Characterization, Selection, and Microassembly of Nanowire Laser Systems.

Semiconductor nanowire (NW) lasers are a promising technology for the realization of coherent optical sources with ultrasmall footprint. To fully realize their potential in on-chip photonic systems, scalable methods are required for dealing with large populations of inhomogeneous devices that are typically randomly distributed on host substrates. In this work two complementary, high-throughput techniques are combined: the characterization of nanowire laser populations using automated optical microscopy, and a high-accuracy transfer-printing process with automatic device spatial registration and transfer. Here, a population of NW lasers is characterized, binned by threshold energy density, and subsequently printed in arrays onto a secondary substrate. Statistical analysis of the transferred and control devices shows that the transfer process does not incur measurable laser damage, and the threshold binning can be maintained. Analysis on the threshold and mode spectra of the device populations proves the potential for using NW lasers for integrated systems fabrication.

Transfer printing of AlGaAs-on-SOI microdisk resonators for selective mode coupling and low-power nonlinear processes.

The transfer printing of aluminum gallium arsenide (AlGaAs) microdisk resonators onto a silicon-on-insulator (SOI) waveguide platform is demonstrated. The integrated resonators exhibit loaded ${Q}$Q-factors reaching $ 4 \times {10^4} $4×104, and the vertical assembly approach allows selective coupling to different spatial mode families. The hybrid platform's nonlinearity is characterized by four-wave mixing with a measured nonlinear coefficient of $ \gamma = 325\;{({\rm Wm})^{ - 1}} $γ=325(Wm)-1, with the devices demonstrating minimal two-photon absorption and free-carrier absorption losses that are inherent to SOI at telecommunications wavelengths.

Vertically Emitting Indium Phosphide Nanowire Lasers.

Semiconductor nanowire (NW) lasers have attracted considerable research effort given their excellent promise for nanoscale photonic sources. However, NW lasers currently exhibit poor directionality and high threshold gain, issues critically limiting their prospects for on-chip light sources with extremely reduced footprint and efficient power consumption. Here, we propose a new design and experimentally demonstrate a vertically emitting indium phosphide (InP) NW laser structure showing high emission directionality and reduced energy requirements for operation. The structure of the laser combines an InP NW integrated in a cat's eye (CE) antenna. Thanks to the antenna guidance with broken asymmetry, strong focusing ability, and high Q-factor, the designed InP CE-NW lasers exhibit a higher degree of polarization, narrower emission angle, enhanced internal quantum efficiency, and reduced lasing threshold. Hence, this NW laser-antenna system provides a very promising approach toward the achievement of high-performance nanoscale lasers, with excellent prospects for use as highly localized light sources in present and future integrated nanophotonics systems for applications in advanced sensing, high-resolution imaging, and quantum communications.

High accuracy transfer printing of single-mode membrane silicon photonic devices.

A transfer printing (TP) method is presented for the micro-assembly of integrated photonic devices from suspended membrane components. Ultra thin membranes with thickness of 150nm are directly printed without the use of mechanical support and adhesion layers. By using a correlation alignment scheme vertical integration of single-mode silicon waveguides is achieved with an average placement accuracy of 100±70nm. Silicon (Si) µ-ring resonators are also fabricated and show controllable optical coupling by varying the lateral absolute position to an underlying Si bus waveguide.

Integration of Semiconductor Nanowire Lasers with Polymeric Waveguide Devices on a Mechanically Flexible Substrate.

Nanowire lasers are integrated with planar waveguide devices using a high positional accuracy microtransfer printing technique. Direct nanowire to waveguide coupling is demonstrated, with coupling losses as low as -17 dB, dominated by mode mismatch between the structures. Coupling is achieved using both end-fire coupling into a waveguide facet, and from nanowire lasers printed directly onto the top surface of the waveguide. In-waveguide peak powers up to 11.8 µW are demonstrated. Basic photonic integrated circuit functions such as power splitting and wavelength multiplexing are presented. Finally, devices are fabricated on a mechanically flexible substrate to demonstrate robust coupling between the on-chip laser source and waveguides under significant deformation of the system.

Waveguide-Integrated Colloidal Nanocrystal Supraparticle Lasers.

