Polarization-controlled Photon Emission from Site-controlled InGaN Quantum Dots

Left to Right: (top row) Chih-Wei Hsu, Anders Lundskog, K. Fredrik Karlsson, Supaluck Amloy. (bottom row) Daniel Nilsson, Urban Forsberg, Per Olof Holtz, Erik Janzén.

Authors: Chih-Wei Hsu¹, Anders Lundskog¹, K. Fredrik Karlsson¹, Supaluck Amloy^1,2, Daniel Nilsson¹, Urban Forsberg¹, Per Olof Holtz¹, Erik Janzén¹

Affiliation:
¹Department of Physics Chemistry and Biology (IFM), Linköping University, Sweden.
²Department of Physics, Faculty of Science, Thaksin University, Phattalung, Thailand.

A common requirement to realize several optoelectronic applications, e.g. liquid-crystal displays, three-dimensional visualization, (bio)-dermatology [1] and optical quantum computers [2], is the need of linearly-polarized light for their operation. For existing applications today, the generation of linearly-polarized light is obtained by passing unpolarized light through a combination of polarization selective filters and waveguides, with an inevitable efficiency loss as the result. These losses could be drastically reduced by employment of sources, which directly generate photons with desired polarization directions.

Quantum dots (QDs) have validated their important role in current optoelectronic devices and they are also seen as promising as light sources for generation of “single-photons-on-demand”. Conventional QDs grown via the Stranski-Krastanov (SK) growth mode are typically randomly distributed over planar substrates and possess different degrees of anisotropies. The anisotropy in the strain field and/or the geometrical shape of each individual QD determines the polarization performance of the QD emission. Accordingly, a cumbersome post-selection of QDs with desired polarization properties among the randomly-distributed QDs is required for device integration [3]. Consequently, an approach to obtain QDs with controlled site and polarization direction is highly desired.

Figure 1. Magnified SEM images of GaN EHPs with various α. The values of α are defined as the angles between the long axis of EHPs and the underlying GaN template.

Here, we demonstrate an approach to directly generate a linearly-polarized QD emission by introducing site-controlled InGaN QDs on top of GaN-based elongated hexagonal pyramids (GaN EHPs). The polarization directions of the QD emission are demonstrated to be aligned with the orientations of the EHPs (Figure 1). The reliability and consistency for this architecture are tested by a statistical analysis of InGaN QDs grown on GaN EHP arrays with different in-plane orientations of the elongations. Details of the process and optical characterizations can be found in our resent publication [4].

Figure 2. a) µPL spectra of EHPs with the polarization analyzer set to θ_max (θ_min), by which the maximum (minimum) intensity of sharp emission peaks are detected. b) Distribution histograms of measured polarization directions from the GaN EHPs for various α.

Figure 2a shows representative polarization-dependent micro-photoluminescence (µPL) spectra from a EHP measured at 4^o K. A broad emission band peaking at 386 nm and several emission peaks in the range between 410 and 420 nm are observed. These sharp emission lines are originating from the multiple QDs formed on top of the GaN EHP. Despite the formation of multiple QDs on a GaN EHP, the emission peaks from all QDs tend to be linearly-polarized in the same direction as revealed in Figure 3a and all peaks have their maximum and minimum intensities in the same direction, θ. The correlation between the outcome of the polarization-resolved measurements and the orientations of GaN EHPs (as defined by α) reveals that the polarization direction is parallel to the elongation (α≅φ in Figure 2b). A polarization guiding (α≅φ) is unambiguously revealed for GaN EHPs with α = 0^o, 60^o and 120^o. For the remaining group of GaN EHPs with α = 30^o, 90^o and 150^o, preferential polarization directions are seemly revealed, but α≅φ is less strictly obeyed. The polarization guiding effect and the high degree of polarization are further elucidated in the following.

Figure 3. a) Statistical histogram showing the overall measured degree of polarization from GaN EHPs. b) The computed degree of polarization plotted as a function of the split-off energy. The QD shape is assumed to be lens-shaped with an in-plane asymmetry of b/a= 0.8. The single particle electron (hole) eigenstates are obtained from an effective mass Schrödinger equation (with a 6 band k•p Hamiltonian), discretized by finite differences. The Hamiltonians include strain and internal electric fields originating from spontaneous and piezoelectric polarizations. The polarized optical transitions are computed by the dipole matrix elements.

