HIV prevalence and HIV clinical outcomes of transgender and gender-diverse people in England.

OBJECTIVES: We provide the first estimate of HIV prevalence among trans and gender-diverse people living in England and compare outcomes of people living with HIV according to gender identity. METHODS: We analysed a comprehensive national HIV cohort and a nationally representative self-reported survey of people accessing HIV care in England (Positive Voices). Gender identity was recorded using a two-step question co-designed with community members and civil society. Responses were validated by clinic follow-up and/or self-report. Population estimates were obtained from national government offices. RESULTS: In 2017, HIV prevalence among trans and gender-diverse people was estimated at 0.46-4.78 per 1000, compared with 1.7 (95% credible interval: 1.6-1.7) in the general population. Of 94 885 people living with diagnosed HIV in England, 178 (0.19%) identified as trans or gender-diverse. Compared with cisgender people, trans and gender-diverse people were more likely to be London residents (57% vs. 43%), younger (median age 42 vs. 46 years), of white ethnicity (61% vs. 52%), under psychiatric care (11% vs. 4%), to report problems with self-care (37% vs. 13%), and to have been refused or delayed healthcare (23% vs. 11%). Antiretroviral uptake and viral suppression were high in both groups. CONCLUSIONS: HIV prevalence among trans and gender-diverse people living in England is relatively low compared with international estimates. Furthermore, no inequalities were observed with regard to HIV care. Nevertheless, trans and gender-diverse people with HIV report poorer mental health and higher levels of discrimination compared with cisgender people.

The development and cognitive testing of the positive outcomes HIV PROM: a brief novel patient-reported outcome measure for adults living with HIV.

BACKGROUND: People living with HIV experience burdensome multidimensional symptoms and concerns requiring person-centred care. Routine use of patient reported outcome measures can improve outcomes. There is no brief patient reported outcome measure (PROM) that currently reflects the breadth of concerns for people living with HIV. This study aimed to develop and cognitively test a brief novel patient reported outcome measure for use within routine adult HIV care- the "Positive Outcomes" HIV PROM. METHODS: Development followed the COSMIN taxonomy and guidance for relevance and comprehensiveness, and Rothrock guidance on development of valid patient reported outcome measures. The Positive Outcomes HIV PROM was developed by a steering group (people living with HIV, HIV professionals and health services researchers) using findings from a previously reported qualitative study of priority outcomes for people living with HIV. The prototype measure was cognitively tested with a purposive sample of people living with HIV. RESULTS: The Positive Outcomes HIV PROM consists of 23 questions (22 structured, and one open question) informed by the priorities of key stakeholders (n = 28 people living with HIV, n = 21 HIV professionals and n = 8 HIV commissioners) to ensure face and content validity, and refined through cognitive testing (n = 6 people living with HIV). Cognitive testing demonstrated high levels of acceptability and accessibility. CONCLUSIONS: The Positive Outcomes HIV PROM is the first brief patient reported outcome measure reflecting the diverse needs of people living with HIV designed specifically for use in the clinical setting to support patient assessment and care, and drive service quality improvement. It is derived from primary data on the priority outcomes for people living with HIV and is comprehensive and acceptable. Further psychometric testing is required to ensure reliability and responsiveness.

Where do we diagnose HIV infection? Monitoring new diagnoses made in nontraditional settings in England, Wales and Northern Ireland.

OBJECTIVES: The objectives of the study were to describe 10-year trends in HIV diagnosis setting and to explore predictors of being diagnosed outside a sexual health clinic (SHC). METHODS: Analyses of national HIV surveillance data were restricted to adults (aged ≥ 15 years) diagnosed in 2005-2014 in England, Wales and Northern Ireland. Logistic regression identified factors associated with diagnosis outside an SHC (2011-2014). RESULTS: Between 2005 and 2014, 63 599 adults were newly diagnosed with HIV infection; 83% had a diagnosis setting reported. Most people were diagnosed in SHCs (69%) followed by: medical admissions/accident and emergency (A&E; 8.6%), general practice (6.4%), antenatal services (5.5%), out-patient services (3.6%), infectious disease units (2.7%) and other settings (4.0%). The proportion of people diagnosed outside SHCs increased from 2005 to 2014, overall (from 27% to 32%, respectively) and among men who have sex with men (MSM) (from 14% to 21%) and black African men (from 25% to 37%) and women (from 39% to 52%) (all trend P < 0.001). Median CD4 increased across all settings, but was highest in SHCs (384 cells/µL) and lowest in medical admissions/A&E (94 cells/µL). Predictors of being diagnosed outside SHCs included: acquiring HIV through heterosexual contact [adjusted odds ratio (aOR) 1.99; 95% confidence interval (CI) 1.81-2.18] or injecting drug use (aOR: 3.28; 95% CI: 2.56-4.19; reference: MSM), being diagnosed late (< 350 cells/µL) (aOR: 2.55; 95% CI: 2.36-2.74; reference: diagnosed promptly) and being of older age at diagnosis (35-49 years: aOR: 1.60; 95% CI: 1.39-1.83; ≥ 50 years: aOR: 2.48; 95% CI: 2.13-2.88; reference: 15-24 years). CONCLUSIONS: The proportion of HIV diagnoses made outside SHCs has increased over the past decade in line with evolving HIV testing guidelines. However, the rate of late diagnosis remains high, indicating that further expansion of testing is necessary, as many people may have had missed opportunities for earlier diagnosis.