Supraparticle (SP) microlasers fabricated by the self-assembly of colloidal nanocrystals have great potential as coherent optical sources for integrated photonics. However, their deterministic placement for integration with other photonic elements remains an unsolved challenge. In this work, we demonstrate the manipulation and printing of individual SP microlasers, laying the foundation for their use in more complex photonic integrated circuits. We fabricate CdSxSe1-x/ZnS colloidal quantum dot (CQD) SPs with diameters from 4 to 20 µm and Q-factors of approximately 300 via an oil-in-water self-assembly process. Under a subnanosecond-pulse optical excitation at 532 nm, the laser threshold is reached at an average number of excitons per CQD of 2.6, with modes oscillating between 625 and 655 nm. Microtransfer printing is used to pick up individual CQD SPs from an initial substrate and move them to a different one without affecting their capability for lasing. As a proof of concept, a CQD SP is printed on the side of an SU-8 waveguide, and its modes are successfully coupled to the waveguide.

Colloidal quantum dot random laser.

We report random laser action in a system where optical amplification is provided by colloidal quantum dots (CQDs). This system is obtained by depositing from solution CdSe/ZnS core-shell CQDs into rough micron-scale grooves fabricated on the surface of a glass substrate. The combination of CQD random packing and of disordered structures in the glass groove enables gain and multiple scattering. Upon optical excitation, random laser action is triggered in the system above a 25-mJ/cm2 threshold. Single-shot spectra were recorded to study the emission spectral characteristics and the results show the stability of the laser mode positions and the dominance of the modes close to the material gain maximum.

Flexible blue-emitting encapsulated organic semiconductor DFB laser.

Mechanically flexible distributed feedback (DFB) lasers are fabricated by a low-cost approach using soft-lithography from a holographic master grating. The gain material is a star-shaped oligofluorene providing laser emission from 425 to 442 nm with a soft pump threshold at 14.4 µJ/cm (2.7 kW/cm). Encapsulation of the devices enables stable operation in ambient atmosphere at a 1/e degradation energy dosage of 53 J/cm.

Automated Nanoscale Absolute Accuracy Alignment System for Transfer Printing.

The heterogeneous integration of micro- and nanoscale devices with on-chip circuits and waveguide platforms is a key enabling technology, with wide-ranging applications in areas including telecommunications, quantum information processing, and sensing. Pick and place integration with absolute positional accuracy at the nanoscale has been previously demonstrated for single proof-of-principle devices. However, to enable scaling of this technology for realization of multielement systems or high throughput manufacturing, the integration process must be compatible with automation while retaining nanoscale accuracy. In this work, an automated transfer printing process is realized by using a simple optical microscope, computer vision, and high accuracy translational stage system. Automatic alignment using a cross-correlation image processing method demonstrates absolute positional accuracy of transfer with an average offset of <40 nm (3σ < 390 nm) for serial device integration of both thin film silicon membranes and single nanowire devices. Parallel transfer of devices across a 2 × 2 mm2 area is demonstrated with an average offset of <30 nm (3σ < 705 nm). Rotational accuracy better than 45 mrad is achieved for all device variants. Devices can be selected and placed with high accuracy on a target substrate, both from lithographically defined positions on their native substrate or from a randomly distributed population. These demonstrations pave the way for future scalable manufacturing of heterogeneously integrated chip systems.

Three-dimensional cross-nanowire networks recover full terahertz state.

Terahertz radiation encompasses a wide band of the electromagnetic spectrum, spanning from microwaves to infrared light, and is a particularly powerful tool for both fundamental scientific research and applications such as security screening, communications, quality control, and medical imaging. Considerable information can be conveyed by the full polarization state of terahertz light, yet to date, most time-domain terahertz detectors are sensitive to just one polarization component. Here we demonstrate a nanotechnology-based semiconductor detector using cross-nanowire networks that records the full polarization state of terahertz pulses. The monolithic device allows simultaneous measurements of the orthogonal components of the terahertz electric field vector without cross-talk. Furthermore, we demonstrate the capabilities of the detector for the study of metamaterials.

High-Throughput Electrical Characterization of Nanomaterials from Room to Cryogenic Temperatures.

