**5. Proposed nanoresonator structures**

(see **Figure 5(b)**) [16], and in three dimensions like nanocavity plasmon laser of

*Different structures of the plasmon nanolasers: (a) a plane plasmon nanolaser [13] (subwavelength in one dimension), (b) typical nanowire-based plasmon nanolaser [16] (subwavelength in two dimension), and (c) quantum well-based nanocavity plasmon laser of [15] (subwavelength in three dimension).*

The gain medium of plasmonic nanolasers can be any material capable of radiative electron decay like any traditional laser. In the proposed structures, a variety of

[17] (see **Figure 5(c)**).

*Nanoplasmonics*

**Figure 4.**

**Figure 5.**

**72**

*Different structures of the plasmonic nanolasers [13].*

According to our most recent publications [17, 31, 32], we have proposed four nanolaser structures that are discussed in this section. All of these structures are electrically pumped in the room temperature, have subwavelength footprints, and have considerable performance characteristics. The first structure is a GaAs quantum dot-based nanocavity integrated into a plasmonic waveguide [31]. The second is a metal strip nanocavity structure which is based in tensile-strained germanium quantum wells [32]. The next one has a notched nanocavity and germanium quantum wells as the gain medium [17] and the last one is a corrugated metal– semiconductor–metal nanocavity structure utilizing two sets of germanium quantum dot arrays as the gain medium [32].

The first structure is a GaAs/AlGaAs QD nanocavity plasmon laser, which can be integrated into plasmonic waveguides for the realization of integrated plasmonic chips. This proposed nanolaser as sketched in **Figure 6** has several advantages over the previously introduced ones.

For instance, it has a high coupling efficiency to the waveguide plasmonic modes because of its thin structure and the monolithic metal layer. In addition, the proposed nanolaser structure benefits from a large beta factor that means lower threshold and also a high Purcell factor, which leads to higher gain and better laser performance. The MSM structure of this device also can provide an efficient heat transfer performance. Therefore, it predicted to efficiently operate without overheating and needs less chip area for fabrication of heatsink. Nevertheless, the threshold pumping current of the proposed device is considerably high, and this structure cannot provide output power in the mW range in the optimal pumping region. Design characteristics related to the first structure can be seen in **Table 1**.

The second device is a germanium/silicon-germanium (Ge/Si0.11Ge0.89) multiple quantum well plasmonic nanolaser as shown in **Figure 7**. This device utilizes a thin gold metal strip layer, sandwiched between Ge quantum wells in order to maximize both field confinement and exciton-plasmon interaction possibility, which means higher Purcell factor and better gain medium with mode overlap factor. Using two aluminum electrical contacts, one on top of the resonator and one beside it, an electrical pump current can be applied. Moreover, it can be coupled into

**Figure 6.** *3D schematic of the GaAs quantum dot-based nanoresonator.*


#### **Table 1.**

*Design characteristics of the first structure.*

#### **Figure 7.**

silicon-based waveguides similar to [15] or used in the far-field configuration in which plasmon modes will be converted into photons through the cavity interface. Our device benefits from a metal–semiconductor–metal–semiconductor (MSMS) structure, which can perform well in the 1550 nm regime by means of incorporating highly doped strained Ge quantum wells as the direct bandgap gain medium [33, 34]. Design characteristics of this structure can be found in **Table 2**.

dots (tensile-strained direct energy bandgap). This structure can be easily integrated into different plasmonic and photonic waveguides with considerable coupling factors. Therefore, it is an appropriate choice for on-chip applications. Also, it can be simply used in the far-field lasing mode. An efficient integration approach can be found in [15]. This device also provides the output free-space wavelength of 1.55 μm, which means it is compatible with commercial photonic devices and systems. The design characteristics of the third structure can be witnessed in

