App Note 44 - Newport Corporation
Contents
1. APPLICATION NOTE Terahertz Spectrometer based on Generation of Ultrafast Terahertz Pulses in Air Plasma Technology and Applications Center Newport Corporation GY Newport Experience Solutions S Spectra Physics A Newport Corporation Brand Introduction Terahertz THz radiation generally considered as electric fields with wavelengths frequencies ranging from 30 um to 3 mm 0 1 to 10 THz has attracted great attention in the past two decades In terms of spectroscopy specific molecular absorptions in the THz region can be utilized in a wide range of applications from probing crystalline structures and protein interactions to discerning traces of explosives A particular strength of THz spectroscopy is that it provides a non invasive tool to help in understanding physical science In terms of imaging the ability to penetrate optically opaque materials such as plastics clothing and biological tissues makes it useful for applications including 2D and or 3D medical imaging pharmaceutical sciences and security screening With all these fields burgeoning novel and neat experimental designs for THz generation are essential to complement the development of THz research In this note we focus on the generation of THz pulses lt 1 ps in air plasma While it is already popular to generate THz pulses using biased photoconductive antennas advancements in laser amplifiers offer a new methodo2. Delay ps Figure 3 The time trace of THz field collected by electro optic sampling a The time trace of the first 20 ps of the THz field b Zoomed view of the first 10 ps shows the main cycle and the small complicated oscillations that follow The time domain trace is Fourier transformed to show the frequency spectrum of the THz radiation figure 4 The spectrum spans from 0 to 3 THz Some sharp absorption lines corresponding to the complicated oscillations after the main cycle are clearly resolved These are the absorption lines of water vapor s rotational transitions This demonstrates that the setup is actually a THz spectrometer In addition by purging the system to remove the water vapor absorption and putting samples into the beam path of the THz pulse we can actually measure the spectra of different molecules in the THz region A lot of explosives and drugs have strong and unique THz absorptions and this feature makes a THz spectrometer much more valuable in detection and spectroscopic applications The amplitude of the THz field with respect to the input energy is shown in figure 4 c By extrapolation the curve would intersect with the positive x axis which means that a certain amount of input energy is required to overcome some threshold before the THz can be generated This threshold is the energy required to rip the electrons from the nuclei and corroborates the statement that the generation of the plasma is nec
3. centered on M when MS is translated Also the distance the probe travels from BS to flip mount S figure 2 d needs to closely match that traveled by the pump from BS toS Figure 2 c Alignment of the probe beam At this point the setup is prepared for recombining pump and probe beams The beam path of the pump will be used as a guide for that of the THz beam generated when the pump power is increased later on The silicon wafer 400 um thick both sides polished is on a flip mount S and used as a combining mirror It is also utilized to filter out and dump 400 nm 800 nm and strong white light generated figure 2 d GO Newport 2 Experience Solutions Figure 2 d Recombination of the probe beam with THz field The flip mount S is set that the pump beam is allowed to hit the center of the parabola PB and folded 90 After inserting an iris I between PB and S it is positioned so that the pump beam passes through its center The lt 110 gt ZnTe crystal on the rotational stage R is positioned normal to the pump beam and at the focus of PB R is on a translation stage TS which is used for fine positioning The pump beam should hit and focus onto the center of the crystal The pump beam power may need to be attenuated to prevent any damage to the crystal The mount S is flipped to its original position it should be 45 to either pump or probe beam at this position Beam dump BD captures the rejecte
4. the THz field utilizes electro optic sampling with the help of a ZnTe crystal By mixing the THz field and the 800 nm probe in the ZnTe crystal the time domain trace of the THz amplitude is recovered directly from the signal intensity difference from a balance detector New Focus Nirvana In addition it is straightforward to measure the absorption spectra of the materials by putting them into the beam path without changing the setup which means we also have a THz spectrometer In this note we will demonstrate this feature by measuring the absorption of water vapor in the THz region GO Newport 1 Experience Solutions Experimental setup The layout of the setup is shown in figure 1 The output of the Spectra Physics Spitfire Pro XP ultrafast amplifier is split see Multimodal Ultrafast Spectroscopy System Based on a 35 Femtosecond Ti Sapphire Chirped Pulse Amplification CPA Laser application note 41 for a recommendation on how to split the beams such that a portion of the output 300 uJ P polarized is used for the experiment Wollaston Polarizer 800nm Probe on Delay Line A 4 Waveplate v ji THz Radiation 4 ZnTe Crystal E Beam Dump Silicon Wafer 7 i i Air na BBO Crystal y p E j a A e a n b oe Pe a G j e r Ca Ta a p ee T e E Figure 1 The experimental layout of the setup a The schematic presentation b The working setup in New
