176 J. Opt. Soc. Am. B/Vol. 18, No. 2 /February 2001 Pietralunga et al.

Experimental characterization of ternaryCd0.9Zn0.1Te as a basic material for

all-optical processing in the 1.5-mm range

Silvia M. Pietralunga,* Antonella Milani, Andrea Zappettini, and Mario Martinelli†

Consorzio Ricerche elaborazione Commutazione Ottica Milano, via Ampere, 30-20131 Milano, Italy

Received January 19, 2000; revised manuscript received June 29, 2000

Bulk II–VI semi-insulating ternary alloy Cd0.9Zn0.1Te is characterized as a basic material for all-optical switch-ing functions on free-space beams at l 5 1.5 mm. The adopted switching mechanism jointly exploits photo-conduction and the linear electro-optic effect. Cd0.9Zn0.1Te proves to be six orders of magnitude faster thanbinary CdTe:In working in the same configuration. Control-beam fluence of less than 0.02 nJ/mm2 is sufficientfor complete switching, and an extinction ratio of 23 dB is obtained, limited by spurious crystal birefringence.The role of material parameters in determining switching performances is discussed. A first estimate of theapplicability of the switching principle in itself to communication signals is also given. © 2001 Optical Societyof America

OCIS codes: 160.5140, 230.1150, 160.2100, 230.5440.

1. INTRODUCTIONThe II–VI semiconductor CdTe and more recently its ter-nary alloy CdZnTe are well known as substrate materialsfor the realization of nuclear and infrared (IR)detectors.1–3 The first experimentation of CdTe-basedmodules for optically activated switching of optical beamswas done by Steier et al. and Ziari et al.,4,5 jointly exploit-ing the efficient Pockels effect and IR photoconductivityin semi-insulating CdTe:In. The possibility of adoptingCdTe:In as a material to operate on optical communica-tion signals at l 5 1.5 mm has been analyzed in more re-cent years by some of the authors.6–10 In particular, aphotonic sampler and an optical time-to-space converterfully transparent to the optical communication signals inthe third window were demonstrated.9,10 A CdTe-basedswitching mechanism is characterized by an extremelylong recovery time, in the hundreds of milliseconds range,owing to the slow recombination rate of photogeneratedexcess carriers trapped into semiconductor intragap deeplevels.6 On the one hand, this feature has been exploitedin itself in order to implement different optical signal-processing functions.7,8 On the other hand, however, itstrongly limits the repetition rate of switching events andhinders effective applications in communication systems.In view of overcoming defect-related CdTe limitations,while keeping electro-optic efficiency and optical trans-mission, we characterize the switching response of bulksamples of ternary alloy Cd0.9Zn0.1Te, operating in thesame architecture as CdTe and activated in the nanosec-ond and subnanosecond range. In this paper we reviewthe most-significant results and evaluate possible applica-tions to communication signals. The paper is organizedas follows: in Section 2 we introduce the optical and elec-tronic properties of the semiconductor. In Section 3 wesummarize the physics of the switching mechanism. InSection 4 we describe the experimental setup and present

0740-3224/2001/020176-06$15.00 ©

the switching results. We end with a discussion ofswitching performance, in view of possible applications.

2. MATERIALMeasurements were performed using commercially avail-able bulk single crystals of undoped Cd0.9Zn0.1Te, grownby the high-pressure Bridgman method. The 10% zinccontent was confirmed by measuring E0 5 1.51 eV for theabsorption edge, defined as the photon energy where ab-sorption steeply increases, and relating E0 to the compo-sition of the ternary alloy at 300 K by E0(x)u300 K5 1.4637 1 0.49613x 1 0.2289x2.11 The absorption co-efficient, measured between 800 and 2000 nm, is reportedin Fig. 1. The measured transparency is acceptable for ap-plications in the third communication window. However,below the bandgap, a broad absorption shoulder is evi-dent. This feature is usually attributed to the presenceof midgap absorption levels.5 The spectral dispersion ofthe refractive index can be extracted from data reportedin Ref. 12. The cubic symmetry of the ternary alloy,which belongs to the point group 43m, make it ideally op-tically isotropic, with no intrinsic birefringence. A smallamount of optical retardation can actually be expected,as resulting from growth-process and samplemanufacturing.11

Electron-transport properties of CdZnTe are affectedby the presence of various intragap defect levels, of differ-ent origin. The determination of nature, position, androle of defect centers, together with corresponding carrier-capture cross sections, is still an open researchsubject.2,13,14

In particular, intrinsic defects can be at the origin ofdeep trap levels, close to half the bandgap, which maycontribute to the pinning of the Fermi level and the semi-insulating character of undoped material.15,16 In the

2001 Optical Society of America

Pietralunga et al. Vol. 18, No. 2 /February 2001 /J. Opt. Soc. Am. B 177

case of a tellurium-antisite defect located at Ec2 0.75 eV, the concentration has been estimated to be1016 cm23.15,17 No external compensation is thereforenecessary to reach high resistivity in undoped CdZnTe.Samples under test in our case reach r ' 1011 V cm.

