† Corresponding author. E-mail:
Project supported by an Australian Research Council Future Fellowship Grant.
In recent years, there have been a significant number of demonstrations of small metallic and plasmonic lasers. The vast majority of these demonstrations have been for optically pumped devices. Electrically pumped devices are advantageous for applications and could demonstrate concepts not amenable for optical pumping. However, there have been relatively few demonstrations of electrically pumped small metal cavity lasers. This lack of results is due to the following reasons: there are limited types of electrically pumped gain media available; there is a significantly greater level of complexity required in the fabrication of electrically pumped devices; finally, the required components for electrical pumping restrict cavity design options and furthermore make it intrinsically more difficult to achieve lasing. This review looks at the motivation for electrically pumped nanolasers, the key issues that need addressing for them to be realized, the results that have been achieved so far including devices where lasing has not been achieved, and potential new directions that could be pursued.
There is interest in the continued miniaturization of light sources from both a scientific perspective of understanding the properties of such small light sources, and also from the utilization of such light sources. In particular, nanoscale lasers have been predicted and shown to have very high modulation speeds and low power,[1] attributes that lend themselves well to future applications such as on-chip optical communications[2] where low power, high speed, and small size are important. Furthermore, nanolasers are needed in the continued miniaturization of integrated photonic systems, which could be employed in diverse areas such as optical sensing systems or digital processing of optical information.[3]
Of all the types of small cavities so far employed to form lasers or resonant cavity light emitting diodes (LEDs), metallic cavities are unique in offering both subwavelength confinement of the light and a small overall footprint for the device,[4] as shown in Fig.
A key issue common to both plasmonics and the use of any small metallic cavity is that of the high optical losses caused by the free electrons in the metal. It has been shown that these metallic losses can be compensated for through the use of an optical gain medium and clever design of the metallic cavity or waveguide.[6–15] The optical loss compensation can occur while still achieving a small device size and subwavelength confinement of the light. Furthermore, coherent light emission has been confirmed in these metallic light emitters by studying the statistics of the emitted light.[15,16] Many of these demonstrations of loss compensation and lasing in metallic/plasmonic devices have occurred at cryogenic temperatures and/or employed optical pumping of the gain medium.[7–15] Operation at cryogenic temperatures has been shown to reduce the metal losses via reduced phonon scattering of the free electrons in the metal.[7,10,17,18] Employing optical pumping allows a wider range of gain medium to be employed, and also avoids the losses and issues introduced with electrical pumping of the gain medium. These electrical pumping issues will be discussed in detail later.
While optical pumping and cryogenic temperatures have been demonstrated in many interesting devices, for most applications it is necessary to have devices that are electrically pumped and can operate at room temperature or higher. Electrical pumping is advantageous for many reasons including the following: electrical pumping avoids the requirement of a secondary light source for pumping, and allows direct electrical modulation of the nanoscale light source. Furthermore, electrical pumping permits gain material buried deeply inside metallic structures to be pumped. Finally, the energy for pumping can be directed efficiently into many possibly spatially separated nanoscale regions of gain material via wires.
Electrically pumped room temperature operation is also an important contemporary topic for the smallest of dielectric lasers and photonic crystal lasers.[19,20] Typically in dielectric lasers, metal contacts must be kept a considerable distance from the dielectric cavity. Metal-based small lasers have an inherent advantage compared to dielectric cavity lasers when considering electrical pumping. Specifically, the metals used to form the cavity itself are available to transport the current to the gain medium.
Loss compensation for plasmonics in particular is an important issue because typically the losses are high when light is tightly confined in plasmonic waveguides or cavities, and light can only propagate a small distance before decaying.[21] (There are plasmon mode waveguides that allow long propagation distances; however, they do not confine the light to small regions.[21]) Widely employed integrated photonic systems typically have means of compensating for the losses in their waveguides and of providing integrated laser sources, for example, InP-based photonic-integrated circuits.[3] Alternatively, the losses in the photonic system need to be sufficiently small so that a meaningful number of manipulations of an optical signal can be performed before it is excessively attenuated, for example, silicon on insulator photonic-integrated circuits.
