Nanocrystals of PbSe core, PbSe�PbS, and PbSe�PbSexS1�x core�shell as saturable absorbers inpassively Q-switched near-infrared lasers

Maya Brumer, Marina Sirota, Ariel Kigel, Aldona Sashchiuk, Ehud Galun, Zeev Burshtein,and Efrat Lifshitz

The saturable optical absorption properties of PbSe core nanocrystals �NCs�, and their correspondingPbSe�PbS core�shell and PbSe�PbSexS1�x core�alloyed-shell NCs, were examined at � � 1.54 �m.Saturation intensities of approximately 100 MW�cm2 were obtained. The NCs act as passive Q switchesin near-infrared pulsed lasers. Q-switched output pulse energies up to 3 mJ, with a pulse duration of40–55 ns were demonstrated. Analysis of the optical transmission versus pulse light intensity was carriedout according to a model that includes ground-state as well as excited-state absorption. For pulses �10 nslong, the NCs act as fast saturable absorbers. The theoretical fits yield a ground-state absorption crosssection of 10�16�10�15 cm2, an excited-state absorption cross section of �es � 10�16 cm2, and an effectivelifetime of �eff � 5 � 10�12 s. © 2006 Optical Society of America

OCIS codes: 140.0140, 140.3070, 140.3540.

1. Introduction

Q switching enables energetic short pulse outputfrom a laser by applying active or passive switchingdevices. The use of a saturable absorber inside a lasercavity as a passive Q-switching device contributes tothe output beam quality, but most importantly, itsimplifies the laser system design. The alternative ofan active device requires, for example, a high-voltage-gated electro-optic crystal or a liquid Kerr cell. Satu-rability of optical absorption is the increase of thematerial optical transmittance with the illuminatedlight energy density (fluence). It occurs when theground-state absorption cross section, �gs, is largeenough, and at the same time, the excited-state ab-sorption cross section, �es, is considerably smaller.The excited-state absorption is a cause for loss in the

laser cavity, and thus limits the efficiency of theQ-switching device. The bleaching factor � � �gs��es

is an important figure of merit for the passiveQ-switching application.

Various materials have already been recognizedas saturable absorbers operating at the infrared(IR) regime, including U2-doped calcium fluoride,1Co2-doped semiconductors, Co2-doped spinels, aswell as epitaxially grown quantum-well and quantum-dot semiconductors, designed as saturable absorbermirrors,2,3 and saturable Bragg reflectors.4 Thesematerials were used to obtain Q-switched pulsesfrom a number of lasers at 1.34, 1.44, and 1.54 �m.5Ground-state absorption cross sections between 0.8and 5.7 � 10�19 cm2 have been determined. The semi-conductor nanostructures are of special interest:These structures can function as saturable absorbersover wide spectral regions, when the size of the crys-tals is in the nanometer range. The optical propertiesof the nanocrystals (NCs) are strongly influenced bythe carriers’ confinement. Thus the NCs’ excitonicabsorption energy changes with their size.

Saturable absorber mirrors and saturable Braggreflectors require using the technique of epitaxialgrowth of semiconductor quantum structures. Whilebeing an already mature and cost-effective technique,it still requires sophisticated and highly expensiveequipment. Recently, efforts were made to produce

M. Brumer, A. Kigel, A. Sashchiuk, and E. Lifshitz (ssefrat@tx.technion.ac.il) are with the Department of Chemistry, SolidState Institute and the Russell Berrie Nanotechnology Institute,Technion–Israel Institute of Technology, Haifa 32000, Israel. M.Sirota and E. Galun are with ElOp Electro Optics Industries,Limited, P.O. Box 1165, Rehovot 76111, Israel. Z. Burshtein iswith the Department of Materials Engineering, Ben-Gurion Uni-versity of the Negev, P.O. Box 653, Beer-Sheva 84105, Israel.

Received 11 January 2006; revised 1 May 2006; accepted 9 May2006; posted 9 May 2006 (Doc. ID 67150).

0003-6935/06/287488-10$15.00/0© 2006 Optical Society of America

7488 APPLIED OPTICS � Vol. 45, No. 28 � 1 October 2006

nonepitaxially grown semiconductor-doped glassesas saturable absorbers. For example, InAs semicon-ductor microcrystals in silica films were fabricatedusing radio-frequency sputtering, which offer avariation in spectral response according to theInAs�silica ratio.6 Some publications describe the useof semiconductor-doped glasses fabricated by low-costbatch melting, which also permit control of the semi-conductor crystallite size within the nanometer scale.Glasses doped with InAs NCs have been used as sat-urable absorbing Q-switching devices in Ti:sapphirelasers, operating between 800 and 880 nm.7 PbSNCs-doped glasses were used as saturable absorbersfor repetitive Q switching of cw Cr:forsterite,8Nd:YVO4,9 Cr4:YAG,10 and Er:glass11–14 lasers, op-erating in the 1–1.5 �m spectral range.

