Generation and Application of Terahertz Pulses

Generation and Application of Terahertz Pulses János Hebling University of Pécs hebling@fizika.ttk.pte.hu 1

Outline Introduction Properties of THz radiation/pulses Importance of THz science and technique Cw THz sources THz detectors, THz imaging Single-cycle THz sources I.: Photoconductive antennae Time-domain terahertz spectroscopy (TDTS) Single-cycle THz sources II.: Optical rectification Optical rectification II.: Tilted-intensity-front-pumping Single-cycle THz sources III.: Laser plasma High intensity THz science and technology Generation and application of THz pulses with extreme high energy and field

Terahertz radiation (T-ray) in the EM spectrum 3

Correspondence of 1 THz frequency 1 THz 4.15 mev photon energy (hν) 1 THz 33.3 cm 1 wave number 1 THz 300 µm wavelength (λ) 1 THz 1 ps temporal period (T) 1 THz 48.1 K temperature (T=hν/k B ) 4

5 0-5 Single-cycle THz pulse 1.0 0.8 0.6 0.4 0. Amplitude FFT (r. e.) 0.0 0 1 3 Frequency (THz) - 0 4 6 8 Time (ps) 5 EOS signal (r. u)

Number of THz publications 000 Number of publications 1500 1000 500 0 1985 1990 1995 000 005 010 015 Year 6

THz imaging It is based on the different transmission or reflection of different materials Metals reflects, some dielectrics absorbs, but most plastics, paper and clothes mostly transmits THz radiation THz imaging is applicable in security screening! THz transmission THz reflexion Optical and THz images of metal, ceramic and explosive hidden into shoe heel 7

THz security screening 8

THz spectroscopy Organic molecules and crystals has specific THz absorption spectrum Spectral fingerprints Intermolecular vibration in the unit cell of RDX crystal Spectra of explosives 9

Spectral imaging MDMA, Methamphetamine, aspirin K. Kawase et al: Opt. Express 11, 549 (003) 10

THz sources Electronic and optical radiation sources. For a long time Terahertz gap existed cw: FIR molecular lasers, FELs Single-cycle THz sources are based on fs lasers 11

Single-cycle (broad-band) terahertz pulse sources I., Emitters and detectors based on photoconductive antennas Time domain THz spectrometer (TDTS) Based on photoconductive switches or photoconductive antennae Photocond. antennae h = R L n n 1 Jepsen: JOSA B 13, 44 (1996) 13

Emitters and detectors based on photoconductive antennae The averaged photocurrent is: di( t) E THz ( t) dt T 1 j( τ ) = E( t) g( t + τ ) dt T 0 where E(t) is the THz electric field and g(t) is the conductivity 14

Measured THz pulse-shape P laser = 30 mw Δt laser = 100 fs E THz =100 V / cm I average = 10 na 15

and the spectrum calculated from that Broad spectral bandwidth Δν ν FWHM mean 16

Time domain terahertz spectroscopy (TDTS) Time domain THz spectrometer (TDTS) is usually based on photoconductive switches (photoconductive antennae) Set-up for transmission measurement The sample is placed between the two off-axis parabolic mirrors Photocond. antennae P. U. Jepsen: JOSA B 13, 44 (1996) 17

Time domain terahertz spectroscopy (TDTS) But time domain THz spectrometer (TDTS) is sometimes based on optical rectification and electro-optic sempling Set-up for transmission measurement A. Nahata et al.: Appl. Phys. Lett. 69, 31 (1996) 18

Time domain terahertz spectroscopy (TDTS) Reflection set-up And time domain THz spectrometer (TDTS) is sometimes based on THz pulse generation and detection in air plasma Set-up for reflection measurement X.-C. Zhang: Intr. to THz wave photonics, Springer (010) 19

Calculation of the spectra In both cases the E(t) temporal shape of the THz pulses are measured by sampling, and the complex spectrum of the THz pulse calculated by (fast) Fourier transformation: ~ iφ ( ω iωt ( ω ) = A( ω) e ) = E( t) e dt E The δω spectral resolution is limited by the T temporal scanning range: The spectral range is limited by the δt temporal step of sampling: In order to have a smooth calculated THz spectrum 0 padding technique (adding several 0 values to one of both side of the measured waveform) can be used. This mathematically increases the value of T, resulting more dense calculated spectral points. However, the resolution will not be better. δω = π T Δω = π δt (Δω is decreased by the finite spectral width of the source and the bandwidth of the detection.) 0

