Volume 82, Issue 7, July 2011
Index of content:
We review the current status of single-photon-source and single-photon-detector technologies operating at wavelengths from the ultraviolet to the infrared. We discuss applications of these technologies to quantum communication, a field currently driving much of the development of single-photon sources and detectors.
- Optics; Atoms and Molecules; Spectroscopy; Photon Detectors
A fiber loop mirror temperature sensor demodulation technique using a long-period grating in a photonic crystal fiber and a band-pass filter82(2011); http://dx.doi.org/10.1063/1.3606432View Description Hide Description
A fiber loop mirror (FLM) temperaturesensor using a long-period grating (LPG) written in a photonic crystal fiber(PCF) and a band-pass filter as a demodulator is proposed. By utilizing the stable filtering function of the LPG in the PCF, the resonant wavelength variation of the FLM with temperature is transferred effectively to the intensity variation of the output light. By monitoring the light intensity of the band-pass of the filter, temperature applied on the FLM is deduced by an optical power meter. Experiment results show that the temperature sensitivity is high as ∼1.742 dB/ °C when a filter with a full width at half maximum 3 nm and the center at 1545 nm is used.
82(2011); http://dx.doi.org/10.1063/1.3606484View Description Hide Description
Before being used in an extreme-ultraviolet (EUV)scanner,photoresists must first be evaluated for sensitivity and tested to ensure that they will not contaminate the scanner optics. The new NIST facility described here provides data on the contamination potential of the outgas products of a candidate resist by simultaneously irradiating a multilayer optic and a nearby resist-coated wafer with EUVradiation. The facility can also be used without changing its configuration to provide accurate resist dose-to-clear measurements. Detailed, real-time information on the rate of contamination growth is given by a unique, in situ imaging ellipsometer. We will describe the optical layout, mechanical design, and capabilities of the beamline, finally presenting experimental examples of its capabilities.
Scanning Fabry-Pérot interferometer with largely tuneable free spectral range for high resolution spectroscopy of single quantum dots82(2011); http://dx.doi.org/10.1063/1.3601016View Description Hide Description
We report on the implementation of a scanning Fabry-Pérot interferometer for photoluminescencespectroscopy investigation. We choose a conveniently small reflectivity of the two planar semitransparent mirrors which, in spite of a moderate cavity finesse, ensures a good mechanical stability over a long time. We also exploit the large tuneability of the cavity length (i.e., of the free spectral range) for changing the spectral resolution over two order of magnitude (from ∼300 μeV to ∼4 μeV in full width at half maximum). Such a characteristic easily allows to scan both sharp and broad luminescence bands. We test our Fabry-Pérot interferometer on sharp photoluminescence lines resulting from excitonic recombination in self-assembled GaAsquantum dots. We demonstrate the ability of our system to resolve linewidth as small as 4 μeV.
82(2011); http://dx.doi.org/10.1063/1.3607438View Description Hide Description
We describe the design, construction, and performance of a 4-circle in-vacuum diffractometer for resonant elastic soft x-ray scattering. The diffractometer, installed on the resonant elastic and inelastic x-ray scattering beamline at the Canadian Light Source, includes 9 in-vacuum motions driven by in-vacuum stepper motors and operates in ultra-high vacuum at base pressure of 2 × 10-10 Torr. Cooling to a base temperature of 18 K is provided with a closed-cycle cryostat. The diffractometer includes a choice of 3 photondetectors: a photodiode, a channeltron, and a 2D sensitive channelplate detector. Along with variable slit and filter options, these detectors are suitable for studying a wide range of phenomena having both weak and strong diffraction signals. Example measurements of diffraction and reflectivity in Nd-doped (La,Sr)2CuO4 and thin film (Ga,Mn)As are shown.
Linewidth reduction of a distributed-feedback diode laser using an all-fiber interferometer with short path imbalance82(2011); http://dx.doi.org/10.1063/1.3606439View Description Hide Description
The linewidth of a distributed-feedback (DFB)diode laser at 1156 nm, of which free-running linewidth was 3 MHz, was reduced to 15 kHz using an all-fiber interferometer with 5-m-long path imbalance. Optical power loss and bandwidth limitation were negligible with this short optical fiber patch cord. This result was achieved without acoustic and vibration isolations, and the frequency lock could be maintained over weeks. In addition to its simplicity, compactness, robustness, and cost-effectiveness, this technique can be applied at any wavelength owing to the availability of DFBdiode lasers and fiber-optic components.