The IV–VI compound (e.g., PbSe, PbS) NCs are afocus of special interest due to unique intrinsic prop-erties.15 Bulk PbSe and PbS materials have a cubic(rock salt) crystal structure and a narrow directbandgap (0.28–0.41 eV at 300 K), fourfold degener-ated at the L point of the Brillouin zone. The highdielectric constant �� � 18.0–24.0� and the smallelectron and hole effective mass (�0.1 of the free-electron mass) create an exciton with a relativelylarge effective Bohr radius �46 nm�, which is eighttimes larger than that of CdSe. Interband opticaltransition studies of colloidal PbSe NCs exhibitwell-defined band-edge excitonic transitions tunablebetween 0.3 and 1.1 eV, small Stokes shift, and sub-nanosecond and nanosecond lifetimes for the intra-band and interband transitions, respectively.16–19

Recently, amplified spontaneous emission from PbSeNCs was demonstrated, with gain parameters similarto those observed in CdSe NCs.20,21 Furthermore, im-pact ionization obtained by photoexcitation with h � 3Eg and its competition with Auger recombinationhas been discussed.19,22 These findings indicate thefeasibility of using the PbSe and PbS NCs in telecom-munications, eye-safe lasers, solar cells, electrolumi-nescence devices, and biological markers.19,23–25

Pulsed Er:glass lasers operating at 1.54 �m areespecially suited to a variety of applications such aslidar, surgery, and telecommunications. This stemsfrom the fact that its emission wavelength coincideswith the so-called eye-safe spectral region, which alsocorresponds to one of the atmospheric transmissionwindows, and to the third transmission window ofsilica fibers. In the past, 20–100 ns pulses were ob-tained by passive Q-switching using U2-doped fluo-rides, Co2-doped garnets such as YAG, and morerecently, using Co2-doped magnetoplumbite, spi-nel, and chalcogenides.26–30

The present study deals with the analysis of op-tical absorption saturation in PbSe core, PbSe�PbScore�shell NCs, and PbSe�PbSexS1�x core�alloyed-shell NCs, and their utilization as passive Q-switching devices in an Er:glass laser at 1.54 �m. Wepresent the synthesis and use of unique core�shellNCs composed of a PbSe core, covered with an epi-taxial layer of a PbS or a PbSexS1�x shell, prepared in

colloidal solutions and embedded into polymermedia. Saturation intensities were obtained, andQ-switched output pulse energies were demon-strated. Analysis of the transmission versus pulselight intensity was carried out according to a modelthat includes ground-state as well as excited-stateabsorption. For pulses �10 ns long, the NCs act asfast saturable absorbers. The theoretical fits yieldground-state absorption cross-section �gs � 10�16–10�15 cm2, excited-state absorption cross-section�es � 10�16 cm2, and effective lifetime �eff � 5 �10�12 s.

2. Experimental

A. Synthesis and Characterization of PbSe, PbSe�PbS,and PbSe�PbSexS1�x Nanocrystals

The synthesis of PbSe core NCs, PbSe�PbS core�shellNCs, and PbSe�PbSexS1�x core�alloyed-shell NCs isdescribed elsewhere in detail.31 A brief description isgiven here for completeness of presentation. The goalwas to synthesize high-quality NCs active as satura-ble absorbers at 1.54 �m, of improved surface qualityand chemical robustness. PbSe and PbS crystals ex-hibit identical crystallographic rock-salt structure,symmetry space group Fm3̄m,32 with only a 3% crys-tallographic mismatch (unit cell dimensions of 6.12 Åfor PbSe and 5.94 Å for PbS).15,33 Their NCs exhibittunability of the band absorption edge as a function ofsize in the vicinity of 1.54 �m. The synthesis involvednew procedures for the formation of PbSe�PbScore�shell NCs and PbSe�PbSexS1�x core�alloyed-shell NCs, both coated by organic ligands [oleic acid(OA) and tri-octyl phosphine (TOP)]. PbSe core NCsare formed by injection of Pb and Se precursors[Pb�Ac�2 and TOP:Se, respectively] into a preheatedmother solution, composed of OA, TOP, and phe-nylether at 180 °C. For the PbSe�PbS core�shellNCs’ preparation, PbSe cores were initially preparedand then were epitaxially coated by PbS shells(hereafter referred to as a two-step procedure).PbSe�PbSexS1�x core�alloyed-shell NCs are pro-duced by simultaneous injection of Pb �Pb�Ac�2�,Se �TOP:Se�, and S �TOP:S� precursors into a pre-heated mother solution, composed of OA, TOP, andphenylether at 180 °C (hereafter referred to as asingle-step procedure). The process involves the cre-ation of embryonic PbSe nuclei, and a delayed pre-cipitation of PbSexS1�x alloyed shells on the PbSe coresurfaces.