Hence, loss compensation in plasmonic systems is important to permit more complex systems to be built and provide integrated sources of coherent plasmons. An electrically pumped means of loss compensation that can provide large numbers of coherent plasmon sources is particularly important.
The rest of this review will look at the following aspects of electrically pumped metallic and plasmonic nanolasers. Firstly, the critical issues that need to be addressed to obtain loss compensation and lasing in small metal structures are examined, and then the key elements required for electrical pumping that typically impact nanolaser design are identified. Examples will be given of some of the optically pumped metal cavity nanolasers and how they achieve lasing. Then, examples of electrically pumped devices will be given, and it will be shown how the requirements for electrical pumping increase the challenge of overcoming the metal losses. A comprehensive review of the latest electrically pumped devices (including those that may fall short of achieving lasing) will be given. There is also a considerable amount of literature providing only simulation or theory results about electrically pumped nanolasers. This simulation and theory literature will also be reviewed to see what is possible in electrically pumped nanolasers and what could be the future directions for their development.
The dielectric function, εm, of the metals employed for metal nanolasers and plasmonic waveguides is both negative and complex, i.e., εm = ε ′m + iε″m. Silver is the metal with the lowest losses at wavelengths of most interest for plasmonics and nanolasers, and thus is employed in many nanolaser demonstrations. However, even for silver, the loss due to ε″m is significant (εm = –130 + i3.3 at a 1550 nm wavelength[22]). The decay of light in a metallic cavity or along a metallic waveguide depends on how much the electric field of the mode overlaps the metal. In general, for optical modes that are tightly confined by the metal structures, propagation lengths and decay times are short.[21] For less-confined optical modes, propagation lengths along waveguides can be in the order of a millimeter.[21]
A nanolaser consists of two main components: a nanocavity or resonator, and an optical gain medium that overcomes the optical losses in the nanocavity. The nanocavity can be characterized via a photon lifetime τp, which gives the time the optical power in the cavity mode decays to 1/e of its initial value.[23] τp is related to the cavity quality factor Q, resonant wavelength λ0, and the speed of light c,[23]
The semiconductor gain medium is characterized by the material gain Ga. The confinement factor Β is defined as the proportion of the laser mode energy that overlaps the optical gain medium.[23]
The Ga required to overcome the cavity losses and so create a laser Gth, is related to the other cavity parameters[23]
In Ref. [24], this tradeoff between metal losses and Β is explored to achieve a sufficiently low Gth so that lasing can be achieved at room temperature,[11] even when higher optical loss metals such as aluminum are employed. In Ref. [11], a thick dielectric spacer between the metal and gain material is employed to reduce the overlap of the optical mode with the confining metal.
In Refs. [10] and[26], the surface plasmon modes typically involve just one surface of the metal or the metal stripe is very thin. Furthermore, the refractive index of the majority of the surrounding dielectric is low, which greatly reduces the amount of optical energy in the metal compared to having a high refractive index material adjacent to the metal.[30] Typically, these plasmon modes are called long range surface plasmons,[21] due to their significant propagation distance.
Another feature that occurs in both Refs. [11] and[10] that enables a large Β is the low refractive index of the dielectric material that completely surrounds the relatively high refractive index semiconductor gain material. In Ref. [11], SiOx (n ∼ 1.5) and air (n ∼ 1) surround the gain material (n ∼ 3.5), which leads to the optical mode being strongly confined on the gain material.
The strategy of having the gain medium surrounded by a low index dielectric has been very successful in producing nanolasers;[10–12] however, it requires that the gain medium is optically pumped. Typically, low index materials are insulators and no pathways exist to carry current to the semiconductor gain medium.
