A wireless power system, comprising: an external module (100; 400) including: multiple sub-wavelength structures (102; 102C; 102D; 102E; 1021; 102J; 102K; 302; 302C; 302D; 302E; 3021; 302J; 302K; 502a-n) configured to transmit respective wireless radio-frequency power signals, in response to respective excitation signals provided to the sub-wavelength structures (102; 102C; 102D; 102E; 1021; 102J; 102K; 302; 302C; 302D; 302E; 3021; 302J; 302K; 502a-n), by manipulating evanescent fields outside of a patient's tissue to generate propagating fields inside the patient's tissue and thereby generate a spatially focused field in tissue, wherein the sub-wavelength structures (102; 102C; 102D; 102E; 1021; 102J; 102K; 302; 302C; 302D; 302E; 3021; 302J; 302K; 502a-n) are defined with respect to a wavelength of the power signals outside of the patient's tissue or in the air; and a controller (520, 522, 524, 526; 524, 528) configured to adjust a phase and/or an amplitude of one or more of the excitation signals provided to the multiple sub-wavelength structures (102; 102C; 102D; 102E; 1021; 102J; 102K; 302; 302C; 302D; 302E; 3021; 302J; 302K; 502a-n) to adjust a focus of the spatially focused field in the tissue; and an implantable module (318; 418; 630, 632, 634, 636; 730, 740, 742, 744; 746, 748, 750, 752) configured to wirelessly receive power from the external module (100; 400) via the transmitted power signals, the implantable module (318; 418; 630, 632, 634, 636; 730, 740, 742, 744; 746, 748, 750, 752) including at least one sensor or stimulator configured to sense a parameter of the tissue or apply stimulation to the tissue. an external module (100; 400) including: multiple sub-wavelength structures (102; 102C; 102D; 102E; 1021; 102J; 102K; 302; 302C; 302D; 302E; 3021; 302J; 302K; 502a-n) configured to transmit respective wireless radio-frequency power signals, in response to respective excitation signals provided to the sub-wavelength structures (102; 102C; 102D; 102E; 1021; 102J; 102K; 302; 302C; 302D; 302E; 3021; 302J; 302K; 502a-n), by manipulating evanescent fields outside of a patient's tissue to generate propagating fields inside the patient's tissue and thereby generate a spatially focused field in tissue, wherein the sub-wavelength structures (102; 102C; 102D; 102E; 1021; 102J; 102K; 302; 302C; 302D; 302E; 3021; 302J; 302K; 502a-n) are defined with respect to a wavelength of the power signals outside of the patient's tissue or in the air; and a controller (520, 522, 524, 526; 524, 528) configured to adjust a phase and/or an amplitude of one or more of the excitation signals provided to the multiple sub-wavelength structures (102; 102C; 102D; 102E; 1021; 102J; 102K; 302; 302C; 302D; 302E; 3021; 302J; 302K; 502a-n) to adjust a focus of the spatially focused field in the tissue; and multiple sub-wavelength structures (102; 102C; 102D; 102E; 1021; 102J; 102K; 302; 302C; 302D; 302E; 3021; 302J; 302K; 502a-n) configured to transmit respective wireless radio-frequency power signals, in response to respective excitation signals provided to the sub-wavelength structures (102; 102C; 102D; 102E; 1021; 102J; 102K; 302; 302C; 302D; 302E; 3021; 302J; 302K; 502a-n), by manipulating evanescent fields outside of a patient's tissue to generate propagating fields inside the patient's tissue and thereby generate a spatially focused field in tissue, wherein the sub-wavelength structures (102; 102C; 102D; 102E; 1021; 102J; 102K; 302; 302C; 302D; 302E; 3021; 302J; 302K; 502a-n) are defined with respect to a wavelength of the power signals outside of the patient's tissue or in the air; and a controller (520, 522, 524, 526; 524, 528) configured to adjust a phase and/or an amplitude of one or more of the excitation signals provided to the multiple sub-wavelength structures (102; 102C; 102D; 102E; 1021; 102J; 102K; 302; 302C; 302D; 302E; 3021; 302J; 302K; 502a-n) to adjust a focus of the spatially focused field in the tissue; and an implantable module (318; 418; 630, 632, 634, 636; 730, 740, 742, 744; 746, 748, 750, 752) configured to wirelessly receive power from the external module (100; 400) via the transmitted power signals, the implantable module (318; 418; 630, 632, 634, 636; 730, 740, 742, 744; 746, 748, 750, 752) including at least one sensor or stimulator configured to sense a parameter of the tissue or apply stimulation to the tissue.