3. SWITCHING MECHANISMThe all-optical switching mechanism is based on the samephysical principle as described in Refs. 4 and 7 and isschematically represented in Fig. 2. Cd0.9Zn0.1Te crystalrods measure 2.5 mm 3 2.5 mm 3 10 mm; they are cutfor amplitude modulation and oriented as shown in theinsert, with copper-indium-gold electrodes on the (110)face. The optical probe beam propagates in the @110# di-rection, [110] linearly polarized. The application of anexternal bias voltage electro-optically modulates the state

Fig. 1. Absorption spectrum of CdZnTe.

Fig. 2. Schematic representation of the switching mechanism inCdZnTe. Pockels linear effect and below-bandgap photoconduc-tion are jointly exploited. In the insert, the cut and orientationof the crystal sample are given, together with the direction of thebiasing electric field and the state of polarization of the probe op-tical beam.

of polarization of the probe beam. At the output of thecrystal, a linear polarizer, cross oriented with respect tothe first one, defines the optical transmission according to

T 5Iout

I in5 sin2S p

ln0

3r41LE D . (1)

In Eq. (1), l is the probe wavelength, n0 5 2.732 is theindex of refraction at 1550 nm, r41 is the electro-optic co-efficient, L is the crystal length, and E is the value of theeffective electric field, averaged on the active volume.

For no electrical bias, no transmission occurs. If a cwelectric field

Ep 5Vp

d5

1

2

l

L~n0

3r41!21

is applied, where d is the interelectrode spacing, the crys-tal behaves like a 45°-oriented half-wave plate, and thelinear input state of polarization of the probe beam isswitched to the orthogonal direction to give unitary trans-mission. If a polarizing beam splitter substitutes for theoutput polarizer, switching of the state of polarization isturned into spatial addressing of the signal beam.7 If anear-IR control beam is used to excite free charge carriersfrom intragap levels, the effect of the spatial distributionof the carrier population is to generate a counterfield thatshields the externally applied bias. This in turn resultsin a local modulation of E and of the transmission of theprobe beam as a consequence. The overall optically acti-vated electro-optical modulation and its dynamics are atthe basis of the switching performances of the material.

4. EXPERIMENTA. Single-Pulse Experimental SetupThe scheme of the setup for the characterization of theswitching performance is reported in Fig. 3. The opticalprobe beam at l 5 1.55 mm, from a semiconductor laser,is collimated to a waist w0 < 100 mm and impinges on theinput face of the sample, equidistant from both electrodes.A beam-steering device allows displacement of the probebeam, to optimize the geometrical configuration. At theoutput of the crystal, a polarization analyzer defines thetransmission transfer function according to Eq. (1).

Two different laser sources were alternatively em-ployed to generate the control beam. An optical para-

Fig. 3. Experimental setup for single-pulse optical switchingcharacterization.

178 J. Opt. Soc. Am. B/Vol. 18, No. 2 /February 2001 Pietralunga et al.

metric oscillator (OPO), pumped by a Q-switched Nd:YAGlaser, delivering 6-ns FWHM pulses ranging from 400 to2000 nm, was used to study the response of the materialas a function of control-beam energy and wavelength. Ifthe Nd:YAG pump is used to operate at 1064 nm, the tem-poral FWHM of the pulses is 10 ns. The pulse energy ischanged by means of interchangeable neutral-density fil-ters. The second laser source, a mode-locked Ti-sapphirelaser delivering 70-fs pulses at 830 nm, at a repetitionrate of 80 MHz, has been used to characterize the ulti-mate temporal speed of response of the switching phe-nomenon.

In both cases the control beam is collimated and sent tothe crystal sample, by means of a dichroic beam splitter,copropagating with respect to the probe. The control-beam waist measures ;250 mm. The temporal evolutionwas studied with a fast IR-sensitive photodiode coupled toa 5-Gs oscilloscope. The characterization was performedin single-shot excitation, by means of a mechanical shut-ter; an interferential filter selects the probe signal in de-tection.

