Characterizing large-area electro crystals two-dimensional real-time terahertz imagingtoward - 副本

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Characterizing large-area electro-optic crystals toward two-dimensional real-time terahertz imaging
Fanzhen Meng,*Mark D.Thomson,Volker Blank,Wolff von Spiegel,
Torsten Löffler,and Hartmut G.Roskos
Physikalisches Institut,Johann Wolfgang Goethe-Universität,Max-von-Laue-Stras1,D-60438Frankfurt,Germany
*Corresponding author:meng@physik.uni‑frankfurt.de
制作面具Received5June2009;accepted13August2009;
梦见头发变白posted2September2009(Doc.ID112375);published15September2009
We have characterized the homogeneity of large-area(>10mm×10mm)CdTeð110Þand ZnTeð110Þcrys-
tals using a raster electro-optic scanning method to asss their usability in two-dimensional electro-
optic terahertz(THz)imaging with parallel read out.The spatial variation in the detected THz signal
(at0.2and0:645THz,respectively)is due to nonuniform residual birefringence and scattering.For CdTe,
this depends critically on the growth method,and has an important contribution from slip planes in the
crystals,as is evident in the scanned images.For the highest-quality CdTeð110Þcrystals investigated,the
rms signal variations are less than15%,comparable to tho for ZnTeð110Þ.For electro-optic scanning,
we introduce a hybrid measurement system bad on a fs Nd:glass lar and a continuous-wave elec-
tronic THz source.©2009Optical Society of America
OCIS codes:300.6495,160.2100,110.6795.
1.Introduction
Radiation in the terahertz frequency range,typically defined as the range0:3–3THz,can be ud for non-destructive measurements and has the ability to penetrate textiles,paper,and plastic materials.Ap-plications of imaging with THz radiation,in fields such as curity and defen[1,2],inspection of defects in plastic pipes,identification of diad skin tissue[3,4],inspection of textile-concealed goods [5],and surface-defect characterization[6,7],have been demonstrated.Recently,key advances have been achieved with advanced THz sources,including continuous-wave(cw)electronic multiplier THz sources[8,9],quantum-cascade lars[10,11],nar-rowband THz-OPO systems[12,13],high-power lar-pumped miconductor THz surface emitters [14–16],and electro-optic emitters[17,18].The emer-gence of such high-power THz sources expedites the development of practical THz imaging systems. Various detection methods have been ud in the systems:electro-optical(EO)sampling[8,9,17,18], time-domain gating with photoconductive antennas [16],bolometric detection[12],direct detection with narrowband Schottky diodes[19],and GaAs and Si field-effect transistors[20,21].Many systems show a significant dynamic range—however,the majority of them still u a raster scanning method for2D imaging.Systems capable of real time(parallel)ac-quisition of2D THz images are highly desirable.This can be readily achieved with multipixel EO sam-pling,where the spatial field modulation of the unfocud THz beam is transferred to the polari-zation of an unfocud optical beam and detected with the help of a VIS/N
IR camera[22–26].The per-formance of such systems depends critically on the lection and quality of the large-area EO crys-tal ud.
Assuming the EO crystal has sufficient trans-parency and nonlinear susceptibility for the optical and THz wavelengths ud,the key lection criter-ion is to achieve pha-matching for the nonlinear EO process.In the past,most of the rearch with broadband THz puls employed800nm femto-cond(fs)Ti:sapphire lars.For EO detection,
0003-6935/09/275197-08$15.00/0
古诗江南春唐杜牧©2009Optical Society of America
20September2009/Vol.48,No.27/APPLIED OPTICS5197
ZnTeð110Þis the most commonly ud EO crystal [25,27–30]due to the natural pha-matching for wavelengths around800nm and THz frequencies ≲3THz.Recently,however,a variety of THz studies
bad on lars working at wavelengths around1μm were reported.For example,powerful THz genera-tion using a diode-pumped Yb:KGW[KGd(WO3)] solid-state fs lar was reported[16],while other groups have employed high-power Yb-doped fiber amplifiers[15,28,31–34].Also,narrowband T
Hz OPO systems bad on nanocond Q-switched Nd: YAG lars were reported by other groups[12,13]. Compared with fs Ti:sapphire lars,the lars can be cheaper,more compact,have a more straight-forward operation,and/or have significantly higher output power.In the reports,the detection of the THz radiation was achieved using GaPð110Þ[27,34],ZnTeð110Þ[27],CdTeð110Þ[28,31–33],or photoconductive detectors[16].For EO sampling, although GaPð110Þshows near-perfect pha match-ing[27,34,35],CdTeð110Þwas found to be the most
efficient EO crystal[28,31–33]owing to its high EO
coefficient(about four times that of GaPð110Þ[25,34]).
