| 1. |
- A Atlasov, Kirill, et al.
(författare)
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1D photonic band formation and photon localization in finite-size photonic-crystal waveguides
- 2010
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Ingår i: OPTICS EXPRESS. - 1094-4087. ; 18:1, s. 117-122
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Tidskriftsartikel (refereegranskat)abstract
- A transition from discrete optical modes to 1D photonic bands is experimentally observed and numerically studied in planar photonic-crystal (PhC) L-N microcavities of length N. For increasing N the confined modes progressively acquire a well-defined momentum, eventually reconstructing the band dispersion of the corresponding waveguide. Furthermore, photon localization due to disorder is observed experimentally in the membrane PhCs using spatially resolved photoluminescence spectroscopy. Implications on single-photon sources and transfer lines based on quasi-1D PhC structures are discussed.
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| 2. |
- Abd Alrahman, Chadi, et al.
(författare)
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Cavity-enhanced optical frequency comb spectroscopy of high-temperature H2O in a flame
- 2014
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Ingår i: Optics Express. - 1094-4087. ; 22:11, s. 13889-13895
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate near-infrared cavity-enhanced optical frequency comb spectroscopy of water in a premixed methane/air flat flame. The detection system is based on an Er:fiber femtosecond laser, a high finesse optical cavity containing the flame, and a fast-scanning Fourier transform spectrometer (FTS). High absorption sensitivity is obtained by the combination of a high-bandwidth two-point comb-cavity lock and auto-balanced detection in the FTS. The system allows recording high-temperature water absorption spectra with a resolution of 1 GHz and a bandwidth of 50 nm in an acquisition time of 0.4 s, with absorption sensitivity of 4.2 x 10 (9) cm(-1) Hz(-1/2) per spectral element.
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| 3. |
- Aghaeipour, Mahtab, et al.
(författare)
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Tunable absorption resonances in the ultraviolet for InP nanowire arrays
- 2014
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Ingår i: Optics Express. - 1094-4087. ; 22:23, s. 29204-29212
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Tidskriftsartikel (refereegranskat)abstract
- The ability to tune the photon absorptance spectrum is an attracting way of tailoring the response of devices like photodetectors and solar cells. Here, we measure the reflectance spectra of InP substrates patterned with arrays of vertically standing InP nanowires. Using the reflectance spectra, we calculate and analyze the corresponding absorptance spectra of the nanowires. We show that we can tune absorption resonances for the nanowire arrays into the ultraviolet by decreasing the diameter of the nanowires. When we compare our measurements with electromagnetic modeling, we generally find good agreement. Interestingly, the remaining differences between modeled and measured spectra are attributed to a crystal-phase dependence in the refractive index of InP. Specifically, we find indication of significant differences in the refractive index between the modeled zinc-blende InP nanowires and the measured wurtzite InP nanowires in the ultraviolet. We believe that such crystal-phase dependent differences in the refractive index affect the possibility to excite optical resonances in the large wavelength range of 345 < lambda < 390 nm. To support this claim, we investigated how resonances in nanostructures can be shifted in wavelength by geometrical tuning. We find that dispersion in the refractive index can dominate over geometrical tuning and stop the possibility for such shifting. Our results open the door for using crystal-phase engineering to optimize the absorption in InP nanowire-based solar cells and photodetectors. (C) 2014 Optical Society of America
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| 4. |
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| 5. |
- Ako, Thomas, et al.
(författare)
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Design of invisibility cloaks with an open tunnel
- 2010
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Ingår i: Optics Express. - 1094-4087. ; 18:26, s. 27060-27066
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we apply the methodology of transformation optics for design of a novel invisibility cloak which can possess an open tunnel. Such a cloak facilitates the insertion (retrieval) of matter into (from) the cloak's interior without significantly affecting the cloak's performance, overcoming the matter exchange bottleneck inherent to most previously proposed cloak designs. We achieve this by applying a transformation which expands a point at the origin in electromagnetic space to a finite area in physical space in a highly anisotropic manner. The invisibility performance of the proposed cloak is verified by using full-wave finite-element simulations. (C) 2010 Optical Society of America
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| 6. |
- Ako, Thomas, et al.
(författare)
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Design of invisibility cloaks with an open tunnel
- 2010
-
Ingår i: Optics Express. - : Optical Society of America. - 1094-4087. ; 18:26, s. 27060-27066
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we apply the methodology of transformation optics for design of a novel invisibility cloak which can possess an open tunnel. Such a cloak facilitates the insertion (retrieval) of matter into (from) the cloak’s interior without significantly affecting the cloak’s performance, overcoming the matter exchange bottleneck inherent to most previously proposed cloak designs. We achieve this by applying a transformation which expands a point at the origin in electromagnetic space to a finite area in physical space in a highly anisotropic manner. The invisibility performance of the proposed cloak is verified by using full-wave finite-element simulations.
