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Optical Characterization and Nanophotonics Laboratory

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Waveguide and Photonic Bandgap

Members: Anna K. Swan

 

Alumni: Bennett B. Goldberg, Josh M. Pomeroy, Greg H. Vander Rhodes, Yan Yin

 

Introduction

Photonic band gap (PBG) are the dielectric structures with a forbidden gap for electromagnetic waves. It is really important for optical communications and optical computing. It can be applied in the optical wave guiding circuits, optoelectronic devices, even optical-optical devices (such as optical switch), make them tinier, cheaper, more effective, integrated and with some incredible characters. This research focuses on the measurement, application and fabrication of PBG devices. Right now, the work is measuring spectral responses of PBG samples from UCLA. By comparing of PBGs without defect and with defect, the effect and effective range of defect in PBG are confirmed. Also, we hope to understand the relationship of the defect states with the parameters of PBG and defect so that we can control the defect states by choosing proper fabrication parameters. A home-built confocal microscope with achromatic lenses is used to measure the far field scattering from PBG devices on a surface with a total internal reflection. After measurements, strong outputs only from the defect are clearly shown as a result of wavelength in the photonic gap. Results of output spectrum of defect also show defect state of the defect. A picture of one defect state shows decay function modulated strips that maybe are the contribution of the defect state wave function.

Abstract

Objective:

We hope to find out spectral responses of Photonic Band-Gap (PBG) structure made by UCLA. By comparing of PBGs without defect and with defect, we can confirm the effect and effective range of defect in PBG. Also, we hope to understand the relationship of the defect states with the parameters of PBG and defect so that we can control the defect states by choosing proper fabrication parameters.

Techniques:

We used a self-built confocal microscope with achromatic lenses to measure the far field scattering from PBG devices on a surface with a total internal reflection.

Preliminary Result:

After measurements, we can see strong outputs only from the defect as a result of wavelength in the photonic gap. Results of output spectrum of defect also show defect state of the defect. A picture of one defect state shows decay function modulated strips that maybe are the contribution of the defect state wave function.

Outline of experiment

Experiment setup:

Figure.1 shows the optical setup of the system. The PBG devices are attached on a prism. An incident light comes from the bottom of the prism with an angel that can make a total internal reflection at the interface of top surface prism and air. The incident light comes from a tunable light source (HP 8168F) that can offer 1440 - 1600 nm wavelength light. The confocal microscope collects the far field scatters from the PBG devices and delivers the collected light to an IR femtowatt photoreciever (New focus 2153). The light intensity signal goes to a PC computer after mixed at a lock-in Amp (Stanford SR850) with the reference signal from tunable laser. The PC computer also collects pictures from an Apogee CCD camera and controls a scanning stage. The samples and prism first are attached on another small moving stage that is independent with the incident light and confocal microscope so that we can measure different devices without changing illumination and collection conditions too much. All of the incident light connector, prism, sample and that small moving stage are mounted on the scanning stage that controlled by computer to achieve scan during measurement.

Figure 1 Optical system sketch 

The lenses of confocal microscope are achromatic from visible wave range to infrared wave. So, we can adjust and focus the system with visible light and do the measurement under the infrared light source. Figure.2 shows a picture of devices captured by the Apogee CCD camera.

Figure 2 A picture of devices captured by Apogee camera

Comparing of devices without defect and with defect:

We measured two devices, one is a device without defect and labeled (550 220), another is a device with defect and labeled (550 220). Comparing scattering for device without defect, much stronger scattering is from device designed with defect. This is because the wavelength is in the designed photonic gap of the device and the scattering light is not allowed to propagate in XY surface. The defect will allow the powers of a large area around it to get in the defect. Those powers after scattering at the edge of the defect are coupled to vertical direction and make a strong output. Also, we can see an output area in graph of device without defect which may comes from an unexpected defect.

The graphs for device without defect are shown at figure 3, 4 and 5. The samples were moved independent with excitation and collection part so that we could maintain about the same incident power. Figure 5 is the cross section graph of pictures in figure 3 and 4. The sections are made at strongest output positions in those two graphs along X scan direction.

Figure 3 Graphs for devices without defect and with defect in different gray scales

Figure 4 Graphs for devices without defect and with defect in same gray scales

Figure 5 Cross section graphs for devices without defect and with defect

Wavelengths scan of the device with defect:

We scanned the device with defect (550 220) from 1440nm to 1590nm with 10nm steps. We can see several dips in spectrum.

First, we started at frequency 1440nm. Made a surface scan and got graph 6. Then offset the confocal position to the strongest output point. Then, swept the wavelength of the tunable laser and got graph 7. In graph7, the black dots are data during increasing wavelength; the red dots are data during decreasing wavelength. After that, we continually scanned the device with 10nm wavelength steps and got graph 8, 9, 10, and 11. Graph 12 is the plots of the integrated outputs of pictures in graph 9 and 11 as a function of wavelength. Graph 13 combines the increasing curve in graph 7 and graph 12 in order to compare them. Graph 14 plots the result of defect output over integrated output.

In graph7, we can see a big dig at 1500nm in the curve. They are the results of defect states generated by the defect. After over the integrated output, the 1500nm dig is clearer and other digs are suppressed. So, only 1500nm dig comes from the behaving of defect and may indict the defect state.

