28 J. Opt. Soc. Am. B/Vol. 19, No. 1 /January 2002 Malyarevich et al.

Glass doped with PbS quantum dots as a saturableabsorber for 1-mm neodymium lasers

A. M. Malyarevich, V. G. Savitski, P. V. Prokoshin, N. N. Posnov, and K. V. Yumashev

International Laser Center, 65/17 F. Skaryna Avenue, 220027 Minsk, Belarus

E. Raaben and A. A. Zhilin

S. I. Vavilov State Optical Institute, 197131, St. Petersburg, Russia

Received March 28, 2001; revised manuscript received July 23, 2001

Saturable-absorber mode locking and Q switching of neodymium-doped lasers at 1.06 mm with a PbS-dopedglass were demonstrated. Q-switched pulses of 0.3 mJ in energy and 15 ns in duration from a cw diode-pumped Nd31:KGd(WO4)2 laser and ultrashort pulses of 18 mJ in energy and 70 ps in duration from aNd31:Y3Al5O12 laser were obtained. The saturation intensity of the PbS-doped glass (with an average semi-conductor crystallite radius of 1.7 nm) was estimated to be 2.3 MW/cm2 at 1.06 mm. The bleaching relaxationtime was measured to be 23 ps. © 2002 Optical Society of America

OCIS codes: 140.4050, 140.3540, 140.3580, 160.2750, 160.6000

Nanosecond and ultrashort laser pulses are finding grow-ing applications in various fields of human activity: tele-communications, biomedicine, chemistry, and materialprocessing. The mode-locking technique is necessaryfor the generation of picosecond or femtosecond laserpulses. Passive modulators for this purpose seem prefer-able to active ones because of their compactness, simplic-ity, and low cost, which is why such systems are exten-sively studied. For passive mode locking of 1-mm lasers,only dyes are accepted as commercial products, in spiteof their well-known disadvantage of rather fast degrada-tion. Other alternatives are color-center crystals(LiF2:F2

2, Ref. 1, RbBr-Ba:Z2, Ref. 2); impurity-dopedcrystals (GSGG:Cr41, Ref. 3); CuInS2xSe2(12x)-dopedglasses, Ref. 4; and semiconductor saturable absorbermirrors (SESAMs), Refs. 5 and 6.

Recently PbS-doped glass was used as a saturable ab-sorber for a 1.2–1.3-mm Cr:forsterite laser, and pulses of4.6 ps in duration at a 110-MHz repetition rate wereobtained.7 Semiconductor PbS was embedded in a glassymatrix as nanometer-sized crystallites. The small sizesof the nanocrystallites (or quantum dots QDs), give rise tothe quantum confinement effect, which is seen for semi-conductor crystallites of sizes comparable with the excitonBohr radius. Quantum confinement manifests itself as ablueshift of the energy bandgap, an enhancement of thenonlinearity in the range of the lowest-energy transition8

(first excitonic resonance). PbS is attractive for QD-doped glass fabrication because it has a large bulk excitonBohr radius (18 nm) and a narrow bandgap (0.28 eV).This combination allows one to obtain strong quantumconfinement with relatively large crystallites and to shiftthe position of the absorption resonance over a wide spec-tral range from 3 to 1 mm. In this paper we examine PbSQD-doped silicate glass as a saturable absorber for mode

0740-3224/2002/010028-05$15.00 ©

locking and Q switching of neodymium solid-state lasersoperating at 1.06 mm.

