Semiconductor nanolasers and the size-energy-efficiency challenge: a review Download: 690次
1 Introduction
The research field of semiconductor lasers is at the very core of the larger field of semiconductor photonics (also known as optoelectronics). This is a field encompassing both fundamental science and a wide range of important technologies. From the scientific perspective, light–semiconductor interaction plays a foundational role in understanding semiconductors as gain media. The development of semiconductor lasers since the early 1960s has played an important role in our understanding of the basic optical properties of semiconductors and has revealed a wealth of important physical phenomena over the last five decades or so. From a technological perspective, semiconductor lasers have fundamentally altered the technology landscape and contributed greatly to our modern lifestyle—from miniature semiconductor lasers that are ubiquitous in many tech gadgets (such as CD/DVD players, sensors in our smartphones, and bar-code scanners) to the lasers that serve as workhorses within the modern communication systems that drive our internet, supercomputers, and data centers. As we stand at the beginning of the second half-century of semiconductor lasers, it is important to review the frontiers of the field, to foresee and analyze any potential challenges, and to develop strategies to meet such challenges. As with the larger field of semiconductor photonics, semiconductor laser research faces three major challenges: device size and energy efficiency, wavelength or bandgap diversity, and system integration. These challenges are explored in the following sections.
1.1 Device Size and Energy-Efficiency Challenge
The sizes of photonic devices are generally limited by the wavelengths involved. Thus, it becomes an important challenge to overcome wavelength limits or diffraction limits. The important questions to ask are whether, how, and to what degree we can break the diffraction limit to create ever smaller and better optoelectronic devices such as lasers. An important related issue is the energy efficiency of photonic devices (such as lasers, modulators, and switches) when used for information transmission,1 in terms of joule per bit of information transmitted. These questions and their related challenges are important for realizing future integrated nanophotonic on-chip circuits.
1.2 Wavelength or Bandgap Diversity Challenge
All semiconductor photonic devices, including lasers, are based on light–semiconductor interaction involving either absorption or emission of photons by semiconductors. Important spectral response (emission, refraction, or absorption) of any semiconductor is ultimately determined by its electronic bandgap and bandstructure. Many applications require (or can significantly benefit from) bandgaps that can be controllably tuned in a wide range, allowing bandgap diversity or flexibility, preferably on a single substrate or monolithically.2 However, our ability to achieve the required diversity of bandgap is rather limited, primarily because of the lattice-matching required in typical planar epitaxial growth of high-quality semiconductors. Such lack of ability to produce the requisite bandgaps severely impedes technological progress in many applications, including displays, solid-state lighting, solar cells, detectors, and widely tunable lasers. Nanoscale semiconductors, such as nanowires,2
1.3 Integration Challenge
A well-recognized long-term challenge is the achievement of integrated photonics on a silicon platform. While most passive devices can be fabricated directly on an Si platform, light sources are still almost exclusively made of III-V materials. Thus, heterogeneous integration of III-V-based lasers, or gain materials onto Si-waveguides, has become a prevailing approach.6
This paper presents an analysis and summary of the aforementioned challenges in semiconductor laser research, with focus on one of the frontiers of the field, namely, semiconductor nanolasers. We focus on the long-term issues and fundamental challenges that are likely to remain unresolved for the foreseeable future, and those that will impact the field in profound ways. In particular, the size-energy-efficiency challenge is emphasized because of its importance and potential impact on the other two challenges and on the entire field of semiconductor photonics. In the following sections, the background of the size-energy-efficiency challenge is introduced, and the relevant progress made in the last decade or so is highlighted. The potential and shortcomings of each approach are analyzed. Finally, we present future perspectives in the resolution of these challenges and the possible impact on the field of semiconductor lasers and photonics.
2 Size and Energy-Efficiency Challenge
To motivate the need for ever-smaller lasers, we recall one of the greatest technological revolutions of our times, namely, the computer revolution. The transformation of the first electrical computers to the present-day laptop or iPad [see
Fig. 1. Comparison of (a) first electrical computer with (b) today’s supercomputer. The similarity in volume and power consumption as well as the dramatic size reduction and improvement in computation power of the electrical computers over the last 70 years naturally raise an interesting question: would we ever be able to achieve similar volume reduction of today’s supercomputers through miniaturization and integration?
Fig. 2. (a) Schematic of a laser-based optical interconnect in a present-day supercomputer based on VCSELs and optical fiber and (b) future on-chip interconnect based on a nanolaser array and a waveguide array.
For such OE integration, it is important to significantly reduce the sizes of the optical devices and reduce the size mismatch between optical devices and electronic devices. Currently, typical optical devices, such as lasers, are two- to three-orders of magnitude larger than electrical devices. It is typically argued that the size mismatch between photonic and electronic devices is due to the much longer wavelengths of photons than electrons. Although it is true that wavelengths of photons are ultimately limited to the sizes of photonic devices, the wavelengths are presently not the limiting factor. Far more important at this stage is the “functionality” determined size limit. For example, the size of a modulator is determined by the length required to achieve a given amount of phase change (functionality length limit); this length is typically much larger than the wavelength of laser light. The functionality limit in a semiconductor laser is the gain length required to overcome the threshold and achieve lasing.2 Presently, the functionality limit is much larger than the wavelengths involved in typical photonic devices; thus, reducing this functionality limit is of highest priority to address size miniaturization.
