SUPERCONDUCTING SINGLE PHOTON DETECTOR

The invention describes an integrated, hybrid nanophotonic- superconducting device that functions as a scalable spectrometer with single photon intensity resolution and picosecond timing resolution. The device is realized on a single chip and contacted optically by optical fibers, thus compatible with optical imaging systems through a fiber coupling port.

Field of research: The invention is the result of research in the areas of nanophotonics and .integrated optics, as well as superconducting thin film technology. The combination of these formerly separate fields yields a new technology platform to realize novel integrated circuits with additional degrees of freedom when compared to traditional photonic integrated circuits (PICs) . In particular, wafer-scale fabrication techniques can be employed to realize functional miniaturized circuits, with the potential for cheap and mass-market production.

The combination of nanophotonics and superconducting technology enables the integration of electronic and microwave components with optical components, in the same device. Both kinds of devices can be fabricated using the same fabrication routines, namely electron beam lithography and dry etching, thus making device design simple and convenient. At the same time, robust fabrication recipes developed for the electronics industry can be used to realize a multitude of devices with high yield.

A particular feature of- the new platform is the possibility to implement scalable circuits, i.e. systems that contain many identical devices. This is of importance when implementing complex circuits, that are otherwise either difficult to fabricate or expensive to add to a given device architectur.

Problem addressed: The invention describes a high-performance spectrometer, which allows measurements of weakest signals on multiple wavelengths simultaneously. The platform described above enables the production of. very high performance single photon detectors as part of advanced nanophotonic circuits. Single photon detectors are of importance in many fields whenever low intensity or non-classical signals, have to be measured, such as quantum optics, biological imaging, chemical analysis, long-distance fiber-optical communication, etc. In these cases either optical loss or low signal strength make it necessary to be able to detect individual photons with high detection efficiency and speed.

Traditional single photon detectors are not available in integrated form and thus have to be purchased as stand-alone units. As a result, these instruments are expensive and unsuitable for applications where many detectors are necessary. Such areas afe for example · optical quantum computing and multi -wavelength applications, such as long-range wavelength division multiplexing in optical fiber communication.

In biological imaging, low intensity signals have to be differentiated when performing single cell or single molecule imaging, as well as during fluorescence imaging. Being able to measure lowest intensities on multiple wavelengths simultaneously will enable the use of advanced image processing techniques to significantly enhance . signal to noise ratio, which is compromised for example by autofluorescence .

This is in particular relevant for the application of optical spectrometers- To date, such devices require a high performance detector for each wavelength used inside the spectrometer, which is not commercially feasible when using high-performance detectors. Alternatively wavelength sweeps can be performed with a single detector. However, this approach severely limits the overall device speed. Furthermore, advanced optical filters and components are required to clean the input signal spectrally, which needs careful optical alignment and design and is thus non-trivial in traditional spectrometer designs.

Problem solution: The technology platform combining nanophotonics and superconducting elements described above enables the fabrication of high-performance single photon detectors directly inside photonic circuits. Therefore arbitrary numbers of detectors can be defined during the same step as the rest of the circuits, making the process scalable. Hence cheap single photon detection on chip can be realized, with unequalled performance compared to other single photon detectors. At the same time high quality integrated optical circuits can be employed to enhance the spectral performance of the device and in ^' particular spectrally design the input into each detector channel .

This approach is therefore ideally suited for the implementation of on-chip spectrometers. In particular, the platform can be used to define low-cost integrated spectrometers that operate at single photon levels for arbitrary wavelength combinations.

Application: The integrated single photon spectrometer comprises a device to measure single photon signals spectrally resolved using an integrated circuit. The optical circuit is used to spectrally separate the input into different channels that are then guided in individual waveguides . Optical filtering on chip is used to engineer the spectral properties of each channel using optical design. Therefore a target spectral output can be selected and adjusted to by photonic computer aided design.

