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Monday, May 7, 2012

Nanolasers are the rising stars

If you attend “QM1H • Spasers and Nanoemitters today, you know exactly what I am talking about. Exciting new materials, including metamaterials, quantum wells, and quantum rods, are used for the realizations of the nanolasers. If you missed it (which is quite possible since there are many other outstanding technical sessions packed today), this short article is your second chance. Three examples of nanolasers are presented here (summarized from today’s speakers) to give you a taste of the flavors.

Nanoscale coaxial lasers:
This is a piece of artwork of nanofabrication. The researchers from UCSD are able to fabricate a nanoscale coaxial laser cavity on an InP substrate (figure 1). It is composed of a metallic rod with different coaxial disks. One of the disks, the gain medium shown in red, is made of 6 quantum wells (each one is made of Inx=0.734Ga1xAsy=0.57P1y / Inx=0.56Ga1xAsy=0.938P1y, with an overall height of 200 nm). They are sandwiched between SiO2 and air plugs. With the help of these two plugs, the entire device behaves like a cavity which supports a few sparse EM like modes (figure 2). If you pump the device in a right way, you can excite these modes and build them up. The lower air plug also allows pump energy into the cavity and couples out the light generated in the coaxial resonator. So once you build up the modes, you can couple the light out. In other words, you can make this device lase.

Figure 1. The structure of a coaxial laser cavity. The enitre thing is ~ 500 nm in all dimensions. (b) and (c) shows the SEM images of two different structures. Courtesy of M. Khajavikhan et al. in Nature 482 204 (2012).
Figure 2. The EM like plasmonic modes that can be supported by the cavity. Two different structures support different modes. Some of the modes can be pump and excited by 1064 nm laser. Courtesy of M. Khajavikhan et al. in Nature 482 204 (2012).
The researchers pump this nanolaser with a 1064 nm laser and it will lase at 1.26 and 1.59 micron at room temperature depending on the overall structures of the nanodevice. I would like to have one of these as souvenir. 
Lasing spasers:
SPASER stands for Surface Plasmons by Stimulated Emission of Radiation -- an idea proposed by Bergman and Stockman in 2003. They suggested that it is possible to construct a nanodevice in which a strong coherent field is built up in a spatial region much smaller than the wavelength. Simply speaking with the help of figure 3, you can induce and build up surface plasmons – the oscillation of electrons on the surface of a nanostructure – by providing it energy. In the example of figure 3, the energy is coming from the excited nanocrystal quantum dots (NQD). Once this oscillation starts, it further drains energy from NQDs into it and builds up strongly. As an analogy, the nano-structure (nano silver shell, for example) confines the plasmons onto its surface, which behaves like a laser cavity confining photons. And the quanta of the plasmons are like photons in the cavity.

Figure 3. (a) A theoretical spaser made of a nano silver sphere coated with NQDs. (b) the energy diagram shows how the energy is transferred from the NQDs to the plamonic modes of the nano silver sphere. (c) and (d) show two different plasmonic modes of it. Courtesy of M. Stockman in Nature Photonics 2 327 (2008).
However, the ideal spaser does not emit light; it simply converts more energy into its in-phase plasmonic modes. One way to make it emit light is to create an array of nanostructures – a principle proposed by professor N. Zheludev et al. and shared at CLEO 2012:
In their “lasing spaser”, the nanostructures are a two dimensional array of metallic nanowires (figure 4). They are situated on the surface of the amplifying medium (gain medium). One of the possible amplifying media can be a substrate packed with quantum dots. A working principle is like this: a pump source is used to excite the amplifying medium (in figure 4, a pump laser is used to excite the quantum dots in the substrate), the nano wires drain the energy from the excited quantum dots into its plasmonic modes, and the currents start to oscillate back and forth within each nanowire (figure 4). Most of the currents cancel each other if viewing the device from the far field. Only the currents on the edge of the array survive the cancellation and behave like an oscillating dipole that emits light (figure 4, bottom). The light would be more intense if the plasmonic modes have more quanta. The entire device can be a few tens of microns while it emits near or mid-IR light (tunable by adjusting the structure of the nanowires).

Figure 4. The device is made of an array of nanowires, gain medium (shown in green), and pumped normally with a laser. At the bottom, it shows how the nanowires behave when viewing them from the far field: the currents in the center of the array look like cancelling each other out, while the currents at the edge can emit light. Courtesy of N. Sheludev in Nature Photonics 2 351 (2008). 
Lasing in self-assembled microcavities of CdSe/CdS core/shell colloidal quantum rods:
This laser is a bit bigger. It is about a few hundred microns but it is something you might be able to do if you have the quantum rods provided by the researchers of Italy. What you have to do is very straightforward. Dissolve the quantum rods in toluene, put a droplet of it on the glass substrate, and wait until it dries. Then you get a microcavity that will lase. Pretty amazing, and this is how:
The building block of this microcavity is a CdS quantum rod (about a few tens of nanometers) with an embedded CdSe nanocrystal (figure 5). What special about this rod is that it is a strong fluorescent little guy with a QE of ~ 70%. In other words, if you pump it with light, it tends to give back its energy through fluorescence. Put a droplet of toluene containing many of the rods on the surface, it will dry out in a special way: The convection created inside the droplet due to the evaporation of it pushes the rods condensate at the border of the droplet. Not only so, the rods will pile up in a regular pattern. Microscopically, you have a wall (tens of microns wide and tens of nanometers thick) of rods piled regularly on the border of the droplet. This wall is very fluorescent (since they are made of rods), and can behave like a cavity (since the wall has very different refraction index compared to the surrounding, the fluorescent light can be bounced back and forth between the wall). If you pump the wall with laser (532 nm) normal to the glass surface, you are able to create a lasing phenomenon at ~ 610 nm (figure 5 bottom). You can proudly say: everyone is able to make a microlaser if the rods are available in the market.
Cheers! And look for more at CLEO 2012!

Figure 5. The building block of the microlaser: a CdSe embeds in a CdS quantuam rods. The lower plot shows how the device is lasing: You pump the dried droplet at its border normally and the device can lase at ~ 610 nm. Courtesy of M. Zavelani-Rossi in Nanoscale 2 931 (2010).

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

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