In this paper,we study the applicability of large-
area CdTe crystals for2D EO detection with optical
wavelengths∼1μm.The results are relevant for
our target of building a real-time THz camera em-
ploying a THz OPO consisting of a Q-switched 1064nm Nd:YVO4lar and a PPLN parametric con-verter[36],which generates narrowband THz radia-
tion.We first examine the pha matching and
absorption properties of CdTe versus THz frequency
and optical wavelength to better define the range of
applicability.We then evaluate the potential perfor-
mance of large-area CdTeð110Þcrystals for2D cross-
岂有此理是什么意思polarizer-type EO detection[22,24]by measuring the
homogeneity of the single-pixel EO signal(with a
focud THz beam)with2D raster scans over the
crystal surfaces.This single-pixel THz tup is bad
on a modification of our existing hybrid THz system
(with a cw electronic source and EO detection with an
asynchronous fs-pul train[8,9])to u the1057nm
fs puls from a Nd:glass lar(as oppod to a fs Ti:
sapphire lar).We compare the spatial uniformity
of the EO signals for CdTe crystals with different
growth/preparation conditions and compare this
with a large-area ZnTeð110Þcrystal.The uniformity
蝙蝠
of the EO signals is affected by spatially varying
residual birefringence,associated with crystal im-perfections such as slip planes[37].The relative rms variations show minimum values(<15%)for CdTe obtained by the vertical-Bridgman technique.
2.CdTe Pha-Matching and Absorption Properties To evaluate the pha-matching coherence length L c [35]in CdTe,we ud the refractive index data pub-lished for the optical[38]and THz[39,40]
ranges.In Fig.1we show a contour plot of L c versus both optical wavelength(λ¼900–1150nm)and THz frequency(ν¼0:5–3THz).As can been en,at 0:5THz one achieves perfect pha matching at an optical wavelength ofλ∼1040nm,with L c remaining above1mm over the rangeλ∼950nm to>1150nm. With increasing THz frequency,the pha-matching
wavelength region shifts toward the visible,and the range narrows,as is characteristic for most EO crys-
tals(as dn THz=dν>0and dn opt;g=dλ<0).Hence,it is evident that CdTe posss good pha-matching properties for the wavelength of∼1:06μm and
THz frequencies≲1:5THz,allowing one to take advantage of the stronger nonlinearity compared to GaP.
We note that the predicted coherence length de-
pends quite nsitively on the preci THz refractive index values ud.Upon comparison of the CdTe data in[39–41],one obrves a variationδn∼0:02, which can readily lead to a factor of2change in L c for situations where L c∼1mm(due to the large gradients in the L c surface in Fig.1).Moreover, t
he different literature data for the THz absorption coefficient vary significantly(which also affects the uful EO crystal length),whereas we found no study on the THz absorption versus CdTe growth technique and doping.In the materials community,it is well established that CdTe prepared using common mod-ern growth ,vertical-Bridgman,Sec-tion3)is inherently p-doped,due to an excess of Cd, and can be compensation-doped,using Group III atoms such as in[42,43].This compensation doping then allows one to achieve the highest electrical re-sistivity,and hence should result in the lowest THz absorption.
