In this chapter, the measurement results of the fabricated devices will be shown.
The lasing modes would be confirmed by the measurement result of the different lattice constant devices. We also bend the laser structure to measure the optical properties with different bending curvatures.
3-1 The Micro-PL Measurement System
In order to measure the optical properties of the photonic crystal micro-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. 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 um 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 most of the photonic crystal devices are only few um or sub-um scale, here we have to 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 Lasing Characterization with a Flat PDMS Substrate
We fabricated arrays of photonic crystal triangular lattices on a PDMS substrate.
One of the fabricated arrays of photonic crystal patterns is shown in Figure 3-2.1. The SEM image shows there are thirty-five patterns in an array. The lattice constants varied from 345 nm to 515 nm. Figure 3-2.2 shows the photonic crystal triangular lattices on a PDMS substrate with 410nm lattice constant. There are about thirty periods of the lattice in both Г-K and Г-M directions.
Figure 3-2.1 The SEM image of the array of patterns with different lattice constants
Figure 3-2.2 The SEM image of the photonic crystals with 410 nm lattice constant
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Figure 3-2.3 shows the lasing spectrum from the triangular lattice photonic crystal band-edge laser with 395 nm lattices constant. The lasing wavelength is about 1581 nm. The side mode suppression ration is about 18 db, which is high enough for optical communication. The light in-light out curve (L-L curve) from the laser is show in Figure 3-2.4. The threshold power is about 3.2 mW. It is higher than the threshold power of photonic crystal defect lasers. The mode volume of the photonic crystal band-edge laser is larger than other defect lasers; therefore it needs higher power to achieve lasing.
Figure 3-2.3 The lasing spectrum from the photonic crystal band-edge laser with 395 nm lattice constant
Figure 3-2.4 The light in light out curve from the 395 nm lattice constant photonic crystal band-edge laser
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In order to characterize the operation mode of the photonic crystal band-edge laser, we measured the photonic crystal band-edge lasers with different lattice constant in the same array of patterns. The photonic crystals with different lattice constant in the array would have the same r/a ratio. Figure 3-2.5 shows the lattice constant versus the lasing wavelength. As the lattice constant increases, the lasing wavelength increases linearly. It indicates that these lasing modes are the same mode with 0.249 normalized frequency.
Figure 3-2.5 The lasing wavelength versus lattice constant of photonic crystal band-edge lasers on a PDMS substrate. The normalized frequency of the lasing mode
is about 0.249.
We also find out other operation modes of the photonic crystal band-edge laser.
Figure 3-2.6 is the lasing spectrum measured from 430 lattice constant triangular lattice photonic crystal band-edge laser. The lasing wavelength is about 1627 nm.
Figure 3-2.7 is the L-L curve from the laser. The threshold power is approximately 2.7 mW.
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Figure 3-2.6 The lasing spectrum from the photonic crystal band-edge laser with 430 nm lattice constant.
Figure 3-2.7 The light in light out curve from the 430 nm lattice constant photonic crystal band-edge laser
By measuring the lasers with lattice constant around 430 nm, we can get the relationship between the lasing wavelength and lattice constant and identify the lasing mode. Figure 3-2.8 shows the lasing wavelength versus lattice constant. The normalized frequency of the mode is about 0.264.
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Figure 3-2.8 The lasing wavelength versus the lattice constant. The normalized frequency of the operation mode is about 0.264.
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3-3 Optical Properties of the Bent Photonic Crystal Laser
After characterizing the operation modes of the photonic crystal band-edge laser with a flat PDMS substrate, we measure the bending structure on a bent mental surface. Figure 3-3.1 illustrates the bent triangular lattice photonic crystal band-edge laser with bending radius R. We bend the structure long the Г-M direction of the triangular lattice. Figure 3-3.2 shows our handmade tool for controlling the bending curvature (1/R). We can increase the bending curvature by rotating the micrometer.
The micrometer would pull the mental slice in and make the mental slice bent. We call the distance that the micrometer pulled in bending depth. For example, the distance between two red lines in Figure 3-3.3 is the bending depth, and the mental slice is bent by rotating the micrometer. Then, we derivate the value of bending curvature from the geometry of the bending tool. Figure 3-3.4 shows the relationship between bending depth and bending curvature.