Methods used : In order to be able to cover a broad wavelength range a new material system for integrated optics is used. This material platform comprises Aluminum nitride (AlN) thin films deposited onto oxidized silicon wafers, thus realizing a substrate stack as AlN-On- Insulator (AOI) , equivalent to silicon-on- insulator wafers. The AlN thin film is ^' deposited by sputtering methods at low temperature, enabling CMOS compatible fabrication at low cost.

AlN offers optical transparency from, 220nm to 13.6μτη, therefore covering the entire visible wavelength range, the telecoms window as well as the fingerprint region in the infrared. AlN can be dry etched after electron-beam lithography using chlorine-based chemistry and is therefore suitable for photonic fabrication.

Optical waveguiding is possible in AlN with low loss and therefore enables the fabrication ^' of large-scale optical circuits and optical resonators. AlN films are sputter-deposited and therefore large wafers can ^' be easily obtained. The AlN film morphology does not need to be crystalline in plane and c-axis oriented film growth out-of- plane can be readily achieved.

To co-fabricate single photon detectors, the AOI substrates are covered with ultra-thin layers of superconducting materials, with a thickness on the order of a few nanometers. Suitable superconducting materials are niobium nitride (NbN) , niobium ^" titanium nitride (NbTiN) or tantalum nitride (TaN) ; among others. From these ultra- thin films high efficiency superconducting single photon detectors (SSPDs) can be fabricated using electron beam lithography and dry etching. The detectors consist of nanowires laid out in a meander pattern.

With the new technology platform, SSPDs are realized directly on top of AlN waveguides. This enables strong coupling of the optical propagating mode to the detector. As a result, the absorption of photons can easily reach 100%, thus providing basically perfect detection efficiency where each waveguided photon is converted to an electrical signal. Due to the strong coupling and propagation of photons along the meander, the absorption length can be drastically reduced compared to traditional SSPDS.

Because the fabricated devices can be very short to reach perfect photon absorption, the resulting detectors are ultra- fast with a timing resolution in the picoseconds range. At the same time, the reset time is very short, thus providing detection rates in the gigahertz range. In summary, the detector reaches 100% efficiency; with unprecedented timing resolution and speed and can be duplicated arbitrarily on chip.

To realize a single photon spectrometer on chip, a. nanophotonic circuit is used to separate different wavelengths into different output waveguides. ^■ Such a circuit can consist of on-chip spectrometers, . such as arrayed waveguide gratings (AWGs) , echelle gratings or devices based ^' on optical ring resonators. The photonic circuit thus performs the spectral shaping and guides photons into different waveguide, arms. Then each waveguide arm is equipped with an on-chip SSPD or waveguide SSPD . (WgSSPD) , which allows counting all photons selected for each particular channel (see figure below) .

The WgSSPDs offer broadband detection efficiency at very low dark count rates. There ore every incoming photon can be detected as long as it is absorbed. The absorption profile can be engineered by designing the coupling between waveguide and detector through modification of the waveguide geometry as well as the detector geometry. The detector is laid out in a nanowire pattern. The width of the nanowires determines the wavelength cutoff of the detectors, where thinner wires allow for the detection of longer wavelengths.

System design: The optical circuit can be easily fabricated ^' on chip. In order to build a ready to use system, the circuit will be optically contacted with optical fibers. This can be achieved by using optical grating couplers, . which allow out-of-plane alignment of the chip to optical fibers. Because the grating couplers do not require much chip area, many optical ports can.be defined to allow for parallel access. This scheme is in particular suitable for connecting different spectrometer circuits to the same optical input access by using, fiber arrays, in which several optical fibers are aligned next to each other.

Alternatively, fiber-butt coupling may be used in silicon V-grooves. Because the AOI substrates are fabricated on silicon carrier wafers, V-grooves . can be directly defined on the same chip by making use of the underlying silicon layer. Therefore precise optical alignment to the photonic circuit can be achieved.

