Organic Broadband Terahertz Sources And Sensors

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Copyright 2007 American Scientific PublishersAll rights reservedPrinted in the United States of AmericaJournal ofNanoelectronics and OptoelectronicsVol. 2, 1–19, 2007Organic Broadband Terahertz Sources and SensorsXuemei Zheng , Colin V. McLaughlin, P. Cunningham, and L. Michael HaydenDepartment of Physics, University of Maryland, Baltimore County, MD 21250, USAREVIEWWe review recent research using organic materials for generation and detection of broadband terahertz radiation (0.3 THz 30 THz). The main focus is on amorphous electrooptic (EO) polymers, withsemiconducting polymers, molecular salt EO crystals, and molecular solutions briefly discussed.The advantages of amorphous EO polymers over other materials for broadband THz generation(via optical rectification) and detection (via EO sampling) include a lack of phonon absorption (goodtransparency) in the THz regime, high EO coefficient and good phase-matching properties, and, ofcourse, easy fabrication (low cost). Our 12-THz, spectral gap-free THz system based on a polymer emitter-sensor pair is an excellent demonstration of the advantages using of EO polymers. Wealso present a model that can predict the performance of a polymer-based THz system. Both thedielectric properties of an EO polymer and laser pulse related parameters are included in the model,making the simulations close to real conditions. From our modeling work, the roles the dielectricproperties play in the THz generation and detection are clearly seen, providing us with a good guideto select and design suitable EO polymers in the future.Keywords: Electrooptic Polymer, Nonlinear Optics, Terahertz, Far Infrared, Spectroscopy.CONTENTS1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2. Generation, Detection, and Application ofBroadband THz Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.1. Basic Broadband THz System . . . . . . . . . . . . . . . . . . . . . .2.2. Broadband THz Generation . . . . . . . . . . . . . . . . . . . . . . . .2.3. Broadband THz Detection . . . . . . . . . . . . . . . . . . . . . . . . .2.4. THz Time-Domain Spectroscopy . . . . . . . . . . . . . . . . . . . .3. Organic Materials for THz Sources and Detectors . . . . . . . . . . .3.1. Conjugated Semiconducting Polymers . . . . . . . . . . . . . . . .3.2. Organic EO Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.3. Organic EO Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.4. Polar Molecules in Solutions . . . . . . . . . . . . . . . . . . . . . . .4. Experimental Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.1. Efficient THz Emitter Based on EO Polymer . . . . . . . . . . .4.2. LAPC Emitter-Sensor Pairs Operated at 800 nm . . . . . .4.3. DAPC Emitter and Multi-Layer LAPCSensor Operated at 1300 nm . . . . . . . . . . . . . . . . . . . . . .4.4. DAST Emitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.5. Dielectric Property Characterization ofEO Polymers Using THz-TDS . . . . . . . . . . . . . . . . . . . . . .5. Modeling a Polymer Emitter-Sensor Pair . . . . . . . . . . . . . . . . . .6. Conclusions and Future Work . . . . . . . . . . . . . . . . . . . . . . . . . .References and Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .133356667899991012141517181. INTRODUCTIONTerahertz (THz) radiation, with a loose definition between0.3 THz and 30 THz (1 THz 1012 Hz), bridges microwave and infrared (IR) radiation. A variety of excitations, Author to whom correspondence should be addressed.J. Nanoelectron. Optoelectron. 2007, Vol. 2, No. 1such as rotational and vibrational states in molecular systems, lattice resonance in dielectric crystalline materials,and confinement states in artificially fabricated nano-structures, occur in this spectral regime, suggesting the versatileapplication of the THz radiation in chemical and biologicaldetection, medical imaging, and spectroscopy. However,the lack of compact, bright THz sources and sensitive THzdetectors slows progress in this field. Even after intensivestudy for nearly two decades, THz science and technologyis still in its infancy.While tunable continuous-wave THz radiation, associated with photomixing or quantum cascade lasers, isuseful for spectroscopy with very high frequency resolution, single-frequency imaging and remote sensing, pulsed(equivalently, broadband) THz radiation associated withthe employment of ultrashort lasers is the optimal choicewhen an overall snapshot of the spectral characteristicsof a sample in the THz regime is important.1 For pulsedTHz systems, a wide bandwidth with a smooth frequencyresponse using low power laser sources would be quitevaluable in many scientific and technological arenas.Currently, optoelectronic and all-optical techniques arecommonly employed for generation and detection ofpulsed THz radiation. The optoelectronic technique relieson the use of photoconductive dipole antennas (PDA)fabricated as micro-striplines or coplanar transmissionlines on photoconductive inorganic substrates.2 3 ThesePDAs have excellent sensitivity and a smooth frequencyresponse but a narrow useable bandwidth. The .2007.0051

