10 µm
DIODE LASERS
Exploring micro- and nanostructures
with tunable diode lasers
RUDOLF NEUHAUS
Micro- and nanostructures are becoming increasingly important for fundamental research and applied quantum
technology. Prominent examples of such
structures are microcavities and quantum dots, and examples of important
applications include single or entan-
gled photon sources, qubits for quantum computers, and various sensors.
The structures also enable investigations
at the quantum limit, such as quantum
oscillations in microcavities, quantum
electrodynamics (QED) with quantum
dots, or even cavity QED studies with
single quantum dots in cavities.
Many applications
require resonant optical
excitation with suitable
tunable continuous-wave
(CW) lasers. By optical-
ly pumping microcavities
at the right wavelength, one can even
create microscopic coherent frequency
combs and short optical pulses—a very
promising application that is expected
to have significant impact on photonics.
Microcavities
Quantum properties are usually not observable in macroscopic objects because
of environmental decoherence unless
specific sample geometries and cooling
are utilized. Employing microcavities,
for example, is one possibility to observe quantum effects in relatively large,
micrometer-scaled structures. Figure 1
illustrates an isolated, donut-shaped,
~30-µm-diameter glass microcavity that
combines a macroscopic mechanical os-
cillator and a ring-shaped, high-Q optical cavity. Light, coupled into the cavity via an evanescent field, bounces off
the walls of the donut by total internal reflection, transferring a small force
on the structure by radiation pressure.
In this way, the coupled light can in-
fluence the vibrational behavior of the
structure and vice versa. This property
turns microcavities into exciting objects
for quantum research. For example,
researchers observed such parametric
coupling between light and mechani-
cal oscillations, 1 and have also used a
sensor that is based on optomechanical
coupling for active feedback cooling of
such a microcavity. 2
Because of their small size, the free
spectral range of microcavities is rela-
tively large and tiny-sized deviations
cause large spectral shifts of the cavi-
ty resonances. So, a widely mode-hop-
free tunable laser is an invaluable tool
to find and study the resonance frequen-
cies of microcavities or to scan across
more than a single free spectral range of
the cavity. In addition, the laser has to
have low noise in power and frequency
to avoid spuriously exciting mechanical
oscillations where they are not wanted.
The dependence of the microcavity
resonance frequencies on size and oth-
er environmental parameters can be ex-
ploited for a promising application: la-
bel-free detection of single biological
molecules in solution. This is enabled
using a microtoroid optical resonator
in combination with a widely tunable
mode-hop-free laser (such as Toptica’s
DLC C TL). Researchers have described
how such a laser is frequency-stabilized
to a microtoroid optical resonator and
how shifts of the optical resonance fre-
quency caused by molecules binding
to the resonator are observed. 3 In this
way, particles with radii between 2 and
100 nm are detected and distinguished.
The results are further extended to-
ward creating a noninvasive tumor biop-
sy assay, and provide a basis for an op-
tical mass spectrometer in solution. For
this application, not only is wide mode-
hop-free tuning required, but also the
ability to conveniently stabilize the laser
Optically pumping microcavities,
advancing quantum technologies,
and exploiting quantum dots are
applications of tunable diode lasers
that may significantly impact photonics.
