Active control of chiral optical near fields on a single metal nanorod

Subscriber access provided by WESTERN SYDNEY U

Article

Active control of chiral optical near fields on a single metal nanorod Shun Hashiyada, Tetsuya Narushima, and Hiromi Okamoto ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b01500 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Active control of chiral optical near fields on a single metal nanorod Shun Hashiyada1,*, Tetsuya Narushima1,2 and Hiromi Okamoto1,2,* 1Center

for Mesoscopic Sciences, Institute for Molecular Science, 38 Nishigonaka, Myodaiji,

Okazaki, Aichi 444-8585, Japan 2The

Graduate University for Advanced Studies, 38 Nishigonaka, Myodaiji, Okazaki, Aichi 444-

8585, Japan

1 ACS Paragon Plus Environment

ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 27

Abstract Chiral optical fields (typified by circularly polarized light) localized on the nanoscale enhance the chiral light-matter interaction, which may provide novel potential applications.

This property

enables the development of an ultrasensitive method for characterization of chiral molecules and nanoscale magnetic control realized by an all-optical method to interconnect spintronic nanooptical devices.

A local chiral light source with switchable handedness or controllable chirality

is indispensable for building such applications for practical use.

In the current major method used

for local chiral light generation, the handedness of the light is controlled by the handedness of the nanomaterial, which is not convenient when we need to change the handedness of the light.

We

experimentally achieve here generation and active control of a highly chiral local optical field by using a combination of an achiral gold nanorod and achiral linearly polarized optical field.

By

tilting the azimuth angle for the incident linear polarization relative to the axis of the nanorod, either left- or right-handed circularly polarized local optical fields can be generated.

Our work

may give us a chance to pioneer analytical applications of chiral optical fields and novel spintronic nano-optical devices.

Keywords chiral plasmonics, near-field optics, nanomaterial, polarimetry, scanning near-field optical microscope


Page 3 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

The polarization state of the optical field is a key parameter in the light-matter interaction [1]. A circularly polarized optical field provides a probe of chirality in chemical and biological systems, as left (LH) and right-handed (RH) circular polarizations are the eigen polarization states for chiral media [2].

The polarization state of the propagating optical field is routinely controlled using

anisotropic crystals.

In contrast, to manipulate the polarization state of the optical field localized

on a nanostructure, not only the incident propagating field but also the secondary field generated by the nanostructure should be controlled.

Establishment of a flexible method to generate and

control spatially localized chiral (typified as circularly polarized) optical fields is also beneficial in view of achieving ultrasensitive characterization of chiral molecules [3].

It also provides potential

applications in high-density all-optical magnetic recording [4] as localized chiral optical fields yield effective spin sources [5].

In most studies on the chiral optical response of matter, chiral

materials and/or chiral optical fields are utilized as chiral light sources.

In the present study, we

propose and experimentally demonstrate a nonconventional way to generate and control a highly circularly polarized (ellipticity close to unity) local field using a simple achiral metal nanostructure and achiral (linearly polarized) optical field.

This method is advantageous over that based on

chiral materials not only for its simplicity but also for controllability of the handedness of the generated field, as described in the following. Localized chiral optical fields are sometimes generated by the optical excitation of plasmon resonances in metal nanostructures with chiral shapes, such as the gammadion and helix [3][6].


ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 27

In this approach, enantiomeric (LH and RH) chiral nanostructures are indispensable for separately generating equivalent LH and RH fields.

The handedness of the localized chiral optical field is

switched by reversing the handedness of the chiral geometry of the nanostructure, which prevents simple tuning of the chiral properties of the local optical fields.

If both LH and RH fields are

generated separately by a single geometry of the nanostructure, great opportunities are potentially provided to extend the application of chiral nano-optics.

A convenient ultrasensitive method for

characterization of chiral molecules will be achievable with simple implementation and higher accuracy, as only one type of nanostructure is necessary to probe the response to both LH and RH optical fields.

The local field with switchable chirality may also be advantageous for practical

construction of nano-optical devices such as sorting of chiral nanomaterials, information processing, and so forth. Recently, it has been theoretically predicted [7][8] and experimentally demonstrated [9][10] that chiral optical fields can be generated near a single achiral metal nanostructure.

When the

longitudinal plasmon of the achiral rod-shaped nanostructure is excited with a linearly polarized optical field parallel to the long axis of the nanorod, both LH and RH fields are created in the periphery of the nanorod.