Our silicate glass sample was prepared with aSiO2 –Al2O3 –NaF–Na2O–ZnO glass system. The opti-mal compositions and technological conditions (tempera-ture, time, and reducing conditions) were developed forthe synthesis of the initial series of colorless silicatesamples with a maximum semiconductor concentration(0.4–0.6 mol.%). The variation of heat treatment re-gimes yielded a high-volume PbS microcrystalline frac-tion with a specific nanocrystal size. This process con-sisted of two or more stages. The first one is a prolongedlow-temperature stage of nucleation (480 °C, 72 h). Thenext stages of sample treatment (for nanocrystal growth)were performed at higher temperatures and shortertimes. Optimal heat-treatment conditions allowed us toobtain samples with a specific spectral position of the firstexcitonic resonance. Figure 1 shows room-temperatureabsorption spectra of PbS QD-doped glasses with variousQD sizes. The correlation between time–temperature re-gimes and the position of the lowest-energy optical reso-nance of the glass materials obtained was found by analy-sis of small-angle x-ray scattering data (in theapproximation of scattering by monodisperse particles ofspherical shape).

Figure 2(a) illustrates the kinetics of bleaching relax-ation at 1.08 mm after laser excitation at the same wave-length for glass sample 1 with an average PbS QD radiusof 1.7 nm (curve 1 of Fig. 1). The kinetics was measuredby the pump–probe technique with 15-ps pulses from apassively mode-locked Nd:YAlO3 laser as pump and probebeams. The bleaching value was defined as the differen-tial absorption DOD 5 2log(T/T0), where T0 and T arethe transmissions of the probe beam without and with thepump beam present, respectively. The characteristic

2002 Optical Society of America

Malyarevich et al. Vol. 19, No. 1 /January 2002 /J. Opt. Soc. Am. B 29

time of bleaching relaxation was estimated to be t 5 236 2 ps. It should be noted that the bleaching relaxationtime of the lowest-energy excited state of PbS QDs in thesilicate glass, with an average QD radius of 2.5 nm, wasmeasured9 to be ;100 ps.

Absorption saturation measurements were been per-formed on the 1.2-mm-thick sample 1 by using aQ-switched Nd:YAG laser at 1.064 mm with a pulse dura-tion tP of 10 ns. The incident intensity was varied by aset of neutral filters. The dependence of the internal ab-sorption on the laser intensity is shown in Fig. 2(b) (filledcircles). This dependence has been modeled within theframework of a two-level-scheme fast saturable absorber(tP is much longer than the lifetime of the charge carriersin the lowest-energy excited state, which equals the

Fig. 1. Room-temperature absorption spectra of PbS QD-dopedsilicate glasses with QD mean radii of 1, 1.7 nm; 2, 2.2, nm; 3, 2.7nm.

Fig. 2. (a) Kinetics of bleaching relaxation at 1.08 mm after la-ser excitation at the same wavelength for glass sample 1 with anaverage QD radius of 1.7 nm; dashed line, linear fit of the experi-mental data (squares). (b) Intensity-dependent absorption ofPbS-doped silicate glass at 1.064 mm for 10-ns pulses from aQ-switched Nd:YAG laser.

bleaching relaxation time t ; 23 ps) with a nonsaturat-ing absorption coefficient aB :

a 5a0

1 1 I/Isat1 aB , (1)

where a0 is the saturating absorption coefficient, I is theinput intensity, and Isat is the saturation intensity. Thebest fit is shown in Fig. 2(b) as the solid curve. The satu-rable absorber parameters estimated are a05 0.87 cm21, aB 5 0.6 cm21, Isat 5 2.3 MW/cm2. Thenonsaturating background absorption coefficient aB in-cludes, probably, the exited-state absorption from the low-est excitonic state.

The fast bleaching relaxation time (23 ps) for the PbSQD-doped glass led us to believe that PbS-QDs dopedglass can be used for mode locking of neodymium solid-state lasers operating near 1 mm. We performed experi-ments with a PbS QD-doped glass saturable absorber anda flash-lamp-pumped Nd:YAG laser. A schematic of thelaser cavity is presented in Fig. 3(a). The �6.33 60 mm Nd:YAG rod was placed between the concave(radius of curvature 105 cm) high reflector and flat outputcoupler, with a transmittance at 1.064 mm of 40%. The1.2-mm-thick sample of PbS-doped glass (SA), with atransmittance at 1.064 mm of 70% with no antireflection(AR) coatings, was placed near the output coupler at thefocus of a 10-mm focal-length lens (L). For this cavityconfiguration laser, mode locking was obtained. Thetypical pulse train had an energy of 2.5 mJ. Without fo-cusing lens L, a pulse of free-running output was ob-served. It should be mentioned that mode locking in theCr:forsterite laser was also obtained with the PbS satu-rable absorber placed in the focus of a spherical mirror.7