Organic Broadband Terahertz Sources and Sensorstechnique uses optical rectification (OR)4 in electro-optic(EO) materials to generate the THz radiation and uses EOsampling5 6 to detect the THz radiation. This method hasgood sensitivity and a large bandwidth, but the conventional systems consisting of crystalline EO materials donot have a smooth frequency response across that bandwidth due in part to phonon absorption associated with thecrystalline nature of the emitters and detectors.Zheng et al.Recently, some attention has been paid to the use oforganic materials for the THz generation and detection.For both the optoelectronic and all-optical techniques,organic materials have shown great potential and broadened the material possibility. Organic EO materials havemade significant contribution to the recent development ofall-optical THz systems. For example, organic crystallineDAST (4-N ,N -dimethylamino-4 -N -methyl stilbazoliumREVIEWXuemei Zheng is currently a postdoctoral research associate in the Physics Departmentat University of Maryland, Baltimore County. She received her Ph.D., MA, and BE fromUniversity of Rochester, the City College of New York, and Tianjin University (China),respectively. Her research interests involve terahertz optoelectronics, nonlinear optics inorganic materials, and studies of ultrafast dynamics of photo-induced carriers in condensedmatters.Colin V. Mclaughlin was born in Portland, Maine, on July 30, 1979. He received his BA,majoring in physics, from Drew University in 2003. He received his MS from Universityof Maryland, Baltimore Co. in 2005. His research interests include EO polymer devices forTHz applications.P. Cunningham was born in Baltimore, MD April 29, 1982. He received his BS in appliedphysics from Towson University in 2004 and his MS from University of Maryland, Baltimore Co. in 2006. His research interests include optical-pump THz-probe studies of photoconductive materials and devices for solar cell and photodetector applications.L. Michael Hayden is a Professor of Physics and the Chairman of the Department ofPhysics at the University of Maryland, Baltimore County. He has a BS from the U. S. NavalAcademy and a Ph.D. from the University of California, Davis. He has had careers in theUS Navy, the private sector, and academia. His current research interests involve ultra-fastoptical studies of organic and polymeric materials, with applications to terahertz science.He is a private pilot with single and multi-engine ratings.2J. Nanoelectron. Optoelectron. 2, 1–19, 2007