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| 7. |
- Akram, Muhammad Nadeem, et al.
(författare)
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Influence of base-region thickness on the performance of Pnp transistor-VCSEL
- 2014
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Ingår i: Optics Express. - 1094-4087. ; 22:22, s. 27398-27414
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Tidskriftsartikel (refereegranskat)abstract
- We have recently reported a 980nm GaAs-based three terminal Pnp transistor-vertical-cavity surface-emitting laser (TVCSEL) operating at room temperature with optical power up to 1.8mW. However, the current gain beta = Delta I-c/Delta I-b was near zero just before lasing and became negative after the lasing threshold. The main cause of the negative current gain was found to be a gradual and position-dependent forward-biasing (saturation) of the base-collector junction with increasing bias even before lasing threshold. In this article, detailed multi-physics device simulations are performed to better understand the device physics, and find ways to avoid the premature saturation of the base-collector junction. We have optimized the thickness of the base region as well as its doping concentration and the location of the quantum wells to ensure that the T-VCSEL is in the active mode throughout its range of operation. That is, the emitter-base junction is forward biased and base-collector junction is reversed biased for sweeping the excess charges out of the base region.
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| 8. |
- Amer, Eynas, et al.
(författare)
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UV laser interaction with a fluorescent dye solution studied using pulsed digital holography
- 2013
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Ingår i: Optics Express. - 1094-4087. ; 21:21, s. 25316-25323
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Tidskriftsartikel (refereegranskat)abstract
- A frequency tripled Q-switched Nd-YAG laser (wavelength 355 nm, pulse duration 12 ns) has been used to pump Coumarin 153 dye solved in ethanol. Simultaneously, a frequency doubled pulse (532 nm) from the same laser is used to probe the solvent perpendicularly resulting in a gain through stimulated laser induced fluorescence (LIF) emission. The resulting gain of the probe beam is recorded using digital holography by blending it with a reference beam on the detector. Two digital holograms without and with the pump beam were recorded. Intensity maps were calculated from the recorded digital holograms and used to calculate the gain of the probe beam due to the stimulated LIF. In addition numerical data of the local temperature rise was calculated from the corresponding phase maps using Radon inversion. It was concluded that about 15% of the pump beam energy is transferred to the dye solution as heat while the rest is consumed in the radiative process. The results show that pulsed digital holography is a promising technique for quantitative study of fluorescent species
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| 9. |
- Amotchkina, Tatiana, et al.
(författare)
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Stress compensation with antireflection coatings for ultrafast laser applications : from theory to practice
- 2014
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Ingår i: Optics Express. - : Optical Society of America. - 1094-4087. ; 22:24, s. 30387-30393
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Tidskriftsartikel (refereegranskat)abstract
- Each complicated coating, in particular, a dispersive mirror consists of dozens of layers. Thin films layers have mechanical stresses. After summing up stresses from all layers, the resulting stress is high enough to bend even a relatively thick substrate. To avoid this effect we suggest depositing an antireflection coating (AR) at the back-side of the substrate which together with suppression of unwanted reflections from the back side will also compensate this stress. We demonstrate unique, extremely thick and sophisticated AR coating consisting of 71 layers with the total physical thickness of 7.5 µm. This AR coating completely compensates stress from the dispersive mirror coated on the front side and minimizes unwanted reflections.
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| 10. |
- Andreasson, Jakob, et al.
(författare)
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Automated identification and classification of single particle serial femtosecond X-ray diffraction data
- 2014
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Ingår i: Optics Express. - 1094-4087. ; 22:3, s. 2497-2510
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Tidskriftsartikel (refereegranskat)abstract
- The first hard X-ray laser, the Linac Coherent Light Source (LCLS), produces 120 shots per second. Particles injected into the X-ray beam are hit randomly and in unknown orientations by the extremely intense X-ray pulses, where the femtosecond-duration X-ray pulses diffract from the sample before the particle structure is significantly changed even though the sample is ultimately destroyed by the deposited X-ray energy. Single particle X-ray diffraction experiments generate data at the FEL repetition rate, resulting in more than 400,000 detector readouts in an hour, the data stream during an experiment contains blank frames mixed with hits on single particles, clusters and contaminants. The diffraction signal is generally weak and it is superimposed on a low but continually fluctuating background signal, originating from photon noise in the beam line and electronic noise from the detector. Meanwhile, explosion of the sample creates fragments with a characteristic signature. Here, we describe methods based on rapid image analysis combined with ion Time-of-Flight (ToF) spectroscopy of the fragments to achieve an efficient, automated and unsupervised sorting of diffraction data. The studies described here form a basis for the development of real-time frame rejection methods, e. g. for the European XFEL, which is expected to produce 100 million pulses per hour. (C)2014 Optical Society of America
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