To move from optically to electrically pumped nanolasers, two key issues must be solved. First, there needs to be a way to efficiently inject electrons and holes into the semiconductor from metal electrodes. Typically, what is desired is to form low-resistance ohmic contacts between metal electrodes and a particular n- or p-doped semiconductor. Considerable effort over the last 50 years has gone into developing such low-resistance ohmic contacts to many types of semiconductor.[31] In fact, the ability to efficiently inject electrons and holes into a particular semiconductor via electrodes determines whether or not it is suitable for use in laser diodes or energy-efficient LEDs. The second key issue is that there often needs to be semiconductor pathways from the electrode contact regions to the semiconductor gain material.
A schematic of an electrically pumped double heterostructure ridge waveguide is given in Fig.
The two requirements of contacts and transporting current from contacts to gain region create three key problems that the optically pumped metal cavity nanolasers do not suffer from.
Firstly, stable low-resistance electrical contacts to semiconductors typically involve a number of metals that have optical losses much higher than metals such as silver. These metals, examples of which are titanium and platinum, among other tasks provide adequate adhesion of the contact to the semiconductor and prevent diffusion into the semiconductor of other metals that form part of the contact. The key point is that the metal contact regions exhibit extremely high optical losses. The optical mode of the nanolaser must be kept away from the contact regions to avoid high cavity losses that cannot be compensated for by the gain medium, similar to a dielectric cavity laser. Some low-optical loss contact metallization schemes have been investigated[32] and may play a role in the future.
Secondly, the refractive index contrast between the semiconductors, which transport the carriers from the contacts and the gain medium, is typically quite low. For example, the refractive index n of InP is 3.17 compared to 3.6 of InGaAs, at 1.55 microns wavelength. Most optically pumped nanolasers have dielectrics such as air or SiO2 (n ∼ 1 to 1.5) adjacent to the semiconductor gain medium. The main result of the much lower index contrast is that it is harder to obtain a high Β in electrically pumped nanolasers. Additionally, the higher refractive index of the materials next to the metal results in a larger amount of optical mode energy in the metal, and therefore metal cavity losses are also increased.[30]
Thirdly, resistive heating from both the electrical contacts and the semiconductors transporting the carriers to the gain medium will be detrimental to the operation of the nanolaser.
While there has been a wide variety of optically pumped metallic nanolaser structures, only a comparatively few forms of electrically pumped metallic nanolaser have been demonstrated. The first metallic nanolasers demonstrated were electrically pumped and based on the concepts of the double heterostructure ridge waveguide laser, as shown in Fig.
Where the etched semiconductor region forms a ridge,[8] light is guided in a metal–insulator–metal (MIM) waveguide. These particular MIM waveguide lasers employ the MIM waveguide concept explored in Ref. [33], but with the MIM structure now vertical instead of horizontal.[33]
The concept shown in Fig.
Another form of etched structure that has similarities to the structure shown in Fig.
A horizontal MIM structure has also been used to create mid and far infrared wavelength lasers (approximately > 3 microns).[42] Here, the semiconductor gain medium is sandwiched between two pieces of metal, which form the electrical n and p-type contacts. Generally, a thin layer of titanium or other adhesive metal is used between the semiconductor and gold to assist adhesion of the metal to the semiconductor. Additionally, there are often specific doped semiconductor layers close to the metal to achieve a low contact resistance between the metal and semiconductor.[42] For the longer wavelengths such devices operate at, the metal losses are low and the extra losses due to the titanium layer are acceptable. Furthermore, the semiconductor layer between the metal is typically several microns thick. The thickness of this layer permits the use of n- and p-doped contact layers in the semiconductor, while still having a significant proportion of the semiconductor as undoped gain medium.
In Ref. [43], a variation of the MIM structure for far infrared lasers involved introducing circuit concepts to create the resonant cavity. Here, sections of MIM with different aspect ratios are used to form inductor- and capacitor-like elements. It would be interesting to see how such resonators based on circuit concepts could be employed for much shorter wavelengths.
The dimensions of these far infrared wavelength MIM laser structures are far from nanoscale. Reducing this horizontal MIM structure down to nanoscale dimensions and shorter wavelengths while still achieving lasing is difficult due to the high optical losses of the metal contact regions close to the optical mode. Nevertheless, these long wavelength MIM lasers highlight an important advantage of electrically pumped devices: confining metal structures themselves can be employed as electrical connections.