Zheng et al.ranges of electromagnetic radiation, it is still quite a recenttechnology and not fully established.A schematic of the optical arrangement of a THz systemis shown in Figure 1. An output laser beam from a femtosecond laser is split into two beams, with most powergoing to the pump beam to drive a THz emitter and verylittle power going to the probe beam that interacts with theTHz wave under investigation in a THz detector. By varying the optical delay line in one arm, the probe pulse (withduration much shorter than the THz pulse) sees differentparts of the THz waveform. With the delay line positionand data acquisition controlled by a computer, we can mapout the electric field (instead of power) of the THz wave.This gated sampling technique allows for jitter-free phasecoherent detection, leading to a high signal-to-noise ratio(SNR) and dynamic range.2.2. Broadband THz GenerationIn general, broadband THz radiation can be generatedeither in an optoeletronic manner involving photogenerated transient currents in photoconductive antennas.14 orin an optical manner involving optical rectification in EOmaterials.15 The two techniques have always been underparallel developments and boasted of different advantages.The technique of using photoconductive antennas togenerate electromagnetic radiation can be traced back to asearly as the middle of the 1970’s when Auston generatedpicosecond microwave pulses on a transmission line byexciting the photoconductor gap bridging the electrodes ofthe transmission line with picosecond pulses2 (see Fig. 2).The mechanism is simple: the optical pulse with the photon energy higher than the bandgap of the photoconductorexcites electrons from the valance band to the conductionband; because of the electric field provided by the biasacross the electrodes, the injection of these photocarrierscloses the switch with the current through the switch rising rapidly (determined by the laser pulse duration) anddecaying with a time constant determined by the carrierlifetime of the photoconductor; according to Maxwell’sequations, E t J t / t, so the transient photocurrentJ t radiates into the free space.16The big step from microwave radiation to THz radiationwas made possible by the availability of single-picosecond2. GENERATION, DETECTION, ANDAPPLICATION OF BROADBAND THzRADIATION2.1. Basic Broadband THz SystemBroadband THz generation is intimately tied to femtosecond laser sources. In fact, THz technology boomed shortlyafter solid-state femtosecond lasers became widely available a little more than two decades ago that could be operated by a relative novice. Yet, compared to other spectralJ. Nanoelectron. Optoelectron. 2, 1–19, 2007Fig. 1. General optical arrangement of a THz system. Four 90 offaxis parabolic mirrors are used to collect, collimate and focus the THzradiation. This arrangement is suitable for spectroscopy study, for whicha sample under study can be placed at the THz focal point.3REVIEWtosylate)7–9 and 2-( -methylbeayl-amino)-5-nitropyridine(MBANP).10 have been shown to exhibit much higherEO coefficients than their inorganic counterparts andefficiently generate and detect the THz radiation. Moreencouragingly, amorphous EO polymers exhibit not onlyhigh EO coefficients but also the absence of phononabsorption (in the THz regime). A THz system based on apolymer emitter-sensor pair has produced spectral gap-freebandwidth up to 12 THz.11 The most exciting thing aboutEO polymers is the tunability of the properties throughcomposite constituent modification and film processing.This should allow these materials to establish excellentsensitivities, extremely wide bandwidths and flat frequencyresponses in the mid- and far-IR THz regimes. With slowerprogress, organic PDAs based on PPV12 and pentacene13have also been successfully made for the THz emission.Compared with a large amount of work done with inorganic crystalline materials, the use of organic materials inthe THz generation and detection is still under-explored.With the great potential that the organic materials haveshown, there is a need, at this point, to review THz sourcesand sensors involved with organic materials. In order tomake this review more readable to researchers who are notquite familiar with but want to get involved in the THzscience and technology, we will give an introduction inSection 2 to the common techniques used for THz generation and detection. We mainly focus on the all-opticaltechnique involved with EO materials, taking our experience into consideration. Principles of THz time-domainspectroscopy (THz-TDS), one of the most important applications of THz radiation, are also presented in this section.Section 3 goes over organic materials for THz sources andsensors. Our focus is on amorphous EO polymers, but wealso briefly discuss organic semiconductors and organicEO crystals and liquids as a complete review of this field.In Section 4, we present the experimental results obtainedfrom our THz systems based on EO polymers operatedat both 800-nm and 1300 nm wavelength. Advantagesof using EO polymers as THz emitters and sensors areclearly shown in this section. Experimental results usingDAST as the THz emitter are also given in this section.Our modeling work on a polymer emitter-sensor pair ispresented in Section 5. In Section 6 we conclude and pointout challenges and future work.Organic Broadband Terahertz Sources and Sensors

