triggered 
 by initial reports on lasers with mirrorless feedback , random lasers with noncoherent ( which essentially is amplified spontaneous 
 emission , ase ) and coherent feedback moved closer in the focus of extensive research activities .
coherent light amplification using high refractive index dielectric 
 or metallic nanostructures inside a gain medium can be seen as a promising 
 alternative to conventional lasers .
while a conventional laser requires 
 accurate fabrication and extremely precise alignment of the resonator , 
 the resonator in a coherent random laser is realized by randomly distributed 
 scattering nanoparticles forming closed - loops for photons .
this permits a cheap and easy fabrication method , making coherent 
 random lasers an attractive alternative to conventional lasers in 
 a manifold of potential applications , ranging from smart sensors and 
 medical diagnostics , to lighting devices and displays .
noble metal nanoparticles exhibiting localized 
 plasmon resonances provide two beneficial 
 features for coherent 
 random lasers : much higher scattering cross sections than dielectric 
 nanoparticles and high field enhancements close 
 to their surface , associated with localized plasmons .
it turns out that star - shaped gold nanoparticles , so - called gold 
 nanostars , outperform conventionally shaped gold nanoparticles such 
 as spheres and rods in terms of coherent random lasing .
gold nanostars exhibit multiple plasmon resonances 
 in the red and near - infrared spectral region .
these plasmon resonances associated 
 with the tips of the gold nanostars lead to large and spectrally broad - band 
 scattering cross sections as well as extremely strong field enhancements 
 localized at the tips , key features for coherent random lasers . however , despite 
 all of the efforts on plasmon - mediated coherent 
 random lasing , there are no reports on hybrid multilayered plasmonic nanostars - based 
 coherent random lasing . here , we report on the development of 
 hybrid multilayered plasmonic 
 nanostars , which can be mixed with gain materials dissolved in either 
 polar or nonpolar solvents .
two different , but in terms of optical 
 properties similar , gain materials , a dye molecule and a light - emitting 
 polymer , were dispersed in polar or nonpolar solvents , respectively . 

for extremely low pumping thresholds , the solutions containing gain 
 material and silver - enhanced and silica - coated nanostars exhibit spectrally 
 narrow lasing modes , typical for coherent random lasing .
further , 
 we show that silver - enhancement of the gold nanostars reduces the 
 pumping threshold for nanostar - based random lasing .
additionally , 
 we reveal that the silica coating nearly does not influence the operation 
 of the silver - enhanced gold nanostar - based random lasers in terms 
 of lasing intensity and pumping threshold , in contrast to gold nanostar - based 
 random lasers , where only ase and no coherent random lasing was observed 
 for the silica - coated gold nanostars .
silver nitrate ( agno3 , 99.9999% ) , gold(iii ) chloride trihydrate ( haucl43h2o ) , l - ascorbic acid ( aa ) , tetraethyl 
 orthosilicate ( teos ) , trisodium citrate dihydrate , 1 n hydrochloric 
 acid solution ( hcl ) , sodium borohydride ( nabh4 , 99% ) , hexadecyltrimethylammonium 
 bromide ( ctab ) , o-[2-(3-mercaptopropionylamino)ethyl]-o-methylpoly(ethylene glycol ) ( mpeg - sh , mw 5k ) , 
 ammonium hydroxide ( nh4oh , 30% ) , and rhodamine 6 g ( r6 g ) 
 were purchased from sigma - aldrich at the highest purity grade available . 

commercial poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene - vinylene ] 
 end - capped with polysilsesquioxane ( ads200re , purchased from american 
 dye source ) was used without further purification .
citrate - capped 
 spherical gold nanoparticles ( 125 nm in diameter ) were purchased from 
 nanopartz .
all glassware and stir bars were thoroughly cleaned with aqua regia 
 and dried prior to use .
ultrapure distilled water ( millipore , 18 m 
 cm ) was used in all steps of nanoparticle preparation .
the hybrid multilayered plasmonic nanostars were 
 synthesized according to the synthesis route in figure 1a .
subsequently , aliquots from the same solution 
 containing freshly synthesized aunsts were branched off for further 
 coating of the aunsts , either for coating with a silver shell for 
 plain silver - enhancement ( auagnsts ) , for silver - enhancement and subsequent 
 coating with a thin silica shell ( auagnsts@sio2 ) , or for 
 coating with a thin silica shell only ( aunsts@sio2 ) .
to 
 avoid aggregation and to improve the stability of the nanostars without 
 silica shell dispersed in ethanol , the ctab was replaced with thiolated 
 mpeg ( mpeg - sh ) in case of the aunsts and auagnsts via a ligand exchange 
 reaction .