To record the pulse train, we used an S1-75 oscilloscopeand a germanium photodiode. (The total temporal resolu-tion of the detecting system was ;2 ns.) A single picosec-

Fig. 3. (a) Schematic of the laser cavity for mode-locking experi-ments: SA, PbS QD-doped silicate glass plate; L, 10-mm focal-length lens. (b) Densitometer trace of the streak camera recordfor two pulses separated by a ;233-ps delay time. The durationof the individual ultrashort pulse is ;70 ps (FWHM).

30 J. Opt. Soc. Am. B/Vol. 19, No. 1 /January 2002 Malyarevich et al.

ond pulse width was measured with a streak camera(AGAT SF-1) with a temporal resolution of 4 ps. Withthe streak camera we detected two pulse trains, obtainedby reflection from the two surfaces of a glass plate andthus separated by an ;233-ps delay time. The pulsetrain was ;500 ns FWHM and consisted of ;135 ul-trashort pulses separated by an ;6.4-ns cavity round-triptime. Each ultrashort pulse had an energy of ;18 mJ.The densitometer trace of the streak camera record fortwo pulses from the trains separated by an ;233-ps delaytime is shown in Fig. 3(b). The pulse duration was mea-sured to be ;70 ps FWHM.

It is known10 that ultrashort-pulse formation in pas-sively mode-locked solid-state lasers is determined byboth the saturable absorber and cavity parameters andthe pump conditions. The analytical expressions thatyield the output pulse duration were deduced in Ref. 10within the framework of a fast-relaxing saturable ab-sorber:

t f 5 Y1/2t i , (2)

t i 52~ ln 2 !1/2Tcav

p1/20.57M@P~P 2 1 !#1/8@LTamp /Tcav#21/41.56

, (3)

Y 511.7@P~P 2 1 !#1/4@LTcav /Tamp#1/2

2 ln~1/T !, (4)

where t f is the output pulse duration; t i is the pulsewidth of the initial fluctuations, with the peak intensityin the laser radiation at the beginning of the nonlinearstage of mode locking; Tcav is the round-trip time of thecavity; M is the number of cavity modes; P is the relativepump energy (such that P 5 1 corresponds to a thresholdpump energy); Tamp is the pump pulse duration; and T isthe saturable absorber transmission, L 5 2 ln(1/T)1 ln(1/R) (R is the output mirror reflectivity).

For the set of our experimental parameters Tcav5 6.4 ns, M 5 1219, P 5 1.1, Tamp 5 150 ms, T 5 0.78,and R 5 0.6, Eqs. (2)–(4) give an initial pulse width oft i 5 91 ps and an output pulse width t f 5 27 ps. Thelatter is somewhat shorter than the pulse width obtainedexperimentally (70 ps) because analytical equations (2)–(4) do not account for the absorber recovery time (23 ps,which is of the same order of magnitude as the initialpulsewidth).

Recently PbS QD-doped silicate glass was used as asaturable absorber for Q switching of a diode-pumped1.5-mm microlaser.11 Therefore we decided to study ourPbS QD-doped glass as a saturable absorber for Q switch-ing of a cw diode-pumped solid-state laser operating near1 mm. Experiments with the diode-pumped laser wereperformed with a Nd:KGd(WO4)2 (KGW) crystal (lasingwavelength of 1.067 mm) as a gain medium, which, com-pared with Nd:YAG, has some advantages in the case of adiode-pumped solid-state laser. Its first advantage isthat a higher Nd31 concentration is achievable in KGWthan in YAG crystal, which leads to a higher slope effi-ciency at low pump powers than in Nd:YAG (Ref. 12) andmakes shorter gain crystals worth exploring, which is es-pecially important for microchip lasers. Second,Nd:KGW is one of the most efficient media for intracavityRaman conversion.13