Calculation of the spectra Measured waveform Calculated spectrum Jepsen: JOSA B 13, 44 (1996) Both for transmission and reflection measurement a reference measurement is needed. In the case of transmission measurement the reference can be a measurement without the sample. In the case of reflection measurement reference is the result of a measurement with a mirror in exactly (with an accuracy of a very small fraction of the wavelength) in the place of the sample. 1

Calculation of the spectra Simplest case For transmission measurement the evaluation is simplest if a thinner sample is used as a reference. In this case the effect of the Fresnel loss at the surfaces will be canceled. If the E S (t) signal and E R (t) reference waveforms are measured on a shorter temporal length than the round-trip time in the reference (t RT =nd r /c), the (field) absorption spectrum is given by: α( ω) = n( ω) 1 A ln d A = 1+ ( ω) ( ω) and the index of refraction is given by: where d=d S -d R. Both amplitude and phase can be measured. R S [ φ )( ω) φ ( ω ] S R ) d ω c Both the real and imaginary part of index of refraction and permittivity can be determined.

THz spectra Typical spectra on the THz range Rotation spectral lines of small molecules Spectral features of molecular crystals (for example explosives) Dynamics of macromolecules, stretching, bending, torsion (typically featureless) Absorption of water (featureless) Solvation (featureless) Quasi-particles in solids: phonons, polarons, excitons, magnons, spin-waves Free-carrier absorption Lee Y.-S.: Principles of THz Sci. & Tech., Springer 009 3

Single-cycle terahertz pulse sources II. Optical rectification. The inverse process: Electro-optic sampling DFG can be used for generation of THz radiation Broad spectrum of ultrashort pump pulse contains spectral content with THz frequency difference Optical rectification (OR) 5

6 Nonlinear optical devices Basic theory Consider monochr. plane wave and linear material response: ) ( ) ( ) ( 0 ω ω χ = ε ω E P From Maxwell s eqs. a wave equation can be derived: t t ε = P E E 0 c 1 c 1-0 ) ( c ) ( - ) ( = ω ω ε ω t z E E where ) n( ) ( 1 ) ( ω = ω + χ = ω ε For nonlinear material response: [ ] ) (... ) ( ) ( ) ( ) ( () (1) 0 ω + ω ω + χ ω χ = ε ω E E P t t z ω ε = ω ω ω ) ( c 1 ) ( c ) n( - ) ( ) ( 0 P E E ) ( ) ( ) ( ) ( () 0 () ω ω ω χ = ε ω E E P where This wave equation is an inhomogeneous diff. eq. r.h.s. is responsible for wave generation (1)

P P () () ( ω) = ε = ε 0 χ 0 () χ () A Optical rectification ( ω) E( ω) E( ω) 1 E( ω, t) = Acos ( ωt - kz) [ ] () cos( ωt kz) = χ A [ 1+ cos(ωt kz) ] E( ω, t) = A cos ( ω1t - k1t) + Acos ( ωt - k P () + ε = ε + ε 0 0 0 χ χ χ = ε () () () 0 χ () 1 t OR SHG { A [ cos( t k z) ] A [ cos( t k z) ] } ω + ω { A A cos( ω t k z) cos( ω t k z) } 1 1 {...1... +... + A A cos( ( ω + ω ) t ( k + k ) z) } { A A cos( ( ω ω ) t ( k k ) z) } 1 ω THz 1 = ω 1 1 1 1 1 ω 1 1 1 ) = 1 + + SFG DFG, OR, THz generation

Optical rectification Using ultrashort laser pulses for excitation: E( ω, t ) = A( t)cos ( ωt - kt) P () + ε 0 χ = ε () 0 A χ () {.1. +.. + A ( t)cos( ( ω + ω ) t ( k + k ) z) } ( t)cos (( ω ω ) t ( k k ) z) 1 1 1 1 + EOS signal (r. u) 5 0-5 Amplitude FFT (r. e.) 1.0 0.8 0.6 0.4 0. 0.0 0 1 3 Frequency (THz) - 0 4 6 8 Time (ps) Single-cycle THz pulse Spectral content is determined by: spectral content of pump pulse phase-matching bandwidth THz absorption Effect of dispersion in THz range