Most of data are reversible. The irreversible data at 1440, 1450, 1460nm in figure 12 maybe are the result of the drift of scan stage after scanned 2 or 3 pictures.

Figure 6 Image of device with defect at 1440nm

Figure 7 Frequency sweep for defect point.

Figure 8 Graphs for increasing wavelength and shown in different gray scales

Figure 9 Graphs for increasing wavelength and shown in same gray scales

Figure 10 Graphs for decreasing wavelength and shown in different gray scales

Figure 11 Graphs for decreasing wavelength and shown in same gray scales

Figure 12 Plot of the integrated outputs of pictures during wavelength sweeping

Figure 13 Combine plots of defect out and integrated outputs

Figure 14 Normalized defect output

Output pattern of defect states:

Figure 15 shows an output pattern of device with defect at 1500nm. It shows a strips structure. The distances between three strongest output strips are about 1.5um. The rest low output strip structures have a period about 700nm (half of wave length) that is result of scattering of wave and only controlled by wavelength. The pattern at defect is a result of interaction of defect state and PBG device. The distances of power peaks of a defect state are not only controlled by wavelength also by the size of defect and model order. It is still early to say the strong output strips indicate the power strong points of defect state; we still need do more analysis and calculations.

Anyway, the pattern shows a kind of oscillation behavior that is the result of oscillation of defect state. And normally, the distance of 0 order peak and 1st order peak is about half of (size of defect + wavelength/2). If we regard the size of defect is about 2um and the wavelength is 1500nm, the distance of 0 order peak and 1st order peak should be 1350um that is identical with our result.

Figure 15 Output pattern of device (550 220) with defect at 1500nm

Publications

G. H. Vander Rhodes, B. B. Goldberg, M. S. Ünlü, S. T. Chu, and B. E. Little, "Internal Spatial Modes in Glass Microring Resonators," IEEE Journal of Selected Topics in Quantum Electronics, Vol. 6, No. 1, January/February 2000, pp. 46-53

B. B. Goldberg, M. S. Ünlü, and G. H. Vander Rhodes, "Internal Spatial Modes And Local Propagation Properties In Optical Waveguides Measured Using Near-Field Scanning Optical Microscopy," Materials Research Society Proceedings Fall 1999, Vol. 588, 29-3 November/December 1999, pp. 3-12

B. B. Goldberg, and M. S. Ünlü, "Mapping Internal Optical Modes by Near-field Scanning Optical Microscopy," Materials Research Society Proceedings Fall 1999, November 1999

G. H. Vander Rhodes, B. B. Goldberg, M. S. Ünlü, S. T. Chu, and B. E. Little, "Near-field Scanning Optical Microscopic Studies of Micro-ring Resonators," Proceedings of IEEE Lasers and Electro-Optics Society 1999 Annual Meeting, Vol. 2, 8-11 November 1999, pp. 552-553

G. H. Vander Rhodes, B. B. Goldberg, M. S. Ünlü, S. T. Chu, W. Pan, T. Kaneko, Y. Kokobun, and B. E. Little, "Measurement of internal spatial modes and local propagation properties in optical waveguides," Applied Physics Letters, Vol. 75, No. 16, October 1999, pp. 2368-2370

G. H. Vander Rhodes, B. B. Goldberg, M. S. Ünlü, B. E. Little, D. J. Ripin, E. P. Ippen, S. T. Chu, W. Pan, T. Kaneko, and Y. Kokobun, "Evanescent Mode Imaging of Micro-ring Resonators using NSOM," Bulletin of APS centennial meeting, March 1999

B. B. Goldberg, M. S. Ünlü, G. H. Vander Rhodes, and W. D. Herzog, "Mapping Internal Optical Fields with Near-Field Scanning Optical Microscopy," Bulletin of APS centennial meeting, March 1999

G. H. Vander Rhodes, M. S. Ünlü, B. B. Goldberg, J. M. Pomeroy, and T. F. Krauss, "Characterisation of Waveguide Microcavities using High-Resolution Transmission Spectroscopy and Near-field Scanning Optical Microscopy," IEE Proceedings Optoelectronics, Vol. 145, No. 6, December 1998, pp. 379-383

G. H. Vander Rhodes, M. S. Ünlü, B. B. Goldberg, J. M. Pomeroy, and T. F. Krauss, "Internal Spatial Modes of One Dimensional Photonic Band Gap Devices Imaged with Near-field Scanning Optical Microscopy," International Quantum Electronics Conference 1998, 3-8 May 1998, pp. 228-229

B. B. Goldberg, and M. S. Ünlü, "Mapping Internal Optical Fields with Near-Field Scanning Optical Microscopy," Materials Research Society Proceedings Spring 1998, April 1998

G. H. Vander Rhodes, J. M. Pomeroy, T. F. Krauss, M. S. Ünlü, and B. B. Goldberg, "Imaging of Spatial Modes of One Dimensional Photonic Band Gap Devices using Near-field Scanning Optical Microscopy," Proceedings of the International Semiconductor Device Research Symposium (ISDRS), 11-13 December 1997


Collaborators

University of California Los Angeles


Support

Army Research Labs


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