In this chapter, the measurement results of the fabricated devices will be shown.
The first of measurement result is compared nano-ring that pumped from InGaAsP with pumped from sapphire. Then we will compare the nano-ring coated Gold with non-coating.
3-1 The Micro-PL Measurement System
In order to measure the optical properties of the nano-ring cavities, the micro-PL measurement system with sub-micrometer scale resolution in space and sub-nanometer scale resolution in spectrum is necessary. Figure 3-1 1 shows the micro-PL measurement system.
Figure 3-1 1 The picture of micro-PL measurement system.
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In the measurement system, the 850 nm diode laser is used as the pump source.
The pump power is modulated by the amplitude modulator, and the laser light goes through the 50/50 beam splitter, 50% of pump power is reflected into the photo-detector to know how much pump power that is used and the other 50% pump power is focused to a spot with 1.5 um to 2 μm in diameter by the 100x NIR objective lens.
The sample is mounted on a high resolution motor control 3-axis stage with 30 nm move resolution. The output power was collected from the top of the sample into the optical spectrum analyzer (OSA) by the objective lens, collective lens and multi-mode fiber. Because the nano-ring devices are only few sub-um scale, here we use the visible light system to observe the position of the pump spot and devices. The visible light system includes the visible light sources, CCD camera and monitor.
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3-2 Nano-Ring Laser on Sapphire
We fabricated nano-ring on the double polishing sapphire substrate that has high transmission is can pump from sapphire. One of the fabricated nano-ring cavity is shown in figure 3-2 1. Its diameter is 4 μm, and width is 420 nm.
(a) (b)
(c)
Figure 3-2 1 SEM image of the nano-ring cavity. Its diameter is 4 μm, and width is 420 nm. (a) Top view and (b) (c) angle view of the nano-ring cavity.
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Figure 3-2 2 shows PL pumped from air side and pumped from sapphire side.
The photo-luminance spectrum (PL) of the QWs that the full-width half-maximum (FWHM) is about 200 nm. The PL is centered at 1530 nm pumped from InGaAsP and 1535nm pumped from sapphire at room temperature.
The PL pumped from sapphire is higher about 10 pW than pumped from InGaAsP. Because the sapphire index is 1.7 that is higher than air. If InGaAsP layer at air layers center, the optics leaks some to both sides of the air. But the below of air layer changes the sapphire layer, the light more leak than the air layer, so the optical intensity is high pumped from sapphire. Figure 3-2 3 shows the light leak distribution diagram. And the sapphire has high transmission in NIR is shown in Figure 3-2 4.
Figure 3-2 2 The PL pumped from air side is black line and pumped from sapphire side is red line.
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(b)
Figure 3-2 3 (a) Air-InGaAsP-Air and (b) Air-InGaAsP-Sapphire optical leak distributions.
Figure 3-2 4 The sapphire the transmission spectrum in NIR
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The leasing spectrum from the nano-ring cavity at room temperature is showed in the figure 3-2 5. The black line is pumped from air side, that lasing wavelength is about 1508.2 nm. And the red line is pumped from sapphire side, that lasing wavelength is about 1507.5 nm. The light in-light out curve (L-L curve) from the laser is shown in figure 3-2 6. The threshold power are 13 mW and 12.5 mW each of pumped from air and sapphire side. We confirm that lasing is worked with pumped from sapphire side.
Figure 3-2 5 The black line is pumped from air side, that lasing wavelength is about 1508.2 nm. The red line is pumped from sapphire side, that lasing wavelength is about 1507.5 nm.
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Figure 3-2 6 The threshold power are 13 mW and 12.5 mW each of pumped from air side and sapphire side.
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3-3 Metal Nano-Ring Laser on Sapphire at 80K
Next, we coat the gold on nano-ring cavity is shown in figure 3-3 1, and the sample is putted in cryostate. It cans drop the temperature and control the temperature with liquid nitrogen in cryostate. The Figure 3-3 2 shows the sample putted in the cryostate.
Figure 3-3. 1 SEM image of the coated gold nano-ring cavity. The thickness of gold is about 200 nm. (a) Top view and (b) (c) angle view of the nano-ring cavity.
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(c)
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Figure 3-3 2 The sample is putted in the crystate.
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Figure 3-3 3 The sample is after coating gold. Viewing of (a) is from gold and (b) is from sapphire.
