2Graphene and related two-dimensional materials are promising candidates for atomically thin, flexible, and transparent optoelectronics 1,2 . In particular, the strong light-matter interaction in graphene 3 has allowed for the development of state-of-the-art photodetectors 4,5 , optical modulators 6 , and plasmonic devices 7 .In addition, electrically biased graphene on SiO 2 substrates can be used as a low-efficiency emitter in the mid-infrared range 8,9 . However, emission in the visible range has remained elusive. Here we report the observation of bright visible-light emission from electrically biased suspended graphenes. In these devices, heat transport is greatly minimised 10 ; thus hot electrons (~ 2800 K) become spatially localised at the centre of graphene layer, resulting in a 1000-fold enhancement in the thermal radiation efficiency 8,9 . Moreover, strong optical interference between the suspended graphene and substrate can be utilized to tune the emission spectrum. We also demonstrate the scalability of this For the realisation of graphene-based bright and broadband light-emitters, a radiative electron-hole recombination process in gapless graphene is not efficient because of the rapid energy relaxation that occurs through electron-electron and electron-phonon interactions [11][12][13] .Alternatively, graphene's superior strength 14 and high-temperature stability may enable efficient thermal light emission. However, the thermal radiation from electrically biased graphene supported on a substrate 8,9,[15][16][17] has been found to be limited to the infrared range and 3 to be inefficient as an extremely small fraction of the applied energy (~ 10 -6 ) 8,9 is converted into light radiation. Such limitations are the direct result of heat dissipation through the underlying substrate 18 and significant hot electron relaxation from dominant extrinsic scattering effects such as charged impurities 19 and surface polar optical phonon interaction 20 , limiting the maximum operating temperatures.On the other hand, a freely suspended graphene is mostly immune to such undesirable vertical heat dissipation 10 and extrinsic scattering effects 21,22 , promising much more efficient and brighter radiation in the infrared-to-visible range. Fortuitously, due to the strong Umklapp phonon-phonon scattering 23 , we find that the thermal conductivity of graphene at high lattice temperatures (1800 ± 300 K) is greatly reduced (~ 65 Wm -1 K -1 ), which additionally suppresses lateral heat dissipation; therefore, hot electrons (~ 2800 K) become spatially localised at the centre of the suspended graphene under modest electric fields (~ 0.4 V/µm), greatly increasing the efficiency and brightness of the light emission. The bright visible thermally emitted light interacts with the reflected light from the separated substrate surface, allowing interference effects that can be utilized to tune the wavelength of the emitted light.We fabricate the freely suspended graphene devices with mechanically exfoliated graphene flakes, and for d...

No abstract

We present an experimental realization of differential spectral responsivity measurement by using a light-emitting diode (LED)-based integrating sphere source. The spectral irradiance responsivity is measured by a Lambertian-like radiation field with a diameter of 40 mm at the peak wavelengths of the 35 selectable LEDs covering a range from 280 to 1550 nm. The systematic errors and uncertainties due to lock-in detection, spatial irradiance distribution, and reflection from the test detector are experimentally corrected or considered. In addition, we implemented a numerical procedure to correct the error due to the broad spectral bandwidth of the LEDs. The overall uncertainty of the DSR measurement is evaluated to be 2.2% (k = 2) for Si detectors. To demonstrate its application, we present the measurement results of two Si photovoltaic detectors at different bias irradiance levels up to 120 mW/cm(2).

No abstract

We present an instrumental realization of linearity measurement of optical detectors based on flux addition of two light-emitting diodes (LEDs) in an integrating sphere. The instrument measures the linearity ratios at pre-defined photocurrent levels according to the algorithm which eliminates the drift effect of LED flux (Shin D J et al 2005 Metrologia 42 154). As a result, the non-linearity correction factors for detector responsivity are determined with their relative standard uncertainties ranging from 2 × 10 −5 to 6 × 10 −5 (k = 1) for photocurrent ranges from 10 −8 A to 10 −3 A. By changing the type of LEDs, the wavelength dependence of non-linearity can also be measured, which is demonstrated at 390 nm, 670 nm and 750 nm for a Si photodiode used as a transfer standard for spectral responsivity scale comparisons.