One face of the 3-at. %-doped 3 mm 3 3 mm 3 3 mmpiece of Nd:KGW was high-reflection (HR) coated at 1.067mm and another was AR coated at 1.067 mm. The small-signal transmission of the gain medium at the pumpingwavelength lp 5 808.5 nm was 14%. To yield an ad-equate absorption at 1.067 mm for Q-switching experi-ments, the PbS QD-doped glass was polished down tolSA 5 0.5 mm. The sample of this thickness without ARcoatings had a small-signal internal transmission of T05 92%. The PbS QD-doped glass plate was placed nearthe gain medium. The output coupler was spherical mir-ror with a radius of curvature of 50 mm and with 2% or6% transmission at 1.067 mm. The total cavity lengthlcav was varied from 5 to 42 mm by movement of the out-put coupler relative to the saturable absorber and thegain medium. A 808.5-nm emitting laser diode mountedon a thermoelectric cooler was used for pumping theNd:KGW crystal. The optical system for pump beam fo-cusing provided a spot with a diameter of ;80 mm.

With 92% of the PbS QD sample internal transmission,a 6%-output coupler in a 5-mm-long cavity, and an ab-sorbed pump power of Pabs 5 260 mW, we obtainedQ-switched 15-ns FWHM pulses with an average outputpower of 5.3 mW at a repetition rate of 22.6 kHz. Themeasured output power and repetition rate dependencieson the absorbed pump power are shown in Figs. 4(a) and4(b). The pulse width was found to be independent of theabsorbed power. The pulse width increased up to 70 ns(at the same pump power) for a 42-mm cavity length,while the Q-switching slope efficiency remaining almostthe same [Figs. 4(a), 4(c)]. The output power, pulsewidth, and repetition rate dependencies on the absorbedpump power for a 42-mm cavity length and a 2%-outputmirror are presented in Figs. 4(a) and 4(d). The main re-sults of Q-switching operation with the PbS QD saturableabsorber are presented in Table 1.

Fig. 4. (a) Input–output characteristics of the Q-switched diode-pumped Nd:KGW laser with PbS QD-doped silicate glass as thesaturable absorber; squares, 42-mm cavity length, 2% outputcoupler; circles, 42-mm cavity length, 6% output coupler; tri-angles, 5-mm cavity length, 6% output coupler. (b)–(d) Outputpulse width (solid squares) and repetition rate (open circles) de-pendencies versus the absorbed pump power of the Q-switcheddiode-pumped Nd:KGW laser with PbS QD-doped silicate glassas the saturable absorber; (b) 5-mm cavity length, 6% output cou-pler; (c) 42-mm cavity length, 6% output coupler; (d) 42-mm cav-ity length, 2% output coupler).

Malyarevich et al. Vol. 19, No. 1 /January 2002 /J. Opt. Soc. Am. B 31

The results obtained were compared with parametersof the same laser Q-switched with Cr41:YAG—one of themost efficient (for the present) saturable absorbers fordiode-pumped 1-mm lasers. The 200 mm-thick Cr:YAGsample, AR coated at 1.06 mm, was placed in the 42-mm-long cavity, which consisted of the 2%-output 50-mm con-cave mirror and the piece of Nd:KGW mentioned above.With an absorbed pump power of 260 mW, we obtainedQ-switched 60-ns FWHM pulses with an average outputpower of 44 mW at a repetition rate of 81 kHz. TheQ-switching slope efficiency was estimated to be ;21%.For the same pumping conditions and cavity design thePbS saturable absorber yielded 70-ns FWHM pulses withan average output power of only 6 mW, at a repetitionrate of 90 kHz, with a Q-switching slope efficiency of;3.5% (Table 1). In our opinion, the higher optical qual-ity and AR coating of the PbS-doped glass saturable ab-sorber will lead to better parameters for Q-switcheddiode-pumped 1-mm neodymium lasers.