Sexualized drug use ('chemsex') and high-risk sexual behaviours in HIV-positive men who have sex with men.

OBJECTIVES: The incidence of sexually transmitted infections (STIs) and HIV infection remains high in gay, bisexual, and other men who have sex with men (MSM) in the UK, and sexualized drug use ("chemsex") and injecting drug use ("slamsex") may play a part in this. We aimed to characterize HIV-positive MSM engaging in chemsex/slamsex and to assess the associations with self-reported STI diagnoses and sexual behaviours. METHODS: Data from a 2014 survey of people attending HIV clinics in England and Wales were linked to clinical data from national HIV surveillance records and weighted to be nationally representative. Multivariable logistic regression assessed the associations of chemsex and slamsex with self-reported unprotected anal intercourse (UAI), serodiscordant UAI (sdUAI) (i.e. UAI with an HIV-negative or unknown HIV status partner), sdUAI with a detectable viral load (>50 HIV-1 RNA copies/mL), hepatitis C, and bacterial STIs. RESULTS: In the previous year, 29.5% of 392 sexually active participants engaged in chemsex, and 10.1% in slamsex. Chemsex was significantly associated with increased odds of UAI [adjusted odds ratio (AOR) 5.73; P < 0.001], sdUAI (AOR 2.34; P < 0.05), sdUAI with a detectable viral load (AOR 3.86; P < 0.01), hepatitis C (AOR 6.58; P < 0.01), and bacterial STI diagnosis (AOR 2.65; P < 0.01). Slamsex was associated with increased odds of UAI (AOR 6.11; P < 0.05), hepatitis C (AOR 9.39; P < 0.001), and bacterial STI diagnosis (AOR 6.11; P < 0.001). CONCLUSIONS: Three in ten sexually active HIV-positive MSM engaged in chemsex in the past year, which was positively associated with self-reported depression/anxiety, smoking, nonsexual drug use, risky sexual behaviours, STIs, and hepatitis C. Chemsex may therefore play a role in the ongoing HIV and STI epidemics in the UK.

A global survey of HIV-positive people's attitudes towards cure research.

OBJECTIVES: Involvement of people living with HIV (PLHIV) in the design of HIV cure studies is important, given the potential risks to participants. We present results of an international survey of PLHIV to define these issues and inform cure research. METHODS: PLHIV were recruited in June-November 2014 through HIV websites, advocacy forums, social media and 12 UK HIV clinics. The survey included questions concerning demographics, HIV disease history, the desirability of types of cure and the patient's willingness to accept potential toxicity and treatment interruption (TI). We examined factors associated with TI and willingness to accept substantial risks. RESULTS: A total of 982 PLHIV completed the survey; 87% were male, 79% white and 81% men who have sex with men (MSM). Fifty-one per cent were aged 25-44 years and 69% were UK residents. The median time since diagnosis was 7 years [interquartile range (IQR) 2-17 years]. Eighty-eight per cent were receiving antiretrovirals (91% reported undetectable viral load). Health/wellbeing improvements (96%) and an inability to transmit HIV (90%) were more desirable cure characteristics than testing HIV-negative (69%). Ninety-five per cent were interested in participating in cure studies, and 59% were willing to accept substantial risks. PLHIV with a low CD4 count [201-350 cells/µL vs. ≥ 350 cells/µL; odds ratio (OR) 2.11; 95% confidence interval (CI) 1.11-4.00] were more likely to accept risks, whereas those with limited knowledge of HIV treatments vs. excellent/good knowledge and those aged ≥ 65 years vs. 45-64 years were less likely to accept risks [OR 0.58 (95% CI 0.37-0.90) and OR 0.18 (95% CI 0.07-0.45), respectively]. TI was acceptable for 62% of participants, with the main concerns being becoming unwell (82%), becoming infectious (76%) and HIV spreading through the body (76%). CONCLUSIONS: Cure research was highly acceptable to the PLHIV surveyed. Most individuals would accept risks, including TI, even in the absence of personal benefit. An optimal cure would improve health and minimize onward transmission risk.