The polarization direction of the ground-state-related emission from the QDs reflects the axis of the in-plane anisotropy of the confining potential, concerning both strain and/or QD shape [5]. The same polarization direction monitored for the different QDs indicates that all grown QDs possess unidirectional in-plane anisotropy. The polarization control observed in our work can be explained in three ways: (1) the GaN EHPs transfer an anisotropic biaxial strain field to the QDs resulting in the formation of elongated QDs. The direction of the strain field in the EHPs should be strongly correlated with α. (2) Given that the top parts of the GaN EHPs are fully strain relaxed, as concluded for the GaN SHPs [6], the asymmetry induced by a ridge will result in an anisotropic relaxation of the in-plane strain of the QDs on the ridge. The degree of relaxation is higher along the smallest dimension of the top area, i.e. along the direction perpendicular to the ridge elongation, resulting in a ground state emission of the QD being polarized in parallel with the ridge. (3) The edges of the ridges form a Schwoebel–Ehrlich barrier, which prevents adatoms of diffusing out from the (0001) facet [7,8]. Since the adatoms have larger probability to interact with an edge barrier parallel rather than orthogonal to the ridge elongation, the adatoms will preferentially diffuse parallel to the ridge. As the strain and the shape of the QDs are not independent factors and accurate structural information of the QDs is currently unavailable, the predominant factors determining the polarization is to be verified.

The polarization degree of the III-Ns is more sensitive to the in-plane asymmetry compared to other semiconductor counterparts due to the significant band mixing and the identical on-axis effective masses of the A and B bands in the III-N [5]. A statistical investigation of the value of P performed on 145 GaN EHPs reveals that 93% of the investigated GaN EHPs possess P > 0.7 with an average value of P = 0.84 (Figure 3a). The polarization of the emissions is related to the QD asymmetry determined by the anisotropy of the internal strain and electric fields, as well as by the structural shape of the QD itself [5]. Numerical computations predict a high degree of polarization for small or moderate in-plane shape anisotropies of GaN and InGaN QDs [9]. This is related to the intrinsic valence band structure of the III-Ns. In particular, the split-off energy has been identified as the key material parameter determining the degree of polarization for a given asymmetry. Figure 3b shows the computed degree of polarization plotted against a variation of the split-off energy. Given a fixed asymmetry of the QDs, it is concluded that the material with the smallest split-off energy exhibits the highest degree of polarization. The high degree of polarization observed for InGaN QDs can be rationalized by the small split-off energies of InN and GaN, resulting in an extreme sensitivity to the asymmetry. Such a characteristic implies its inherent advantage for the generation of photons possessing a specific polarization.

In summary, we have demonstrated an effective method to achieve site-controlled QDs emitting linearly-polarized emission with controlled polarization directions by growing InGaN QDs on top of elongated GaN pyramids in a MOCVD (metal organic chemical vapor deposition) system. The polarization directions of the QD emission can be guided by the orientations of the underlying elongated GaN pyramids. Such an effect can be realized as the elongated GaN pyramids provide additional in-plane confinement for the InGaN QDs implanting unidirectional in-plane anisotropy into the QDs, which subsequently emit photons linearly-polarized along the elongated direction of the GaN EHPs.

References:
[1] Zeng Nan, Jiang Xiaoyu, Gao Qiang, He Yonghong, Ma Hui, "Linear polarization difference imaging and its potential applications". Applied Optics, 48, 6734-6739 (2009). Abstract.
[2] E. Knill, R. Laflamme, G.J. Milburn, "A scheme for efficient quantum computation with linear optics". Nature, 409, 46-52 (2001). Abstract.
[3] Robert J. Young, D.J.P. Ellis, R.M. Stevenson, Anthony J. Bennett, "Quantum-dot sources for single photons and entangled photon pairs". Proceedings of the IEEE, 95, 1805–1814 (2007). Abstract.
[4] Anders Lundskog, Chih-Wei Hsu, K Fredrik Karlsson, Supaluck Amloy, Daniel Nilsson, Urban Forsberg, Per Olof Holtz, Erik Janzén, "Direct generation of linearly-polarized photon emission with designated orientations from site-controlled InGaN quantum dots". Light: Science & Applications 3, e139 (2014). Full Article.
[5] R. Bardoux, T. Guillet, B. Gil, P. Lefebvre, T. Bretagnon, T. Taliercio, S. Rousset, F. Semond, "Polarized emission from GaN/AlN quantum dots: single-dot spectroscopy and symmetry-based theory". Physical Review B, 77, 235315 (2008). Abstract.
[6] Q.K.K. Liu, A. Hoffmann, H. Siegle, A. Kaschner, C. Thomsen, J. Christen, F. Bertram, "Stress analysis of selective epitaxial growth of GaN". Applied Physics Letters, 74, 3122-3124 (1999). Abstract.
[7] O. Pierre-Louis, M.R. D’Orsogna, T.L. Einstein, "Edge diffusion during growth: The kink Schwoebel-Erhlich effect and resulting instabilities". Physical Review Letters, 82, 3661-3664 (1999). Abstract.
[8] S.J. Liu, E.G. Wang, C.H. Woo, Hanchen Huang, "Three-dimensional Schwoebel–Ehrlich barrier". Journal of Computer-Aided Materials Design, 7, 195–201 (2001). Abstract.
[9] S. Amloy, K.F. Karlsson, T.G. Andersson, P.O. Holtz, "On the polarized emission from exciton complexes in GaN quantum dots". Applied Physics Letters, 100, 021901 (2012). Abstract.