The colloidal NCs were embedded in a polymethylmethacrylate (PMMA, �-CH2C�CH3��CO2CH3�-�n, an-alytical grade, Aldrich) polymer film for the opticalmeasurements and as the laser Q-switching device.The polymer was prepared by mixing PbSe NCs withPMMA in a chloroform solution. The mixture wasthen spread on a quartz substrate, and left to dry over24 h to form a uniform film with a thickness of a fewhundred micrometers.

The entire procedure described is based on a wetchemistry technique, utilizing cheap materials, and

1 October 2006 � Vol. 45, No. 28 � APPLIED OPTICS 7489

inexpensive equipment, and the reaction takes onlyseveral hours to complete. These facts would rendertechnologies based on the process very cost effective.

The morphology and crystallography of the colloi-dal NCs were examined by transmission electron mi-croscopy (TEM), high-resolution TEM (HR-TEM) andselected-area electron diffraction (SAED). TEM stud-ies, combined with SAED, were carried out on aJEOL 2000FX instrument, operated at 200 kV. High-spatial frequency noise of the SAED image wasfiltered out using the built-in numerical package soft-ware of the measuring system, based on the Fouriertransform of the original image. HR-TEM imageswere recorded with a JEOL 3010 instrument oper-ated at 300 kV. The TEM specimens were preparedby injecting small liquid droplets of the solution on aCu grid (300 mesh) coated with amorphous C film andthen dried at room temperature.

The absorbance spectra were recorded using anultraviolet–visible–near-infrared (UV–VIS–NIR)Jasco V-570 spectrometer. The photoluminescence(PL) spectra were obtained by exciting the sampleswith a Ti:sapphire laser (wavelength between 690and 820nm). Emission was recorded using an Actonmonochromator (Model 300) equipped with a cooledGe detector. All the measurements were carried outat room temperature.

The PL quantum yield was measured utilizing anintegrating sphere technique, described by de Melloet al.34 A solution of NCs was placed inside an inte-grating sphere (Labsphere, Incorporated IS-040-SLwith UV–VIS–NIR reflectance coating) and excitedby a monochromatic light (Xe lamp, fiber coupled to amonochromator). The spectra were detected by afiber-coupled spectrometer equipped with a liquidnitrogen-cooled Ge photodetector, coupled to a lock-inamplifier. The entire system response was normal-ized against a calibrated detector and care was takento ensure that the sample absorption was more than20%.

B. Saturable Absorption Measurements

For transmission saturation measurements, we used1.537 �m signal output pulses from a 1.064 �mpumped KTiOPO4 (KTP) optical parametric oscillator(OPO). The output (signal) beam was vertically polar-ized with a near-TEM00 transverse energy distribu-tion, energy up to 150 mJ�pulse, and duration of10 ns (FWHM). A lens of 15 cm focal length focusedthe beam. The laser pulse fluence (energy density)hitting the sample was varied by moving the samplealong the propagation axis of the laser beam. Thebeam transverse energy distribution at each pointwas measured using the knife-edge scanning method.The incident and the transmitted-beam energieswere measured with an Ophir energy meter.

C. Laser Passive Q-Switching Experiment

Laser passive Q-switching experiments were per-formed by inserting a NC saturable absorber sampleinside the Er:glass laser resonator (Fig. 1). One sam-ple consisted of PbSe NCs with an average diameter

of 6.0 nm suspended in chloroform and placed in a1.54 �m antireflective (AR) -coated quartz cuvette.Two other samples consisted of the NCs incorporatedpolymer film sandwiched between two AR-coatedglass windows. The Er:glass laser consisted of a4.0 cm long laser rod with a 3.0 mm diameter, housedin a diffused reflector cavity equipped with a linear Xeflash tube. The flash tube was powered by a currentpulse with a duration of approximately 700 �s(FWHM). The laser resonator consisted of a concaveback mirror, 100% reflecting at 1.54 �m, and a flatoutput coupler with a reflectivity of 85%. The dis-tance between the mirrors was approximately 7 cm.Due to the large absorption cross section ofthe NCs at 1536 nm as compared to the stimulatedemission cross section of Er3 in phosphate glass,intraresonator focusing was not necessary for Q-switching action. The laser output energy was mea-sured using an Ophir Model PE50BB energy meter.The pulse temporal behavior was measured using afast p-i-n InGaAs photodiode, with a response time of�175 ps, and recorded by a Tektronix TDS 350 oscil-loscope with a bandwidth of 200 MHz.