To investigate the THz dispersion/absorption of the CdTe crystals under study here,we performed THz transmission measurements on two CdTeð110Þcrys-tals,both from the same supplier(Keystone Crystal Corporation),only with and without indium compen-sation doping.The measurements were carried out using a THz time-domain spectroscopy(TDS)tup bad on a1kHz amplifier lar(Clark-MXR CPA-2101)with ZnTe EO crystals for emission and
0.51  1.52  2.53
900
950
1000
1050
1100
11500.1mm
0.3mm
1mm
3mm
10mm
Fig.1.(Color online)Contour plot of the calculated coherence length of CdTeð110Þversus THz frequency(0:5–3THz)and optical wavelength(900–1150nm)bad on refractive index spectral data from the literature[38,39].The inner and outer pair of black curves show the3and1mm border lines,respectively.
5198APPLIED OPTICS/Vol.48,No.27/20September2009
detection [17,18].The refractive index and absorption spectra of the two CdTe samples extracted from the TDS data are shown in Fig.2(each with thickness L ≈1mm —the preci thickness determined with fine mechanical calipers to within ≲5μm),along with measurements extracted from [39,40](crystal growth not specified).As can be en,the refractive index data for the In-doped sample here agrees very well with that reported previously ,while the data for the undoped sample is shifted to smaller values by about δn ∼0:013.While this relative shift is compar-able with the precision in the thickness measure-ment,the negative direction is consistent with uncompensated dopant absorption expected from Drude theory [44].
A much more pronounced effect is apparent in the THz absorption coefficient α[Fig.2,bottom].For the In-compensation-doped sample,we measure similar results to tho reported previously [40],with a low residual absorption α<0:05mm −1below 1THz ,which increas at higher frequencies due to one-/two-phonon absorption.For the undoped sample,there is a significant vertical shift to higher absorp-tion,with a residual level of α∼0:5mm −1below 1THz.We note that in another report on the THz properties of high-resistivity CdTe [45]the resi-dual low-frequency absorption was actually found to be clor to our undoped sample ,α∼0:5mm −1.However,in that report,a thinner CdTe sa
mple was ud for this range (320μm)such that corrections for the Fresnel reflections would af-fect the derived bulk properties more strongly ,and no details of the growth or compensation doping method were specified.For near-IR wavelengths >1μm,the absorption of CdTe is negligible [41],and hence the effective absorption length for EO detection is domi-nated by the THz (field),L a ¼ðα=2Þ−1
[30].Hence,below 1THz we have L a ∼4mm and ∼8mm for the undoped and compensation-doped crystals,respectively .As shown in Fig.1,for wave-lengths near 1040nm the coherence length exceeds 10mm,such that in this regime the EO detection is absorption limited and the difference in THz ab-sorption between undoped and compensation-doped crystals can be important if thick crystals are ud.For our target application,with wavelengths around 1060nm and THz frequencies of 1–1:5THz,both L c and L a come clo to 1mm,such that a 1mm crystal thickness is a nearly optimal choice.
3.Details on Commercial CdTe Growth and Preparation
CdTe is a II –VI group compound with a zinc blende crystal structure,which ideally has no birefringence.The Cd –Te chemical bond has a high ionicity ,which leads to a low thermal conductivit
y and a very small formation energy for dislocations,twins and stacking faults [46].Many miconductor growth techniques have been tested in the pursuit of high-quality CdTe single crystals [46,47].The vertical-Bridgman (VB)method in a high-pressure furnace is the most com-mon method ud by commercial CdTe single-crystal suppliers.Tellurium precipitates [48]and inclusions [49],twins [50],grain boundaries [51],and slip planes [37]often appear in commercial CdTe crystals [52].The defects degrade the uniformity of the CdTe crystal and lead to residual birefringence.In general,high-quality large-area CdTe ð110Þcrystals are relatively rare on the market due to such growth/preparation difficulties.Moreover,the most common applications for CdTe crystals are for x-and γ-ray detectors,and as substrates for IR detec-tors (HgCdTe),hence many developments to improve the crystal properties may not be in the proper direc-tion for THz EO applications.
In this paper,we examine four CdTe ð110Þcrystals to find tho with the most favorable properties for our application.The CdTe ð110Þsingle crystals obtained from different suppliers (which we label as C −n ,n ¼1−4):Keystone Crystal Corporation (USA,growth:VB,15×15×1mm,either undoped or In-compensation doped,C-1and C-2,respec-tively);Cradley Crystals (Russia,growth:VB,10×10×2mm,C-3);and Moltech GmbH (Germany ,growth:high-pressure vertical zone melting (HP-VZM),10×10×1mm,C-4).Note that by simple in-spection by eye,all CdTe crystals appear to have a very high surface quality and mirrorlike reflection.