Polarization conversion-based molecular sensing using anisotropic plasmonic metasurfaces.

Anisotropic media induce changes in the polarization state of transmitted and reflected light. Here we combine this effect with the refractive index sensitivity typical of plasmonic nanoparticles to experimentally demonstrate self-referenced single wavelength refractometric sensing based on polarization conversion. We fabricated anisotropic plasmonic metasurfaces composed of gold dimers and, as a proof of principle, measured the changes in the rotation of light polarization induced by biomolecular adsorption with a surface sensitivity of 0.2 ng cm(-2). We demonstrate the possibility of miniaturized sensing and we show that experimental results can be reproduced by analytical theory. Various ways to increase the sensitivity and applicability of the sensing scheme are discussed.

Directional Light Extinction and Emission in a Metasurface of Tilted Plasmonic Nanopillars.

Plasmonic optical antennas and metamaterials with an ability to boost light-matter interactions for particular incidence or emission angles could find widespread use in solar harvesting, biophotonics, and in improving photon source performance at optical frequencies. However, directional plasmonic structures have generally large footprints or require complicated geometries and costly nanofabrication technologies. Here, we present a directional metasurface realized by breaking the out-of-plane symmetry of its individual elements: tilted subwavelength plasmonic gold nanopillars. Directionality is caused by the complex charge oscillation induced in each individual nanopillar, which essentially acts as a tilted dipole above a dielectric interface. The metasurface is homogeneous over a macroscopic area and it is fabricated by a combination of facile colloidal lithography and off-normal metal deposition. Fluorescence excitation and emission from dye molecules deposited on the metasurface is enhanced in specific directions determined by the tilt angle of the nanopillars. We envisage that these directional metasurfaces can be used as cost-effective substrates for surface-enhanced spectroscopies and a variety of nanophotonic applications.

Optical magnetism and plasmonic Fano resonances in metal-insulator-metal oligomers.

The possibility of achieving optical magnetism at visible frequencies using plasmonic nanostructures has recently been a subject of great interest. The concept is based on designing structures that support plasmon modes with electron oscillation patterns that imitate current loops, that is, magnetic dipoles. However, the magnetic resonances are typically spectrally narrow, thereby limiting their applicability in, for example, metamaterial designs. We show that a significantly broader magnetic response can be realized in plasmonic pentamers constructed from metal-insulator-metal (MIM) sandwich particles. Each MIM unit acts as a magnetic meta-atom and the optical magnetism is rendered quasi-broadband through hybridization of the in-plane modes. We demonstrate that scattering spectra of individual MIM pentamers exhibit multiple Fano resonances and a broad subradiant spectral window that signals the magnetic interaction and a hierarchy of coupling effects in these intricate three-dimensional nanoparticle oligomers.

Mutually synchronized bottom-up multi-nanocontact spin-torque oscillators.

Spin-torque oscillators offer a unique combination of nanosize, ultrafast modulation rates and ultrawide band signal generation from 100 MHz to close to 100 GHz. However, their low output power and large phase noise still limit their applicability to fundamental studies of spin-transfer torque and magnetodynamic phenomena. A possible solution to both problems is the spin-wave-mediated mutual synchronization of multiple spin-torque oscillators through a shared excited ferromagnetic layer. To date, synchronization of high-frequency spin-torque oscillators has only been achieved for two nanocontacts. As fabrication using expensive top-down lithography processes is not readily available to many groups, attempts to synchronize a large number of nanocontacts have been all but abandoned. Here we present an alternative, simple and cost-effective bottom-up method to realize large ensembles of synchronized nanocontact spin-torque oscillators. We demonstrate mutual synchronization of three high-frequency nanocontact spin-torque oscillators and pairwise synchronization in devices with four and five nanocontacts.

Sertraline inhibits the transport of PAT1 substrates in vivo and in vitro.