This principle is illustrated in the figures, wherein:

Fig. 1 is a schematic perspective view of an embodiment of on-chip spectrometer (1) with output waveguide arms having a photon detector (2) on each arm and a polymer covered (3) input port coupled to a lensed optical fiber (5) positioned in a V-groove (4) cut in the Si0 ₂ (6) /Silicon ' (7) wafer chip;

Fig. 2a is ^' an enlarged perspective view of one of the output waveguide arms in Fig. 1, showing an embodiment of a detector in more detail which includes a meandering superconductive NbN nanowire (9) on top of an AlN waveguide arm (8) which is oh top of a substrate comprising an oxidized silicon (6) layer on a silicon base (7);

Fig. 2b is a cross section of Fig. 2a;

Fig. 3a is an enlarged side view of the input port of shown in FLg. 1, coupled to the lensed optical fiber (5) ;

Fig. 3b is a cross section of Fig. 3a; and

Fig. 3c is a top view of Fig. 3a.

Referring to the figures, a lensed fiber is aligned to the optical circuit using an inverse optical taper on chip for high coupling efficiency. The spectral selectivity, is provided by an AWG, which splits the optical input into multiple waveguide outputs, each designed for a different wavelength. The waveguides are then terminated with a WgSSPD to detect the propagating single photons.

The device design coupled to optical fibers enables in particular the use of the spectrometer with optical microscopes. The free fiber end can be efficiently connected to a fiber coupling port in a microscope and is thus directly compatible with existing imaging equipment. This will also enable the implementation of a scanning mechanism for 2D-Image acquisition, by using a- motorized microscope stage. Because of the high detection rate of the WgSSPDs fast scanning rates and thus high image repetition rates are possible.

Technical features: The detectors fabricated so far provide close to 100% detection efficiency on chip, with' a timing jitter below 19ps and a detection rate above 2GHz. These values can be further enhanced by fabricating shorter detectors and by using resonant enhancement in optical cavities. The current detectors show a length of 20μπι to reach 99% absorption efficiency at a rate of ldB/μιη. Thus higher absorption efficiency can be reached by increasing the detector length.

The superconducting thin films . are based on NbN layers with a typical thickness around 4nm, leading to a critical temperature around 11K. The nanowire detector width is typically on the order of 50nm-100nm, which can easily be achieved with electron beam lithography.

The AlN waveguides demonstrated so far show propagation loss below 0.6dB/cm and optical transparency above .22 Onm to 13. βμιτ.. The refractive index of Al is on the order of 2,1, thus enabling tight optical confinement and small bend radii on chip, which is a prerequisite for optical circuits with small footprint.

Comparison to existing spectrometers: . The new device is fully integrated and fully scalable and therefore a low-cost fabrication technology. The . device operates, at single photon levels with individual detectors, thus maintaining high detection rates at lowest signal levels. The device enables an operational bandwidth far above currently available devices. Spectral resolution can be freely chosen ^' by optical design. Using ring resonators, a demonstrated line width of 2.6pm therefore enables extremely high optical . resolution on chip. In addition, the high timing resolution is preserved, thus making the device inherently fast.

Traditional devices require either . many detectors or an optical wavelength scan. In the first case, only low-cost detectors are feasible,' which are not available for single photon applications. In the second case, the scanning approach severely limits the operation speed and leads to optical alignment and handling difficulties. Furthermore, optical resolution is typically not comparable to the performance of the devices described above.

Extensions: The scalability of the platform enables optical filter shaping on chip and the combination of many circuits to form complex systems. Therefore multi-pixel imaging is possible by fabricating many devices on the same chip, thus enabling full 2D-image acquisition at single photon levels on unlimited wavelength channels simultaneously.

The wavelength range can be further extended by combining optical materials with either UV transparency or further IR transparence with the approach described above. Fabrication routines will remain unaffected since the design itself is freely transferrable to other material platforms.

List of reference numbers

1 on-chip spectrometer

2 single photon detectors

3 polymer cover

4 V-groove

5 lensed optical fiber Silicon A1N

NbN

taper region