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The PL are pumped non coating from sapphire, pumped from gold, and pumped gold-coating from sapphire with the same input power at 80K that is shown in figure 3-3 4. The peak 1 PL at 1450nm is the InGaAsP quantum walls, and the peak 2 at 1250nm is the InGaAsP bulk. The PL of gold-coating is higher than non-coating because the gold is a very well reflection mirror in NIR, which can addition intensity from sapphire. Figure 3-3 5 shows reflection of Gold in NIR [32]. And the PL is pumped effect not well from gold side, but the PL intensity is outstanding in output efficiency from sapphire side. Because of the gold has low transmission in NIR that figure 3-3 6 shows gold transmission. Then we confirm the advantage of using the double polishing sapphire.
Figure 3-3 4 The PL pumped without gold from sapphire is black line, pumped from gold is red line, and pumped from sapphire is blue line.
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The pumped condition of each other is 6% duty cycle and 500 ns period at 80K.
Because the sapphire-bonded device has better thermal conductivity, it can use high intensive intensity to pump, even use continuous wave. The figure 3-3 7 shows the lasing spectrums and L-L curve for the ring gold-coating and non-coating. The lasing wavelength is 1475.2 nm and threshold power is 1.3 mW for non-coating ring. The other is 1311 nm and threshold power is 8 mW.
Figure 3-3 5 Reflection of gold in NIR
Figure 3-3 6 Transmission of gold in NIR
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(b)
Figure 3-3 7 (a) The lasing wavelength are 1475.2 nm for non-coating ring, and 1311 nm for ring gold-coating ring. (b) The threshold power are 1.3 mW for non-coating, and 8 mW for ring gold-coating.
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3-4 Simulation
We simulate the ring gold-coating and non-coating to confine the lasing mode is like mode profile use the finite element method (FEM). We simulate the mode profile with 3-D. Figure 3-4 1 (a) is illustrations of the non-coating ring on sapphire substrate, the diameter and width are 4 μm and 420 nm. Thickness of InGaAsP layer is 240nm and sapphire substrate thickness is 1μm. We use a quadrant of ring to instead of full ring, which can save memory and time. On the other hand, figure 3-4 1 (b) is illustrations of the non-coating ring on sapphire substrate, the diameter and width are 4 μm and 420 nm. Thickness of InGaAsP layer is 240nm, sapphire substrate thickness is 1 μm, and the gold thickness is 200 nm.
Figure 3-4. 1 The simulation diagram. (a) is non-coating ring on sapphire and (b) is gold-coating ring on sapphire.
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The simulation result of the non-coating ring is shown in figure 3-4 2, which is Whispering-Gallery ( WG ) mode. The lasing wavelength of measurement compare to the wavelength of simulation is shown in figure 3-4 3, the small difference between the experimental and simulation results. Because of it is not perfect in the fabrication, it has some roughness in the fabrication process, but it is smooth when simulation.
And the simulation is not think about thermal. In fact, when we pumped the ring, and provide thermal at the same time.
Figure 3-4 2 The normalize H field of the non-coating ring. (a) top view (b) side view.
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(b)
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Figure 3-4 3 The measurement compares with simulation result. The lasing wavelength is 1478.9 nm and quality factor about 12300 for measurement. The wavelength is 1470 nm and quality factor about 1000 for simulation.
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On the other hand, figure 3-4 4 shows the simulation result of the gold-coating ring. The mode profile shows normalize H field, which is WG mode at the same. The measurement result compares with the simulation is shown in figure 3-4 5, which are have error the same.
Figure 3-4 4 The normalize of H field. (a) top view (b) side view.
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(b)
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Figure 3-4 5 The measurement compares with simulation result. The lasing wavelength is 1311 nm and quality factor about 3700 for measurement. The wavelength is 1300 nm and quality factor about 400 for simulation.
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The gold-coating ring compare with non-coating ring from side view with simulation. The figure 3-4 6 and the figure 3-4 7 show the direction of Z versus the intensity from side view. The intensity is strongest in center of MQWs/InGaAsP for the non-coating ring. but the gold-coating ring is not. The intensity is strongest at InGaAsP and gold interface, so we think that is hybird plasmon mode, it is not only at interface but aslo at InGaAsP. And simulation shows the intensity not leak to gold layer. Because of the gold absorption is high in NIR. The energy is not easy transmission.
Figure 3-4 6 (a) The mode profile of total energy. (b) The distribution of energy.
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Figure 3-4 7 (a) The mode profile of total energy. (b) The distribution of energy.
(a) (b
)
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3-5 Conclusion
In the chapter, we make a metal ring laser on sapphire at 80K. The sapphire has low index and high transmission in NIR. And it has higher pumped effect from sapphire side then from gold side. The lasing wavelength is 1311 nm and threshold power is 8 mW. We use FEM which can easy show approximately the mode profile for the gold-coating ring. The intensity is not only at InGaAsP but also at InGaAsP and gold interface, which shows side view of total energy. Next the chapter will discuss that the metal ring laser of thermal properties.
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