Invisibility Cloak Goes Three-Dimensional for Heat

Authors: Hongyi Xu¹, Xihang Shi¹, Fei Gao¹, Handong Sun^1,2, Baile Zhang^1,2

Affiliation:
¹Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore.
²Centre for Disruptive Photonic Technologies, Nanyang Technological University, Singapore.

While the topic of invisibility has long been investigated in optics-related topics, it is for the first time that invisibility cloaking is realized for heat in a three-dimensional (3D) thermal space, according to a recent research result[1] published by our group at Nanyang Technological University, Singapore.

Thermal invisibility was initially inspired by the concept of Transformation Optics [2, 3], a method that can control light propagation with a coordinate transformation in a 3D optical space, which in general requires optical metamaterials with exotic constitutive parameters (e.g. extremely large or extremely small anisotropic permittivity and permeability). Despite the inspiring elegance of 3D optical invisibility cloaking theory, its experimental realizations have been mainly limited to two dimensions (2D), because of the widely acknowledged tremendous difficulties in constructing optical metamaterials with stringent parameters in 3D.

Similarly, the recent development of thermal invisibility cloaking based on transformation thermodynamics were firstly demonstrated in 2D [4, 5]. The method, similar to transformation optics, requires thermal metamaterials with anisotropy and inhomogeneity, being difficult in 3D.

Researchers in Nanyang Technological University successfully bypassed the problem by taking advantage of the difference between heat (a diffusion phenomenon) and light (a wave phenomenon), and experimentally demonstrated the world’s first ultra-thin 3D thermal cloak that shields an air bubble in a stainless steel from external conductive heat flux [1]. The technology can protect a 3D object from heat flux without distorting the external temperature distribution by simply using an ultra-thin layer of thermal metamaterial made of copper with carefully designed thickness.

The implementation process of thermal cloak is illustrated in Fig.1. A hemi-spherical hole with radius of 0.51 cm was drilled by electrical discharge machining in a half stainless steel block with dimension of 2×2×1 cm. A thin disk of copper was punched into the hemi-spherical hole by a molding rod (Fig 1a), to form a copper shell (Fig. 1b) with homogeneous thickness of 100 μm. Two identical half blocks were further combined together to form a complete 3D thermal cloak (Fig. 1c), with dimension of 0.5/0.51 cm for the inner/outer radius of the copper spherical layer, and 2×2×2 cm for the complete stainless steel block.

Figure 1. Illustration of the Fabrication of a 3D thermal cloak. a, Molding process of half of the 3D thermal cloak: (a) Thin copper disk is punched into the hemispherical hole in the stainless steel block. (b) Illustration and snapshot of half of the thermal cloak after molding. (c) Illustration and snapshot of the full cloak by combining two half blocks. The red/blue plate represents high/low temperature at the bottom/top surface [1].

In the experimental characterization, a hot plate (red color, Fig. 1c) and an ice tank (blue color, Fig. 1c) were closely attached to the bottom and top surface of the thermal cloak. When heat diffused from bottom to top, the temperature at the cross-section surface was captured by a thermal camera. The dynamic process of heat transfer from the beginning to the moment near thermal equilibrium was recorded in a movie clip:
 

The temperature distributions at the beginning time and at the moment near thermal equilibrium are shown in Fig. 2. In Fig. 2a and 2d (cases of background), the temperature distribution is homogeneous across the entire surface, indicating that heat diffuses through the stainless steel smoothly. In Fig. 2b and 2e (cases without thermal cloak), the distribution of temperature is distorted (being ‘bent’ towards the air bubble) and a relatively cool region is left behind the air bubble, indicating that part of heat flux has been blocked by the air bubble. In Fig. 2c and Fig. 2f (cases with thermal cloak), the temperature distribution outside the air bubble is restored to norm, as if the air bubble did not exist, indicating the cloaking effect for heat flux.

