Metal-Cavity Nanolasers

CHAPTER I

INTRODUCTION

Since the invention of the first laser by Maiman in 1960, different lines of development have yielded lasers the size of buildings, or as small as a few tens of nanometers. Perhaps the greatest impact on society has been had with making lasers smaller. In particular, the invention of the semiconductor laser has allowed small, electrically driven lower power coherent light sources. One of the later developments in the miniaturization of the laser has been the vertical cavity surface-emitting laser or VCSEL.

The VCSEL was the first laser with dimensions which approached the wavelength scale. The VCSEL has found many applications due in part to the following characteristics: electrical pumping, room temperature operation, reasonable efficiency, small threshold current, useful output beam characteristics, and ease of test and manufacture owing to the surface normal output.

With the increase of demand for internet bandwidth, photonic devices have gained a lot of attention in the past decades and much significant progress has been made. A few years ago, researchers only considered two types of small lasers: micro disk lasers and photonic crystal lasers (PhCs). The former have been developed for many years and are the most mature among small lasers. Moreover, shows the feasibility of integrating micro disk lasers with straight waveguides using wafer boding. However, one of their disadvantages is whispering gallery mode operation, with light circulating around the edge. A waveguide needs to be placed right next to it to couple light out. Thus, the other design parameter, the gap between the waveguide and micro disk, has to be optimized, thus increasing the challenge in fabrication.

Fig1.1: A nanoscale laser.

Nanoscale lasers possess advantages such as low power consumption, an ultra small footprint, and ultrafast switching. Potential applications include biochemical sensing, imaging, and intrachip and interchip short-distance optical interconnects. Practical nanolasersrequire electrical injection operation at room temperature in continuous-wave mode. Independent nanolasers can form dense arrays of sub wavelength pitch for possible near-field scanning and optical atom traps. The smallest laser based on dielectric cavities requires an optical cavity with a dimension of half a wavelength in all three directions, which is often called the diffraction limit.

During the last decade, photonic crystal lasers have been extensively studied as candidates for small lasers. However, to have a large quality factor for laser action, many periods of photonic crystal are required, making the size on the order of several wavelengths. To produce a laser breaking the diffraction limit, one approach is to use the plasmonic effect formed at the interfaces between the metal and semiconductor. In this case, both the physical and effective volume of the optical cavity can be reduced, although it would be at the expense of modal absorption due to the metal loss.

By positioning the active materials such as quantum dots or quantum wells (QWs) at the peak of optical fields with an emission wavelength near the cavity resonance, it is possible to enhance the spontaneous and stimulated emission and reduce the lasing threshold. There has been excellent progress in micro- and nanolasers, especially metallic and plasmonic nanolasers. Plasmonic nanolasers via optical pumping have been reported by using a CdS nanowire as the gain medium on top of a silver surface with a 5-nm insulator gap. Nanoparticles with a gold core and dye-doped silica shell have been used to realize spaser-based nanolasers via optical pumping

*****CHAPTER II

HISTORY

In 2010, significant progress on micro- and nanolasers has been made, i.e., sub wavelength nanolasers via optical pumping, nanopillarlasers on silicon substrate, electrical injection FabryPerot metal-cavity lasers at 240 K, and substrate-free metal-cavity surface emitting micro lasers at room temperature.

At the University of California at San Diego, metallo-dielectric sub wavelength lasers using an InGaAsP multiple quantum well (QW) active layer disk surrounded by an aluminum/silica bilayer shield as the cavity were made by optical pumping at room temperature. The importance of the optimized thickness of the insulating silica is emphasized to reduce the threshold gain for optical pumping at room temperature. The feedback is provided by a mode cutoff plug-in structure which forbids the propagating mode inside, thus achieving a high reflectivity mirror.

At University of California at Berkeley, sub wavelength nanopatch lasers using top and bottom metals (gold) to form the nanocavity with InP/InGaAsP/InP materials with a physical volume were demonstrated at 78 K by optical pumping. Due to their resemblance to patch antennas in microwave technology, the structures emit light from the sidewalls with constructive/destructive interferences in the surface normal direction and are suitable for beam divergence control.

Polarization controllability has been demonstrated by tuning the geometry of the nanopatches. Silver nanopanplasmonic lasers have also been demonstrated at 8 K with a subnanometerlinewidth by optical pumping. Whispering gallery modes in silver defined cavity were identified in nanopanplasmonic lasers. Nanolasers using InGaAs nanopillars grown on silicon substrate by optical pumping at room temperature have also been reported by UC Berkeley.