**Description Symbol Value Unit** Resonator size *WR* 270 nm Resonator height *HR* 130 nm Bottom metal thickness *XStrip* 10 nm Metal thickness *XContact* 40 nm Bottom buffer thickness *XBottom* 15 nm Top buffer thickness *XTop* 15 nm Number of QWs *NQW* 4 — QW thickness *XQW* 7 nm Barrier wall thickness *XBarrier* 10 nm Thickness of p-doped Ge buffer *XBuffer* 16 nm Ge alloy percent *x* 89 % Doping concentration of the QWs and barriers *ND* 7.6 1019 cm<sup>3</sup> Doping concentration of the Ge buffer *NA* <sup>1</sup> 1019 cm<sup>3</sup>

*Nanoscale Plasmon Sources: Physical Principles and Novel Structures*

*DOI: http://dx.doi.org/10.5772/intechopen.90842*

The last proposed structure is a corrugated metal-semiconductor-metal nanocavity device that can be seen in **Figure 9**. The specific design of the cavity leads to a significant plasmonic mode with gain medium interaction and also an increase in mode confinement and quality factor. Furthermore, this cavity design using the two-side contacts can provide efficient electrically pumping with a reasonable threshold current. The gain medium of our nanolaser consists of several germanium quantum dots provided on both sides for maximizing the output power.

**Table 3**.

**75**

**Figure 8.**

**Table 2.**

*Design characteristics of the first structure.*

*3D schematic of the notched cavity nanolaser structure.*

It should be noticed that for transforming germanium into a direct bandgap material, strong tensile strain levels could be applied in the fabrication process. This will reduce the Г-valley direct bandgap of the material below the L-valley indirect bandgap (0.664 eV) [33, 34]. This will result in an output wavelength about several micrometers in which efficient plasmonic nanocavities cannot be designed. Alternatively, much lower strain level can be utilized, which in combination with extreme level of donor doping for occupying the remaining indirect L-valley states below the Г-valley results in a direct energy gap diagram [33, 34].

The third structure as shown in **Figure 8** has a cubic nanoresonator with two parabolic notches at both sides. This device provides a high-quality factor and Purcell factor because of the notches which can effectively decrease the output loss (amount of energy escaping the resonator) and improve energy confinement in the cavity. The gain medium of this structure consists of four Germanium quantum

*<sup>3</sup>D schematic of the metal strip nanocavity structure with germanium quantum wells.*

*Nanoscale Plasmon Sources: Physical Principles and Novel Structures DOI: http://dx.doi.org/10.5772/intechopen.90842*


#### **Table 2.**

silicon-based waveguides similar to [15] or used in the far-field configuration in which plasmon modes will be converted into photons through the cavity interface. Our device benefits from a metal–semiconductor–metal–semiconductor (MSMS) structure, which can perform well in the 1550 nm regime by means of incorporating highly doped strained Ge quantum wells as the direct bandgap gain medium [33, 34]. Design characteristics of this structure can be found in **Table 2**.

*3D schematic of the metal strip nanocavity structure with germanium quantum wells.*

Cavity height HR 50 nm Cavity size WR 260 nm QD size DQD 5 nm QD separation DQD2QD 10 nm Distance of QDs from gold plate HQD 25 nm Top/bottom metal thickness Hmetal 50 nm Doping level (p-type) NA 1017 cm<sup>3</sup> Doping level (n-type) ND 10<sup>19</sup> cm<sup>3</sup> Number of QDs NQD 256 QD volume VQD 1.25 1019 cm<sup>3</sup>

**Table 1.**

*Nanoplasmonics*

**Figure 7.**

**74**

*Design characteristics of the first structure.*

**Symbol Value**

It should be noticed that for transforming germanium into a direct bandgap material, strong tensile strain levels could be applied in the fabrication process. This will reduce the Г-valley direct bandgap of the material below the L-valley indirect bandgap (0.664 eV) [33, 34]. This will result in an output wavelength about several micrometers in which efficient plasmonic nanocavities cannot be designed. Alternatively, much lower strain level can be utilized, which in combination with extreme level of donor doping for occupying the remaining indirect L-valley states

The third structure as shown in **Figure 8** has a cubic nanoresonator with two parabolic notches at both sides. This device provides a high-quality factor and Purcell factor because of the notches which can effectively decrease the output loss (amount of energy escaping the resonator) and improve energy confinement in the cavity. The gain medium of this structure consists of four Germanium quantum

below the Г-valley results in a direct energy gap diagram [33, 34].