The LH and RH local field amplitudes are balanced due to the

symmetry of the system consisting of the nanostructure and the radiation, with no chiral field generated as a whole.

When the symmetry of the whole system is broken by rotating the azimuth

angle (θin) of the incident linear polarization relative to the axis of the nanorod (Figure 1), the


Page 5 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

creation of either LH or RH fields in excess is anticipated [7][8][11].

This is because the system

formed by the wave vector and the polarization vector of the incident light and the vector representing the anisotropy of the nanorod breaks the parity symmetry.

We experimentally

demonstrate here that a chiral local optical field is in fact generated with a single achiral gold nanorod excited by a linearly polarized optical field, with the chirality (handedness) of the field controllable by tuning θin.

Chirality control of the optical field is achieved not by switching the

geometrical chirality of the nanostructure but by switching the chirality of the whole system including the material and the optical field.

Since the degree of the chiral asymmetry of the

system depends on the polarization angle θin, the degree of circular polarization (PCP) of the localized optical field can, in principle, be continuously controlled from 0 (linear polarization) to even ±1 (pure circular polarization).

By using near-field polarimetry [9], we experimentally

found that highly circularly polarized (|PCP| > 0.5) local optical fields were in fact generated at the position above the center of the nanorod.

We also investigate theoretically the conditions for

generating a pure (|PCP| = 1) circularly polarized local optical field on the nanorod based on a simple point dipole model.

These results may give us a fundamental principle not only for designing a

chiral optical field in nano space but also for developing a new strategy for ultrasensitive chirooptical spectroscopy and spintronic nano-optical devices.


ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1.

Page 6 of 27

Strategy for introducing symmetry breaking in the system and active control

of the local chiral optical field.

By adjusting the azimuth angle (θin) of the incident linearly

polarized optical field relative to the axis of the nanorod, we can tune the degree of asymmetry for the whole system consisting of the gold nanorod and incident field.

Inset: Scanning

electron micrograph of a gold nanorod (160 nml × 40 nmw × 55 nmt) used in this study.

Scale

bar: 100 nm.

Results and Discussion Near-Field Polarimetry.

We evaluated the polarization state of the optical field generated near

the gold nanorod using a near-field polarimetry (NFP) imaging system [9] based on an aperturetype scanning near field optical microscope.

In the NFP measurements, a linearly polarized

optical near field (azimuth angle θin, wavelength λin = 850 nm) illuminates the nanostructured sample through a gold-coated near-field aperture probe (aperture diameter ≈ 60 nm), and the polarization states of the scattered optical field were analyzed in the far field regime.

In this 6

ACS Paragon Plus Environment

Page 7 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

scheme of measurement, the polarization states of the scattered light, rather than the optical activity of the nanostructured sample, were analyzed.

If optical reciprocity [12] is valid in the NFP

imaging system, polarization maps taken by the NFP measurements should be identical to those of the optical fields in the vicinity of the nanostructured sample.

The results of previous studies

based on near-field experiments [9][13][14] support the validity of the optical reciprocity.

The

polarization state of the optical field was characterized by the degree of circular polarization (PCP ≡ (IL – IR)/(IL + IR), where IL and IR denote the intensities of the LH and RH circular polarization components of the optical field, respectively).

The PCP value ranges from +1 (LH circular

polarization) to –1 (RH circular polarization) and PCP = 0 indicates linear polarization (or totally unpolarized). Near-Field Polarimetry Imaging of a Single Gold Nanorod.

As shown in Figures 2a and 2l, a

four lobe (d-orbital like) spatial structure was observed for LH (PCP > 0) and RH (PCP < 0) chiral optical fields near the gold nanorod when the nanorod was excited by a linearly polarized optical field of θin ≈ 0 or 90° (thus, the whole system was achiral). structure qualitatively coincides with previous reports [7][8][9].

This observation of the four lobe Such symmetric distributions for

positive and negative PCP lead to approximately null signals when averaged over space, which correctly reflected the geometrical symmetry of the system.

On the other hand, when the

symmetry of the system was broken by tilting the incident polarization in the anticlockwise direction (θin > 0), negative PCP was dominant in the observation area (Figures 2b-e).

When the


ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 27

incident polarization was rotated in the clockwise direction (θin < 0), positive PCP was dominant (Figures 2h-k).

