We demonstrate the generation of near-single cycle longitudinally polarized terahertz radiation using a large-area radially biased photoconductive antenna with a longitudinal field amplitude in excess of 2 kV/cm. The 76 mm diameter antenna was photo-excited by a 0.5 mJ amplified nearinfrared femtosecond laser system and biased with a voltage of up to 100 kV applied over concentric electrodes. Amplitudes for both the transverse and longitudinal field components of the source were measured using a calibrated electro-optic detection scheme. By tightly focusing the radiation emitted from the photoconductive antenna, we obtained a maximum longitudinal field amplitude of 2.22 kV/cm with an applied bias field of 38.5 kV/cm

Cliffe, M.J.

Graham, D.M.

Jamison, S.P.

Rodak, A.

M. J. Cliffe

A. Rodak

D. M. Graham

S. P. Jamison

Crossref

Applied Physics Letters

Generation of longitudinally polarized terahertz pulses with field amplitudes exceeding 2 kV/cm

We demonstrate the generation of near-single cycle longitudinally polarized terahertz radiation using a large-area radially biased photoconductive antenna with a longitudinal field amplitude in excess of 2 kV/cm. The 76 mm diameter antenna was photo-excited by a 0.5 mJ amplified near-infrared femtosecond laser system and biased with a voltage of up to 100 kV applied over concentric electrodes. Amplitudes for both the transverse and longitudinal field components of the source were measured using a calibrated electro-optic detection scheme. By tightly focusing the radiation emitted from the photoconductive antenna, we obtained a maximum longitudinal field amplitude of 2.22 kV/cm with an applied bias field of 38.5 kV/cm