3. Results and Discussion

A. Characterization of the PbSe, PbSe�PbS, andPbSe�PbSexS1�x Nanocrystals

Figure 2(a) shows a HR-TEM image of PbSe�PbScore�shell NCs with a 4.8 nm core diameter, pre-pared by a two-step procedure. HR-TEM image of aPbSe�PbSexS1�x NC, prepared by a single-step proce-dure, is presented in Fig. 2(b). These images revealthe formation of spherical NCs with well-resolvedcrystal planes, without distinct boundaries at thecore–shell interface. The increase in the core–shellNC size by 1.2 nm as compared to the preliminarycore NCs is consistent with a PbS or PbSexS1�x shellthickness of one monolayer (ML). Figure 2(c) providesSAED measurement of the sample shown in Fig. 2(a).The image is of a cubic single crystal, with a latticespacing of 6.12 Å relevant to the rock-salt structure ofspace group Fm3̄m. Figure 2(d) presents a TEM im-age of PbSe�PbSexS1�x NCs with 6.7 nm diameter,prepared by a single-step procedure. The ensemble ofNCs in the image exhibits a 5% diameter distribu-tion.

The room-temperature absorption and PL spectra ofPbSe�PbS core�shell NCs, prepared by a two-stepprocedure and composed of a 4.9 nm PbSe core,n-monolayered by PbS shell �n � 0, 1, 3�, are pre-sented in Fig. 3(a). These spectra exhibit a systematic

Fig. 1. Schematic of an Er:glass laser cavity, including a passiveQ switch. For details, see text.

7490 APPLIED OPTICS � Vol. 45, No. 28 � 1 October 2006

redshift with the increase of the shell thickness andof the core diameter [Fig. 3(b)]. A cubic rock salt PbSesemiconductor exhibits direct band transitions at the L

point of the Brillouin zone, with a fourfold degeneracy.Currently, there are two main suggestions for the elec-tronic configuration of the IV–VI compound NCs,based either on an effective mass approximation35 oron a tight-binding model.36 According to the effectivemass approximation, and ignoring the L point degen-eracy, the allowed transitions in core PbSe NCs obeythe �j � 0, �1, and �e, �h � �1 selection rules, whenj and � designate the total angular momentum andparity of the electronic state. The lowest absorptionband corresponds to the (j � 1�2, � � 1) to �j � 1�2,� � �1) transition (indicated hereon as 1S-excitontransition). The third lowest absorption band corre-sponds to the (j � 1�2 or 3�2, � � �1) to �j � 1�2 or3�2, � � 1) transition (indicated as 1P-exciton tran-sition). The parity-forbidden transition (j � 1�2, �� 1) to �j � 1�2, 3�2, � � 1) or (j � 1�2 or 3�2, �� �1) to �j � 1�2, � � �1) becomes partially alloweddue to mixing of the degenerate states and is respon-sible for the second lowest absorption band. Alterna-tively, tight-binding calculations suggested that theso-called forbidden transitions actually correspond tothe allowed 1P transitions, with energy just abovethat of the 1S exciton. The 1S exciton �|1�) and anexcited state above it �|2�), relevant to the forthcom-ing discussion, are drawn schematically in Fig. 4.Other notations in this diagram and their relevancyto the saturable absorption will be further discussedlater in this paper.

Both absorption and PL bands of the PbSe�PbScore�shell NC samples are redshifted by up to150 meV with an increase of the PbS shell thickness

Fig. 2. (a) HR-TEM image of a 6.1 nm PbSe�PbS core�shell NCwith a 4.8 nm core using 1:1:1.5 Pb:Se:S molar ratio in the mothersolution; (b) HR-TEM image of a 6.1 nm PbSe�PbSexS1�x core�alloyed-shell NC with Pb:Se:S molar ratio similar to the sample in(a). Bar scales in (a) and (b) are 5.0 nm. (c) SAED picture of thesample in (a); (d) TEM image of 6.7 nm PbSe�PbSexS1�x core�alloyed-shell NCs, arranged in an ordered assembly on a TEM grid.Bar scale is 20.0 nm.

Fig. 3. (Color online) (a) Absorbance (dashed curves) and PL (bold curves) spectra of PbSe�PbS core�shell NCs with 4.9 nm PbSe coreand n MLs of a PbS shell �n � 0, 1, 3�; (b) three-dimensional plot of the 1S-exciton emission energy versus PbSe core diameter and versusnumber of PbS MLs.

