms from Fe|Pt bilayers 3 3 -20 mT Pt Fe 2 2 Signal (10 -5 ) Signal (10 -5 ) 1 1 0 0 -1 -1 -2 -2 Fe Pt Fe +20 mT ext. field -3 -3 0 0.5 1 0 0.5 1 Time (ps) Time (ps) Further findings Consistent with scenario ƒ Signal × pump power spin transfer + ISHE ƒ THz electric field ] sample magnetization Need more evidence for the spin Hall scenario

Ultrafast inverse spin Hall effect Idea: Ta vs Ir: vary nonmagnetic cap layer opposite spin Hall angles, Ir larger 2 The inverse Signal (arb. units) Fe Ir spin Hall effect is 1 still operative at THz frequencies Fe Ta 0 Kampfrath, Battiato, Oppeneer, Freimuth, Mokrousov, Radu, Wolf, Münzenberg et al ., -1 Nature Nanotech (2013) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Time t (ps) This has interesting applications… tomorrow Today: how can we determine the THz-emitting source current?

THz source current Fs Photo- THz pump current field Sample impedance, usually known Sample � = � ∗ � � � � ( � ) Ohm‘s law: � ⇑ Yields photocurrent � ( � ) Issue: we do not measure � ( � )

The transfer function of the THz setup THz Photo- THz Collimation, current field focusing detector Actually measured signal Sample ℎ � ( � ) ? � ( � ) � = ∫ d � � ℎ � − � � � � � � depends linearly on � : � � = ℎ ∗ � Convolution Transfer function: response to � -pulse � � = ℎ � � ⋅ � � � � Simpler in frequency space: How can we get the transfer function ℎ ?

How to determine the transfer function? � = ℎ ∗ � Goal: determine ℎ over large bandwidth (0.3 to 40 THz) 1) Calculate ℎ : requires approximations, e.g. idealized setup 2) Measure ℎ : use a broadband THz reference emitter � ��� = ℎ ∗ � ��� Measure Calculate Use optically transparent THz emitter: ƒ � ( � ) and � ( � ) are well known ⇑ � ��� is quite well predictable ƒ We choose ZnTe and GaP Calculate � ��� and measure � ���

Reference emitter: GaP(110), 50 µm thick � ��� | | � Calculated � ��� ( � ) Measured � ��� ( � ) � ��� | | � 0 10 20 30 -0.2 0 0.2 0.4 0.6 Time t (ps) Frequency (THz) � ��� ∗ ℎ = � ��� Solve for ℎ —directly in the time domain

Experiment vs theory � ( � )| Spectral amplitude |ℎ Transfer function ℎ ( � ) Reference emitter: GaP(110), 50 µm thick (arb. untis) (arb. untis) Reference emitter: ZnTe(110), 50 µm thick Calculated: ƒ Extended Gaussian beam propagation ƒ Detector response -0.2 0 0.2 0.4 0 10 20 30 Time � (ps) Frequency � / 2� (THz) Highly consistent results for ℎ

Understanding the structure of � � ( � )| Spectral amplitude |ℎ Transfer function ℎ ( � ) (arb. untis) (arb. untis) Calculated -0.2 0 0.2 0.4 0 10 20 30 Time � (ps) Frequency � / 2� (THz) ƒ � = 0 : remainder of input � -peak ƒ High pass: DC cannot propagate ƒ � < 0 : faster THz components ƒ Low pass: e.g. probe duration ƒ � > 0 : slower components, e.g. ƒ 8…12 THz: Restrahlen band of GaP in Reststrahlen region Ready to apply ℎ

Spintronic THz emitter: from � to � Measured signal � ( � ) Extracted field � ( � ) Electrooptic detector: ZnTe(110) GaP(110) (arb. units) (10 -5 ) Detector out of focus ≥ 5 Aperture ( ⊕ =2 cm) in collimated THz path -0.4 -0.2 0 0.2 0.4 0.6 0 0.2 0.4 Demonstrates consistent Time � (ps) Time � (ps) extraction of THz field

Conclusion Developed reliable extraction method: ℎ THz electric field � � Measured electro- optic signal �(� ) directly behind the sample Application: quantitative measurement of ultrafast charge transfer in e.g. ƒ Spintronic multilayers ƒ Photovoltaic structures �(� ) A + - ƒ Molecules: photochemical processes �(� ) … so far very rarely implemented B - Future extensions: ƒ Better reference emitters: thinner, stronger, flat spectral output