These results indicate that the chirality of the optical field generated by the

nanorod is determined by the geometrical chirality of the whole system including both the nanorod and the incident field.


Page 9 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Figure 2 | Experimentally observed maps for the degree of circular polarization of the optical fields (PCP) near a single gold nanorod.

A single gold nanorod was excited with a linearly

polarized optical field with a positive (a-f) and negative (g-l) azimuth angle (θin: polarization direction is indicated by arrows).

Dashed line indicates the approximate position of the

nanostructure. Scale bars: 100 nm.


ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 27

The PCP value was most significantly dependent on θin at the position above the center of the nanorod, while the dependence was minor at the outer area of the nanorod. central position of the nanorod is plotted as a function of θin in Figure 3. continuously with θin, with its sign reversed at θin ≈ 0º. θin ≈ ±40º, with |PCP| reaching a value of 0.5 or more.

The PCP value at the

The PCP value changed

The extremal values for PCP are found at For the ellipticity angle η (|η| ≤ 45º), which

is often used as a measure of the magnitude of a circular dichroism signal [2], |PCP| = 0.5 corresponds to |η| = 15º.

We measured 14 nanorods in total and confirmed the reproducibility of

the θin dependence for PCP (Figure S4).

These results show that active and continuous control of

the polarization state (from linear polarization to LH or RH elliptical polarization with high PCP values) of the local optical field is achievable by adjusting the relative angle between the incident linear polarization and the long axis of the nanorod.

Figure 3 | The degree of circular polarization (PCP) at the center above the gold nanorod vs. incident azimuth angle (θin).

Experimentally obtained PCP (red circles) and a fit to the

model function in Eq. 2 (black solid curve). 10 ACS Paragon Plus Environment

Page 11 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Model Calculation of the Localized Chiral Optical Field.

To discuss the mechanism for

polarization control of the localized optical field generated by a single gold nanorod, we simulated the spatial distribution of PCP based on a simple model where the longitudinal plasmon excited on the nanorod is approximated as an oscillating point dipole p parallel to the x-axis (Figure 4a) [7][8][9].

Here we neglected the effect of the transverse plasmon mode, because the resonant

wavelength of the transverse mode is ~600 nm which is much shorter than our excitation wavelength of 850 nm and is under off-resonance condition.

Suppose that the point dipole is

induced by a linearly polarized time-harmonic plane-wave electric field propagating in +z direction, Ein(θin) = E0J(θin)e –iωteikz, where ω and k = 2π/λ represent the angular frequency and wavenumber, respectively.

Quantities with tildes are complex numbers.

is determined by the Jones vector J(θin).

The polarization state

In the present case, J(θin) = t(cosθin, sinθin, 0) to

reproduce the incident linear polarization of the azimuth angle θin with respect to the x-axis.

We

analyzed the polarization state of the electric field at the observation plane (z = +d; Figure 4a) located in the near- and intermediate-field regime (kd ≲ 1).

The optical system assumed here

for the simulation (i.e., far-field excitation and near- and intermediate-field detection) is reciprocal to that used in the experiment (near- and intermediate-field excitation and far-field detection).

If

the optical reciprocity is valid, the result of simulation should be equivalent to that observed under the experimental optical arrangement. As shown in Figures 4b-l, the calculated spatial distributions for PCP near the point dipole


ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 27

(details in Supporting Information) were qualitatively in good agreement with the experimentally obtained spatial distributions (Figure 2).

Four-lobe structures for PCP were observed at the outer

area of the dipole, with significant changes in PCP with respect to θin observed at the central region above the dipole.

In the following, we analyze the behavior of PCP(θin) at the position/point above

the center of the dipole (x = y = 0, z = +d, Figure 4m) and discuss the mechanism for elliptically polarized light generation. The induced-dipole field in the near- and intermediate-field regime (kd ≲ 1) at a position (0, 0, d) can be approximately described by Edipole(θin) = t(–(|α| d3) 1 + k2d2ei(δ – Δ)E0e –iωtcos θin, 0, 0) (see Eq. S2), where |α| and δ denote the amplitude and phase of the complex electric polarizability (α = |α|eiδ) of the dipole, and tan∆ = kd.

We note that the phase of the dipole-

induced field shifts from that of the incident external field when the induced oscillating dipole is resonant with the incident field.