1 October 2006 � Vol. 45, No. 28 � APPLIED OPTICS 7491

from 1.2 nm (1 ML) to 3.6 nm (3 MLs). The 1S-excitonic band FWHM of the PbSe�PbS core�shellNCs is narrower by a factor of �1.5 compared withthat of the PbSe core. The measured 1S-excitonemission energies versus core diameter and versusshell thickness are plotted in Fig. 3(b), corroborat-ing the redshift of the 1S-excitonic emission uponincrease in the shell thickness. That redshift is ex-plained by the low potential barrier of only �0.1 eVat the PbSe–PbS interface, allowing the carriers’wavefunctions to extend over the entire core–shellstructure.37–39 The redshift may alternatively be ex-plained by the anomalous sequence of the valenceand conduction band-edge energies in the bulk com-pounds, when EC�PbS� � EC�PbSe� � EV�PbS� �EV�PbSe�.40 This suggests the optical transitions inthe core–shell PbSe�PbS NCs to be of type II, withthe electron distributed mostly at the PbSe conduc-tion band and the hole distributed mostly in the PbSvalence band. This type-II alignment could be pre-served upon confinement in the nanoscale regime,depending on the ratio between the core radius andthe shell thickness. It is worth noting that the Stokesshift between absorption and emission bands in thecore�shell samples decreases gradually to zero withthe shell thickness, occasionally even showing anti-Stokes behavior, as discussed below.

In general, the robustness of PbSe�PbS NCs, sus-tained over several months, is substantially betterthan equivalent core samples; the latter exhibit vari-ation in the absorption energies and fluorescencequantum yield after storage of only several weeks.

The room-temperature absorption and PL spectraof PbSe�PbSexS1�x core�alloyed-shell NCs, preparedby a single-step procedure with increasing S contentand reaction duration, are shown in Fig. 5(a). Energydispersive x-ray analyses (EDAX) were performed onaliquots drawn from the PbSe�PbSexS1�x formationreaction using a Pb:Se:S molar ratio of 1:1:0.5. Theanalyses indicate that, up to 5–8 min after the onsetof the reaction, only the core NCs had formed.

The reaction then continues by deposition of thePbSexS1�x alloyed shell. The 1S-exciton absorptionand emission energies of the core�alloyed-shell sam-ples are slightly redshifted with respect to those ofthe core NCs, most likely due to a spread of the car-riers’ wave functions over the entire NCs’ volume. Ablueshift rather than a redshift would have been an-ticipated if the single-step procedure produced simpleuniform PbSexS1�x-alloyed NCs.

Quite unexpectedly, the emission bands show aStokes shift ��12 meV� that is converted into an anti-Stokes shift ��52 meV� with an increase in the Scontent [compare the emission bands in Fig. 5(a)]. Asmall anti-Stokes shift ��9 meV� is also resolved inthe spectra of core PbSe with the reduction in size, asshown in Fig. 5(b). Mechanisms causing the occur-rence of an anti-Stokes shift are not clear, and thiseffect calls for further investigation. It could, per-haps, be caused by an initial excitation from an im-purity state, as proposed by Poles et al.41 It couldalternatively be associated with a small electron-holeexchange interaction.

The measured PL quantum yield � of PbSe coreNCs, and PbSe�PbS core�shell NCs was typically40%, while that of the PbSe�PbSexS1�x core�alloyed-shell NCs showed a typical value of 60%, as discussedin Ref. 31. Resolution of the 1S absorption peak al-lows calculation of its radiative lifetime, �R, using thefollowing expression42:

�R�1 �

8�n2

Nc2 0

�

2�� �d , (1)

Fig. 4. Schematic of the ground and excited states in PbSe NCs.�gs and �es correspond to the ground-state and excited-state ab-sorption cross sections, respectively. �i �i � 1–4� refers to the non-radiative decay times between the designated states, while �R

corresponds to the radiative emission lifetime.

Fig. 5. (a) Absorbance (dashed curves) and PL spectra (solidcurves) of PbSe�PbSexS1�x core�alloyed-shell NCs, with a gradualincrease of the S content, the molar fraction x varying between 1.0and 0.5 with approximately �0.1 steps from bottom to top. (b)Absorbance (dashed curves) and PL spectra (solid curves) of PbSecore NCs, with a diameter D ranging between 4.0 and 6.0 nm, with0.25 nm steps from bottom to top.

7492 APPLIED OPTICS � Vol. 45, No. 28 � 1 October 2006

where �� � is the frequency � dependence of the 1Sabsorption coefficient �, n is the medium refractiveindex, c is the speed-of-light constant, and N is thevolume density of absorbing states. N is composed ofthe NCs’ density multiplied by an eightfold degener-acy of a single NC (fourfold degeneracy of the L pointof the Brillouin zone times the two spin states). Itshould be noted that the eightfold degeneracy hasbeen shown experimentally by Schaller et al.20,43 Theobtained values of �R for the various samples aresummarized in Table 1. The results range between 80and 170 ns, which are shorter than values reportedby Du et al.17 and Wehrenberg et al.18, who proposeda radiative lifetime of �200–800 ns. The expectedactual exciton decay time (fluorescence lifetime) isgiven by �f � ��R. These values are also listed in Table1 and range between 36 and 76 ns.