For constructive THz generation k THz Optical rectification That part of the nonlinear polarization which responsible for THz pulse generation is: (( ω ω ) t ( k k z) () () PTHz = ε0χ A ( t)cos 1 1 ) THz pulse: ETHz ( ωthz, t) = A( t)cos ( ωthzt - kthzt) ω THz = ω 1 ω = k 1 k is needed Velocity matching (phase matching) is needed: n = gr p n THz Without velocity matching generation only at the two surfaces of the nonlinear crystal Example: generation in 0.5 mm LiNbO 3 l. Xu et al.: Appl. Phys. Lett. 61, 1784 (199)

Optical rectification Velocity matching Isotropic crystals: only possibility (if any) for velocity matching is choosing appropriate pump and/or THz wavelength (frequency) Refractive indices of ZnTe n p n THz 3.01 λ ( λ) = 4.7 + λ 0.14 ( ν) = 89.7 6 ν 9.16 ν A. Nahata et al., Appl. Phys. Lett. 61, 31 (1996) Lee Y.-S.: Principles of THz Sci. & Tech., Springer 009

Optical rectification Velocity matching Anisotropic (birefrigent) crystal: velocity matching by angle tuning Example: GaSe (see lecture 3) Other phase/velocity matching methods: Cherenkov (small spot size needed limited THz energy) Waveguide (large dispersion, practically no single-cycle generation) Tilted-pulse-front pumping (LiNbO 3, see lecture 8) Quasi-phase matching (no velocity matching, no single-cycle generation, as many cycles as the number of periods in the crystal)

In the case of velocity matching and no dispersion Using the dil = 1 χ () ijk Optical rectification Efficiency η THz ω d = ε n n contracted matrix 0 v eff THz L I 3 c exp α L sinh α THz L 4 α THz L 4 where l = 1 3 4 5 6 for jk = 11 33 3,3 31,13 1,1 THz ω deff L I For α THz L << 1 η THz = 3 n n c ε 0 v THz ω deff I For α THz L >> 1 η THz = 3 ε n n α c 0 v THz THz 3

Optical rectification FOMs of a few crystals supposing 1 mm crystal length and 1 THz Material n gr n THz α (cm -1 ) at 1 THz FOM (pm cm /V ) CdTe 3.73 3.3 4.8.10 GaAs 4.18 3.61 0.5 0.78 GaP 3.57 3.34 1.9 0.18 ZnTe 3.31 3.17 1.3 1.71 GaSe 3.13 3.7 0.07 0.6 LiTaO 3. 6.4 46 0.44 LiNbO 3.3 5.16 16 4.57 DAST 3.31.4 150 1.

Electro-optic effect Electro-optic effect Pockels effect Pockels effect: static electric field induced change of refractive index (birefrigence) Pockels effect is used for example for Q-switching of lasers P () i ( ω) where = () ε = 0χijk ( ω, ω,0) j ( ω) Ek (0) j, k () () χij = χijk ( ω, ω,0) E k ( ω) k E ε χ ( ω) E ( ω) j 0 () ij j Eq. EO Use Pockels effect for the quasi-static field of THz pulse: 0 ω THz Optical rectification: P () i ( ω THz ) = j, k ε 0 χ () ijk ( ω THz, ω, ω ω THz ) E j ( ω) E k ( ω ω THz )

Electro-optic sampling Using EO effect for measuring THz pulse shape The quasi-static field of THz pulse can induce birefrigence through the Pockels effect Lee Y.-S.: Principles of THz Sci. & Tech., Springer 009

Electro-optic sampling in ZnTe-type materials In the case of velocity matching the phase retardation difference between the two orthogonal component of the probe is: where n o is the refractive index of the optical probe, and r 4,1 =4d 1,4 /n o4 is the EO coefficient. The intensities at the two photodetector are: The signal of the balanced photodetector is:

Velocity matching methods Limitations of previous velocity matching methods: Birefringence possible for GaSe, but decreases the effective nonlinear coefficient, not possible at all for LN Periodic poling (PPLN) only (quasi-) phase matching, no velocity matching, THz pulse duration > 10 ps, reduced efficiency Cherenkov radiation D.H. Auston et al.: Phys. Rev. Lett. 53, 1555 (1984) inconvenient emission characteristics, energy can not be scaled-up