Cliffe, Matthew J

Rodak, Aniela

Graham, Darren M

Jamison, Steven P

The University of Manchester - Institutional Repository

The University of Manchester ResearchGeneration of longitudinally polarized terahertz pulses withfield amplitudes exceeding 2 kV/cmDOI:10.1063/1.4901904Document VersionAccepted author manuscriptLink to publication record in Manchester Research ExplorerCitation for published version (APA):Cliffe, M. J., Rodak, A., Graham, D. M., & Jamison, S. P. (2014). Generation of longitudinally polarized terahertzpulses with field amplitudes exceeding 2 kV/cm. Applied Physics Letters, 105(10).https://doi.org/10.1063/1.4901904Published in:Applied Physics LettersCiting this paperPlease note that where the full-text provided on Manchester Research Explorer is the Author Accepted Manuscriptor Proof version this may differ from the final Published version. If citing, it is advised that you check and use thepublisher's definitive version.General rightsCopyright and moral rights for the publications made accessible in the Research Explorer are retained by theauthors and/or other copyright owners and it is a condition of accessing publications that users recognise andabide by the legal requirements associated with these rights.Takedown policyIf you believe that this document breaches copyright please refer to the University of Manchester’s TakedownProcedures [http://man.ac.uk/04Y6Bo] or contact uml.scholarlycommunications@manchester.ac.uk providingrelevant details, so we can investigate your claim.Download date:12. Nov. 2022Generation of longitudinally-polarized terahertz pulses with field amplitudesexceeding 2 kV/cmM. J. Cliffe,1, 2, a) A. Rodak,1, 2 D. M. Graham,1, 2 and S. P. Jamison2, 31)School of Physics and Astronomy and the Photon Science Institute,The University of Manchester, Manchester M13 9PL, United Kingdom2)The Cockcroft Institute, Sci-Tech Daresbury, Keckwick Lane, Daresbury,Warrington WA4 4AD, United Kingdom3)Accelerator Science and Technology Centre, Science and Technology FacilitiesCouncil, Darebsury Laboratory, Keckwick Lane, Daresbury, Warrington WA4 4AD,United Kingdom(Dated: 4th November 2014)We demonstrate the generation of near-single cycle longitudinally-polarized terahertzradiation using a large-area radially-biased photoconductive antenna with a longitu-dinal field amplitude in excess of 2 kV/cm. The 76 mm diameter antenna was photo-excited by a 0.5 mJ amplified near-infrared femtosecond laser system and biased witha voltage of up to 100 kV applied over concentric electrodes. Amplitudes for boththe transverse and longitudinal field components of the source were measured usinga calibrated electro-optic detection scheme. By tightly focusing the radiation emit-ted from the photoconductive antenna we obtained a maximum longitudinal fieldamplitude of 2.22 kV/cm with an applied bias field of 38.5 kV/cm.a)Matthew.Cliffe@manchester.ac.uk1Longitudinally-polarized radiation is commonly used in many applications including elec-tron acceleration1, laser machining2, optical tweezers3 and microscopy4. The above applica-tions exploit the enhancement of the longitudinally-polarized field component5,6 by the tightfocusing of a radiially-polarized beam. In recent years there has been a push to develop suchsources in the terahertz spectral region and a number of laser-driven terahertz sources, withstrong longitudinally-polarized field components, have been reported including: velocity-mismatched optical rectification7, a segmented GaP crystal8, an air plasma filament9 andphoto-conductive antennas with radial electrode geometries, both single gap10 and interdigi-tated InP11 and GaAs12,13 structures. To date all demonstrations of longitudinally-polarizedterahertz generation have produced longitudinal-field amplitudes of < 30 V/cm, with the ex-ception of Minami et al.9 who reported a field amplitude of 1 kV/cm by tightly focusing theterahertz radiation generated from an air plasma filament induced by a 4.5 mJ femtosecondoptical laser pulse.In this letter we report the results of a detailed characterization of a radially-polarizedterahertz radiation source generated from a large-area GaAs photo-conducting antenna. Byfocusing the terahertz radiation we were able to obtain a longitudinal, on axis, componentwith a peak amplitude of 2.22 kV/cm using an optical laser pulse energy of 0.5 mJ; the laserfluence was below the photoconductive antenna saturation threshold, and the peak terahertzfield amplitude was monitored as a function of laser fluence to further confirm this.The experimental setup is shown schematically in Fig. 1. The terahertz pulses weregenerated using a radially-biased large-area GaAs photoconductive antenna. The antennaconsisted of a 76 mm diameter, 500 µm thick semi-insulating GaAs (SI-GaAs) wafer witha circular central electrode, radius r = 10 mm, and a concentric annular outer electrode,r = 36 mm. A high-voltage bias was applied to the antenna by a custom-built pulsed voltagesupply that provided up to 100 kV in a 9 µs pulse at a repetition rate of 250 Hz. A two-resistor potential divider was wired in parallel to the antenna to enable the monitoring ofthe voltage pulse on an oscilloscope, this allowed for both calibration of the voltage appliedto the antenna and temporal synchronization of the photo-exciting laser. The laser systememployed in this work was a