*Design characteristics of the first structure.*

**Figure 8.** *3D schematic of the notched cavity nanolaser structure.*

dots (tensile-strained direct energy bandgap). This structure can be easily integrated into different plasmonic and photonic waveguides with considerable coupling factors. Therefore, it is an appropriate choice for on-chip applications. Also, it can be simply used in the far-field lasing mode. An efficient integration approach can be found in [15]. This device also provides the output free-space wavelength of 1.55 μm, which means it is compatible with commercial photonic devices and systems. The design characteristics of the third structure can be witnessed in **Table 3**.

The last proposed structure is a corrugated metal-semiconductor-metal nanocavity device that can be seen in **Figure 9**. The specific design of the cavity leads to a significant plasmonic mode with gain medium interaction and also an increase in mode confinement and quality factor. Furthermore, this cavity design using the two-side contacts can provide efficient electrically pumping with a reasonable threshold current. The gain medium of our nanolaser consists of several germanium quantum dots provided on both sides for maximizing the output power.


rates in the QDs. This structure also has two aluminum contacts for electrical pumping into the gain medium. In this structure, the electrical pump current flows perpendicular to the plasmonic mode propagation direction into the germanium quantum dots (QDs) in order to produce excitons. This electron–hole pairs

*Nanoscale Plasmon Sources: Physical Principles and Novel Structures*

*DOI: http://dx.doi.org/10.5772/intechopen.90842*

**Table 4.**

Area (μm<sup>2</sup>

output power

threshold

threshold

**Table 5.**

**77**

*Design characteristics of the lateral MSMSM structure.*

**Parameter 1st**

Pump current in mA for 1 mW

Modulation bandwidth in GHz at

*Performance characteristics of the proposed nanolasers.*

Spectral bandwidth in THz at

**structure**

Output wavelength (nm) 850 1550 1550 1550 850

Threshold current (mA) 4.7 29 21 1.9 1.87 Output power in mW at threshold 0.198 4.16 15.6 10.59 0.08 Output power in μW at 10 μA 0.44 2.8 3 50 0.25

Purcell factor (lasing mode) 66 291 700 2965 15 Quality factor (Q) 30 26 58 138 32

**2nd structure**

) 0.07 0.073 0.1125 0.076 0.06

**3rd structure**

20 7 3 0.2 N.A

3.37 5.7 2.98 28.5 —

0.541 1.46 1.98 21.68 >0.08

**4th structure** **Liu et al. [7]**

**Description Symbol Value Unit** Width of the thick area of the strip *WStrip,1* 60 nm Width of the thin area of the strip *WStrip,2* 50 nm Height of Ge virtual substrate *HVS* 200 nm Resonator height *HC* 40 nm Al contact width *WCont* 40 nm Ge buffer width *WBuff* 40 nm Si-Ge barrier width *WBarrier* 30 nm Quantum dot size *WQD* 5 nm Quantum dot distance from the strip *DQD* 10 nm Quantum dot distance from the top *HQD* 20 nm Number of QDs *NQD* 34 — Spacing between QDs *SQD* 20 nm Spacing between two notches *Snotch* 5 nm Diameter of notches *Dnotch* 10 nm Cavity length *LC* 270 nm Ge alloy percent *x* 85 % Doping level of the QDs and barriers *ND* <sup>4</sup> <sup>10</sup><sup>19</sup> cm<sup>3</sup> Doping level of the Ge buffer *NA* <sup>1</sup> 1019 cm<sup>3</sup>

## **Table 3.**

*Design parameters of the third structure.*

#### **Figure 9.**

*Schematic illustration of the corrugated lateral MSMSM structure: (a) 3D schematic, (b) transverse cross section, and (c) top view.*

The proposed structure for the nanolaser according to **Figure 9** consists of a corrugated metal nanostrip with two arrays of n + doped tensile-strained germanium quantum dots (QDs) at both sides. In addition, two high-doped p + germanium layers are used for better field confinement and providing higher carrier generation