4.
Two-Dimensional EO Raster Imaging
In 2D EO THz imaging with cross-polarizer-type de-tection,the uniformity of the crystal plays an impor-tant role,as has been studied previously for (large-area)ZnTe crystals [22,24].A typical configuration places the EO crystal between two nearly crosd polarizers;the addition of a quarter-wave plate after the EO crystal provides a polarization bias such that the transmitted optical intensity
感谢老师的词语
modulation
Fig.2.(Color online)THz refractive index and absorption coeffi-cient of two CdTe ð110Þcrystals (undoped and In-compensation-doped),including a comparison with literature data [39,40].
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is esntially proportional to the THz field (not inten-sity).The resultant differential signal is given by ΔI ≈I 0 ð1=2ÞΓ0þδ Γ,where Γ¼n 3rk 0LE THz is the EO retardance in the crystal,Γ0is the residual bire-fringence,and δis the tilt angle of the cond polarizer [22].
Although effective methods have been demon-strated to correct the distortion induced by residual birefringence (and scattering)in the EO crystal [22,24],the methods can only be applied with some restrictions.For instance,the polarization bias δmust be chon large enough such that no image re-gions occur where the residual birefringence and polarization bias cancel (i.e.,ð1=2ÞΓ0þδ¼0,which physically eliminates the linear THz signal compo-nent).For EO crystals with large residual birefrin-gence variations,this then requires one to u a large δ,as oppod to that for optimal relative mod-u
lation depth [53].Moreover,the corresponding vari-ation in optical intensity on the camera prevents one from using its full capacity for most of the pixels (as the brightest pixels must still remain below satura-tion),which reduces the average signal-to-noi ratio (SNR).In addition,the correction methods are bad on having direct access to both the reference (no THz)and the signal image frames.For certain detection modalities where only a differential signal is ac-quired (such as the single-pixel hybrid detection ud here)this is not readily achieved.Hence it is impor-tant that the residual birefringence variations are kept to a minimum,even when correction methods are ud.
In the experiments here,we simulate the situation of a 2D EO detection scheme by using single-pixel crosd-polarizer EO detection (with a focud THz beam)while raster-scanning the EO crystal.The scheme of the experimental tup is shown in Fig.3.In contrast to a typical fs optoelectronic THz system (with emitter and EO detection driven synchronously by the same fs puls),here we employ a hybrid sys-tem [7–9]with a cw electronic THz source and asyn-chronous EO detection using a fs optical pul train.
Two different narrowband terahertz multiplier sources were ud here (both from Radiometer Phys-i
cs GmbH),with operation frequencies (f THz )of 0.20and 0:645THz (and cw output powers of ∼2mW and 0:5mW,respectively).The fs lar is a 1057nm Nd:glass lar (Time Bandwidth Products GLX-200)with a repetition rate (f rp )of 100MHz and 150fs pul duration.The concept of the EO detection with an asynchronous source and puld lar can be de-scribed as heterodyne detection with the fs pul train acting as a local oscillator (LO)[8,9].The mod-ulation of the optical polarization that occurs due to EO mixing with the THz field generates a sideband comb with frequencies at f THz −nf rp (extending down to baband frequencies).The lowest modulation sideband for the 0:645THz source occurs at an inter-mediate frequency (IF)f IF ¼f THz −6450f rp clo to 10MHz.This IF has a bandwidth of about 250kHz and drifts by less than 1MHz over an hour,under the highly stable condition of the temperature-controlled and vibration-isolated rearch environ-ment.To generate a pha-locked reference of the IF for lock-in detection,a fraction of the THz radia-tion is nt to a reference EO detection arm (using balanced detection with a 1mm CdTe ð110Þcrystal,Fig.3).The remainder of the THz radiation is nt to the single-pixel crosd-polarizer signal detection arm,where the different large-area EO crystals un-der study are mounted on a computer-controlled x –y scanner.The lock-in amplifier then demodulates the EO signal at the IF with the aid of the reference EO signal.As the cw THz pha is transferred to the IF electronic signal,the detected lock-in pha repre-nts the relative pha of the THz wave between the signal and reference detectors.