Under the rigorous resonance excitation condition, where the

wavelength of the incident field λin exactly matches with the resonance wavelength λr of the dipole (λin = λr), the shift is δ = 90º.

In the case of a preresonance excitation condition (λin > λr), the shift

takes a value of 0 < δ < 90º.

The x- and y-components of the total field, i.e., the superposition of

the incident field and the dipole field, can be written as: Ex(θin) = (1 – (|α| d3) 1 + k2d2ei(δ – Δ))E0e –iωtcos θin iξ

= e

2 1 – 2(|α| d3) 1 + k2d2cos (δ – Δ) + (|α| d3) (1 + k2d2) E0e –iωtcos θin,

Ey(θin) = E0e –iωtsin θin,

(1a)

(1b)


Page 13 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

2 2 where, tanξ = – (|α| d3) 1 + k d sin (δ – Δ) 1 – (|α| d3) 1 + k2d2cos (δ – Δ) .

(

)

As shown

in Figures 4c-l, an elliptically polarized field was generated at the center above the dipole (except for the case of θin = 0º (Figure 4b)).

In Eq. 1, we find that the total field becomes elliptical when

ξ ≠ 0º, i.e., the dipole is resonantly excited with the incident field (δ ≠ 0) and the azimuth angle θin of the incident field is θin ≠ 0º, 90º.

This is because the phase of the x-component of the total

field, which is a superposition of the induced-dipole field and the incident field, shifts from that of the y-component of the field, which is independent of the induced dipole at the center (x,y = 0) (Figure 4m).


ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 27

Figure 4 | Model calculation of the localized chiral optical field.

a, The model for the

polarization analysis of the electric field near an oscillating point dipole.

b-l, Maps for the

degree of circular polarization of the electric field PCP near the dipole. polarization directions are indicated by arrows.

Scale bar: 100 nm.

The incident

m, Schematic illustration

for the time development of the incident, dipole, and total electric field vectors at the evaluation point (x,y = 0, z = +d). 14 ACS Paragon Plus Environment

Page 15 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

We have further found that it is possible in principle to generate a pure circularly polarized (i.e., no linear polarization component involved) local field at the center above the dipole, based on Eq. 1.

In general, the field is circularly polarized if (i) the phase difference between the x- and y-

components of the electric field is ±90º and (ii) the amplitude of the x- and y-components are the same.

For condition (i), ξ = 90º can be achieved when the evaluation plane is located at d = dc,

which satisfies the condition:

(|α| dc3)

1 + k2dc2cos (δ – Δ) = 1 (where δ – Δ < 90º).

Condition (ii) can be satisfied under condition (i) when θin = δ – Δ ≡ θinc.

Therefore, pure

circular polarization can be generated at a certain point (0, 0, dc) when the induced-dipole is resonantly excited with a linearly polarized field of azimuth angle θinc.

We need information on

the complex electric polarizability (α = |α|eiδ) for the dipolar plasmon of the nanorod to obtain the values for dc and θinc.

PCP (θin) at the point above the dipole is given by the following equation

(see Supporting Information for derivation), PCP(x = 0, y = 0, z = d, θin) =

(|α| d3) 1 +

((|α| d3)2(1

1 + k2d2sin (δ – Δ)sin 2θin

+ k2d2) – 2(|α| d3) 1 + k2d2cos (δ – Δ))cos2 θin

.

(2)

The value of PCP and its variation against θin is strongly dependent on |α| and δ, and, thus, these parameters can be estimated by fitting this functional form of PCP to the experimentally obtained PCP (θin) to this equation.

As shown in Figure 3, the fitting curve reasonably reproduced the

measured data when we used the following parameters: |α| = 1.33×10–21 m3 and δ = 21º, λ = 2π/k = 850 nm, and d = 100 nm.

In the experimental system, d corresponds to the distance between 15 ACS Paragon Plus Environment

ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the near-field probe aperture and the sample surface.

Page 16 of 27

Although the value of d used (100 nm) is of

the same order of magnitude as the actual value (typically 20-50 nm), they are quantitatively not consistent.

This may be because of oversimplification of the dipole model presently adopted.

From the obtained value of the phase δ = 21º (0 < δ < 90º), the plasmon of the nanorod is considered to be excited under the preresonance excitation condition (λin > λr).