sem images of a representative ( b ) aunst and of representative ( c ) 
 auagnsts .
the scale bars are 100 nm . insets : 
 schematics of the corresponding nanostars illustrating the layered 
 composition . for 
 the synthesis of the gold nanostars ( aunsts )
sodium citrate - functionalized gold nanoparticles of 35 nm 
 in diameter were used as seeds for the growth of the aunsts .
the gold 
 seed particles were prepared by adding 0.6 ml of ice cold 0.1 m nabh4 to a 20 ml aqueous solution containing 0.25 mm haucl4 and 0.25 mm sodium citrate under vigorous stirring .
for the growth of aunsts , 100 l of the aged seed 
 solution was added to 10 ml of an aqueous solution of 0.25 mm gold 
 chloride ( haucl4 ) , hereafter called growth solution .
the 
 ph was adjusted by adding 10 l of 1 n hydrochloric acid ( hcl ) . 

subsequently , 100 l of 2 mm silver nitrate ( agno3 ) and 50 l of 0.1 m ascorbic acid ( aa ) were simultaneously 
 added to the growth solution under vigorous stirring .
the nanostars 
 were functionalized with hexadecyltrimethylammonium bromide ( ctab ) ; 
 that is , 100 l of 0.1 m ctab was added to the growth solution , 
 which was left stirring for 5 min .
the solutions containing aunsts 
 were washed by centrifugation ( 2000 rcf for 20 min ) , and redispersed 
 in a 10 ml of 1 mm aqueous ctab solution .
such aunsts with long and 
 sharp tips , pointing in arbitrary directions , exhibit multiple plasmon 
 resonances , strong field enhancements located at their tips , as well 
 as large scattering cross sections .
for the silver - enhanced gold nanostars ( auagnsts ) , we followed 
 a method that was inspired by the method reported by fales et al .
5 l of 0.1 m agno3 and an 
 equivalent volume of 0.1 m aa were added as a precursor and as a reducing 
 agent , respectively , under vigorous stirring to 1 ml of ctab - functionalized 
 aunsts in solution .
after 15 min , the silver enhanced gold nanostars 
 were purified by centrifugation at 2000 rcf for 10 min , discarding 
 the supernatant , and redispersed in 1 ml of distilled water .
. the auagnsts still exhibit sharp tips , 
 but the branch length of the previously long gold nanostar tips protruding 
 from the core of the nanostars is reduced while the core diameter 
 is increased due to the silver - enhancement .
the exact 
 silver shell thickness of the auagnsts can hardly be measured precisely 
 with a tem , due to the limited material contrast .
however , we can 
 distinguish between the gold nanostar and the silver shell in high 
 angle annular dark field tem images , so that we estimate the silver 
 shell thickness to be in the range of 20 nm around the core of the 
 gold nanostar ( an example is shown in figure s3 ) .
further , from elemental line and mapping analysis of representative 
 auagnsts using energy dispersive x - ray spectroscopy ( edx ) , we assume 
 that the apexes of the tips are also covered by a very thin ( < 8 
 nm ) silver shell ( figure s3 ) .
the presented 
 synthesis methods for the aunsts and auagnsts allow a high reproducibility 
 with negligible batch - to - batch variations in the optical ensemble 
 spectra ( figure s4 ) . to obtain 
 a homogeneous and thin ( < 10 nm )
silica shell that 
 follows the morphology of the nanostars , the original recipe , which provides substantially thicker shells , 
 had to be modified .
the freshly synthesized and washed aunsts and 
 auagnsts were functionalized with thiolated mpeg ( mpeg - sh ) via a ligand 
 exchange reaction .
mpeg - sh was mixed with 1 ml of aqueous solution 
 containing nanostars to a final concentration of 5 m under 
 gentle stirring for 1 h. the pegylated nanostars were washed once 
 more by centrifugation ( 2500 rcf for 10 min ) to remove the residual 
 ligands and were redispersed in a mixed solution of ethanol ( 0.9 ml ) 
 and water ( 0.2 ml ) . under gentle stirring , silica ( teos ) coating of 
 the nanostars was initiated by adding 10 l of nh4oh followed by 3 l of 10% teos in ethanol .
the reaction was 
 allowed to proceed for 12 h. after that , the nanostars were washed 
 three times by centrifugation at 3000 rcf for 5 min and redispersed 
 in ethanol or chlorobenzene . in figure 1c and d , tem images of representative silica - coated 
 nanostars , aunst@sio2 and auagnst@sio2 ,
are 
 shown ( see figures s5 and s6 for more examples 
 of aunst@sio2 and auagnst@sio2 , respectively ) . 