82(2011); http://dx.doi.org/10.1063/1.3610465View Description Hide Description
We report on a new design of a vacuum ultra violet (VUV) lamp for direct optical excitation of high laying atomic states, e.g., for excitation of metastable rare gas atoms. The lamp can be directly mounted to ultra-high vacuum vessels (p ⩽ 10−10mbar). It is driven by a 2.45 GHz microwave source. For optimum operation, it requires powers of ∼20 W. The VUV light is transmitted through a magnesium fluoride window, which is known to have a decreasing transmittance for VUVphotons with time. In our special setup, after a run-time of the VUV lamp of 550 h the detected signal continuously decreased to 25% of its initial value. This corresponds to a lifetime increase of two orders of magnitude compared to previous setups or commercial lamps.
82(2011); http://dx.doi.org/10.1063/1.3611005View Description Hide Description
We use a waveguide-based electro-optic phase modulator, driven by a nanosecond-timescale arbitrary waveform generator, to produce an optical spectrum with an arbitrary pattern of peaks. A programmed sequence of linear voltage ramps, with various slopes, is applied to the modulator. The resulting phase ramps give rise to peaks whose frequency offsets relative to the carrier are equal to the slopes of the corresponding linear phase ramps. This simple extension of the serrodyne technique provides multi-line spectra with peak spacings in the 100 MHz range.
82(2011); http://dx.doi.org/10.1063/1.3610454View Description Hide Description
A new type of collector optics was developed for grazing incident x-ray emission spectrometer. The collector optics used two cylindrical mirrors to add two extra light paths while keeping the center light path that directly illuminates the grating. The design and properties of the spectrometer using the triple-path collector optics were evaluated using ray-tracing simulations, and validity of this design in terms of throughput and energy resolution was confirmed by the experimentally obtained spectra.
82(2011); http://dx.doi.org/10.1063/1.3613952View Description Hide Description
Here we present a new fast two-bit quantum random number generator based on the intrinsic randomness of the quantum physical phenomenon of photon statistics of coherent light source. Two-bit random numbers were generated according to the number of detected photons in each light pulse by a photon-number-resolving detector. Poissonian photon statistics of the coherent light source guaranteed the complete randomness of the bit sequences. Multi-bit true random numbers were generated for the first time based on the multi-photon events from a coherent light source.
82(2011); http://dx.doi.org/10.1063/1.3615245View Description Hide Description
We describe our implementation of a high repetition rate (54 kHz–6.5 MHz), high power (>10 W), laser system at the 7ID beamline at the Advanced Photon Source for laser pump/x-ray probe studies of optically driven molecular processes. Laser pulses at 1.06 μm wavelength and variable duration (10 or 130 ps) are synchronized to the storage ring rf signal to a precision of ∼250 fs rms. Frequency doubling and tripling of the laser radiation using nonlinear optical techniques have been applied to generate 532 and 355 nm light. We demonstrate that by combining a microfocused x-ray probe with focused optical laser radiation the requisite fluence (with <10 μJ/pulse) for efficient optical excitation can be readily achieved with a compact and commercial laser system at megahertz repetition rates. We present results showing the time-evolution of near-edge x-ray spectra of a well-studied, laser-excited metalloporphyrin, Ni(II)-tetramesitylporphyrin. The use of high repetition rate, short pulse lasers as pump sources will dramatically enhance the duty cycle and efficiency in data acquisition and hence capabilities for laser-pump/x-ray probe studies of ultrafast structural dynamics at synchrotron sources.
- Particle Sources, Optics and Acceleration; Particle Detectors
Calibration of a Thomson parabola ion spectrometer and Fujifilm imaging plate detectors for protons, deuterons, and alpha particles82(2011); http://dx.doi.org/10.1063/1.3606446View Description Hide Description
A Thomson parabola ion spectrometer has been designed for use at the Multiterawatt (MTW) laser facility at the Laboratory for Laser Energetics (LLE) at the University of Rochester. This device uses parallel electric and magnetic fields to deflect particles of a given mass-to-charge ratio onto parabolic curves on the detector plane. Once calibrated, the position of the ions on the detector plane can be used to determine the particle energy. The position dispersion of both the electric and magnetic fields of the Thomson parabola was measured using monoenergetic proton and alpha particle beams from the SUNY Geneseo 1.7 MV tandem Pelletron accelerator. The sensitivity of Fujifilm BAS-TR imaging plates, used as a detector in the Thomson parabola, was also measured as a function of the incident particle energy over the range from 0.6 MeV to 3.4 MeV for protons and deuterons and from 0.9 MeV to 5.4 MeV for alpha particles. The device was used to measure the energy spectrum of laser-produced protons at MTW.
