A diamond nano-crystal hosting a single nitrogen vacancy (NV) center is optically selected with a confocal scanning microscope and positioned deterministically onto the subwavelength-diameter waist of a tapered optical fiber (TOF) with the help of an atomic force microscope. Based on this nano-manipulation technique we experimentally demonstrate the evanescent coupling of single fluorescence photons emitted by a single NV-center to the guided mode of the TOF. By comparing photon count rates of the fiber-guided and the free-space modes and with the help of numerical FDTD simulations we determine a lower and upper bound for the coupling efficiency of (9.5 ± 0.6)% and (10.4 ± 0.7)%, respectively. Our results are a promising starting point for future integration of single photon sources into photonic quantum networks and applications in quantum information science.PACS numbers: 03.67. 42.50.Ex, 78.67.Bf Efficient collection of single photons radiated by a single solid state quantum emitter -like the nitrogen vacancy (NV) center in diamond 1 -is an important prerequisite for future applications in applied physical and quantum information science, like ultra-sensitive fluorescence spectroscopy and linear optical quantum computation 2-4 . A standard technique for fluorescence collection is confocal microscopy. However, when applied to defect centers in bulk diamond, total internal reflection limits the collection efficiency to few percent. Recently, the collection efficiency of NV-fluorescence has been increased by one order of magnitude by combining confocal microscopy with solid immersion lenses (SILs) 5-7 , respectively photonic nanowires 8 . In the latter system the improvement is based on efficient coupling of NV-fluorescence photons to the strongly confined mode (HE 11 ) 9-11 of diamond nanowires. For defect centers in diamond nano-crystals, tapered optical fibers (TOFs) 12 with a subwavelength diameter waist are a particularly attractive alternative platform. Due to the strong evanescent field at the surface, such TOFs promise coupling efficiencies up to 36%13,14 and approaching unity when combined with Bragg-grating cavities 15,16 . Until now, evanescent coupling of fluorescence photons to a single guided mode of a TOF has been achieved for a) email: lars.liebermeister@physik.uni-muenchen.de b) email: markusweber@lmu.de various solid state quantum emitters 17-20 , molecules 21 , and laser-cooled atomic vapors 22 . To bring these emitters into the strong evanescent optical field at the surface of the nano-fiber several non-deterministic deposition techniques like dip-coating 17,18 , picoliter-dispensers 19,20 , and optical surface traps 23 have been applied. However, for real applications in quantum information science, e. g., the photonic quantum-bus mediated coupling of NVcenters in a lattice 24 , deterministic positioning of single solid state quantum emitters onto the submicron waist of a TOF with nm position control is desirable. In this letter we demonstrate significant steps towards deterministic coup...

We investigate photoconductive terahertz (THz) emitters compatible with 1550 nm excitation for THz time-domain spectroscopy (TDS). The emitters are based on rhodium (Rh) doped InGaAs grown by molecular beam epitaxy. InGaAs:Rh exhibits a unique combination of ultrashort trapping time, high electron mobility, and high resistivity. THz emitters made of InGaAs:Rh feature an emitted THz power of 637 μW at 28 mW optical power and 60 kV/cm electrical bias field. In particular for a fiber coupled photoconductive emitter, this is an outstanding result. When these emitters are combined with InGaAs:Rh based receivers in a THz TDS system, 6.5 THz bandwidth and a record peak dynamic range of 111 dB can be achieved for a measurement time of 120 s.

Broadband terahertz spectroscopy enables many promising applications in science and industry alike. However, the complexity of existing terahertz systems has as yet prevented the breakthrough of this technology. In particular, established terahertz time-domain spectroscopy (TDS) schemes rely on complex femtosecond lasers and optical delay lines. Here, we present a method for optoelectronic, frequency-modulated continuous-wave (FMCW) terahertz sensing, which is a powerful tool for broadband spectroscopy and industrial non-destructive testing. In our method, a frequency-swept optical beat signal generates the terahertz field, which is then coherently detected by photomixing, employing a time-delayed copy of the same beat signal. Consequently, the receiver current is inherently phase-modulated without additional modulator. Owing to this technique, our broadband terahertz spectrometer performs (200 Hz measurement rate, or 4 THz bandwidth and 117 dB peak dynamic range with averaging) comparably to state-of-the-art terahertz-TDS systems, yet with significantly reduced complexity. Thickness measurements of multilayer dielectric samples with layer-thicknesses down to 23 µm show its potential for real-world applications. Within only 0.2 s measurement time, an uncertainty of less than 2 % is achieved, the highest accuracy reported with continuous-wave terahertz spectroscopy. Hence, the optoelectronic FMCW approach paves the way towards broadband and compact terahertz spectrometers that combine fiber optics and photonic integration technologies.

We carried out an experimental comparison study of the two most established optoelectronic emitters for continuous-wave (cw) terahertz generation: a uni-traveling-carrier photodiode (UTC-PD) and a pin-photodiode (PIN-PD). Both diodes are commercially available and feature a similar package (fiber-pigtailed housings with a hyper-hemispherical silicon lens). We measured the terahertz output as a function of optical illumination power and bias voltage from 50 GHz up to 1 THz, using a precisely calibrated terahertz power detector. We found that both emitters were comparable in their spectral power under the operating conditions specified by the manufacturers. While the PIN-PD turned out to be more robust against varying operating parameters, the UTC-PD showed no saturation of the emitted terahertz power even for 50 mW optical input power. In addition, we compared the terahertz transmission and infrared (IR) blocking ratio of four different filter materials. These filters are a prerequisite for correct measurements of the absolute terahertz power with thermal detectors.

Continuous wave THz (cw THz) systems define the state-of-the-art in terms of spectral resolution in THz spectroscopy. Hitherto, acquisition of broadband spectra in a cw THz system was always connected with slow operation. Therefore, high update rate applications like inline process monitoring and non-destructive testing are served by time domain spectroscopy (TDS) systems. However, no fundamental restriction prevents cw THz technology from achieving faster update rates and be competitive in this field. In this paper, we present a fully fibercoupled cw THz spectrometer. Its sweep speed is two orders of magnitude higher compared to commercial state-of-the-art systems and reaches a record performance of 24 spectra per second with a bandwidth of more than 2 THz. In the single-shot mode, the same system reaches a peak dynamic range of 67 dB and exceeds a value of 100 dB with averaging of 7 min, which is among the highest values ever reported. The frequency steps can be as low as 40 MHz. Due to the fully homodyne detection, each spectrum contains full amplitude and phase information. This demonstration of THz-spectroscopy at video-rate is an essential step towards applying cw THz systems in non-destructive, in line testing.