5. Broadband THz 20 Mlejnek M E M Wright and J V emission from gas plasmas induced by Moloney Femtosecond pulse femtosecond optical pulses From propagation in argon A pressure fundamentals to applications Laser amp Photonics Reviews 2007 1 4 p 349 368 16 Kress M et al Determination of the carrier envelope phase of few cycle laser pulses with terahertz emission spectroscopy Nature Physics 2006 2 5 dependence study Physical Review E 1998 58 4 p 4903 4910 21 Federici J F et al THz imaging and sensing for security applications explosives weapons and drugs Semiconductor Science and Technology 2005 20 7 p S266 S280 low temperature grown GaAs and semi insulating GaAs Applied Optics 1997 36 30 p 7853 7859 6 Xu J K W Plaxco and S J Allen Probing the collective vibrational dynamics of a This Application Note has been prepared based on development activities and experiments conducted in Newport s Technology and Applications Center and the results associated therewith Actual results may vary based on laboratory environment and setup conditions the type and condition of actual components and instruments used and user skills Nothing contained in this Application Note shall constitute any representation or warranty by Newport express or implied regarding the information contained herein or the products or software described herein Any and all representations warranties and obl
6. ation Worldwide Headquarters 1791 Deere Avenue Irvine CA 92606 In U S 800 222 6440 Tel 949 863 3144 Fax 949 253 1680 Email sales newport com Newport Experience Solutions Visit Newport Online at www newport com protein in liquid water by terahertz absorption spectroscopy Protein science 2006 15 5 p 1175 1181 References l Ferguson B and X C Zhang Materials for terahertz science and technology Nature Materials 2002 1 1 p 26 33 7 Shen Y C et al Detection and identification of explosives using terahertz pulsed spectroscopic imaging Applied Physics Letters 2005 86 24 2 Strachan C J et al Using terahertz pulsed spectroscopy to quantify pharmaceutical polymorphism and crystallinity Journal of Pharmaceutical 8 Chan W L J Deibel and D M Sciences 2005 94 4 p 837 846 Mittleman Imaging with terahertz radiation Reports on Progress in 3 Zeitler J A et al Terahertz pulsed Physics 2007 70 8 p 1325 1379 spectroscopy and imaging in the pharmaceutical setting a review 9 Wallace V P et al Three dimensional imaging of optically opaque materials Journal of Pharmacy and Pharmacology using nonionizing terahertz radiation 2007 59 2 p 209 223 dependence of terahertz pulse detection in ZnTe Journal of the Optical Society of America B Optical Physics 2001 18 3 p 313 317 12 Hamster H et al Subpicosecond electromagnetic pu
7. d pump beam Mirror M is used to center the probe through I and subsequently S is adjusted such that the probe beam hits the center of the ZnTe crystal Some iteration is required After the above procedure a lens with 25 mm focal length L on a delay stage TS is inserted right after R to collimate the probe beam figure 2 e TS is adjusted for the probe beam to be collimated The collimated probe beam is then sent through a quarter wave plate QW and a Wollaston polarizer WP to create two beams with perpendicular polarizations QW is rotated such that the two beams exiting WP have the same intensity Figure 2 e Sending the probe beam into the balance detector for electro optic sampling After routing the beams to the entrance ports of the balance detector D using mirrors Mg and Mo respectively a co axial cable is connected from the differential output of the balance detector to an oscilloscope At this point the balance detector should indicate null response since the intensities of the two beams should be equal If not rotate QW such that a null response is achieved Increasing the pump energy to around 150 uJ will generate air plasma At this point when the delay is adjusted using MS within its full range and the timing is right you will see the signal intensity difference figure 3 is oscillating between negative and positive This is caused by the THz that is generated from the air plasma The distance be