Figure 4 reports the switched transmission of the sig-nal beam at l 5 1.55 mm, for different values of control-beam fluence at l 5 1.064 mm. Characteristic time con-stants of the process are the activation time and recoverytime of the optically induced counterfield, ton and toff , re-spectively. ton is mainly a function of the photonic flux ofthe control beam and the electrical biasing,4 toff is relatedto transport dynamics of photogenerated charge carriers,with particular regard to trapping and recombinationphenomena. A third temporal parameter, top , can be de-fined as the duration of the gating window, at constanttransmission and for maximum extinction ratio.

If relative positions and dimensions of optical beamsare optimized, complete switching is reached at a thresh-old control energy of E > 5 mJ, corresponding to a fluenceof less than 0.02 nJ/mm2. The best geometrical configu-ration is reached for collinear and superposed beams, thecontrol-beam waist being three times wider than thesignal-beam waist. The estimated density of opticallygenerated charge carriers at threshold corresponds to ageneration rate G 5 2 3 1023 cm23 s21 for 10-ns FWHMpulses. Extinction ratios up to 23 dB have been ob-tained, for control-pulse energy E 5 9.4 mJ. This valuecoincides with the extinction ratio owing to the birefrin-gence of the crystal sample, at zero bias.

As the switching energy is increased above threshold,the gating time top and the overall recovery time increase.The dependence of switching response times top and toffon activating energy is detailed in Fig. 4. The activationtime ton is not reported because it is independent of en-ergy in our experimental conditions. To calculate the re-covery time of the switching events toff , the closing tran-sient of the optical gate is fitted with a single exponential.Results are reported in Fig. 5(b). The lowest toff valuesare related to incomplete switching at low control ener-gies. By tuning control-beam wavelength, switching per-formance in the range from 800 to 1600 nm was tested inthe nanosecond range. The dependence of photonic fluxrequired for complete optical switching, as a function ofthe control-beam wavelength, is reported in Fig. 6. Theenergy of the pulses is set to completely shield the exter-

nal dc voltage. The switch is efficient on a wide range ofwavelengths of the control pulse (870–1300 nm), wherepulse energies in the microjoule range are used. By tak-ing into account the dependence of the absorption coeffi-cient on wavelength, the density of calculated photogener-

Fig. 4. All-optical switching of a l 5 1.55 mm signal beam, con-trolled by a pump beam at l 5 1.064 mm, of FWHM ' 10 ns, atdifferent pump-beam energies. Threshold energy correspondsto the minimum amount leading to complete transmissionswitching; in this case E ' 5 mJ.

Fig. 5. Dependence of switching characteristic times on the ac-tivating energy: (a) duration of the gating window top , mea-sured at constant transmission; (b) recovery-time constant of theoptically generated electric field distribution, obtained fromsingle-exponential fitting of the switching-off transient.

Pietralunga et al. Vol. 18, No. 2 /February 2001 /J. Opt. Soc. Am. B 179

ated carriers is constant in this region. In going from1300 nm toward longer wavelengths, the required photo-nic flux slightly increases. Around l 5 1580 nm, that is,for photon energy comparable to half the band gap (Egap5 0.8 eV), there is an abrupt change, and the switchingeffect practically vanishes.

The temporal evolution of transmitted-light intensity,for crystals exposed to the subpicosecond pulsed controlbeam, is reported in Fig. 7. The limited amount of en-ergy available (2 nJ/pulse) is not sufficient for completeswitching. Nevertheless, it allows evaluating character-istic times. The activating time ton can be defined as thetime required for the transmitted amplitude to pass from10% to 90% of the peak excursion. In this configurationwe estimate ton ' 0.8 ns, which actually depends on theenergy delivered. Faster ton can be expected when in-creasing control-beam fluence.4 On the contrary, byevaluating the recovery-time constant toff , the ultimatevelocity limit of switching dynamics can be inferred. Asingle negative exponential fitting of the slope providestoff ' 2.5 ns, which can be considered to be the fastest ofthe time constants ruling the turn-off process.

B. Double-Pulse Experimental SetupIn order to give a first estimate of the possibility of ex-ploiting CdZnTe-based switches for optical signal process-

Fig. 6. Threshold values of control photonic flux for completetransmission switching at different activating wavelengths.