Therefore,coher-ent THz field detection is realized without actively synchronizing the lar and the THz source.A lar power of about 25mW was ud in the signal arm,as limited by the saturation level of the photodiode.The scanned images of the different CdTe ð110Þcrystals are shown in Figs.4(a)–4(f),including mea-surements with 0:2THz and 0:645THz.The gray scales indicate the signal strength,which is (ideally)linearly proportional to the THz field magnitude (not intensity).In each image,the signals are normalized to their maximum values.While the u of 0:645THz more cloly approaches our target applications in the range of 1–1:5THz,the u of 0:2THz allows us to perform a comparison with ZnTe (where the co-herence length is still above 3mm for 1057nm).A 0:2THz image of a ZnTe ð110Þcrystal (2mm thick-ness,diameter 25mm)is shown in Fig.4(g).As the focud optical beam samples the axial THz field,the lateral resolution of the scan images is dictated by the optical focal spot size (well below 100μm).The slanting striplike structures shown in Figs.4(a)–4(e)(VB growth)are due to residual birefringence in the crystal induced by slip planes formed during growth.The structure of this residual birefringence is also evident in transmission images obtained with a conventional optical cross-polarizer tup.
An
Fig.3.(Color online)Schematic of the hybrid THz system,includ-ing 2D raster scanning of the large-ar
ea EO crystals in the cross-polarizer signal detection arm.5200
APPLIED OPTICS /Vol.48,No.27/20September 2009
example is shown in Fig.5(for the CdTe crystal C-3),measured by illuminating the whole crystal surface with an expanded lar beam from the Nd:glass lar.The peripheral optical diffraction patterns in Fig.5also demonstrate that one should not u the entire EO crystal surface for 2D THz imaging.
Returning to Figs.4(d)–4(f),a significantly larger variation in signal strength is apparent for the CdTe ð110Þcrystal C-4,implying additional sources of inhomogeneity ,probably owing to inner structural vacancies,thickness variations,and/or grain bound-aries associated with the different growth technique for this crystal (HP-VZM,Section 3).To provide a more quantitative analysis of the signal homogene-ity ,in Fig.4(h)we plot a t of histograms for the sig-nal level in the 0:2THz measurements [normalized to the mean values,analysis regions as indicated in Figs.4(d)–4(g)].As can be en,the signal distri-butions posss a fairly well-behaved statistical char-acter (except for some outlier points for the CdTe crystal C-4).The relative rms variations σin the THz signal obtained for the CdTe ð110Þcrystals are 13%(C-1),23%(C-3),and 49%(C-4),with 12%for the ZnTe ð110Þcryst
主试al.The rms values for the 0:645THz measurements (histograms not included here)are esntially the same.
In comparing the homogeneity of the various sig-nals,we should consider the differences in crystal thickness (1mm for C-1,C-2,C-4;2mm for C-3,ZnTe).We have performed simple modeling of the expected crosd-polarizer EO signal [22]with a ran-dom variation σΓ0in the net residual birefringence Γ0.It is straightforward to show that the resultant relative signal variation σis proportional to
the
Fig.4.(Color online)2D raster-scan images of the THz signal for different EO crystals,as detailed in text:undoped and In-compensation-doped 1mm-thick CdTe ð110Þ(C-1and C-2,respectively ,Keystone Crystal Corporation);2mm-thick CdTe ð110Þ(C-3,Cradley Crystals);1mm-thick CdTe ð110Þ(C-4,Moltech GmbH);and a 2mm-thick ZnTe ð110Þcrystal.(a)–(c)Measurements with 0:645THz;(d)–(g)with 0:2THz.(h)Histogram of relative EO signal strengths for the images in (d)–(g)(data region ud for analysis indicated by dashed
rectangles).
Fig.5.Residual optical birefringence of the CdTe ð110Þcrystal from Cradley Crystals [(c)and (e)in Fig.4]measured by a con-ventional crosd-polarizer tup using the expanded Nd:glass lar beam.
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