Lasers are widely used in household appliances, medicine, industry, telecommunications and more. A nanolaser is a laser that has nanoscale dimensions. These tiny lasers can be modulated quickly and, combined with their small footprint, this makes them ideal candidates for on-chip optical computing.
Several years ago, scientists introduced nanolasers that generate coherent light at the nanoscale.. Their design is similar to that of the conventional semiconductor lasers based on heterostructures in common use for several decades. The difference is that the cavities of nanolasers are exceedingly small, on the order of the wavelength of the light they emit. Since they mostly generate visible and infrared light, the size is on the order of one millionth of a meter.
In 2012, researchers at Northwestern University published a description of a working room-temperature nanolaser “based on three-dimensional (3D) Au bowtie (nanoparticles) supported by an organic gain material,” constructs which were thought to be suitable for inclusion in photonic circuit architectures. In February 2012, researchers at University of California, San Diego demonstrated the first thresholdless laser and the smallest room temperature nanolaser using plasmonic nanoscale coaxial structures
In the past decade, they have attracted intense interest, because they are more compact, faster and more power-efficient than conventional lasers. The nanolaser also can operate in extremely confined spaces, including quantum circuits and microprocessors for ultra-fast and low-power electronics. Nanolasers are now an emergent tool for a variety of practical applications such as optical interconnects, near-field spectroscopy and sensing, optical probing for biological systems and far-field beam synthesis through near-field eigenmode engineering.
In the near future, nanolasers will be incorporated into integrated optical circuits, where they are required for a new generation of high-speed interconnects based on photonic waveguides, which would boost the performance of CPUs and GPUs by several orders of magnitude. In a similar way, the advent of fiber optic internet has enhanced connection speeds, while also boosting energy efficiency.
They will also be used as miniature sensors, Researchers are already developing chemical and biological sensors, mere millionths of a meter large, and mechanical stress sensors as tiny as several billionths of a meter. Nanolasers are also expected to be used for controlling neuron activity in living organisms, including humans.
In a normal laser, each photon (light particle) is ‘cloned’ many times in a medium that is located inside a cavity (e.g. a pair of mirrors between which the photon moves back and forth producing other photons with the same characteristics). This process is known as Light Amplification by Stimulated Emission of Radiation (LASER). To achieve laser emission an electrical current is usually injected through the medium, or it is illuminated with high energy light. The minimum energy needed for a laser to emit is called the lasing threshold.
Lasers provide the unique capability to concentrate energy in the form of coherent radiation in the smallest phase-space volume possible in optics. This allows the formation of coherent beams with a minimum angular divergence or the focusing of radiation to the smallest possible spots, with sizes down to a half-wavelength.
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. 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, perovskite platelets, and quantum dots, offer many potential benefits and have enabled many exciting developments, such as widely tunable emission in the entire visible spectrum, or white lasers from a single substrate, or a single monolithic semiconductor. Such nanomaterials must be fully and systematically explored to develop more mature devices.
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. 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.
Presently, semiconductor lasers consume typically more than 1 pJ/bit (or milliwatt per Gbs). According to IBM’s estimate, 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. We see that device sizes on the orders of a few hundred nanometers in diameter are required for energy efficiency on the order of 1 fJ/bit. This analysis clearly demonstrates why nanolasers are required for such applications.
Thus, realizing lasers of small size and high energy efficiency is the major challenge faced by the semiconductor photonics community. The intense optical fields of such a laser also enable the enhancement effect in non-linear optics or surface-enhanced-raman-scattering (SERS), and therefore paves the way toward integrated nanophotonic circuitry.
Nanoscale laser types
Photonic crystal lasers are promising candidates for use in energy-efficient applications because they have very small volumes of optical modes and represent a type of laser based on a very fruitful wave-analogy between electrons and photons. These lasers can have very low thresholds as a result. Recently, direct modulation at 5.5 GHz at room temperature via optical pumping has been demonstrated. The energy efficiency was estimated to be 13 fJ, with one of the lowest threshold pumping powers, estimated at 1.5 microwatts. Such lasers are of great importance both for studying basic quantum optics of nanolasers and for potential use in integrated photonic applications because of the ability to achieve unprecedented tight three-dimensional (3-D) mode confinement. The total device sizes including the large periodic dielectric structures are very large, however.
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 greater than 1. Such nanowires with the two ends exposed to air provide a unique structure both for high-reflective laser cavity and a gain medium at the same time, an ideal combination for laser miniaturization. After the initial wave of laser demonstration in the UV and visible wavelength regime, the first near IR lasing was demonstrated using a single GaSb nanowire at the telecom wavelengths. Recently, several realizations of lasing were demonstrated in the short wavelength NIR regime.
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 cavities, albeit according to different scaling laws. Therefore, it is important to constantly search for better gain materials that provide high optical gain within a small volume. In this regard, the newly emerging 2-D materials, such as transition metal dichalcogenides (TMDCs), show great promise.