Fig. 7. Temporal evolution of the transmitted probe beam, forsubpicosecond control pulses. Incident pulse energy is ;2 nJ.

ing, we set up a laboratory simulation of the gating of apacket of digitally encoded information. In particular, anoperating window of 170 ns can be used to gate a singlecell of data at 2.5 Gbits/s transmitted in asynchronoustransfer mode (ATM). For an adequate choice of thecontrol-beam energy, top-cell 5 170 ns can be obtained anda whole data cell can be switched in a single event. Oneexample of this case is shown in Fig. 4. In case top mul-tiples of top-cell are necessary, for instance, when switch-ing of the sequence of several cells is required, multiplecontrol pulses delayed by top-cell must be sent to the de-vice. The setup described in Fig. 8 enables one to createtwo control pulses, delayed by 170 ns. The optical benchis modified by addition of a beam splitter and a fiber-opticdelay line. A 50-mm-core multimode fiber is used, toavoid Raman nonlinearities. The choice for a single de-lay line is justified by the fact that a greater number oflines would have greatly increased the complexity of theoptical bench and the difficulty in implementing a reliableexperiment, without adding substantial information to amore-detailed explanation of the switching response.The optical splitting ratio and additional losses are cali-brated, so that the two delayed control pulses deliverequal amounts of energy. The results of this double-pulse excitation are shown in Fig. 9 for complete switch-ing. If the overall recovery time of the single switchingevent tr 5 (top 1 toff) measures less than the delay be-tween pulses, two subsequent events behave indepen-dently of each other, showing the same characteristic re-sponse times. As the control energy increases, tr exceedsthe delay between control pulses, and the second pulse op-erates on the crystal still in an excited state. In thiscase, ton remains the same, but the second switchingevent shows a longer toff , resulting in a quasi memory ef-fect, and the complete return of the system to the groundstate is delayed.

5. DISCUSSIONWe discuss hereafter the obtained experimental results,starting from the switching responses reported in Figs. 4and 7, which show the speed of the recovery mechanism of

Fig. 8. Experimental setup for characterization of the switchingresponse to the cascade of two activating pulses, delayed by 170ns.

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the switching phenomenon that constitutes the great ad-vantage of CdZnTe switches over CdTe. Optical activa-tion using subpicosecond control pulses allows evaluationof the ultimate-velocity limit of switching dynamics. Asingle-exponential fitting of the closing transient in Fig. 7is consistent with a recovery-time constant toff ' 2.5 ns.This is representative of the fast-time constant governingthe first part of the temporal evolution of an optically ex-cited space-charge field, with switching dynamics beingruled by transport of photoexcited excess carrier popula-tion. For complete switching off to happen and in orderto avoid long-lived shielding effects, free and trapped car-riers must finally recombine.

Complete switching of probe-beam transmission isshown in Fig. 4, for optical activation in the nanosecondrange. In this case, if we focus on the curve obtained foractivating energy E 5 6.01 mJ slightly above thresholdconditions and already showing some saturation of theoptical gate, we notice that complete recovery of theswitching event occurs within 200 ns. This is six ordersof magnitude faster than for highly resistive CdTe-basedswitches, tested in equivalent experimental conditions.7

As explained in Subsection 3.A, the probe and controlbeams are too distant from the electrodes to ascribe theswitching behavior to an electrode nonlinearity.18 Fast

Fig. 9. All-optical switching response to the cascade of two ac-tivating pulses at l 5 1.064, 170-ns delayed: (a) low activatingenergy such that top-cell . tr 5 (top 1 toff); (b) higher activatingenergy top-cell , tr 5 (top 1 toff).

evolution of switching can be ascribed to the combinationof band-to-band and defect-assisted recombination, sinceAuger recombination is not an efficient recovery mecha-nism under present conditions of optical excitation.19 Ac-tually, band-to-band recombination characteristic timesin CdTe and CdZnTe seems to be comparable. Thereforethe presence of a deep, efficient recombination center inthe ternary alloy can be inferred, which prevents chargecapture into deep trapping states, at the origin of thefreezing of the field-shielding distribution in CdTe. Ex-perimental evidences for the existence of deep recombina-tion centers have been recently obtained from spectro-scopic studies as reported in Refs. 2 and 13. Moreindications regarding the role of deep intragap levels inthe switching mechanism can be obtained by analyzingresults of Figs. 4 and 5. Figure 4 reports the evolution ofrecovery-time constants top and toff with the control-pulseenergy. At threshold optical control energy, an opticalgate is formed. If the control energy is increased further,the duration of the gating window top increases, corre-sponding to a delay in restoring excited free-carrier den-sity threshold conditions. This increasing trend showsno saturation, which suggests that the density of the de-fect center involved in optical excitation should be at leastof the order of 1016 cm23, thus accommodating the in-crease in free-carrier population, with no bleaching effect.The recovery time toff , which corresponds to the turn-offtime of the optically excited electric field spatial distribu-tion, shows an exponential increase with activating en-ergy, as reported in Fig. 5(b). The explanation for thisbehavior can be twofold. On the one hand, we can makethe hypothesis of saturation of available fast-recombination centers, which in this case can be esti-mated to be in the range of 1015 cm23 or less, so that thecontribution of slower-recombination mechanisms andtrapping acquire importance at higher-excitation flu-ences. In Ref. 12 the concentration of efficient recombi-nation centers is estimated from deep-level transientspectroscopy measurements in 8.5 3 1012 cm23. On theother hand, the temporal evolution of the optically gener-ated local-field distribution results from the interplay ofelectron-transport phenomena, which respond in a com-plex way to the increase in excess-carrier generation.The combination of both aspects, together with form anddimensions of the device, should be considered and ana-lyzed in more detail, to formulate a detailed description ofthe switching response at varying control beam energies.