Velocity matching methods

Comparison of Cherenkov-type and tilted-pulse-front velocity matching Cherenkov Tilted-pulse-front t pump < T THz w pump < λ THz Cone shaped radiation t pump < T THz No limit for w pump Plane-wave radiation

Comparison of line focusing and tilted-pulse-front velocity matching Line focusing: the THz wave transversal size increasing THz energy ~ crystal thickness Tilted-pulse-front: electric field of the THz pulse increases THz energy ~ square of crystal thickness

Experimental set-up

Experimental results Appl. Phys. Lett. 83, 3000 (003), Appl. Phys. B 78, 593 (004) THz pulse energy at ( 77/300 K ) : 400/100 pj peak electric field: 7 kv/cm, photon conversion efficiency: 3.4 % EOS signal (r. u) 5 0 Amplitude FFT (r. e.) 1.0 0.8 0.6 0.4 0. 0.0 0 1 3 Frequency (THz) -5-0 4 6 8 Time (ps)

Scaling-up the THz pulse energy 0 100 00 300 400 500 Pump energy (μj) 100 THz pulse energy (nj) 10 1 Tilted pulse Line focus Tilted pulse 5 4 3 1 Efficiency (10-4 ) 0.1 Pump energy (μj) 0 100 00 300 400 500 0 Opt. Express 13, 576 (005) E THz = 40 nj, η ph = 10 %, electric field (unfocused) 0.1 MV/cm

Classification of THz pulses by peak electric field and energy Electric field (a. u.) 1,0 0,5 0,0 E max Spectral amplitude (a. u.) 1,0 0,8 0,6 0,4 0, 0,0 0 1 3 Frequency (THz) -0,5-0 4 6 8 time (ps) Linear (TDTS) THz spectroscopy (E max 100 V/cm 10 fj energy) High field THz pulses (E max 100 kv/cm 1 µj energy) THz pump probe measurement, nonlinear THz optics Extreme high field THz pulses (E max 100 MV/cm 10 mj energy) enhancement of HHG, particle manipulation, etc.

High intensity terahertz science and technology Nonlinear terahertz optics Nonlinear spectroscopy (THz pump THz/NIR probe measurement) Keldysh parameter THz photoemission THz Kerr-effect THz streaking

THz pump THz probe set-up Hoffmann et al.: Phys. Rev. B 79, 16101R (009) 49

Following the carrier dynamics by THz pump THz probe measurement germanium Hebling et al.: Phys. Rev. B 81, 03501 (010)

Following the carrier dynamics by THz pump THz probe measurement n-type Ge, n c = 5x10 14 cm -3 α ( ω) = c α ε bω pγ nc c c * ( ω + γ ) ε ncm ( ω + γ ) ε nc( ω + γ ) t n-type GaAs, n c = 8 x 10 15 cm -3 = 0 e N τ r ( t) e τ r =.7 ps γ S3 = ps -1, γ 3S = 0 ps -1, γ 3 = 9 ps -1 γ = 0 en γ μ α( t ) μini ( t) i τ r = 1.9 ps α n-type Si, n c = 5 x 10 14 cm -3 t τ t r1 τ ( t) e + e τ r1 = 0.8 ps, τ r = 4 ps Hebling et al.: Phys. Rev. B 81, 03501 (010)

Impact ionization in InSb Undoped sample, N=5x10 13 cm -3, T=80 K Used model (avalanche ionization): Appl. Phys. A 9, 15 (198), τ e =7 ps, ε 0 =1.3 ev Zener effect (tunneling) plays any role? Accordings to Phys. Rev. B 8, 07504 (010), YES it does 5

Terahertz manipulation of excitons Hirori et al.: Phys. Rev. B 81, 081305R (010) Ogawa et al.: Appl. Phys. Lett. 97, 041111 (010) Excitons in carbon nanotubes Doi et al.: Opt. Express 18, 18415 (010) THz free induction decay from tyrosine 53

Keldysh parameter The so-called Kaldysh parameter can be used to describe the strength of nonlinearity. Keldysh parameter: γ = W U B P where W B is the binding energy, and U P = e E THz 4mωTHz is the cycle-averaged kinetic energy of the wiggling electron, the so-called ponderomotive potential. Perturbative nonlinear optics for γ 1 Strong field nonlinear optics for γ <<1 For example for the Hirori experiment The THz field dominates the excitonic Coulomb field. 19.6meV γ = 0.4 64meV e E The amplitude of the wiggling motion of an electron: aw = mω THz THz 54