BACKGROUND AND PURPOSE: Intestinal nutrient transporters may mediate the uptake of drugs. The aim of this study was to investigate whether sertraline interacts with the intestinal proton-coupled amino acid transporter 1 PAT1 (SLC36A1). EXPERIMENTAL APPROACH: In vitro investigations of interactions between sertraline and human (h)PAT1, hSGLT1 (sodium-glucose linked transporter 1) and hPepT1 (proton-coupled di-/tri-peptide transporter 1) were conducted in Caco-2 cells using radiolabelled substrates. In vivo pharmacokinetic investigations were conducted in male Sprague-Dawley rats using gaboxadol (10 mg·kg(-1), p.o.) as a PAT1 substrate and sertraline (0-30.6 mg·kg(-1)). Gaboxadol was quantified by hydrophilic interaction chromatography followed by MS/MS detection. KEY RESULTS: Sertraline inhibited hPAT1-mediated L-[(3)H]-Pro uptake in Caco-2 cells. This interaction between sertraline and PAT1 appeared to be non-competitive. The uptake of the hSGLT1 substrate [(14)C]-α-methyl-D-glycopyranoside and the hPepT1 substrate [(14)C]-Gly-Sar in Caco-2 cells was also decreased in the presence of 0.3 mM sertraline. In rats, the administration of sertraline (0.1-10 mM, corresponding to 0.3-30.6 mg·kg(-1), p.o.) significantly reduced the maximal gaboxadol plasma concentration and AUC after its administration p.o. CONCLUSIONS AND IMPLICATIONS: Sertraline is an apparent non-competitive inhibitor of hPAT1-mediated transport in vitro. This inhibitory effect of sertraline is not specific to hPAT1 as substrate transport via hPepT1 and hSGLT1 was also reduced in the presence of sertraline. In vivo, sertraline reduced the amount of gaboxadol absorbed, suggesting that the inhibitory effect of sertraline on PAT1 occurs both in vitro and in vivo. Hence, sertraline could alter the bioavailability of drugs absorbed via PAT1.

Ten-year mortality trends among persons diagnosed with HIV infection in England and Wales in the era of antiretroviral therapy: AIDS remains a silent killer.

OBJECTIVES: We present national trends in death rates and the proportion of deaths attributable to AIDS in the era of effective antiretroviral therapy (ART), and examine risk factors associated with an AIDS-related death. METHODS: Analyses of the national HIV-infected cohort for England and Wales linked to death records from the Office of National Statistics were performed. Annual all-cause mortality rates were calculated by age group and sex for the years 1999-2008 and rates for 2008 were compared with death rates in the general population. Risk factors associated with an AIDS-related death were investigated using a case-control study design. RESULTS: The all-cause mortality rate among persons diagnosed with HIV infection aged 15-59 years fell over the decade: from 217 per 10 000 in 1999 to 82 per 10 000 in 2008, with declines in all age groups and exposure categories except women aged 50-59 years and persons who inject drugs (rate fluctuations in both of these groups were probably a result of small numbers). Compared with the general population (15 per 10 000 in 2008), death rates among persons diagnosed with HIV infection remained high, especially in younger persons (aged 15-29 years) and persons who inject drugs (13 and 20 times higher, respectively). AIDS-related deaths accounted for 43% of all deaths over the decade (24% in 2008). Late diagnosis (CD4 count < 350 cells/µL) was the most important predictor of dying of AIDS [odds ratio (OR) 10.55; 95% confidence interval (CI) 8.22-13.54]. Sixty per cent of all-cause mortality and 81% of all AIDS-related deaths were attributable to late diagnosis. CONCLUSIONS: Despite substantial declines, death rates among persons diagnosed with HIV infection continue to exceed those of the general population in the ART era. Earlier diagnosis could have prevented 1600 AIDS-related deaths over the decade. These findings highlight the need to intensify efforts to offer and recommend an HIV test in a wider range of clinical and community settings.

Function and expression of the proton-coupled amino acid transporter PAT1 along the rat gastrointestinal tract: implications for intestinal absorption of gaboxadol.