Plasmonic lasers or spasers are the newest member of the laser family; such lasers can realize the smallest sizes of any lasers and thus can be potentially energy-efficient lasers when small size is essential. Plasmonic lasers utilize the coupling between photons and plasmon excitation at metal–dielectric interfaces to confine photons to the smallest possible spatial volumes.
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. 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. Plasmonic lasers can be designed to have the smallest values of all three volumes among all the proposed types of nanolaser.
New type of low-energy nanolaser that shines in all directions
Different kind of laser is the so-called polariton laser. This works on the principle not of cloning photons but making non-identical photons identical in much the same way as water vapor molecules, moving in all directions with different velocities, are condensed into a single drop. Condensation of photons gives rise to the intense and directional emission characteristic of a laser. An important advantage of polariton lasers is that they have a much lower lasing threshold, which makes them excellent candidates for many applications.
Researchers in Eindhoven have developed a new type of low-energy, nanoscale laser that shines in all directions. Published in the prestigious journal Physical Review Letters, scientists from Eindhoven University of Technology (TU/e) and the Dutch Institute for Fundamental Energy Research (DIFFER) have investigated the role of imperfections and disorder in nanolasers. By introducing a slight degree of disorder, they have observed a dramatic change: the laser no longer emits in one specific direction, but in all directions.
However, a major problem of polariton lasers has been that they need to operate at very low temperatures (like vapor condensation that takes place only when the temperature is lowered) but by using organic materials, it is possible to obtain polariton laser emission even at ambient temperature. The Eindhoven researchers demonstrated last year that they can realize nanoscale polariton lasers that function at ambient temperature, using metallic nanoparticles instead of mirrors as in normal lasers.
The TU/e-DIFFER researchers have now discovered a new kind of polariton laser that consists of a regular pattern of silver nanostripes covered with colored PMMA-polymer whose dye comprises organic emitting molecules. However, the silver stripes deliberately have some degree of imperfection and disorder. The emission from this non-perfect nanolaser is omnidirectional and mainly is determined by the properties of the organic molecules. This result is not expected in the framework of condensation, as omnidirectional emission requires emissions from independent organic molecules instead of the collective emission that is typical for condensation. The demonstration of omnidirectional emission defines new boundaries for the development of nanoscale lasers at ambient temperatures.
The researchers think their laser may eventually be applied in many areas. Compared to a LED, the omnidirectional laser light is much brighter and better defined. That’s why it is a good candidate for microscopy lighting, which currently uses LEDs. LIDAR (Laser Imaging Detection And Ranging) is another potential application. Current LIDAR use one or more lasers and a set of fast moving mirrors in order to cover large areas to image distant objects. An omnidirectional laser does not require the moving mirrors, thereby significantly reducing the complexity. And also general illumination is an option, says lead researcher professor Jaime Gomez Rivas. “But the research is still very fundamental. We hope that our results will stimulate other researchers to improve them by further reducing the lasing threshold or increasing the range of emitted colors.”
Shrunken nanolasers enable on-chip optical connections
The challenge is to connect optics and electronics at the nanoscale. To achieve this, the optical components cannot be larger than hundreds of nanometres. This size restriction also applies to on-chip lasers, which are necessary for converting information from electrical signals to optical pulses that carry the bits of the data. However, light is a kind of electromagnetic radiation with a wavelength of hundreds of nanometres. And the quantum uncertainty principle says there is a certain minimum volume that light particles, or photons, can be localized in. It cannot be smaller than the cube of the wavelength. In crude terms, if one makes a laser too small, the photons will not fit into it. That said, there are ways around this restriction on the size of optical devices, which is known as the diffraction limit. The solution is to replace photons with surface plasmon-polaritons, or SPPs.
Researchers from the Moscow Institute of Physics and Technology and King’s College London cleared the obstacle that had prevented the creation of electrically driven nanolasers for ICs. The approach enables a coherent light source design on the scale smaller than the wavelength of light emitted by the laser. This enables ultrafast optical data transfer in microprocessors.
SPPs are collective oscillations of electrons that are confined to the surface of a metal and interact with the surrounding electromagnetic field. Only a few metals known as plasmonic metals are good to work with SPPs: gold, silver, copper, and aluminum. Just like photons, SPPs are electromagnetic waves, but at the same frequency they are much better localized — that is, they occupy less space. Using SPPs instead of photons makes it possible to “compress” light and thus overcome the diffraction limit. The design of truly nanoscale plasmonic lasers is already possible with current technologies. However, these nanolasers are optically pumped, that is, they have to be illuminated with external bulky and high-power lasers.