Photoemission Cs. Lombosi et al., ELI Workshop 014 55

Photoemission Photocurrent vs. peak THz field Photocurrent vs. THz field polarization Cs. Lombosi et al., ELI Workshop, Szeged 014 56

Coherent control of soft mode Lattice anharmonicity SrTiO 3 is a ferroelectric crystal soft mode at 1.5 THz 300-nm-thick SrTiO 3 on 0.5 mm MgO Tilted-pulse-front pumped THz source E THz,max =80 kv/cm Increasing E THz increasing amplitude, decreasing period Sam t ( ω) = E t ( ω) / E Re f t ( ω) I. Katayama et al., Phys. Rev. Lett. 108, 097401 (01)

Coherent control of soft mode Lattice anharmonicity Effective susceptibility: χ eff ( ω) = c( n + 1)( t( ω) 1) iωt( ω) d Resonance peak shifts to higher frequency for increased THz field. The bandwidth decreases. Resonance peak shifts to higher frequency for increased temperature, but the bandwidth also increases. The THz pulse drives the soft mode, but not the other modes.

Lattice anharmonicity Numerical simulation * d Q dq e E film ( t) 3 + Γ + Ω Q + λq = e*=7.37 e, M=18.6m (proton) dt dt M 1 d P( t) P( ω) E film ( t) = Ei ( t) χeff ( ω) = Γ = Γ0 +αq n + 1 ε 0 c t E film( ω) λ = 35 pm - THz, α = -3.5 pm - THz

Coherent control of antiferromagnetic spin-waves T. Kampfrath et al.: Nature Photonics 5, 31 (011)

THz streaking Characterization of ultrashort electron bunches or X-ray pulses THz field streaks the electron bunch created by the X-ray (HHG) beam Δ The momentum of electrons born at time t will be changed by the THz field by: r r,, Δp ( r, t) = e E ( r, t ) dt = ea ( r, t) where A r THz t THz THz is the THz vector potential. The change of the kinetic energy of electrons having velocity parallel to the THz field is: ΔW U. Frühling et al., Single-shot terahertz-field-driven X-ray streak camera, Nature Photonics 3, 53 (009) B. Schütte etal., Electron wave packet sampling, Opt. Express 19, 18833 (011) W 0 ( t) eathz ( t) m

THz streaking Characterization of ultrashort electron bunches or X-ray pulses The electric field of a linearly chirped (chirp rate: c) Gaussian XUV pulse is: E XUV ( t) = E ln t 0 τ X ct X exp i( ω0t+ exp ) If the XUV pulse duration is much shorter than the THz, the temporal shape of the THz pulse can be measured as shown left. If the XUV is much shorter than the THz pulse and the delay chosen such that the slope of A THz is approximately linear the streaking induce a linear energy chirp of the electron bunch lengthening it to: σ = σ + τ ( s 4 cs ) 0 Xst X X ± Neon p electron kinetic energy spectra vs. delay between ionizing XUV and streaking THz ΔW t W 0 t m where s = ee THz ( ) and ± corresponds to the two observation direction. B. Schütte etal., Electron wave packet sampling, Opt. Express 19, 18833 (011)

THz streaking Characterization of ultrashort electron bunches or X-ray pulses By measuring the field-free spectrum and the streaked spectra in two opposite directions, the XUV pulse duration and its linear chirp rate can be determined. Photoelectron spectra measured at the left (left column) and right (right column) detector, respectively. (a,b) field free case, (c,d, e,f) using THz streak field. In the case of (e,f) a negative chirp of the pump pulse of the HHG generation increased the negative chirp of the harmonic pulse. The large and small peaks corresponds to the 59th and 61st harmonic, respectively. B. Schütte etal., Electron wave packet sampling, Opt. Express 19, 18833 (011)

Velocity matching by tilting of the pump-pulse-front Tilted-Pulse-Front Pumping (TPFP) Setup Hebling et al., Opt. Expr. 10, 1161 (00) tanγ = n n g dε λ dλ lens pulse front tilt angular dispersion ~100 fs typical fs laser

