This page contains a Flash digital edition of a book.
Technology  GaN lasers


innovation and research institute CSEM, which is co- ordinating the project; Fraunhofer IAF; EPFL; the Lebedev Physical Institute of the Russian Academy of Sciences; the University of Cambridge; and the Technical University of Berlin. The mission for this multi- national team is to create an ultra-fast semiconductor laser diode that produces sub-picosecond optical pulses in the blue and violet spectral range.


Figure 1.Multiple section cavity arrangement (a) and driving conditions for operation in self-starting mode-locking (b),active mode- locking (c) and superradiance (d) regimes .Figure show images of Femtoblue devices grown and processed by researchers at EPFL.The cavity design is made by researchers at CSEM,LPI and UCAM


the obvious question to ask this: Is there an easier way to generate blue-green picosecond pulses?


Nitride solutions Well, there isn’t a unit that you could buy off the shelf for doing this today – but there soon could be. That’s because it has been recently shown that is possible to generate really short pulses with a GaN-based laser, which has the great attribute that its spectral range is a perfect match for the absorption spectrum of many organic components. This wide bandgap source also has many key advantages over the Ti:Sapphire incumbent, including low cost, small size and maintenance-free operation. These characteristics enable the GaN laser to provide the first portable source of blue-green ultra-short pulses, opening up the opportunity for a portable, time-resolved fluorescence measurement system to be placed at the point of care for biomedical diagnostics.


Within Europe we are trying to create such a laser through a project called FemtoBlue, which is funded through the European Commission. This effort, which kicked off in September 2009 and is backed by € 2 million of funding, is drawing on a diverse set of talents held by researchers at six institutions: The Swiss


If successful, the benefits could extend beyond activities in the biological sciences. Blue and violet lasers with ultra-fast pulses could unlock the door to a new, three- dimensional optical data storage disc technology that replaces the Blu-ray standard. Other possible applications include multiphoton nano-processing and nano-imaging (see “Further Reading” for details).


Our development of ultrafast GaN lasers is not the only work in this field. A Japanese collaboration between Sony Corporation and Tohoku University’s New Industry Creation Hatchery Center has recently reported the output of 2 ps pulses with a peak power of 20 W and a 1 GHz repetition rate from an external-cavity, multi- section laser diode.


By feeding this output into a semiconductor optical amplifier, this research team has boosted peak power to 300 W. Their approach is markedly different from ours, using a relatively large laser system rather than a monolithic cavity design, but it provides another example of the capability of GaN lasers for providing pico-second pulses in a spectral range suited to biomedicine.


Forming ultra-short pulses The good news for anyone trying to develop pico- second GaN lasers is that they don’t have to re-invent the wheel. Instead, they can exploit all that has been learnt in the evolution of arsenide and phosphide lasers that deliver ultra-short optical pulses. With these AlGaAs and InGaAsP lasers, it has been possible to realize a wide variety of ultra-fast dynamic regimes by applying multiple p-contacts to the top of the device (see Figure 1).


In these modified lasers, one section is positively biased to provide optical gain, while another section takes on the role of a saturable absorber, which is driven as a photodiode with negative bias. With this design, lasing characteristics are governed by the driving conditions for each cavity and its geometry. Through a complex interplay of numerous phenomena, lasing is possible in eight different dynamic regimes, three of which produce ultrafast pulses.


Figure 2.Wurtzite lattice structure of nitrides (left) and Zinc blende lattice structure of conventional III-V compounds (right)


28 www.compoundsemiconductor.net January/February 2012


By applying an appropriate combination of applied voltages, the laser can operate in a self-starting passive mode-locking regime. Operating in this fashion, researchers at University of Cambridge have produced a InAs-GaAs quantum dot source emitting pulses with a


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  |  Page 161  |  Page 162  |  Page 163  |  Page 164  |  Page 165  |  Page 166  |  Page 167  |  Page 168  |  Page 169  |  Page 170  |  Page 171  |  Page 172  |  Page 173  |  Page 174  |  Page 175  |  Page 176  |  Page 177  |  Page 178  |  Page 179  |  Page 180  |  Page 181  |  Page 182  |  Page 183  |  Page 184  |  Page 185  |  Page 186  |  Page 187  |  Page 188  |  Page 189  |  Page 190  |  Page 191  |  Page 192  |  Page 193  |  Page 194  |  Page 195  |  Page 196  |  Page 197  |  Page 198  |  Page 199  |  Page 200  |  Page 201  |  Page 202  |  Page 203  |  Page 204  |  Page 205  |  Page 206  |  Page 207  |  Page 208  |  Page 209  |  Page 210  |  Page 211  |  Page 212  |  Page 213  |  Page 214  |  Page 215  |  Page 216  |  Page 217  |  Page 218  |  Page 219  |  Page 220  |  Page 221  |  Page 222  |  Page 223  |  Page 224  |  Page 225  |  Page 226  |  Page 227  |  Page 228  |  Page 229  |  Page 230  |  Page 231  |  Page 232  |  Page 233  |  Page 234  |  Page 235  |  Page 236  |  Page 237  |  Page 238  |  Page 239  |  Page 240  |  Page 241