We present multiplexer methodology and hardware for nanoelectronic device characterization. This high-throughput and scalable approach to testing large arrays of nanodevices operates from room temperature to milli-Kelvin temperatures and is universally compatible with different materials and integration techniques. We demonstrate the applicability of our approach on two archetypal nanomaterials-graphene and semiconductor nanowires-integrated with a GaAs-based multiplexer using wet or dry transfer methods. A graphene film grown by chemical vapor deposition is transferred and patterned into an array of individual devices, achieving 94% yield. Device performance is evaluated using data fitting methods to obtain electrical transport metrics, showing mobilities comparable to nonmultiplexed devices fabricated on oxide substrates using wet transfer techniques. Separate arrays of indium-arsenide nanowires and micromechanically exfoliated monolayer graphene flakes are transferred using pick-and-place techniques. For the nanowire array mean values for mobility µFE = 880/3180 cm2 V-1 s-1 (lower/upper bound), subthreshold swing 430 mV dec-1, and on/off ratio 3.1 decades are extracted, similar to nonmultiplexed devices. In another array, eight mechanically exfoliated graphene flakes are transferred using techniques compatible with fabrication of two-dimensional superlattices, with 75% yield. Our results are a proof-of-concept demonstration of a versatile platform for scalable fabrication and cryogenic characterization of nanomaterial device arrays, which is compatible with a broad range of nanomaterials, transfer techniques, and device integration strategies from the forefront of quantum technology research.

A wearable phototherapy device utilizing micro-LEDs.

A conformable device for wearable phototherapy applications is presented. The device consists of a 1 mm thick elastomeric membrane edge-lit by specially fabricated micro-sized LEDs. Nanoparticle based scattering films are utilized to extract light and a uniform emission of 15 µW/cm2 is reported over an area of 2 cm2.

Transfer Printing of Semiconductor Nanowires with Lasing Emission for Controllable Nanophotonic Device Fabrication.

Accurate positioning and organization of indium phosphide (InP) nanowires (NWs) with lasing emission at room temperature is achieved using a nanoscale transfer printing (TP) technique. The NWs retained their lasing emission after their transfer to targeted locations on different receiving substrates (e.g., polymers, silica, and metal surfaces). The NWs were also organized into complex spatial patterns, including 1D and 2D arrays, with a controlled number of elements and dimensions. The developed TP technique enables the fabrication of bespoke nanophotonic systems using NW lasers and other NW devices as building blocks.

Hybrid organic semiconductor lasers for bio-molecular sensing.

Bio-functionalised luminescent organic semiconductors are attractive for biophotonics because they can act as efficient laser materials while simultaneously interacting with molecules. In this paper, we present and discuss a laser biosensor platform that utilises a gain layer made of such an organic semiconductor material. The simple structure of the sensor and its operation principle are described. Nanolayer detection is shown experimentally and analysed theoretically in order to assess the potential and the limits of the biosensor. The advantage conferred by the organic semiconductor is explained, and comparisons to laser sensors using alternative dye-doped materials are made. Specific biomolecular sensing is demonstrated, and routes to functionalisation with nucleic acid probes, and future developments opened up by this achievement, are highlighted. Finally, attractive formats for sensing applications are mentioned, as well as colloidal quantum dots, which in the future could be used in conjunction with organic semiconductors.

An oligofluorene truxene based distributed feedback laser for biosensing applications.

The first example of an all-organic oligofluorene truxene based distributed feedback laser for the detection of a specific protein-small molecule interaction is reported. The protein avidin was detected down to 1 µg mL(-1) using our biotin-labelled biosensor platform. This interaction was both selective and reversible when biotin was replaced with desthiobiotin. Avidin detection was not perturbed by Bovine Serum Albumin up to 50,000 µg mL(-1). Our biosensor offers a new detection platform that is both highly sensitive, modular and potentially re-usable.

Tunable single-mode fiber-VCSEL using an intracavity polymer microlens.

We report a tunable, single-mode vertical cavity surface-emitting laser (VCSEL) format suitable for array operation, power scaling, fiber coupling, and operation in isolated environments such as those required by atom optics. The devices are fiber VCSELs, consisting of a semiconductor gain and mirror structure separated from a mirror-coated optical fiber by an air (or vacuum) gap. The gain structure has polymer microlenses fabricated on its surface, of characteristics suitable to focus the oscillating mode on both cavity mirrors, ensuring stable fundamental mode emission and high fiber coupling efficiency. We demonstrate such devices in continuous-wave operation at 1.03 microm at room temperature, with a single-mode tuning range of 13 nm, laser threshold as low as 2.5 mW, and a maximum fiber-coupled output power of 10 mW.