B. Saturable Absorption in PbSe, PbSe�PbS, andPbSe�PbSexS1�x Nanocrystals

Figure 6 shows the 1.54 �m pulse energy transmis-sion versus the laser peak pulse intensity (upperscale) through the NCs solution. For example, in thecase of PbSe core NCs (top curve) the transmissionstarts at about T0 � 62% in the low-power range,rises with increasing pulse power, and saturates atabout Tmax � 77% in the high-power range. This gen-eral behavior is obtained for all the samples. We in-terpret this behavior as the result of depletion of theground state by the photons that arrive first andreduce the absorption probability for the photons thatfollow. The fact that the transmission never reaches100% is explained by the existence of excited-stateabsorption.

At first glance, conditions for considering the samplestudied as a slow absorber are approximately satisfiedin the present experiments: the pulse duration of�10 ns is indeed shorter than the 1S singlet fluores-cence lifetime, �f of 35–75 ns, estimated from the ab-sorption spectrum (Table 1). This approach, however,yields a serious inconsistency: consider, for example,the data of the top curve in Fig. 6. On the one hand,the ground-state absorption coefficient, �gs ���1�NL�ln T0, where N is eight times the NCs den-sity and L � 1 mm is the sample thickness, yields�gs � 2.05 � 10�16 cm2. The saturation fluence thusexpected, JS h ��gs, would then be approximately

600 �J�cm2. This result is 3 to 4 orders of magnitudelower than the experimental value of �1 J�cm2. Theconclusion is then that the sample acts, in fact, as afast saturable absorber, namely, that its effective 1Slifetime �eff is short compared to the pulse duration of�10 ns.

Theoretical expressions for transmission of a fastsaturable absorber, which also exhibits excited-stateabsorption, have been developed.44 They are given asa closed equation for the transmission T Itr�I0,where Itr and I0 are the transmitted and incidentpulse power densities, respectively,

I0 �S�1 � �T0�T�1�D�

�T0�T�1�D � T , (2)

where S 1��eff �gs and D ��gs � �es���es. A bestfit of the data in Fig. 6 (top) to Eq. (2) yields�gs � 2.05 � 10�16 cm2, �es � 1.1 � 10�16 cm2, and

Fig. 6. Transmission versus laser intensity of 5.6 nm PbSe NCs(top), 5.8 nm PbSe�PbS 1.5 monolayered core�shell NCs (middle),and 6.0 nm PbSe�PbSexS1�x core�alloyed-shell NCs (bottom), allsuspended in chloroform. The dots represent the experimentaldata; solid curves correspond to the simulation, utilizing Eq. (2).

Table 1. Fluorescence Quantum Yields �, Radiative Lifetimes �R, andFluorescence Lifetimes �f for Several Representative NC Samplesa

CompositionD

(nm)N0

(1015 NCs�mL)�R

(ns)�

(%)�f

(ns)

PbSe 5.6 2.9 170 40 68PbSe 5.4 3.1 160 40 64PbSe�PbS

1.5 MLb5.8 1.5 80 45 36

PbSe�PbSe0.5S0.5 6.0 1.3 140 55 76PbSe�PbSe0.5S0.5 6.0 2.2 80 55 44

aThe NCs’ diameters D and concentrations N0 are indicated.bCorresponds to a PbS shell thickness of 0.9 nm on average.

1 October 2006 � Vol. 45, No. 28 � APPLIED OPTICS 7493

�eff � 4.8 ps. One should address these estimates withsome caution. Should there exist an inhomogeneouscontribution to the absorption peak width, only afraction of the NCs actually take part in the absorp-tion (such broadening may be caused, for example,by deviations in the NCs’ diameter). Allan andDelerue36 recently calculated a homogeneous widthof approximately 30 meV of the 1S exciton in PbSecaused by splitting among its four degenerate states.Our measured absorption curves exhibit widths ofapproximately 120 meV (Fig. 5). Accordingly, the fit-ted �gs and �es should be regarded as lower limits tothe true values, and the fitted �eff as an upper limitone. The saturation power density defined as IS h S � h ���eff�gs� is then calculated as IS � 137MW�cm2. Similar results were obtained for differentsamples, those shown in Fig. 6, as well as others, andare summarized in Table 2. In all cases, the effective1S-exciton lifetime �eff is of the order of several pico-seconds.