the average silica shell thickness was about 10 nm based on tem image 
 analysis of several nanoparticles ( figure s7 ) . in both cases ,
the shell was uniformly and completely formed around 
 the nanostars , maintaining their morphology . as a major benefit ,
this 
 thin silica shell allows mixing of both nanostar types , aunsts@sio2 and auagnsts@sio2 , with polar ( ethanol ) as well 
 as nonpolar organic solvents ( chlorobenzene ) without causing aggregation . 

in addition , the silica shell of around 10 nm is thick enough to prevent 
 chemical degradation such as oxidation of silver in the case of auagnsts@sio2 .
it is worth rementioning that all of the displayed nanostars 
 stem from the same original batch of gold nanostars . for a period 
 of 10 h without stirring ,
figure s8 shows the corresponding 
 extinction spectra of auagnsts with and without silica shell dispersed 
 in ethanol recorded every hour for 10 h. all nanostars have similar 
 tip - to - tip diameters of about 100150 nm ( the corresponding 
 histograms can be found in figure s9 ) . 
 for reference ,
citrate - functionalized spherical nanoparticles ( aunps ) 
 with diameters of about 120 nm were also coated with silica using 
 the same procedure as for the nanostars ( examples of sem and tem images 
 of aunps and aunps@sio2 are displayed in figure s10 ) .
extinction 
 spectra were measured by a varian cary 500 scan uv  vis  nir 
 spectrophotometer . for the characterization of the morphology , solutions 
 containing aunsts and auagnsts were drop casted on cleaned ito substrates , 
 dried , and analyzed with a zeiss smart sem supra 55 vp scanning electron 
 microscope ( sem ) . for the transmission electron microscope ( tem ) characterization , 
 solutions containing silica - coated auagnsts and aunsts
were drop casted 
 on carbon - coated copper tem grids , dried , and analyzed using a jeol 
 jem-2010 tem . scanning tem ( stem ) was performed with bright 
 field ( bf ) and high angle annular dark field ( haadf ) detectors .
elemental 
 line and mapping analyses were carried out using energy dispersive 
 x - ray spectroscopy ( edx ) .
the samples were investigated with a jeol 
 jem-2200fs transmission electron microscope in stem mode operated 
 at 200 kv , equipped with an oxford sdd x - maxn ( 80 mm ) 
 edx - system . for preparation 
 of the randomly lasing systems ,
rhodamine 6 g ( r6 g ) and ads200re ( a 
 meh - ppv derivate ) were used without further treatment .
r6 g ( 2 mg ml ) or meh - ppv ( 7 mg ml ) dissolved 
 in ethanol or chlorobenzene , respectively , were used as stock solutions . 
 in the following ,
we refer to the r6 g dissolved in ethanol as to the 
 r6 g solution and to the ads200re dissolved in chlorobenzene as to 
 the meh - ppv solution .
the nanoparticle concentrations were controlled 
 by gentle centrifugation and redispersion in ethanol or chlorobenzene . 

aunsts and aunsts@sio2 in ethanol exhibit the same optical 
 density ( od ) at 405 nm ( d - band absorption of gold ) .
the auagnsts and 
 auagnsts@sio2 were directly synthesized out of a part of 
 the aunsts , so that the concentrations are considered to be similar . 

because all four types of the nanostars can be 
 dispersed in ethanol without aggregation , 0.3 ml of each type of the 
 nanostars dispersed in ethanol was mixed with 1 ml of the r6 g solution . 
 as a reference without any nanoparticles ,
0.3 ml of ethanol was mixed 
 with 1 ml of r6 g solution . in the case of the meh - ppv solutions , either 
 0.3 ml of aunsts@sio2 , or 0.3 ml of auagnsts@sio2 or silica - coated spherical nanoparticles ( aunps@sio2 ) 
 dispersed in chlorobenzene
the concentration of aunp@sio2 dispersion was adjusted 
 in such a way that its od at 405 nm ( d - band absorption of gold ) was 
 the same as the od of the aunst@sio2 dispersion ( figure s11c ) . as reference mixture of meh - ppv 
 without any nanoparticles ,
0.3 ml of chlorobenzene was mixed with 
 1 ml of meh - ppv solution .
the final mixtures contain similar concentrations 
 of nanostars / nanoparticles and either r6 g ( 1.5 mg ml ) or meh - ppv ( 5.4 mg ml ) . for more clarity
, we 
 denote each suspension as nanostar type / gain material , for example , 
 aunst / r6 g . for random 
 lasing experiments , quartz cuvettes with four transparent windows 
 ( inner dimensions 10 mm  4 mm ) containing 0.5 ml of the solutions 

we used a setup , which basically resembles 
 the variable stripe length method for 
 cuvettes , that is , a stripe - like excitation illumination of the cuvette 
 through the 10 mm window .
