The Surface Plasmon Resonance (SPR) Sensing experiments and computations will immerse participants into the interdisciplinary field of nanoplasmonics, where they will “surf” surface plasmon–polaritons (SPPs)—hybrid electric polarization–photon evanescent waves that travel along the gold–dielectric-fluid interface of an optomicrofluidic sensor device when excited by laser light under specific illumination conditions (wavelength, polarization, incident angle, prism). The devices that Immersion participants will build can register tiny variations (< 10^–3) in the fluid’s refractive index thanks to the resonant nature of the SPP waves, generated from the coupling of the incident electromagnetic waves to the collective plasma oscillations of the metal’s conduction electrons, and because most of the SPP field is concentrated in the dielectric fluid. Understanding the physical principles behind the SPR sensor and optimizing its operation via computational modeling entail learning about several advanced concepts of classical electrodynamics, such as attenuated total internal reflection, optical, evanescent waves and optical tunneling, the frequency-wavevector dispersion of SPP eigenmodes (and the coupling role of the prism), polarization-dependent interfacial boundary conditions imposed on Maxwell’s equations, the Fresnel equations, and the complex-valued, frequency-dependent optical constants of plasmonic metals within the framework of Drude–Lorentz dispersion models.

Figure 1. (a) Exploded-view perspective of the custom-made SPR sensor chip to be assembled by the Immersion participants from off-the-shelf components. (b) Diagram and (c) photograph of a variation of the SPR sensing optical setup. The SPR curve in the upper left of (c) shows a Fresnel-equations fit to actual student-acquired data of reflectance as a function of angle. Adapted from “Surface plasmon resonance sensing in the advanced physics laboratory” by E. U. Donev and students, Am. J. Phys. 90, 865–880 (2022); https://doi.org/10.1119/5.0070022

 The SPR sensor to be assembled, tested, optimized and pushed to its sensitivity limits in this Immersion is illustrated in Fig. 1(a). The key elements are a 50-nm-thin gold film on a glass slide affixed to a microfluidic cell and pressed against a glass prism with index-matching liquid. A typical optical setup incorporating the SPR sensor is laid out in Fig. 1(b, c). Its versatility fosters creativity through variations; for example: dispensing with the polarizer and half-wave plate in favor of a polarized laser emitted from a rotatable cylindrical cavity (as common HeNe are); “physics storeroom” vs. commercial options for a goniometric platform (e.g., substituting a precision rotation stage for the vintage spectroscope shown in Fig. 1(c)); educational vs. research-grade light detectors; forgoing a reference light detector by opting for an (expensive) intensity-stabilized laser; commercial vs. custom-built (e.g., LabVIEW-interfaced) data acquisition systems; and a spectrum of laser diodes as possible incident-light sources. The plastic tubing in the lower right corner of Fig. 1(c) leads to a syringe and a reservoir of dielectric fluid (e.g., water) to be pumped through the microfluidic channel and sensed via the angular shift in the SPR reflectance curve, a measured example of which is plotted in the upper left corner of Fig. 1(b), together with a Fresnel-equations fit to the data.

 The ALPhA Lab Immersion on SPR sensing will offer a hands-on introduction to intricate light-matter interactions at the nanoscale, in particular the conversion of freely propagating transverse electromagnetic waves into evanescent waves under total internal reflection, followed by tunneling through the thin plasmonic-metal film and coupling to longitudinal charge-density oscillations at the other metallic surface. Fundamentally, excitation of these SPP waves requires both momentum- and energy-matching between the incident light and the surface plasmons—a sharply resonant phenomenon that underlies the exceptional sensitivity of this type of SPR sensing to environmental conditions. SPPs, being inherently near-field electromagnetic modes, are central to active areas of research into transcending the limits of classical far-field optics, such as super-resolution imaging, near-field optical spectroscopy, negative-refractive-index metasurfaces, tunable plasmonic coloration, and quantum plasmonics. SPP-based (i.e., utilizing contiguous films rather than nanoparticles) nanoplasmonic biosensors are some of the most sensitive devices for detecting refractive-index changes (smaller than 10^–6 refractive index units have been reported) as well as enabling high-specificity measurements of binding (on-rate) and dissociation (off-rate) kinetics of molecular interactions. Variations of the technique also find extensive use in pharmaceutical research (e.g., ‘lead optimization’ of promising drug candidates), clinical diagnostics (e.g., early detection and monitoring of viral loads), food safety (e.g., pesticide control), explosives detection, hydrogen gas sensing, etc. As of the end of 2025, companies marketing SPP-based SPR sensors and systems include Cytiva, Bruker, Carterra, Sartorius, Reichert (AMETEK), and IBIS Technologies. In a nutshell, film-based SPR sensing is a technologically and commercially important enterprise rooted in the fundamental electrodynamics of light-matter interactions at a dielectric-metal interface. 