Figure 2. Characterization of conductive thermal cloaking for transient homogeneous thermal flux. (a-c) Temperature distributions for the moment of 0.5 min at the beginning of heat transfer. (d-f) Temperature distributions for the moment of 4.5 min close to thermal equilibrium. (a&d) Temperature distributions in the pure background without any air bubble or cloak. (b&e) Temperature distributions when an air bubble without the cloak is present. (c&f) Temperature distributions when the air bubble is cloaked by the ultra-thin cloak. In b-c and e-f, the dotted circles indicate the position of the air bubble, while the dotted circles in a and d are merely for comparison [1].

This thermal invisibility cloak is the first demonstration in 3D that heat flux can be effectively controlled by thermal metamaterials. Application wise, effective control of heat is an important subject in modern semiconductor industries, where the exponential increase of package density is generating more and more heat in a unit space. The heat generated jeopardized the performance and lifetime of semiconductor devices, accounting for over 50 percent of electronic failures [6]. With effective heat control technologies based on thermal metamaterials, it is possible to develop efficient heat dissipation solutions to thermal problems in semiconductor industries.

References: 
[1] Hongyi Xu, Xihang Shi, Fei Gao, Handong Sun, Baile Zhang, "Ultrathin Three-Dimensional Thermal Cloak", Physical Review Letters, 112, 054301 (2014). Abstract.
[2] Ulf Leonhardt, "Optical Conformal Mapping", Science, 312, 1777-1780 (2006). Abstract.
[3] J. B. Pendry, D. Schurig, D. R. Smith, "Controlling Electromagnetic Fields", Science, 312, 1780-1782 (2006). Abstract.
[4] Supradeep Narayana, Yuki Sato, "Heat Flux Manipulation with Engineered Thermal Materials", Physical Review Letters, 108, 214303 (2012). Abstract.
[5] Robert Schittny, Muamer Kadic, Sebastien Guenneau, Martin Wegener, "Experiments on Transformation Thermodynamics: Molding the Flow of Heat", Physical Review Letters, 110, 195901 (2013). Abstract.
[6] Shanmuga Sundaram Anandan, and Velraj Ramalingam, "Thermal Management of Electronics: A Review of Literature," Thermal Science, 12, 5-26 (2008). Full Article.

Trasparent Electronics Wrapped Around Hairs and Transferred on Plastic Contact Lens

(Left to Right) Giovanni A. Salvatore, Niko Münzenrieder, Gerhard Tröster

Authors: Giovanni A. Salvatore, Niko Münzenrieder, Gerhard Tröster

Affiliation: Electronics Laboratory, Swiss Federal Institute of Technology, Zurich, Switzerland.

While silicon is bulky and rigid, plastic electronics is soft, deformable and lightweight. Flexible electronic devices like rollable displays, conformable sensors, plastic solar cells and flexible batteries could enable applications that would be impossible to achieve by using the hard electronics of today. So far, the effective commercialization of such technology has been mainly prevented by cost and performance constraints. However, for some applications, the specific functionalities provided by flexible, biocompatible, conformable and light plastic electronics are much more important than the aforementioned obstacles and future scenarios can be realistically foreseen. The development of flexible electronic circuits plays a crucial role in all such applications. Flexibility can be achieved by direct fabrication on plastic foil [1], by peeling off a polymer layer spin coated on a rigid substrate [2], by transfer printing [3] or by spalling the thin top layer from a crystalline silicon wafer after device fabrication [4].

The scenario is even more heterogeneous when looking at the variety of materials employed for the active layers. Inorganic materials including silicon nanomembranes [5] or nanowires [2], organic materials [6] or amorphous oxides [7] are all possible options. Recently electronics on very thin substrates has shown remarkable bendability, conformability and lightness, which are important attributes for biological tissues sensing, wearable or implantable devices [5,8,9]. All the mentioned approaches and materials offer advantages and suffer from limitations. A unique and universal integration scheme has not been developed yet and at the moment the choice mainly depends on specific applications.

In our recent work [10] we propose a wafer-scale process scheme to realize ultra-flexible, lightweight and transparent electronics on top of a 1μm thick parylene film which is released from the carrier substrate after the dissolution in water of a Poly-Vinyl-Alcohol (PVA) layer. The thin substrate ensures extreme flexibility, which is demonstrated by transistors which continue to work when wrapped around human hairs. In parallel, the use of amorphous Indium Gallium Zinc Oxide (a-IGZO) as semiconductor and high-k dielectric enables the realization of analog amplifiers operating at 12V and above 1MHz. The possibility to implement such scheme at wafer scale and the use of a-IGZO could represent a good compromise between large area integration and performance and, hence, a valid alternative to similar reported approaches [5, 9]. After the release, electronics can be transferred onto any object, surface and on biological tissues like human skin and plant leaves.