the detection through the 4 mm side window 
 was almost perpendicular to the excitation .
the cuvette was slightly 
 tilted with respect to the direction of the excitation stripe .
thus , 
 any signals stemming from multiple reflections are not fed into the 
 detector .
figure 2 shows 
 the excitation and detection scheme for the random lasing experiments . 
 a diode - pumped , passively q - switched solid state laser ( crylas gmbh
, 
 pulse length shorter than 1.3 ns , maximal pulse energy 20 j ) 
 was used to excite the suspensions at a wavelength of 532 nm .
the excitation beam was shaped into a thin stripe by two crossed cylindrical 
 lenses and focused in the plane of two razor blades . to establish 
 a 3 mm long , 55 m wide excitation stripe on the suspensions 
 in the cuvette ,
the emission from the pumped excitation volume 
 was coupled out of the side window and focused onto the entrance slit 
 of the spectrometer ( newport ms260i ) , which was equipped with a peltier - cooled 
 charge - coupled device camera ( andor ivac ) .
a 532 nm notch filter was 
 used to suppress detection of scattered excitation light .
to quantitatively 
 compare the random lasing from the different gain - nanostar solutions , 
 all measurements were performed under exactly the same experimental 
 conditions . in all random lasing experiments , the size of the excitation 
 stripe , the position of the cuvette in the setup , the position of 
 the excitation stripe on the cuvette s window , and the integration 
 time ( 1 s ) for the detection were fixed . ( a ) illustration of the 
 excitation stripe and the detection of 
 the emission .
( b ) the nanostars can randomly form resonant cavities 
 in the gain medium and hence the necessary feedback mechanism for 
 random lasing .
figure 3a displays 
 the absorption ( abs ) and photoluminescence ( pl ) spectra of r6 g dispersed 
 in ethanol .
the dashed orange curve in figure 3b zooms into the pl spectrum of r6 g from 
 545 to 580 nm , taken at a pumping fluence of 0.24 mj cm . for this fluence ,
the pl intensity was magnified 30 times to be 
 visible on the scale of the ordinate in figure 3b . for fluences above 0.30 mj cm , the emission spectrum of the reference r6 g solution shows amplified 
 spontaneous emission ( ase ) with a substantially reduced spectral bandwidth 
 as compared to pl ( see figure s12 ) .
the 
 curves shaded from black to red in figure 3b show the ase spectra of the reference r6
g 
 solution without nanostars excited by single pulses at different excitations 
 fluences , ranging from 0.33 to 0.66 mj cm , increasing 
 with equal steps of 0.03 mj cm .
( a ) normalized absorption 
 ( green ) and photoluminescence ( pl ) ( orange ) 
 spectra of r6 g ; ( b ) ase spectra of the reference r6 g solution without 
 nanostars , excited by single pulses at different excitation fluences 
 from 0.33 to 0.66 mj cm in steps of 0.03 mj cm .
the intensity of the pl spectra of r6 g at a pumping 
 fluence of 0.24 mj cm ( dashed orange curve ) was 
 magnified 30 times for comparison .
( c , e , g , i ) spectral overlap of the 
 r6 g absorption and emission spectra ( dotted curves ) with the extinction 
 spectra of respective nanostars in ethanol ( solid lines ) and ( d , f , h , j ) 
 the emission ( random lasing or ase ) spectra at different excitations 
 fluences for nanostars / r6 g mixtures .
( c , d ) aunsts , ( e , f ) aunsts@sio2 , ( g , h ) auagnsts , and ( i , j ) auagnsts@sio2 , respectively
. aunsts dispersed in ethanol exhibit 
 a spectrally broad extinction 
 spectrum with a maximum around 630 nm , due to the multiple plasmon 
 resonances of each individual nanostar ( figure 3c , blue curve ) .
the plasmon resonances of 
 the aunsts spectrally overlap partially with the pl emission of r6 g 
 ( dashed orange curve in figure 3c ) , but they hardly overlap with the r6 g absorption . at lower 
 pumping fluences , ase is observed from the aunst / r6 g sample ( figure 3d ) , similar to the 
 ase of the r6 g reference ( figure 3b )
. for higher fluences , sharper features are emerging 
 out of the ase background .