Until recently, the electrical injection of metal-cavity semiconductor lasers has demonstrated significant progress, such as high-temperature (240 K) continuous-wave (CW) operation using a FabryPerot type with emission from the bottom aperture by Arizona State University and Technical University of Eindhoven, as well as CW room temperature surface emitting micro laser bonded on silicon by the University of Illinois and the Technical University of Berlin.

*****CHAPTER III

NANO LASERS

A nanolaser, also referred to as a miniature laser is a laser, namely a light amplifier by stimulated emission of radiation that has nanoscale dimensions. While the word nano originated from Greek which means dwarf, the international system of units has adapted the prefix as a quantifier equal to ten raise to the power of minus nine. The nanolaser concept was developed by mark stockman at Georgia state university in 2003.

Fig 3.1: Nano lasers This tiny laser can be modulated quickly and combined with its small footprint, makes it an ideal candidate for onchip optical computing. the intense optical fields of such a nanolaser also enables the enhancement effect in non linear optics or surface enhanced Raman scattering, and the therefore paves the way toward integrated nanophotonic circuitry. In 2012, researchers at northwestern university published a description of a working room temperature nanolaser based on three dimensional Au bowite(nanoparticles) supported by an organic gain material, constructs which were thought to be suitable for inclusion in photonic circuit architectures. For a long time it was thought that direction effects made it impossible for lasers and other photonic devices to be small than about half the wavelength of the light they emitted or processed. Between 2000-2005 years many intriguing designs of microscopic lasers based on tiny pillars, nanowires and photonic crystals have all approached this limit but exceeded it. For example, using a photonic crystal, it is possible to construct a nanometer scale laser shown in figure with a modal volume close to the direction limit of light. Nanolasers are small size laser which goes beyond the direction limit by using special mechanisms and geometric. By nanolaser we are not only achieve small size laser, but also we can obtain almost all threshold less laser. Nanolasers are important partner in light and matter interaction. That is why many research from the world wide devoted to that important subject. Actually this is hot topic for researchers in the end of the nanophoton ICs. The advantages of nanolasers lie not only in their low power consumption due to high single-mode spontaneous emission coupling into the cavity mode, but also in their high modulation bandwidth. One of the figures of the merit for a laser is the energy per bit, which is defined as the ratio of the supplied power at which the maximum bandwidth is reached to the maximum bandwidth. Obviously, a smaller value indicates better energy efficiency. To evaluate the energy per bit of nanolasers, however, a rigorous treatment of the rate equations is needed to guide the future direction.

Fig 3.2: A typical view of nanolaser

*****CHAPTER IV

Metal-Cavity Lasers and the State of the Art

One new small laser is the metal cavity laser. The permittivity of real metal is negative in the frequency domain and this characteristic can allow the plasmonic mode to exist. The most important feature of the plasmonic mode is that it can be confined at the interface and behaves as a surface wave. Therefore, the optical modal volume of the plasmonic mode can break the diffraction limit, which is defined as, where is the wavelength in free space and n is the refractive index of the material surrounded by PEC. However, metal is dispersive and has great loss at room temperature; therefore, researchers used to think the metal could not be used to form the semiconductor laser cavity, especially in infrared regime.

In 2007, Hill et al. successfully demonstrated the first metal-cavity nanolaser with cavity size 0.018 , equivalent to 0.38, breaking the diffraction limit and dispelling the belief that metal is too lossy to be the laser cavity. Indeed, metal used to be a mirror to provide high reflectivity, but it has never been considered a candidate for a laser cavity able to confine light in a sub wavelength region. Some papers studied plasmonic effects on metal-coated waveguides before 2000; since then, related work on theory and experiments has been more intensive. The advent of Martins laser finally demonstrates the feasibility of metal-cavity lasers and paves the way for future practical application, such as optical interconnection, although this first-generation metal cavity nanolaser operates at cryogenic temperature and the output power goes through the substrate so that the power is too small to measure.

Due to Martins success, more and more groups started to develop different types of metal-cavity lasers and theoretically and experimentally demonstrated their work. We summarize the performance of the experimentally demonstrated state-of-the-art 4 devices in Fig 4.1. We notice the volume and operation condition is from cryogenic temperature to room temperature according to the cavity volume. The laser with the smaller cavity volume only works at lower temperatures.