We fabricate Thin Film Transistors (TFTs) and circuits on top of a 1μm thick parylene film. Parylene has been chosen because it is biocompatible and resistant to acetone and chemicals necessary for lift-off and etching process steps. In our experiments, PVA constitutes the first soluble layer needed to release, in water, the silicon wafer used as support during the fabrication (Fig.1b-e). Depending on the application, an additional Poly-Vinyl-Acetate (PVAc) film is added to improve the adhesion and enable the removal of electronics after use.

The devices are formed by Indium Gallium Zinc Oxide which is used as semiconductors and ensure electron mobility greater than 10cm²/Vs, transparency in the visible spectrum and large area deposition. The insulator is Al2O3 which has excellent dielectric properties. The contacts are made up by thin layer of chromium and gold. We also fabricated fully transparent devices in which metal contacts are replaced by Indium Tin Zinc Oxide (Fig.1). The maximum process temperature reached during the fabrication is 150°C. The final thickness of the TFT structure is 145nm for the non-transparent TFTs and 175nm for the transparent ones. In order to test and prove the reliability of our approach, we also designed and fabricated analog amplifiers based on the previously described TFTs.

Figure 1: Structure of thin film transistors fabricated on a silicon carrier chip covered with two sacrificial layers and a non-soluble layer which is parylene in our experiments. Poly-Vinyl-Alcohol (PVA) is water soluble and enables the release of the thin parylene membrane from the silicon substrate. Poly-Vinyl-Acetate (PVAc) film is added to improve the adhesion and enable the removal of electronics after use. Transistors and circuits are fabricated by using a-IGZO as semiconductor and Al₂O₃ as dielectric. Gold is used for the contacts in non-transparent devices (left in the picture) while ITO is used in case of the transparent version of the device (right).

It is worth mentioning that the use of rigid supports (glass or silicon) during the fabrication, mitigate some issues, like substrate thermal expansion or water absorption, which are encountered in case of direct fabrication on foils and which limit the feature resolution and alignment, therefore, yield and performance.

After the fabrication, the chip is put in water to selectively dissolve the PVA layer and release the parylene membrane. For a 2-inch wafer this operation takes approximately 30 minutes after which the circuits are floating on water. The membrane can then be fished and transferred onto the final destination substrate which can be any arbitrary rigid or flexible support or any organic or inorganic surface (Fig. 2).

Figure 2: The proposed process scheme can be implemented at wafer scale. Here, we demonstrate the feasibility of such approach in the case of a 2-inch wafer. The water starts dissolving the PVA layer from the borders of the wafer and slowly proceeds towards the center. The whole releasing procedure takes approximately 30 minutes after which the wafer sinks while the membrane floats on water. After the release of the hosting support, the membrane is fished and transferred onto flexible and elastic foils, textiles, biological tissues and implantable devices.

The thin substrate ensures extreme flexibility and conformability. In order to experimentally investigate the ultimate limit of the bending stability, we transferred the membrane on top of a glass substrate where some fragments of human hairs, which have a radius of about 50μm, had been previously placed. TFTs having the gate region bent around the hairs are fully operational. The mechanical properties of the thin substrate are further investigated by transferring the devices onto a 100μm thick polypropylene foil. This facilitates the handling and manipulation of the membrane while minimizing the strain induced by the substrate, thanks to the poor adhesion between polypropylene and parylene. The membrane on the polypropylene foil is folded to a radius of 750μm (Fig.3c), then repeatedly crumpled in the hands (Fig.3d) and finally re-flattened (Fig.3e). After the curling, we were able to measure only the non-transparent transistors, while none of the transparent devices was functioning. We, in fact, observed cracks in the ITO layer -- most probably due to induced strain after the release.

Figure 3: (a) Scanning electron microscopy picture of transistors wrapped around three fragments of human hairs placed on a glass support. The thin membrane ensures high flexibility and conformability and it wraps around the hairs which have a radius of approximately of 50μm (tensile strain, ε=0.4%). (b) Optical microscope picture of the area highlighted by the white box in a. (c, d, e) The membrane is transferred on a 100μm thick polypropylene foil which is folded in hand (bending radius is about 750μm) aggressively crumpled in hands and then re-flattened. The devices are still working either when bent around the hair and when curled by hands.

