Multicolour nanowire lasers
Independent optimization of the core and the shellofamulti-quantum well (MQW) nanowire laser leads toanunprecedentedlevel of control and tunability over thesetinysources of laserlight. That's according to researchers in theUSwho see this as astep towards free-standing injectionnanolasers(NatureMaterialsdoi:10.1038/nmat2253).
“Our work represents the first synthesis ofconcentricMQWssurrounding the nanowire core,” Charles LieberofHarvardUniversity told optics.org. “These MQWstructuresrepresentan unprecedented level of structural complexity,andimportantlyenabled us to separately investigate two keycomponentsof ananowire laser: the cavity and gain medium.”
Although nanowire lasers are not a new concept,allpreviousstudies have concentrated on homogeneoussemiconductorssuch asGaN. This means that the laser wavelength isdictated bythematerial's bandgap, and there is no way to designandtuneproperties of the laser.
Nanowire cross-section
In contrast, Lieber and colleagues used MOCVD growthtomakesources containing a GaN nanowire core on asapphiresubstrate.Those cores act as the optical cavity and aresurroundedbyInGaN/GaN MQW shells that serve as acomposition-tunablegainmedium.
Varying the indium content tunes the emissionwavelengthfrom365 nm through to 494 nm, with all devicesoperatingatroom temperature.
“We have focused on one issue at a time, and this hasenabledusfind the best structures efficiently,” explainedLieber.“Thepurpose of decoupling the gain medium and the cavity isnottoseparate but to combine the best respective solutions.”
The team's MQW nanowire heterostructures contained between3and26 quantum wells. Typical nanowires were 200 to400 nmin“diameter” and 20 to 60 µm in length.AQ-switchedNd:YVO4 laser emitting at266 nm(35 kHzrepetition rate, 7 ns pulse duration,typicalexcitation power0.4–2 mW) then optically pumped thenanowires.
Lieber and colleagues are now pursuing several linesofresearch.First, they are working to achieve lower laserthresholdsby growingsmaller core and/or additional cladding layers.
“More quantum wells resulted in a lowerlaserthreshold,”commented Lieber. “This is due to bettercouplingbetween the gainmedium and resonant modes, whichsubsequentlylowers the threshold.We believe that lasing could beachieved withfewer QWs, givenoptimized cavity structures.”
Second, they believe that electrical excitation isfeasible.“Weare working on the combination of MQW and dopantmodulation inasingle nanowire to achieve an injection laser,” saidLieber. “Afewfundamental issues need to be addressed to achievethis goalsuchas reducing cavity loss via optimizing the nanowirestructure.Weare very optimistic about this direction based onourexcellentcontrol of nanowire structure and composition.”
In the long term, Lieber says that a specific milestonewillbethe successful demonstration of electricallydriven,multicolour,low-threshold nanolasers arrays, and theirapplicationinheterogeneously integrated optical-electronic chips.
Author
Jacqueline Hewett is editor of Optics&LaserEurope magazine.