From the amplitude of the

polarizability |α| = 1.33×10–21 m3, the absorption cross section at the resonance wavelength (assuming λr = 800 nm), Qabs (= k|α|), is estimated to be 1.0×104 nm2.

The estimated Qabs value

is of the same order of magnitude as the experimentally obtained Qabs ≈ 0.9×104 nm2 for a single small gold nanorod [15]. observed PCP.

This result shows the validity of Eq. (2) to describe the behavior of the

The result also indicates that the NFP imaging experiment correctly visualizes the

polarization of the optical fields in the near- and intermediate-field regime of the metal nanostructure and that the optical reciprocity is valid (details in Supporting Information).

Then,

by solving equation |PCP| = 1 with the expression of PCP in Eq. (2), we find the condition to obtain a pure circularly polarized field with linearly polarized excitation to be dc ≈ 116 nm and θinc ≈ ±20º. To experimentally verify this expectation for pure circularly polarized local field generation, we need measurements of PCP at various distances between the near-field probe aperture and the sample surface, d. Correlation between the optical chirality and degree of circular polarization.

The strength

of interaction between the chiral optical field and chiral matter depends on the magnitude of the


Page 17 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

time-averaged optical chirality C = –ε0ωIm(E *∙ B)/2 of the field [16][17][18][19][20], where ε0, E and B denote the permittivity in vacuum, and the complex time-harmonic electric and magnetic fields, respectively.

The use of the optical field possessing larger C has been thus considered as

a promising approach to enhance the efficiency of the chiral light-matter interaction.

Although

the optical chirality is useful to characterize chiral optical field and to evaluate the strength of the chiral light-matter interaction, this quantity cannot be directly observed by an experiment.

Here,

we consider interaction between chiral matter and the localized chiral field based on the observable quantities to maximize the efficiency of the interaction. The efficiency of the chiral optical interaction is expected to be larger as the degree of circular polarization (PCP) as well as the averaged intensity Iave (= (IL + IR)/2) of the optical field (both of them are experimentally observable) increase. and Iave at the point above the dipole.

Figure 5 shows the θin- and d-dependence of PCP

As seen in Figure 5, PCP shows extremal values at certain

values of d, whereas Iave becomes larger as d decreases.

This is because the intensity of the dipole-

induced field near the dipole is much stronger than that of the incident field (Iave → ∞ when d → 0).

Under this condition, the polarization of the total field is close to that of the dipole-induced

field, which is linearly polarized (i.e., PCP → 0) at any θin.

The strength of the chiral light-matter

interaction may be maximized when |PCP • Iave| takes its maximum value.

Interestingly, we found

that the product of PCP and Iave/I0 (where I0 is the intensity of the incident field) is identical to C CLCP at the position above the dipole under the present model, where CLCP is the optical


ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 27

chirality of the propagating LH circularly polarized field in free space: PCP

Iave I0

=

|α|sin δsin 2θin 2d3

=

C CLCP

(3)

.

From Eq. 3, the product of PCP and Iave/I0 or C CLCP shows an extremal value when the nanorod is excited with a linearly polarized optical field with θin = ±45º under the rigorous resonance condition (δ = 90º), and, thus, the efficiency of the chiral light-matter interaction is expected to be maximized under this condition.

Under this condition, the nanorods used in this study (|α| =

1.33×10–21 m3) generates superchiral fields that satisfies the relation |C CLCP| > 1 in their proximity (d < 87.3 nm). Electromagnetic modeling in previous studies revealed that the optical field localized on the metal nanostructure displays superchirality [3][6][7][8][11].

Experimental studies demonstrated

that chiral fields localized on chiral metal nanostructures greatly enhance the chiro-optical detection sensitivity of chiral molecules [3][21].

The controllable chiral optical field localized on

a single gold nanorod excited with a linearly polarized field may thus provide a basis for new schemes for highly efficient methods of characterization and discrimination of chiral molecules.


Page 19 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Figure 5 | The degree of circular polarization (PCP) and the intensity (Iave) of the electric field at the center above the dipole.

The degree of circular polarization (a-d) and the

intensity (e-h) of the electric field at the point above the dipole are plotted as functions of the distance from the evaluation plane to the dipole (d) and the azimuth angle of the incident linear polarization (θin).


ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 27

Conclusions We experimentally demonstrated the generation of a local chiral optical field and control of chirality with a very simple system that consists of a gold nanorod illuminated with linearly polarized light.