regenerative amplifier system which provided 100 fs pulses witha central wavelength of 800 nm and a repetition rate of 500 Hz. The laser pulse train wasdivided into a pump and probe beam with the pump beam providing pulse energies of 500 µJto photo-excite the antenna. An average photo-excitation fluence of 6.1 µJ/cm2 was obtained2across the annular active surface area of the antenna by expanding the pump beam usinga combination of a diverging and focusing lens. Placing the antenna in the focusing pumpenabled the terahertz phase front to adopt the radial time delay of the optical excitationpulse, generating a focusing terahertz wave. The emitted terahertz radiation was then routedto a 90o off-axis parabolic mirror with an effective focal length of 76.2 mm where it wasfurther focused; providing an effective numerical aperture of 0.32. The terahertz radiationwas detected using an electro-optic detection scheme which utilized a co-linear focused probebeam that was combined with the terahertz radiation via a pellicle beamsplitter. The probebeam was focused by a 1 m focal length lens and had a 1/e2 diameter of 155 µm which wasmeasured by a knife-edge scan at the detection crystal position. Temporal waveforms wererecorded at multiple sampling positions along the terahertz focal plane by translating boththe probe beam and the electro-optic detection optics using manually-driven translationstages. To detect the radial components of the terahertz field a 2 mm thick (110) ZnTecrystal was employed, while a 2 mm thick (100) ZnTe crystal was utilized to detect thelongitudinal field components. In both cases the azimuthal rotation of the crystals were setto maximize the terahertz signal as described by Winnerl et al.12.FIG. 1. (Color) Schematic diagram of the experimental setup. Inset shows the concentric arrange-ment of the antenna electrodes.Figure 2 shows the transverse terahertz field as a function of sampling position and timedelay. A polarity flip in the field is observed as the sampling position is scanned through the3origin of the cylindrical beam. The radial field pattern was found to rotate at double theangle of the probe beam polarization when imaged using an electro-optical two dimensionalcamera setup14, therefore confirming the cylindrical symmetry of the polarization structureof the beam.-10 0 10 20-0.10-0.050.000.050.10I/I0Time Delay (ps)-10 -5 0 5 10 15 20-4-3-2-1012-2.50Time Delay (ps)Sampling Position (mm)-0.13-0.10-0.08-0.05-0.030.000.030.050.080.100.13I/I02.5kV/cm(a)(b)FIG. 2. (Color) (a) A transverse electric field component of the terahertz source measured at thefocal point. (b) Individual terahertz waveforms obtained at sampling positions of +/- 0.5 mm fromthe center of the focus.The features seen in the waveforms of Fig. 2 at a time delay of approximately 14.3 psare due to a reflection of the emitted terahertz radiation in the GaAs wafer. The flip inrelative polarity is attributed to a phase change of π upon reflection at the larger refractiveindex of the photo-excited plasma layer.15,16 Following the approach of Planken et al.17, theterahertz field amplitude can be related to the probe intensity in orthogonal polarizations,ETHz =∆II0λ2πn3r41LT (1)4where L = 2 mm is the detection crystal thickness, n = 2.85 is the refractive index ofZnTe at λ = 800 nm, r41 = 4.0 pm/V is the electro-optic coefficient of ZnTe18 and T is theFresnel transmission coefficient. For low signal-to-noise ratio measurements we observed theterahertz induced intensity changes with a lock-in amplifier. The differential probe signal,∆S250 ∝ ∆I, was measured at 250 Hz, while the reference signal, S500 ∝ I0, was measuredat 500 Hz. A correction factor, C = 1.36, was determined to account for the different lock-inresponses at 250 Hz and 500 Hz, where C is dependent on the photo-diode response time.With this approach the lock-in signal was consistent with direct observation of the photo-diode change, ∆V /V . A maximum differential terahertz signal, ∆I/I0, of 0.13 was obtainedwith an applied bias voltage of 40 kV. Using Eq. 1 and taking T = 2.08 we obtained amaximum field amplitude of 2.5 kV/cm at a bias voltage of 100 kV.Careful inspection of the χ(2) tensor for both (110) and (100) cut crystals as well both po-larization geometries confirms the validity of Eq. 1 for both transversely- and longitudinally-polarized radiation detection schemes. The usual Fresnel transmission coefficient howeveris not valid for longitudinally-polarized radiation as the component of the electric field per-pendicular to the interface is not continuous. By using the boundary condition for theelectric displacement vector, i.e. that the perpendicular component is continuous across theinterface, we can obtain the following transmission coefficient for longitudinally-polarizedradiation,Tlong =2nTHz(nTHz + 1). (2)The transmission co-efficient of Eq. 2 is also consistent with the spatial compression of apulsed field in the longitudinal direction, due to the reduced pulse velocity, and the necessarymaintenance of ∂∂zEz = −∇⊥E⊥ within the crystal. Figure 3 (a) shows the measuredlongitudinal field as a function of sampling position and time delay for an applied bias of40 kV. It exhibits a bipolar temporal structure and a unipolar transverse profile. For anapplied bias of 40 kV the maximum differential terahertz signal, ∆I/I0, measured was 