The membrane is also light, and biocompatible. To design a concrete application which takes advantages of such capabilities, we transferred the parylene membrane, with on top transparent transistors and strain gauge sensors, on a commercially available plastic contact lens (Fig.4). After transferring the lens onto an artificial eye, TFTs continue to function and could be used for signal amplification in future developments. In the design of a real system particular attention must be paid to the packaging which has to mitigate or eliminate the effects of the humor aqueous of the eye.

Figure 4: The membrane with on top transparent TFTs and gold strain gauge sensors is transferred on a plastic contact lens held between two fingers. Such technology could find application as smart contact lenses able to monitor and diagnose glaucoma disease.

Thereby, the developed technology could find application as smart contact lenses able to monitor and diagnose glaucoma disease and it could offer significant advantages over existing solutions [11] in terms of thickness, lightness and transparency and, hence, comfort for the patient. Future works should be focused on the development of wireless communication schemes and on the powering of the system.

References:
[1] N. Münzenrieder, L. Petti, C. Zysset, G.A. Salvatore, T. Kinkeldei, C. Perumal, C. Carta, F. Ellinger, G. Troster, “Flexible a-IGZO TFT amplifier fabricated on a free standing polyimide foil operating at 1.2 MHz while bent to a radius of 5 mm”, Proceedings of International Device meeting IEDM (2012). Abstract.
[2] Kuniharu Takei, Toshitake Takahashi, Johnny C. Ho, Hyunhyub Ko, Andrew G. Gillies, Paul W. Leu, Ronald S. Fearing, Ali Javey, “Nanowire active-matrix circuitry for low-voltage macroscale artificial skin”, Nature Materials, 9, 821-826 (2010). Abstract.
[3] Matthew A. Meitl, Zheng-Tao Zhu, Vipan Kumar, Keon Jae Lee, Xue Feng, Yonggang Y. Huang, Ilesanmi Adesida, Ralph G. Nuzzo, John A. Rogers, “Transfer printing by kinetic control of adhesion to an elastomeric stamp", Nature Materials, 5, 33 - 38 (2006). Abstract.
[4] Davood Shahrjerdi, Stephen W. Bedell, “Extremely Flexible Nanoscale Ultrathin Body Silicon Integrated Circuits on Plastic”, Nano Letters, 13, 315-320 (2012). Abstract.
[5] Dae-Hyeong Kim, Jong-Hyun Ahn, Won Mook Choi, Hoon-Sik Kim, Tae-Ho Kim, Jizhou Song, Yonggang Y. Huang, Zhuangjian Liu, Chun Lu, John A. Rogers, "Stretchable and Foldable Silicon Integrated Circuits". Science, 320, 507-511 (2008). Abstract.
[6] Tsuyoshi Sekitani, Ute Zschieschang, Hagen Klauk, Takao Someya, "Flexible organic transistors and circuits with extreme bending stability". Nature Materials, 9, 1015-1022 (2010). Abstract.
[7] Kenji Nomura, Hiromichi Ohta, Akihiro Takagi, Toshio Kamiya, Masahiro Hirano, Hideo Hosono, “Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors”, Nature, 432, 488-492 (2004). Abstract.
[8] Martin Kaltenbrunner, Matthew S. White, Eric D. Głowacki, Tsuyoshi Sekitani, Takao Someya, Niyazi Serdar Sariciftci, Siegfried Bauer, "Ultrathin and lightweight organic solar cells with high flexibility". Nature Communications, 3:770, (2012). Abstract.
[9] Martin Kaltenbrunner, Tsuyoshi Sekitani, Jonathan Reeder, Tomoyuki Yokota, Kazunori Kuribara, Takeyoshi Tokuhara, Michael Drack, Reinhard Schwödiauer, Ingrid Graz, Simona Bauer-Gogonea, Siegfried Bauer, Takao Someya, "An ultra-lightweight design for imperceptible plastic electronics". Nature, 499, 458-463, (2013). Abstract.
[10] Giovanni A. Salvatore, Niko Münzenrieder, Thomas Kinkeldei, Luisa Petti, Christoph Zysset, Ivo Strebel, Lars Büthe, Gerhard Tröster, “Wafer-scale design of lightweight and transparent electronics that wraps around hairs”, Nature communications, 5:2982 (2014). Abstract.
[11] www.sensimed.ch

Quantum Up-Conversion of Squeezed Vacuum States

From Left to Right: Christina E. Vollmer, Christoph Baune, and Aiko Samblowski

Authors: Christina E. Vollmer, Christoph Baune, Aiko Samblowski, Tobias Eberle, Vitus Händchen, Jaromír Fiurášek, Roman Schnabel

Affiliation: Institut für Gravitationsphysik der Leibniz Universität Hannover, Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut), Hannover, Germany

Squeezed vacuum states of light belong to the special class of ‘nonclassical states’ that can not be fully described by either a classical or a semi-classical model. Overlapped with a (bright) coherent laser beam, they are able to reduce (to ‘squeeze’) the photon counting statistics, i.e. the light’s shot noise, thereby enhancing the sensitivity of optical measurement devices. There are applications in spectroscopy [1] and imaging [2] and in particular in interferometric length measurements in gravitational wave detectors [3-6], once the kick-off for research in squeezed states. With squeezed vacuum states it is also possible to teleport quantum states [7], to generate so-called Schrödinger kitten states [8], and to improve quantum cryptography [9].