We succeeded in active control of chiral optical fields localized on a single gold

nanorod, from a linearly polarized to elliptically polarized (both LH and RH) field, by adjusting the relative angle between the long axis of the nanorod and the incident linear polarization.

Our

experimental results show that a chiral optical field with a high degree of circular polarization (|PCP| > 0.5) is localized at the center above the nanorod.

Theoretical considerations revealed the

condition for generating a pure (|PCP| = 1) circularly polarized local optical field and the condition for maximizing the chiral light-mater interaction.

Since a single gold nanorod generates a

continuously controllable chiral optical field, this technique may lead to the realization of a nanoscale polarization modulator, enabling high-sensitivity detection and characterization of chiral molecules, for instance, by simply combining with a lock-in detection method.

This technique

may also provide a fundamental principle to construct a circularly polarized light emitting device by combining with a nano-sized emitter, or other polarization converting devices.

Methods Nanostructured Sample Fabrication.

Gold nanorods used in this study were fabricated on a

glass substrate using the electron-beam lithography and lift-off technique (Figure S1).

A gold


Page 21 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

film with a thickness of 55 nm was formed by vacuum deposition onto an underlying 2-nm-thick chromium adhesion layer.

The measured dimensions for the fabricated nanorod were 160 nm

long and 40 nm wide (aspect ratio ~4).

Theoretical calculations predict that gold nanorods with

aspect ratios of 4 yield a longitudinal dipolar plasmon resonance at a wavelength of ~800 nm [22]. The glass substrate possibly induces the shift of plasmon resonance wavelength, but does not cause the other effect. Near-Field Polarimetry Measurements.

The experimental scheme for the near-field

polarimetry measurement is explained in the Results and Discussion section; Near-Field Polarimetry.

To generate a linearly polarized optical field with an azimuth angle θin at the tip

aperture of the probe, we precompensated the polarization characteristics of the fiber probe with a combination of a linear polarizer, half-wave plate, and quarter-wave plate.

The scattered optical

field to be analyzed travelled through an objective lens (NA = 0.4), photoelastic modulator (PEM) and linear polarizer before detection by a photomultiplier tube [23].

The PEM modulated the

phase of the y-component of the electric field at the mechanical resonance frequency (Ω = 42 kHz), and, thus, the detected signal contains a frequency component of nΩ (where n is an integer).

By

demodulating the detected signal for the nΩ-components with a lock-in detection technique, information for the amplitude and phase of the electric field are simultaneously obtained.

We

note that the absolute values of the x- and y-components of the electric field cannot be measured because of the intrinsic limitation of this method, and, therefore, the range of the values for the


ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 27

azimuth angle is limited from +45º to –45º (0º and 90º cannot be distinguished from each other, for example).

Associated content Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Supporting notes, figures, tables and equations (PDF)

Author Information Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] ORCID Shun Hashiyada: 0000-0002-3229-538X Hiromi Okamoto: 0000-0003-0082-8652 Notes The authors declare no competing financial interests.


Page 23 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Acknowledgements The authors thank Ms. A. Ishikawa (IMS) for the nanostructured sample fabrication and Dr. S. Nakao (IMS) for his help in operation of the scanning electron microscope.

This work was

supported by Grants-in-Aid for Scientific Research (KAKENHI) (No. JP16H06505 in Scientific Research on Innovative Areas “Nano-Material Optical-Manipulation”, Nos. JP22225002, JP15H02161, JP15K13683 to H.O., JP17H07330, JP15J01261 to S.H., and JP17H03014 to T.N.) from the Japan Society for the Promotion of Science (JSPS), JSPS Core-to-Core Program (A. Advanced Research Networks), JST PRESTO Grant (No. JPMJPR14KB to T.N.), and the Photon Frontier Network Program of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan.

References [1] Atkins, P. W.; de Paula, J. Physical Chemistry, 10th ed.; Oxford Univ. Press: Oxford, U.K., 2014.

[2] Barron, L. D. Molecular Light Scattering and Optical Activity, 2nd ed.; Cambridge Univ. Press: Cambridge, U.K., 2004.

[3] Hendry, E. et al. Ultrasensitive detection and characterization of biomolecules using superchiral fields. Nat. Nanotechnol. 2010, 5, 783–787.


ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 27

[4] Stanciu, C. D. et al. All-optical magnetic recording with circularly polarized light. Phys. Rev. Lett. 2007, 99, 047601.

[5] Mamyouda, S. et al. Circularly polarized near-field optical mapping of spin-resolved quantum Hall chiral edge states. Nano Lett. 2015, 15, 2417–2421.

[6] Schäferling, M.; Dregely, D.; Hentschel, M.; Giessen, H. Tailoring enhanced optical chirality: design principles for chiral plasmonic nanostructures. Phys. Rev. X 2012, 2, 031010.

[7] Schäferling, M.; Yin, X.; Giessen, H. Formation of chiral fields in a symmetric environment. Opt. Express 2012, 20, 26326–26336.

[8] Davis, T. J.; Hendry, E. Superchiral electromagnetic fields created by surface plasmons in nonchiral metallic nanostructures. Phys. Rev. B 2013, 87, 085405.

[9] Hashiyada, S.; Narushima, T.; Okamoto, H. Imaging chirality of optical fields near achiral metal nanostructures excited with linearly polarized light. ACS Photon. 2018, 5, 1486–1492.

[10] Zhang, S. et al. Chiral surface plasmon polaritons on metallic nanowires. Phys. Rev. Lett. 2011, 107, 096801.

[11] Tian, X.; Fang, Y.; Sun, M. Formation of enhanced uniform chiral fields in symmetric dimer nanostructures. Sci. Rep. 2015, 5, 17534; doi: 10.1038/srep17534.


Page 25 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

[12] Born, M.; Wolf, E. Principle of Optics, 7th ed.; Cambridge Univ. Press: Cambridge, U.K., 1999.

[13] Imura, K.; Okamoto, H. Reciprocity in scanning near-field optical microscopy: illumination and collection modes of transmission measurements. Opt. Lett. 2006, 31, 1474–1476.

[14] Pham, A.; Zhao, A.; Genet, C.; Drezet, A. Optical chirality density and flux measured in the local density of states of spiral plasmonic structures. Phys. Rev. A 2018, 98, 013837.

[15] Muskens, O. L. et al. Quantitative absorption spectroscopy of a single gold nanorod. J. Phys. Chem. C 2008, 112, 8917–8921.

[16] Tang, Y.; Cohen, A. E. Optical chirality and its interaction with mater. Phys. Rev. Lett. 2010, 104, 163901.

[17] Tang, Y.; Cohen, A. E. Enhanced enantioselectivity in excitation of chiral molecules by superchiral light. Science 2011, 332, 333.

[18] Choi, J. S.; Cho, M. Limitations of a superchiral field. Phys. Rev. A 2012, 86, 063834.

[19] Canaguier-Durand, A.; Hutchison, J. A.; Genet, C.; Ebbesen, T. W. Mechanical separation of chiral dipoles by chiral light. New J. Phys. 2013, 15, 123037.

[20] Davis, T. J.; Gómez, D. E. Interaction of localized surface plasmons with chiral molecules. Phys. Rev. B 2014, 90, 235424. 25 ACS Paragon Plus Environment

ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 27

[21] Tullius, R. et al. “Superchiral” spectroscopy: detection of protein higher order hierarchical structure with chiral plasmonic nanostructures. J. Am. Chem. Soc. 2015, 137, 8380–8383.

[22] Vernon, K. C. et al. Influence of particle-substrate interaction on localized plasmon resonances. Nano Lett. 2010, 10, 2080–2086.

[23] Sato, K. Measurement of magneto-optical Kerr effect using piezo-birefringent modulator. Jpn. J. Appl. Phys. 1981, 20, 2403–2409.


Page 27 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

For Table of Contents Use Only Manuscript title: Active control of chiral optical near fields on a single metal nanorod

Names of authors: Shun Hashiyada, Tetsuya Narushima, and Hiromi Okamoto

Brief synopsis: The graphic shows experimental evidence that a simple system that consists of an achiral nanostructure (gold nanorod) and an achiral incident radiation field (linearly polarized light) with normal incidence can generate circularly polarized optical fields localized in a nanometer region. Its chirality (handedness) can be actively controlled simply by changing the incident polarization direction.

Through this result we propose a novel concept of creation of localized chiral

electromagnetic fields with chiral plasmons that has not been hitherto discussed.

TOC graphic:


Active control of chiral optical near fields on a single metal nanorod

Recommend Documents