BACKGROUND AND PURPOSE: Intestinal absorption via membrane transporters may determine the pharmacokinetics of drug compounds. The hypothesis is that oral absorption of gaboxadol (4,5,6,7-tetrahydroisoxazolo [5,4-c] pyridine-3-ol) in rats occurs via the proton-coupled amino acid transporter, rPAT1 (encoded by the gene rSlc36a1). Consequently, we aimed to elucidate the in vivo role of rPAT1 in the absorption of gaboxadol from various intestinal segments obtained from Sprague-Dawley rats. EXPERIMENTAL APPROACH: The absorption of gaboxadol was investigated following its administration into four different intestinal segments. The intestinal expression of rSlc36a1 mRNA was measured by quantitative real-time PCR. Furthermore, the hPAT1-/rPAT1-mediated transport of gaboxadol or L-proline was studied in hPAT1-expressing Xenopus laevis oocytes, Caco-2 cell monolayers and excised segments of the rat intestine. KEY RESULTS: The absorption fraction of gaboxadol was high (81.3-91.3%) following its administration into the stomach, duodenum and jejunum, but low (4.2%) after administration into the colon. The pharmacokinetics of gaboxadol were modified by the co-administration of L-tryptophan (an hPAT1 inhibitor) and L-proline (an hPAT1 substrate). The in vitro carrier-mediated uptake rate of L-proline in the excised intestinal segments was highest in the mid jejunum and lowest in the colon. The in vitro uptake and the in vivo absorption correlated with the expression of rSlc36a1 mRNA along the rat intestine. CONCLUSIONS AND IMPLICATIONS: These results suggest that PAT1 mediates the intestinal absorption of gaboxadol and therefore determines its oral bioavailability. This has implications for the in vivo role of PAT1 and may have an influence on the design of pharmaceutical formulations of PAT1 substrates.

HIV testing in community settings in resource-rich countries: a systematic review of the evidence.

OBJECTIVES: Community HIV testing represents an opportunity for diagnosing HIV infection among individuals who may not have contact with health services, especially in hard-to-reach groups. The aim of this review was to assess the evidence for feasibility, acceptability and effectiveness of HIV testing strategies in community settings in resource-rich countries. METHODS: The PubMed database was searched for English language studies of outreach HIV testing in resource-rich countries. Studies were included if they reported one of the following outcome measures: uptake of testing; seropositivity; client acceptability; or provider acceptability. RESULTS: Forty-four studies were identified; the majority took place in the USA and targeted men who have sex with men. Uptake of HIV testing varied between 9 and 95% (in 14 studies). Seropositivity was ≥ 1% in 30 of 34 studies. In 16 studies the proportion of patients who received their test results varied from 29 to 100% and rapid testing resulted in a higher proportion of clients receiving their results. Overall, client satisfaction with community HIV testing was high. However, concern remained over confidentiality, professional standards and the need for post-test counselling. Staff reported positive attitudes towards community testing. CONCLUSIONS: In the majority of studies, the reported seropositivity was higher than 1/1000, the threshold deemed to be cost-effective for routinely offering testing. Rapid testing improved the return of HIV test results to clients. HIV testing in outreach settings may be important in identifying undiagnosed infections in at-risk populations, but appropriate data to evaluate these initiatives must be collected.

Safe travels? HIV transmission among Britons travelling abroad.

OBJECTIVES: The aim of the study was to identify and describe the characteristics of persons born in the UK who acquire HIV infection abroad. METHODS: Analyses using case reports and follow-up data from the national HIV database held at the Health Protection Agency were performed. RESULTS: Fifteen per cent (2066 of 13 891) of UK-born adults diagnosed in England, Wales and Northern Ireland between 2002 and 2010 acquired HIV infection abroad. Thailand (534), the USA (117) and South Africa (108) were the countries most commonly reported. As compared with UK-born adults acquiring HIV infection in the UK, those acquiring HIV infection abroad were significantly (P < 0.01) more likely to have acquired it heterosexually (70% vs. 22%, respectively), to be of older age at diagnosis (median 42 years vs. 36 years, respectively), and to have reported sex with a commercial sex worker (5.6% vs. 1%, respectively). Among men infected in Thailand, 11% reported sex with a commercial sex worker. CONCLUSIONS: A substantial number of UK-born adults are acquiring HIV infection in countries with generalized HIV epidemics, and in common holiday destinations. Of particular concern is the high proportion of men infected reporting sex with a commercial sex worker. We recommend HIV prevention and testing efforts be extended to include travellers abroad, and that sexual health advice be provided routinely in travel health consultations and in occupational health travel advice packs, particularly to those travelling to high HIV prevalence areas and destinations for sex tourism. Safer sex messages should include an awareness of the potential detrimental health and social impacts of the sex industry.

Fano interference in supported gold nanosandwiches with weakly coupled nanodisks.