Past 2Physics articles by this group:
September 25, 2011: "A Gravitational Wave Observatory Operating Beyond the Quantum Shot-Noise Limit" by Hartmut Grote, Roman Schnabel, Henning Vahlbruch.
April 03, 2008: "Squeezed Light – the first real application starts now" by Roman Schnabel and Henning Vahlbruch

Fig.1: Schematic of the experiment. A squeezed vacuum state at 1550 nm is overlapped with a bright pump field at 810 nm inside a periodically poled KTP crystal inside an optical resonator for sum-frequency generation. The output is a squeezed vacuum state at 532 nm.

In our recent work [10] we demonstrated for the first time the frequency up-conversion of squeezed vacuum states of light in an external setup, i.e. ‘on the fly’. Our scheme can be applied to quantum networks that first use a squeezing wavelength of 1550 nm for transmission through optical fibres and then use a shorter wavelength to meet the requirements of a quantum memory for storing the squeezed state. In our experiment we converted a 4dB squeezed state at 1550nm to a 1.5dB squeezed state at 532nm. The degradation was due to optical loss and in full agreement with our model.

With our experiment we also demonstrated a scheme that provides access to short squeezing wavelengths. Today, squeezed states are most efficiently produced at near-infrared wavelengths. Due to the lack of appropriate nonlinear media it is difficult to produce them with conventional techniques at visible or even ultra-violet wavelengths. In future work we plan to reduce the optical loss of our setup to be able to demonstrate strong squeezing at visible wavelengths.

Fig. 2: Photograph of parts of the experiment. In total, the experiment required five frequency conversion steps. First, a 1064 nm continuous-wave laser beam was frequency doubled. The produced 532 nm beam was used to generate two beams at 1550 nm and 810 nm via optical parametric oscillation. The 1550 nm light was frequency doubled and the generated 775 nm light used to pump a parametric down converter to produce squeezed vacuum states at 1550nm. The final step was the up-conversion as shown in Fig. 1.

References:
[1] E. Polzik, J. Carri, H. Kimble, “Spectroscopy with squeezed light”. Physical Review Letters, 68, 3020 (1992). Abstract.
[2] G. Brida, M. Genovese, I. Ruo Berchera, “Experimental realization of sub-shot-noise quantum imaging”. Nature Photonics 4, 227 (2010). Abstract.
[3] Carlton M. Caves, “Quantum-mechanical noise in an interferometer”. Physical Review D,  23, 1693 (1981). Abstract.
[4] Roman Schnabel, Nergis Mavalvala, David E. McClelland, Ping K. Lam, “Quantum metrology for gravitational wave astronomy”. Nature Communications, 1:121 (2010). Abstract.
[5] The LIGO Scientific Collaboration, “A gravitational wave observatory operating beyond the quantum shot-noise limit”. Nature Physics, 7, 962 (2011). Abstract. 2Physics Article.
[6] The LIGO Scientific Collaboration, “Enhanced sensitivity of the LIGO gravitational wave detector by using squeezed states of light”. Nature Photonics, 7, 613 (2013). Abstract.
[7] A. Furusawa, J. L. Sørensen, S. L. Braunstein, C. A. Fuchs, H. J. Kimble, E. S. Polzik, “Unconditional Quantum Teleportation,” Science 282, 706 (1998). Abstract.
[8] Alexei Ourjoumtsev, Rosa Tualle-Brouri, Julien Laurat, Philippe Grangier, “Generating Optical Schrödinger Kittens for Quantum Information Processing,” Science 312, 83 (2006). Abstract.
[9] Christian Weedbrook, Stefano Pirandola, Raúl García-Patrón, Nicolas J. Cerf, Timothy C. Ralph, Jeffrey H. Shapiro, Seth Lloyd, “Gaussian quantum information,” Review of Modern Physics, 84, 621 (2012). Abstract.
[10] C. E. Vollmer, C. Baune, A. Samblowski, T. Eberle, V. Händchen, J. Fiurášek, and R. Schnabel, “Quantum Up-Conversion of Squeezed Vacuum States from 1550 to 532 nm”, Physical Review Letters, 112, 073602 (2014). Abstract.

