Digital laser for on-demand intracavity selective excitation of second harmonic higher-order modes.

In this article, we demonstrate selective excitation of second harmonic higher-order modes inside a diode end-pumped solid-state laser resonator that comprises of a nonlinear potassium titanyl phosphate (KTP) crystal and a digitally addressed holographic end-mirror in a form of a reflective phase-only spatial light modulator (SLM). The emitted second harmonic higher-order modes at 532 nm are generated by an intracavity nonlinear KTP crystal that is pumped by high-order fundamental modes operating at 1064 nm. The fundamental modes are digitally controlled by displaying a computer-generated hologram in the form of a grey-scale image to the SLM screen for on-demand high-order modes. The phase matching of the fundamental mode to the generated frequency-doubled mode is achieved by controlling the phase of the digital hologram to either achieve a high or quasi-degree of orbital angular momentum conservation. We show that we can intracavity generate frequency-doubled high-order Laguerre-Gaussian modes and Hermit-Gaussian modes that are either quasi or fully reproducible in the far-field. To the best of our knowledge, this is the first laser to generate frequency-doubled on-demand higher-order modes inside the cavity at the visible (green) wavelength of 532 nm.

Direct fiber excitation with a digitally controlled solid state laser source.

Mode division multiplexing has been mooted as a future technology to address the impending data crunch of existing fiber networks. Present demonstrations delineate the light source from the mode creation steps, potentially inhibiting integrated solutions. Here we create fiber modes on demand with a digital laser and couple them directly into a few-mode fiber, where after transmission they are decoupled by modal decomposition. This is the first demonstration of an integrated source for encoding information into the spatial modes of light.

Doughnut laser beam as an incoherent superposition of two petal beams.

Laguerre-Gaussian beams with a nonzero azimuthal index are known to carry orbital angular momentum (OAM), and are routinely created external to laser cavities. The few reports of obtaining such beams from laser cavities suffer from inconclusive evidence of the real electromagnetic field. In this Letter we revisit this question and show that an observed doughnut beam from a laser cavity may not be a pure Laguerre-Gaussian azimuthal mode but can be an incoherent sum of petal modes, which do not carry OAM. We point out the requirements for future analysis of such fields from laser resonators.

Tuneable Gaussian to flat-top resonator by amplitude beam shaping.

We outline a simple laser cavity comprising an opaque ring and a circular aperture that is capable of producing spatially tuneable laser modes, from a Gaussian beam to a Flat-top beam. The tuneability is achieved by varying the diameter of the aperture and thus requires no realignment of the cavity. We demonstrate this principle using a digital laser with an intra-cavity spatial light modulator, and confirm the predicted properties of the resonator experimentally.

Exciting higher-order radial Laguerre-Gaussian modes in a diode-pumped solid-state laser resonator.

In this paper we experimentally demonstrate the intracavity generation of selected Laguerre-Gaussian modes of variable radial order, from 0 to 5. Our technique requires only an amplitude mask made up of absorbing rings to be placed inside the cavity, with the ring radii selected to coincide with the zeros of the desired Laguerre-Gaussian mode. We demonstrate high mode purity and a mode volume proportional to the order of the mode. Our results suggest a possible route to high brightness diode-pumped solid-state laser sources.

Modal decomposition without a priori scale information.

The modal decomposition of an arbitrary optical field may be done without regard to the spatial scale of the chosen basis functions, but this generally leads to a large number of modes in the expansion. While this may be considered as mathematically correct, it is not efficient and not physically representative of the underlying field. Here we demonstrate a modal decomposition approach that requires no a priori knowledge of the spatial scale of the modes, but nevertheless leads to an optimised modal expansion. We illustrate the power of the method by successfully decomposing beams from a diode-pumped solid state laser resonator into an optimised Laguerre-Gaussian mode set. Our experimental results, which are in agreement with theory, illustrate the versatility of the approach.

Graphene for improved femtosecond laser based pluripotent stem cell transfection.

Pluripotent stem cells are hugely attractive in the tissue engineering research field as they can self-renew and be selectively differentiated into various cell types. For stem cell and tissue engineering research it is important to develop new, biocompatible scaffold materials and graphene has emerged as a promising material in this area as it does not compromise cell proliferation and accelerates specific cell differentiation. Previous studies have shown a non-invasive optical technique for mouse embryonic stem (mES) cell differentiation and transfection using femtosecond (fs) laser pulses. To investigate cellular responses to the influence of graphene and laser irradiation, here we present for the first time a study of mES cell fs laser transfection on graphene coated substrates. First we studied the impact of graphene on Chinese Hamster Ovary (CHO-K1) cell viability and cell cytotoxicity in the absence of laser exposure. These were tested via evaluating the mitochondrial activity through adenosine triphosphates (ATP) luminescence and breakages on the cell plasma membrane assessed using cytosolic lactate dehydrogenase (LDH) screening. Secondly, the effects of fs laser irradiation on cell viability and cytotoxicity at 1064 and 532 nm for cells plated and grown on graphene and pure glass were assessed. Finally, optical transfection of CHO-K1 and mES cells was performed on graphene coated versus plain glass substrates. Our results show graphene stimulated cell viability whilst triggering a mild release of intracellular LDH. We also observed that compared to pure glass substrates; laser irradiation at 1064 nm on graphene plates was less cytotoxic. Finally, in mES cells efficient optical transfection at 1064 (82%) and 532 (25%) nm was obtained due to the presence of a graphene support as compared to pristine glass. Here we hypothesize an up-regulation of cell adhesion promoting peptides or laminin-related receptors of the extracellular matrix (ECM) in cell samples grown and irradiated on graphene substrates. By bringing together advances in optics and nanomaterial sciences we demonstrate pathways for enhancement of pluripotent stem cell biology.

A digital laser for on-demand laser modes.

Customizing the output beam shape from a laser invariably involves specialized optical elements in the form of apertures, diffractive optics and free-form mirrors. Such optics require considerable design and fabrication effort and suffer from the further disadvantage of being immutably connected to the selection of a particular spatial mode. Here we overcome these limitations with the first digital laser comprising an electrically addressed reflective phase-only spatial light modulator as an intra-cavity digitally addressed holographic mirror. The phase and amplitude of the holographic mirror may be controlled simply by writing a computer-generated hologram in the form of a grey-scale image to the device, for on-demand laser modes. We show that we can digitally control the laser modes with ease, and demonstrate real-time switching between spatial modes in an otherwise standard solid-state laser resonator. Our work opens new possibilities for the customizing of laser modes at source.