Fig 4.1: state of the art: The operation temperatures and conditions for recent experimentally demonstrated metal cavity laser.

The reason is that although metal can shrink the optical field in a sub wavelength volume, more and more field penetrates into the metal, and the field suffers from more metal loss. The metal loss can be larger than the loss in conventional dielectric materials by two orders. It implies that when we work on metal-cavity lasers, higher material gain has to be overcome and many parameters, such as the optical confinement factor and the quality factor, have to be well designed and optimized. Those concerns are also related to the polarization of the optical mode excited.

Use of HE11 modes, in which the Ezcomponent is present and can couple to the plasmonic mode. Such a mode suffers high material loss in a diameter smaller than a subwavlength radius, and can only work at cryogenic temperature due to the suppression of metal loss in that condition. But it can behave as conventional fiber modes in micro cavity (radius larger than 1 m) with considerably small material loss and work at room temperature.

In addition, TE modes in metal cavities cannot couple to plasmonic mode and, thus, the material loss is much smaller than others and can operate at room temperature. The smallest cavity volume at room temperature, which is TE mode. Because of being decoupled to plasmonic loss, TE mode has the cutoff condition, which means that there is a bottom limit on the volume reduction. As for, the modes presented in their work are purely TM modes, i.e., plasmonic modes, except to one TE mode found in their devices. Due to plasmonic modes and the small cavity volume, those devices have to work at cryogenic temperature.

*****CHAPTER V

DEMONSTRATIONA demonstration of a metal cavity surface-emitting micro laser with metal on the top and surrounding sidewall and a bottom distributed Bragg reflector (DBR), which lases at room temperature under CW operation. The active region consists of 14 pairs of GaAs/AlGaAs QWs. Multiple QWs uniformly distributed in the active region are used to provide enough optical gain without worrying about the longitudinal standing wave (node/peak) effects. A17.5 pair n-doped quarter-wavelength DBR acts as both the feedback and the electron injector. (a) (b)

Fig 5.1: A metal cavity nanolaser. The figure shows a fabricated device having an active region of 14 GaAs/Al0.2Ga0:8 as quantum wells. Optical feedback is from the bottom silver and top hybrid/DBR mirrors with the surrounding metal sidewall. Silver encapsulation helps mode confinement and scattering reduction.

The integration to silicon was demonstrated by flip chip bonding to a gold coated silicon substrate with the complete removal of the GaAs substrate to allow its surface emission. The physical size after substrate removal is only 2.0 m in diameter and 2.5 m in total thickness, including the overall p i (QWs) n (DBR) regions. Flip chip bonding with metal allows the integration of our metal-cavity lasers to various substrates, including silicon in our devices.

Metal serves as a multifunction medium for reflector, contact, and heat sink. The round-trip resonance phase condition is satisfied by choosing the active layer thickness to match the boundary conditions at both top metal and bottom DBR for the metal confined fundamental optical mode. Also, with a broadband reflector using metal, the detuning of the cavity mode with the gain peak can thus be reduced, compared with standard vertical-cavity surface emitting lasers.

The devices were mounted on a thermoelectrically cooled copper heat sink for measurements at 300 K under CW operation. Thermal management has been largely improved as a result of efficient heat removal from the surrounding metal and the substrate-free configuration with bonding. We have measured the light output power as a function of the injection current at temperatures from 10 to 27C, showing temperature-stable operation with a characteristic temperature of 425 K.

The light output power is up to 7.5 W at 4.5 mA. We have also measured the laser line width and obtained a value of 0.67 A (full-width at half maximum) at a bias of 2.8 mA. This is probably the narrowest measured laser line width among metal-cavity lasers with electrical injection, which are typically hard to measure due to their low power. A kink at 3.2 mA bias current shows polarization switching behavior, which is confirmed by measuring the polarization resolved LI curves and emission spectra at various bias currents.

We have also developed a rigorous theoretical model, which takes into account the plasmonic dispersion in a nanocavity and pointed out the importance of using the energy (instead of power) confinement factor. Our theoretical formulation and the resultant rate equations have been applied to study nanolasers such as a nanobowtie laser and a metal-cavity edge-emitting laser for the prediction of lasing threshold and light output power versus injection current (LI curve).