300K Single Photons from Site Controlled GaN Nanowire Quantum Dots

From left to right: S. Kako, Y. Arakawa, M. Holmes, K. Choi and M. Arita.

Authors: M. Holmes, K. Choi, S. Kako, M. Arita, and Y. Arakawa

Affiliation: Institute for Nano Quantum Information Electronics, Institute of Industrial Science, University of Tokyo, Japan

Sources of single photons are vital for the future realization of several technologies including quantum key distribution, linear optical quantum computing, quantum teleportation, and also for quantum metrology. An ideal single photon source will provide one and only one photon on demand, and ideally would operate at room temperature (to avoid the necessity of large and cumbersome cooling systems). From a device engineering point of view, the technology to control the location of each single photon emitter would also be of great importance, as it would enable the fabrication of arrays of emitters.

To date, examples of solid state single photon sources in the literature that are able to operate at 300K rely on emitters which have not had their physical location determined, (in short: they form at random locations). Examples include defects in diamond [1] and SiC [2], molecules [3] and quantum dots [4,5]. Whilst effort is being made to control the location of single photon emitters, particularly with quantum dots, all examples of site controlled emitters in the literature must be cryogenically cooled before they can be operated.

In our recent publication [6], we describe a device that is both site controlled, and also operates at 300K. Furthermore, it is created from III-Nitride semiconductors, so that it can in principle be integrated with other semiconductor systems for future device applications. Our device is based on a GaN quantum dot embedded in a site controlled nanowire. These devices are grown by selective area metal organic chemical vapour deposition, meaning that the emitters can be fabricated into arrays of almost any pitch. For our optical experiments we utilized a low density of nanowires so that we could easily excite, and isolate the emission from, a single device.

Figure 1: A schematic of a single nanowire quantum dot emitting a single photon, and also the autocorrelation function of the collected emission. The low intensity peak (at time delay 0) is proof of the single photon emission from the site controlled quantum dot.

Because of the strong quantum confinement in this type of quantum dot, it should theoretically be possible to keep excitons inside them for long enough that they can recombine and emit single photons even at room temperature. We found that this is indeed the case by directing the light emitted from a quantum dot into a detection system consisting of a 50/50 beam splitting mirror and two photon detectors, finding a large suppression in the number of simultaneous detection events in the two detectors (a single photon could only ever be detected in a single detector).

An intriguing property of this device is that the quality of the single photons (the degree to which we can say we have a single photon source) seems to be independent of the operation temperature. Many previous studies on single photon sources showed a degradation of the single photon statistics with elevated temperatures, which we have avoided in this case by using low density, site controlled, small GaN quantum dots providing a high spectral purity, even at high temperatures.

We believe that this is a big step forward towards the eventual realization of future quantum technologies that may work at room temperature.

References:
[1] N. Mizuochi, T. Makino, H. Kato, D. Takeuchi, M. Ogura, H. Okushi, M. Nothaft, P. Neumann, A. Gali, F. Jelezko, J. Wrachtrup, S. Yamasaki, "Electrically driven single-photon source at room temperature in diamond", Nature Photonics, 6, 299 (2012). Abstract.
[2] S. Castelletto, B. C. Johnson, V. Ivády, N. Stavrias, T. Umeda, A. Gali, T. Ohshima, "A silicon carbide room-temperature single-photon source", Nature Materials, 13, 151 (2014). Abstract.
[3] B. Lounis and W. E. Moerner, "Single photons on demand from a single molecule at room temperature", Nature, 407, 491 (2000). Abstract.
[4] P. Michler, A. Imamog brevelu, M. D. Mason, P. J. Carson, G. F. Strouse, S. K. Buratto, "Quantum correlation among photons from a single quantum dot at room temperature", Nature, 406, 968 (2000). Abstract.
[5] S. Bounouar, M. Elouneg-Jamroz, M. den Hertog, C. Morchutt, E. Bellet-Amalric, R. André, C. Bougerol, Y. Genuist, J.-Ph. Poizat, S. Tatarenko, K. Kheng, "Ultrafast Room Temperature Single-Photon Source from Nanowire-Quantum Dots", Nano Letters, 12, 2977 (2012). Abstract.
[6] Mark J. Holmes, Kihyun Choi, Satoshi Kako, Munetaka Arita, Yasuhiko Arakawa, "Room-Temperature Triggered Single Photon Emission from a III-Nitride Site-Controlled Nanowire Quantum Dot", Nano Letters, 14, 982 (2014). Abstract.