82(2011); http://dx.doi.org/10.1063/1.3608454View Description Hide Description
We have designed and constructed a superconducting beta spectrometer with a momentum resolution of about 2% and a peak solid angle of 0.5 sr. The performance of the spectrometer is described and the results of calibrations with line sources are presented.
Development of a compact fast CCD camera and resonant soft x-ray scattering endstation for time-resolved pump-probe experiments82(2011); http://dx.doi.org/10.1063/1.3609862View Description Hide Description
The designs of a compact, fast CCD (cFCCD) camera, together with a resonant soft x-rayscattering endstation, are presented. The cFCCD camera consists of a highly parallel, custom, thick, high-resistivity CCD, readout by a custom 16-channel application specific integrated circuit to reach the maximum readout rate of 200 frames per second. The camera is mounted on a virtual-axis flip stage inside the RSXS chamber. When this flip stage is coupled to a differentially pumped rotary seal, the detector assembly can rotate about 100°/360° in the vertical/horizontal scattering planes. With a six-degrees-of-freedom cryogenic sample goniometer, this endstation has the capability to detect the superlattice reflections from the electronic orderings showing up in the lower hemisphere. The complete system has been tested at the Advanced Light Source, Lawrence Berkeley National Laboratory, and has been used in multiple experiments at the Linac Coherent Light Source, SLAC National Accelerator Laboratory.
- Nuclear Physics, Fusion and Plasmas
82(2011); http://dx.doi.org/10.1063/1.3605485View Description Hide Description
The laser megajoule (LMJ) and the National Ignition Facility (NIF) plan to demonstrate thermonuclear ignition using inertial confinement fusion(ICF). The neutron yield is one of the most important parameters to characterize ICF experiment performance. For decades, the activation diagnostic was chosen as a reference at ICF facilities and is now planned to be the first nuclear diagnostic on LMJ, measuring both 2.45 MeV and 14.1 MeV neutron yields. Challenges for the activation diagnostic development are absolute calibration, accuracy, range requirement, and harsh environment. At this time, copper and zirconium material are identified for 14.1 MeV neutron yield measurement and indium material for 2.45 MeV neutrons. A series of calibrations were performed at Commissariat à l'Energie Atomique (CEA) on a Van de Graff facility to determine activation diagnostics efficiencies and to compare them with results from calculations. The CEA copper activation diagnostic was tested on the OMEGA facility during DT implosion. Experiments showed that CEA and Laboratory for Laser Energetics (LLE) diagnostics agree to better than 1% on the neutron yield measurement, with an independent calibration for each system. Also, experimental sensitivities are in good agreement with simulations and allow us to scale activation diagnostics for the LMJ measurement range.
The coincidence counting technique for orders of magnitude background reduction in data obtained with the magnetic recoil spectrometer at OMEGA and the NIF82(2011); http://dx.doi.org/10.1063/1.3605483View Description Hide Description
A magnetic recoil spectrometer (MRS) has been built and successfully used at OMEGA for measurements of down-scattered neutrons (DS-n), from which an areal density in both warm-capsule and cryogenic-DT implosions have been inferred. Another MRS is currently being commissioned on the National Ignition Facility (NIF) for diagnosing low-yield tritium-hydrogen-deuterium implosions and high-yield DT implosions. As CR-39 detectors are used in the MRS, the principal sources of background are neutron-induced tracks and intrinsic tracks (defects in the CR-39). The coincidence counting technique was developed to reduce these types of background tracks to the required level for the DS-n measurements at OMEGA and the NIF. Using this technique, it has been demonstrated that the number of background tracks is reduced by a couple of orders of magnitude, which exceeds the requirement for the DS-n measurements at both facilities.
82(2011); http://dx.doi.org/10.1063/1.3608551View Description Hide Description
An integrated data analysis system based on Bayesian inference has been developed for the TJ-II stellarator. It reconstructs the electron density profile at a single time point, using data from interferometry,reflectometry, Thomson scattering, and the Helium beam, while providing a detailed error analysis. In this work, we present a novel analysis of the ambiguity inherent in profile reconstruction from reflectometry and show how the integrated data analysis approach elegantly resolves it. Several examples of the application of the technique are provided, in both low-density discharges with and without electrode biasing, and in high-density discharges with an (L-H) confinement transition.