We studied the far-field optical response of supported gold-silica-gold nanosandwiches using spectroscopic ellipsometry, reflectance and transmittance measurements. Although transmittance data clearly shows that the gold nanodisks in the sandwich structure interact very weakly, oblique reflectance spectra of s- and p-polarized light show clearly asymmetric line-shapes of the Fano type. However, all experimental results are very well described by modeling the gold nanodisks as oblate spheroids and by employing a 2 × 2 scattering matrix formulation of the Fresnel coefficients provided by an island film theory. In particular, the Fano asymmetry can be explained in terms of interference between the scattered waves from the decoupled nanodisks in the spectral range limited by their respective plasmon resonances. We also show that the reflectance and ellipsometry spectra can be described by a three-layer system with uniaxial effective dielectric functions.

Optical response of supported gold nanodisks.

It is shown that the ellipsometric spectra of short range ordered planar arrays of gold nanodisks supported on glass substrates can be described by modeling the nanostructured arrays as uniaxial homogeneous layers with dielectric functions of the Lorentz type. However, appreciable deviations from experimental data are observed in calculated spectra of irradiance measurements. A qualitative and quantitative description of all measured spectra is obtained with a uniaxial effective medium dielectric function in which the nanodisks are modeled as oblate spheroids. Dynamic depolarization factors in the long-wavelength approximation and interaction with the substrate are considered. Similar results are obtained calculating the optical spectra using the island-film theory. Nevertheless, a small in-plane anisotropy and quadrupolar coupling effects reveal a very complex optical response of the nanostructured arrays.

Ultrahigh sensitivity made simple: nanoplasmonic label-free biosensing with an extremely low limit-of-detection for bacterial and cancer diagnostics.

We present a simple and robust scheme for biosensing with an ultralow limit-of-detection down to several pg cm(-2) (or several tens of attomoles cm(-2)) based on optical label-free biodetection with localized surface plasmon resonances. The scheme utilizes cost-effective optical components and comprises a white light source, a properly functionalized sensor surface enclosed in a simple fluidics chip, and a spectral analyzer. The sensor surface is produced by a bottom-up nanofabrication technique with hole mask colloidal lithography. Despite its simplicity, the method is able to reliably detect protein-protein binding events at low picomolar and femtomolar concentrations, which is exemplified by the label-free detection of the extracellular adherence protein (EAP) found on the outer surface of the bacterium Staphylococcus aureus and of prostate-specific antigen (PSA), which is believed to be a prostate cancer marker. These experiments pave the way towards an ultra-sensitive yet compact biodetection platform for point-of-care diagnostics applications.

Optically controlled interparticle distance tuning and welding of single gold nanoparticle pairs by photochemical metal deposition.

We report on the in-situ controlled tuning of the particle gap in single pairs of gold nanodisks by photochemical metal deposition. The optically induced growth of nanodisk dimers fabricated by electron beam lithography leads to a decrease of the interparticle gap width down to 0 nm. Due to the increasing particle size and stronger plasmonic coupling, a smooth redshift of the localized surface plasmon (LSP) resonances is observed in such particle pairs during the growth process. The interparticle gap width, and hence the LSP resonance, can be tuned to any desired spectral position. The experimental results we obtain with this nanoscale fabrication technique are well described by the so-called plasmon ruler equation. Consequently, both the changes in particle diameter as well as in gap width can be characterized in-situ via far-field read-out of the optical properties of the dimers.

Shape effects in the localized surface plasmon resonance of single nanoholes in thin metal films.

We report on the polarization-dependent optical response of elongated nanoholes in optically thin gold films. We measured elastic scattering spectra of spatially isolated ellipsoidal nanoholes with varying aspect ratio and compared the results to electrodynamic simulations. Both experiments and theory show that the plasmon mode that is polarized parallel to the short axis of the ellipsoidal hole red-shifts with increasing aspect ratio. This behavior is completely opposite to the case of elongated metal particles. We present a simple analytical model that qualitatively explains the observations in terms of the different orientations of the induced dipole moments in holes and particles.

Nanometric control of the distance between plasmonic nanoparticles using optical forces.

We theoretically analyze the optical forces between two nearby silver nanoparticles for the case when the wavelength of the incoming light is close to the localized surface plasmon resonance (LSPR). It is shown that the optical force between the nanoparticles is enhanced by the LSPR and that it changes from attractive to repulsive for wavelengths slightly shorter than the resonance when the polarization of the incident light is parallel to the axis of the dimer. This behavior can be utilized to generate a stable separation distance between the nanoparticles. In the Rayleigh limit, the equilibrium distance is uniquely determined by the real part of the particle polarizability and the wavelength of the incident light. The results suggest that near-field optical forces can be used to manipulate and organize plasmonic nanoparticles with a tunable spatial resolution in the nanometer regime.