Figure 3-3.1 Illustration of the bent photonic crystal band-edge laser
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Figure 3-3.2 The measurement tools for bending the structure. The upper is a mental slice, which is served as a bending platform here. The lower is a homemade
tool with a micrometer. A flat mental slice is equipped.
Figure 3-3.3 The bending measurement tools with a bent metal slice. The distance between the two red lines is the bending depth.
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Figure 3-3.4 Bending depth versus bending curvature.
In the measurement, we apply the device on the varying bent mental surface with fixed pump condition and pump position. At first, we measure the PL spectrum of the MQWs with different bending curvatures. Figure 3-3.5 is the PL spectrums of the MQWs with different bending curvatures. The PL spectrums do not shift when the bending curvature is increased.
Figure 3-3.5 The PL spectrums of the MQWs with different bending curvature.
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We measure the 395 nm lattice constant triangular lattice photonic crystal laser with the lasing mode of 0.249 normalized frequency. Figure 3-3.6 shows the light in light out curve of the laser with different bending curvature. The structure does achieve lasing at varied bending curvature. From the Figure 3-3.7, we can find out that as the bending curvature increases, the threshold power increases. Figure 3-3.8 shows that the output power decreases as the bending curvature increases. When the curvature is increased to 0.06 (1/mm), an increase of 20% in threshold power and a drop of 70% in output power are observed. These behaviors are attributed to the increase of optical loss due to the structure deformation. The output power drops more rapidly than the increase of the threshold power because we do not collect the entire laser light due to larger divergent angle of the bent laser. In other words, we only collect small part of the laser light in our OSA when the laser structure is bent.
Figure 3-3.6 The light in light out curve of the 395 nm photonic crystal band-edge laser with different bending curvature.
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Figure 3-3.7 The threshold power versus the bending curvature of the 395 nm photonic crystal band-edge laser
Figure 3-3.8 The output power versus the bending curvature of the 395 nm photonic crystal band-edge laser
The lasing wavelength is another important optical property. After bending the structure, we observe the lasing wavelength red-shifts. Figure 3-3.9 shows the lasing wavelength red-shifts as the bending curvature is increased.
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Figure 3-3.9 The lasing wavelength versus the bending curvature of the 395 nm photonic crystal band-edge laser
The lasing wavelength red-shifts when the structure is bent. It implies that it can be applied as a wavelength tunable laser light source in optical communication. The cost of the semiconductor laser is high; once the structure is defined, the lasing wavelength would hard to be altered. It means if we want a multi-wavelength light source, we must fabricate many devices and fine tune their geometry parameter. Here, we demonstrate the multi-wavelength light source in a single device by bending the structure; therefore, the cost for the multi-wavelength light source becomes lower.
Besides, the flexible laser can be a local curvature sensor with a very compact size.
Base on the experiment result, the sensibility is about 29 nm/mm-1.
We also measure the photonic crystal band-edge laser with 430 nm lattice constant. The normalized frequency of the lasing mode is 0.264, which is shown beforehand. Figure 3-3.10 shows the light in light out curve of the 430 nm lattice constant photonic crystal laser with different bending curvature. The structure achieves lasing at varied bending curvature. Figure 3-3.11 is the threshold power versus the bending curvature. The threshold power increases as the bending curvature, too.
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Figure 3-3.10 The light in light out curve of the 430 nm lattice constant photonic crystal laser with different bending curvature.
Figure 3-3.11 The threshold power versus the bending curvature of the 430 nm lattice constant photonic crystal laser
Figure 3-3.12 The lasing wavelength red-shifts as the curvature is increased.
Compared with the previous optical mode of 0.249 normalized frequency, the red-shifts of this mode is about 40% less than the previous mode at the same bending curvature.
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Figure 3-3.12 The lasing wavelength red-shifts as the curvature is increased of the 430 nm lattice constant photonic crystal laser
3-4 Conclusion
In this chapter, we demonstrate the triangular lattice photonic crystal lasers on a flexible PDMS substrate. The lasing actions of the lasers are observed with different lattice constant and the two lasing modes of 0.249 and 0.264 normalized frequencies are identified. The optical properties of the lasers with different bending curvature are also shown in this chapter
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