this behavior is typical for random lasers : 
 ordinary pl is observed for low pumping fluences , ase is observed 
 for a fairly broad range of pumping fluences , followed by coherent 
 laser peaks for even higher pumping fluences . upon silica - coating , 

the plasmon resonances of the aunsts red - shift 
 by about 20 nm due to the introduction of a higher refractive index 
 medium around the aunsts as compared to the solvent ( see figure 3e ) . to underline 
 our findings , we performed three - dimensional finite difference time 
 domain ( fdtd ) calculations on the optical properties of the hybrid 
 multilayered nanostars .
the calculated plasmon resonance of a single 
 aunst with silica shell is red - shifted as compared to the plasmon 
 resonance of the same single aunst without the silica shell ( figure s14 ) . for a detailed description of the 
 numerical calculation on the plasmonic properties of hybrid multilayered 
 plasmonic nanostars , refer to the corresponding section in the supporting information .
surprisingly , the 
 aunst@sio2/r6 g sample exhibits only 
 pronounced ase even at the highest pumping fluence ( figure 3f ) .
the silica shell affects 
 the gold nanostar - based random lasers in several ways .
first , the 
 local field enhancements at the surface of the aunsts@sio2 are slightly reduced by the silica coating .
the maximal field intensity 
 is localized close to the gold tip in the silica shell ( see figure s15 ) , so that outside the silica shell 
 the efficient excitation of the nearby gain material might also be 
 reduced .
second , besides the spectral red - shift , the silica shell 
 induces a spectral broadening of the plasmon resonances , typically 
 for chemical damping of the plasmon resonances , leading to a small reduction in the spectral overlap between 
 the pl of r6 g and the plasmon resonances of the aunsts@sio2 .
third , the silica shell acts as a spacer between the gain material 
 and the gold reducing quenching . while the latter is beneficial for 
 random lasing , the small reduction in the spectral overlap between 
 aunsts@sio2 scattering and the pl of r6 g as well as slight 
 reduction of the field enhancements
the silver - enhancement of the aunsts 
 leads to a blue - shift of the 
 plasmon resonances from around 630 nm ( aunsts , figure 3b ) down to 530 nm ( auagnsts , figure 3 g ) .
hence , the extinction spectra 
 of the auagnsts in ethanol spectrally overlap with both the absorption 
 and the pl of the r6 g .
the blue - shift of the plasmon resonances can 
 be attributed to a mixed gold  silver dielectric function , as well as to the reduced length of the nanostars 
 tips protruding from the core and to an increased diameter of the 
 core induced by the silver - enhancement ( see figures s14 and s16 ) . additionally , 
 due to the high scattering efficiency of silver , the auagnsts in ethanol 
 exhibit 4 times higher extinction as compared to the aunsts 
 ( note the different scales of the ordinate in figure 3b and g ) , although the concentrations of 
 the nanostars were similar in both dispersions .
spectrally very 
 sharp and intense random lasing peaks , characteristic 
 for coherent random lasing , are observed for the auagnst / r6 g sample . 

surprisingly , no pronounced ase background is detected even at low 
 pumping fluences ( figure 3h and figure s13 ) , in contrast 
 to the previously mentioned samples .
the investigation of the underlying 
 physical mechanisms for this experimental observation is beyond the 
 scope of this article .
the maximally observed random lasing intensities 
 are more than 3-fold higher than the random lasing intensities detected 
 from the aunst / r6 g .
silica coating of the auagnsts leads to 
 a spectral red - shift ( 20 
 nm ) of the plasmon resonances , so that the extinction spectra of the 
 auagnsts@sio2 in ethanol perfectly overlap with the absorption 
 and the pl of the r6 g ( figure 3i ) .
no sign of a possible detrimental effect of the silica 
 coating on the random lasing characteristics was observed in the case 
 of the auagnst@sio2/r6 g sample ( figure 3j ) , in contrast to the aunst@sio2/r6 g case ( figure 3f ) . moreover , the random lasing intensities are comparable for both 
 samples . as for the auagnst / r6 g sample , coherent random lasing is 
 observed also for the auagnst@sio2/r6 g sample even at low 
 pumping fluences ( figure 3j ) . for the sake of better visualization
, the random 
 lasing spectra 
 of the auagnst / r6 g and auagnst@sio2/r6 g from figure 3h and j are redisplayed in figure 4a and b , respectively . 
 for both samples ,
the measured spectral width of the coherent random 
 lasing peaks is as small as 0.3 nm , corresponding to the resolution 
 of the spectrometer .
hence , the values represent an upper limit , while 
 the actual line widths of the lasing modes might be well below .
the 
 spectral position , number , and relative intensities of the random 
 lasing peaks vary from pulse to pulse .