To compare our theory with experimental data, we first calculate the band structure of the GaAs/AlGaAs QW lasers and the optical gain spectrum as a function of increasing carrier density. We also compared the amplified spontaneous emission spectra in the metal cavity with the measured asymmetrical electroluminescence spectra [see Fig] at various injection currents below threshold and obtained good agreement.

Fig 5.2: Current dependent spectra of the device lasing under CW current injection at300 K.

The band edge of the QW spontaneous emission spectrum and the cavity resonance spectrum creates an asymmetrical line shape. Our model result of the quality factor Q of 556 of the cold cavity is close to the measured value of 580 at low injection current. We then model the measured light output power as a function of the injection current based on our rate equations and show our theory agrees well with the experimental data shown in Fig. 5.3.

We found that at a very small bias current below 0.5 mA, there is no light emission until the spontaneous emission peak wavelength merges with the cavity resonance wavelength. Above 0.5 mA, the spontaneous emission starts to amplify significantly with increasing gain as the current increases. When the optical gain reaches threshold at 1.75 mA, the laser action starts to occur.

Fig 5.3: Light output power as a function of injection current (LI) curves at various temperatures (10 C27 C) and the IV curve at room temperature (27 C)

We have further reduced the size of our metal-cavity surface-emitting lasers by either shrinking the diameter or reducing the number of DBR pairs to only five or even zero, while maintaining a reasonable quality factor for laser action.

5.1. THE RATE EQUATIONS OF NANO LASERS.

Although the rate equations have been discussed widely, most of them do not consider the dispersive material and treat the normalization of the optical field properly. In this paper, we introduce the rate equations based on our rigorous derivations, taking into account plasmonic dispersion and negative permittivity of the metal plasma.

We should point out that these rate equations are applicable to both metal and dielectric cavities, from nano-, to micro-, to macroscale lasers:

eqn 5.1, 5.2.

Wheren = carrier density ()I = injection current (A)i = current injection efficiencyq = electron unit charge (Coulomb)Va= active volume ()Rnr(n) = nonradiative recombination rate ()Rsp(n) = total spontaneous emission rate ()Rst(n) = stimulated emission coefficient ()S = photon density ()E= optical energy confinement factorsp(n) = spontaneous emission coupling factorp= photon lifetime (ns)

Here, the optical energy confinement factor, E, is used to correctly account for the negative permittivity and dispersive properties of the metal plasma:

.eqn 5.3, 5.4.

WhereR= the real part of the relative permittivityg= the real part of the relative group permittivityVeff = the effective optical modal volume () (= Va/E)m(r) = the phasor of the electric field

The subscript a indicates the active region. The nonradiative recombination rate accounting for the surface recombination and Auger recombination is:eqn 5.4WhereVs= the surface velocity (cm)Aa= the surface area of the active material ()C = Auger recombination coefficient ()

The total spontaneous emission rate contains all of the discrete cavity modes and continuous modes:eqn 5.5

Here, the first term is from the discrete cavity modes and will be defined later. The second term is from the continuous modes and modeled as the ratio of the carrier recombination from the density of states available in the active material to a background radiative time sp rad. Although there should exist only one cavity mode in a nanocavity, we keep m to distinguish Rsp,m(n) from Rsp(n) and refer Rsp,m(n) to the single-mode spontaneous emission rate.

With a rigorous treatment of the stimulated emission and spontaneous emission, our rate equations are derived for nanolasers and NanoLEDs with the dispersion.

The importance of our rate equations is the introduction of the optical energy confinement factor to take into account the plasma dispersion and the negative permittivity of metal; therefore, the optical energy is always positive.

*****CHAPTER VI

THE CHALLENGE OF METAL CAVITY NANOLASERS

There are a few difficulties for the realization of metal-cavity lasers, especially in fabrication.First of all, the purpose of using metal is to confine light in a sub wavelength cavity. In other words, if the cavity size is much larger than a wavelength, metal helps little but incurs material loss since in such a cavity the optical field is already well confined and the metal imposes the loss upon the tail of the field. Therefore, metal-cavity lasers imply micro- or nanolasers.

Fabrication of a wavelength dimension or nanostructure device is a great challenge. Many considerations have to be addressed, such as how to produce smooth and conformal surfaces of the semiconductor and metal at such a tiny size and the heuristic process is inevitable. The uniformity in one chip is another issue, and the low yield can bar metal-cavity lasers from practical applications.