0.0205.This differential signal increased to 0.036 by increasing the applied bias to 100 kV. A peaklongitudinal field amplitude of 1.26 kV/cm was therefore determined from the data shownin Fig. 3 (a), which increased to 2.22 kV/cm at a 100 kV bias. As it is known that a slightlytilted electro-optic sampling crystal can lead to a transverse field component being detectedas a longitudinal field component,8 it is important to ensure that any tilt of the detection5crystal was minimized. This was achieved by ensuring the back reflection of probe beamfrom the ZnTe detection crystal followed the same path as the incoming probe beam. Tofurther verify that the correct polarization state is being detected the expected longitudinalfield was calculated from the measured transverse field components. From ∇ · E = 0 itfollows that,Ez(r, zobs) = −∫ ∞zobs1r∂∂r[rEr(r, z)] dz, (3)where Er(r, z) is the radial component of the field. In order to calculate an expected lon-gitudinal field from the data shown in Fig. 2. We integrate over time, t, instead of z bymaking the assumption that E(r, z, t) ≈ E(r, z − ct). Spatial profiles of the terahertz beamwere taken at various z positions by moving the detection crystal through the focus; verylittle change in the spatial profile was observed within our scan range confirming the validityof this assumption. Using Eq. 3 and the measured transverse field data shown in Fig. 2 (a),which was measured at a transverse slice through the origin, r = 0, we calculate the ex-pected longitudinal field as a function of r and z which is shown in Fig. 3 (b). Numericalartifacts at r = 0 that are due to 1rhave been removed by the truncation of r around thisregion. There is good agreement of both the spatial and temporal properties between themeasured and calculated datasets as can be seen. This shows that any contribution fromthe transverse field components in the longitudinal detection due to a tilted electro-opticcrystal has been minimized.In order to verify that the detection crystal was positioned at the focal point of theterahertz radiation beam we recorded the spatial profile of the longitudinal terahertz fieldwhile moving the (100) ZnTe detection crystal along the direction of beam propagation witha micrometer-driven stage. We observed an increase in the beam diameter and a decreasein the peak field amplitude on both sides of the nominal focal position, hence confirmingthe optimal positioning of our detection crystal and the measurement of the maximumlongitudinal field amplitude.An average photo-excitation fluence of 6.1 µJ/cm2 was used in this work, which is farbelow the saturation fluence of 40 µJ/cm2 as determined by You et al.19. The terahertz fieldamplitude continuously decreased as the excitation laser fluence was decreased hence con-firming we were operating below saturation. It therefore follows that higher field amplitudescould be obtained with modest laser pulse energies increasing towards saturation levels. We6a)-0.025-0.020-0.015-0.010-0.0050.0000.0050.0100.0150.0200.025-10 -5 0 5 10 15 20-4-3-2-1012-1.230.00Time Delay (ps)Sampling Position (mm) I/I01.23kV/cm-10 -5 0 5 10 15 20-4-3-2-1012 kV/cmTime Delay (ps)Sampling Position (mm)-0.032-0.0070.0180.0320.0001.574-1.574(a)(b)FIG. 3. (Color) (a) Longitudinal electric field component of the terahertz source measured at thefocal point. (b) Calculated longitudinal electric field component, truncation of r occurs within thedotted lines.estimate that for our experimental arrangement a 6 times increase in the longitudinally-polarized field amplitude could be obtained before reaching the saturation fluence; poten-tially increasing the longitudinally-polarized field amplitude to 13.3 kV/cm.In summary we have shown that it is possible to generate large longitudinally-polarizedterahertz field amplitudes, in excess of 2 kV/cm, from a large-area photo-conductive antenna.Expressions commonly used for transversely-polarized field analysis have been modified andcorrectly applied to longitudinally-polarized radiation. Scaling the pump laser towards sat-uration, this method is estimated to be capable of 6 fold higher longitudinal fields, with thepossibility of even higher still if larger diameter photo-conductive antennas are considered.This work was supported by the United Kingdom Science and Technology Facilities Coun-cil [grant numbers ST/L502558/1, ST/G008248/1]; the Engineering and Physical Sciences7Research Council [grant number EP/J002518/1]; and the Accelerator Science and Technol-ogy Centre through contract number PR110140. The authors also wish to thank R. Smithfrom the Accelerator Science and Technology Centre for developing the high-voltage pulsedpower supply used in this work.REFERENCES1S. Payeur, S. Fourmaux, B. E. Schmidt, J. P. MacLean, C. Tchervenkov, F. Legare,M. Piche, and J. C. Kieffer, Appl. Phys. Lett. 101, 41105 (2012).2M. Meier, V. Romano, and T. Feurer, Appl. Phys. A 86, 329 (2007).3S. E. Skelton, M. Sergides, R. Saija, M. A. Iat̀ı, O. M. Maragó, and P. H. 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Generation of longitudinally polarized terahertz pulses with field amplitudes exceeding 2 kV/cm

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Generation of longitudinally polarized terahertz pulses with field amplitudes exceeding 2 kV/cm

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