 As described in the article cited underneath Fig. 1, our take on SPR sensing combines “physical insights, theoretical models, student-programmed analytical computations, and numerical simulations with student-performed precision angular measurements of SPR reflectance curves and real-time sensing of large and small refractive-index changes—measured using readily customizable optomicrofluidic setups constructed by students with affordable off-the-shelf components”. Participants in this Immersion will deploy their apparatus-building skills and resourcefulness by designing, assembling, aligning, troubleshooting, operating and optimizing a prism-coupled microfluidic plasmonic sensor, based on the angle-dependent SPR reflectance of a gold (Au) nanofilm (see Fig. 1(a)). Best of all, each participant gets to take home their sensor assembly (minus the prism):  the Au-coated glass slide affixed to a microfluidic cell is reusable. To optically interrogate the SPR sensor, participants will construct a versatile, modular setup (see Fig. 1(b, c)) capable of measuring rotational adjustments of 0.1° or better (e.g., the SPR reflectance curve in Fig. 1(b) has a width of approximately 5° at half minimum). Participants will also utilize computations implementing the Fresnel-equations (or, equivalently, the transfer-matrix) formalism, which will guide the initial angular orientation of the SPR sensor for “first light dip” and the subsequent optimizations of its sensitivity for detecting different combinations of dielectric fluids. In order to practice their programming skills as well as (re)acquaint themselves with the physics, optics and mathematics of surface plasmon–polaritons, participants will be encouraged to creating their own SPR reflectance algorithms prior to attending the Immersion; copies of ready-made MATLAB, Mathematica, Maple and Microsoft Excel scripts will be provided on site. Once participants have successfully distinguished air and water with their SPR setups, they will test their precision-measurement skills by attempting lower signal-to-noise detections such as varying concentrations of salt or sugar solutions, the dependence on temperature of the refractive index of water and differentiating between distilled water and commercial mineral waters. The latter challenge will (re)introduce participants to a sophisticated signal-processing technique, homodyne phase-sensitive detection, implemented by means of a lock-in amplifier instrument.

 Below is an overview of the scientific activities prior to and during the three-day Immersion. For convenience, though not necessary, participants should bring a notebook and a laptop with one or more of the above-mentioned software installed (MATLAB, Mathematica, Maple, Microsoft Excel ). The primary safety considerations involve the handling of common organic solvents (ethanol, isopropanol) and the use of Class 2 (< 1 mW) and Class 3R (1–5 mW) lasers.

 ~ Pre-Immersion:
(0.1) Read “Surface plasmon resonance sensing in the advanced physics laboratory” by E. U. Donev and students, Am. J. Phys. 90, 865–880 (2022); https://doi.org/10.1119/5.0070022

(0.2) Simulate the SPR reflectance curve using the Fresnel equations or transfer-matrix method (TMM).

 ~ Immersion Day 1:
(1.1) Assemble an SPR sensor chip for monitoring refractive-index changes of a dielectric fluid by affixing a microfluidic cell to an Au-coated glass slide.
(1.2) Construct an SPR optical setup in the Kretschmann-Raether configuration: HeNe laser, linear polarizer, half-wave waveplate, glass prism with the SPR sensor chip attached to the prism base via index-matching liquid, manual rotation stage, light detector, computer interface.
(1.3) Observe the sharp reflectance dip for p-polarization (and its absence for s-polarization) at the resonant angle of incidence.

 ~ Immersion Day 2:
(2.1) Measure the SPR reflectance curve for air in the microfluidic channel as a function of incident angle.
(2.2) Fit the SPR curve with the Fresnel/TMM formalism to compare the best-fit optical constants and thickness of the Au layer to the nominal/literature values.

(2.3) Iteratively flow air and water through the sensor’s microfluidic channel while monitoring the reflected light at a fixed, simulation-informed angle of incidence (sensorgram).

(2.4) Measure SPR sensorgrams for organic solvents (ethanol, isopropanol), water and air at two different angles of incidence (“naïve” vs. simulation-informed) and compare to Fresnel-equations/TMM simulations.

 ~ Immersion Day 3:

(3.1) Measure SPR sensorgrams for saline/sugar solutions with decreasing concentrations below and above the resonant angle of incidence.

(3.2) Enhance (with a lock-in amplifier) and optimize (selecting the best incident angle based on simulations) the SPR sensor to detect very small changes in the fluid’s refractive index: e.g., water at different temperatures, tap water vs. distilled water vs. mineral water, or even distinguishing between mineral water brands.

 Overall, the SPR sensing setup is very versatile; the only must-haves to observe variations in the SPR reflectance are the Au-coated glass (plus index-matching fluid), prism, microfluidic cell, and polarized monochromatic light source. The upfront equipment cost for the first setup would be at most $2000, sans the ubiquitous HeNe laser. Each additional setup, however, would cost less than $1000 because the two most expensive items are multipacks: Au(50 nm)-coated coverslips at $850/20 and microfluidic cells at $500/45. The next most expensive items, a precision rotation stage at $400 (e.g., Thorlabs) and a computer-interfaces light detector at $300 (e.g., PASCO) are needed separately for each setup, though suitable substitutes are often available in a general optics laboratory (e.g., a vintage three-legged spectrometer with a goniometer turntable). Glass prisms are readily available at prices in the $10–$100+ range. As a low-cost benchmark, one of us (Donev) has routinely had students measure air-vs-water SPR sensorgrams with a $1 polarized laser module and without a polarizer, waveplate, rotation stage or reference detector.