Organic Broadband Terahertz Sources and SensorsREVIEWFig. 2. A photoconductive switch integrated in a microstrip transmission line. When a laser pulse with photon energy higher than the photoconductor’s bandgap energy illuminates the gap of the transmission linebiased by an external field, photogenerated carriers close the switch andthe transient current consequently radiates into free space.and subpicoscecond photoconductors (LT-GaAs, ionimplanted GaAs, etc.), micro-lithography (allowing fabrication of smaller radiating devices and consequentlyhigher frequency electromagnetic radiation), and ultrashortlasers providing shorter and shorter pulses (down to 12 fs, commercially available). The achievable bandwidth from most PDA-based THz systems is usually afew THz and is often attributed to the limit of the carrier lifetime of the photoconductor involved. In a fewcases, however, ultrabroad bandwidths ( 10 THz) havebeen reported. Kono et al.17 reported a THz detection upto 20 THz with a low-temperature-grown GaAs (LT-GaAs)PDA gated with 15 fs light pulse. It was quite a surprisingresult as the LT-GaAs they used exhibited a relatively longcarrier lifetime of 1.4 ps. According to the authors, thefast response of the PDA was explained by the fast risein the photocurrent upon excitation by the ultrashort laserpulse,17 and the physical origin of the fast photocurrentwithin 100 fs might be explained by the ballistic transport of the photoexcited electrons in the biased electricfield.18 In this picture, the PDA works as an integrationdetector, so the photocurrent from the antenna should beproportional to the time integration of the incident THzradiation. With a post-measurement analysis where boththe number of photocarriers (a function of time) and theintegration mode of the PDA detector were taken into consideration, the same authors obtained even broader detection bandwidth, up to 40 THz.19 On the other hand,ultrabroadband THz generation from LT-GaAs PDAs wasdemonstrated by Shen et al.20 Using a backward collection scheme to minimize the THz absorption by the LTGaAs substrate, THz radiation with frequency componentsover 30 THz was achieved, and the transverse optical (TO)phonon absorption band of GaAs was clearly identified.The ultrabroad bandwidth might be due to the specificscheme where the pump beam was illuminated on the edgeof one of the PDA electrodes.21–23 For the edge illumination scheme, the transient current in the PDA results fromthe dielectric relaxation of the space-charge field such thatits dynamics is not determined by the carrier lifetime.24So far, the broadest bandwidth from a PDA emitter-sensorpair is 15 THz,25 although a very distinguished spectral4Zheng et al.gap related to the TO phonon band of the substrate wasexhibited. In addition to the operation complexity, anotherdisadvantage of this technique is the necessity of the complex and expensive lithography facility.Compared with the technique involved with PDAs, theadvantage of using nonlinear optical rectification to generate THz radiation is a possible broader bandwidth, as wellas the availability of a variety of EO materials. Pioneeringwork done by Shen et al.4 demonstrated the possibility ofusing picosecond laser pulses in EO materials to generate far infrared radiation via optical rectification. Austonet al. extended this technique by using shorter laser pulsesand observed a generated electromagnetic wave in the THzregime.26 Since then, many researchers have followed andfurther developed this technique by exploiting numerousmaterials and geometries.7 15 27–29Optical rectification can be understood as mixing of twodifferent frequency components in the frequency spectrumof an incident ultrashort optical pulse in an EO medium.The difference frequency mixing results in a nonlinearpolarization and consequently a radiation at the beat frequency. The bandwidth of the radiation in OR is limitedby the bandwidth of the optical pulse, as well as the relevant properties of the nonlinear medium. Mathematically,the difference frequency mixing process via optical rectification is describe as follows: 2 d 24 ETHz z 2 2 PNL z (1) dz2c2cwhere c is the speed of light, is the THz frequency, is the dielectric constant of the nonlinear medium inthe THz region [ n2THz , if there does not existTHz absorption in the NLO medium], ETHz z is thepropagating THz field generated in the nonlinear medium,and PNL z is the nonlinear polarization propagatingalong the z-axis expressed by (assuming the optical wavelength is far away from the material’s electronic resonanceregion): E z · E z d PNL z eff eff · I z (2)where is the optical frequency. It should be noted thatI z is the autocorrelation of the optical electric field,or, actually, the Fourier transform of the intensity profileof the optical pulse—I z t . Dispersion and absorption ofthe nonlinear medium in both the THz and optical regimemake analytically solving Eq. (1) very difficult, if notimpossible. However, analytical solutions based on certainassumptions simplifying the problem can be obtained.30It is found, from these solutions, that the phase-mismatch(the difference between the optical group index ng nopt dn opt opt , where nopt and opt are the optical index and optioptcal wavelength, respectively, and the THz index nTHz inthe material) limits the amplitude and bandwidth of theJ. Nanoelectron. Optoelectron. 2, 1–19, 2007

Zheng et al.THz generation. In fact, the desired phase-matching condition ng nTHz can also be identified by carefully scrutinizing Eq. (1): the nonlinear polarization involves the opticalgroup index while the THz electric field involves the THzindex. Alternatively, the phase-matching requirement canbe derived by resorting to a quantum picture.31 As is wellknown, all particles involved in an interaction must obeythe laws of both energy conservation and momentum conservation. There are two optical photons (with the energydifference being the THz photon energy) and one THzphoton in the OR process, so the phase-matching conditionis: k kopt kopt kTHz ng nT Hz /c 0. In the case of phase-mismatching ( k 0), the coherence length (optimal interaction length) canbe expressed as:31 clc k ng nTHz J. Nanoelectron. Optoelectron. 2, 1–19, 20072.3. Broadband THz DetectionLike THz generation, THz detection can be done in anoptoelectronic manner called photoconductive sampling oran optical manner called EO sampling.Historically, the appearance of photoconductive sampling was almost as early as the first use of a PDA togenerate electromagnetic pulses.34 A pair of PDAs and afemtose

Organic Broadband Terahertz Sources and Sensors Zheng et al. technique uses optical rectification (OR)4 in electro-optic . crystalline nature of the emitters and detectors. Xuemei Zheng is currently a postdoctoral research associate in the Physics Department at University of Maryland, Baltimore County. She received her Ph.D., MA, and BE from .

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