the closed loop resonator cavities 
 of the random laser , formed by the nanostars , change from shot to 
 shot , due to brownian motion .
there are several effects that can contribute 
 to the fact that the auagnsts as well as the auagnsts@sio2 could facilitate the buildup of coherent lasing modes .
first , the 
 plasmon resonances of the auagnsts and auagnsts@sio2 spectrally 
 overlap with both the absorption and the emission of r6 g in ethanol , 
 in contrast to aunsts .
second , the stronger hot spots at the tips 
 of the auagnsts or auagnsts@sio2 as compared to hot spots 
 at the tips of the aunsts ( see figure s15 ) might provide a more efficient pumping of the gain material , promoting 
 inversion .
third , the auagnsts could provide coherent feedback for 
 random lasing at 560 nm , due to their high scattering cross 
 sections in the spectral region coinciding to the spectral position 
 of the main pl maximum of r6 g ( 560 nm ) ( see figure s14 ) .
random lasing spectra of the ( a ) auagnst / r6 g and ( b ) auagnst@sio2/r6 g samples excited by single pulses at different excitation 
 fluences .
next , we show that 
 silica - overcoated nanostars can not only be 
 used as scatterers for random lasing in the polar solvent ethanol , 
 but also in the nonpolar solvent chlorobenzene .
the normalized absorption 
 and pl spectra of meh - ppv dissolved in chlorobenzene are displayed 
 in figure 5a .
figure 5b shows the ase spectra 
 of the reference meh - ppv solution without nanostars excited by single 
 pulses at different excitation fluences , ranging from 0.18 to 0.39 
 mj cm increasing with equal steps of 0.03 mj cm . in the case of the meh - ppv reference sample and 
 for small excitations fluences ( < 0.18 mj cm )
the dashed orange curve in figure 5b shows the pl spectra of meh - ppv 
 at a pumping fluence of 0.12 mj cm .
the pl intensity 
 was magnified 45 times for comparison with ase . for excitation fluences 
 above 0.18 mj cm , the meh - ppv reference sample 
 shows ase ( see figure s17 ) .
( a ) normalized absorption 
 ( blue ) and photoluminescence ( pl ) ( orange ) 
 spectra of the meh - ppv in chlorobenzene solution ; ( b ) ase spectra 
 of the reference meh - ppv solution without nanostars , excited by single 
 pulses at different excitation fluences .
the intensity of the pl spectra 
 of meh - ppv at a pumping fluence of 0.12 mj cm ( dashed 
 orange curve ) was magnified 45 times for comparison .
( c  h ) 
 spectral overlap of the meh - ppv absorption and emission spectra ( dotted 
 curves ) with the extinction spectra of respective silica - coated nanoparticles 
 in chlorobenzene ( solid lines ) and the emission ( ase or random lasing ) 
 spectra at different excitations fluences for nanoparticles / meh - ppv 
 mixtures : ( c , d ) aunps@sio2 , ( e , f ) aunsts@sio2 , ( g , h ) auagnsts@sio2 , respectively .
the inset in ( d ) 
 shows the tem image of a representative aunp@sio2 nanoparticle . 

the scale bar is 100 nm . as a further reference , spherical nanoparticles with dimension 
 similar to that of the nanostars ( 125 nm in diameter ) and coated with 
 a 10 nm silica shell ( aunps@sio2 )
the plasmon resonances of the 
 aunps@sio2 in chlorobenzene spectrally overlap with the 
 pl spectra of the meh - ppv ( figure 5c ) .
the inset in figure 5d displays the tem image of a representative aunp@sio2 nanoparticle . as shown in figure 5d ,
coherent random lasing arising from the 
 ase background could be observed for the aunp@sio2/meh - ppv 
 sample only for the highest fluences .
the random lasing intensity 
 for a pumping fluence of 0.39 mj cm is increased 
 by a factor of 3 with respect 
 to ase intensities detected for the other pumping fluences , which 
 are comparable to the ase intensities observed for the reference meh - ppv 
 sample ( figure 5d ) .
the extinction spectrum of the aunsts@sio2 dispersed 
 in chlorobenzene ( black curve ) is shown in figure 5e .
the aunst@sio2/meh - ppv sample 
 exhibits prominent ase ( figure 5f ) , as was previously observed also for the aunst@sio2/r6 g sample ( figure 3f ) . for fluences above 0.30 mj cm
, coherent 
 random lasing peaks can be observed on top of the pronounced ase background . 