The conclusion of �eff being that short contradictsthe previously presented results and analyses con-cerning the nature of the 1S excitons, namely, thatthe radiative lifetime �R is of the order of tens ofnanoseconds, and the fluorescence quantum yield isabout 50%. Note, however, that these results relate toa case of excitation by very low light intensity, assur-ing that the number of excitons in a single NC is nomore than one. Schaller and Klimov19 and Ellingsonet al.22 recently showed that biexcitons in a single NC

recombine very rapidly, on a time scale of 20–50 ps.Since every NC can accommodate eight excitons,multiexciton interactions set in under saturation,which apparently renders their effective recombina-tion time even shorter—consistent with the 2–5 psfigure obtained by the transmission saturation exper-iments (Fig. 6). In addition, Harbold et al. recentlyobserved picosecond time-scale relaxation of chargecarriers in PbSe NCs.45

C. Use of the Saturable Absorbers in an Er:Glass Laser

The occurrence of absorption saturation in the NCsamples (Fig. 6) suggested testing their use as pas-sive Q-switching devices in an Er:glass laser. Thispoint is not a trivial matter, since being a fast satu-rable absorber also implies losses when inserted intothe cavity.

Figure 7 provides an oscilloscope trace of the Er:glass laser output under Q-switching conditions.Under low-energy pumping above threshold, only asingle pulse appeared. When pumped by higher en-ergy, a second pulse joined in. The duration of eachpulse is about 50 ns (FWHM) and the wavelength(separately measured) was 1.54 �m. These pulses are

Fig. 7. Oscilloscope trace of a single output pulse of an Er:glasslaser cavity (plot of intensity versus time) using PbSe NCs colloidalsolution as a Q switch.

Fig. 8. Output versus input pump energies of an Er:glass laserunder free-running and under passive Q-switching conditions.Solid squares, free-running; open circles, PbSe�PbS NCs inPMMA; solid circles, PbSe�PbS NCs in chloroform. Scale for theQ-switched output is on the right.

Table 2. Best Fit Values of �gs, �es, �eff, and IS, Using Eq. (2) for Several Representative NCs’ Samplesa

CompositionD

(nm)N0

(1015 NCs�mL)IS

(MW�cm2)�gs

(10�16 cm2)�es

(10�16 cm2)�eff

(ps)

PbSeb 5.6 2.9 137.0 2.05 1.1 4.8PbSe 5.4 3.1 139.7 1.25 0.55 7.4PbSe�PbS 1.5 MLb 5.8 1.5 45.8 4.7 2.45 6.0PbSe�PbSe0.5S0.5

b 6.0 1.3 45.2 2.6 1.15 11.0PbSe�PbSe0.5S0.5 6.0 2.2 117.0 2.4 1.45 4.6

aThe NCs’ diameters D and concentrations N0 are indicated.bExperimental data are shown in Fig. 6.

7494 APPLIED OPTICS � Vol. 45, No. 28 � 1 October 2006

typical for passive Q switching of the laser due togradual bleaching of the saturable absorber, whichthen allows the development of lasing in the reso-nance cavity. Short recovery of the excited NCs bydecay to the ground state enables the buildup of anew population inversion in the laser rod by the con-tinuing pumping flash, which then established con-ditions for a new lasing pulse.

In Fig. 8 we show the laser output versus inputpump energy of the free-running and of the passivelyQ-switched laser. The free-running laser exhibitsthreshold energy of 4.5 J, and a slope efficiency of1.8%. Pulse durations were approximately 500 �s.The two passively Q-switched cases (using a PbSe�PbS 6.0 nm diameter NCs in the polymer, and PbSe5.5 nm diameter NCs in chloroform) exhibit in-creased thresholds of 5.8 and 7.3 J, respectively, andslope efficiencies of 0.17% and �0.2%, respectively,and pulse durations of approximately 50 ns (Fig. 7).Both Q-switched curves reveal two steep steps, eachstep raiser reflecting the appearance of a new pulse.Similar steps were observed in the passive Q switch-ing of Nd:YAG lasers.46 Table 3 presents a summaryof the Q-switching performance of different sampletypes.

Diode laser pumping, where pump wavelength isaccurately tailored to the absorption band of the Er:glass laser system, will be more efficient. In that re-spect, our results indicate a lower limit for thecapability of the PbSe�PbS NCs to function as Q-switching devices. In other words, diode pumping ofthe same laser system should result in higherQ-switched pulse outputs. It is our estimation, thatmeasures like insertion of a long pass optical filter infront of the Q switch that cuts out the VIS and NIRstray light (below �1 �m) from the broad flashlampemission, and optimization of the Q-switch darkoptical density, could also considerably increase theflashlamp pumped laser efficiency.