There have been a number of demonstrations of light (or surface plasmon) emitters that employ such horizontal MIM structures, with nanoscale dimensions and shorter wavelengths. It is worthwhile looking at these structures considering that the number and variety of demonstrated electrically pumped metallic and plasmonic nanolasers is so small. As highlighted by a number of authors,[44] nanoscale metal cavity surface plasmon emitting diodes (SPEDs) or LED sources may be useful sources. At the very least they can provide an interim solution for small plasmon sources until technology develops sufficiently for the challenges of metal cavity nanolasers to be overcome in a wide variety of configurations.
One of the first demonstrations of surface plasmon emission in an MIM structure was reported in Ref. [45] (Fig.
Compound III–V semiconductors have also been employed for small surface plasmon emitters, forming SPEDs. In Ref. [46] (Fig.
Another demonstration of a nanoscale SPED was given in Ref. [44] (Fig.
In Ref. [47], an electrically pumped quantum well emitter has was also placed next to a thin gold layer. The idea was to fully compensate for the metal losses for the long range surface plasmon supported by the gold layer. However, loss compensation was not achieved.
Finally, a recent demonstration of an LED involving a metal nanocavity was given in Ref. [48] (Fig.
There has been a considerable amount of theory and simulation articles published about metal and plasmonic cavity nanolasers. It is worthwhile briefly examining some of the literature pertinent to electrically pumped devices and in particular those focused on semiconductor gain medium, as shown in Fig.
Some of the initial theory to look at semiconductors amplifying surface plasmons at visible and near infrared wavelengths was given in Ref. [30], showing it was indeed likely that metallic losses could be overcome. Further work by the same group showed that by using less-confined transverse electric (TE) modes and dielectric spacers, some metal-coated laser resonators could be formed with quite low threshold gain requirements that could be readily satisfied by semiconductors.[24] Other modeling work looked at Fabry–Perot metal nanocavities.[23] Again, it was shown in theory that with current semiconductor gain media it would be possible to obtain lasing in these cavities. However, all of the above-mentioned simulations did not incorporate structures to permit electrical pumping of the gain medium.
A number of later simulations also looked at coupling metal nanocavities to waveguides and examining their potential as laser sources for on-chip communications,[34,49] as shown in Figs.
A number of simulation studies have highlighted that considerable improvements in the performance of metal cavity devices are possible by optimizing or modifying their structure. In Ref. [51], it is shown how a simple change in the shape of the ends of a Fabry–Perot cavity, constructed with the waveguide of Fig.
A number of theory and simulation studies have also examined the intrinsic modulation speed of both metal nanocavity lasers and LEDs.[1,53,54] All studies suggest that the metallic nanocavities can provide useful and high-speed devices with distinct advantages in terms of size and speed over dielectric-based cavity devices.
Some studies have made broad dismissive statements about a particular single nanoparticle cavity[55] or a simplified one-dimensional metal structure[56,57] being used for devices. However, it is dangerous to generalize these results to all possible structures that could exist. The theory and simulation studies cited above already indicate that metallic and plasmonic nanolasers are worth pursuing, and they can have advantages in certain respects to dielectric cavity lasers. It is true, particularly for electrically pumped devices, that the current experimental device performance is far from what theory predicts. However, with improving manufacturing technology, device performance should improve to approach that of theory; additionally, new device structures with better performance may be found. Indeed, optically pumped devices are already showing interesting results, which indicates that plasmonic laser devices do indeed have particular performance advantages over dielectric cavity devices since their size goes below the diffraction limit.[58]
From the published relevant simulation studies, it appears possible to achieve useful small metal cavity lasers. However, many materials are pushed close to their limits to obtain lasing and therefore it is technologically difficult to make the laser structures. Improvements could occur on two fronts to make achieving electrically pumped small metal cavity lasers easier. Firstly, improvements could be made in the materials employed in the laser. Secondly, the design of the laser cavity could be improved so that useful small metal lasers could be made with current materials.