Faraday cup with nanosecond response and adjustable impedance for fast electron beam characterization82(2011); http://dx.doi.org/10.1063/1.3610649View Description Hide Description
A movable Faraday cup design with simple structure and adjustable impedance is described in this work. This Faraday cup has external adjustable shunt resistance for self-biased measurement setup and 50 Ω characteristic impedance to match with 50 Ω standard BNC coaxial cable and vacuum feedthroughs for nanosecond-level pulse signal measurements. Adjustable shunt resistance allows self-biased measurements to be quickly acquired to determine the electron energy distribution function. The performance of the Faraday cup is validated by tests of response time and amplitude of output signal. When compared with a reference source, the percent difference of the Faraday cup signal fall time is less than 10% for fall times greater than 10 ns. The percent difference of the Faraday cup signal pulse width is below 6.7% for pulse widths greater than 10 ns. A pseudospark-generated electron beam is used to compare the amplitude of the Faraday cup signal with a calibrated F-70 commercial current transformer. The error of the Faraday cup output amplitude is below 10% for the 4–14 kV tested pseudospark voltages. The main benefit of this Faraday cup is demonstrated by adjusting the external shunt resistance and performing the self-biased method for obtaining the electron energy distribution function. Results from a 4 kV pseudospark discharge indicate a “double-humped” energy distribution.
- Microscopy and Imaging
82(2011); http://dx.doi.org/10.1063/1.3605664View Description Hide Description
We demonstrate a simple capacitance-based method to quickly and efficiently locate micron-sized conductive samples, such as graphene flakes, on insulating substrates in a scanning tunneling microscope(STM). By using edge recognition, the method is designed to locate and to identify small features when the STM tip is far above the surface, allowing for crash-free search and navigation. The method can be implemented in any STM environment, even at low temperatures and in strong magnetic field, with minimal or no hardware modifications.
82(2011); http://dx.doi.org/10.1063/1.3606442View Description Hide Description
We discuss the problem of signal diffusion among neighbouring pixels in x-ray phase contrastimaging (XPCi) specifically for coded-aperture (CA) XPCi, but many of the discussed observations are directly transferable to other XPCi modalities. CA XPCi exploits the principle of pixel edge illumination by means of two CA masks. The first mask, placed in contact with the detector, creates insensitive regions between adjacent pixels; the second one, placed immediately before the sample, creates individual beams impinging on the boundaries between sensitive and insensitive regions on the detector, as created by the detector mask. In this way, edge illumination is achieved for all pixels of an area detector illuminated by a divergent and polychromatic beam generated by a conventional source. As the detector mask redefines the resolution properties of the detector, sample dithering can be used to effectively increase the system spatial resolution, without having to apply any post-processing procedure (e.g., deconvolution). This however creates artifacts in the form of secondary fringes (which have nothing to do with phase-related secondary fringes) if there is signal diffusion between adjacent pixels. In non-dithered images, signal diffusion between adjacent pixels causes a reduction in imagecontrast. This effect is investigated both theoretically and experimentally, and its direct implications on image quality are discussed. The interplay with the sample positioning with respect to the detector pixel matrix, which also has an effect on the obtained imagecontrast, is also discussed.
Modification of a commercial atomic force microscopy for low-noise, high-resolution frequency-modulation imaging in liquid environment82(2011); http://dx.doi.org/10.1063/1.3606399View Description Hide Description
A key issue for high-resolution frequency-modulation atomic force microscopyimaging in liquids is minimizing the frequency noise, which requires a detailed analysis of the corresponding noise contributions. In this paper, we present a detailed description for modifying a commercial atomic force microscope (Bruker MultiMode V with Nanoscope V controller), aiming at atomic-resolution frequency-modulation imaging in ambient and in liquid environment. Care was taken to maintain the AFMs original stability and ease of operation. The new system builds upon an optimized light source, a new photodiode and an entirely new amplifier. Moreover, we introduce a home-built liquid cell and sample holder as well as a temperature-stabilized isolation chamber dedicated to low-noise imaging in liquids. The success of these modifications is measured by the reduction in the deflection sensornoise density from initially 100 to around 10 after modification. The performance of our instrument is demonstrated by atomically resolved images of calcite taken under liquid conditions.