Let us consider now the variation of switching effi-ciency with wavelength, as reported in Fig. 6. Efficientswitching in the microjoule energy range is obtained on awide range of control-pulse wavelengths (870–1300 nm).If we take into account the dependence of the absorptioncoefficient on wavelength, the density of photogeneratedcarriers can be estimated to be constant in this spectralregion, whereas the required photonic flux increasesslightly in going from 1300 nm toward longer wave-lengths. At photon energy comparable to half the band-gap (Egap 5 0.8 eV) the switching effect practically van-ishes. The spectral behavior of switching efficiencyshown in Fig. 6 suggests that a deep level close to midgapis mainly involved in photogeneration. In this frame thedifferences in required photonic flux, within one order of

Pietralunga et al. Vol. 18, No. 2 /February 2001 /J. Opt. Soc. Am. B 181

magnitude, can be ascribed to spectral variations of thecross section of optical excitation of the level. Thereforeresults reported in Figs. 5(a) and 6 jointly suggest thatoptical generation is performed from a defect center lo-cated at half the bandgap, the density of which is at leastequal to 1016 cm23. This is compatible with identifyingthe center with a deep level approximately 0.75–0.78 eVbelow the conduction band, which accounts for pinning ofthe Fermi level at midgap.15,17 No significant differencescan be found in the relaxation times of the switching pro-cess, as the control-beam wavelength is varied. This inturn suggests either that the same intragap levels are themost effective in photogeneration and recombination ofcarriers at all wavelengths, or, that the overall efficiencyof the mechanism, responsible for switching relaxation, isindependent of the particular level from which photoge-neration occurs.

From the results of Fig. 9 the possibility of exploitingCdZnTe-based switches for optical signal processing canbe evaluated. By comparing results in Figs. 9(a) and9(b), it can be seen that, if the control-beam energy hasbeen chosen so that tr 5 (top 1 toff) , top-cell , as in Fig.9(a), then complete return to the equilibrium state isreached before the arrival of the second control pulse. Inthis case, however, switching of the overall cell durationis not possible. If, on the contrary, tr 5 (top 1 toff). top-cell , as in Fig. 9(b), the second control pulse oper-ates on the material, excited by the first pulse. In thiscase, doubling of top and approximate doubling of toff isobtained. As a consequence, we can estimate that, in thepresent experimental configuration, the overall CdZnTeswitching behavior enables gating of signals with an av-erage 50% duty cycle and can operate efficiently on opticalcommunication channels with a 50% occupation probabil-ity.

6. CONCLUSIONSThe semi-insulating II–VI ternary alloy Cd0.9Zn0.1Te hasbeen tested as a material to implement efficient all-optical switching of communication signals at l5 1.5 mm. The adopted switching principle can be used

both for gating and for spatial switching of beams in afree-space architecture. Cd0.9Zn0.1Te overcomes the ma-jor drawback related to CdTe-based devices, slow recoverytimes. The nature of intragap defects enables one to con-textually obtain high resistivity, which is necessary forelectro-optic applications, and fast-evolving IR photocon-ductivity. In this way, recovery time for all-opticalswitching can be limited to the 100-ns range. This fea-ture, together with low energy consumption for a singleswitching event and a good extinction ratio, make intrin-sic CdZnTe an interesting candidate as a basic materialfor photonic applications. In particular, an operatingwindow of 170 ns can be used to gate a single cell of dataat 2.5 Gbits/s transmitted in asynchronous transfer mode.The design of devices, which may optimize operating con-ditions, deserves more attention and future research.

*Corresponding author: Consorzio Ricerche elaborazi-one Commutazione Ottica Milano, via Ampere, 30-20131

Milano, Italy. Telephone: 39-02-2369-131. Fax: 39-02-2369-1322. E-mail: pietralunga@corecom.it.

†Mario Martinelli is also with Dipartimento diElettronica ed Informazione, Politecnico di Milano, PiazzaL. da Vinci, 32-20133 Milano, Italy

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