With regards to improvements in materials, plasmonic materials with lower loss than noble metals, such as silver, could be employed or created. In fact, considerable effort in recent years has gone into developing new materials,[59] in part due to the difficulty in compensating loss in metals such as silver. Indium tin oxide (ITO) is one material that has been suggested for use as a plasmonic material for plasmon mode lasers.[59,60] Indeed, there have been proposals for using ITO for plasmonic lasers.[61] However, metals such as silver have many advantages such as good thermal and electrical properties and limited penetration of the electric field into the metal, allowing truly nanoscale devices. More studies should shed light on how well alternatives to silver will enable nanolasers to be realized. Recent results on using single-crystal aluminum and silver have also shown considerable improvements in optically pumped nanolaser performance.[62,63] Applying single crystal metals to electrically pumped designs would be another avenue for improvement that could be pursued.
Other materials in the nanolaser where improvements have been investigated are increasing the maximum gain from the semiconductor gain medium,[64] and reducing the loss of the electrical contacts.[32]
Considering now the improvements to the structure of the laser cavity, it is not trivial to find the optimum structure that minimizes the threshold gain required to overcome losses in a nanocavity or waveguide. Finding an optimum structure is particularly difficult when taking into account the electrical contact issues and if the cavity is a complex three-dimensional structure consisting of metals, semiconductors, and insulators. For example, two-dimensional structures that employ low refractive index materials in each side of a semiconductor gain medium on a metal can have comparatively low amounts of energy loss in the metal, with good confinement of the optical mode in the gain medium.[10,52] More work could be done searching for structures that localize the optical mode well on the gain medium, but minimize the amount of energy in the metal while maintaining small overall size. Looking at small groups or arrays of plasmonic scatters[65] may prove useful, or even circuit-based cavity concepts could be looked at Ref. [43].
This review has examined the key issues involved in producing electrically pumped metal cavity nanolasers, as well as how these key issues can impact on the already difficult task of obtaining lasing in metal nanocavities. It looked at the approaches taken to solve electrical pumping by the limited number of actually working laser devices fabricated.
Simulation studies were also reviewed to show that in theory useful electrically pumped nanolaser devices could be made with current materials. However, often the material gain required for lasing is close to the maximum possible, which leads to tight fabrication and material tolerances. In light of the difficult manufacturing technology, resonant cavity plasmons or LEDs[44] may provide useful avenues for development in the interim.
Apart from improving the technology to build current nanolaser concepts, the review looked at possible advances in materials and cavity designs that could ease the difficulties involved in realizing electrically pumped metal nanolasers.
Providing room temperature continuous wave electrically pumped metallic and plasmonic nanolasers with useful lifetimes and output power, typically coupled into a waveguide, is still a challenge to be solved. Furthermore, the structures and physics involved with such nanolasers are not trivial, and thus predicting what is achievable is not easy. However, recent simulation and experimental studies suggest that useful devices are indeed possible. Hence, metallic and plasmonic nanolasers are worth pursuing because no other technology allows lasers and photonic systems to be miniaturized to dimensions smaller than the wavelength of light.
[1] | |
[2] | |
[3] | |
[4] | |
[5] | |
[6] | |
[7] | |
[8] | |
[9] | |
[10] | |
[11] | |
[12] | |
[13] | |
[14] | |
[15] | |
[16] | |
[17] | |
[18] | |
[19] | |
[20] | |
[21] | |
[22] | |
[23] | |
[24] | |
[25] | |
[26] | |
[27] | |
[28] | |
[29] | |
[30] | |
[31] | |
[32] | |
[33] | |
[34] | |
[35] | |
[36] | |
[37] | |
[38] | |
[39] | |
[40] | |
[41] | |
[42] | |
[43] | |
[44] | |
[45] | |
[46] | |
[47] | |
[48] | |
[49] | |
[50] | |
[51] | |
[52] | |
[53] | |
[54] | |
[55] | |
[56] | |
[57] | |
[58] | |
[59] | |
[60] | |
[61] | |
[62] | |
[63] | |
[64] | |
[65] | |
[66] |