8. essary for the THz generation On the other hand at high pulse energies the defocusing of the laser beam by the plasma becomes important and limits the peak intensity of the laser pulse at the focal point As a result the efficiency drops From our design it is seen that around 250 uJ input energy is optimal in terms of efficiency and the THz intensity Arrows Show the Absorption w of Water Vapor THz Power Spectrum with Water Vapor Absorption e Reconstructed THz Power Spectrum Spectral Density a u Amplitude a u 0 50 100 150 200 250 300 350 400 450 500 Input Energy uJ Figure 4 a and b The spectrum of the generated THz field c The amplitude of the generated THz field is plotted against the input energy Conclusion The setup shows a straightforward implementation of a THz generation tool based on a laser amplifier The damage threshold is not a concern based on this design and the fact that air plasma generates THz The strong strength and the pulsed nature of the THz field make it applicable not only to linear spectroscopy in THz domain but also to time resolved nonlinear experiments such as optical pump and THz probe experiments Furthermore being able to recover the phase and the amplitude information of the THz field through electro optic sampling adds the functionality of a THz Spectrometer without further modification GO Newport 4 Experience Solutions Newport Corpor
9. igations of Newport with respect to its products and software shall be as set forth in Newport s terms and conditions of sale in effect at the time of sale or license of such products or software Newport shall not be liable for any costs damages and expenses whatsoever including without limitation incidental special and p 327 331 17 Planken P C M et al Measurement and calculation of the orientation consequential damages resulting from any use of or reliance on the information contained herein whether based on warranty contract tort or any other legal theory and whether or not Newport has been advised of the possibility of such damages Newport does not guarantee the availability of any products or software and reserves the right to discontinue or modify its products and software at any time Users of the products or software described herein should refer to the Users Manual and other documentation accompanying such products or software at the time of sale or license for more detailed information regarding the handling operation and use of such products or software including but not limited to important safety precautions This Application Note shall not be copied reproduced distributed or published in whole or in part without the prior written consent of Newport Corporation Copyright 2011 Newport Corporation All Rights Reserved Spectra Physics the Spectra Physics S logo the Newport N logo are registe
10. logy based on air plasma generation Pioneered by Hamster Sullivan and coworkers with unbiased air plasma in 1993 and by Cook and Hochstrasser with AC biased air plasma in 2000 further adoption of this method has taken place due to several advantages including straightforward implementation of the setup practically no damage threshold of the ambient air and strong field strength comparable with antenna based generation In this note we basically follow the work done by Cook and Hochstrasser to demonstrate the feasibility of THz generation based on a sub 35 fs 800 nm ultrafast laser amplifier In brief the 800 nm laser pulse 1 kHz 300 uJ per pulse is focused into ambient air to generate the air plasma Right before the focal point a BBO crystal is inserted to create second harmonic generated SHG 400 nm converted from the 800 nm fundamental The oscillating 400 nm field acts as AC bias at the focal point to polarize the plasma by drifting the electrons away from the nuclei This process combined with the re collision of the electrons collapsing toward the nuclei creates a transient current and as a result a strong THz field is generated This highly nonlinear process makes the efficiency of the THz generation sensitive to the carrier envelope phase CEP of the ultrafast laser pulses and it has been demonstrated that this effect can be used to monitor and stabilize the CEP of a laser amplifier system The detection of
11. lses from intense laser plamsa interaction Physical Review Letters 1993 71 17 p 2725 2728 18 Xin X et al Terahertz absorption 13 Cook D J and R M Hochstrasser Intense spectrum of para and ortho water vapors terahertz pulses by four wave at different humidities at room rectification in air Optics Letters 2000 temperature Journal of Applied Physics 25 16 p 1210 1212 2006 100 9 19 Zhou Z et al Terahertz generation and detection setup based on pump probe 14 Loffler T et al Comparative performance of terahertz emitters in 4 Ebbinghaus S et al An extended dynamical hydration shell around proteins Proceedings of the National Academy of Sciences of the United States of America 2007 104 52 p 20749 20752 5 Plusquellic D E et al Applications of terahertz spectroscopy in biosystems Chemphyschem 2007 8 17 p 2412 2431 Journal of the Optical Society of America a Optics Image Science and Vision 2008 25 12 p 3120 3133 10 Kemp M C et al Security applications of terahertz technology in Terahertz for Military and Security Applications R J Hwu and D L Woodlard Editors 2003 p 44 52 11 Tani M et al Emission characteristics of photoconductive antennas based on amplifier laser based systems scheme Microwave and Optical Semiconductor Science and Technology Technology Letters 2009 51 7 p 1617 2005 20 7 p 8134 5141 1619 15 Thomson M D et al