Miniaturization of photonic devices is not merely required for the sake of size parity with electronic devices. Such miniaturization is more importantly related to energy efficiency, or the amount of energy an optical device consumes for each bit of information it transmits, also known as the energy-data rate (EDR), often expressed in the unit of femtojoule per bit.2,22 According to various system level analyses, photonic devices used for on-chip communications require the energy efficiency to be better than 10 fJ/bit and less than 1 fJ/bit in the near future, to be competitive with electronic interconnects.2 Presently, semiconductor lasers consume typically more than 1 pJ/bit (or milliwatt per Gbs). According to IBM’s estimate,23 exascale computers would require 800 million optical channels of 25 Gbs each for interconnects, representing a total power consumption of 20 MW for optical interconnects alone if an energy efficiency of 1 pJ/bit is assumed. Such a level of power consumption is obviously too high to tolerate. Thus, it is important to reduce the energy consumption of an optical transmitter, i.e., a semiconductor laser in the case of a directly modulated transmitter.
There is a close relationship between size, speed, and energy efficiency, as we demonstrated recently.22 A special case of this study22 is shown in
Fig. 3. Relationship between energy efficiency (EDR, in fJ/bit) and modulation bandwidth for various values of diameter for a cylindrical laser (adapted from Ref. 22).
3 Miniaturization of Semiconductor Lasers
Semiconductor lasers are the smallest and most energy efficient lasers among all types of lasers. They are best suited for applications involving on-chip or onboard integration because of their compact sizes and energy efficiency, and the possibility of operation under the convenient electrical bias. Such intrinsic advantages are, however, not sufficient to meet the much more stringent requirements of future optoelectronic-integrated chips. For these and many other reasons, constant size reduction has been one of the most recognizable features in the development of optoelectronics via constant inventions of paradigms of laser cavities over the past five decades.
Fig. 4. Device volume normalized by the wavelength cubed for several types of semiconductor lasers: EEL, edge emitting lasers; MD, microdisk; P-laser, plasmonic lasers. Symbols represent typical values for these lasers; colored bars indicate the ranges of values found in the literature. Red marks along the axis indicate the years when the type of laser was first experimentally demonstrated. The yellow bar is extended intentionally downward beyond the data symbols to indicate the potential for further size reduction for plasmonic laser and spaser.
3.1 Vertical Cavity Surface Emitting Lasers
VCSELs, which are one of the most important types of microsize lasers, were invented24 many years after the initial demonstration of conventional EELs. VCSELs offer a size reduction of at least an order of magnitude in total device volume compared to EELs (see
3.2 Microdisk Lasers
Microdisk lasers were initially developed28
3.3 Photonic Crystal Lasers
Photonic crystal lasers30,32
3.4 Nanowire or Nanopillar Lasers
Semiconductor nanowires or nanopillars in air provide one of the best semiconductor optical cavities via the large index contrast (similar to that of microdisk lasers). As with the microdisk lasers, the mode confinement is much better than in typical double-heterostructures, with the possibility of achieving a confinement factor of
3.5 Two-Dimensional Material Nanolasers
With the reduction of laser cavity size or volume, the cavity quality factor decreases. This decrease in cavity quality factor occurs for both pure dielectric cavities and metallic or plasmonic (see Sec.
4 Plasmonic Nanolasers and Spasers
4.1 Progress Overview
Plasmonic lasers or spasers61
One of the first demonstrations of plasmonic mode lasing63 was reported soon afterward; such lasing, which is closer to the original proposal,62 involved the use of an InP/InGaP/InP core and silver shell, with the core having a rectangular cross section etched from an InP-InGaAs wafer with thickness varying between 80 and 340 nm. The device operated in the so-called plasmonic-gap mode, with a semiconductor core as thin as 80–90 nm. The entire optical thickness of the device, including the SiN insulating layer and the metal penetration, is
4.2 Benefits of Plasmonic Nanolasers
Several important advantages of plasmonic nanolasers that are relevant to size and energy efficiency are worth mentioning here. One advantage is the size or volume. When comparing the sizes or volumes of various lasers, it is important to note that there are three types of volume that are relevant: the volume of the active region, the modal volume, and the total volume of devices. The volume of the active region determines the total number of electron-hole pairs that need to be injected. For a given transparency carrier density, the smaller the volume of the active region is, the smaller the threshold current, and thus, the smaller the total electrical energy input (I-V product). For low energy operation, a small active region is preferred. The small modal volume often results in a large confinement factor if the optical modes enclose the active region. A large confinement factor corresponds to a large modal gain. The total device volume is often limited by the precious availability of real estate on chip for integrated photonics applications and is also related to the heat dissipation efficiency. Thus, all three volumes must be small for lasers to be used as on-chip light sources. An efficient small laser will preferably have all three volumes as small and as closely equal as possible. Plasmonic lasers can be designed to have the smallest values of all three volumes among all the proposed types of nanolaser.92 For example, PC lasers, which have small modal volumes, often have large volumes of the active region and large total device volumes. Currently, most of the plasmonic lasers demonstrated have not been miniaturized in all three dimensions and can be further optimized to further reduce sizes in one or two more dimensions.92
Because energy efficiency is defined as the energy consumed per bit of information transmitted, energy efficiency benefits greatly from high-speed operation. Purcell enhancement (see
Low lasing threshold is generally desired for low power consumption. This might lead people to believe that plasmonic or metallic cavity lasers are unsuited for energy-efficient applications. However, low threshold alone does not always translate into high energy efficiency. Many conventional lasers, such as VCSELs, have extremely low thresholds; however, their energy efficiency is too low for future integrated photonics applications, as mentioned earlier.