it is worth mentioning that , although the plasmon resonances of the 
 aunsts@sio2 dispersed in chlorobenzene spectrally overlap 
 with the second pl emission maximum of meh - ppv ( at 630 nm ) , no ase 
 or random lasing was observed from the aunst@sio2/meh - ppv 
 sample in this spectral region .
the plasmon resonances of the auagnsts@sio2 in chlorobenzene 
 spectrally overlap with both the absorption and the pl spectra of 
 meh - ppv . for the auagnst@sio2/meh - ppv sample ,
coherent 
 random lasing was observed starting with pumping fluences above 0.24 
 mj cm ( figure 5h ) . in figure s18
, the random 
 lasing spectra of the auagnst@sio2/meh - ppv sample excited 
 by single pulses at different excitations fluences are displayed in 
 a fashion similar to those in figure 4 .
for a more 
 quantitative comparison of the different samples , we analyze the maximal 
 emission intensity , that is , the maximal intensity of the ase or coherent 
 random lasing in figure 3 and figure 5 , as 
 a function of the applied pumping fluence for the samples containing 
 r6 g ( figure 6a ) or 
 meh - ppv ( figure 6b ) 
 as a gain material , respectively .
( a ) maximal emission intensities of the 
 ase or random lasing spectra 
 ( figure 3 ) as a function 
 of the corresponding pumping fluence for r6 g reference sample without 
 nanoparticles ( orange ) , for aunst / r6 g ( blue ) , aunst@sio2/r6 g ( black ) , auagnst / r6 g ( purple ) , and auagnst@sio2/r6 g 
 samples ( red ) .
( b ) maximum emission intensities of the ase or random 
 lasing spectra ( figure 5 ) as a function of the corresponding pumping fluence for meh - ppv 
 reference solution ( orange ) , for aunp@sio2/meh - ppv ( dark 
 cyan ) , aunst@sio2/meh - ppv ( black ) , and auagnst@sio2/meh - ppv ( red ) samples , respectively . for the r6 g reference sample without nanoparticles ,
the ase 
 intensity 
 increases with increasing pumping fluences ( the orange curve in figure 6a ) .
as expected from figure 3b , there is no hint 
 for a coherent random lasing threshold .
the blue curve in figure 6a refers to the spectra 
 of aunst / r6 g sample in figure 3d .
one is able to identify two distinctively different slopes 
 of the blue curve . for low fluences ( < 0.45 mj cm ) , the aunst / r6 g sample exhibits ase similar to that of the r6 g reference 
 sample .
at around 0.45 mj cm , the slope of the 
 blue curve changes , indicating the coherent random lasing threshold 
 of the aunst / r6 g sample . the black curve in figure 6a , depicting the maximal intensities of the 
 spectra displayed in figure 3h for aunst@sio2/r6 g samples , shows a trend with 
 increasing pumping fluences similar to that of the r6 g reference sample . 
 as one can deduct from the steeper slope of the black curve as compared 
 to the orange curve ,
the 
 red and purple curves in figure 6a correspond to the maximal random lasing intensities 
 from the auagnst / r6 g ( red ) and auagnst@sio2/r6 g ( purple ) 
 samples , respectively .
the threshold fluences for coherent lasing 
 are at 0.39 mj cm ( red curve ) and at 0.42 mj cm ( purple curve ) , respectively .
saturation of peak 
 intensities is observed for high fluences . in the case of auagnst@sio2/meh - ppv
, the threshold fluence 
 for coherent lasing is at 0.24 mj cm , while the 
 onset for coherent random lasing for the aunp@sio2/meh - ppv 
 sample ( dark cyan curve in figure 6b ) is at much higher pumping fluences ( 0.36 mj cm ) . at a fluence of 0.39 mj cm , 

the coherent random lasing emission intensity from the auagnst@sio2/meh - ppv sample is 8.3 times higher than the emission 
 intensities observed from the samples containing aunps@sio2 ( dark cyan curve ) or aunsts@sio2 ( black curve ) .
moreover , the 
 maximal detected random lasing intensity for the auagnst@sio2/meh - ppv sample is boosted by a factor of 25 as compared 
 to maximal ase intensity from the reference meh - ppv sample ( orange 
 curve ) .
our observations can be explained by the unique properties 
 of the 
 auagnsts as compared to other particles .
the auagnsts and the auagnsts@sio2 exhibit higher scattering cross sections than the aunps@sio2 and the aunsts with and without silica shell , due to the 
 silver - enhancement .
the higher scattering cross section might be one 
 of the main reasons for the low pumping thresholds at which the solutions 
 containing auagnsts or auagnsts@sio2 show coherent random 
 lasing .
another reason could be the spectral overlap of the plasmon 
 resonances with the absorption spectra of the gain material and the 
 excitation wavelength .
the hot - spots at the tips of the auagnsts could 
 facilitate the creation of inversion by providing more efficient pumping 
 of the gain material .
similar correlations between the lasing thresholds 
 and the field enhancement were reported also by others .
finally , we turn our discussion to the 
 comparison of pumping thresholds 
 for the auagnst@sio2/r6 g and auagnst@sio2/meh - ppv - based 
 random lasers . because the concentrations of auagnsts@sio2 in the r6 g and meh - ppv solutions were the same , we analyze the lasing 
 thresholds with respect to the extinction a = cd of the gain materials in the stock solutions .