In terms of device physics, work has to be done as well. Ideally, a metal-cavity laser can have a small threshold current. To understand this, we start with the rate equations for a single mode laser...eqn 6.1, 6.2, 6.3Where, are carrier density, electron charge, active material volume, current injection efficiency, spontaneous emission rate, single mode spontaneous emission rate, single mode spontaneous emission coupling factor, nonradioactive emission rate, stimulated emission rate, optical energy confinement factor, and resonant angular frequency respectively.

Further, the threshold current can be written as:.eqn 6.4 Below the threshold, spontaneous and nonradiative emission rates are dominant and the stimulated emission rate can be ignored. The nonradiative emission includes two terms: one is surface or defect recombination and the other is Auger recombination. We can see that once sp(n) is large or equal to 1, the threshold current can be significantly reduced and even zero when i= 1 and Rnr= 0. If there is only one resonant mode in the cavity, then sp(n) is unity theoretically. This is the purpose of metal-cavity nanolasers. The small cavity volume reduces the number of resonant modes and enhances the sp (n) and, thus, reduces the power consumption. When sp (n) is 1, it means the radiation from carrier recombination completely couples to the stimulated emission and is not consumed in the spontaneous emission. In this ideal case, the device is a threshold less laser.

Fig 6.1: The internal structure of a metal cavity nano-scale laser.

In reality, however, the nonradiative emission cannot be zero. Nanostructures tend to suffer from surface recombination because the surface recombination rate is proportional to the ratio of the surface area to a volume of the active region, and this ratio is high in the nanostructure. Therefore, good passivation in nanostructures is very important. Auger recombination happens to the light-emitting device working at long wavelength and/or with high carrier density operation since it is proportional to the cube of the carrier density. Due to the nonradiative emission, the threshold current can be increased by these two leakage paths. Other defects introduced by the fabrication can further deteriorate the device performance.

If the nonradiative emission is dominant in the injection current, it means the power conversion efficiency is low and most of the input electrical power is converted to heat instead of optical power. The generated heat raises the temperature inside the cavity and the gain decreases with the temperature. Thus, more carriers are needed to compensate for the reduction of the gain.

However, the increase in carrier density can further increase the temperature since more current is consumed in nonradiative current and more power converts to heat. The close cycle forms a positive feedback loop in terms of the temperature rise and prevents the devices from working at high bias. If the device is working at cryogenic temperature, the nonradiative emission can be reduced, but the device becomes less practical.

Based on the above, in order for the metal cavity lasers to work well at room temperature, the threshold material gain has to be as small as possible so that the carrier density can be reduced. The challenge is that the modal loss increases as the cavity size decreases. The design parameters such as insulator thickness, the active material thickness, and mode polarization alternatives play important roles.

6.1. SIGNIFICANCE

Nanoscale lasers possess advantages such as low power consumption, an ultra small footprint, and ultrafast switching.

Potential applications include biochemical sensing, imaging, and intrachip and interchip short-distance optical interconnects.

Nanolasers will have a large impact on our technology if they are integrable to current electronic architecture.

From an application point of view, nanolasers with integrability to current electronic platforms (i.e., silicon) will lead to advanced photonic integrated circuits.

Several nanolasers have shown a promising future for integration either by direct growth of nanopillars (without metal coating) on a silicon substrate or by stacking the devices onto the electronic platform.

*****CHAPTER VII

CONCLUSION

We have demonstrated experimentally a room-temperature metal-cavity surface emitting micro laser and developed a rigorous model for nanolasers with further reduction in size. Our theory explains the observed asymmetrical optical emission spectrum below threshold and the light output versus injection current (LI curve).

Nanolasers pose intriguing challenges for researchers in photonics, both intellectually and technologically. Due to their compactness in size and substrate-free and/or silicon compatibility, they are promising elements to bridge the gap between nanophotonics and silicon electronics. They have potential applications for ultrahigh density photonic integrated circuits with ultralow power consumption and footprint and ultrafast switching speed.

The ultrahigh modulation bandwidth of nanolasers has yet to be demonstrated experimentally. Further research is necessary to reduce the metal losses in the cavity and to overcome the technological challenges of nanofabrication of nanoscale semiconductor lasers with electrical injection.

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*****Dept. of EC, Dr MVSIT.Page 14