Fig. 5. Comparison of a dielectric cavity and a metallic cavity. (a) The dependence of the cavity factor on its diameter for two cavities: a pure dielectric cavity and a dielectric cylinder with a metal shell (adapted from Ref. 84). (b) Laser performance comparison of a semiconductor pillar cavity [denoted with (D)] and a semiconductor pillar cavity with metal shell [denoted with (M)] (from the supporting information of Ref. 22): the laser output power (P) and temperature (T) are shown.
Heat dissipation is another important factor for high-density on-chip integration and thus must be considered when comparing various lasers. Many of the other proposed laser miniaturization solutions suffer from poor heat dissipation, such as nanowire lasers, PC lasers, and microdisk lasers, because of the long or poor heat conduction channels involved in these lasers. For example, the airgaps involved in most of the PC lasers prevent a more efficient downward heat dissipation to the substrate. Plasmonic lasers provide potential advantages in this respect via the close proximity of metal shell to active region22,63 and much improved heat conduction by the metal. These advantages are illustrated in
Another counterintuitive result is the plasmonic enhancement of optical gain due to an unusual feature of confinement factor near plasmonic resonance,97 resulting in slowing down of energy propagation. This slow energy propagation leads to a giant enhancement of optical gain,98 leading to much higher optical gain than material gain. Such giant optical gain could lead to a significant decrease of laser threshold. However, the slowed-down energy propagation may affect the modulation speed when extreme high speed is desired. A detailed optimization of the trade-off between low threshold and high modulation speed is required to achieve optimal performance.
5 Issues and Perspectives
Although VCSELs are deployed widely in many applications, including optical interconnects in supercomputers and datacenters, their applications for on-chip interconnects in the long run are hindered by the relatively large volume, which limits the energy efficiency to above tens to hundreds of fJ/bit. Significant reduction below this level is not likely. Furthermore, thick DBRs in VCSELs result in very high profile and inefficient heat dissipation, a significant disadvantage for on-chip integration.
To date, microdisk lasers are among the smallest lasers, as shown in
PC lasers are among the best developed nanoscale lasers to date, with high-speed modulation, reasonable energy efficiency, and electrical injection pumping demonstrated; however, their current power efficiency is well below that of typical EELs or VCSELs. Significant research is still required to improve electrical injection efficiency and device wall plug efficiency. Similar to microdisk lasers, PCs typically involve under-etching to increase mode confinement vertical to the PC plane, making heat dissipation inefficient and the mechanical stability quite poor. In addition, the total sizes of typical PC lasers are still on the order of 10s of microns, even though the modal volumes are much smaller. One major advantage of PC lasers compared to plasmonic nanolasers is the low threshold. PC lasers are quite strong candidates for use as on-chip lasers for interconnects, especially where the large total volume can be accepted.
There is still significant room for size reduction of semiconductor nanowire or nanopillar lasers. The great advantages of such lasers are their 1-D morphology and high index contrast (allowing them to serve as natural waveguides and cavities) and their material flexibility in energy bandgaps (via the larger tolerance to lattice mismatch). However, nanowire lasers are not yet mature enough for systematic examination in terms of high-speed modulation and low-power applications. Electrical injection remains difficult based on the bottom-up manufacturing approach. The full potential of nanowire lasers is difficult to estimate accurately, leaving more research opportunities for device engineers.
Plasmonic nanolasers promise to provide high-speed operation. However, systematic experimental work on high-speed modulation is lacking. Demonstration of electrical injection plasmonic nanolasers remains a significant challenge, especially at high temperature. The demonstrated electrical injection operation at higher operation temperature is usually based on dielectric modes. Innovative designs of plasmonic nanolasers are required that take full consideration of low loss, high injection efficiency, high speed, and low energy consumption.
Most of the discussions of this article have focused on the cavity or mode confinement mechanism. In the long run, the ultimate solutions to the size-energy-efficiency challenge require design and fabrication from both the photon aspect (i.e., cavity design or mode confinement) and the charge carrier aspect (i.e., gain medium). As shown in
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Article Outline
Cun-Zheng Ning. Semiconductor nanolasers and the size-energy-efficiency challenge: a review[J]. Advanced Photonics, 2019, 1(1): 014002.