12. or efficient generation of 400 nm It is then detuned from this position by adjusting R 35 to create enough 400 nm with the same polarization as the fundamental This adjustment will be fine tuned later for the optimization of the THz field The power is increased by rotating the neutral density filter such that air plasma is just observed at the focal point A beam dump BD with the back plate removed is then mounted on TS to prevent the strong white light from reaching the eyes Reduce the power back to 10 yJ after BD is set A parabola PB 50 mm EFL is positioned about 50 mm away from the focal point such that the pulse is folded 90 and travels along the side of the breadboard and parallel to the table TS is adjusted such that the beam after the parabola is collimated The probe beam is routed by M to a retro reflector consisting of M and M figure 2 c This retro reflector is mounted on a motorized delay stage MS for the purpose of controlling the electro optic sampling The probe beam is then redirected by M Initially each mirror from M to M will fold the probe beam exactly 90 When inserting a business card into the probe beam bouncing off M and adjusting MS no displacement of the probe beam should be observed on the business card Adjusting M may be necessary so that the probe beam does not walk on M when MS is translated Subsequently mirrors M and Mg are adjusted such that the probe beam remains
13. port s Technology and Applications Center Initially the energy of the laser pulse is reduced to about 10 uJ by use of a variable neutral density filter before mirror M not shown Then as shown in Figure 2 a mirrors M and M are used to align the laser pulse parallel to the table and the side of the breadboard The laser pulse should be kept parallel to the table at all times 5 inches in this setup This minimizes the aberration caused by other optics in the beam path An iris I is used to help meet this requirement Subsequently a beam sampler BS is installed at a 45 angle to the incoming beam passing through its center The beam sampler reflects 3 of the energy for use as the probe beam Figure 2 a Alignment of the beam into the setup The pump beam the pulse train that transmits through BS is folded 90 by M such that it travels parallel to the side of the breadboard Figure 2 b A lens L with 125 mm focal length mounted on a translational stage TS is positioned in a way that the pump beam passes through its center Figure 2 b Alignment of the pump beam to generate air plasma A 100 um thick type I BBO crystal mounted in a rotational stage R and on a linear stage TS is placed after the lens L This assembly is positioned normal to the beam path with the BBO crystal about 25 mm prior to the focal point and used to generate 400 nm The BBO crystal is rotated to the optimum angle f
14. red trademarks of Newport Corporation Newport is a trademarks of Newport Corporation Newport Corporation Irvine California has been certified compliant with ISO 9001 by the British Standards Institution DS 011107
15. tween the BBO crystal and the air plasma determines the phase relationship between SHG and the fundamental Changing the distance between them directly relates to how far the 400 nm can drift away the electrons which relates to how strong the THz will be as the electrons recombine So once the THz signal is observed it can be optimized by adjusting TS and R the detune angle Tweaking S for better spatial overlap between the probe and THz beams may also improve the signal After any of these steps adjusting MS will be required for the optimization of the THz signal Results The generated THz field is shown in figure 3 Several features can be observed in the time domain trace First of all the main oscillation is only one cycle and spans about ps It contains most of the THz energy and is estimated to be about 0 1 0 3 n per pulse Secondly small and complicated oscillations after the main cycle are observed which indicate something absorbs the THz while it is propagating in air Thirdly there is a second smaller peak around 11 5 ps as shown in figure 3 a This is actually resulting from the second reflection of the THz within the Silicon wafer Since it is about 0 4 mm in thickness and it has n 3 4 in the THz region it would cause the delay that is observed in the plot GY Newport 3 Experience Solutions ane a I I D IA IA A 0 2 4 6 8 10 12 14 16 18 20 Amplitude a u a A 2 3 4 5 6 7 8 9 10 Time
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