Longer Pump Pulse Duration for Longer THz Generation Length Pump pulse duration, τ [ps] THz generation efficiency [%] 1 0 4 0-0 -10 0 10 0 (a) (b) Pump propagation distance, ζ [mm] τ 0 = 50 fs τ 0 = 350 fs τ 0 = 600 fs L eff L d -5 0 5 THz propagation distance, z [mm] LiNbO 3 λ p = 800 nm F p = 5.1 mj/cm Ω pm = 1 THz Pulse front tilt: GVD parameter: D = d 1 ( v ) g d λ tanγ = angular dispersion λ dε = n c d λ Martínez et al., JOSA A, 1984 Hebling, Opt. Quantum Electron., 1996 Fülöp et al., Opt. Express, 010 n n g material dispersion dε λ dλ d n d λ

Generation of THz pulses with extremely high energy 1., Using pump pulses with optimal duration Fülöp et al. Opt. Expr. 19, 15090 (011) tanγ = λ d θ dλ D v dv = 1 cs λ dθ = n dλ c dλ d n dλ Hebling, Opt. & QE 8, 1759 (1996)., Decreasing THz absorption via cooling the LiNbO 3 nonlinear crystal Fülöp et al. Opt. Expr. 19, 15090 (011) 3., Using contact grating setup Pálfalvi et al., Appl. Phys. Lett. 9, 171107(008) Ollmann et., Appl. Phys. B 108, 81 (01) (c) LN crystal x [mm] 4 pump THz 0 - -4 >0 mj THz energy and >10 MV/cm peak electric field is predicted for 00 mj pump pulse energy

Sub-mJ THz pulse energy for close to optimal pump duration pump intensity [GW/cm ] 0 50 100 150 00 50 THz energy [μj] 400 300 00 measured, RT y = ax 1.53 y = ax 1.40 y = ax 1.67 Ref. [15] 436 μj 100 0 0 10 0 30 40 50 60 pump energy [mj] Fülöp et al., Opt. Express (014)

Compact set-up for high energy THz pulse generation Material λ p = 800 nm λ p = 1064 nm λ p = 1560 nm MPA γ MPA γ MPA γ ZnTe PA collinear PA.4 3PA 8.6 GaSe PA 16.5 PA 5.9 3PA 30. LiNbO 3 3PA 6.7 4PA 63.4 5PA 63.8 Tilted-pulse-front-excitation without 10 1 Ω 0 = 1 THz imaging 10 0 Pump spot diameter can be increased For semiconductors: smaller tilt, longer pump wavelength Efficiency [%] 10-1 10-10 -3 ZnTe, 1560 nm, 80 K (β 3 = 3.99x10-4 cm 3 GW - ) ZnTe, with 5 cm beam diameter: 10-4 ZnTe, 1064 nm GaSe, 1560 nm LN, 1064 nm W THz =. mj, I THz, focus = 630 GW/cm, 0.1 1 10 100 E THz, focus 17 MV/cm Pump intensity [GW cm - ] Pálfalvi et al., Appl. Phys. Lett. 9 171107(008) Fülöp et al., Opt. Express, 18, 1311 (010)

THz pulses with MV/cm electric field generated in DSTMS Optical rectification of 95 fs, 30 mj, 130 nm pulses of Cr:forsterite (Cr:Mg SiO4) laser in collinear geometry. Pump fluence: 10 mj/cm. Low-pass filter is used with cut-off fr. at 10 THz. Spectrum determined from the linear autocorrelation (inset) measured with a Michelson interferometer and THz detector. C. Vicario et al., Opt. Lett. 39, 663 (014)

THz pulses with MV/cm electric field generated in DSTMS Efficiency up to 3 %, THz energy up to 0.9 mj! Focusing down to 60 μm the calculated THz field strength is 40 MV/cm. C. Vicario et al., Opt. Lett. 39, 663 (014)

THz pulses with MV/cm electric field generated in air-plasma Efficiency scaling with pump wavelength Clerici M. et al., Phys. Rev. Lett. 110, 53901 (013) For 1850 nm pump from the 0.6 μj THz energy, from the 85 μm focused THz spot size and from the sub-ps THz pulse duration 4.4 MV/cm field amplitude is calculated Broad spectrum peaking at 5 THz

Applications of THz pulses having extreme high electric field strength Increasing the cut-off frequency of HHG (E THz = 30 100 MV/cm) Quasi-phase-matched attosecond pulse generation (E THz = 6 MV/cm) Ultrashort X-ray pulse generation by THz driven undulators ( and FEL s) Electron acceleration Proton acceleration (hadron therapy)