 
 is the extinction coefficient of r6 g ( 219 ml mg cm taken from ref ( 41 ) ) or meh - ppv ( 106 ml mg cm from ref ( 42 ) assuming that  of meh - ppv in chlorobenzene is in 
 the same order of magnitude as in chloroform ) , c is 
 the concentration of the gain materials , and d is 
 the optical path of 4 mm .
the extinction of the auagnst@sio2/r6 g solutions ( c = 1.5 mg ml ) is ar6 g  132 , while the auagnst@sio2/meh - ppv solutions ( 5.4 mg ml ) have an 
 extinction ameh 
ppv  
 229 . for the auagnst@sio2/meh - ppv sample , coherent random 
 lasing without pronounced ase background
was observed starting with 
 pumping fluences above 0.24 mj cm . considering 
 that ameh 
ppv  1.73 ar6 g , the lasing threshold for the auagnst@sio2/r6 g should be 1.73 higher than the lasing threshold 
 for auagnst@sio2/meh - ppv . indeed , the experimental threshold 
 for coherent random lasing of auagnst@sio2/r6 g ( 0.42 mj 
 cm ) is 1.75 times higher than the threshold for 
 auagnst@sio2/meh - ppv .
in summary , we synthesized hybrid multilayered plasmonic nanostars , 
 comprised of gold , silver , and silica in various compositions .
the 
 silver - enhanced and silica - coated gold nanostars can be mixed with 
 polar or nonpolar solutions containing gain material .
we find that 
 auagnsts and auagnsts@sio2 provide sufficient feedback 
 for coherent random lasing . in single pulse experiments ,
we show that 
 the silver - enhancement of the gold nanostars reduces the pumping threshold 
 for nanostar - based coherent random lasers substantially in different 
 gain media .
we attribute this to the plasmon resonances of auagnsts 
 and auagnsts@sio2 , which spectrally overlap perfectly with 
 both the absorption and pl emission of the gain materials as well 
 as the excitation wavelength .
first , hot - spots 
 at the tips of the auagnsts and of auagnsts@sio2 can be 
 resonantly excited by the pumping laser , and hence the gain material 
 in the vicinity of the tips can be excited with a higher effective 
 intensity .
therefore , the auagnsts and auagnsts@sio2 can 
 facilitate inversion of the gain material more easily .
second , the 
 auagnsts and the auagnsts@sio2 can provide the necessary 
 feedback for coherent . considering the perfect spectral overlap of 
 the plasmon resonances with the pl of the gain material and the high 
 scattering cross sections of the auagnsts and auagnsts@sio2 , the emitted photons can be scattered efficiently .
further , we reveal 
 that the lasing intensity and pumping threshold of a silver - enhanced 
 gold nanostar - based random laser are basically not influenced by the 
 silica coating , in contrast to the gold nanostar - based random laser , 
 where for the silica - coated gold nanostars only mediate ase but no 
 coherent random lasing was observed .
the large scattering cross 
 sections and easily accessible hot - spots 
 provided by auagnsts and the simultaneous compatibility of the auagnsts@sio2 with polar and nonpolar solvents render hybrid multilayered 
 plasmonic nanostars one of the most promising candidates for an effective 
 photon - management in organic and inorganic optoelectronic devices 
 as well as for biosensing and analytical applications .
 here , 
 we report that hybrid multilayered plasmonic nanostars can 
 be universally used as feedback agents for coherent random lasing 
 in polar or nonpolar solutions containing gain material . we show that 
 silver - enhancement of gold nanostars reduces the pumping threshold 
 for coherent random lasing substantially for both a typical dye ( r6 g ) 
 and a typical fluorescent polymer ( meh - ppv ) . 
 further , we reveal that 
 the lasing intensity and pumping threshold of random lasers based 
 on silver - enhanced gold nanostars are not influenced by the silica 
 coating , in contrast to gold nanostar - based random lasers , where silica - coated 
 gold nanostars support only amplified spontaneous emission but no 
 coherent random lasing . 