In Table 4 we compare the spectroscopic charac-teristics of some other saturable absorbers studiedfor the near-1.54 �m spectral region to those ofthe PbSe�PbS core�shell NCs of our present study.The latter differ by two prominent basic features: theeffective lifetime (relaxation time) �eff is many ordersof magnitude shorter, and the ground-state absorp-tion cross-section �gs is many orders of magnitudelarger. Thus the other materials [except for the semi-conductor saturable absorber mirror (SESAM) case]can be regarded as slow absorbers (effective lifetimelonger than the typical Q-switched laser output pulsewidth of �50 ns). The PbSe�PbS core�shell NCs, onthe other hand, are definitely fast absorbers under allcircumstances. The property that renders them suit-able for Q switching of the Er:glass laser is their hugeground-state absorption cross section, which allowsthe saturation of their absorption at reasonable pulseintensities. While the PbSe�PbS core�shell NCs seemto be equivalent to other alternatives for Er:glasslaser passive Q switching, they seem to have a higherchemical stability, and they can be tuned to operatein different spectral regimes. We may also considerthat the same material may serve for laser modelocking in the NIR spectral region; this is a straight-forward idea based on the fact that the material is afast saturable absorber with a recovery time of onlyseveral picoseconds. We intend to investigate thisaspect in the future.

4. Summary

The functionality of PbSe core NCs, PbSe�PbS core�shell NCs, and PbSe�PbSexS1�x core�alloyed-shellNCs, as passive Q switches in Er:glass laser cavity

Table 3. Results of Q-Switching Performance Tests

Parameter

PbSe CoreNCs in

Chloroforma

PbSe NCCore inPMMAa

PbSe�PbSCore�Shell

NCs inPMMAa

Transmissionat 1.54 �m

90% 87.4% 86.0%

Pumpingenergy

7 J 7.5 J 8.8 J

Free runningpulse energy

45 mJ 50 mJ 77 mJ

Q-switchedpulse energy

0.8 mJ 2.0 mJ 3.5 mJ

Pulse FWHM 50 ns 53 ns 40 nsQ-switched

pulse power1.6 104 W 3.8 104 W 8.75 104 W

Q-switchingefficiencyb

1.7% 4% 4.5%

aNCs external surface was covered with OA–TOP surfactants.bEfficiency defined as (Q-switched pulse energy�free-running

pulse energy) 100%.

Table 4. Spectroscopic Characteristics Summary of Various Saturable Absorber Q Switches Demonstrated for Er:Glass Lasers

Q Switch

RelaxationTime(ns)

�gs

�cm2��es��gs

�cm2� References

Co:LaMgAl11O10 220 1.2 10�19 0.07 28Co:MgAl2O4 350 3.5 10�19 0.03 29Co:ZnSe 2.9 105 5 to 5.7 10�19 0.05–0.22 47, 48Cr:ZnSe 8 103 2.7 10�19 0.075 47U:CaF2 4.7 103 7 to 9 10�19 0.4 1, 48V:YAG (at 1.34 �m) 20–30 7.2 10�18 0.1 49, 50PbSe NCs 10�3–10�2 2 10�16 0.5 Present study

1 October 2006 � Vol. 45, No. 28 � APPLIED OPTICS 7495

operating at � � 1.54 �m was examined. Prior to theQ-switching experiments, the saturable absorbingproperties of the indicated NCs were investigated.Theoretical fits to the experimental saturation curvesrevealed �gs values of 10�15�10�16 cm2 and �eff of5–10 ps, confirming that the NCs act as fast satura-ble absorbers for the used 10 ns long laser pulses.Although the NCs effective lifetimes under intensepumping were found to be much shorter than theexciting laser pulse, they showed comparable perfor-mances for Q switching regarding other inorganicmaterials (Table 4). This is a result of the relativelyhigh �gs values, which compensate for the short life-time. The advantages of the PbSe�PbS core�shelland PbSe�PbSexS1�x core�alloyed-shell NCs are thechemical stability over a long period of time, the op-tical stability for many illumination cycles, and thepossibility of tuning their wavelength of operationwithin the NIR range. These NCs showed promisingfunctionality as passive Q-switching devices. Thepossibility of using them for laser mode-locking ap-plications is predicted to be more beneficial and willbe examined in the future.

This project was supported by the Israel Sci-ence Foundation (ISF), project 156�03-12.6, by theGerman–Israel Project Cooperation (DIP) grant D3.2, and by the Ministry of Industry, Trade andLabor, Magneton grant 1000052. M. Brumer ex-presses her gratitude to the Israel Ministry of Sci-ence and Education, Eshkol grant 867. We thankLilac Amirav for performing the TEM and HR-TEMmeasurements.

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