An electronic chip intended for mass production and real-life applications has to incorporate hundreds of nanolasers and operate on an ordinary printed circuit board. A practical laser needs to be electrically pumped, or, in other words, powered by an ordinary battery or DC power supply. So far such lasers are only available as devices that operate at cryogenic temperatures, which is not suitable for most practical applications, since maintaining liquid nitrogen cooling is not typically possible. The physicists from the Moscow Institute of Physics and Technology (MIPT) and King’s College London have proposed an alternative to the conventional way electrical pumping works.
Usually the scheme of electrical pumping of nanolasers requires an ohmic contact made of titanium, chromium, or a similar metal. Moreover, that contact has to be a part of the resonator — the volume where the laser radiation is generated. The problem with that is titanium and chromium strongly absorb light, which harms resonator performance. Such lasers suffer from high pump current and are susceptible to overheating. This is why the need for cryogenic cooling emerges, along with all the inconveniences it entails.
The proposed new scheme for electrical pumping is based on a double heterostructure with a tunneling Schottky contact. It makes the ohmic contact with its strongly absorbing metal redundant. The pumping now happens across the interface between the plasmonic metal and semiconductor, along which SPPs propagate. The plasmonic nanolaser proposed by the researchers is smaller — in each of its three dimensions — than the wavelength of the light it emits. Moreover, the volume occupied by SPPs in the nanolaser is 30 times smaller than the light wavelength cubed.
According to the researchers, their room-temperature plasmonic nanolaser could be easily made even smaller, making its characteristics even more impressive, but that would come at the cost of the inability to effectively extract the radiation into a bus waveguide. Thus, while further miniaturization would render the device poorly applicable to on-chip integrated circuits, it would be still convenient for chemical and biological sensors and near-field optical spectroscopy or optogenetics. Despite its nanoscale dimensions, the predicted output power of the nanolaser amounts to over 100 microwatts, which is comparable to much larger photonic lasers.
Such a high output power allows each nanolaser to be used to transmit hundreds of gigabits per second, eliminating one of the most formidable obstacles to higher-performance microchips. And that includes all sorts of hi-end computing devices: supercomputer processors, graphic processors, and perhaps even some gadgets to be invented in the future.
ITMO University has been awarded one projects, centered around nanolasers and microlasers based on next-gen nanomaterials and modern optical architectures. Sergey Makarov, a professor at ITMO’s , represents Russia in the interdisciplinary project on creating new nano- and microlasers. This research will be carried out at the intersection of physics, chemistry, and materials science by the joint efforts of the faculty’s leading young scientists.The researchers aim to create new ultra-compact high-speed lasers that will operate in the visible rather than the infrared range. They plan to apply advanced nanomaterials, such as gallium phosphide, which can be potentially used in the production of compact optoelectronic devices for various applications – from communication lines to optical computers.
“We want to create working prototypes of optoelectronic devices,” emphasizes Sergey Makarov. “We want to start by conducting fundamental research. For example, lasers will be first pumped by light from other lasers, and systems will operate at low temperatures. But our end goal will be to switch to systems capable of working at room temperature and lasers pumped by current. This will allow us to fully integrate them in optical chips.” Hilmi Volkan Demir, a renowned specialist at the Nanyang Technological University in Singapore and a major expert in optoelectronics, became one of the international partners of the project.
“He is a pioneer in nanomaterials and his inventions were widely used in various displays and flexible electronics,” says Sergey Makarov. “Currently, he is engaged in combining next-gen nanomaterials with the new concepts of nanophotonics. He has a great deal of experience in bridging science and industry. Many of his works can be found in many top-rated journals.”
Tiny, Biocompatible Laser Could Function Inside Living Tissues
Researchers have developed a tiny nanolaser that can function inside living tissues without harming them. Just 50 to 150 nanometers thick, the laser is about 1/1,000th the thickness of a single human hair. At this size, the laser can fit and function inside living tissues, with the potential to sense disease biomarkers or perhaps treat deep-brain neurological disorders, such as epilepsy.
Developed by NSF-funded researchers at Northwestern and Columbia Universities, the nanolaser shows promise for imaging in living tissues. Not only is the laser made mostly of glass, which is intrinsically biocompatible, but it can also be excited with longer wavelengths of light and emit at shorter wavelengths.
“Longer wavelengths of light are needed for bioimaging because they can penetrate farther into tissues than visible wavelength photons,” said Northwestern’s Teri Odom, who co-led the research. “But shorter wavelengths of light are often desirable at those same deep areas. We have designed a system that can deliver visible laser light at penetration depths accessible to longer wavelengths.”
While many applications require increasingly small lasers, researchers continually run into the same roadblock: Nanolasers tend to be much less efficient than their macroscopic counterparts. And these lasers typically need shorter wavelengths, such as ultraviolet light, to power them. Odom, Schuck and their teams were able to achieve a nanolaser platform that solves these issues by using photon upconversion. In upconversion, low-energy photons are absorbed and converted into one photon with higher energy.
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