news digest ♦ Lasers
will accelerate further improvements to widen the gap in efficiency, reliability and cost of ownership over the incumbents.”
Diode lasers - used in laser pointers, barcode scanners, DVD players, and other low-power applications - are amongst the most efficient, compact, and low-cost lasers available. Attempts have been made over the years to increase their brightness for industrial applications, such as welding and cutting metal, but boosting power usually meant decreasing beam quality, or focus.
TeraDiode has made these lasers powerful enough to cut steel using a combination of semiconductor diode laser array chips and a power-scaling technique, called wavelength beam combining (WBC), developed at MIT. WBC manipulates individual diode laser beams into a single output ray to boost the power of a diode laser, while preserving a very focused beam. TeraDiode believes that direct-diode lasers using this technology will, in time, replace fibre, disk and other lasers for the most demanding material processing applications.
WBC can be thought of as the spatial and directional superposition of many independent diode laser external cavities, says the company. The angle-to-wavelength conversion property of a diffraction grating is used to provide feedback to each emitter in an array, via a series of lenses, at different wavelengths. The laser resonator is formed between the HR coated back facet of the emitter and the output coupler. WBC allows for brightness scaling of an emitter array because all of the laser elements are spatially overlapped at the output coupler, maintaining the output beam quality of a single element while scaling the output power by the number of elements in the array.
Laser sensor could lead way to handheld bomb-detectors
Tiny plasmon-based sensor detects minute traces of explosives in the air
A team at University of California, Berkeley led by Xiang Zhang, professor of mechanical engineering, has shown that a plasmon laser sensor can be used to detect minute concentrations of explosives in the air, including a hard-to-detect plastic explosive
102
www.compoundsemiconductor.net Issue VI 2014
called PETN popular among terrorists. The results were published in the journal Nature Nanotechnology.
Plasmon lasers work by coupling electromagnetic waves with the electrons that oscillate at the surface of metals to squeeze light into nanoscale spaces far past its natural diffraction limit of half a wavelength. The UC Berkeley plasmon laser is based on a cadmium suphide semiconductor square measuring around 50nm thick and 1000nm long, placed on a silver surface and separated by a 8nm gap of magnesium fluoride. The most intense electric fields of the device reside in the magnesium fluoride gap.
In designing the sensor device, the researchers took advantage of the chemical makeup of many explosives, particularly nitro-compounds such as DNT and its more well-known relative, TNT. Their unstable nitro groups are characteristically electron deficient, which increases the interaction of the molecules with natural surface defects on the semiconductor. The sensor works by detecting the increased intensity in the light signal that occurs as a result of this interaction.
The engineers put the sensor to the test with various explosives - 2,4-dinitrotoluene (DNT), ammonium nitrate and nitrobenzene - and found that the device successfully detected the airborne chemicals at concentrations of 0.67 parts per billion, 0.4 parts per billion and 7.2 parts per million, respectively. One part per billion would be akin to a blade of grass on a football field. These results, which are much more sensitive than those published to date for other optical sensors, were published in the advanced online publication of the journal Nature Nanotechnology.
The researchers hope that their plasmon laser sensor could detect pentaerythritol tetranitrate, or PETN, a plastic explosive favoured by terrorists because small amounts of it pack a powerful punch and it escapes x-ray machines when not connected to a detonator. It is the explosive found in Richard Reid’s shoe bomb in 2001 and Umar
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160