Increasing the cut-off frequency of HHG HHG in the presence of THz electric field Combined THz + IR fields ( ω t) E cos( ω t) E( t) = E0 cos IR + 1 THz IR THz broken the symmetry of IR field Hong et al., Opt. Expr. 009 One as pulse per IR cycle Electric field 0 THz Spectrum consists both odd and even harmonics Time Lewenstein, PRA 1994 E. Balogh et al.: Phys. Rev. A (011), University of Szeged

Increasing the cut-off frequency of HHG 1560 nm 800 nm Both odd and even harmonics only one as pulse per IR period I IR = 10 14 W cm - E IR = 388 MV/cm E. Balogh et al.: Phys. Rev. A (011) E THz = 0 40 MV/cm

Quasi-phasematched attosecond pulse generation Ne, L = mm, p = 5 Torr IR: 800 nm, 0 fs, 0, mj f = 1,6 THz E THz = 5 MV/cm W THz = 5 mj λ MH =7, nm K. Kovács et al.: Phys. Rev. Lett. 108, 193903 (01) 800 nm 1,5 μm: λ MH < 4 nm (víz ablak) 78

Electron manipulation by laser THz beam Plettner et al.: Phys. Rev. Spec. Top. Accel. and Beams 9, 111301 (006) acceleration 11, 030704 (008) undulator 1, 10130 (009) deflection, focusing 1 GV/m = 10 MV/cm peak field strength is needed!

Motivation for THz-pulse-driven particle acceleration Suggestion of laser-driven (non-plasma) particle accelerators (dielectric grating, photonic crystal, etc.): Dielectric laser accelerators (DLA) Dielectric double grating accelerator T. Plettner et al., Phys. Rev. ST-AB, 9, 111301 (006) Demonstration of acceleration in DLA E. A. Peralta et al., Nature, 503, 91-97 (013) J. McNeur, today 11.30 Up to 300 MeV/m acceleration gradient was demonstrated

Motivation for THz-pulse-driven particle acceleration Disadvantages of short wavelength and short oscillation period of laser used for driving DLA: 1. small gap (400 nm) between dielectric grating small bunch charge and low throughput. different parts of the bunch see different field phases both acceleration and deceleration E. A. Peralta et al., Nature, 503, 91-97 (013) 100 500 times longer wavelength of THz pulses can solve both problem

THz driven DLA for relativistic electron acceleration Model Driving pulse Electric field z-component 1.0 Amplitude / E 0 0.5 0.0-0.5-1.0 100 00 300 400 Time (fs) A. Aimidula et al., Nucl. Instr. and Meth. A, 19, 15090 (014) Computer Simulation Technology (CST) 1.0 Amplitude / E 0 0.5 0.0-0.5-1.0 5 10 15 0 5 30 35 Time (ps)

Optimized parameters of THz DLA (Depend on the used material) Material: Silicon (n = 3.41) A = B = λ / We optimized C, D and L parameter. C = 0.1 λ D = 0.167 λ L = 0.8 λ λ = 0.9 mm (0.33 THz)

The evanescent THz wave accelerator setup Material requirements High optical damage threshold Large index of refraction Low absorption Low dispersion Cooled LiNbO 3 (LN) Palfalvi et al., Phys. Rev. ST-AB 17, 031301 (014)

Acceleration and monochromatization of a proton bunch 43 50 % parameter setting for optimization proton energy (MeV) 4 41 0.5% input output 4.37 MeV E 0 = 1.71 MV/cm l acc = 1.5 cm v THz = 0.871 c φ 0 = 3 rad ω/π = 0.5 THz 40 0 0 40 60 80 100 ordinal number of the proton (i)

Acceleration and monochromatization of a proton bunch in multi-stages at 0.5 THz frequency output energy (MeV) 55 E 0 = 1.71 MV/cm 50 45 ω/π = 0.5 THz d = 50 μm 1.stage.stage 3.stage 4.stage 5.stage 40 0 0 40 60 80 100 ordinal number of the proton (i) 4 5 MeV monochromatization rate: 10 %

Estimation of neded time for hadron therapy 1 shot typically has 10 9 proton/mev at 40 MeV This means 10 9 10% 70 MeV = 0.00175 J energy / single shot Tens of Gy, required for hadron therapy is reached in a couple of minutes