The Q switching of the Nd:KGW laser with the PbS QDsaturable absorber was simulated numerically by use ofthe rate equations for a Q-switched four-level gain me-dium derived by Szabo and Stein14 and modified for thecase of the fast saturable absorber (when the Q-switchedpulse width is longer than the lifetime of the lowest-energy excited state):

df~t !

dt5 H 2sgN~t !lg 2 2alSA 2 F lnS 1

R D 1 LG J f~t !

tr,

dN~t !

dt5 2gsgcf~t !N~t ! 1 Wp 2

N~t !

t,

a 5a0

1 1 f~t !/fsat1 aB , (5)

where f is the photon density (per cubic centimeter), N isthe population inversion density in the gain medium (percubic centimeter), sg is the stimulated-emission cross sec-tion of the gain medium (per square centimeter), lg andlSA are the gain medium and the saturable absorberlengths (centimeters), a is the saturable absorber internalabsorption coefficient (per centimeter), tr is the round-tripphoton transit time in the cavity (seconds), R is the out-put couple reflectivity, L is the unusable round-trip opti-cal losses, g is the inversion reduction factor,15 t is thegain medium upper-level lifetime (seconds), Wp5 hpPabslp /Vhc is the pumping rate (inverse secondsper cubic centimeter), hp is the pumping efficiency coeffi-

Table 1. Results of Q-Switched Operation for cwDiode-Pumped Nd:KGW 1.067 mm Laser with PbS

QD-Doped Silicate Glass Saturable Absorbera

Cavity length, mm 42 42 5

Output coupler reflectivity, % 98 94 94Pulsewidth, ns 70 70 15Output power, mW 6.0 6.0 5.3Pulse repetition rate, kHz 90 35 22.6Slope efficiency, % 3.5 8.3 8.1

a The absorbed pump power is 260 mW.

cient, V the pumping volume of the gain medium (percubic centimeter), fSAT 5 ISATltr /lSAhc is the saturationphoton density (per cubic centimeter), and a05 2ln T0 /lSA 2 aB . Equations (5) were solved numeri-cally with the initial conditions N(0) 5 @22 ln(T0)1 ln(1/R) 1 L#/2sglg , f(0) 5 1.

The parameters hp and g were adjusted to fit the pulsewidth and the repetition rate. The best agreement withthe experimental values for a cavity length of lcav5 4.2 cm, output mirror reflectivity of R 5 0.94, and ab-sorbed pump power of Pabs 5 250–325 mW was obtainedfor the next set of parameters: sg 5 3.8 3 10219 cm2

(Ref. 12), t 5 99 3 1026 s (Ref. 12), L 5 0.17, g 5 0.075,and hp 5 0.4.

For the above set of parameters the numerically simu-lated Q-switched laser demonstrated good agreement ofthe pulse width (;40–110 ns) with the corresponding ex-perimental values (;70–65 ns). The repetition rate wassomewhat higher (;70 kHz) than the corresponding ex-perimental value (;40 kHz).

In summary, saturable-absorber mode locking and Qswitching of neodymium-doped lasers at 1.06 mm with aPbS-doped glass were demonstrated. Q-switched pulsesof 15 ns (0.3 mJ) in duration (energy) from a cw diode-pumped Nd31:KGW laser and ultrashort pulses of 18 mJin energy and 70 ps in duration from a Nd31:YAG laserwere obtained. The numerically simulated Nd31:KGWlaser Q switched with PbS-doped glass demonstratedgood agreement of the pulse width and repetition ratewith experimental results. The saturation intensity ofthe PbS-doped glass (average radius of QDs, 1.7 nm) wasestimated to be 2.3 MW/cm2 at 1.06 mm. The bleachingrelaxation time was measured to be 23 6 2 ps.

A. M. Malyarevich may be reached at malyar@ilc.unibel.by.

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